**Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes**

Sergey Pogodin, Shmuel Cohen, Tahani Mala'bi and Israel Agranat *Organic Chemistry, Institute of Chemistry, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Jerusalem Israel* 

#### **1. Introduction**

"Acylation differs from alkylation in being virtually irreversible" [Olah, 1973], free of rearrangements and isomerizations [Wang, 2009; Norman & Taylor, 1965]. This authoritative exposition of the state of the art of Friedel–Crafts chemistry in 1973 close to the centennial of the invention of the Friedel–Crafts reaction has been long recognized and not without reason. The difference in behavior between Friedel–Crafts acylation and Friedel– Crafts alkylation was attributed to the resonance stabilization existing between the acyl group and the aromatic nucleus [Buehler & Pearson, 1970], which may serve as a barrier against rearrangements and reversible processes. However, if the acyl group is tilted out of the plane of the aromatic nucleus, e.g., by bulky substituents, the resonance stabilization is reduced and the pattern of irreversibility of Friedel–Crafts acylation may be challenged [Buehler & Pearson, 1970; Pearson & Buehler, 1971; Gore, 1974]. Under these conditions deacylations and acyl rearrangements become feasible [Buehler & Pearson, 1970; Pearson & Buehler, 1971; Gore, 1974].

Fig. 1. The Friedel–Crafts acyl rearrangement of 1- and 2-benzoylnaphthalenes in PPA

The concept of reversibility in Friedel–Crafts acylations [Gore, 1955, 1964] was put forward in 1955 by Gore, who proposed that "the Friedel–Crafts acylation reaction of reactive hydrocarbons is a reversible process" [Gore, 1955]. Gore concluded that "Reversibility is an important factor in acylation reactions" [Gore, 1955]. The reversibility studies have been

Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes 5

mechanism involves the two dibenzoylnaphthalenes, their *O*-protonates and their σcomplexes. In the kinetically controlled pathway 1σ-BzNAH+ is more stable than 2σ-BzNAH+ and by virtue of the Hammond–Leffler postulate [Muller, 1994] the transition state leading to 1σ-BzNAH+ is lower in energy than the transition state leading to 2σ-BzNAH+. Thus, 1-BzNA is the kinetically controlled product. By contrast, in the thermodynamically controlled pathway, 1-BzNAH+ and 1-BzNA are less stable than 2-BzNAH+ and 2-BzNA, respectively. Therefore, 2-BzNA is the thermodynamically controlled product. Under conditions of thermodynamic control, the relative stabilities of the constitutional isomers of a given PAK are detrimental to the products of the Friedel–Crafts acyl rearrangement of the

HO O

1-BzNAH+ 1*Z*-BzNAH<sup>+</sup>

OH

Fig. 3. The mechanism of the Friedel–Crafts acyl rearrangement of representative PAKs 1-

Anthracene (AN) is essentially a planar PAH. Due to its *D*2*<sup>h</sup>* symmetry, three constitutional isomers of acetylanthracenes (AcAN) are possible: 1-acetylanthracene (1-AcAN), 2 acetylanthracene (2-AcAN), and 9-acetylanthracene (9-AcAN) (see Fig. 4). These isomers differ in the position of the acetyl substituent at the anthracene ring system. The three constitutional isomers of AcAN can be categorized, depending on the degree of their overcrowding: (i) the non-overcrowded isomer 2-AcAN, in which the acetyl group is flanked by two *ortho*-hydrogens (H1, H3); (ii) the overcrowded isomer 1-AcAN, in which the overcrowding is due to the presence of one hydrogen atom (H9) *peri* to the acetyl group; (iii) the severely overcrowded isomer 9-AcAN (assuming the planar conformation), in which the overcrowding is due to the presence of two *peri*-hydrogens (H1, H8) to the acetyl group. The

2*E*-BzNAH<sup>+</sup>

**2. Structures of monoacetylanthracenes and diacetylanthracenes** 

O

NA

Bz<sup>+</sup>

O

H

2-BzNAH<sup>+</sup>

H

PAK and of the Friedel–Crafts acylation of the corresponding PAH.

PPA


and 2-benzoylnaphthalenes.

O

1*Z*-BzNA

2*E*-BzNA

focused mainly on unusual aspects of selectivity, including deacylations, one-way rearrangements and kinetic versus thermodynamic control [Gore, 1974]. Under classical Friedel–Crafts conditions (e. g., AlCl3 and a trace of water), the pattern of irreversibility (e. g., in the naphthalene series) has been highlighted [Gore, 1964, 1974; Andreou et al., 1978; Dowdy et al., 1991].

The incursion of reversibility in Friedel–Crafts acylations was revealed by Agranat, et al. in the benzoylation of naphthalene in polyphosphoric acid (PPA) at elevated temperatures (Fig. 1) [Agranat et al., 1974]. The kinetically controlled 1-benzoylnaphthalene rearranged to the thermodynamically controlled 2-benzoylnaphthalene (PPA, 140 °C) (*vide infra*). The reversibility concept was then applied to the synthesis of linearly annelated polycyclic aromatic ketones by intramolecular Friedel–Crafts rearrangements of their angularly annelated constitutional isomers [Agranat & Shih, 1974a; Heaney, 1991]. The Haworth synthesis of PAHs, which previously had allowed access to angularly annelated PAHs could thus be applied to the synthesis of linearly annelated PAHs [Agranat & Shih, 1974b]. Further experimental evidence in support of true reversibility of Friedel–Crafts acylation is limited [Frangopol et al., 1964; Balaban, 1966; Nenitzescu & Balaban, 1964; Effenberger et al., 1973; Levy et al., 2007; Mala'bi et al,. 2009; Titinchi et al., 2008; Adams et al., 1998; Okamoto & Yonezawa, 2009]. Notable cases are the report by Balaban [Frangopol et al., 1964; Balaban, 1966; Nenitzescu & Balaban, 1964] on the reversibility of Friedel–Crafts acetylation of olefins to β-chloroketones, the report by Effenberger [Effenberger et al., 1973] of the retro-Fries rearrangement of phenyl benzoates (CF3SO3H, 170 °C) and the reversible ArSE aroylation of naphthalene derivatives [Okamoto & Yonezawa, 2009]. Additional examples are the acyl rearrangements of acetylphenanthrenes [Levy et al., 2007] and acetylanthracenes [Mala'bi et al., 2009] in PPA, the acetylation of fluorene [Titinchi et al., 2008], the disproportionation of 9 acetylanthracene into 1,5- and 1,8-diacetylanthracenes in an ionic liquid systems [Adams et al., 1998]. Complete reversibility of Friedel–Crafts acylation was established in the intramolecular *para ortho* acyl rearrangements of fluorofluorenones in PPA (Fig. 2) [Agranat et al., 1977]. Friedel–Crafts acyl rearrangement of polycyclic aromatic ketones (PAKs) has been referred to as the Agranat–Gore rearrangement [Levy et al., 2007; Mala'bi et al., 2009]. The Friedel–Crafts acylation can be adjusted to give a kinetically controlled ketone or a thermodynamically controlled ketone [Buehler & Pearson, 1970]. Acyl rearrangements and reversibility in Friedel– Crafts acylations have been associated with thermodynamic control [Pearson & Buehler, 1971; Andreou et al., 1978; Agranat et al., 1977]. The contributions of kinetic control vs. thermodynamic control in Friedel–Crafts acyl rearrangements remain an open question, in spite of the rich chemistry of Friedel–Crafts acylations. We have recently shown that kinetic control wins out over thermodynamic control in the Friedel–Crafts acyl rearrangement of diacetylanthracenes in PPA [Mala'bi et al., 2011].

Fig. 2. The Friedel–Crafts intramolecular acyl rearrangements of fluorofluorenones in PPA

A plausible mechanism of the Friedel–Crafts acyl rearrangement of 1-benzoylnaphthalene (1-BzNA) into 2-benzoylnaphthalene (2-BzNA) in PPA, is presented in Fig. 3. The

focused mainly on unusual aspects of selectivity, including deacylations, one-way rearrangements and kinetic versus thermodynamic control [Gore, 1974]. Under classical Friedel–Crafts conditions (e. g., AlCl3 and a trace of water), the pattern of irreversibility (e. g., in the naphthalene series) has been highlighted [Gore, 1964, 1974; Andreou et al., 1978;

The incursion of reversibility in Friedel–Crafts acylations was revealed by Agranat, et al. in the benzoylation of naphthalene in polyphosphoric acid (PPA) at elevated temperatures (Fig. 1) [Agranat et al., 1974]. The kinetically controlled 1-benzoylnaphthalene rearranged to the thermodynamically controlled 2-benzoylnaphthalene (PPA, 140 °C) (*vide infra*). The reversibility concept was then applied to the synthesis of linearly annelated polycyclic aromatic ketones by intramolecular Friedel–Crafts rearrangements of their angularly annelated constitutional isomers [Agranat & Shih, 1974a; Heaney, 1991]. The Haworth synthesis of PAHs, which previously had allowed access to angularly annelated PAHs could thus be applied to the synthesis of linearly annelated PAHs [Agranat & Shih, 1974b]. Further experimental evidence in support of true reversibility of Friedel–Crafts acylation is limited [Frangopol et al., 1964; Balaban, 1966; Nenitzescu & Balaban, 1964; Effenberger et al., 1973; Levy et al., 2007; Mala'bi et al,. 2009; Titinchi et al., 2008; Adams et al., 1998; Okamoto & Yonezawa, 2009]. Notable cases are the report by Balaban [Frangopol et al., 1964; Balaban, 1966; Nenitzescu & Balaban, 1964] on the reversibility of Friedel–Crafts acetylation of olefins to β-chloroketones, the report by Effenberger [Effenberger et al., 1973] of the retro-Fries rearrangement of phenyl benzoates (CF3SO3H, 170 °C) and the reversible ArSE aroylation of naphthalene derivatives [Okamoto & Yonezawa, 2009]. Additional examples are the acyl rearrangements of acetylphenanthrenes [Levy et al., 2007] and acetylanthracenes [Mala'bi et al., 2009] in PPA, the acetylation of fluorene [Titinchi et al., 2008], the disproportionation of 9 acetylanthracene into 1,5- and 1,8-diacetylanthracenes in an ionic liquid systems [Adams et al., 1998]. Complete reversibility of Friedel–Crafts acylation was established in the intramolecular *para ortho* acyl rearrangements of fluorofluorenones in PPA (Fig. 2) [Agranat et al., 1977]. Friedel–Crafts acyl rearrangement of polycyclic aromatic ketones (PAKs) has been referred to as the Agranat–Gore rearrangement [Levy et al., 2007; Mala'bi et al., 2009]. The Friedel–Crafts acylation can be adjusted to give a kinetically controlled ketone or a thermodynamically controlled ketone [Buehler & Pearson, 1970]. Acyl rearrangements and reversibility in Friedel– Crafts acylations have been associated with thermodynamic control [Pearson & Buehler, 1971; Andreou et al., 1978; Agranat et al., 1977]. The contributions of kinetic control vs. thermodynamic control in Friedel–Crafts acyl rearrangements remain an open question, in spite of the rich chemistry of Friedel–Crafts acylations. We have recently shown that kinetic control wins out over thermodynamic control in the Friedel–Crafts acyl rearrangement of

<sup>O</sup> <sup>O</sup> <sup>F</sup>

Fig. 2. The Friedel–Crafts intramolecular acyl rearrangements of fluorofluorenones in PPA A plausible mechanism of the Friedel–Crafts acyl rearrangement of 1-benzoylnaphthalene (1-BzNA) into 2-benzoylnaphthalene (2-BzNA) in PPA, is presented in Fig. 3. The

PPA

F

Dowdy et al., 1991].

diacetylanthracenes in PPA [Mala'bi et al., 2011].

mechanism involves the two dibenzoylnaphthalenes, their *O*-protonates and their σcomplexes. In the kinetically controlled pathway 1σ-BzNAH+ is more stable than 2σ-BzNAH+ and by virtue of the Hammond–Leffler postulate [Muller, 1994] the transition state leading to 1σ-BzNAH+ is lower in energy than the transition state leading to 2σ-BzNAH+. Thus, 1-BzNA is the kinetically controlled product. By contrast, in the thermodynamically controlled pathway, 1-BzNAH+ and 1-BzNA are less stable than 2-BzNAH+ and 2-BzNA, respectively. Therefore, 2-BzNA is the thermodynamically controlled product. Under conditions of thermodynamic control, the relative stabilities of the constitutional isomers of a given PAK are detrimental to the products of the Friedel–Crafts acyl rearrangement of the PAK and of the Friedel–Crafts acylation of the corresponding PAH.

Fig. 3. The mechanism of the Friedel–Crafts acyl rearrangement of representative PAKs 1 and 2-benzoylnaphthalenes.

#### **2. Structures of monoacetylanthracenes and diacetylanthracenes**

Anthracene (AN) is essentially a planar PAH. Due to its *D*2*<sup>h</sup>* symmetry, three constitutional isomers of acetylanthracenes (AcAN) are possible: 1-acetylanthracene (1-AcAN), 2 acetylanthracene (2-AcAN), and 9-acetylanthracene (9-AcAN) (see Fig. 4). These isomers differ in the position of the acetyl substituent at the anthracene ring system. The three constitutional isomers of AcAN can be categorized, depending on the degree of their overcrowding: (i) the non-overcrowded isomer 2-AcAN, in which the acetyl group is flanked by two *ortho*-hydrogens (H1, H3); (ii) the overcrowded isomer 1-AcAN, in which the overcrowding is due to the presence of one hydrogen atom (H9) *peri* to the acetyl group; (iii) the severely overcrowded isomer 9-AcAN (assuming the planar conformation), in which the overcrowding is due to the presence of two *peri*-hydrogens (H1, H8) to the acetyl group. The

Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes 7

Of the three monoacetylanthracenes and eleven diacetylanthracenes included in the present study, only the crystal structures of 1-AcAN [Langer & Becker, 1993], 9-AcAN [Anderson et al., 1984; Zouev et al., 2011] and 1,5-AcAN [Li & Jing, 2006] have previously been described. The molecular and crystal structures of 2-AcAN, 1,6-Ac2AN, 1,7-Ac2AN, 1,8-Ac2AN, 2,7- Ac2AN and 9,10-Ac2AN are reported here for the first time, along with the previously

Table 1 shows the crystallographic data for 2-AcAN, 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN, 1,8- Ac2AN, 2,7-Ac2AN and 9,10-Ac2AN.1 The ORTEP diagrams of 2-AcAN and of the six diacetylanthracenes as determined by X-ray crystallography are presented in Fig. 6–10. Ketones 2-AcAN, 1,5-Ac2AN, 1,8-Ac2AN and 2,7-Ac2AN crystallize in the monoclinic space groups *P*21/*n* , *P*21/*c*, *P*2/*n* and *I*2/*a*, respectively. The unit cell dimensions of the crystal structure of 1,5-Ac2AN are essentially identical to those reported in the literature [Li & Jing, 2006]. Ketones 1,6-Ac2AN and 1,7-Ac2AN crystallize in the triclinic space group *P*-1. Ketone 9,10-Ac2AN crystallizes in the orthorhombic space group *Pna*21. Table 2 gives selected geometrical parameters derived from the X-ray crystal structures of the mono- and diacetylanthracenes under study. The following geometrical parameters were considered: the twist angles τ(Carom–Carom–Ccarb–O) (divided into τ1, τ2 and τ9 depending on the position of the acetyl group) and υ(Carom–Carom–Ccarb–O) around the anthracenyl-carbonyl bond; the dihedral angle θ between the least-square planes of the carbonyl group and the anthracene system; the dihedral angle φ between the least-square planes of two side rings of the anthracene system; the pyramidalization angles χ at Carom and Ccarb. Table 3 gives the bond lengths in the mono- and diacetylanthracenes under study, as compared with the parent

The data presented in Table 3 indicate the considerable variation in bond lengths in monoand diacetylanthracenes. The bond lengths may be classified into several types: four C1–C2, or α-β, bonds (134.2–137.7 pm), two C2–C3, or β-β, bonds (138.7–144.4 pm), four C1–C9a bonds (141.8–145.5 pm), four C9a–C9 bonds (138.3–140.9 pm), and two C4a–C9a bonds (142.8– 145.3 pm). These values are in the same range as the respective bond lengths in the X-ray crystal structure of anthracene, which are 136.1, 142.8, 143.4, 140.1 and 143.6 pm [Brock & Dunitz, 1990]. It has previously been shown that the bond lengths in anthracene are in agreement with the superposition of its four Kekulé structures and with the free valence numbers [Sinclair et al., 1950]. Table 3 also shows that the bonds adjacent to the acetyl group are elongated as compared to the respective bonds in anthracene, e.g. the C2–C3 bond in 2- AcAN (143.2 pm vs. 142.8 pm), the C1–C2 bonds in 1,5-Ac2AN and 1,8-Ac2AN (137.5 pm vs. 136.1 pm), the C5–C6 bonds in 1,6-Ac2AN (137.0 pm vs. 136.1 pm), the C7–C8 bond in 1,7- Ac2AN (137.4 pm vs. 136.1 pm) and the C2–C3 bond in 2,7-Ac2AN (144.4 pm vs. 142.8 pm). This elongation effect stems from the contributions of dipolar Kekulé structures, in which the anthracene bonds adjacent to the acetyl group are necessarily single. This effect, however, is not noticeable in 9,10-Ac2AN, because the carbonyl groups are almost perpendicular to the aromatic plane and are hardly conjugated with the anthracene system.

1 CCDC 839159 – 839165 contains the supplementary crystallographic data for this paper. These data

can be obtained free of charge from The Cambridge Crystallographic Data Centre via

**2.1 Molecular and crystal structure of monoacetylanthracenes and** 

**2.1.1 Geometries of monoacetylanthracenes and diacetylanthracenes** 

**diacetylanthracenes** 

reported structures.

anthracene.

www.ccdc.cam.ac.uk/data\_request/cif.

overcrowding in 1-AcAN and 9-AcAN should result in significant deviations of the acetyl groups from the plane of the anthracene nucleus, which is expected to encourage reversibility and rearrangements.

Fig. 4. Constitutional isomers of monoacetylanthracenes (*E* and *Z* stereodescriptors are omitted)

There are 15 constitutional isomers of diacetylanthracenes (Ac2AN), shown in Fig. 5. These isomers differ in the position of the acetyl substituents at the anthracene ring system. The present study encompasses the three monoacetylanthracenes 1-AcAN, 2-AcAN, 9-AcAN and the following eleven diacetylanthracenes: 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN, 1,8-Ac2AN, 1,9- Ac2AN, 1,10-Ac2AN, 2,6-Ac2AN, 2,7-Ac2AN, 2,9-Ac2AN, 2,10-Ac2AN and 9,10-Ac2AN. The remaining diacetylanthracenes, 1,2-Ac2AN, 1,3-Ac2AN, 1,4-Ac2AN and 2,3-Ac2AN were not included in the present study. These constitutional isomers are not expected to be formed in the Friedel–Crafts acylations of 1-AcAN and 2-AcAN, due to the deactivation effect of the electron-withdrawing acetyl group towards further acetylation. This effect is not necessarily operating with respect to the "remote" unsubstituted benzene ring.

In 1-AcAN and 2-AcAN, the *E*- and *Z*-diastereomers should be considered. *E* and *Z* are the stereodescriptors applied to monoacetylanthracenes and diacetylanthracenes with a fractional bond order of the bond between the carbonyl carbon and the corresponding aromatic carbon [Moss, 1996]. In diacetylanthracenes, four diastereomers should be considered: *ZZ*, *ZE*, *EZ* and *EE*. Depending on the symmetry of a given diacetylanthracene, *ZE* and *EZ* diastereomers could be equivalent. 9,10-AcAN is a special case: only one stereodescriptor, *Z* or *E*, is required. In this case, *Z* or *E* refers to whether the carbonyl bonds lie on the same or on the opposite sides of the plane containing the C9–C11 and C10–C12 bonds and perpendicular to the aromatic plane.

Acetylanthracene 2-AcAN and diacetylanthracenes 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN, 1,8- Ac2AN, 2,7-Ac2AN and 9,10-Ac2AN have been synthesized in the present study and their crystal structures have been determined. Ketones 2-AcAN, 1,5-Ac2AN and 1,8-Ac2AN have been prepared by the Friedel–Crafts acetylation of anthracene. Ketones 1,6-Ac2AN, 1,7- Ac2AN and 2,7-Ac2AN have been prepared by the Friedel–Crafts acetylation of 2-AcAN. Ketone 9,10-Ac2AN has been synthesized by methylation (MeLi) of 9,10 dicarbomethoxyanthracene (prepared from 9,10-dibromoanthracene via 9,10 anthracenedicarboxylic acid). Ketones 1,7-Ac2AN and 2,7-Ac2AN are reported here for the first time.

The present study encompasses the crystal and molecular structures of monoacetylanthracenes (AcANs) and diacetylanthracenes (Ac2ANs), the results of a systematic DFT study of the structures and the conformational spaces of AcANs and Ac2ANs, as well as the comparison between the calculated and the experimental structures of these PAKs.

#### **2.1 Molecular and crystal structure of monoacetylanthracenes and diacetylanthracenes**

6 Current Trends in X-Ray Crystallography

overcrowding in 1-AcAN and 9-AcAN should result in significant deviations of the acetyl groups from the plane of the anthracene nucleus, which is expected to encourage

Fig. 4. Constitutional isomers of monoacetylanthracenes (*E* and *Z* stereodescriptors are

operating with respect to the "remote" unsubstituted benzene ring.

bonds and perpendicular to the aromatic plane.

There are 15 constitutional isomers of diacetylanthracenes (Ac2AN), shown in Fig. 5. These isomers differ in the position of the acetyl substituents at the anthracene ring system. The present study encompasses the three monoacetylanthracenes 1-AcAN, 2-AcAN, 9-AcAN and the following eleven diacetylanthracenes: 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN, 1,8-Ac2AN, 1,9- Ac2AN, 1,10-Ac2AN, 2,6-Ac2AN, 2,7-Ac2AN, 2,9-Ac2AN, 2,10-Ac2AN and 9,10-Ac2AN. The remaining diacetylanthracenes, 1,2-Ac2AN, 1,3-Ac2AN, 1,4-Ac2AN and 2,3-Ac2AN were not included in the present study. These constitutional isomers are not expected to be formed in the Friedel–Crafts acylations of 1-AcAN and 2-AcAN, due to the deactivation effect of the electron-withdrawing acetyl group towards further acetylation. This effect is not necessarily

In 1-AcAN and 2-AcAN, the *E*- and *Z*-diastereomers should be considered. *E* and *Z* are the stereodescriptors applied to monoacetylanthracenes and diacetylanthracenes with a fractional bond order of the bond between the carbonyl carbon and the corresponding aromatic carbon [Moss, 1996]. In diacetylanthracenes, four diastereomers should be considered: *ZZ*, *ZE*, *EZ* and *EE*. Depending on the symmetry of a given diacetylanthracene, *ZE* and *EZ* diastereomers could be equivalent. 9,10-AcAN is a special case: only one stereodescriptor, *Z* or *E*, is required. In this case, *Z* or *E* refers to whether the carbonyl bonds lie on the same or on the opposite sides of the plane containing the C9–C11 and C10–C12

Acetylanthracene 2-AcAN and diacetylanthracenes 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN, 1,8- Ac2AN, 2,7-Ac2AN and 9,10-Ac2AN have been synthesized in the present study and their crystal structures have been determined. Ketones 2-AcAN, 1,5-Ac2AN and 1,8-Ac2AN have been prepared by the Friedel–Crafts acetylation of anthracene. Ketones 1,6-Ac2AN, 1,7- Ac2AN and 2,7-Ac2AN have been prepared by the Friedel–Crafts acetylation of 2-AcAN. Ketone 9,10-Ac2AN has been synthesized by methylation (MeLi) of 9,10 dicarbomethoxyanthracene (prepared from 9,10-dibromoanthracene via 9,10 anthracenedicarboxylic acid). Ketones 1,7-Ac2AN and 2,7-Ac2AN are reported here for the

The present study encompasses the crystal and molecular structures of monoacetylanthracenes (AcANs) and diacetylanthracenes (Ac2ANs), the results of a systematic DFT study of the structures and the conformational spaces of AcANs and Ac2ANs, as well as the comparison between the calculated and the experimental structures

2-AcAN 9-AcAN

CH3

O

O CH3

reversibility and rearrangements.

