Catalytic Activity of Iron N-Heterocyclic Carbene Complexes

*Badri Nath Jha, Nishant Singh and Abhinav Raghuvanshi*

## **Abstract**

Recent research towards development of more efficient as well as cost effective catalyst as a substitute to traditional precious metal catalysts has witnessed significant growth and interest. Importance has been given to catalyst based on 3d-transition metals, especially iron because of the broad availability and environmental compatibility which allows its use in various environmentally friendly catalytic processes. N-Heterocyclic carbene (NHC) ligands have garnered significant attention because of their unique steric and electronic properties which provide substantial scope and potential in organometallic chemistry, catalysis and materials sciences. In the context of catalytic applications, iron-NHC complexes have gained increasing interest in the past two decades and could successfully be applied as catalysts in various homogeneous reactions including C–C couplings (including biaryl cross-coupling, alkyl-alkyl cross-coupling, alkyl-aryl cross-coupling), reductions and oxidations. In addition to this, iron-NHC complexes have shown the ability to facilitate a variety of reactions including C-heteroatom bond formation reactions, hydrogenation and transfer-hydrogenation reactions, polymerization reactions, etc. In this chapter, we will discuss briefly recent advancements in the catalytic activity of iron-NHC complexes including mono-NHC, bis-NHC (bidentate), tripodal NHC and tetrapodal NHC ligands. We have chosen iron-NHC complexes because of the plethora of publications available, increasing significance, being more readily available, non-toxic and economical.

**Keywords:** N-heterocyclic carbene (NHC), singlet carbenes, triplet carbenes, percent buried volume (% Vbur), *σ-donation*, *π-donation*, CO complexes, NO complexes, halide complexes, donor-substituted NHCs, pincer motifs, scorpionato motifs, macrocyclic ligands, piano stool motifs, iron-sulfur clusters, C-C bond formations, allylic alkylations, C-X (heteroatom) bond formations, reduction reactions, cyclization reactions, polymerization

## **1. Introduction**

Story of N-heterocyclic carbene builds up from an unstable non-isolable reactive species to a stable and highly flourished ligand for the synthesis of a variety of organometallic compounds and many important catalytic reactions. Based on the orbital occupancy of the electrons, carbenes can be classified as singlet and triplet carbenes. In singlet carbene, a lone pair of electron occupies sp2 -hybrid orbital (**Figure 1A**)

whereas, in triplet carbene, two single electrons occupy two different p-orbitals (**Figure 1B**). Carbenes are inherently unstable, hence highly reactive species due to incomplete electron octet. Initial reports of isolable carbene came in the late 1980s, where the carbene is stabilized by adjacent silicon and phosphorus substituents.

Credit for the discovery of stable and isolable carbene goes to Arduengo, where carbene carbon is a part of a nitrogen heterocycle and gave the first N-heterocyclic carbene (NHC) compound called 1,3-di(adamantyl)imidazol-2-ylidene briefly called IAd (**Figure 2A**) [1]. Since then NHC compounds are enjoying their success to several dimensions of synthesis and organic transformations.

#### **1.1 Structure and general properties of NHCs**

Thus, a heterocyclic compound with a carbene carbon and at least a nitrogen atom adjacent to it within the ring can be termed as NHC [2]. NHCs are singlet carbenes and their remarkable stability is contributed by both steric and electronic effects. Dimerization of carbene carbon is kinetically frustrated by keeping bulky groups on the two sides of the carbene carbon, as is the case with IAd (**Figure 2A**) where two adamantyl groups are attached to the nitrogen atoms (adjacent to the carbene center). Nolan and his co-workers have quantified the steric properties in terms of the 'buried volume' parameter (% Vbur) (**Figure 2B**) [3]. Metal ion of the NHC-metal complex is assumed to be at the center of a sphere and then % Vbur is calculated as the portion of the sphere occupied by the NHC ligand (**Figure 2B**). Larger the value of % Vbur, greater is the steric repulsion at the metal center. The buried volume is usually determined from crystallographic data of the NHC-metal complex [4] or directly from theoretical calculations with the free NHC.

**183**

**Figure 3.**

*Molecular orbital diagram of an NHC.*

*Catalytic Activity of Iron N-Heterocyclic Carbene Complexes*

electronic factor. Carbene carbon of NHC has three sp2

forcing the carbene carbon to adopt a more sp2

(1) and (2)], respectively [9, 10].

The value of % Vbur is affected by both the nature of the NHC ligand as well as the geometry of the NHC-metal complex; therefore, data is useful only for the comparison within the same family of complexes. A small change in the structure of ligands may bring more than 10% increase or decrease in percent buried volume [5]. Caution should also be paid as the calculation of % Vbur is carried out in solid-phase through crystallographic data analysis or in gas phase by DFT calculation. In both the methods the behavior of the complexes in solution and solvation is not considered where ligand may adopt several conformations. The stability of an NHC is far more affected by the

lar planar fashion and one p-orbital (pz) perpendicular to the plane of the NHC ring.