1-AcAN

omitted)

first time.

of these PAKs.

O CH3

Of the three monoacetylanthracenes and eleven diacetylanthracenes included in the present study, only the crystal structures of 1-AcAN [Langer & Becker, 1993], 9-AcAN [Anderson et al., 1984; Zouev et al., 2011] and 1,5-AcAN [Li & Jing, 2006] have previously been described. The molecular and crystal structures of 2-AcAN, 1,6-Ac2AN, 1,7-Ac2AN, 1,8-Ac2AN, 2,7- Ac2AN and 9,10-Ac2AN are reported here for the first time, along with the previously reported structures.

#### **2.1.1 Geometries of monoacetylanthracenes and diacetylanthracenes**

Table 1 shows the crystallographic data for 2-AcAN, 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN, 1,8- Ac2AN, 2,7-Ac2AN and 9,10-Ac2AN.1 The ORTEP diagrams of 2-AcAN and of the six diacetylanthracenes as determined by X-ray crystallography are presented in Fig. 6–10. Ketones 2-AcAN, 1,5-Ac2AN, 1,8-Ac2AN and 2,7-Ac2AN crystallize in the monoclinic space groups *P*21/*n* , *P*21/*c*, *P*2/*n* and *I*2/*a*, respectively. The unit cell dimensions of the crystal structure of 1,5-Ac2AN are essentially identical to those reported in the literature [Li & Jing, 2006]. Ketones 1,6-Ac2AN and 1,7-Ac2AN crystallize in the triclinic space group *P*-1. Ketone 9,10-Ac2AN crystallizes in the orthorhombic space group *Pna*21. Table 2 gives selected geometrical parameters derived from the X-ray crystal structures of the mono- and diacetylanthracenes under study. The following geometrical parameters were considered: the twist angles τ(Carom–Carom–Ccarb–O) (divided into τ1, τ2 and τ9 depending on the position of the acetyl group) and υ(Carom–Carom–Ccarb–O) around the anthracenyl-carbonyl bond; the dihedral angle θ between the least-square planes of the carbonyl group and the anthracene system; the dihedral angle φ between the least-square planes of two side rings of the anthracene system; the pyramidalization angles χ at Carom and Ccarb. Table 3 gives the bond lengths in the mono- and diacetylanthracenes under study, as compared with the parent anthracene.

The data presented in Table 3 indicate the considerable variation in bond lengths in monoand diacetylanthracenes. The bond lengths may be classified into several types: four C1–C2, or α-β, bonds (134.2–137.7 pm), two C2–C3, or β-β, bonds (138.7–144.4 pm), four C1–C9a bonds (141.8–145.5 pm), four C9a–C9 bonds (138.3–140.9 pm), and two C4a–C9a bonds (142.8– 145.3 pm). These values are in the same range as the respective bond lengths in the X-ray crystal structure of anthracene, which are 136.1, 142.8, 143.4, 140.1 and 143.6 pm [Brock & Dunitz, 1990]. It has previously been shown that the bond lengths in anthracene are in agreement with the superposition of its four Kekulé structures and with the free valence numbers [Sinclair et al., 1950]. Table 3 also shows that the bonds adjacent to the acetyl group are elongated as compared to the respective bonds in anthracene, e.g. the C2–C3 bond in 2- AcAN (143.2 pm vs. 142.8 pm), the C1–C2 bonds in 1,5-Ac2AN and 1,8-Ac2AN (137.5 pm vs. 136.1 pm), the C5–C6 bonds in 1,6-Ac2AN (137.0 pm vs. 136.1 pm), the C7–C8 bond in 1,7- Ac2AN (137.4 pm vs. 136.1 pm) and the C2–C3 bond in 2,7-Ac2AN (144.4 pm vs. 142.8 pm). This elongation effect stems from the contributions of dipolar Kekulé structures, in which the anthracene bonds adjacent to the acetyl group are necessarily single. This effect, however, is not noticeable in 9,10-Ac2AN, because the carbonyl groups are almost perpendicular to the aromatic plane and are hardly conjugated with the anthracene system.

<sup>1</sup> CCDC 839159 – 839165 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data\_request/cif.

Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes 9

 2-AcAN 1,5-Ac2AN 1,6-Ac2AN 1,7-Ac2AN 1,8-Ac2AN 2,7-Ac2AN 9,10-Ac2AN Formula C16H12O C18H14O2 C18H14O2 C18H14O2 C18H14O2 C18H14O2 C18H14O2 Temp, K 173(1) 173(1) 173(1) 123(2) 173(1) 173(1) 173(1)

Space group *P*21/*n P*21/*c P*-1 *P*-1 *P*2/*n I*2/*a Pna*21 *a*, Å 6.031(2) 9.8394(9) 7.5776(12) 7.9765(6) 12.6504(9) 17.003(4) 10.4235(14) *b*, Å 7.394(3) 6.2073(6) 8.8581(14) 8.2802(6) 8.5008(6) 5.8390(13) 7.9835(10) *c*, Å 24.847(9) 10.8876(10) 10.1653(16) 11.2321(8) 12.6756(9) 26.395(6) 16.164(2) , deg 90.0 90.0 92.062(3) 96.661(1) 90.0 90.0 90.0 , deg 90.051 107.630(1) 94.348(3) 96.863(1) 108.605(1) 94.592(4) 90.0 , deg 90.0 90.0 110.604(3) 115.295(1) 90.0 90.0 90.0 Volume, Å3 1108.07(7) 633.7(1) 637.9(2) 654.28(8) 1291.9(2) 2612.1(10) 1345.1(3) *Z* 4 2 2 2 4 8 4 Calc density 1.320 1.375 1.365 1.331 1.349 1.334 1.295

max, mm 0.16 0.27 0.40 0.25 0.42 0.37 0.27 mid, mm 0.14 0.26 0.20 0.22 0.40 0.18 0.24 min, mm 0.06 0.23 0.15 0.13 0.28 0.09 0.17 Reflections 6443 6860 5077 7510 14485 14453 13845

Independent 2581 1494 2875 3033 3089 3124 2936 reflections *R*int=0.0676 *R*int=0.0231 *R*int=0.0181 *R*int=0.0181 *R*int=0.0257 *R*int=0.0390 *R*int=0.0263

1307 1429 2336 2689 2841 2275 2865

[I>2I] w*R*2=0.1803 w*R*2=0.1253 w*R*2=0.1498 w*R*2=0.1312 w*R*2=0.1847 w*R*2=0.1747 w*R*2=0.1242 *R* indices *R*1=0.1691 *R*1=0.0518 *R*1=0.0710 *R*1=0.0523 *R*1=0.0748 *R*1=0.1051 *R*1=0.0520 (all data) w*R*2=0.2185 w*R*2=0.1271 w*R*2=0.1588 w*R*2=0.1362 w*R*2=0.1900 w*R*2=0.1896 w*R*2=0.1248

Table 1. Crystallographic data for acetylanthracenes 2-AcAN, 1,5-Ac2AN, 1,6-Ac2AN, 1,7-

Ac2AN, 1,8-Ac2AN, 2,7-Ac2AN and 9,10-Ac2AN.

*R*1=0.0880 *R*1=0.0496 *R*1=0.0583 *R*1=0.0470 *R*1=0.0705 *R*1=0.0771 *R*1=0.0510

Monoclinic Monoclinic Triclinic Triclinic Monoclinic Monoclinic Orthorhombic

Crystal system

Mg/m3 Crystal size

collected

Reflections with I>2σ(I)

Final *R* indices

Fig. 5. Constitutional isomers of diacetylanthracenes (*E* and *Z* stereodescriptors are omitted)

O CH3

O CH3

CH3 O

O

CH3

O CH3

H3C

H3C O O CH3

O

H3C

CH3

O

<sup>O</sup> CH3 <sup>O</sup>

1,10-Ac2AN H3C O

2,7-Ac2AN

O CH3

9,10-Ac2AN H3C O

CH3 O

1,4-Ac2AN

O CH3

O CH3

1,6-Ac2AN

1,9-Ac2AN

2,6-Ac2AN

2,10-Ac2AN

Fig. 5. Constitutional isomers of diacetylanthracenes (*E* and *Z* stereodescriptors are omitted)

H3C O

1,3-Ac2AN

1,5-Ac2AN 1,7-Ac2AN

O CH3

H3C O

O

CH3

1,2-Ac2AN

O CH3

H3C O

H3C O

CH3 O

O

CH3

O

CH3

O

O

H3C

H3C

O CH3

2,9-Ac2AN

2,3-Ac2AN

1,8-Ac2AN


Table 1. Crystallographic data for acetylanthracenes 2-AcAN, 1,5-Ac2AN, 1,6-Ac2AN, 1,7- Ac2AN, 1,8-Ac2AN, 2,7-Ac2AN and 9,10-Ac2AN.

Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes 11

Fig. 6. ORTEP drawing of the crystal structure of 2-AcAN, scaled to enclose 50% probability

Fig. 7. ORTEP drawings of the crystal structures of 1,5-Ac2AN (left) and 1,6-Ac2AN (right),

scaled to enclose 50% probability


<sup>a</sup> τ1(C9a–C1–C11–O15) for 1-AcAN, 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN and 1,8-Ac2AN, τ2(C1–C2–C11–O15) for 2-AcAN and 2,7-Ac2AN, τ2(C5–C6–C13–O16) for 1,6-Ac2AN, τ2(C8–C7–C13–O16) for 1,7-Ac2AN, τ9(C9a– C9–C11–O15) for 9-AcAN and 9,10-Ac2AN.

<sup>b</sup> υ1(C2–C1–C11–O15) for 1-AcAN, 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN and 1,8-Ac2AN, υ2(C3–C2–C11–O15) for 2-AcAN and 2,7-Ac2AN, υ2(C7–C6–C13–O16) for 1,6-Ac2AN, υ2(C6–C7–C13–O16) for 1,7-Ac2AN, υ9(C8a– C9–C11–O15) for 9-AcAN and 9,10-Ac2AN.

c C1–C11 for 1-AcAN, 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN and 1,8-Ac2AN, C2–C11 for 2-AcAN and 2,7- Ac2AN, C6–C13 for 1,6-Ac2AN, C7–C13 for 1,7-Ac2AN, C9–C11 for 9-AcAN and 9,10-Ac2AN.

Table 2. Selected geometrical parameters of the X-ray molecular structures of acetylanthracenes 2-AcAN, 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN, 1,8-Ac2AN, 2,7-Ac2AN and 9,10-Ac2AN.

Canc

1-AcAN *Z Ci* 27.1 -152.7 28.6 3.2 149.3 -0.1 -0.2 223.4 H9 223.4 H2 2-AcAN *E C*1 173.1 -5.3 5.9 0.4 148.6 -1.6 0.4 249.2 H3 244.3 H1 9-AcAN -- *C*1 87.9 -91.8 89.2 5.8 150.4 0.3 -0.8 294.0 H1 263.1 H8

1,5-Ac2AN *ZZ Ci* 20.0 -156.8 22.7 0.0 149.4 -3.2 3.4 226.9 H9 243.7 H2

1,6-Ac2AN *ZE C*1 30.0 -147.1 32.2 1.3 150.1 -2.9 3.0 228.8 H9 230.0 H2

1,7-Ac2AN *ZE C*1 -15.2 162.9 16.0 2.3 149.8 1.9 -1.9 221.3 H9 229.2 H2

1,8-Ac2AN *ZZ C*2 -34.0 145.4 36.0 0.3 149.3 0.6 0.3 228.2 H9 231.1 H2

2,7-Ac2AN *EZ C*1 171.9 -3.4 9.9 2.9 149.0 -4.7 3.0 253.9 H3 239.0 H1

87.0 -93.7 86.5 151.5 -0.6 0.1 290.5 H4 264.4 H5

<sup>a</sup> τ1(C9a–C1–C11–O15) for 1-AcAN, 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN and 1,8-Ac2AN, τ2(C1–C2–C11–O15) for 2-AcAN and 2,7-Ac2AN, τ2(C5–C6–C13–O16) for 1,6-Ac2AN, τ2(C8–C7–C13–O16) for 1,7-Ac2AN, τ9(C9a–

<sup>b</sup> υ1(C2–C1–C11–O15) for 1-AcAN, 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN and 1,8-Ac2AN, υ2(C3–C2–C11–O15) for 2-AcAN and 2,7-Ac2AN, υ2(C7–C6–C13–O16) for 1,6-Ac2AN, υ2(C6–C7–C13–O16) for 1,7-Ac2AN, υ9(C8a–

c C1–C11 for 1-AcAN, 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN and 1,8-Ac2AN, C2–C11 for 2-AcAN and 2,7-

acetylanthracenes 2-AcAN, 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN, 1,8-Ac2AN, 2,7-Ac2AN and

Ac2AN, C6–C13 for 1,6-Ac2AN, C7–C13 for 1,7-Ac2AN, C9–C11 for 9-AcAN and 9,10-Ac2AN.

Table 2. Selected geometrical parameters of the X-ray molecular structures of

178.6 -0.7 1.9 149.3 -0.7 -0.3 249.9 H7 230.1 H5



0.9 -178.5 2.0 148.9 0.7 -1.3 246.0 H8 226.4 H3

*E C*1 -85.0 94.0 86.7 1.6 151.3 -1.0 -1.2 288.2 H8 257.4 H1

Compound deg deg deg deg pm deg deg pm pm

χ(Carom) χ(Ccarb) O…H CH3

…H

τa υb θ φ Ccarb–

C9–C11–O15) for 9-AcAN and 9,10-Ac2AN.

C9–C11–O15) for 9-AcAN and 9,10-Ac2AN.

9,10- Ac2AN

9,10-Ac2AN.

Fig. 6. ORTEP drawing of the crystal structure of 2-AcAN, scaled to enclose 50% probability

Fig. 7. ORTEP drawings of the crystal structures of 1,5-Ac2AN (left) and 1,6-Ac2AN (right), scaled to enclose 50% probability

Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes 13

 C10–C10a C10a–C8a C10a–C5 C5–C6 C6–C7 C7–C8 C8–C8a ANa 140.1 143.6 143.4 136.1 142.8 136.1 143.4 1-AcANb 139.5(3) 142.8(3) 143.0(3) 134.6(3) 140.8(3) 135.1(3) 142.2(3) 2-AcAN 139.3(4) 143.6(4) 142.4(4) 136.1(4) 141.3(4) 135.0(4) 143.0(4) 9-AcANc 139.1(3) 142.9(3) 142.4(3) 134.5(3) 140.4(3) 135.6(3) 142.7(3) 1,5-Ac2AN 139.7(2) 143.8(2) 145.0(2) 137.5(2) 141.4(2) 135.6(2) 142.9(2) 1,6-Ac2AN 139.2(2) 143.7(2) 142.5(2) 137.0(2) 143.9(2) 135.5(2) 143.1(2) 1,7-Ac2AN 139.0(2) 142.8(2) 143.3(2) 135.6(2) 142.8(2) 137.4(1) 142.7(1) 1,8-Ac2AN 139.2(2) 143.7(2) 142.8(2) 135.5(2) 141.4(3) 137.3(2) 144.4(2) 1,8-Ac2AN 139.2(2) 143.7(2) 142.9(2) 135.2(3) 141.2(3) 137.5(2) 144.9(2) 2,7-Ac2AN 139.4(3) 144.3(3) 142.3(3) 134.9(3) 142.8(3) 136.6(3) 141.8(3) 9,10-Ac2AN 139.4(3) 143.8(3) 143.3(3) 135.2(4) 141.1(4) 136.5(4) 143.0(3)

 C8a–C9 C9–C9a C9a–C1 Car–C11 C11–C12 C11-O ANa 140.1 140.1 143.4 – – – 1-AcANb 139.0(3) 138.9(3) 144.8(3) 149.3(3) 148.8(3) 121.7(3) 2-AcAN 139.3(4) 139.7(4) 142.3(4) 148.6(4) 149.5(4) 121.5(3) 9-AcANc 140.3(3) 140.2(3) 142.4(3) 150.3(3) 148.5(3) 120.8(2) 1,5-Ac2AN 139.9(2) 139.7(2) 145.0(2) 149.4(2) 150.9(2) 121.8(2) 1,6-Ac2AN 139.5(2) 140.0(2) 144.6(2) 150.1(2) 150.5(2) 121.7(2) 149.3(2) 150.6(2) 121.8(2) 1,7-Ac2AN 140.5(1) 140.1(1) 145.5(1) 149.8(2) 151.6(2) 121.7(1) 149.0(2) 150.1(2) 122.1(1) 1,8-Ac2AN 140.1(2) 140.1(2) 144.4(2) 149.3(2) 151.2(2) 121.5(2) 1,8-Ac2AN 139.9(2) 139.9(2) 144.9(2) 148.9(2) 151.4(2) 121.4(2) 2,7-Ac2AN 139.3(3) 139.6(3) 141.9(3) 149.0(4) 150.0(4) 122.1(3) 148.9(3) 149.5(3) 121.0(3) 9,10-Ac2AN 140.0(3) 140.1(3) 142.9(3) 151.4(3) 149.4(3) 121.3(3) 151.5(3) 149.4(4) 120.0(3)

Table 3. Bond lengths (pm) in the X-ray structures of anthracene, monoacetylanthracenes

a Brock & Dunitz, 1990; averaged bonds lengths

b Langer & Becker, 1993 c Zouev et al., 2011

and diacetylanthracenes.

 C1–C2 C2–C3 C3–C4 C4–C4a C4a–C9a C4a–C10 ANa 136.1 142.8 136.1 143.4 143.6 140.1 1-AcANb 137.6(3) 138.7(3) 134.6(3) 143.1(3) 143.1(3) 139.2(3) 2-AcAN 136.4(4) 143.2(4) 134.6(4) 144.10(4) 143.0(4) 138.7(4) 9-AcANc 134.9(3) 141.2(3) 135.0(3) 142.9(3) 143.2(3) 138.8(3) 1,5-Ac2AN 137.5(2) 141.4(2) 135.6(2) 142.9(2) 143.8(2) 139.9(2) 1,6-Ac2AN 136.9(2) 142.0(2) 135.5(2) 142.7(2) 144.1(2) 140.0(2) 1,7-Ac2AN 137.7(2) 141.5(2) 135.6(2) 143.0(2) 144.0(2) 140.0(2) 1,8-Ac2AN 137.3(2) 141.4(3) 135.5(2) 142.8(2) 143.7(2) 139.2(2) 1,8-Ac2AN 137.5(2) 141.2(3) 135.2(3) 142.9(2) 143.7(2) 139.2(2) 2,7-Ac2AN 137.1(3) 144.4(3) 134.2(4) 142.5(3) 144.6(3) 138.3(3) 9,10-Ac2AN 136.3(3) 141.8(4) 134.6(4) 142.7(3) 143.8(3) 140.9(3)

Fig. 8. ORTEP drawings of the crystal structures of 1,7-Ac2AN (left) and 1,8-Ac2AN (right), scaled to enclose 50% probability

Fig. 9. ORTEP drawings of the crystal structure of 2,7-Ac2AN, scaled to enclose 50% probability



a Brock & Dunitz, 1990; averaged bonds lengths

b Langer & Becker, 1993

c Zouev et al., 2011

12 Current Trends in X-Ray Crystallography

Fig. 8. ORTEP drawings of the crystal structures of 1,7-Ac2AN (left) and 1,8-Ac2AN (right),

Fig. 9. ORTEP drawings of the crystal structure of 2,7-Ac2AN, scaled to enclose 50%

scaled to enclose 50% probability

probability

Table 3. Bond lengths (pm) in the X-ray structures of anthracene, monoacetylanthracenes and diacetylanthracenes.

Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes 15

those of 1,6-Ac2AN and 1,7-Ac2AN but with a larger twist of the *E*-acetyl group, τ2(C1–C2– C11–O15)=171.9°, and a smaller twist of the *Z*-acetyl group, τ2(C6–C7–C12–O16)=0.9°. There are four pairs of enantiomeric molecules in the unit cell of 2,7-Ac2AN. Ketone 9,10-Ac2AN crystallizes in the *E* conformation with the twist angles of τ9(C9a–C9–C11–O15)=–85.0° and τ9(C10a–C10–C13–O16)=87.0°. There are two pairs of enantiomeric molecules in the unit cell of 9,10-Ac2AN. According to the literature structure [Langer & Becker, 1993], 1-AcAN crystallizes in the *Z*-conformation with a twist angle of τ1(C9a–C1–C11–O13)=27.1°. The carbonyl group of ketone 9-AcAN [Zouev2011] is nearly orthogonal to the aromatic plane,

None of the mono- and diacetylanthracenes under study adopts a planar conformation in their crystal structures. The absolute values of the twist angles in the mono- and diacetylanthracenes vary, depending on the position of substitution and on the conformation of the acetyl groups: |τ1|=15.2–34.0° for the 1*Z*-acetyl groups, |τ2|=0.9° for the 2*Z*-acetyl group, |τ2|=171.9–178.6° for the 2*E*-acetyl groups, and |τ9|=85.0–87.9°. The higher twist angles of the 1*Z*-acetyl groups are caused by the repulsive interactions between the carbonyl oxygen and the respective *peri*-hydrogen. The acetyl groups themselves are nealry planar (excluding the methyl hydrogens), and the pyramidalization angles χ at the carbonyl carbon atom are small, 0.1–3.4°. The dihedral angles θ between the plane of the carbonyl group and the aromatic plane are very close to the respective twist angles τ (Table 2). The anthracene systems in the mono- and diacetylanthracenes under study are also essentially planar: the dihedral fold angles φ between the side six-membered rings of the anthracene unit are 0.0–5.8°. The pyramidalization angles χ at the carbon atom bonded to the acetyl substituent are small, 0.1–4.7°. Thus, the twist of the acetyl group(s) is the main

The diacetylanthracenes under study can be arranged in the order of decreasing twist angles τ:

9,10-Ac2AN>>1,8-Ac2AN>1,6-Ac2AN>1,5-Ac2AN>1,7-Ac2AN>2,7-Ac2AN. The magnitude ot the twist angle of the acetyl group is important. It has been shown that if an acyl group is tilted out of the plane of the aromatic ring of an aromatic ketone by neighboring bulky groups, the resonance stabilization is reduced and the pattern irreversibility of Friedel–Crafts acylation may be challenged, allowing deacylation, transacylation and acyl rearrangments [Buehler & Pearson, 1970; Gore, 1974; Mala'bi, et al., 2011]. Thus, the twist angle τ may define the ability of diacetylanthracenes to undergo

Another factor that may influence the tilting of the acetyl group and, as a consequence, the feasibility of acyl rearrangements, is the overcrowding. Its main source is the short contact distances between the carbonyl oxygen and the *peri*-hydrogen, or between the methyl group and *peri*-hydrogen. The intramolecular O...H distances in the crystal structures of the monoand diacetylanthracenes under study are not particularly short, 221–246 pm, for the *Z*-acetyl groups, which corresponds to 0–9% penetration. There are no close contact distances caused

**2.1.2 Intermolecular interactions in monoacetylanthracenes and diacetylanthracenes**  Aromatic–aromatic interactions are non-covalent intermolecular forces similar to hydrogen bonding [Janiak, 2000]. Aromatic systems may be arranged in three principal configurations:

feature of non-planarity in the mono- and diacetylanthracenes.

deacylations and rearrangements according to Agranat-Gore rearrangement.

τ9(C1–C2–C11–O13)=87.9°.

by the *E*-acetyl groups.