respectively). In the molecular orbital model, sp2

described as HOMO (A1 non-bonding molecular orbital) and LUMO (B2

houses the lone pair of electrons. The two nitrogen atoms stabilize the carbene carbon in two ways: (i) by withdrawing the sigma-electrons through inductive effect and (ii) through a π-electron donation to the empty pz-orbital of the carbene carbon (mesomeric effect). This π-electron donation is so strong that NHCs are also described by its zwitterionic resonance structure and is evident by the intermediate bond length of carbene C-N bond (1.37 Å) in IAd, which falls in between C-N single bond length (1.49 Å) and C-N double bond length (1.33 Å) of the corresponding analog compounds (IAdH2

molecular orbital), respectively (**Figure 3**) [6, 7]. The cyclic nature of NHCs is also an important structural aspect as it creates a preferable situation for the singlet state by

Like the phosphines, the electron-donating capability of NHCs is evaluated using Tolman electronic parameter (TEP) [8]. Any build-up of electron density on the metal center of the complex [Ni(CO)3(NHC)] due to electron donation by the NHC is reflected by the decrease in the infrared-stretching frequency of CO bonded with the metal ion. Now-a-days, instead of [Ni(CO)3(NHC)], less toxic [(NHC) IrCl(CO)2] and [(NHC)RhCl(CO)2] are used and a correlation formula is used [Eqs.

*TEP* = 0.847. νCO(Ir) + 336 cm−1 (1)



and pz-orbital can be

\*

bonding


*DOI: http://dx.doi.org/10.5772/intechopen.90640*

Two sp2

and IAdH+

**Figure 2.** *(A) Structure of IAd; (B) percent buried volume (% Vbur).*

#### *Catalytic Activity of Iron N-Heterocyclic Carbene Complexes DOI: http://dx.doi.org/10.5772/intechopen.90640*

*Organic Synthesis - A Nascent Relook*

whereas, in triplet carbene, two single electrons occupy two different p-orbitals (**Figure 1B**). Carbenes are inherently unstable, hence highly reactive species due to incomplete electron octet. Initial reports of isolable carbene came in the late 1980s, where the carbene is stabilized by adjacent silicon and phosphorus substituents.

to several dimensions of synthesis and organic transformations.

**1.1 Structure and general properties of NHCs**

Credit for the discovery of stable and isolable carbene goes to Arduengo, where carbene carbon is a part of a nitrogen heterocycle and gave the first N-heterocyclic carbene (NHC) compound called 1,3-di(adamantyl)imidazol-2-ylidene briefly called IAd (**Figure 2A**) [1]. Since then NHC compounds are enjoying their success

Thus, a heterocyclic compound with a carbene carbon and at least a nitrogen atom adjacent to it within the ring can be termed as NHC [2]. NHCs are singlet carbenes and their remarkable stability is contributed by both steric and electronic effects. Dimerization of carbene carbon is kinetically frustrated by keeping bulky groups on the two sides of the carbene carbon, as is the case with IAd (**Figure 2A**) where two adamantyl groups are attached to the nitrogen atoms (adjacent to the carbene center). Nolan and his co-workers have quantified the steric properties in terms of the 'buried volume' parameter (% Vbur) (**Figure 2B**) [3]. Metal ion of the NHC-metal complex is assumed to be at the center of a sphere and then % Vbur is calculated as the portion of the sphere occupied by the NHC ligand (**Figure 2B**). Larger the value of % Vbur, greater is the steric repulsion at the metal center. The buried volume is usually determined from crystallographic data of the NHC-metal

complex [4] or directly from theoretical calculations with the free NHC.

**182**

**Figure 2.**

**Figure 1.**

*(A) Singlet carbenes; (B) triplet carbenes.*

*(A) Structure of IAd; (B) percent buried volume (% Vbur).*

The value of % Vbur is affected by both the nature of the NHC ligand as well as the geometry of the NHC-metal complex; therefore, data is useful only for the comparison within the same family of complexes. A small change in the structure of ligands may bring more than 10% increase or decrease in percent buried volume [5]. Caution should also be paid as the calculation of % Vbur is carried out in solid-phase through crystallographic data analysis or in gas phase by DFT calculation. In both the methods the behavior of the complexes in solution and solvation is not considered where ligand may adopt several conformations. The stability of an NHC is far more affected by the electronic factor. Carbene carbon of NHC has three sp2 -orbitals orientated in triangular planar fashion and one p-orbital (pz) perpendicular to the plane of the NHC ring. Two sp2 -orbitals are bonded with two nitrogen atoms in the ring and one sp2 -orbital houses the lone pair of electrons. The two nitrogen atoms stabilize the carbene carbon in two ways: (i) by withdrawing the sigma-electrons through inductive effect and (ii) through a π-electron donation to the empty pz-orbital of the carbene carbon (mesomeric effect). This π-electron donation is so strong that NHCs are also described by its zwitterionic resonance structure and is evident by the intermediate bond length of carbene C-N bond (1.37 Å) in IAd, which falls in between C-N single bond length (1.49 Å) and C-N double bond length (1.33 Å) of the corresponding analog compounds (IAdH2 and IAdH+ respectively). In the molecular orbital model, sp2 and pz-orbital can be described as HOMO (A1 non-bonding molecular orbital) and LUMO (B2 \* bonding molecular orbital), respectively (**Figure 3**) [6, 7]. The cyclic nature of NHCs is also an important structural aspect as it creates a preferable situation for the singlet state by forcing the carbene carbon to adopt a more sp2 -like arrangement.

Like the phosphines, the electron-donating capability of NHCs is evaluated using Tolman electronic parameter (TEP) [8]. Any build-up of electron density on the metal center of the complex [Ni(CO)3(NHC)] due to electron donation by the NHC is reflected by the decrease in the infrared-stretching frequency of CO bonded with the metal ion. Now-a-days, instead of [Ni(CO)3(NHC)], less toxic [(NHC) IrCl(CO)2] and [(NHC)RhCl(CO)2] are used and a correlation formula is used [Eqs. (1) and (2)], respectively [9, 10].

$$\text{TPP} = \text{0.847} \nu\_{\text{CO}} \text{(Ir)} + \text{336 cm}^{-1} \tag{1}$$

**Figure 3.** *Molecular orbital diagram of an NHC.*

where, *ν*CO(Ir) = average IR-stretching frequency of CO in [(NHC)IrCl(CO)2] complex.

$$
\nu\_{\rm CO} \,\mathrm{(Ir)} = 0.8695 \,\nu\_{\rm CO} \,\mathrm{(Rh)} + 250.7 \,\mathrm{cm}^{-1} \tag{2}
$$

where, *ν*CO(Ir) = average IR-stretching frequency of CO in [(NHC)IrCl(CO)2] complex, and *ν*CO(Rh) = average IR-stretching frequency of CO in [(NHC) RhCl(CO)2] complex.

### **1.2 Synthesis of NHCs precursor and generation of carbene**

Azolium or dihydroimidazolium salts are sufficiently stable solids and the generation of NHCs can be carried out *in situ* by their deprotonation using nonnucleophilic bases such as sodium hydride, butyllithium or *t*-butoxide. Alkoxides form an adduct with azolium salt, however, in presence of transition metal precursor, NHC is transferred to the metal and usually moves toward complex formation rather than the disruption of the azolium ring. Generation of NHCs is also carried out using mild metal oxides like silver (I) or copper (I) oxides where after generation, NHC forms NHC-silver(I) or copper(I) complexes and *in situ* transfer of NHC occurs to the desired metal center. A general protocol for the synthesis of NHCs and NHC precursor **11** is outlined below in **Figures 4** and **5**, respectively [11, 12].

**Figure 4.** *General protocol for the synthesis of unsymmetrical substituted NHCs.*

**185**

**Figure 6.**

*Catalytic Activity of Iron N-Heterocyclic Carbene Complexes*

base is shown in **Figure 6A** and **B**, respectively [13].

largely through the *strong σ-donation* of the formal sp2

**1.4 Coordination of NHCs to transition metals**

Formation of saturated and unsaturated NHCs upon treatment with an alkoxide


Thus, the coordination of NHC ligand to the transition metal ion occurs

NHCs are being compared with strong sigma donating ligands like phosphines and cyclopentadienes. As a ligand, NHCs edge ahead of phosphines on several points:

i.*Electron donor*: NHCs are relatively stronger electron-donor than phosphines and produce thermodynamically stronger metal-ligand bonds, except when there are steric constraints interfere with metal-ligand

ii. *Steric properties*: Whereas the spatial arrangement of steric bulk takes up a

iii.*Ease of varying their steric and electronic properties*: There are several wellestablished synthetic routes to tune the steric and electronic properties of NHCs, whereas it is usually difficult to tune the properties to the desired

*Treatment with an alkoxide base leads to formation of (A) saturated NHCs; and (B) unsaturated NHCs.*

nitrogen atoms and the heterocyclic ring, if required.

in *umbrella-shaped* steric bulk and the orientation of the substituent on the two nitrogen atoms are more toward the metal center. Thus, the steric crowd around the metal center can be tuned by changing the substituent on the two


to a *σ*-accepting orbital of the transition metal and a *weak but not inconsiderable π-donation* [14] either in the form of π-back donation from metal to the pz orbital of the ligand or vice versa [15, 16]. However, in practice a single bond is drawn since the free rotation energy across the M-C bond is very low

*DOI: http://dx.doi.org/10.5772/intechopen.90640*

**1.3 Generation of NHCs**

(**Figures 2B** and **6**).

**1.5 Phosphine versus NHCs**

binding [17].

*cone-shape* due to sp3

level for the phosphines.

**Figure 5.** *Synthesis of NHC precursor 11.*

## **1.3 Generation of NHCs**

*Organic Synthesis - A Nascent Relook*

complex.

RhCl(CO)2] complex.

where, *ν*CO(Ir) = average IR-stretching frequency of CO in [(NHC)IrCl(CO)2]

where, *ν*CO(Ir) = average IR-stretching frequency of CO in [(NHC)IrCl(CO)2]

Azolium or dihydroimidazolium salts are sufficiently stable solids and the generation of NHCs can be carried out *in situ* by their deprotonation using nonnucleophilic bases such as sodium hydride, butyllithium or *t*-butoxide. Alkoxides form an adduct with azolium salt, however, in presence of transition metal precursor, NHC is transferred to the metal and usually moves toward complex formation rather than the disruption of the azolium ring. Generation of NHCs is also carried out using mild metal oxides like silver (I) or copper (I) oxides where after generation, NHC forms NHC-silver(I) or copper(I) complexes and *in situ* transfer of NHC occurs to the desired metal center. A general protocol for the synthesis of NHCs and

NHC precursor **11** is outlined below in **Figures 4** and **5**, respectively [11, 12].

complex, and *ν*CO(Rh) = average IR-stretching frequency of CO in [(NHC)

**1.2 Synthesis of NHCs precursor and generation of carbene**

*General protocol for the synthesis of unsymmetrical substituted NHCs.*

νCO(Ir) = 0.8695. νCO(Rh) + 250.7 cm−1 (2)

**184**

**Figure 5.**

**Figure 4.**

*Synthesis of NHC precursor 11.*

Formation of saturated and unsaturated NHCs upon treatment with an alkoxide base is shown in **Figure 6A** and **B**, respectively [13].