Fig. 10. ORTEP drawings of the crystal structure of 9,10-Ac2AN, scaled to enclose 50% probability

Ketone 2-AcAN crystallizes in the *E* conformation with a small twist angle of τ2(C1–C2–C11– O13)=173.1° and a small dihedral angle θ=5.9°. There are two pairs of enantiomeric molecules in the unit cell of 2-AcAN. The structure does not contain any short contact distances. Ketone 1,5-Ac2AN adopts the *Z*,*Z* conformation with large twist angles, τ1(C9a– C1–C11–O15)=20.0°, τ1(C10a–C5–C13–O16)=–20.0°. The O15...H9 and O16...H10 contact distances are 226.9 pm, which is slightly shorter (7% penetration) than the sum of the respective van der Waals radii of hydrogen (115 ppm) and oxygen (129ppm) [Zefirov, 1997]. There are two identical molecules of 1,5-Ac2AN in the unit cell, each posessing the *Ci* symmetry. Ketone 1,6-Ac2AN crystallizes in the *Z*,*E* conformations, with a large twist angle of the *Z* carbonyl group, τ1(C9a–C1–C11–O15)=30.0° and a small twist of the *E* carbonyl group, τ2(C5–C6–C13– O16)=178.6°. The O15...H9 contact distance is 228.8 pm (6% penetration), while the O16...H7 contact distance is 249.9 pm. There are two enantiomeric molecules in the unit cell of 1,6- Ac2AN. Ketone 1,7-Ac2AN also crystallizes in the *Z*,*E* conformations, with the twist angles of τ1(C9a–C1–C11–O15)=–15.2° and τ2(C8–C7–C13–O16)=–176.6°. The O15...H9 contact distance is 221.3 pm (9% penetration). There are two enantiomeric molecules in the unit cell of 1,7- Ac2AN. Ketone 1,8-Ac2AN adopts the *Z*,*Z* conformation with two large twist angles, due to the repulsive *peri*-interactions O15...H9 and O16...H9 (225.9 and 228.2 pm) between two carbonyl oxygens and the same aromatic hydrogen. There are two enantiomeric pairs of non-equivalent molecules, A and B, in the unit cell of 1,8-Ac2AN, each of them posessing the *C*2 symmetry. The respective twist angles are τ1(C9a–C1–C11–O15)=–34.0° (A) and τ1(C9a–C1– C11–O15)=–32.4° (B). Ketone 2,7-Ac2AN adopts the *E*,*Z* conformation, which is similar to

Fig. 10. ORTEP drawings of the crystal structure of 9,10-Ac2AN, scaled to enclose 50%

Ketone 2-AcAN crystallizes in the *E* conformation with a small twist angle of τ2(C1–C2–C11– O13)=173.1° and a small dihedral angle θ=5.9°. There are two pairs of enantiomeric molecules in the unit cell of 2-AcAN. The structure does not contain any short contact distances. Ketone 1,5-Ac2AN adopts the *Z*,*Z* conformation with large twist angles, τ1(C9a– C1–C11–O15)=20.0°, τ1(C10a–C5–C13–O16)=–20.0°. The O15...H9 and O16...H10 contact distances are 226.9 pm, which is slightly shorter (7% penetration) than the sum of the respective van der Waals radii of hydrogen (115 ppm) and oxygen (129ppm) [Zefirov, 1997]. There are two identical molecules of 1,5-Ac2AN in the unit cell, each posessing the *Ci* symmetry. Ketone 1,6-Ac2AN crystallizes in the *Z*,*E* conformations, with a large twist angle of the *Z* carbonyl group, τ1(C9a–C1–C11–O15)=30.0° and a small twist of the *E* carbonyl group, τ2(C5–C6–C13– O16)=178.6°. The O15...H9 contact distance is 228.8 pm (6% penetration), while the O16...H7 contact distance is 249.9 pm. There are two enantiomeric molecules in the unit cell of 1,6- Ac2AN. Ketone 1,7-Ac2AN also crystallizes in the *Z*,*E* conformations, with the twist angles of τ1(C9a–C1–C11–O15)=–15.2° and τ2(C8–C7–C13–O16)=–176.6°. The O15...H9 contact distance is 221.3 pm (9% penetration). There are two enantiomeric molecules in the unit cell of 1,7- Ac2AN. Ketone 1,8-Ac2AN adopts the *Z*,*Z* conformation with two large twist angles, due to the repulsive *peri*-interactions O15...H9 and O16...H9 (225.9 and 228.2 pm) between two carbonyl oxygens and the same aromatic hydrogen. There are two enantiomeric pairs of non-equivalent molecules, A and B, in the unit cell of 1,8-Ac2AN, each of them posessing the *C*2 symmetry. The respective twist angles are τ1(C9a–C1–C11–O15)=–34.0° (A) and τ1(C9a–C1– C11–O15)=–32.4° (B). Ketone 2,7-Ac2AN adopts the *E*,*Z* conformation, which is similar to

probability

those of 1,6-Ac2AN and 1,7-Ac2AN but with a larger twist of the *E*-acetyl group, τ2(C1–C2– C11–O15)=171.9°, and a smaller twist of the *Z*-acetyl group, τ2(C6–C7–C12–O16)=0.9°. There are four pairs of enantiomeric molecules in the unit cell of 2,7-Ac2AN. Ketone 9,10-Ac2AN crystallizes in the *E* conformation with the twist angles of τ9(C9a–C9–C11–O15)=–85.0° and τ9(C10a–C10–C13–O16)=87.0°. There are two pairs of enantiomeric molecules in the unit cell of 9,10-Ac2AN. According to the literature structure [Langer & Becker, 1993], 1-AcAN crystallizes in the *Z*-conformation with a twist angle of τ1(C9a–C1–C11–O13)=27.1°. The carbonyl group of ketone 9-AcAN [Zouev2011] is nearly orthogonal to the aromatic plane, τ9(C1–C2–C11–O13)=87.9°.

None of the mono- and diacetylanthracenes under study adopts a planar conformation in their crystal structures. The absolute values of the twist angles in the mono- and diacetylanthracenes vary, depending on the position of substitution and on the conformation of the acetyl groups: |τ1|=15.2–34.0° for the 1*Z*-acetyl groups, |τ2|=0.9° for the 2*Z*-acetyl group, |τ2|=171.9–178.6° for the 2*E*-acetyl groups, and |τ9|=85.0–87.9°. The higher twist angles of the 1*Z*-acetyl groups are caused by the repulsive interactions between the carbonyl oxygen and the respective *peri*-hydrogen. The acetyl groups themselves are nealry planar (excluding the methyl hydrogens), and the pyramidalization angles χ at the carbonyl carbon atom are small, 0.1–3.4°. The dihedral angles θ between the plane of the carbonyl group and the aromatic plane are very close to the respective twist angles τ (Table 2). The anthracene systems in the mono- and diacetylanthracenes under study are also essentially planar: the dihedral fold angles φ between the side six-membered rings of the anthracene unit are 0.0–5.8°. The pyramidalization angles χ at the carbon atom bonded to the acetyl substituent are small, 0.1–4.7°. Thus, the twist of the acetyl group(s) is the main feature of non-planarity in the mono- and diacetylanthracenes.

The diacetylanthracenes under study can be arranged in the order of decreasing twist angles τ:

#### 9,10-Ac2AN>>1,8-Ac2AN>1,6-Ac2AN>1,5-Ac2AN>1,7-Ac2AN>2,7-Ac2AN.

The magnitude ot the twist angle of the acetyl group is important. It has been shown that if an acyl group is tilted out of the plane of the aromatic ring of an aromatic ketone by neighboring bulky groups, the resonance stabilization is reduced and the pattern irreversibility of Friedel–Crafts acylation may be challenged, allowing deacylation, transacylation and acyl rearrangments [Buehler & Pearson, 1970; Gore, 1974; Mala'bi, et al., 2011]. Thus, the twist angle τ may define the ability of diacetylanthracenes to undergo deacylations and rearrangements according to Agranat-Gore rearrangement.

Another factor that may influence the tilting of the acetyl group and, as a consequence, the feasibility of acyl rearrangements, is the overcrowding. Its main source is the short contact distances between the carbonyl oxygen and the *peri*-hydrogen, or between the methyl group and *peri*-hydrogen. The intramolecular O...H distances in the crystal structures of the monoand diacetylanthracenes under study are not particularly short, 221–246 pm, for the *Z*-acetyl groups, which corresponds to 0–9% penetration. There are no close contact distances caused by the *E*-acetyl groups.

#### **2.1.2 Intermolecular interactions in monoacetylanthracenes and diacetylanthracenes**

Aromatic–aromatic interactions are non-covalent intermolecular forces similar to hydrogen bonding [Janiak, 2000]. Aromatic systems may be arranged in three principal configurations:

Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes 17

the π...π interactions are not possible due to very long distances between molecules lying in

the parallel planes (>600 pm). The unit cell of 1,5-Ac2AN is shown in Figure 12.

Fig. 11. The unit cell of 2-AcAN (view along *c* axis)

Fig. 12. The unit cell of 1,5-Ac2AN (view along special axis *1*,*0*,*1*)

The molecules of 1,6-Ac2AN are packed by β type, forming a layered structure made up of "graphitic" planes with zero interplanar angle. From the point of view of aromatic–aromatic interactions, the anthracene moieties in the crystal structure of 1,6**-**Ac2AN are stacked by the **D**-type, with the centroid–centroid separation of 359.2 and 385.6 pm. The slippage distances


The crystal structure of the parent anthracene (AN) has been studied [Brock & Dunitz, 1990; Sinclair et al., 1950; Murugan & Jha, 2009]. It crystallizes in the monoclinic space group *P*21/*a*. Within the unit cell, the anthracene molecules are packed in a "herringbone" pattern, similar to the parent PAH naphthalene [Desiraju & Gavezzotti, 1989]. In this motif, the C...C non-bonded interactions are between non-parallel nearest neighbor molecules. The herringbone packing is one of four basic structural types for PAH, which are defined depending on the shortest cell axis and the interplanar angle [Desiraju & Gavezzotti, 1989]. The structures with herringbone packing, "sandwich herringbone" packing and γ packing obtain crystal stabilization mainly from C...C interactions, but also from C...H interactions [Desiraju & Gavezzotti, 1989]. The "graphitic", or β, packing characterized by strong C...C interactions without much contribution from C...H contacts [Desiraju & Gavezzotti, 1989]. The selected geometric parameters of aromatic interactions in the mono- and diacetylanthracenes under study are presented in Table 4. *Cg*1 is the centroid for the C1–C2– C3–C4–C4a–C9a ring, *Cg*2 is the centroid for the C4a–C10–C10a–C8a–C9–C9a ring and *Cg*3 is the centroid for the C5–C6–C7–C8–C8a–C10a ring; *Cg*4–6 are the respective centroids of the second non-equivalent molecule in the unit cell, if it exists. Interplanar angle is the angle between the planes of adjacent molecules. Slippage distance is distance of one centroid from the projection of another centroid. Displacement angle is the angle between the ring normal and the centroid vector.

The molecules of 2-AcAN are packed in a "herringbone" pattern, with the interplanar angle of 51.0°. The anthracene moieties in the crystal structure of 2-AcAN adopt the **T**-configuration with the shortest centroid-centroid separation of 464.7 pm. The shortest distances between the centroids of one molecule and the carbon atoms of the other molecule are *Cg*3'...C4=343.7 pm, *Cg*2'...C8=351.2 pm, *Cg*2'...C10=351.2 pm, *Cg*3'...C9=357.6 pm and *Cg*1'...C5=358.2 pm. The respective centroid–hydrogen distances are *Cg*3'...H4=271.5 pm, *Cg*2'...H8=283.3 pm, *Cg*2'...H10=280.8 pm, *Cg*3'...H9=288.2 pm and *Cg*1'...H5=287.9 pm. The π...π interactions in 2- AcAN are very weak despite close lying parallel planes, as reflected in very long distances between the respective centroids (>584 pm). Thus, the aryl C–H...π interactions dominate in the crystal structure of 2-AcAN. The unit cell of 2-AcAN is shown in Figure 11.

The molecules of 1,5-Ac2AN are packed in a "herringbone" pattern, with the interplanar angle of 56.2°. The anthracene moieties in the crystal structure of 1,5-Ac2AN adopt the **T**configuration with the shortest centroid-centroid separation of 462.9 and 470.5 pm. The shortest distances between the centroids of one molecule and the carbon atoms of the other molecule are *Cg*1'...C4=341.9 pm, *Cg*1'...C3=353.6 pm and *Cg*2'...C4=376.3 pm. The respective centroid–hydrogen distances are *Cg*1'...H4=264.3 pm, *Cg*1'...H3=293.7 pm and *Cg*2'...H4=342.9 pm. Thus, the aryl C–H...π interactions dominate in the crystal structure of 1,5-Ac2AN, while the π...π interactions are not possible due to very long distances between molecules lying in the parallel planes (>600 pm). The unit cell of 1,5-Ac2AN is shown in Figure 12.

Fig. 11. The unit cell of 2-AcAN (view along *c* axis)

16 Current Trends in X-Ray Crystallography

 A stacked (**S**) configuration, or a π...π interaction, in which aromatic rings are face-toface aligned, with the interplanar distances of about 3.3–3.8 Å [Janiak, 2000]. This configuration has the maximal overlap but it is rarely observed in real systems

The T-shaped configuration (**T**), or a C–H...π interaction, where one aromatic ring points

 The parallel displaced (**D**), or offset stacked, configuration; it is reached from the stacked configuration by the parallel shift of one aromatic ring relative to the other [Sinnokrot & Sherrill, 2006], and features both π-π and C–H...π interactions. The **T**- and **D**-type configurations are often observed in small aromatic compounds [Dahl, 1994]

The crystal structure of the parent anthracene (AN) has been studied [Brock & Dunitz, 1990; Sinclair et al., 1950; Murugan & Jha, 2009]. It crystallizes in the monoclinic space group *P*21/*a*. Within the unit cell, the anthracene molecules are packed in a "herringbone" pattern, similar to the parent PAH naphthalene [Desiraju & Gavezzotti, 1989]. In this motif, the C...C non-bonded interactions are between non-parallel nearest neighbor molecules. The herringbone packing is one of four basic structural types for PAH, which are defined depending on the shortest cell axis and the interplanar angle [Desiraju & Gavezzotti, 1989]. The structures with herringbone packing, "sandwich herringbone" packing and γ packing obtain crystal stabilization mainly from C...C interactions, but also from C...H interactions [Desiraju & Gavezzotti, 1989]. The "graphitic", or β, packing characterized by strong C...C interactions without much contribution from C...H contacts [Desiraju & Gavezzotti, 1989]. The selected geometric parameters of aromatic interactions in the mono- and diacetylanthracenes under study are presented in Table 4. *Cg*1 is the centroid for the C1–C2– C3–C4–C4a–C9a ring, *Cg*2 is the centroid for the C4a–C10–C10a–C8a–C9–C9a ring and *Cg*3 is the centroid for the C5–C6–C7–C8–C8a–C10a ring; *Cg*4–6 are the respective centroids of the second non-equivalent molecule in the unit cell, if it exists. Interplanar angle is the angle between the planes of adjacent molecules. Slippage distance is distance of one centroid from the projection of another centroid. Displacement angle is the angle between the ring normal and

The molecules of 2-AcAN are packed in a "herringbone" pattern, with the interplanar angle of 51.0°. The anthracene moieties in the crystal structure of 2-AcAN adopt the **T**-configuration with the shortest centroid-centroid separation of 464.7 pm. The shortest distances between the centroids of one molecule and the carbon atoms of the other molecule are *Cg*3'...C4=343.7 pm, *Cg*2'...C8=351.2 pm, *Cg*2'...C10=351.2 pm, *Cg*3'...C9=357.6 pm and *Cg*1'...C5=358.2 pm. The respective centroid–hydrogen distances are *Cg*3'...H4=271.5 pm, *Cg*2'...H8=283.3 pm, *Cg*2'...H10=280.8 pm, *Cg*3'...H9=288.2 pm and *Cg*1'...H5=287.9 pm. The π...π interactions in 2- AcAN are very weak despite close lying parallel planes, as reflected in very long distances between the respective centroids (>584 pm). Thus, the aryl C–H...π interactions dominate in the

The molecules of 1,5-Ac2AN are packed in a "herringbone" pattern, with the interplanar angle of 56.2°. The anthracene moieties in the crystal structure of 1,5-Ac2AN adopt the **T**configuration with the shortest centroid-centroid separation of 462.9 and 470.5 pm. The shortest distances between the centroids of one molecule and the carbon atoms of the other molecule are *Cg*1'...C4=341.9 pm, *Cg*1'...C3=353.6 pm and *Cg*2'...C4=376.3 pm. The respective centroid–hydrogen distances are *Cg*1'...H4=264.3 pm, *Cg*1'...H3=293.7 pm and *Cg*2'...H4=342.9 pm. Thus, the aryl C–H...π interactions dominate in the crystal structure of 1,5-Ac2AN, while

crystal structure of 2-AcAN. The unit cell of 2-AcAN is shown in Figure 11.

containing aromatic rings [Sinnokrot & Sherrill, 2006].

at the center of another ring.

and proteins [Hunter et al., 1991].

the centroid vector.

Fig. 12. The unit cell of 1,5-Ac2AN (view along special axis *1*,*0*,*1*)

The molecules of 1,6-Ac2AN are packed by β type, forming a layered structure made up of "graphitic" planes with zero interplanar angle. From the point of view of aromatic–aromatic interactions, the anthracene moieties in the crystal structure of 1,6**-**Ac2AN are stacked by the **D**-type, with the centroid–centroid separation of 359.2 and 385.6 pm. The slippage distances

Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes 19

the plane of the acetyl group (containing C1', C11', C13', O15') of molecule B is nearly parallel to the aromatic plane of molecule A, 3.8°. The distances between the anthracene systems and the carbonyl group are sufficiently small to consider the intermolecular π...π interactions: *Cg*4'...O1=353.8 pm, *Cg*4'...C11=384.3 pm, *Cg*3...O1'=363.3 pm and *Cg*3...C11'=398.2 pm. Thus, the crystal structure of 1,8-Ac2AN features π–π-interactions not between two aromatic systems, but between the aromatic system and the carbonyl π-bond. The unit cell of 1,8-Ac2AN is

shown in Figure 15.

Fig. 14. The unit cell of 1,7-Ac2AN (view along *b* axis)

Fig. 15. The unit cell of 1,8-Ac2AN (view along *b* axis)

of the centroids are relatively short, 94.0 and 107.1 pm. The shortest contact distances between the aromatic carbons in 1,6-Ac2AN are C5…C7'=355.1 and C6…C8a'=358.5. The unit cell of 1,6-Ac2AN is shown in Figure 13.

Fig. 13. The unit cell of 1,6-Ac2AN (view along *c* axis)

The molecules of 1,7-Ac2AN are also packed by β type. The anthracene moieties in the crystal structure of 1,7-Ac2AN adopt the **D**-configuration, with the shortest centroidcentroid separation of 370 pm. Despite the longer slippage distance between centroids (154.4–154.8 pm), the contact distances in 1,7-Ac2AN are shorter than those in 1,6-Ac2AN: C3…C8'=333.3, C4a…C9'=336.4, C8…C9'=337.1 and C1…C10'=340.9. In both 1,6-Ac2AN and 1,7- Ac2AN the aromatic interactions are mainly those of the π...π type. The unit cell of 1,7- Ac2AN is shown in Figure 14.

The molecules of 1,8-Ac2AN are packed in a "herringbone" pattern, with the interplanar angle of 34.7°. The centroids of the anthracene molecules lying onto the parallel planes are separated by 580–581 pm. These distances together with the slippage distance of 493-494 pm render the aromatic interactions of either **S**- or **D**-type impossible. The **T**-type interactions in 1,8-Ac2AN are too weak to be of any importance, due to the long distances between centroids (546–562 pm). However, the plane of the acetyl group (containing C1, C11, C13, O15) of molecule A forms the angle of 4.0° with the aromatic plane of molecule B. Analogously, the plane of the acetyl group (containing C1', C11', C13', O15') of molecule B is nearly parallel to the aromatic plane of molecule A, 3.8°. The distances between the anthracene systems and the carbonyl group are sufficiently small to consider the intermolecular π...π interactions: *Cg*4'...O1=353.8 pm, *Cg*4'...C11=384.3 pm, *Cg*3...O1'=363.3 pm and *Cg*3...C11'=398.2 pm. Thus, the crystal structure of 1,8-Ac2AN features π–π-interactions not between two aromatic systems, but between the aromatic system and the carbonyl π-bond. The unit cell of 1,8-Ac2AN is shown in Figure 15.

Fig. 14. The unit cell of 1,7-Ac2AN (view along *b* axis)

18 Current Trends in X-Ray Crystallography

of the centroids are relatively short, 94.0 and 107.1 pm. The shortest contact distances between the aromatic carbons in 1,6-Ac2AN are C5…C7'=355.1 and C6…C8a'=358.5. The unit

The molecules of 1,7-Ac2AN are also packed by β type. The anthracene moieties in the crystal structure of 1,7-Ac2AN adopt the **D**-configuration, with the shortest centroidcentroid separation of 370 pm. Despite the longer slippage distance between centroids (154.4–154.8 pm), the contact distances in 1,7-Ac2AN are shorter than those in 1,6-Ac2AN: C3…C8'=333.3, C4a…C9'=336.4, C8…C9'=337.1 and C1…C10'=340.9. In both 1,6-Ac2AN and 1,7- Ac2AN the aromatic interactions are mainly those of the π...π type. The unit cell of 1,7-

The molecules of 1,8-Ac2AN are packed in a "herringbone" pattern, with the interplanar angle of 34.7°. The centroids of the anthracene molecules lying onto the parallel planes are separated by 580–581 pm. These distances together with the slippage distance of 493-494 pm render the aromatic interactions of either **S**- or **D**-type impossible. The **T**-type interactions in 1,8-Ac2AN are too weak to be of any importance, due to the long distances between centroids (546–562 pm). However, the plane of the acetyl group (containing C1, C11, C13, O15) of molecule A forms the angle of 4.0° with the aromatic plane of molecule B. Analogously,

cell of 1,6-Ac2AN is shown in Figure 13.

Fig. 13. The unit cell of 1,6-Ac2AN (view along *c* axis)

Ac2AN is shown in Figure 14.

Fig. 15. The unit cell of 1,8-Ac2AN (view along *b* axis)

Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes 21

The anthracene moieties in the crystal structure of 9,10-Ac2AN adopt the **T**-configuration, similarly to 1,5-Ac2AN and 2,7-Ac2AN. The planes of the adjacent molecules form the angle of 73.6°. The shortest distances between the centroids and the carbon atoms are *Cg*2...C7=356.1 pm, *Cg*2...C8=379.6 pm and *Cg*1...C8=384.7 pm. The respective shortest centroid–aryl hydrogen distances are *Cg*2...H7=287.9 pm, *Cg*2...H8=329.4 pm and *Cg*1...H8=294.4 pm. The **D**-type interactions in 9,10-Ac2AN are non-existent. The molecules lying in the parallel planes are separated by >720 pm, probably due to the considerable twist angles of the acetyl group in 9,10-Ac2AN (–85.0° and 87.0°), making the tighter arrangement

> Interplanar distance pm

*Cg*1 *Cg*3'b 477.2 – 51.0 103.6 12.5 *Cg*1 *Cg*3'c 499.5 – 51.0 209.7 24.8 *Cg*1 *Cg*3'd 511.1 – 51.0 205.9 23.8 *Cg*1 *Cg*2'e 584.5 251.6 0.0 544.4 64.5

*Cg*2 *Cg*1'h 601.5 292.2 0.0 526.4 61.1 *Cg*3 *Cg*2'h 601.5 292.2 0.0 525.8 60.9

*Cg*1 *Cg*1'j 385.6 370.4 0.0 107.1 16.1

*Cg*1 *Cg*2'i 370.4 335.9 0.0 154.8 24.7 *Cg*2 *Cg*2'i 370.2 335.9 0.0 154.4 24.7

*Cg*1 *Cg*1'i 580.5 307.2 0.0 492.6 58.1 *Cg*1 *Cg*1'm 580.9 305.9 0.0 493.9 58.2

*Cg*3 *Cg*3'n 432.7 354.6 0.0 241.8 35.2

*Cg*3 *Cg*2'r 721.2 478.0 0.0 540.1 48.5 *Cg*2 *Cg*1'r 722.3 478.0 0.0 541.5 48.6 *Cg*3 *Cg*1'r 724.2 478.0 0.0 544.0 48.7 Symmetry codes: a 0.5–*x*, 0.5+*y*, 1.5–*z*; b 0.5–*x*, –0.5+*y*, 1.5–*z*; c 1.5–*x*, 0.5+*y*, 1.5–*z*; d 1.5–*x*, –0.5+*y*, 1.5–*z*; e –1+*x*,

1–*x*, 1–*y*, 1–*z*; j

1–*x*, –*y*, 1–*<sup>z</sup>*; m 0.5+*<sup>x</sup>*, –*y*, –0.5+*z*; n 0.5–*x*, 0.5–*y*, 0.5–*z*; o –x, 0.5+*y*, 0.5–*z*; p –0.5+*x*, 1.5–*y*, *z*; q 0.5+*x*, 0.5–*y*, *z*; r *<sup>x</sup>*, –1+*y*, *z*.

Table 4. Aromatic interactions in monoacetylanthracenes and diacetylanthracenes

1–*x*, –*y*, –*z*; k 1.5–*x*, 1+*y*, 1.5–*z*; l

Interplanar angle deg

Slippage distance pm

Displacement

angle deg

impossible. The unit cell of 9,10-Ac2AN is shown in Figure 17.

1,5-Ac2AN *Cg*2 *Cg*1'f 462.9 – 56.2

1,8-Ac2AN *Cg*1 *Cg*4'k 546.4 – 34.7

9,10-Ac2AN *Cg*2 *Cg*3'p 475.5 – 73.6

*y*, *z*; f *x*, 0.5–*y*, 0.5–*z*; g 1–*x*, 0.5+*y*, 0.5–*z*; h *x*, 1+*y*, *z*; i

centroid distance pm

*Cg*1 *Cg*1'g 470.5 – 56.2

*Cg*1 *Cg*4'l 561.5 – 34.7

*Cg*1 *Cg*3'o 481.0 – 58.1 *Cg*2 *Cg*2'o 486.6 – 58.1

*Cg*2 *Cg*1'q 481.8 – 73.6

2-AcAN *Cg*1 *Cg*3'a 464.7 – 51.0 110.8 13.8

1,6-Ac2AN *Cg*3 *Cg*3'i 359.2 346.1 0.0 94.0 15.2

1,7-Ac2AN *Cg*1 *Cg*3'i 370.1 335.9 0.0 154.6 24.7

2,7-Ac2AN *Cg*2 *Cg*3'n 419.8 354.6 0.0 226.2 32.6

Centroid Centroid Centroid

The molecules of 2,7-Ac2AN are packed in a "herringbone" pattern. The anthracene moieties in the crystal structure of 2,7-Ac2AN adopt the **T**-configuration, similarly to 1,5-Ac2AN. The planes of the adjacent molecules form the angle of 58.1°. The shortest distances between the centroids and the carbon atoms are *Cg*3...C4=358.4 pm and *Cg*1...C5=375.5 pm on the one side of the anthracene system, and *Cg*3...C9=374.0 pm, *Cg*2...C8=374.8 pm on the other side. The respective shortest centroid–aryl hydrogen distances are *Cg*1...H5=299.3 pm and *Cg*3...H4=283.5 pm. The **D**-type interactions in 2,7-Ac2AN are very weak due to the large separation of centroids (420–433 pm) and large slippage distances (226–242 pm). The unit cell of 2,7-Ac2AN is shown in Figure 16.

Fig. 16. The unit cell of 2,7-Ac2AN (view towards plane *1*,*0*,*–5*)

Fig. 17. The unit cell of 9,10-Ac2AN (view along special axis *1*,*0*,*1*)

The molecules of 2,7-Ac2AN are packed in a "herringbone" pattern. The anthracene moieties in the crystal structure of 2,7-Ac2AN adopt the **T**-configuration, similarly to 1,5-Ac2AN. The planes of the adjacent molecules form the angle of 58.1°. The shortest distances between the centroids and the carbon atoms are *Cg*3...C4=358.4 pm and *Cg*1...C5=375.5 pm on the one side of the anthracene system, and *Cg*3...C9=374.0 pm, *Cg*2...C8=374.8 pm on the other side. The respective shortest centroid–aryl hydrogen distances are *Cg*1...H5=299.3 pm and *Cg*3...H4=283.5 pm. The **D**-type interactions in 2,7-Ac2AN are very weak due to the large separation of centroids (420–433 pm) and large slippage distances (226–242 pm). The unit

cell of 2,7-Ac2AN is shown in Figure 16.

Fig. 16. The unit cell of 2,7-Ac2AN (view towards plane *1*,*0*,*–5*)

Fig. 17. The unit cell of 9,10-Ac2AN (view along special axis *1*,*0*,*1*)

The anthracene moieties in the crystal structure of 9,10-Ac2AN adopt the **T**-configuration, similarly to 1,5-Ac2AN and 2,7-Ac2AN. The planes of the adjacent molecules form the angle of 73.6°. The shortest distances between the centroids and the carbon atoms are *Cg*2...C7=356.1 pm, *Cg*2...C8=379.6 pm and *Cg*1...C8=384.7 pm. The respective shortest centroid–aryl hydrogen distances are *Cg*2...H7=287.9 pm, *Cg*2...H8=329.4 pm and *Cg*1...H8=294.4 pm. The **D**-type interactions in 9,10-Ac2AN are non-existent. The molecules lying in the parallel planes are separated by >720 pm, probably due to the considerable twist angles of the acetyl group in 9,10-Ac2AN (–85.0° and 87.0°), making the tighter arrangement impossible. The unit cell of 9,10-Ac2AN is shown in Figure 17.


Symmetry codes: a 0.5–*x*, 0.5+*y*, 1.5–*z*; b 0.5–*x*, –0.5+*y*, 1.5–*z*; c 1.5–*x*, 0.5+*y*, 1.5–*z*; d 1.5–*x*, –0.5+*y*, 1.5–*z*; e –1+*x*, *y*, *z*; f *x*, 0.5–*y*, 0.5–*z*; g 1–*x*, 0.5+*y*, 0.5–*z*; h *x*, 1+*y*, *z*; i 1–*x*, 1–*y*, 1–*z*; j 1–*x*, –*y*, –*z*; k 1.5–*x*, 1+*y*, 1.5–*z*; l 1–*x*, –*y*, 1–*<sup>z</sup>*; m 0.5+*<sup>x</sup>*, –*y*, –0.5+*z*; n 0.5–*x*, 0.5–*y*, 0.5–*z*; o –x, 0.5+*y*, 0.5–*z*; p –0.5+*x*, 1.5–*y*, *z*; q 0.5+*x*, 0.5–*y*, *z*; r *<sup>x</sup>*, –1+*y*, *z*.

Table 4. Aromatic interactions in monoacetylanthracenes and diacetylanthracenes

Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes 23

monoacetylanthracenes and diacetylanthracenes: Δδ(H1, ppm)=0.70 (2-AcAN), 0.72 (2,7- Ac2AN); Δδ(H2, ppm)=0.59 (1-AcAN), 0.63 (1,6-Ac2AN), 0.67 (1,5-Ac2AN), 0.67 (1,7-Ac2AN), 0.73 (1,8-Ac2AN); Δδ(H3, ppm)=0.61 (2-AcAN), 0.65 (2,7-Ac2AN); Δδ(H5, ppm)=0.62 (1,6- Ac2AN); Δδ(H6, ppm)=0.63 (1,7-Ac2AN), 0.65 (2,7-Ac2AN), 0.67 (1,5-Ac2AN); Δδ(H7, ppm)=0.58 (1,6-Ac2AN), 0.73 (1,8-Ac2AN); Δδ(H8, ppm)=0.72 (2,7-Ac2AN), 0.77 (1,7-Ac2AN). The protons at *peri*-positions to an acetyl group are deshielded with even greater magnitudes: Δδ(H9, ppm)=1.06 (1,6-Ac2AN), 1.08 (1-AcAN), 1.17 (1,5-Ac2AN), 1.27 (1,7- Ac2AN), 1.78 (1,8-Ac2AN). The latter case is special because of the presence of two acetyl groups at *peri*-positions to H9, which nearly double its low field chemical shift. Note that in 2-AcAN, 1,6-Ac2AN, 1,7-Ac2AN and 2,7-Ac2AN both protons *ortho* to the acetyl groups demonstrate similar low field shifts, suggesting that these protons are located above the plane of the carbonyl group and near the oxygen atom [Martin et al., 2003]. Thus, the twist angles of the acetyl groups of mono- and diacetylanthracenes are small, in accordance with their respective X-ray crystal structures, and *E*,*Z*-diastereomerizations of the acetyl groups

H1 H2 H3 H4 H5 H6 H7 H8 H9 H10 CH3 CH3

1-AcAN 7.998 7.469 8.169 7.998 7.528– 7.528– 8.083 9.482 8.446 2.810

2-AcAN 8.646 8.054– 8.054– 8.054– 7.546 7.516 8.054– 8.573 8.432 2.763

9-AcAN 7.859 7.556– 7.556– 8.027 8.027 7.556– 7.556– 7.859 8.473 2.822

1,5-Ac2AN 8.083 7.530 8.262 8.083 7.530 8.262 9.570 9.570 2.818 2.818 1,6-Ac2AN 8.040 7.490 8.153 8.570 7.994 8.064 9.457 8.523 2.796 2.730 1,7-Ac2AN 8.080 7.559 8.199 8.036 8.036 8.719 9.673 8.460 2.836 2.773 1,8-Ac2AN 8.140 7.514 7.964 7.964 7.514 8.140 10.175 8.471 2.840 2.840 2,7-Ac2AN 8.670 8.063 8.063 8.063 8.063 8.670 8.718 8.449 2.775 2.775

Table 5. The 1H-NMR chemical shifts (δ, ppm) of aromatic and methyl protons in anthracene

Ketones 9-AcAN and 9,10-Ac2AN differ from the rest of the mono- and diacetylanthracenes. The protons at *peri*-positions to the acetyl groups of 9-AcAN and 9,10-Ac2AN are slightly shielded: Δδ(H1, ppm)= –0.09 (9-AcAN), –0.09 (9,10-Ac2AN). This suggests that the carbonyl groups in 9-AcAN and 9,10-Ac2AN are turned away of the protons at *peri*-positions, and these protons are located near the carbonyl carbon atoms, which implies high twist angles of the acetyl groups. It corresponds well to the respective X-ray crystal structures of 9-AcAN

7.881– 7.571– 7.571– 7.881– 7.881– 7.571– 7.571– 7.881– 2.816 2.816

at both α (1, 5, 8) and β (2, 6, 7) positions are swift on the NMR time scale.

7.495 7.495

7.477 7.477 7.477 7.477

9,10- Ac2AN

and 9,10-Ac2AN.

7.982 7.982 7.984 7.984

7.845 7.537 7.537 7.845 7.845 7.537 7.537 7.845

(AN), monoacetylanthracenes and diacetylanthracenes under study.

AN 7.95 7.41 7.41 7.95 7.95 7.41 7.41 7.95 8.40 8.40

Thus, the monoacetylanthracenes and diacetylanthracenes under study may be divided into two groups, based on the aromatic–aromatic interactions in their crystal structures. The anthracene units in 1,6-Ac2AN and 1,7-Ac2AN are offset stacked (the **D**-type arrangement) and feature aromatic–aromatic π...π interactions. The anthracene molecules in ketones 2- AcAN, 1,5-Ac2AN, 2,7-Ac2AN and 9,10-Ac2AN adopt the **T**-type arrangement, and feature aryl C–H...π interactions. The analysis of the literature crystal structures of 1-AcAN and 9- AcAN shows that these ketones also adopt the **T**-type arrangement. In 1-AcAN, 9-AcAN, 1,5-Ac2AN and 9,10-Ac2AN the considerable twist angles of the acetyl groups prevents the molecules from being arranged in close lying parallel planes. The exception is the crystal structure of 1,8-Ac2AN, which features π…π-interactions between the aromatic system and the carbonyl π-bond. Most likely the methyl groups are the reason for the lack of more examples of slipped-stacking and also in some cases the competing ketone–π system as well. It should be noted, however, that the centroid–centroid analysis can be misleading, and its limitations should not be overlooked.

Another kind of intermolecular interactions that could exist in acetylanthracenes is hydrogen bonds. No particular strong intermolecular aryl C–H…O bonds have been found in the diacetylanthracenes under study. The shortest contact distances between an oxygen and an aromatic hydrogen are O15...H5=242.2 pm (9,10-Ac2AN), O15...H1=247.4 pm and O16...H9=259.4 pm (2,7-Ac2AN), O15...H5=255.6 pm (1,6-Ac2AN), O16...H3=256.4 pm and O15...H4=260.7 pm (1,7-Ac2AN), O15...H2'=260.5 pm (1,8-Ac2AN), O15...H2=284.6 pm (1,5- Ac2AN). The shortest contact distances between an oxygen and a methyl hydrogen are of a similar magnitude: O15...H14c=240.8 pm (2,7-Ac2AN), O16...H12c=254.7 pm (1,6-Ac2AN), O16...H12b=257.5 pm (1,7-Ac2AN), O15...H12c=259.4 pm (1,5-Ac2AN), O15...H12c=265.9 pm (9,10- Ac2AN).

#### **2.2 NMR Study of monoacetylanthracenes and diacetylanthracenes**

The structure of a compound in crystal is not necessarily the same as that in solution. More often, in the case of substances that are not conformationally homogeneous, e.g. diacetylanthracenes, the crystal has a unique conformation and the conformational heterogeneity appears in fluid phases [Eliel & Wilen, 1994]. An insight into the conformations of mono- and diacetylanthracenes in solution may be gained from the chemical shifts of the aromatic protons adjacent to the carbonyl groups. The magnetic shielding (or deshielding) effect on the chemical shifts of protons that lie in or near the plane of the carbonyl group is well known. The McConnell equation [McConnel, 1957] predicts shielding for protons lying above the center of a carbon–oxygen double bond and deshielding for protons located within a cone aligned with the carbon–oxygen bond axis. The McConnell model, however, takes into account only the effect of magnetic anisotropy.

Recently, more detailed shielding model has been proposed [Martin et al., 2003]. According to this model, shielding is predicted for protons located in the region from over the center of the carbon–oxygen double bond to beyond the carbon atom; deshielding is predicted for protons located above and beyond the oxygen atom. Table 5 gives 1H-NMR chemical shifts for the monoacetylanthracenes and diacetylanthracenes under study, together with the chemical shifts in parent anthracene (AN).

The data presented in Table 5 show that the protons at *ortho*-positions to an acetyl group are considerably deshielded as compared with the protons of unsubstituted anthracene. The magnitudes of the low field shifts of the *ortho*-protons are similar among

Thus, the monoacetylanthracenes and diacetylanthracenes under study may be divided into two groups, based on the aromatic–aromatic interactions in their crystal structures. The anthracene units in 1,6-Ac2AN and 1,7-Ac2AN are offset stacked (the **D**-type arrangement) and feature aromatic–aromatic π...π interactions. The anthracene molecules in ketones 2- AcAN, 1,5-Ac2AN, 2,7-Ac2AN and 9,10-Ac2AN adopt the **T**-type arrangement, and feature aryl C–H...π interactions. The analysis of the literature crystal structures of 1-AcAN and 9- AcAN shows that these ketones also adopt the **T**-type arrangement. In 1-AcAN, 9-AcAN, 1,5-Ac2AN and 9,10-Ac2AN the considerable twist angles of the acetyl groups prevents the molecules from being arranged in close lying parallel planes. The exception is the crystal structure of 1,8-Ac2AN, which features π…π-interactions between the aromatic system and the carbonyl π-bond. Most likely the methyl groups are the reason for the lack of more examples of slipped-stacking and also in some cases the competing ketone–π system as well. It should be noted, however, that the centroid–centroid analysis can be misleading, and its

Another kind of intermolecular interactions that could exist in acetylanthracenes is hydrogen bonds. No particular strong intermolecular aryl C–H…O bonds have been found in the diacetylanthracenes under study. The shortest contact distances between an oxygen and an aromatic hydrogen are O15...H5=242.2 pm (9,10-Ac2AN), O15...H1=247.4 pm and O16...H9=259.4 pm (2,7-Ac2AN), O15...H5=255.6 pm (1,6-Ac2AN), O16...H3=256.4 pm and O15...H4=260.7 pm (1,7-Ac2AN), O15...H2'=260.5 pm (1,8-Ac2AN), O15...H2=284.6 pm (1,5- Ac2AN). The shortest contact distances between an oxygen and a methyl hydrogen are of a similar magnitude: O15...H14c=240.8 pm (2,7-Ac2AN), O16...H12c=254.7 pm (1,6-Ac2AN), O16...H12b=257.5 pm (1,7-Ac2AN), O15...H12c=259.4 pm (1,5-Ac2AN), O15...H12c=265.9 pm (9,10-

The structure of a compound in crystal is not necessarily the same as that in solution. More often, in the case of substances that are not conformationally homogeneous, e.g. diacetylanthracenes, the crystal has a unique conformation and the conformational heterogeneity appears in fluid phases [Eliel & Wilen, 1994]. An insight into the conformations of mono- and diacetylanthracenes in solution may be gained from the chemical shifts of the aromatic protons adjacent to the carbonyl groups. The magnetic shielding (or deshielding) effect on the chemical shifts of protons that lie in or near the plane of the carbonyl group is well known. The McConnell equation [McConnel, 1957] predicts shielding for protons lying above the center of a carbon–oxygen double bond and deshielding for protons located within a cone aligned with the carbon–oxygen bond axis. The McConnell model, however, takes into account only the effect of magnetic anisotropy. Recently, more detailed shielding model has been proposed [Martin et al., 2003]. According to this model, shielding is predicted for protons located in the region from over the center of the carbon–oxygen double bond to beyond the carbon atom; deshielding is predicted for protons located above and beyond the oxygen atom. Table 5 gives 1H-NMR chemical shifts for the monoacetylanthracenes and diacetylanthracenes under study, together with the

The data presented in Table 5 show that the protons at *ortho*-positions to an acetyl group are considerably deshielded as compared with the protons of unsubstituted anthracene. The magnitudes of the low field shifts of the *ortho*-protons are similar among

**2.2 NMR Study of monoacetylanthracenes and diacetylanthracenes** 

limitations should not be overlooked.

chemical shifts in parent anthracene (AN).

Ac2AN).

monoacetylanthracenes and diacetylanthracenes: Δδ(H1, ppm)=0.70 (2-AcAN), 0.72 (2,7- Ac2AN); Δδ(H2, ppm)=0.59 (1-AcAN), 0.63 (1,6-Ac2AN), 0.67 (1,5-Ac2AN), 0.67 (1,7-Ac2AN), 0.73 (1,8-Ac2AN); Δδ(H3, ppm)=0.61 (2-AcAN), 0.65 (2,7-Ac2AN); Δδ(H5, ppm)=0.62 (1,6- Ac2AN); Δδ(H6, ppm)=0.63 (1,7-Ac2AN), 0.65 (2,7-Ac2AN), 0.67 (1,5-Ac2AN); Δδ(H7, ppm)=0.58 (1,6-Ac2AN), 0.73 (1,8-Ac2AN); Δδ(H8, ppm)=0.72 (2,7-Ac2AN), 0.77 (1,7-Ac2AN). The protons at *peri*-positions to an acetyl group are deshielded with even greater magnitudes: Δδ(H9, ppm)=1.06 (1,6-Ac2AN), 1.08 (1-AcAN), 1.17 (1,5-Ac2AN), 1.27 (1,7- Ac2AN), 1.78 (1,8-Ac2AN). The latter case is special because of the presence of two acetyl groups at *peri*-positions to H9, which nearly double its low field chemical shift. Note that in 2-AcAN, 1,6-Ac2AN, 1,7-Ac2AN and 2,7-Ac2AN both protons *ortho* to the acetyl groups demonstrate similar low field shifts, suggesting that these protons are located above the plane of the carbonyl group and near the oxygen atom [Martin et al., 2003]. Thus, the twist angles of the acetyl groups of mono- and diacetylanthracenes are small, in accordance with their respective X-ray crystal structures, and *E*,*Z*-diastereomerizations of the acetyl groups at both α (1, 5, 8) and β (2, 6, 7) positions are swift on the NMR time scale.


Table 5. The 1H-NMR chemical shifts (δ, ppm) of aromatic and methyl protons in anthracene (AN), monoacetylanthracenes and diacetylanthracenes under study.

Ketones 9-AcAN and 9,10-Ac2AN differ from the rest of the mono- and diacetylanthracenes. The protons at *peri*-positions to the acetyl groups of 9-AcAN and 9,10-Ac2AN are slightly shielded: Δδ(H1, ppm)= –0.09 (9-AcAN), –0.09 (9,10-Ac2AN). This suggests that the carbonyl groups in 9-AcAN and 9,10-Ac2AN are turned away of the protons at *peri*-positions, and these protons are located near the carbonyl carbon atoms, which implies high twist angles of the acetyl groups. It corresponds well to the respective X-ray crystal structures of 9-AcAN and 9,10-Ac2AN.

Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes 25

1-AcAN *Z Cs* –692.17301155 14.66 15.79 0.00 0.0 180.0 0.0 0.0 149.9 0.0 1-AcAN d *Z C*1 – – – – 27.1 –152.7 28.6 3.2 149.3 –0.1 1-AcAN *E C*1 –692.16815672 27.41 28.80 13.01 150.8 –31.1 36.0 3.7 150.7 1.9 2-AcAN *E Cs* –692.17859715 0.00 0.00 0.00 180.0 0.0 0.0 0.0 149.6 0.0 2-AcAN d *E* – – – – 173.1 -5.3 5.9 0.4 149.6 –1.6 2-AcAN *Z Cs* –692.17777414 2.16 2.24 2.24 0.0 180.0 0.0 0.0 149.9 0.0 9-AcAN *– C*1 –692.16381815 38.80 36.94 0.00 –67.0 113.9 69.8 1.7 151.3 –1.0 9-AcAN d *– C*1 – – – – 87.9 –91.8 89.2 5.8 150.4 0.3 1,5-Ac2AN *ZZ C*<sup>2</sup>*<sup>h</sup>* –844.81621983 24.25 27.69 0.00 0.0 180.0 0.0 0.0 149.8 0.0 1,5-Ac2AN d *ZZ Ci* – – – – 20.0 –156.8 22.7 0.0 149.4 –3.2 1,5-Ac2AN *ZE C*1 –844.81074449 38.62 40.48 12.79 152.4 –29.3 34.1 3.8 150.6 1.6 –1.1 178.9 2.3 150.0 0.0 1,5-Ac2AN *EEanti Ci* –844.80527143 52.99 55.06 27.36 150.6 –30.9 34.8 0.0 150.8 1.5 1,5-Ac2AN *EEsyn C*2 –844.80559961 52.13 55.74 28.05 151.9 –29.9 35.6 7.4 150.7 1.7 1,6-Ac2AN *ZE Cs* –844.82056764 12.83 13.45 0.00 0.0 180.0 0.0 0.0 150.0 0.0 180.0 0.0 0.0 149.7 0.0 1,6-Ac2AN d *ZE C*1 – – – – 30.0 –147.1 32.2 150.1 –2.9 178.6 –0.7 1.9 1.3 149.3 –0.7 1,6-Ac2AN *ZZ Cs* –844.82005388 14.18 15.14 1.69 0.0 180.0 0.0 0.0 150.0 0.0 0.0 180.0 0.0 150.0 0.0 1,6-Ac2AN *EE C*1 –844.81569542 25.63 27.03 13.57 150.6 –31.1 36.0 3.6 150.8 1.7 179.9 –0.1 1.4 149.8 –0.1 1,6-Ac2AN *EZanti C*1 –844.81493981 27.61 28.88 15.43 150.6 –31.2 36.2 150.7 1.7 –0.3 179.8 1.7 150.0 –0.1 1,7-Ac2AN *ZE Cs* –844.82110775 11.42 12.34 0.00 0.0 180.0 0.0 0.0 150.0 0.0 180.0 0.0 0.0 149.7 0.0 1,7-Ac2AN d *ZE C*1 – – – – –15.2 162.9 16.0 2.3 149.8 1.9 –176.6 3.7 4.5 149.0 0.3 1,7-Ac2AN *ZZ Cs* –844.81939574 15.91 15.83 3.50 0.0 180.0 0.0 0.0 150.1 0.0 0.0 180.0 0.0 150.0 0.0 1,7-Ac2AN *EE C*1 –844.81562930 25.80 26.79 14.45 150.3 –31.4 36.3 3.7 150.8 1.7 179.6 –0.5 1.7 149.8 0.0 1,7-Ac2AN *EZanti C*1 –844.81488173 27.76 28.96 16.62 150.8 –31.0 35.8 3.5 150.9 1.7 0.2 –179.8 1.1 150.1 0.0 1,8-Ac2AN *ZZanti C*2 –844.81111292 37.66 38.89 0.00 –17.3 160.4 19.3 2.2 150.2 2.3 1,8-Ac2AN d *ZZ C*2 – – – – –34.0 145.4 36.0 0.3 149.3 0.6 –32.4 144.9 35.4 3.4 148.9 2.7 1,8-Ac2AN *EZ C*1 –844.81126554 37.26 39.25 0.35 150.4 –31.2 36.1 3.5 151.1 1.6 1.5 –178.3 2.6 150.0 0.2 1,8-Ac2AN *EEsyn Cs* –844.80423404 55.72 56.49 17.60 147.9 –33.8 40.2 7.0 150.7 1.7 1,8-Ac2AN *EEanti C*2 –844.80485619 54.08 56.56 17.66 148.1 –33.7 38.6 5.1 150.7 1.8 1,9-Ac2AN *ZZanti C*1 –844.79904569 69.34 70.32 0.00 –50.9 120.3 59.9 7.5 150.8 8.8 –59.6 114.1 62.8 151.5 –6.3 1,9-Ac2AN *EZsyn C*1 –844.78990701 93.33 96.19 25.88 –141.2 45.3 56.7 10.7 151.2 –6.5 44.8 –128.4 48.1 151.1 6.8 1,10-Ac2AN *ZE C*1 –844.80536578 52.75 49.45 0.00 0.2 –180.0 1.0 1.8 150.1 0.2 –108.0 73.0 75.3 151.6 –1.0 1,10-Ac2AN *ZZ C*1 –844.80575464 51.73 50.43 0.98 1.8 –178.2 2.7 2.4 150.1 0.0 –65.9 115.1 68.5 151.4 1.0

Hartree kJ/mol kJ/mol kJ/mo

*E*Tot Δ*E*Tot Δ*G*298 ΔΔ*G*298 τa υb θ φ C11–Caromc χ

<sup>l</sup>deg deg deg deg pm deg

#### **2.3 DFT computational study of monoacetylanthracenes and diacetylanthracenes**

DFT methods are capable of generating a variety of isolated molecular properties quite accurately, especially via the hybrid functional, and in a cost-effective way [deProft & Geerlings, 2001, Koch & Holthausen, 2000]. The B3LYP hybrid functional was successfully employed to treat overcrowded BAEs [Biedermann et al., 2001, Pogodin et al., 2006] and overcrowded naphthologues of BAEs-1, i.e. mono-bridged tetraarylethylenes [Assadi et al., 2009]. The monoacetylanthracenes and diacetylanthracenes under study were subjected to a systematic computational DFT study of their conformational spaces and of their relative stabilities. The B3LYP/6-31G(d) relative energies of the global minima conformations of certain diacetylanthracenes have been previously reported [Mala'bi et al., 2011]. The total and relative B3LYP/6-31G(d) energies (*E*Tot and Δ*E*Tot) and Gibbs free energies (Δ*G*298 and ΔΔ*G*298) of the acetylanthracenes are presented in Table 6. Selected calculated geometrical parameters of the acetylanthracenes are also given in Table 6. The following geometrical parameters were considered: the twist angles τ1, τ2 and τ9 and the respective twist angles υ around the anthracenyl–carbonyl bond; the dihedral angle θ between the least-square planes of the carbonyl group and the anthracene system; the dihedral angle φ between the leastsquare planes of two side rings of the anthracene system; the pyramidalization angles χ at C1, C2 and C9.