## **1.4 Coordination of NHCs to transition metals**

Thus, the coordination of NHC ligand to the transition metal ion occurs largely through the *strong σ-donation* of the formal sp2 -hybridized lone pair to a *σ*-accepting orbital of the transition metal and a *weak but not inconsiderable π-donation* [14] either in the form of π-back donation from metal to the pz orbital of the ligand or vice versa [15, 16]. However, in practice a single bond is drawn since the free rotation energy across the M-C bond is very low (**Figures 2B** and **6**).

## **1.5 Phosphine versus NHCs**

NHCs are being compared with strong sigma donating ligands like phosphines and cyclopentadienes. As a ligand, NHCs edge ahead of phosphines on several points:


iv.In the case of phosphines, changing the substituent on the phosphorus inevitably changes both steric and the electronic properties whereas each parameter can be modified independently through modifying the substituents on nitrogen, functionalities on the heterocycle and the type of heterocycle itself.

## **2. Various motifs of Fe-NHC complexes**

The structural diversity in various motifs of Fe-NHC complexes is shown in **Figure 7** and each of them is explained below along with their known applications in different areas.

## **2.1 Mono- and bis-(mono- or chelating) carbene ligands**

## *2.1.1 CO complexes*

The chemistry of Fe-NHC complexes began with the synthesis of their unsaturated and saturated ligand precursors with carbonyl as their motifs, and extensive studies on molecular structure determination and reactivity (**Figure 8A–D**). These CO complexes were further subjected to substitution reaction, e.g. ligand exchange with monophosphines and oxidation, to develop newer Fe-NHC complexes (on oxidation their geometry tends to change from trigonal bipyramidal to distorted square pyramidal). These transformations, in progression, led to the formation of new classes of complexes with novel attributes viz. monocarbene, bis-monocarbene, and chelating biscarbene ligands having variable oxidation states of iron from Fe(0) to Fe(II), which contributed to new horizons in bioinorganic chemistry and biomimetic systems e.g. Novel Fe(II) monocarbene complexes (**Figure 8C**) as models for basic structure of the monoiron hydrogenase [18].

## *2.1.2 NO complexes*

Synthesis of novel and intriguing Fe-NHC complexes in the field of biomimetic chemistry e.g. dinitrosyliron complexes (DNICs) (**Figure 8G**) displaying a variety of vital biological functions [18], forced the scientific community to shift their attention toward novel monocarbenes and bis-monocarbene ligands having nitrosyl as their structural attributes (**Figure 8E–G**). Not only as biomimetic structural models, these nitrosyl complexes can act as catalyst in chemical transformations e.g. allylic alkylation [18, 19].

**187**

*Catalytic Activity of Iron N-Heterocyclic Carbene Complexes*

Just like carbonyl and nitrosyl motifs in Fe-NHCs chemistry, halides do play a major role in influencing the role of Fe-NHC complexes in both catalysis as well as biomimetics. Halide complexes catalytic role varies from polymerization catalysis by bis-monocarbene dihalide Fe-NHC complexes [18, 20] C-C cross-coupling reactions catalyzed by dinuclear Fe-NHC imido complexes [18, 21] to catalytic hydrosilylation by ethylenediamine-derived Fe-NHC complex [18]. Depending upon the structural versatility in halide complexes, many subclasses have been synthesized and studied, namely monocarbene ligands, bis-monocarbene ligands, chelating biscarbene ligands, dinuclear Fe-NHC imido complexes, halide-bridged Fe-NHC complexes, immobilized

Fe-NHC complexes, three-coordinate Fe-NHC complexes (**Figure 9A–G**).

Effects on the reactivity of organometallic iron complexes could be observed when the ligand environment changes from CO, NO, halides to donor-substituted NHC ligands (**Figure 10A**). These donor-substituted NHC ligands possess nitrogen or oxygen as heteroatoms, thus present themselves as potential coordinating "arms" attached to the NHCs and exhibit coordination from bi- to pentadentate as ligand systems. These complexes have shown their catalytic role in ring-opening polymer-

Chelating biscarbene pincer ligands (**Figure 10B**) are an extension of donorsubstituted NHCs in Fe-NHC chemistry, where instead of the presence of heteroatoms as "arms", two NHC units are linked by a pyridyl moiety and hence "chelation". Structurally, pincer motifs exhibit two coordination geometries predominantly, octahedral and square pyramidal, due to their strict binding mode to three adjacent coplanar centers. Catalysis by Fe-NHC complexes bearing pincer motifs has been demonstrated by their catalytic role in concerted C-H oxidation addition reaction

[18], hydroboration reaction [18, 23], and hydrogenation reaction [18].

*DOI: http://dx.doi.org/10.5772/intechopen.90640*

*2.1.3 Halide complexes*

*(A–D) CO complexes; (E–G) NO complexes.*

**Figure 8.**

**2.2 Donor-substituted NHCs**

ization of ε-caprolactone [18, 22].

**2.3 Pincer motifs**

**Figure 7.** *Different motifs of Fe-NHC complexes.*

*Catalytic Activity of Iron N-Heterocyclic Carbene Complexes DOI: http://dx.doi.org/10.5772/intechopen.90640*

*(A–D) CO complexes; (E–G) NO complexes.*

## *2.1.3 Halide complexes*

*Organic Synthesis - A Nascent Relook*

in different areas.

*2.1.1 CO complexes*

*2.1.2 NO complexes*

**2. Various motifs of Fe-NHC complexes**

**2.1 Mono- and bis-(mono- or chelating) carbene ligands**

iv.In the case of phosphines, changing the substituent on the phosphorus inevitably changes both steric and the electronic properties whereas each parameter can be modified independently through modifying the substituents on nitrogen, functionalities on the heterocycle and the type of heterocycle itself.

The structural diversity in various motifs of Fe-NHC complexes is shown in **Figure 7** and each of them is explained below along with their known applications

The chemistry of Fe-NHC complexes began with the synthesis of their unsaturated and saturated ligand precursors with carbonyl as their motifs, and extensive studies on molecular structure determination and reactivity (**Figure 8A–D**). These CO complexes were further subjected to substitution reaction, e.g. ligand exchange with monophosphines and oxidation, to develop newer Fe-NHC complexes (on oxidation their geometry tends to change from trigonal bipyramidal to distorted square pyramidal). These transformations, in progression, led to the formation of new classes of complexes with novel attributes viz. monocarbene, bis-monocarbene, and chelating biscarbene ligands having variable oxidation states of iron from Fe(0) to Fe(II), which contributed to new horizons in bioinorganic chemistry and biomimetic systems e.g. Novel Fe(II) monocarbene complexes (**Figure 8C**) as models for basic structure of the monoiron hydrogenase [18].

Synthesis of novel and intriguing Fe-NHC complexes in the field of biomimetic chemistry e.g. dinitrosyliron complexes (DNICs) (**Figure 8G**) displaying a variety of vital biological functions [18], forced the scientific community to shift their attention toward novel monocarbenes and bis-monocarbene ligands having nitrosyl as their structural attributes (**Figure 8E–G**). Not only as biomimetic structural models, these nitrosyl complexes can act as catalyst in chemical transformations e.g. allylic alkylation [18, 19].

**186**

**Figure 7.**

*Different motifs of Fe-NHC complexes.*

Just like carbonyl and nitrosyl motifs in Fe-NHCs chemistry, halides do play a major role in influencing the role of Fe-NHC complexes in both catalysis as well as biomimetics. Halide complexes catalytic role varies from polymerization catalysis by bis-monocarbene dihalide Fe-NHC complexes [18, 20] C-C cross-coupling reactions catalyzed by dinuclear Fe-NHC imido complexes [18, 21] to catalytic hydrosilylation by ethylenediamine-derived Fe-NHC complex [18]. Depending upon the structural versatility in halide complexes, many subclasses have been synthesized and studied, namely monocarbene ligands, bis-monocarbene ligands, chelating biscarbene ligands, dinuclear Fe-NHC imido complexes, halide-bridged Fe-NHC complexes, immobilized Fe-NHC complexes, three-coordinate Fe-NHC complexes (**Figure 9A–G**).

#### **2.2 Donor-substituted NHCs**

Effects on the reactivity of organometallic iron complexes could be observed when the ligand environment changes from CO, NO, halides to donor-substituted NHC ligands (**Figure 10A**). These donor-substituted NHC ligands possess nitrogen or oxygen as heteroatoms, thus present themselves as potential coordinating "arms" attached to the NHCs and exhibit coordination from bi- to pentadentate as ligand systems. These complexes have shown their catalytic role in ring-opening polymerization of ε-caprolactone [18, 22].

#### **2.3 Pincer motifs**

Chelating biscarbene pincer ligands (**Figure 10B**) are an extension of donorsubstituted NHCs in Fe-NHC chemistry, where instead of the presence of heteroatoms as "arms", two NHC units are linked by a pyridyl moiety and hence "chelation". Structurally, pincer motifs exhibit two coordination geometries predominantly, octahedral and square pyramidal, due to their strict binding mode to three adjacent coplanar centers. Catalysis by Fe-NHC complexes bearing pincer motifs has been demonstrated by their catalytic role in concerted C-H oxidation addition reaction [18], hydroboration reaction [18, 23], and hydrogenation reaction [18].

#### **2.4 Scorpionato motifs**

Scorpionato-type motifs (**Figure 10C**) means boron linked anionic chelating triscarbene ligands and on complexation with iron results in a new class of Fe-NHC complexes. Therefore, if any iron complex/compound is bearing two scorpionatotype ligands, it will be, (a) coordinated by six carbenes, (b) highly stable, and (c) showing *S6* symmetry along Fe-B-H axis [18]. Different types of scorpionato-type motifs have also been synthesized e.g. tripodal borane NHC iron complexes [18], amine-bridged scorpionato Fe-NHC motifs [18].

#### **2.5 Macrocyclic ligands**

Macrocyclic ligands, despite well-investigated other cyclic ligands such as cyclam, porphyrin, on complexation with iron developed a new class of complexes in Fe-NHC coordination chemistry (**Figure 10D**). Their catalytic aspect has been successfully employed in aziridination of alkenes with aryl azides [18, 24].