#### **2.3.1 Conformational space of monoacetylanthracenes and diacetylanthracenes**

Monoacetylanthracenes may adopt two conformations, *Z* and *E*, defined by the twist angle of the carbonyl group. Diacetylanthracenes may adopt four conformations, i. e. *ZZ*, *ZE*, *EZ* and *EE*; in certain cases, *ZE* is identical to *EZ*. In addition, the oxygen atoms of two carbonyl groups may be located on the same side of the aromatic plane, or on the opposite sides, potentially resulting in *syn*- and *anti*-*ZZ*, *ZE*, *EZ* and *EE* conformations, respectively. Depending on the symmetry constraints and the twist angle τ, not all of the abovementioned conformations exist for a given diacetylanthracene. The possible conformations of diacetylanthracene are shown in Fig. 18.

Fig. 18. Schematic representation of the eight possible conformations of a diacetylanthracene (view along the aromatic plane).

**2.3 DFT computational study of monoacetylanthracenes and diacetylanthracenes**  DFT methods are capable of generating a variety of isolated molecular properties quite accurately, especially via the hybrid functional, and in a cost-effective way [deProft & Geerlings, 2001, Koch & Holthausen, 2000]. The B3LYP hybrid functional was successfully employed to treat overcrowded BAEs [Biedermann et al., 2001, Pogodin et al., 2006] and overcrowded naphthologues of BAEs-1, i.e. mono-bridged tetraarylethylenes [Assadi et al., 2009]. The monoacetylanthracenes and diacetylanthracenes under study were subjected to a systematic computational DFT study of their conformational spaces and of their relative stabilities. The B3LYP/6-31G(d) relative energies of the global minima conformations of certain diacetylanthracenes have been previously reported [Mala'bi et al., 2011]. The total and relative B3LYP/6-31G(d) energies (*E*Tot and Δ*E*Tot) and Gibbs free energies (Δ*G*298 and ΔΔ*G*298) of the acetylanthracenes are presented in Table 6. Selected calculated geometrical parameters of the acetylanthracenes are also given in Table 6. The following geometrical parameters were considered: the twist angles τ1, τ2 and τ9 and the respective twist angles υ around the anthracenyl–carbonyl bond; the dihedral angle θ between the least-square planes of the carbonyl group and the anthracene system; the dihedral angle φ between the leastsquare planes of two side rings of the anthracene system; the pyramidalization angles χ at

**2.3.1 Conformational space of monoacetylanthracenes and diacetylanthracenes**  Monoacetylanthracenes may adopt two conformations, *Z* and *E*, defined by the twist angle of the carbonyl group. Diacetylanthracenes may adopt four conformations, i. e. *ZZ*, *ZE*, *EZ* and *EE*; in certain cases, *ZE* is identical to *EZ*. In addition, the oxygen atoms of two carbonyl groups may be located on the same side of the aromatic plane, or on the opposite sides, potentially resulting in *syn*- and *anti*-*ZZ*, *ZE*, *EZ* and *EE* conformations, respectively. Depending on the symmetry constraints and the twist angle τ, not all of the abovementioned conformations exist for a given diacetylanthracene. The possible conformations

Me

Me

O

O

O

O

Fig. 18. Schematic representation of the eight possible conformations of a diacetylanthracene

O

O

Me

(*E*,*E*)-*syn*

Me

Me

(*E*,*E*)-*anti*

Me

O

O

Me

Me

O

O

Me

(*E*,*Z*)-*syn*

Me

(*E*,*Z*)-*anti*

O

O

C1, C2 and C9.

Me

Me

of diacetylanthracene are shown in Fig. 18.

Me

Me

(*Z*,*Z*)-*syn* (*Z*,*E*)-*syn*

(*Z*,*Z*)-*anti* (*Z*,*E*)-*anti*

Me

Me

O

O

(view along the aromatic plane).

O

O


Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes 27

radii of oxygen and hydrogen, 244 pm [Zefirov, 1997]). The non-planar *C*1-1*E*-AcAN conformation (the twist angle τ1(C9a–C1–C11–O13)=150.8°) is higher in energy by 13.0 kJ/mol. The energy barrier for the *E*,*Z*-diastereomerization *Cs*-1*Z*-AcAN→*C*1-1*E*-AcAN by the rotation of the acetyl group via a nearly orthogonal transition state is 19.5 kJ/mol. As mentioned above, 1-AcAN [Langer1993] crystallizes as the *Z*-diastereomer, which is correctly described by the calculated structure of *Cs*-1*Z*-AcAN. However, the carbonyl group in the crystal structure of 1-AcAN is considerably twisted out of the plane of the anthracene ring system, τ1=27.1°. As a result, the calculated *Cs*-1*Z*-AcAN structure is more overcrowded

Ketone 2-AcAN adopts a *Cs*-*E* conformation as its global minimum. Its local minimum *Cs*-2*Z*-AcAN conformation is 2.2 kJ/mol higher in energy. Both conformations are not overcrowded, lacking any *peri*-interactions. The energy barrier for the *E*,*Z*diastereomerization *Cs*-2*E*-AcAN→*Cs*-2*Z*-AcAN by the rotation of the acetyl group via a nearly orthogonal transition state is 31.5 kJ/mol. The calculated *Cs*-2*E*-AcAN conformation corresponds well to the *E*-conformation of the crystal structure. The latter, however, features a small twist angle of τ2(C1–C2–C11–O13)=173.1°, in contrast to the planar (excluding the

In the global minimum conformation of 9-AcAN the twist angle τ9(C9a–C9–C11–O13) is –67.0°. This conformations cannot be defined as either *E* or *Z*, and no other minimum conformation was located. Comparing the calculated structure of 9-AcAN with the crystal structure of 9- AcAN reported in the literature [Zouev2011], the carbonyl group in the latter is almost orthogonal to the plane of the anthracene ring system: the twist angle τ9(C9a–C9–C11–O13)=87.9° is considerably larger than the twist angle predicted by the DFT calculations. The energy barrier for the enantiomerization of 9-AcAN via the orthogonal [*Cs*-9-AcAN] transition state is only 3.6 kJ/mol. The low enantiomerization barrier as compared to the diastereomerization

Ketone 1,5-Ac2AN adopt a *C*2*<sup>h</sup>*-1*Z*,5*Z* conformation as its global minimum. The geometry optimizations under *C*2 or *Ci* symmetry constraints converged to the *C*2*<sup>h</sup>* symmetry structure. *C*2*<sup>h</sup>*-1*Z*,5*Z*-Ac2AN is considerably overcrowded due to the short O15...H9/O16...H10 contact distances (14% penetration). The *C*2*h*-1*Z*,5*Z*-Ac2AN conformation corresponds to the *Z,Z* Xray structure of 1,5-Ac2AN. However, the calculated structure is planar (excluding the methyl hydrogens), while the X-ray structure has the twist angle τ1(C9a–C1–C11–O15)=20.0° and the dihedral angle θ=22.7°, and, as a result, is less overcrowded. In addition to the global minimum, there are three local minima conformations of 1,5-Ac2AN: *C*1-1*Z*,5*E*-Ac2AN, *Ci*-1*E*,5*E-anti*-Ac2AN and *C*2-1*E*,5*E-syn*-Ac2AN. The four conformations of 1,5- Ac2AN undergo diastereomerizations by the rotation of one of the acetyl groups via "nearly orthogonal" transition states, in which the rotating acetyl group has the twist angle of τ=85– 97°, and the other acetyl group retains its *E*- or *Z*-conformation. The rotation of an acetyl group of *C*2*<sup>h</sup>*-1*Z*,5*Z*-Ac2AN via [*C*1-1*Z*,90-Ac2AN] leads to the *C*1-1*Z*,5*E*-Ac2AN conformation, which is 12.8 kJ/mol higher in energy than the global minimum. The *E*orientation of the acetyl group at the 5-position and the *peri*-interactions of its methyl hydrogens with H10 force the acetyl group out of the aromatic plane, thus decreasing the conjugation. Due to the twist angle τ1(C10a–C5–C13–O16)=152.4° which differs from either 0° or 180°, rotation of the 1*Z*-acetyl group of *C*1-1*Z*,5*E*-Ac2AN may be realized in either *anti*- (via [*C*1-90,5*E*-*anti*-Ac2AN]) or in *syn*-direction (via [*C*1-90,5*E*-*syn*-Ac2AN]) relative to the 5*E*acetyl group. These processes lead to the different local minima *Ci*-1*E*,5*E*-*anti*-Ac2AN and *C*2- 1*E*,5*E*-*syn*-Ac2AN conformations, respectively, which are 27.4 and 28.0 kJ/mol higher in

barriers in 1-AcAN and 2-AcAN is due to an already high twist angle in 9-AcAN.

than the X-ray structure (in the latter the O13...H9 distance is 223 pm).

methyl hydrogens) calculated structure.


<sup>a</sup> τ1(C9a–C1–C11–O15) for 1-AcAN, 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN and 1,8-Ac2AN, τ2(C1–C2–C11–O15) for 2-AcAN and 2,7-Ac2AN, τ2(C5–C6–C13–O16) for 1,6-Ac2AN, τ2(C8–C7–C13–O16) for 1,7-Ac2AN, τ9(C9a–

C9–C11–O15) for 9-AcAN and 9,10-Ac2AN. b <sup>υ</sup>1(C2–C1–C11–O15) for 1-AcAN, 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN and 1,8-Ac2AN, υ2(C3–C2–C11–O15) for 2-AcAN and 2,7-Ac2AN, υ2(C7–C6–C13–O16) for 1,6-Ac2AN, υ2(C6–C7–C13–O16) for 1,7-Ac2AN, υ9(C8a–

C9–C11–O15) for 9-AcAN and 9,10-Ac2AN. c C1–C11 for 1-AcAN, 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN and 1,8-Ac2AN, C2–C11 for 2-AcAN and 2,7- Ac2AN, C6–C13 for 1,6-Ac2AN, C7–C13 for 1,7-Ac2AN, C9–C11 for 9-AcAN and 9,10-Ac2AN. d the selected geometrical parameters derived from the corresponding X-ray structures.

Table 6. Total energies (*E*Tot), relative energies (Δ*E*Tot) and Gibbs free energies (Δ*G*298) and selected geometric parameters of mono- and diacetylanthracenes.

Ketone 1-AcAN adopts a *Cs*-*Z* conformation as its global minimum. The planar (excluding the methyl hydrogens) *Cs*-1*Z*-AcAN is overcrowded due to the short O13...H9 contact distance (the O13...H9 distance is 215 pm, 14% penetration, based on the sum of the wan-der-Vaals

1,10-Ac2AN *EZanti C*1 –844.80066416 65.09 63.37 13.91 148.6 –33.3 38.0 2.9 150.7 1.9 –70.6 110.9 72.5 151.6 –1.5 1,10-Ac2AN *EEanti C*1 –844.80064430 65.14 63.38 13.92 148.5 –33.6 37.9 2.8 150.8 2.1 –106.6 75.1 76.7 151.6 1.7 1,10-Ac2AN *EEsyn C*1 –844.80035871 65.89 63.70 14.25 149.9 –31.7 37.8 5.4 150.8 1.6 111.3 –68.9 71.9 151.6 0.2 9,10-Ac2AN *E Ci* –844.79648217 76.07 71.57 0.00 –72.6 108.5 74.7 0.0 151.6 –1.1 9,10-Ac2AN d *E C*1 – – – – –85.0 94.0 86.7 1.6 151.3 –1.0 87.0 –93.7 86.5 151.5 –0.6 9,10-Ac2AN *Z Cs* –844.79616186 76.91 71.63 0.06 71.8 –108.9 74.2 2.9 151.6 –0.7 9,10-Ac2AN *E C*2 –844.79637404 76.35 72.55 0.99 75.4 –105.7 76.9 0.1 151.6 –1.1 9,10-Ac2AN *Z C*2 –844.79619082 76.84 73.69 2.13 –71.9 108.8 73.9 3.1 151.6 –0.7 2,6-Ac2AN *EE C*<sup>2</sup>*<sup>h</sup>* –844.82603788 –1.53 0.40 0.00 180.0 0.0 0.0 0.0 149.8 0.0 2,6-Ac2AN *ZE Cs* –844.82517129 0.75 0.79 0.40 0.0 180.0 0.0 0.0 150.1 0.0 180.0 0.0 149.8 0.0 2,6-Ac2AN *ZZ C*<sup>2</sup>*<sup>h</sup>* –844.82448815 2.54 4.19 3.79 0.0 –180.0 0.0 0.0 150.1 0.0 2,7-Ac2AN *EZ Cs* –844.82545585 0.00 0.00 0.00 180.0 0.0 0.0 0.0 149.7 0.0 0.0 180.0 150.0 0.0 2,7-Ac2AN d *EZ C*1 – – – – 171.9 –3.3 9.8 2.7 149.0 –4.8 0.9 –178.8 1.6 148.9 0.3 2,7-Ac2AN *EE C*<sup>2</sup>*<sup>v</sup>* –844.82612845 –1.77 0.20 0.20 180.0 0.0 0.0 0.0 149.8 0.0 2,7-Ac2AN *ZZ C*<sup>2</sup>*<sup>v</sup>* –844.82444406 2.66 4.29 4.29 0.0 180.0 0.0 0.0 150.1 0.0 2,9-Ac2AN *EE C*1 –844.81120818 37.41 32.29 0.00 –178.9 1.4 2.1 1.6 149.8 –0.4 1.50 –106.9 73.9 75.8 151.6 0.8 2,9-Ac2AN *EZ C*1 –844.81178130 35.90 35.90 3.61 –178.9 1.3 1.8 2.5 149.9 –0.2 0.00 –63.0 118.1 66.2 151.3 1.1 2,9-Ac2AN *ZE C*1 –844.81042981 39.45 37.37 5.08 –1.8 178.5 2.6 1.8 150.1 –0.3 3.55 –114.4 66.0 69.1 151.5 0.4 2,10-Ac2AN *EE C*1 –844.81146291 36.74 34.49 0.00 179.9 –0.7 0.7 1.8 149.7 0.5 –113.9 66.7 70.5 151.5 –0.6 2,10-Ac2AN *EZ C*1 –844.81128488 37.21 34.81 0.32 179.6 –0.5 0.3 1.7 149.7 –0.1 0.32 –68.8 112.0 71.4 151.5 0.9 2,10-Ac2AN *ZE C*1 –844.81074527 38.62 36.64 2.15 0.9 –179.8 1.9 2.0 150.0 0.7 2.15 –114.3 66.4 69.3 151.4 –0.8 2,10-Ac2AN *ZZ C*1 –844.81084444 38.36 38.36 3.87 1.0 –179.2 1.7 2.1 150.0 –0.2 3.87 –65.7 115.3 68.6 151.4 1.0 <sup>a</sup> τ1(C9a–C1–C11–O15) for 1-AcAN, 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN and 1,8-Ac2AN, τ2(C1–C2–C11–O15) for 2-AcAN and 2,7-Ac2AN, τ2(C5–C6–C13–O16) for 1,6-Ac2AN, τ2(C8–C7–C13–O16) for 1,7-Ac2AN, τ9(C9a– C9–C11–O15) for 9-AcAN and 9,10-Ac2AN. b <sup>υ</sup>1(C2–C1–C11–O15) for 1-AcAN, 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN and 1,8-Ac2AN, υ2(C3–C2–C11–O15) for 2-AcAN and 2,7-Ac2AN, υ2(C7–C6–C13–O16) for 1,6-Ac2AN, υ2(C6–C7–C13–O16) for 1,7-Ac2AN, υ9(C8a– C9–C11–O15) for 9-AcAN and 9,10-Ac2AN. c C1–C11 for 1-AcAN, 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN and 1,8-Ac2AN, C2–C11 for 2-AcAN and 2,7-

Ac2AN, C6–C13 for 1,6-Ac2AN, C7–C13 for 1,7-Ac2AN, C9–C11 for 9-AcAN and 9,10-Ac2AN. d the selected geometrical parameters derived from the corresponding X-ray structures.

selected geometric parameters of mono- and diacetylanthracenes.

Table 6. Total energies (*E*Tot), relative energies (Δ*E*Tot) and Gibbs free energies (Δ*G*298) and

Ketone 1-AcAN adopts a *Cs*-*Z* conformation as its global minimum. The planar (excluding the methyl hydrogens) *Cs*-1*Z*-AcAN is overcrowded due to the short O13...H9 contact distance (the O13...H9 distance is 215 pm, 14% penetration, based on the sum of the wan-der-Vaals radii of oxygen and hydrogen, 244 pm [Zefirov, 1997]). The non-planar *C*1-1*E*-AcAN conformation (the twist angle τ1(C9a–C1–C11–O13)=150.8°) is higher in energy by 13.0 kJ/mol. The energy barrier for the *E*,*Z*-diastereomerization *Cs*-1*Z*-AcAN→*C*1-1*E*-AcAN by the rotation of the acetyl group via a nearly orthogonal transition state is 19.5 kJ/mol. As mentioned above, 1-AcAN [Langer1993] crystallizes as the *Z*-diastereomer, which is correctly described by the calculated structure of *Cs*-1*Z*-AcAN. However, the carbonyl group in the crystal structure of 1-AcAN is considerably twisted out of the plane of the anthracene ring system, τ1=27.1°. As a result, the calculated *Cs*-1*Z*-AcAN structure is more overcrowded than the X-ray structure (in the latter the O13...H9 distance is 223 pm).

Ketone 2-AcAN adopts a *Cs*-*E* conformation as its global minimum. Its local minimum *Cs*-2*Z*-AcAN conformation is 2.2 kJ/mol higher in energy. Both conformations are not overcrowded, lacking any *peri*-interactions. The energy barrier for the *E*,*Z*diastereomerization *Cs*-2*E*-AcAN→*Cs*-2*Z*-AcAN by the rotation of the acetyl group via a nearly orthogonal transition state is 31.5 kJ/mol. The calculated *Cs*-2*E*-AcAN conformation corresponds well to the *E*-conformation of the crystal structure. The latter, however, features a small twist angle of τ2(C1–C2–C11–O13)=173.1°, in contrast to the planar (excluding the methyl hydrogens) calculated structure.

In the global minimum conformation of 9-AcAN the twist angle τ9(C9a–C9–C11–O13) is –67.0°. This conformations cannot be defined as either *E* or *Z*, and no other minimum conformation was located. Comparing the calculated structure of 9-AcAN with the crystal structure of 9- AcAN reported in the literature [Zouev2011], the carbonyl group in the latter is almost orthogonal to the plane of the anthracene ring system: the twist angle τ9(C9a–C9–C11–O13)=87.9° is considerably larger than the twist angle predicted by the DFT calculations. The energy barrier for the enantiomerization of 9-AcAN via the orthogonal [*Cs*-9-AcAN] transition state is only 3.6 kJ/mol. The low enantiomerization barrier as compared to the diastereomerization barriers in 1-AcAN and 2-AcAN is due to an already high twist angle in 9-AcAN.

Ketone 1,5-Ac2AN adopt a *C*2*<sup>h</sup>*-1*Z*,5*Z* conformation as its global minimum. The geometry optimizations under *C*2 or *Ci* symmetry constraints converged to the *C*2*<sup>h</sup>* symmetry structure. *C*2*<sup>h</sup>*-1*Z*,5*Z*-Ac2AN is considerably overcrowded due to the short O15...H9/O16...H10 contact distances (14% penetration). The *C*2*h*-1*Z*,5*Z*-Ac2AN conformation corresponds to the *Z,Z* Xray structure of 1,5-Ac2AN. However, the calculated structure is planar (excluding the methyl hydrogens), while the X-ray structure has the twist angle τ1(C9a–C1–C11–O15)=20.0° and the dihedral angle θ=22.7°, and, as a result, is less overcrowded. In addition to the global minimum, there are three local minima conformations of 1,5-Ac2AN: *C*1-1*Z*,5*E*-Ac2AN, *Ci*-1*E*,5*E-anti*-Ac2AN and *C*2-1*E*,5*E-syn*-Ac2AN. The four conformations of 1,5- Ac2AN undergo diastereomerizations by the rotation of one of the acetyl groups via "nearly orthogonal" transition states, in which the rotating acetyl group has the twist angle of τ=85– 97°, and the other acetyl group retains its *E*- or *Z*-conformation. The rotation of an acetyl group of *C*2*<sup>h</sup>*-1*Z*,5*Z*-Ac2AN via [*C*1-1*Z*,90-Ac2AN] leads to the *C*1-1*Z*,5*E*-Ac2AN conformation, which is 12.8 kJ/mol higher in energy than the global minimum. The *E*orientation of the acetyl group at the 5-position and the *peri*-interactions of its methyl hydrogens with H10 force the acetyl group out of the aromatic plane, thus decreasing the conjugation. Due to the twist angle τ1(C10a–C5–C13–O16)=152.4° which differs from either 0° or 180°, rotation of the 1*Z*-acetyl group of *C*1-1*Z*,5*E*-Ac2AN may be realized in either *anti*- (via [*C*1-90,5*E*-*anti*-Ac2AN]) or in *syn*-direction (via [*C*1-90,5*E*-*syn*-Ac2AN]) relative to the 5*E*acetyl group. These processes lead to the different local minima *Ci*-1*E*,5*E*-*anti*-Ac2AN and *C*2- 1*E*,5*E*-*syn*-Ac2AN conformations, respectively, which are 27.4 and 28.0 kJ/mol higher in

Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes 29

and τ2(C8–C7–C13–O16)=-176.6. The relative stabilities of the conformations of 1,7-Ac2AN and its conformational space are very similar to those of 1,6-Ac2AN**,** both being α,βdiacetylanthracenes. The local minima conformations *Cs*-1*Z*,7*E*-Ac2AN, *C*1-1*E*,7*E*-Ac2AN and *C*1-1*E*,7*Z-anti*-Ac2AN are higher in energy than the global minimum by 3.5, 14.5, and 16.6 kJ/mol, respectively. The diastereomerization processes in 1,7-Ac2AN are shown in Fig. 21.