#### **2.6 Piano stool motifs**

The term "piano stool Fe-NHC complexes" states that all such complexes bear both, (a) N-heterocyclic carbene motif and (b) cyclopentadienyl (Cp) ligand. The structural variations in these complexes are well explained by (a) mono- and dimeric piano stool Fe-NHC complexes [18], (b) donor-substituted piano stool Fe-NHC complexes [18], (c) biscarbene-chelated piano stool complexes [18], (d) alkyl piano stool Fe-NHC complexes [18], (e) three coordinate piano stool Fe-NHC complexes [18], and many more [18] (**Figure 10E**–**G**). These have shown their catalytic activities in C-H bond activation [18], borylation reactions [18, 23], hydrosilylation [18, 25–27], transfer hydrogenation [18], C-N bond formation [18, 24].

**189**

**2.7 Iron-sulfur clusters**

*piano stool motifs; (H) iron-sulfur clusters*

**Figure 10.**

were based on all-ferrous [Fe4S4]

Diiron dithiolate complexes (**Figure 10H**) have been reported to mimic the active site of [FeFe] hydrogenase [18]. Also, the substitution of carbonyl motifs (one or more) in the diiron dithiolate complexes by σ-donor ligands (in this case NHCs) is shown to influence the redox potential of the iron center [18]. Further, donor-substituted NHCs motifs were included in the molecular framework of [FeFe] hydrogenase model compounds to extend its molecular assembly [18]. Another notable characteristic presence of Fe-NHC complexes bearing iron-sulfur clusters was demonstrated in synthesis of nitrogenase model compounds, which

*(A) Donor-substitutes NHCs; (B) pincer motifs; (C) Scorpionato motifs; (D) macrocyclic ligands; (E–G)* 

0 [18].

**3. Catalysis by Fe-NHC complexes: important transformations**

Even if there are a tremendous number of catalysts based on rare/heavy transition metals such as palladium, platinum, ruthenium, rhodium, iridium, and gold

*Catalytic Activity of Iron N-Heterocyclic Carbene Complexes*

*DOI: http://dx.doi.org/10.5772/intechopen.90640*

*Catalytic Activity of Iron N-Heterocyclic Carbene Complexes DOI: http://dx.doi.org/10.5772/intechopen.90640*

**Figure 10.**

*Organic Synthesis - A Nascent Relook*

**2.4 Scorpionato motifs**

*(A–G) Halide complexes.*

**Figure 9.**

**2.5 Macrocyclic ligands**

**2.6 Piano stool motifs**

C-N bond formation [18, 24].

amine-bridged scorpionato Fe-NHC motifs [18].

Scorpionato-type motifs (**Figure 10C**) means boron linked anionic chelating triscarbene ligands and on complexation with iron results in a new class of Fe-NHC complexes. Therefore, if any iron complex/compound is bearing two scorpionatotype ligands, it will be, (a) coordinated by six carbenes, (b) highly stable, and (c) showing *S6* symmetry along Fe-B-H axis [18]. Different types of scorpionato-type motifs have also been synthesized e.g. tripodal borane NHC iron complexes [18],

Macrocyclic ligands, despite well-investigated other cyclic ligands such as cyclam, porphyrin, on complexation with iron developed a new class of complexes in Fe-NHC coordination chemistry (**Figure 10D**). Their catalytic aspect has been successfully employed in aziridination of alkenes with aryl azides [18, 24].

The term "piano stool Fe-NHC complexes" states that all such complexes bear both, (a) N-heterocyclic carbene motif and (b) cyclopentadienyl (Cp) ligand. The structural variations in these complexes are well explained by (a) mono- and dimeric piano stool Fe-NHC complexes [18], (b) donor-substituted piano stool Fe-NHC complexes [18], (c) biscarbene-chelated piano stool complexes [18], (d) alkyl piano stool Fe-NHC complexes [18], (e) three coordinate piano stool Fe-NHC complexes [18], and many more [18] (**Figure 10E**–**G**). These have shown their catalytic activities in C-H bond activation [18], borylation reactions [18, 23], hydrosilylation [18, 25–27], transfer hydrogenation [18],

**188**

*(A) Donor-substitutes NHCs; (B) pincer motifs; (C) Scorpionato motifs; (D) macrocyclic ligands; (E–G) piano stool motifs; (H) iron-sulfur clusters*

#### **2.7 Iron-sulfur clusters**

Diiron dithiolate complexes (**Figure 10H**) have been reported to mimic the active site of [FeFe] hydrogenase [18]. Also, the substitution of carbonyl motifs (one or more) in the diiron dithiolate complexes by σ-donor ligands (in this case NHCs) is shown to influence the redox potential of the iron center [18]. Further, donor-substituted NHCs motifs were included in the molecular framework of [FeFe] hydrogenase model compounds to extend its molecular assembly [18]. Another notable characteristic presence of Fe-NHC complexes bearing iron-sulfur clusters was demonstrated in synthesis of nitrogenase model compounds, which were based on all-ferrous [Fe4S4] 0 [18].