*Cs*-(1*Z*,6*E*)

[*C*1-(1*Z*,90)] 30.4

[*C*1-(1*Z*,90)] 31.5

[*C*1-(90,7*Z*)] 23.5

[*C*1-(90,6*Z*)] 21.6

*Cs*-(1*Z*,6*Z*) 1.7

energies (Δ*G*298, kJ/mol)

energies (Δ*G*298, kJ/mol)

*Cs*-(1*Z*,7*Z*) 3.5

0.0

*C*1-(1*E*,6*Z*)-*anti*

15.4

Fig. 20. The interconversion of conformations of 1,6-Ac2AN and their relative Gibbs free

*Cs*-(1*Z*,7*E*)

0.0

Fig. 21. The interconversion of conformations of 1,7-Ac2AN and their relative Gibbs free

The most interesting diacetylanthracene is 1,8-Ac2AN. *Peri*-interactions O15...H9 and O16...H9 tilt both carbonyl groups out of the aromatic plane, rendering a planar conformation such as *C*2*h*-1*Z*,5*Z*-Ac2AN energetically highly unfavorable. Ketone 1,8-Ac2AN adopts a *C*2-1*Z*,8*Zanti* conformation as its global minimum. It is overcrowded due to the short O15...H9 contact distance (12% penetration). The *C*2-1*Z*,8*Z*-*anti*-Ac2AN conformation corresponds to the *Z,Z* X-ray structure of 1,8-Ac2AN. Both structures feature twisted carbonyl groups; however, in the X-ray structure the twist angles are more pronounced (τ1(C9a–C1–C11–O15)=-32.4° and - 34.0°, θ=35.4° and 36.0°) than in the calculated structure (τ1(C9a–C1–C11–O15)=-17.3°, θ=19.3°). Although the conformational space of 1,8-Ac2AN resembles that of another α,αdiacetylanthracene, 1,5-Ac2AN, it is more complicated. There are three local minima conformations of 1,8-Ac2AN: *C*1-1*Z*,8*E*-Ac2AN, *Cs*-1*E*,8*E-syn*-Ac2AN and *C*2-1*E*,8*E*-*anti*-Ac2AN. Rotation of an acetyl group of *C*2-1*Z*,8*Z*-Ac2AN via [*C*1-1*Z*,90-Ac2AN] leads to the *C*1-1*Z*,8*E*-Ac2AN conformation, which is only 0.4 kJ/mol higher in energy. The tilting of the 8*E*-acetyl group (τ2(C8a–C8–C13–O16)=150.4°) allows the 1*Z*-acetyl group to align itself with the aromatic plane (τ1(C9a–C1–C11–O15)=1.5°), restoring the conjugation and thus stabilizing this conformation. The rotation of the 1*Z*-acetyl group of *C*1-1*Z*,8*E*-Ac2AN may be realized

*C*1-(1*E*,7*Z*)-*anti*

[*C*1-(1*E*,90)-*syn*]

44.6

[*C*1-(1*E*,90)-*anti*]

44.5

[*C*1-(1*E*,90)-*syn*]

45.9

[*C*1-(1*E*,90)-*anti*]

45.1

16.6 14.5

[*C*1-(90,7*E*)]

20.7

[*C*1-(90,6*E*)]

19.9

*C*1-(1*E*,6*E*)

13.6

*C*1-(1*E*,7*E*)

energy than *C*2*<sup>h</sup>*-1*Z*,5*Z*-Ac2AN, due to both acetyl groups being forced out of the aromatic plane: τ1(C9a–C1–C11–O15)=150.6° and 151.9°, respectively. In addition, the *Ci*-1*E*,5*E*-*anti*-Ac2AN and *C*2-1*E*,5*E*-*syn*-Ac2AN conformations may undergo *syn*,*anti*-diastereomerization via the [*C*1-1*E*,5*E*180-Ac2AN] transition state. It is a "nearly planar" transition state of a different type than the "nearly orthogonal" ones; the twist angle of the rotating acetyl group is close to zero, and the other acetyl group retains its *E*- or *Z*-conformation. The [*C*1-1*E*,5*E*180- Ac2AN] transition state is considerably strained due to the short O16...H10 distance (205.3 pm) and the distorted *sp*2 angles C13–C5–C10a (127.9°) and C13–C5–C6 (113.4°). The diastereomerization processes in 1,5-Ac2AN are shown in Fig. 19.

Fig. 19. The interconversion of conformations of 1,5-Ac2AN and their relative Gibbs free energies (Δ*G*298, kJ/mol)

Ketone 1,6-Ac2AN adopts a *Cs*-1*Z*,6*E* conformation as its global minimum. Like *C*2*h*-1*Z*,5*Z*-Ac2AN, it is overcrowded due to the short O15...H9 contact distance (14% penetration). The *Cs*-1*Z*,6*E*-Ac2AN conformation corresponds to the *Z,E* X-ray structure of 1,6-Ac2AN. As in the case of 1,5-Ac2AN, the DFT calculations predict a planar structure for 1,6-Ac2AN, while the X-ray geometry features the twisted 1*Z*-acetyl group: the twist angle τ1(C9a–C1–C11– O15)=30.0° and the dihedral angle θ=32.2°. The 6*E*-acetyl group remains in the aromatic plane in both calculated and the X-ray geometries. The rotation of the 1*Z*-acetyl group leads from *Cs*-1*Z*,6*E*-Ac2AN via [*C*1-90,6*E*-Ac2AN] to the local minimum *C*1-1*E*,6*E*-Ac2AN, which is 13.6 kJ/mol higher in energy. The 6*E*-acetyl group, in contrast to the 1*E*-acetyl group, lies in the aromatic plane: τ1(C9a–C1–C11–O15)=150.6° and τ2(C5–C6–C13–O16)=179.9°. The rotation of the 6*E*-acetyl group of *C*1-1*E*,6*E*-Ac2AN may be realized either via [*C*1-1*E*,90-*syn*-Ac2AN] or via [*C*1-1*E*,90-*anti*-Ac2AN] transition states; both pathways lead to *C*1-1*E*,6*Z-anti*-Ac2AN, which is 15.4 kJ/mol higher in energy than the global minimum. The rotation of the 6*E*acetyl group in *Cs*-1*Z*,6*E*-Ac2AN via [*C*1-1*Z*,90-Ac2AN] leads to the local minimum *C*s-1*Z*,6*Z*-Ac2AN, which is only 1.7 kJ/mol higher in energy than the global minimum. The rotation of the 1*E*-acetyl group in *C*1-1*E*,6*Z*-Ac2AN via [*C*1-90,6*Z*-Ac2AN] also leads to *C*s-1*Z*,6*Z*-Ac2AN. The diastereomerization processes in 1,6-Ac2AN are shown in Fig. 20.

Ketone 1,7-Ac2AN, similarly to 1,6-Ac2AN, adopts a *Cs*-1*Z*,7*E* conformation as its global minimum. It is overcrowded due to the short O15...H9 contact distance (15% penetration). The *Cs*-1*Z*,7*E*-Ac2AN conformation corresponds to the *Z,E* X-ray structure of 1,7-Ac2AN. The differences between the geometries of the planar DFT calculated structure of *Cs*-(1*Z*,7*E*)- Ac2AN and the twisted X-ray structure of 1,7-Ac2AN are smaller than in 1,5-Ac2AN and 1,6- Ac2AN. In the X-ray structure of 1,7-Ac2AN the twist angles are τ1(C9a–C1–C11–O15)=-15.2°

energy than *C*2*<sup>h</sup>*-1*Z*,5*Z*-Ac2AN, due to both acetyl groups being forced out of the aromatic plane: τ1(C9a–C1–C11–O15)=150.6° and 151.9°, respectively. In addition, the *Ci*-1*E*,5*E*-*anti*-Ac2AN and *C*2-1*E*,5*E*-*syn*-Ac2AN conformations may undergo *syn*,*anti*-diastereomerization via the [*C*1-1*E*,5*E*180-Ac2AN] transition state. It is a "nearly planar" transition state of a different type than the "nearly orthogonal" ones; the twist angle of the rotating acetyl group is close to zero, and the other acetyl group retains its *E*- or *Z*-conformation. The [*C*1-1*E*,5*E*180- Ac2AN] transition state is considerably strained due to the short O16...H10 distance (205.3 pm) and the distorted *sp*2 angles C13–C5–C10a (127.9°) and C13–C5–C6 (113.4°). The

*C*2*h*-(1*Z*,5*Z*) [*C*1-(1*Z*,90)] *C*1-(1*Z*,5*E*)

0.0 19.9 12.8

[*C*1-(90,5*E*)-*anti*]

33.8

[*C*1-(90,5*E*)-*syn*]

33.7

diastereomerization processes in 1,5-Ac2AN are shown in Fig. 19.

[*C*1-(1*E*,5E180)] 35.6

energies (Δ*G*298, kJ/mol)

*C*2-(1*E*,5*E*)-syn

28.0

*Ci*-(1*E*,5*E*)-anti

27.4

The diastereomerization processes in 1,6-Ac2AN are shown in Fig. 20.

Fig. 19. The interconversion of conformations of 1,5-Ac2AN and their relative Gibbs free

Ketone 1,6-Ac2AN adopts a *Cs*-1*Z*,6*E* conformation as its global minimum. Like *C*2*h*-1*Z*,5*Z*-Ac2AN, it is overcrowded due to the short O15...H9 contact distance (14% penetration). The *Cs*-1*Z*,6*E*-Ac2AN conformation corresponds to the *Z,E* X-ray structure of 1,6-Ac2AN. As in the case of 1,5-Ac2AN, the DFT calculations predict a planar structure for 1,6-Ac2AN, while the X-ray geometry features the twisted 1*Z*-acetyl group: the twist angle τ1(C9a–C1–C11– O15)=30.0° and the dihedral angle θ=32.2°. The 6*E*-acetyl group remains in the aromatic plane in both calculated and the X-ray geometries. The rotation of the 1*Z*-acetyl group leads from *Cs*-1*Z*,6*E*-Ac2AN via [*C*1-90,6*E*-Ac2AN] to the local minimum *C*1-1*E*,6*E*-Ac2AN, which is 13.6 kJ/mol higher in energy. The 6*E*-acetyl group, in contrast to the 1*E*-acetyl group, lies in the aromatic plane: τ1(C9a–C1–C11–O15)=150.6° and τ2(C5–C6–C13–O16)=179.9°. The rotation of the 6*E*-acetyl group of *C*1-1*E*,6*E*-Ac2AN may be realized either via [*C*1-1*E*,90-*syn*-Ac2AN] or via [*C*1-1*E*,90-*anti*-Ac2AN] transition states; both pathways lead to *C*1-1*E*,6*Z-anti*-Ac2AN, which is 15.4 kJ/mol higher in energy than the global minimum. The rotation of the 6*E*acetyl group in *Cs*-1*Z*,6*E*-Ac2AN via [*C*1-1*Z*,90-Ac2AN] leads to the local minimum *C*s-1*Z*,6*Z*-Ac2AN, which is only 1.7 kJ/mol higher in energy than the global minimum. The rotation of the 1*E*-acetyl group in *C*1-1*E*,6*Z*-Ac2AN via [*C*1-90,6*Z*-Ac2AN] also leads to *C*s-1*Z*,6*Z*-Ac2AN.

Ketone 1,7-Ac2AN, similarly to 1,6-Ac2AN, adopts a *Cs*-1*Z*,7*E* conformation as its global minimum. It is overcrowded due to the short O15...H9 contact distance (15% penetration). The *Cs*-1*Z*,7*E*-Ac2AN conformation corresponds to the *Z,E* X-ray structure of 1,7-Ac2AN. The differences between the geometries of the planar DFT calculated structure of *Cs*-(1*Z*,7*E*)- Ac2AN and the twisted X-ray structure of 1,7-Ac2AN are smaller than in 1,5-Ac2AN and 1,6- Ac2AN. In the X-ray structure of 1,7-Ac2AN the twist angles are τ1(C9a–C1–C11–O15)=-15.2° and τ2(C8–C7–C13–O16)=-176.6. The relative stabilities of the conformations of 1,7-Ac2AN and its conformational space are very similar to those of 1,6-Ac2AN**,** both being α,βdiacetylanthracenes. The local minima conformations *Cs*-1*Z*,7*E*-Ac2AN, *C*1-1*E*,7*E*-Ac2AN and *C*1-1*E*,7*Z-anti*-Ac2AN are higher in energy than the global minimum by 3.5, 14.5, and 16.6 kJ/mol, respectively. The diastereomerization processes in 1,7-Ac2AN are shown in Fig. 21.

Fig. 20. The interconversion of conformations of 1,6-Ac2AN and their relative Gibbs free energies (Δ*G*298, kJ/mol)

Fig. 21. The interconversion of conformations of 1,7-Ac2AN and their relative Gibbs free energies (Δ*G*298, kJ/mol)

The most interesting diacetylanthracene is 1,8-Ac2AN. *Peri*-interactions O15...H9 and O16...H9 tilt both carbonyl groups out of the aromatic plane, rendering a planar conformation such as *C*2*h*-1*Z*,5*Z*-Ac2AN energetically highly unfavorable. Ketone 1,8-Ac2AN adopts a *C*2-1*Z*,8*Zanti* conformation as its global minimum. It is overcrowded due to the short O15...H9 contact distance (12% penetration). The *C*2-1*Z*,8*Z*-*anti*-Ac2AN conformation corresponds to the *Z,Z* X-ray structure of 1,8-Ac2AN. Both structures feature twisted carbonyl groups; however, in the X-ray structure the twist angles are more pronounced (τ1(C9a–C1–C11–O15)=-32.4° and - 34.0°, θ=35.4° and 36.0°) than in the calculated structure (τ1(C9a–C1–C11–O15)=-17.3°, θ=19.3°). Although the conformational space of 1,8-Ac2AN resembles that of another α,αdiacetylanthracene, 1,5-Ac2AN, it is more complicated. There are three local minima conformations of 1,8-Ac2AN: *C*1-1*Z*,8*E*-Ac2AN, *Cs*-1*E*,8*E-syn*-Ac2AN and *C*2-1*E*,8*E*-*anti*-Ac2AN. Rotation of an acetyl group of *C*2-1*Z*,8*Z*-Ac2AN via [*C*1-1*Z*,90-Ac2AN] leads to the *C*1-1*Z*,8*E*-Ac2AN conformation, which is only 0.4 kJ/mol higher in energy. The tilting of the 8*E*-acetyl group (τ2(C8a–C8–C13–O16)=150.4°) allows the 1*Z*-acetyl group to align itself with the aromatic plane (τ1(C9a–C1–C11–O15)=1.5°), restoring the conjugation and thus stabilizing this conformation. The rotation of the 1*Z*-acetyl group of *C*1-1*Z*,8*E*-Ac2AN may be realized

Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes 31

*Cs*-(2*E*,6*Z*) 0.4

Fig. 24. The interconversion of conformations of 2,6-Ac2AN and their relative Gibbs free

plane. The diastereomerization processes in 9,10-Ac2AN are shown in Fig. 25.

1.0

energies (Δ*G*298, kJ/mol)

[*C*1-90-*sy n*]

*Cs*-*Z C*2-*Z*

[*C*1-(9*Z*,10*Z*0)]

0.1 2.1

*Ci C*2-*E* -*E*

Fig. 25. The interconversion of conformations of 9,10-Ac2AN and their relative Gibbs free

4.4 3.7

[*C*1-(9*E*,10*E*180)]

0.0

[*C*1-90-*anti*]

[*C*1-(90,6*Z*)] [*C*1-(2*E*,90)]

Ketone 9,10-Ac2AN stands out of the other diacetylanthracenes by virtue of its acetyl groups being each flanked by two *peri*-hydrogens. In order to avoid short non-contact distances to H1/H4/H5/H8, the acetyl groups in all the conformations of 9,10-Ac2AN are considerably twisted. Another mode for the relief of the steric strain in 9,10-Ac2AN is elongation of the C11–C9 and C12–C10 carbonyl bonds, 151.6 pm, as compared to 149.7 pm in planar *Cs*-(2*E*,7*Z*)- Ac2AN and 149.8 pm in *Cs*-(2*E*,6*E*)-Ac2AN. The global minimum of 9,10-Ac2AN is a *Ci*-*E* conformation, with the twist angles τ9(C9a–C9–C11–O15)=–72.6°, τ9(C10a–C10–C13–O16)=72.6° and the dihedral angle θ=74.7°. It corresponds well to the X-ray structure, which features even higher twist angles τ9=–85.0°, 87.0° and the dihedral angles θ=86.7°, 86.5°. The local minima conformations of 9,10-Ac2AN are *Cs*-*Z* (0.1 kJ/mol), *C*2-*E* (1.0 kJ/mol) and *C*2-*Z* (2.1 kJ/mol). They all have high twist angles, ±71.8°, 75.4° and –71.9°, respectively. The similarity of the energies and the geometries of the four conformations of 9,10-Ac2AN stems from the fact that in 9,10-Ac2AN, each of the *Z* and *E* conformations is defined relative to the other acetyl group, and not by the twist angles of the carbonyl groups relative to the anthracene system, which are very similar for all four conformations of 9,10-Ac2AN. The *Ci*-*E* global minimum undergoes diastereomerization to the *C*2-*E* conformation via [*C*1- (9*E*,10*E*180)] transition state, in which one of the carbonyl groups lies in the aromatic plane. The *Cs*-*Z* and *C*2-*Z* conformations interconvert via the analogous [*C*1-(9*Z*,10*Z*0)] transition state. The *Ci*-*E* conformation diastereomerizes to the *C*2-*Z* conformation and the *C*2-*E* conformation diastereomerizes to the *Cs*-*Z* conformation via the pair of transition states [*C*1- 90-*syn*] and [*C*1-90-*anti*], in which one of the carbonyl groups is orthogonal to the aromatic

*C*2*h*-(2*Z*,6*Z*) 3.8

energies (Δ*G*298, kJ/mol)

*C*2*h*-(2*E*,6*E*) 0.0

in either *syn*- (via [*C*1-90,8*E*-*syn*-Ac2AN]) or in *anti*-direction (via [*C*1-90,8*E*-*anti*-Ac2AN]) relative to the 8*E*-acetyl group. These pathways lead to the local minima *Cs*-1*E*,8*E*-*syn*-Ac2AN and *C*2-1*E*,8*E*-*anti*-Ac2AN conformations, respectively, which are 17.6 and 17.7 kJ/mol higher in energy than the global minimum. These two conformations undergo interconversion via the [*C*1-1*E*,8*E*180-Ac2AN] transition state. The diastereomerization processes in 1,8-Ac2AN are shown in Fig. 22.

Fig. 22. The interconversion of conformations of 1,8-Ac2AN and their relative Gibbs free energies (Δ*G*298, kJ/mol)

Ketone 2,7-Ac2AN adopts a *Cs*-2*E*,7*Z* conformation as its global minimum. It is not overcrowded, lacking *peri*-interactions. The *Cs*-(2*E*,7*Z*)-Ac2AN conformation corresponds well to the *E,Z* X-ray structure of 2,7-Ac2AN. The differences between the geometries of the planar DFT calculated structure of *Cs*-2*E*,7*Z*-Ac2AN and the twisted X-ray structure of 2,7- Ac2AN are not large: in the latter structure the twist angles are τ2(C1–C2–C11–O15)=171.9° and τ2(C8–C7–C13–O16)=0.9° (θ=9.8° and 1.6°, respectively). There are only two local minima conformations of 2,7-Ac2AN, both are planar like the global minimum. Due to the twist angles τ2 being either 0° or 180°, no *anti*-, *syn*-conformations are possible. The rotation of the 7*Z*-acetyl group in *Cs*-2*E*,7*Z*-Ac2AN via [*C*1-2*E*,90-Ac2AN] leads to *C*2*<sup>v</sup>*-2*E*,7*E*-Ac2AN conformation, which is only 0.2 kJ/mol higher in energy. The rotation of the 2*E*-acetyl group of *Cs*-2*E*,7*Z*-Ac2AN via [*C*1-90,7*Z*-Ac2AN] leads to the *C*2*<sup>v</sup>*-2*Z*,7*Z*-Ac2AN conformation, which is 4.3 kJ/mol higher in energy than the global minimum. The diastereomerization processes in 2,7-Ac2AN are shown in Fig. 23.

$$\begin{array}{c} \mathsf{C}\_{2\mathsf{V}} \text{(2\mathsf{Z},7\mathsf{Z})} \rightleftharpoons \begin{bmatrix} \mathsf{C}\_{1} \text{(90,7\mathsf{Z})} \end{bmatrix} \rightleftharpoons \begin{array}{c} \mathsf{C}\_{3} \text{(2\mathsf{E},7\mathsf{Z})} \rightleftharpoons \begin{bmatrix} \mathsf{C}\_{1} \text{(2\mathsf{E},90)} \end{bmatrix} \rightleftharpoons \begin{array}{c} \mathsf{C}\_{2\mathsf{V}} \text{(2\mathsf{E},7\mathsf{E})} \\ 0.2 \end{array} \end{array}$$

Fig. 23. The interconversion of conformations of 2,7-Ac2AN and their relative Gibbs free energies (Δ*G*298, kJ/mol)

The conformational space of 2,6-Ac2AN is similar to that of 2,7-Ac2AN. The global minimum is the *C*2*h*-2*E*,6*E*-Ac2AN conformation. Rotation of the 6*E*-acetyl group leads to *Cs*-2*E*,6*Z*-Ac2AN conformation, which is only 0.4 kJ/mol higher in energy than the global minimum. The rotation of the 2*E*-acetyl group in *Cs*-2*E*,6*Z*-Ac2AN leads to *C*2*<sup>h</sup>*-2*Z*,6*Z*-Ac2AN conformation, which is 3.8 kJ/mol higher in energy than the global minimum. The diastereomerization processes in 2,6-Ac2AN are shown in Fig. 24.

in either *syn*- (via [*C*1-90,8*E*-*syn*-Ac2AN]) or in *anti*-direction (via [*C*1-90,8*E*-*anti*-Ac2AN]) relative to the 8*E*-acetyl group. These pathways lead to the local minima *Cs*-1*E*,8*E*-*syn*-Ac2AN and *C*2-1*E*,8*E*-*anti*-Ac2AN conformations, respectively, which are 17.6 and 17.7 kJ/mol higher in energy than the global minimum. These two conformations undergo interconversion via the [*C*1-1*E*,8*E*180-Ac2AN] transition state. The diastereomerization

*C*2-(1*Z*,8*Z*)-*anti* [*C*1-(1*Z*,90)] *C*1-(1*Z*,8*E*)

0.0 0.4

9.8

[*C*1-(90,8*E*)-*anti*]

21.8

*C*2*v*-(2*E*,7*E*) 0.2

[*C*1-(90,8*E*)-*syn*]

23.8

processes in 1,8-Ac2AN are shown in Fig. 22.

processes in 2,7-Ac2AN are shown in Fig. 23.

31.6

diastereomerization processes in 2,6-Ac2AN are shown in Fig. 24.