### **3. Catalysis by Fe-NHC complexes: important transformations**

Even if there are a tremendous number of catalysts based on rare/heavy transition metals such as palladium, platinum, ruthenium, rhodium, iridium, and gold

[28–30] are available for various different kind of organic transformations and they are very successful; the scientific community is trying hard to replace these metals by some environment and biological friendly metals because they are highly expensive and very toxic in nature therefore not compatible with biological systems. Iron becomes the obvious choice since it is the most abundant transition metal on the earth's crust, relatively inexpensive, environmentally benign [31] and relatively less toxic to the biological systems [32, 33]. There are several very successful examples of iron-based catalysts like Fischer-Tropsch and the Haber-Bosch processes [34] and are capable of catalysis in numerous different reactions [35, 36]. Reports related to the iron-NHC complexes started coming just after the publication of first metal-NHC complex in 1968, the growth in the research was almost ceased for next three decades and picks up the pace after the success of Grubb's catalyst for various organic transformations and polymerization reactions [20, 37]. Iron-NHC complexes are reported to have found applications in different classes of reactions such as substitution, addition, oxidation, reduction, cycloaddition, isomerization, rearrangement and polymerization reactions (**Figure 11**).

## **3.1 C-C bond formations**

Negishi, Suzuki, and Heck were awarded the Nobel Prize in 2010 for their pioneer work in the area of cross-coupling reactions, as it provides a very effective tool for C-C bond formation. Several different protocols have been reported mainly based on palladium and, to some extent, Ni and copper metal ions. Iron-NHC complex based catalysts have been used for various Kumada-type cross-couplings such as C(sp3 )-C(sp2 ), C(sp2 )-C(sp3 ), C(sp2 )-C(sp2 ), C(sp3 )-C(sp3 ) bond formations, and C(sp2 )-C(sp2 ) homo-couplings. NHC can either be generated *in situ* in a reaction or a resynthesized iron-NHC complex can be used. Bedford and co-workers, in a first, introduced the NHCs ligands and iron-NHC complexes along with FeCl3 to improve the yield of Kumada-type coupling reactions (**Figure 12A**) [38]. Among

**191**

**Figure 13.**

iron-NHC complex **12** (94% yield).

the oxidative addition) of alkyl radical (R.

*Catalytic Activity of Iron N-Heterocyclic Carbene Complexes*

*(A) Aryl Grignard reagents-bromoalkanes cross-coupling [38]; (B) proposed mechanism.*

several carbene ligand precursors, *tert*-butylimidazolinium chloride **12a** was found to give the best results (97% yield) and the performance was almost matched by the

*(A) Primary alkyl fluorides-aryl Grignard reagents Kumada-type coupling [21]; (B) proposed mechanism.*

The proposed mechanism suggests that reaction does not follow the classical oxidative addition mechanism, but rather involves a radical intermediate produced through single electron transfer (SET) (**Figure 12B**) [39, 40]. Reaction mechanism involves the following processes: (i) generation of active catalyst through reduction of Fe(III) to Fe(II, I, or 0), (ii) generation and association (not

(iii) transmetalation, where aryl group is transferred from ArMgX to the iron

) with the iron center through SET,

*DOI: http://dx.doi.org/10.5772/intechopen.90640*

**Figure 12.**

**Figure 11.** *Important transformations catalyzed by Fe-NHC complexes.*

#### **Figure 12.**

*Organic Synthesis - A Nascent Relook*

**3.1 C-C bond formations**

)-C(sp2

)-C(sp2

), C(sp2

as C(sp3

and C(sp2

[28–30] are available for various different kind of organic transformations and they are very successful; the scientific community is trying hard to replace these metals by some environment and biological friendly metals because they are highly expensive and very toxic in nature therefore not compatible with biological systems. Iron becomes the obvious choice since it is the most abundant transition metal on the earth's crust, relatively inexpensive, environmentally benign [31] and relatively less toxic to the biological systems [32, 33]. There are several very successful examples of iron-based catalysts like Fischer-Tropsch and the Haber-Bosch processes [34] and are capable of catalysis in numerous different reactions [35, 36]. Reports related to the iron-NHC complexes started coming just after the publication of first metal-NHC complex in 1968, the growth in the research was almost ceased for next three decades and picks up the pace after the success of Grubb's catalyst for various organic transformations and polymerization reactions [20, 37]. Iron-NHC complexes are reported to have found applications in different classes of reactions such as substitution, addition, oxidation, reduction, cycloaddition, isomerization,

Negishi, Suzuki, and Heck were awarded the Nobel Prize in 2010 for their pioneer work in the area of cross-coupling reactions, as it provides a very effective tool for C-C bond formation. Several different protocols have been reported mainly based on palladium and, to some extent, Ni and copper metal ions. Iron-NHC complex based catalysts have been used for various Kumada-type cross-couplings such

)-C(sp2

tion or a resynthesized iron-NHC complex can be used. Bedford and co-workers, in a first, introduced the NHCs ligands and iron-NHC complexes along with FeCl3 to improve the yield of Kumada-type coupling reactions (**Figure 12A**) [38]. Among

), C(sp3

) homo-couplings. NHC can either be generated *in situ* in a reac-

)-C(sp3

) bond formations,

rearrangement and polymerization reactions (**Figure 11**).