27.1 [*C*1-(1*E*,8*E*180)]

energies (Δ*G*298, kJ/mol)

*C*2*v*-(2*Z*,7*Z*) 4.3

energies (Δ*G*298, kJ/mol)

*Cs*-(1*E*,8*E*)-syn

17.6

*C*2-(1*E*,8*E*)-anti

17.7

Fig. 22. The interconversion of conformations of 1,8-Ac2AN and their relative Gibbs free

Ketone 2,7-Ac2AN adopts a *Cs*-2*E*,7*Z* conformation as its global minimum. It is not overcrowded, lacking *peri*-interactions. The *Cs*-(2*E*,7*Z*)-Ac2AN conformation corresponds well to the *E,Z* X-ray structure of 2,7-Ac2AN. The differences between the geometries of the planar DFT calculated structure of *Cs*-2*E*,7*Z*-Ac2AN and the twisted X-ray structure of 2,7- Ac2AN are not large: in the latter structure the twist angles are τ2(C1–C2–C11–O15)=171.9° and τ2(C8–C7–C13–O16)=0.9° (θ=9.8° and 1.6°, respectively). There are only two local minima conformations of 2,7-Ac2AN, both are planar like the global minimum. Due to the twist angles τ2 being either 0° or 180°, no *anti*-, *syn*-conformations are possible. The rotation of the 7*Z*-acetyl group in *Cs*-2*E*,7*Z*-Ac2AN via [*C*1-2*E*,90-Ac2AN] leads to *C*2*<sup>v</sup>*-2*E*,7*E*-Ac2AN conformation, which is only 0.2 kJ/mol higher in energy. The rotation of the 2*E*-acetyl group of *Cs*-2*E*,7*Z*-Ac2AN via [*C*1-90,7*Z*-Ac2AN] leads to the *C*2*<sup>v</sup>*-2*Z*,7*Z*-Ac2AN conformation, which is 4.3 kJ/mol higher in energy than the global minimum. The diastereomerization

> *Cs*-(2*E*,7*Z*) 0.0

Fig. 23. The interconversion of conformations of 2,7-Ac2AN and their relative Gibbs free

[*C*1-(90,7*Z*)] [*C*1-(2*E*,90)]

The conformational space of 2,6-Ac2AN is similar to that of 2,7-Ac2AN. The global minimum is the *C*2*h*-2*E*,6*E*-Ac2AN conformation. Rotation of the 6*E*-acetyl group leads to *Cs*-2*E*,6*Z*-Ac2AN conformation, which is only 0.4 kJ/mol higher in energy than the global minimum. The rotation of the 2*E*-acetyl group in *Cs*-2*E*,6*Z*-Ac2AN leads to *C*2*<sup>h</sup>*-2*Z*,6*Z*-Ac2AN conformation, which is 3.8 kJ/mol higher in energy than the global minimum. The

$$\begin{array}{c} \mathsf{C\_{2\tilde{\mathsf{T}}}(2\mathsf{Z},6\mathsf{Z})} \xleftarrow{} [\mathsf{C\_{1}}(90,6\mathsf{Z})] \xleftarrow{} \mathsf{C\_{3}}(2\mathsf{E},6\mathsf{Z}) \xleftarrow{} [\mathsf{C\_{1}}(2\mathsf{E},90)] \xleftarrow{} \mathsf{C\_{2\tilde{\mathsf{N}}}(2\mathsf{E},6\mathsf{E})} \\\ \mathsf{3.8} \end{array}$$

Fig. 24. The interconversion of conformations of 2,6-Ac2AN and their relative Gibbs free energies (Δ*G*298, kJ/mol)

Ketone 9,10-Ac2AN stands out of the other diacetylanthracenes by virtue of its acetyl groups being each flanked by two *peri*-hydrogens. In order to avoid short non-contact distances to H1/H4/H5/H8, the acetyl groups in all the conformations of 9,10-Ac2AN are considerably twisted. Another mode for the relief of the steric strain in 9,10-Ac2AN is elongation of the C11–C9 and C12–C10 carbonyl bonds, 151.6 pm, as compared to 149.7 pm in planar *Cs*-(2*E*,7*Z*)- Ac2AN and 149.8 pm in *Cs*-(2*E*,6*E*)-Ac2AN. The global minimum of 9,10-Ac2AN is a *Ci*-*E* conformation, with the twist angles τ9(C9a–C9–C11–O15)=–72.6°, τ9(C10a–C10–C13–O16)=72.6° and the dihedral angle θ=74.7°. It corresponds well to the X-ray structure, which features even higher twist angles τ9=–85.0°, 87.0° and the dihedral angles θ=86.7°, 86.5°. The local minima conformations of 9,10-Ac2AN are *Cs*-*Z* (0.1 kJ/mol), *C*2-*E* (1.0 kJ/mol) and *C*2-*Z* (2.1 kJ/mol). They all have high twist angles, ±71.8°, 75.4° and –71.9°, respectively. The similarity of the energies and the geometries of the four conformations of 9,10-Ac2AN stems from the fact that in 9,10-Ac2AN, each of the *Z* and *E* conformations is defined relative to the other acetyl group, and not by the twist angles of the carbonyl groups relative to the anthracene system, which are very similar for all four conformations of 9,10-Ac2AN. The *Ci*-*E* global minimum undergoes diastereomerization to the *C*2-*E* conformation via [*C*1- (9*E*,10*E*180)] transition state, in which one of the carbonyl groups lies in the aromatic plane. The *Cs*-*Z* and *C*2-*Z* conformations interconvert via the analogous [*C*1-(9*Z*,10*Z*0)] transition state. The *Ci*-*E* conformation diastereomerizes to the *C*2-*Z* conformation and the *C*2-*E* conformation diastereomerizes to the *Cs*-*Z* conformation via the pair of transition states [*C*1- 90-*syn*] and [*C*1-90-*anti*], in which one of the carbonyl groups is orthogonal to the aromatic plane. The diastereomerization processes in 9,10-Ac2AN are shown in Fig. 25.

Fig. 25. The interconversion of conformations of 9,10-Ac2AN and their relative Gibbs free energies (Δ*G*298, kJ/mol)

Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes 33

1*Z*-acetyl group, the *C*1-1*Z*,10*E*-Ac2AN conformation may undergo diastereomerization to either *C*1-1*E*,10*E*-*anti*-Ac2AN or *C*1-1*E*,10*E*-*syn*-Ac2AN via the respective "nearly orthogonal" transition states. Analogously, *C*1-1*Z*,10*Z*-Ac2AN may undergo diastereomerization to either *C*1-1*E*,10*Z*-*anti*-Ac2AN or *C*1-1*E*,10*Z*-*syn*-Ac2AN. The *C*1-1*E*,10*E*-*anti*-Ac2AN and *C*1-1*E*,10*Zanti*-Ac2AN conformations are interconnected via the [*C*1-1*E*,90-*anti*-Ac2AN] transition state, while *C*1-1*E*,10*E*-*syn*-Ac2AN and *C*1-1*E*,10*Z*-*syn*-Ac2AN are interconnected via the [*C*1-1*E*,90 *syn*-Ac2AN] transition state. Finally, *C*1-1*E*,10*E*-*anti*-Ac2AN is interconnected with *C*1-1*E*,10*Esyn*-Ac2AN and *C*1-1*E*,10*Z*-*anti*-Ac2AN is interconnected with *C*1-1*E*,10*Z*-*syn*- Ac2AN, via the "nearly planar" transition states [*C*1-1*E*,10*E*180-Ac2AN] and [*C*1-1*E*,10*Z*0-Ac2AN],

respectively. The diastereomerization processes in 1,10-Ac2AN are shown in Fig. 26.

diastereomerization processes in 2,9-Ac2AN are shown in Fig. 27.

energies (Δ*G*298, kJ/mol)

processes in 2,10-Ac2AN are shown in Fig. 28.

the acetyl groups lead to an enhanced degree of overcrowding.

Ketone 2,9-Ac2AN (which has never been synthesized) adopts a *C*1-2*E*,9*E* conformation as its global minimum. The acetyl groups in 2,9-Ac2AN do not affect directly one another, and twist angles are similar to the respective twist angles in 2-AcAN and 9-AcAN: τ2(C1–C2–C11– O15)=–178.9° and τ9(C4a–C10–C13–O16)=–106.9°. There are two local minima conformations of 2,9-Ac2AN, *C*1-2*E*,9*Z*-Ac2AN (3.6 kJ/mol above the global minimum) and *C*1-2*Z*,9*E*-Ac2AN (5.1 kJ/mol). Surprisingly, the search after the *C*1-2*Z*,9*Z*-Ac2AN conformation was not successful. The acetyl groups in the putative *C*1-2*Z*,9*Z*-Ac2AN conformation are not expected to cause a steric hindrance more severe than in the *C*1-1*Z*,9*Z*-*anti*-Ac2AN conformation. Nevertheless, the latter conformation exists and even was found to be a global minimum, while the former does not seem to exist. The global minimum *C*1-2*E*,9*E*-Ac2AN conformation may diastereomerize either to the *C*1-2*E*,9*Z*-Ac2AN conformation via the [*C*1-2*E*,90-Ac2AN] transition state, or to the *C*1-2*Z*,9*E*-Ac2AN conformation via the [*C*1-90,9*E*-Ac2AN] transition state. The

*C*<sup>1</sup> *C* -(2*Z*,9*E*) 1-(2*E*,9*Z*) *C*<sup>1</sup> [*C* -(2*E*,9*E*) 1-(2*E*,90)] [*C*1-(90,9*E*)] 3.6 0.0 5.1

Ketone 2,10-Ac2AN (which has never been synthesized) adopts a *C*1-2*E*,10*E* conformation as its global minimum. The twist angles are τ2(C1–C2–C11–O15)=179.9° and τ9(C4a–C10–C13– O16)=–113.9°. The global minimum *C*1-2*E*,10*E*-Ac2AN conformation may diastereomerize either to the *C*1-2*Z*,10*E*-Ac2AN conformation (the relative energy of 2.1 kJ/mol) via the [*C*1- 90,10*E*-Ac2AN] transition state, or to the *C*1-2*E*,10*Z*-Ac2AN conformation (0.3 kJ/mol) via the [*C*1-2*E*,90-Ac2AN] transition state. Both these local minima configurations undergo diastereomerization to the *C*1-2*Z*,10*Z*-Ac2AN conformation (3.9 kJ/mol) via the [*C*1-2Z,90- Ac2AN] and [*C*1-90,10*Z*-Ac2AN] transition states, respectively. The diastereomerization

The comparison between the X-ray structures of mono- and diacetylanthracenes and their respective calculated geometries deserves a brief discussion. The absolute values of the twist angles of the B3LYP/6-31G(d) calculated conformations (including the local minima conformations) of mono- and diacetylanthracenes may be summarized as follows: |τ1|=0– 17.3°2 for the 1*Z*-acetyl groups, |τ1|=141.2–152.4° for the 1*E*-acetyl groups, |τ2|=0.0–1.8° for

2 The 1,9-Ac2AN is an outlier, having unusually high twist angle of the 1*Z*-acetyl group, |τ1|=51.9°, due to the steric strain caused by its interaction with the *peri* 9Z-acetyl group. The mutual *peri*-positions of

Fig. 27. The interconversion of conformations of 2,9-Ac2AN and their relative Gibbs free

Ketone 1,9-Ac2AN has never been isolated. Recently 1,9-Ac2AN has been claimed to be a putative intermediate in the Friedel–Crafts acyl rearrangements of 1,5-Ac2AN, 1,8-Ac2AN and 9,10-Ac2AN in PPA to give 3-methylbenz[de]anthracen-1-one [Mala'bi et al., 2011]. Ketone 1,9-Ac2AN adopts a *C*1-1*Z*,9*Z*-*anti* conformation as its global minimum. Both acetyl groups are considerably twisted because of their mutual *peri*-positions: τ1(C9a–C1–C11–O15)=– 50.9°, τ9(C9a–C9–C13–O16)=–59.6°. The local minimum conformation *C*1-1*E*,9*Z*-*syn*-Ac2AN is considerably higher in energy than the global minimum, 25.9 kJ/mol. Potentially, two more conformations may exist due to the twist angles τ1 and τ9 being different from 0° or 180°, i.e. *C*1-1*Z*,9*Z*-*syn*-Ac2AN and *C*1-1*E*,9*Z*-*anti*-Ac2AN. However, the search after these conformations has not resulted in any additional stationary points. The *C*1-1*Z*,9*E*-Ac2AN and *C*1-1*E*,9*E*-Ac2AN conformations have also not been found in the conformational space of 1,9-Ac2AN, probably due to the considerable steric strain caused by the *peri*-interactions between the methyl of the 9*E*-acetyl group and the 1-acetyl group.

Ketone 1,10-Ac2AN (which has never been synthesized [Mala'bi et al., 2011]) adopts a *C*1- 1*Z*,10*E* conformation as its global minimum. Contrary to 1,9-Ac2AN, its acetyl groups do not affect directly each other. Hence, their twist angles, τ1(C9a–C1–C11–O15)=0.2°, τ9(C4a–C10–C13– O16)=–108.0°, are very close to the twist angles of the lone acetyl groups in *Cs*-1*Z*-AcAN (0.0°) and *C*1-9-AcAN (–67.0°), respectively. Another consequence of the non-interacting acetyl groups in 1,10-Ac2AN is the abundance of conformations – six minima conformations have been identified. The local minimum *C*1-1*Z*,10*Z*-Ac2AN conformation is only 1.0 kJ/mol less stable than the global minimum, and differs from it in the twist angle τ9(C4a–C10–C13–O16)=– 65.9°. There are four 1*E* conformations of 1,10-Ac2AN, which have the twist angles τ1(C9a– C1–C11–O15) of 148–150° and the relative energy of 13.9–15.3 kJ/mol. The conformational behavior of 1,10-Ac2AN is complicated. Depending on the rotational direction of the

Fig. 26. The interconversion of conformations of 1,10-Ac2AN and their relative Gibbs free energies (Δ*G*298, kJ/mol)

Ketone 1,9-Ac2AN has never been isolated. Recently 1,9-Ac2AN has been claimed to be a putative intermediate in the Friedel–Crafts acyl rearrangements of 1,5-Ac2AN, 1,8-Ac2AN and 9,10-Ac2AN in PPA to give 3-methylbenz[de]anthracen-1-one [Mala'bi et al., 2011]. Ketone 1,9-Ac2AN adopts a *C*1-1*Z*,9*Z*-*anti* conformation as its global minimum. Both acetyl groups are considerably twisted because of their mutual *peri*-positions: τ1(C9a–C1–C11–O15)=– 50.9°, τ9(C9a–C9–C13–O16)=–59.6°. The local minimum conformation *C*1-1*E*,9*Z*-*syn*-Ac2AN is considerably higher in energy than the global minimum, 25.9 kJ/mol. Potentially, two more conformations may exist due to the twist angles τ1 and τ9 being different from 0° or 180°, i.e. *C*1-1*Z*,9*Z*-*syn*-Ac2AN and *C*1-1*E*,9*Z*-*anti*-Ac2AN. However, the search after these conformations has not resulted in any additional stationary points. The *C*1-1*Z*,9*E*-Ac2AN and *C*1-1*E*,9*E*-Ac2AN conformations have also not been found in the conformational space of 1,9-Ac2AN, probably due to the considerable steric strain caused by the *peri*-interactions

Ketone 1,10-Ac2AN (which has never been synthesized [Mala'bi et al., 2011]) adopts a *C*1- 1*Z*,10*E* conformation as its global minimum. Contrary to 1,9-Ac2AN, its acetyl groups do not affect directly each other. Hence, their twist angles, τ1(C9a–C1–C11–O15)=0.2°, τ9(C4a–C10–C13– O16)=–108.0°, are very close to the twist angles of the lone acetyl groups in *Cs*-1*Z*-AcAN (0.0°) and *C*1-9-AcAN (–67.0°), respectively. Another consequence of the non-interacting acetyl groups in 1,10-Ac2AN is the abundance of conformations – six minima conformations have been identified. The local minimum *C*1-1*Z*,10*Z*-Ac2AN conformation is only 1.0 kJ/mol less stable than the global minimum, and differs from it in the twist angle τ9(C4a–C10–C13–O16)=– 65.9°. There are four 1*E* conformations of 1,10-Ac2AN, which have the twist angles τ1(C9a– C1–C11–O15) of 148–150° and the relative energy of 13.9–15.3 kJ/mol. The conformational behavior of 1,10-Ac2AN is complicated. Depending on the rotational direction of the

*C*<sup>1</sup> *C*1-(1*E*,10*Z*)-*anti* -(1*E*,10*Z*)-*syn*

*C*1-(1*Z*,10*Z*)

1.0

[*C*<sup>1</sup> [*C* -(90,10*Z*)-*sy n*] 1-(90,10*Z*)-*anti*]

[*C*1-(1*E*,10*Z*0)]

15.3

*C*1-(1*E*,10*E*)-*anti C*1-(1*E*,10*E*)-*syn*

13.9 14.3

[*C*<sup>1</sup> [*C* -(90,10*E*)-*sy n*] 1-(90,10*E*)-*anti*]

*C*1-(1*Z*,10*E*)

0.0

Fig. 26. The interconversion of conformations of 1,10-Ac2AN and their relative Gibbs free

[*C*1-(1*E*,90)-*anti*] [*C*1-(1*E*,90)-*syn*]

[*C*1-(1*E*,10*E*180)]

between the methyl of the 9*E*-acetyl group and the 1-acetyl group.

13.9

energies (Δ*G*298, kJ/mol)

1*Z*-acetyl group, the *C*1-1*Z*,10*E*-Ac2AN conformation may undergo diastereomerization to either *C*1-1*E*,10*E*-*anti*-Ac2AN or *C*1-1*E*,10*E*-*syn*-Ac2AN via the respective "nearly orthogonal" transition states. Analogously, *C*1-1*Z*,10*Z*-Ac2AN may undergo diastereomerization to either *C*1-1*E*,10*Z*-*anti*-Ac2AN or *C*1-1*E*,10*Z*-*syn*-Ac2AN. The *C*1-1*E*,10*E*-*anti*-Ac2AN and *C*1-1*E*,10*Zanti*-Ac2AN conformations are interconnected via the [*C*1-1*E*,90-*anti*-Ac2AN] transition state, while *C*1-1*E*,10*E*-*syn*-Ac2AN and *C*1-1*E*,10*Z*-*syn*-Ac2AN are interconnected via the [*C*1-1*E*,90 *syn*-Ac2AN] transition state. Finally, *C*1-1*E*,10*E*-*anti*-Ac2AN is interconnected with *C*1-1*E*,10*Esyn*-Ac2AN and *C*1-1*E*,10*Z*-*anti*-Ac2AN is interconnected with *C*1-1*E*,10*Z*-*syn*- Ac2AN, via the "nearly planar" transition states [*C*1-1*E*,10*E*180-Ac2AN] and [*C*1-1*E*,10*Z*0-Ac2AN], respectively. The diastereomerization processes in 1,10-Ac2AN are shown in Fig. 26. Ketone 2,9-Ac2AN (which has never been synthesized) adopts a *C*1-2*E*,9*E* conformation as its global minimum. The acetyl groups in 2,9-Ac2AN do not affect directly one another, and twist angles are similar to the respective twist angles in 2-AcAN and 9-AcAN: τ2(C1–C2–C11– O15)=–178.9° and τ9(C4a–C10–C13–O16)=–106.9°. There are two local minima conformations of 2,9-Ac2AN, *C*1-2*E*,9*Z*-Ac2AN (3.6 kJ/mol above the global minimum) and *C*1-2*Z*,9*E*-Ac2AN (5.1 kJ/mol). Surprisingly, the search after the *C*1-2*Z*,9*Z*-Ac2AN conformation was not successful. The acetyl groups in the putative *C*1-2*Z*,9*Z*-Ac2AN conformation are not expected to cause a steric hindrance more severe than in the *C*1-1*Z*,9*Z*-*anti*-Ac2AN conformation. Nevertheless, the latter conformation exists and even was found to be a global minimum, while the former does not seem to exist. The global minimum *C*1-2*E*,9*E*-Ac2AN conformation may diastereomerize either to the *C*1-2*E*,9*Z*-Ac2AN conformation via the [*C*1-2*E*,90-Ac2AN] transition state, or to the *C*1-2*Z*,9*E*-Ac2AN conformation via the [*C*1-90,9*E*-Ac2AN] transition state. The diastereomerization processes in 2,9-Ac2AN are shown in Fig. 27.

$$\begin{array}{c} \mathsf{C\_{1}\cdot(2\mathsf{E},9\mathsf{Z})} \xleftarrow{} [\mathsf{C\_{1}\cdot(2\mathsf{E},90)]} \xleftarrow{} \mathsf{C\_{1}\cdot(2\mathsf{E},9\mathsf{E})} \xleftarrow{} [\mathsf{C\_{1}\cdot(90,9\mathsf{E})]} \xleftarrow{} \mathsf{C\_{1}\cdot(2\mathsf{Z},9\mathsf{E})} \\\ \mathsf{3.6} \end{array}$$

Fig. 27. The interconversion of conformations of 2,9-Ac2AN and their relative Gibbs free energies (Δ*G*298, kJ/mol)

Ketone 2,10-Ac2AN (which has never been synthesized) adopts a *C*1-2*E*,10*E* conformation as its global minimum. The twist angles are τ2(C1–C2–C11–O15)=179.9° and τ9(C4a–C10–C13– O16)=–113.9°. The global minimum *C*1-2*E*,10*E*-Ac2AN conformation may diastereomerize either to the *C*1-2*Z*,10*E*-Ac2AN conformation (the relative energy of 2.1 kJ/mol) via the [*C*1- 90,10*E*-Ac2AN] transition state, or to the *C*1-2*E*,10*Z*-Ac2AN conformation (0.3 kJ/mol) via the [*C*1-2*E*,90-Ac2AN] transition state. Both these local minima configurations undergo diastereomerization to the *C*1-2*Z*,10*Z*-Ac2AN conformation (3.9 kJ/mol) via the [*C*1-2Z,90- Ac2AN] and [*C*1-90,10*Z*-Ac2AN] transition states, respectively. The diastereomerization processes in 2,10-Ac2AN are shown in Fig. 28.

The comparison between the X-ray structures of mono- and diacetylanthracenes and their respective calculated geometries deserves a brief discussion. The absolute values of the twist angles of the B3LYP/6-31G(d) calculated conformations (including the local minima conformations) of mono- and diacetylanthracenes may be summarized as follows: |τ1|=0– 17.3°2 for the 1*Z*-acetyl groups, |τ1|=141.2–152.4° for the 1*E*-acetyl groups, |τ2|=0.0–1.8° for

<sup>2</sup> The 1,9-Ac2AN is an outlier, having unusually high twist angle of the 1*Z*-acetyl group, |τ1|=51.9°, due to the steric strain caused by its interaction with the *peri* 9Z-acetyl group. The mutual *peri*-positions of the acetyl groups lead to an enhanced degree of overcrowding.

Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes 35

The order of stabilities of the global minima of the diacetylanthracenes is 2,7-Ac2AN≈2,6- Ac2AN>1,7-Ac2AN≈1,6-Ac2AN>1,5-Ac2AN>2,9-Ac2AN>2,10-Ac2AN>1,8-Ac2AN>1,10- Ac2AN>>1,9-Ac2AN>9,10-Ac2AN. The acetyl groups at positions 1, 5 and 8 destabilize the diacetylanthracenes, while acetyl groups at positions 9 and 10 cause even greater destabilization. The destabilization of the 1, 5, 8, 9 and 10-substituted diacetylanthracenes relative to their 2, 6 and 7-substituted constitutional isomers stems from the overcrowding due to repulsive non-bonding interactions between the carbonyl oxygen/methyl group and the aromatic hydrogens in *peri*-positions, and from the decreased resonance stabilization between the carbonyl and the aromatic system. Thus, the acetyl groups in 9,10-Ac2AN, 1,9- Ac2AN, 1,10-Ac2AN and 1,8-Ac2AN, being considerably tilted out of the aromatic plane, reduce the relative stabilities of these diacetylanthracenes, potentially allowing deacylation, transacylation and acyl rearrangements. By contrast, 2,7-Ac2AN and 2,6-Ac2AN are not

As noted above, monoacetylanthracenes and diacetylanthracenes may undergo *E*,*Z*diastereomerizations and *syn*,*anti*-diastereomerizations by rotation of their acetyl groups. The diastereomerization of the first type connects an *E*-diastereomer with a *Z*-diastereomer and proceeds via a "nearly orthogonal" transition state, in which the acetyl group, rotating around the Carom–Ccarb bond, has the twist angle of τ=85–97° (the other acetyl group in diacetylanthracenes retains its *E*- or *Z*-orientation). The diastereomerization of the second type occurs only in diacetylanthracenes and connects either an *E*-*syn*-diastereomer with an *E*-*anti*-diastereomer, or a *Z*-*syn*-diastereomer with a *Z*-*anti*-diastereomer. It proceeds via a "nearly planar" transition states, in which the twist angle of the rotating acetyl group is close to either 180° (*E*-diastereomer) or zero (*Z*-diastereomer), and the other acetyl group retains its *E*- or *Z*-orientation. Fig. 29 and Fig. 30 show the *E*,*Z*-diastereomerization and

Table 7 gives the energy barriers for the *E*,*Z*-diastereomerization and *syn*,*anti*diastereomerization in the monoacetylanthracenes and diacetylanthracenes under study by rotation of the acetyl groups via the respective nearly orthogonal or nearly planar transition states. The E,Z-diastereomerization energy barriers may be divided into three groups, depending on the position of the acetyl group that undergoes rotation and on its conformation. *E*-Acetyl groups at positions 1, 5 and 8 and acetyl groups at positions 9 and 10 have the lowest energy barriers, 3.6–9.5 kJ/mol, due to their already significant twist angles (τ=141–152° for *E*-acetyl groups at positions 1, 5 and 8 and τ=67–73° for acetyl groups at positions 9 and 10). *Z*-Acetyl groups at the same positions 1, 5 and 8 have medium energy barriers, 19.5–23.5 kJ/mol. The twist angles of these acetyl groups are smaller (τ=0–17°), but the *E*,*Z*-diastereomerization in this case is facilitated by the steric strain due to repulsive *peri*-interactions between the carbonyl oxygen and aromatic H9/H10 hydrogens. Finally, both *E*- and *Z*-acetyl groups at positions 2, 6 and 7 have the highest energy barriers for diastereomerization, 27.3–31.6 kJ/mol, due to the lack of steric strain and negligible twist angles (less than 1°). For comparison, the experimental rotational energy barrier for methyl 1-(2-methylnaphthyl) ketone is 33.9 kJ/mol (–110 °C, [Wolf, 2008]). Table 8 gives the relative Gibbs free energies of the global minima and the most stable local minima of the acetylanthracenes whose crystal structures have been determined in this study or reported in the literature, i.e. 1-AcAN, 2-AcAN, 9-AcAN, 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN, 1,8- Ac2AN, 2,7-Ac2AN and 9,10-Ac2AN, as well the energy barriers for their *E*,*Z*-

expected to undergo the Friedel–Crafts acyl rearrangements.

*syn*,*anti*-diastereomerization on the example of 1,8-Ac2AN.

**2.3.2 Activation barriers** 

Fig. 28. The interconversion of conformations of 2,10-Ac2AN and their relative Gibbs free energies (Δ*G*298, kJ/mol)

the 2Z-acetyl groups, |τ2|=178.9–180.0° for the 2*E*-acetyl groups, and |τ9|=44.8–75.4° (180– |τ9| values were taken for |τ9|>90°). The respective twist angles derived from the X-ray structures are |τ1|=15.2–34.0° for the 1*Z*-acetyl groups, |τ2|=0.9° for the 2*Z*-acetyl group, |τ2|=177.9–178.6° for the 2*E*-acetyl groups, and |τ9|=85.0–87.9°. There is no X-ray structure of acetylanthracenes featuring a 1*E*-acetyl group, and such conformations are always found to be local minima by the DFT calculations. The B3LYP/6-31G(d) calculations seem to underestimate the twist angles of the 1*Z*- and 9-acetyl groups in mono- and diacetylanthracenes. In a number of cases it leads to predicting planar and more overcrowded conformations than the respective twisted X-ray geometries. There is a limited number of reports in the literature that DFT methods, including B3LYP, could overstabilize planar conformations of biphenyl and related heteroaromatic compounds [Viruela et al., 1997; Karpfen et al., 1997]. As in the X-ray structures, the acetyl groups and the anthracene systems in the mono- and diacetylanthracenes under study are essentially planar. Thus, B3LYP/6-31G(d) satisfactorily predicts the overall conformations of mono- and diacetylanthracenes under study, i.e. the *Z*-conformation of 1-AcAN, the *E*-conformation of 2-AcAN, the twisted conformation of 9-AcAN, the *Z*,*Z* conformations of 1,5-Ac2AN and 1,8- Ac2AN, the *Z*,*E* conformations of 1,6-Ac2AN, 1,7-Ac2AN and 2,7-Ac2AN, and the *E*,*E* conformation of 9,10-Ac2AN. It has not escaped our mind, however, that the packing interactions in crystals can readily dominate and suppress any preference for one conformation or another, especially in the cases of low diastereomerization barriers and low energy differences. We also note the limitations of the DFT calculations in the gas phase and in the comparison of the computational results with the crystal structures.

The relative free Gibbs energies of the diacetylanthracenes under study are given in Table 6. It should be noted that 1,5-Ac2AN is 11.2 kJ/mol more stable than its constitutional isomer 1,8-Ac2AN. The acetyl groups of 1,5-Ac2AN are attached to a starred and an unstarred aromatic carbons of alternant anthracene, while the acetyl groups of 1,8-Ac2AN are both attached to starred aromatic carbons. Simple resonance considerations would favor the stabilization of 1,8-Ac2AN over 1,5-Ac2AN, due to the better delocalization of the partial positive charge in the dipolar Kekulé structures. However, the twist angle of the acetyl groups in 1,8-Ac2AN are notably larger than that in 1,5-Ac2AN, in both the crystal structures (32.4°/34° vs. 20.0°) and the DFT calculated geometries (17.3° vs. 0.0°). This increased twist angle decreases the conjugation between the acetyl group and the aromatic system, thus destabilizing 1,8-Ac2AN relative to 1,5-Ac2AN.

The order of stabilities of the global minima of the diacetylanthracenes is 2,7-Ac2AN≈2,6- Ac2AN>1,7-Ac2AN≈1,6-Ac2AN>1,5-Ac2AN>2,9-Ac2AN>2,10-Ac2AN>1,8-Ac2AN>1,10- Ac2AN>>1,9-Ac2AN>9,10-Ac2AN. The acetyl groups at positions 1, 5 and 8 destabilize the diacetylanthracenes, while acetyl groups at positions 9 and 10 cause even greater destabilization. The destabilization of the 1, 5, 8, 9 and 10-substituted diacetylanthracenes relative to their 2, 6 and 7-substituted constitutional isomers stems from the overcrowding due to repulsive non-bonding interactions between the carbonyl oxygen/methyl group and the aromatic hydrogens in *peri*-positions, and from the decreased resonance stabilization between the carbonyl and the aromatic system. Thus, the acetyl groups in 9,10-Ac2AN, 1,9- Ac2AN, 1,10-Ac2AN and 1,8-Ac2AN, being considerably tilted out of the aromatic plane, reduce the relative stabilities of these diacetylanthracenes, potentially allowing deacylation, transacylation and acyl rearrangements. By contrast, 2,7-Ac2AN and 2,6-Ac2AN are not expected to undergo the Friedel–Crafts acyl rearrangements.

#### **2.3.2 Activation barriers**

34 Current Trends in X-Ray Crystallography

*C*<sup>1</sup> *C* -(2*Z*,10*E*) 1-(2*Z*,10*Z*)

[*C*1-(2*Z*,90)]

3.9 2.1

[*C*1-(90,10*Z*)] [*C*1-(90,10*E*)]

*C*1-(2*E*,10*Z*) *C*1-(2*E*,10*E*)

0.3 0.0

the 2Z-acetyl groups, |τ2|=178.9–180.0° for the 2*E*-acetyl groups, and |τ9|=44.8–75.4° (180– |τ9| values were taken for |τ9|>90°). The respective twist angles derived from the X-ray structures are |τ1|=15.2–34.0° for the 1*Z*-acetyl groups, |τ2|=0.9° for the 2*Z*-acetyl group, |τ2|=177.9–178.6° for the 2*E*-acetyl groups, and |τ9|=85.0–87.9°. There is no X-ray structure of acetylanthracenes featuring a 1*E*-acetyl group, and such conformations are always found to be local minima by the DFT calculations. The B3LYP/6-31G(d) calculations seem to underestimate the twist angles of the 1*Z*- and 9-acetyl groups in mono- and diacetylanthracenes. In a number of cases it leads to predicting planar and more overcrowded conformations than the respective twisted X-ray geometries. There is a limited number of reports in the literature that DFT methods, including B3LYP, could overstabilize planar conformations of biphenyl and related heteroaromatic compounds [Viruela et al., 1997; Karpfen et al., 1997]. As in the X-ray structures, the acetyl groups and the anthracene systems in the mono- and diacetylanthracenes under study are essentially planar. Thus, B3LYP/6-31G(d) satisfactorily predicts the overall conformations of mono- and diacetylanthracenes under study, i.e. the *Z*-conformation of 1-AcAN, the *E*-conformation of 2-AcAN, the twisted conformation of 9-AcAN, the *Z*,*Z* conformations of 1,5-Ac2AN and 1,8- Ac2AN, the *Z*,*E* conformations of 1,6-Ac2AN, 1,7-Ac2AN and 2,7-Ac2AN, and the *E*,*E* conformation of 9,10-Ac2AN. It has not escaped our mind, however, that the packing interactions in crystals can readily dominate and suppress any preference for one conformation or another, especially in the cases of low diastereomerization barriers and low energy differences. We also note the limitations of the DFT calculations in the gas phase and

Fig. 28. The interconversion of conformations of 2,10-Ac2AN and their relative Gibbs free

in the comparison of the computational results with the crystal structures.

system, thus destabilizing 1,8-Ac2AN relative to 1,5-Ac2AN.

The relative free Gibbs energies of the diacetylanthracenes under study are given in Table 6. It should be noted that 1,5-Ac2AN is 11.2 kJ/mol more stable than its constitutional isomer 1,8-Ac2AN. The acetyl groups of 1,5-Ac2AN are attached to a starred and an unstarred aromatic carbons of alternant anthracene, while the acetyl groups of 1,8-Ac2AN are both attached to starred aromatic carbons. Simple resonance considerations would favor the stabilization of 1,8-Ac2AN over 1,5-Ac2AN, due to the better delocalization of the partial positive charge in the dipolar Kekulé structures. However, the twist angle of the acetyl groups in 1,8-Ac2AN are notably larger than that in 1,5-Ac2AN, in both the crystal structures (32.4°/34° vs. 20.0°) and the DFT calculated geometries (17.3° vs. 0.0°). This increased twist angle decreases the conjugation between the acetyl group and the aromatic

energies (Δ*G*298, kJ/mol)

[*C*1-(2*E*,90)]

As noted above, monoacetylanthracenes and diacetylanthracenes may undergo *E*,*Z*diastereomerizations and *syn*,*anti*-diastereomerizations by rotation of their acetyl groups. The diastereomerization of the first type connects an *E*-diastereomer with a *Z*-diastereomer and proceeds via a "nearly orthogonal" transition state, in which the acetyl group, rotating around the Carom–Ccarb bond, has the twist angle of τ=85–97° (the other acetyl group in diacetylanthracenes retains its *E*- or *Z*-orientation). The diastereomerization of the second type occurs only in diacetylanthracenes and connects either an *E*-*syn*-diastereomer with an *E*-*anti*-diastereomer, or a *Z*-*syn*-diastereomer with a *Z*-*anti*-diastereomer. It proceeds via a "nearly planar" transition states, in which the twist angle of the rotating acetyl group is close to either 180° (*E*-diastereomer) or zero (*Z*-diastereomer), and the other acetyl group retains its *E*- or *Z*-orientation. Fig. 29 and Fig. 30 show the *E*,*Z*-diastereomerization and *syn*,*anti*-diastereomerization on the example of 1,8-Ac2AN.

Table 7 gives the energy barriers for the *E*,*Z*-diastereomerization and *syn*,*anti*diastereomerization in the monoacetylanthracenes and diacetylanthracenes under study by rotation of the acetyl groups via the respective nearly orthogonal or nearly planar transition states. The E,Z-diastereomerization energy barriers may be divided into three groups, depending on the position of the acetyl group that undergoes rotation and on its conformation. *E*-Acetyl groups at positions 1, 5 and 8 and acetyl groups at positions 9 and 10 have the lowest energy barriers, 3.6–9.5 kJ/mol, due to their already significant twist angles (τ=141–152° for *E*-acetyl groups at positions 1, 5 and 8 and τ=67–73° for acetyl groups at positions 9 and 10). *Z*-Acetyl groups at the same positions 1, 5 and 8 have medium energy barriers, 19.5–23.5 kJ/mol. The twist angles of these acetyl groups are smaller (τ=0–17°), but the *E*,*Z*-diastereomerization in this case is facilitated by the steric strain due to repulsive *peri*-interactions between the carbonyl oxygen and aromatic H9/H10 hydrogens. Finally, both *E*- and *Z*-acetyl groups at positions 2, 6 and 7 have the highest energy barriers for diastereomerization, 27.3–31.6 kJ/mol, due to the lack of steric strain and negligible twist angles (less than 1°). For comparison, the experimental rotational energy barrier for methyl 1-(2-methylnaphthyl) ketone is 33.9 kJ/mol (–110 °C, [Wolf, 2008]). Table 8 gives the relative Gibbs free energies of the global minima and the most stable local minima of the acetylanthracenes whose crystal structures have been determined in this study or reported in the literature, i.e. 1-AcAN, 2-AcAN, 9-AcAN, 1,5-Ac2AN, 1,6-Ac2AN, 1,7-Ac2AN, 1,8- Ac2AN, 2,7-Ac2AN and 9,10-Ac2AN, as well the energy barriers for their *E*,*Z*-

Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes 37

*E*-AcAN→1*Z*-AcAN 6.51

*E*-AcAN→2*Z*-AcAN 31.52

*Z*,5*E*-Ac2AN→1*Z*,5*Z*-Ac2AN 7.14

*E*,5*E*-*anti*-Ac2AN→1*E*,5*E*-*syn*-Ac2AN 8.22

*E*,5*E*-syn-Ac2AN→1*Z*,5*E*-Ac2AN 5.67

*E*,5*E*-*anti*-Ac2AN→1*Z*,5*E*-Ac2AN 6.44

*Z*,6*E*-Ac2AN→1*Z*,6*Z*-Ac2AN 30.35

*E*,6*Z*-Ac2AN→1*Z*,6*Z*-Ac2AN 6.13

*E*,6*E*-Ac2AN→1*Z*,6*E*-Ac2AN 6.29

*E*,6*E*-Ac2AN→1*E*,6*Z*-Ac2AN 30.88

*E*,6*E*-Ac2AN→1*E*,6*Z*-Ac2AN 31.01

*Z*,7*E*-Ac2AN→1*Z*,7*Z*-Ac2AN 31.54

*E*,7*Z*-Ac2AN→1*Z*,7*Z*-Ac2AN 6.83

*E*,7*E*-Ac2AN→1*Z*,7*E*-Ac2AN 6.26

*E*,7*E*-Ac2AN→1*E*,7*Z*-Ac2AN 30.69

*E*,7*E*-Ac2AN→1*E*,7*Z*-Ac2AN 31.45

*Z*,8*E*-Ac2AN→1*Z*,8*Z*-*anti*-Ac2AN 9.46

*E*,*Z* diastereomerization Δ*G*‡ Transition state *E*Tot or *syn*,*anti*-diastereomerization kJ/mol Hartree *Z*-AcAN*→*1*E*-AcAN 19.52 [1-AcAN] –692.16527547

*Z*-AcAN*→*2*E*-AcAN 29.28 [2-AcAN] –692.16644237

9-AcAN→9-AcAN\* 3.64a [9-AcAN] –692.16355087 *Z*,5*Z*-Ac2AN→1*Z*,5*E*-Ac2AN 19.93 [1*Z*,90-Ac2AN] –844.80776475

*E*,5*E*-*syn*-Ac2AN→1*E*,5*E*-*anti*-Ac2AN 7.54 [1*E*,5*E*180-Ac2AN] –844.80415197

*Z*,5*E*-Ac2AN→1*E*,5*E*-*syn*-Ac2AN 20.93 [90,5*E*-*syn*-Ac2AN] –844.80261144

*Z*,5*E*-Ac2AN→1*E*,5*E*-*anti*-Ac2AN 21.01 [90,5*E*-*anti*-Ac2AN] –844.80259178

*Z*,6*Z*-Ac2AN→1*Z*,6*E*-Ac2AN 28.66 [1*Z*,90-Ac2AN] –844.80884650

*Z*,6*Z*-Ac2AN→1*E*,6*Z*-Ac2AN 19.86 [90,6*Z*-Ac2AN] –844.81218832

*Z*,6*E*-Ac2AN→1*E*,6*E*-Ac2AN 19.86 [90,6*E*-Ac2AN] –844.81289093

*E*,6*Z*-Ac2AN→1*E*,6*E*-Ac2AN 29.02 [1*E*,90-*anti*-Ac2AN] –844.80381925

*E*,6*Z*-Ac2AN→1*E*,6*E*-Ac2AN 29.16 [1*E*,90-*syn*-Ac2AN] –844.80371345

*Z*,7*Z*-Ac2AN→1*Z*,7*E*-Ac2AN 28.05 [1*Z*,90-Ac2AN] –844.80879074

*Z*,7*Z*-Ac2AN→1*E*,7*Z*-Ac2AN 19.95 [90,7*Z*-Ac2AN] –844.81202172

*Z*,7*E*-Ac2AN→1*E*,7*E*-Ac2AN 20.72 [90,7*E*-Ac2AN] –844.81303091

*E*,7*Z*-Ac2AN→1*E*,7*E*-Ac2AN 28.53 [1*E*,90-*anti*-Ac2AN] –844.80384032

*E*,7*Z*-Ac2AN→1*E*,7*E*-Ac2AN 29.29 [1*E*,90-*syn*-Ac2AN] –844.80370612

*Z*,8*Z*-*anti*-Ac2AN→1*Z*,8*E*-Ac2AN 9.81 [1*Z*,90-Ac2AN] –844.80732674

Fig. 29. *E*,*Z*-Diastereomerization of *C*2-1*Z*,8*Z*-Ac2AN to *C*2-1*Z*,8*E*-Ac2AN via [1*Z*,90-Ac2AN] transition state

Fig. 30. *syn*,*anti*-Diastereomerization of *C*2-1*E*,8*E*-*anti*-Ac2AN to *Cs*-1*E*,8*E*-*syn*-Ac2AN via [1*E*,8*E*180-Ac2AN] transition state

diastereomerizations. The energy barriers are in the range of 20–32 kJ/mol (relative to the respective global minima) for the rotation of the acetyl groups at 1, 2, 5, 6 and 7 positions. The lower energy barrier in 1,8-Ac2AN (9.8) may be rationalized by destabilization of the global minimum due to the larger twist of the acetyl groups. This effect is even more pronounced in the case of 9-AcAN and 9,10-Ac2AN, which have large twist values (67° and 73°, respectively) and remarkably low *E*,*Z*-diastereomerization barriers (less than 4 kJ/mol). All these barriers are sufficiently low to allow a swift *E*,*Z*-diastereomerizations on the NMR time scale (at room temperature), in accordance with the results of the NMR experiments (*vide supra*). The differences in the relative energies of the global minimum and the most stable local minimum of these acetylanthracenes are relatively small, 0.06-3.5 kJ/mol (with the exception of 1-AcAN and 1,5-Ac2AN), which suggests the presence of both *E*- and *Z*diastereomers in equilibrium mixture at ambient temperature.

Fig. 29. *E*,*Z*-Diastereomerization of *C*2-1*Z*,8*Z*-Ac2AN to *C*2-1*Z*,8*E*-Ac2AN via [1*Z*,90-Ac2AN]

Fig. 30. *syn*,*anti*-Diastereomerization of *C*2-1*E*,8*E*-*anti*-Ac2AN to *Cs*-1*E*,8*E*-*syn*-Ac2AN via

diastereomers in equilibrium mixture at ambient temperature.

diastereomerizations. The energy barriers are in the range of 20–32 kJ/mol (relative to the respective global minima) for the rotation of the acetyl groups at 1, 2, 5, 6 and 7 positions. The lower energy barrier in 1,8-Ac2AN (9.8) may be rationalized by destabilization of the global minimum due to the larger twist of the acetyl groups. This effect is even more pronounced in the case of 9-AcAN and 9,10-Ac2AN, which have large twist values (67° and 73°, respectively) and remarkably low *E*,*Z*-diastereomerization barriers (less than 4 kJ/mol). All these barriers are sufficiently low to allow a swift *E*,*Z*-diastereomerizations on the NMR time scale (at room temperature), in accordance with the results of the NMR experiments (*vide supra*). The differences in the relative energies of the global minimum and the most stable local minimum of these acetylanthracenes are relatively small, 0.06-3.5 kJ/mol (with the exception of 1-AcAN and 1,5-Ac2AN), which suggests the presence of both *E*- and *Z*-

transition state

[1*E*,8*E*180-Ac2AN] transition state


Polycyclic Aromatic Ketones – A Crystallographic and Theoretical Study of Acetyl Anthracenes 39

The monoacetylanthracenes and diacetylanthracenes under study adopt non-planar conformations in their crystal structures. The twist angles are maximal for the 9-acetyl groups (|τ9|=85.0–87.9°) and significant for the 1Z-acetyl groups (|τ1|=15.2–34.0°), but very small for 2-acetyl groups. The conformations in solution are in agreement with the X-ray crystal structure conformations, according to the NMR data. The crystal structures are stabilized by intermolecular interactions: aromatic–aromatic π–π interactions (1,6-Ac2AN and 1,7-Ac2AN), C...H-π interactions (2-AcAN, 1,5-Ac2AN, 2,7-Ac2AN and 9,10-Ac2AN), or π–π interactions between the anthracene unit and the carbonyl bond (1,8-Ac2AN). The B3LYP/6-31G(d) calculated conformations of the monoacetylanthracenes and diacetylanthracenes are in good agreement with the X-ray crystal structures. The acetyl groups in the crystal structures and the B3LYP/6-31G(d) calculated global minima of the monoacetylanthracenes and diacetylanthracenes preferentially adopts 1*Z* and 2*E* conformations. The order of stabilities of the diacetylanthracenes under study is 2,7- Ac2AN>1,7-Ac2AN≈1,6-Ac2AN>1,5-Ac2AN>1,8-Ac2AN>9,10-Ac2AN. The acetyl groups at positions 1, 5 and 8 destabilize the diacetylanthracenes because of the repulsive interactions between the carbonyl oxygen/methyl group and the aromatic *peri*-hydrogens, and because of the decreased resonance stabilization. This effect is even more pronounced for the acetyl groups at positions 9 and 10. The B3LYP/6-31G(d) calculated energy barriers for the E,Zdiastereomerizations show that the *E*,*Z*-diastereomerizations is swift on the NMR time scale (at room temperature), in accordance with the results of the NMR experiments. The present results of the crystallographic and theoretical study of monoacetylanthracenes and diacetylanthracenes contribute to our understanding of the motifs of reversibility and thermodynamic control in the Friedel–Crafts acyl rearrangements of these representative

Table 9 summarizes the applied methods of preparation of the monoacetylanthracenes and diacetylanthracenes. Melting points are uncorrected. All NMR spectra were recorded with Bruker DRX 500 MHz spectrometer. 1H-NMR spectra were recorded at 500.13 MHz using CDCl3 as solvent and as internal standard, δ(CDCl3)=7.263 ppm. 13C-NMR spectra were recorded at 125.75 MHz using CDCl3 as a solvent with internal standard, δ(CDCl3)=77.008 ppm. Complete assignments were made through 2-dimensional correlation spectroscopy (COSY, HSQC, HBMC and NOESY). Anthracene and nitrobenzene were obtained from Sigma-Aldrich; acetyl chloride and aluminum chloride were obtained from Acros. All the solvents were AR grade. Chloroform and dichloromethane were distilled before use. Single crystal X-ray diffraction was carried out on a Bruker SMART APEX CCD X-ray diffractometer, equipped with graphite monochromator and using MoKα radiation (λ=0.71073 Å). Low temperature was maintained with a Bruker KRYOFLEX nitrogen cryostat. The diffractometer was controlled by a Pentium-based PC running the SMART software package [Bruker AXS GmbH, 2002a]. Immediately after collection, the raw data frames were transferred to a second PC computer for integration and reduction by the SAINT program package [Bruker AXS GmbH, 2002b]. The structures were solved and

refined by the SHELXTL software package [Bruker AXS GmbH, 2002c].

**3. Conclusions** 

PAKs.

**4. Experimental section** 


a enantiomerization barrier

Table 7. Energy barriers (Δ*G*‡, kJ/mol) for diastereomerizations of monoacetylanthracenes and diacetylanthracenes


Table 8. Relative energies (Δ*G*298 and ΔΔ*G*298, kJ/mol) of selected monoacetylanthracenes and diacetylanthracenes and respective energy barriers for *E*,*Z*-diastereomerizations (Δ*G*‡, kJ/mol)