)-C(sp3

), C(sp2

**190**

**Figure 11.**

*Important transformations catalyzed by Fe-NHC complexes.*

*(A) Aryl Grignard reagents-bromoalkanes cross-coupling [38]; (B) proposed mechanism.*

#### **Figure 13.**

*(A) Primary alkyl fluorides-aryl Grignard reagents Kumada-type coupling [21]; (B) proposed mechanism.*

several carbene ligand precursors, *tert*-butylimidazolinium chloride **12a** was found to give the best results (97% yield) and the performance was almost matched by the iron-NHC complex **12** (94% yield).

The proposed mechanism suggests that reaction does not follow the classical oxidative addition mechanism, but rather involves a radical intermediate produced through single electron transfer (SET) (**Figure 12B**) [39, 40]. Reaction mechanism involves the following processes: (i) generation of active catalyst through reduction of Fe(III) to Fe(II, I, or 0), (ii) generation and association (not the oxidative addition) of alkyl radical (R. ) with the iron center through SET, (iii) transmetalation, where aryl group is transferred from ArMgX to the iron

center, and (iv) attack of alkyl radical (R. ) to the aryl group (Ar) leading to the generation of coupled product and the catalyst [38].

It was proved through a control experiment that particularly primary and secondary alkyl halides favor iron-catalyzed reactions, in comparison to most of the Pd or Ni systems, because of their sluggish tendency toward the β-hydride elimination and hence less susceptibility to the olefin formation. Therefore, it plausibly indicated the limitations of the catalytic role of the Fe-NHC complexes, in case of *in situ* formation of an iron NHC complex or the deprotonation of the imidazolium salt. Besides Alkyl bromide, dinuclear Fe-NHC imido complexes such as **13** have been reported to be effective in activating other alkyl halides and most challenging alkyl fluoride (**Figure 13A**). Here again, the use of the substrates such as (fluoromethyl)cyclopropane suggested a radical-mediated mechanistic pathway (**Figure 13B**). The first step is the dissociation of one NHC

#### **Table 1.**

*Other examples of C-C bond formation and allylic alkylation reactions [41–44].*

**193**

**Figure 14.**

*Catalytic Activity of Iron N-Heterocyclic Carbene Complexes*

ligand followed by the second step as transmetalation (note: dinuclear iron imido subunit stays intact during the process). The further mechanism involves the usual mechanistic protocol, which includes firstly the formation of radical species and secondly, attack of the radical on the aryl moiety [21]. Several more iron-NHC complex catalyzed carbon-carbon coupling reactions have been given

In a seminal work by Plietker group [19], allylic alkylation by the catalyst **14** was shown through the reaction of allyl carbonate and a Michael donor resulting into two isomeric products, i.e. (i) Product **X**, through the *ipso* substitution, and (ii) Product **Y**, via a σ−π−σ isomerization (**Figure 14A**). Mechanistic investigation suggests that the product ratio is greatly influenced by the steric crowd around the metal center, created due to the substituents on the nitrogen atoms of NHC moiety. Increased steric crowd hinders the isomerization process and thus favoring *ipso* substitution product **X**. For example, if *tert-*butyl group is present on the N atom of NHC, *ipso* substitution is favored, on the other hand, mesitylene group, which creates less steric hindrance around the metal center, favors isomerized product **Y**. In addition, stronger nucleophilicity of Michael donor favors the *ipso*-substitution. A plausible mechanism is outlined in **Figure 14B**. Few more allylic alkylation reac-

Catalytic C-H bond activation has been one of the major tools to perform effective chemical transformations. Applicability of Fe-NHC complex as the catalyst for C-H bond activation has gained momentum since it can produce the formation of a range of different C-X bonds such as C-N, C-B, C-Mg, and C-S bond. Fe-NHC complex catalyzed C-N bond formation is important because of the three very basic

*DOI: http://dx.doi.org/10.5772/intechopen.90640*

in **Table 1**.

**3.2 Allylic alkylations**

tions are presented in **Table 1**.

**3.3 C-X (heteroatom) bond formations**

*(A) (TBA)Fe/NHC catalyzed allylic alkylation [19]; (B) proposed mechanism.*

ligand followed by the second step as transmetalation (note: dinuclear iron imido subunit stays intact during the process). The further mechanism involves the usual mechanistic protocol, which includes firstly the formation of radical species and secondly, attack of the radical on the aryl moiety [21]. Several more iron-NHC complex catalyzed carbon-carbon coupling reactions have been given in **Table 1**.

## **3.2 Allylic alkylations**

*Organic Synthesis - A Nascent Relook*

center, and (iv) attack of alkyl radical (R.

generation of coupled product and the catalyst [38].

It was proved through a control experiment that particularly primary and secondary alkyl halides favor iron-catalyzed reactions, in comparison to most of the Pd or Ni systems, because of their sluggish tendency toward the β-hydride elimination and hence less susceptibility to the olefin formation. Therefore, it plausibly indicated the limitations of the catalytic role of the Fe-NHC complexes, in case of *in situ* formation of an iron NHC complex or the deprotonation of the imidazolium salt. Besides Alkyl bromide, dinuclear Fe-NHC imido complexes such as **13** have been reported to be effective in activating other alkyl halides and most challenging alkyl fluoride (**Figure 13A**). Here again, the use of the substrates such as (fluoromethyl)cyclopropane suggested a radical-mediated mechanistic pathway (**Figure 13B**). The first step is the dissociation of one NHC

) to the aryl group (Ar) leading to the

**192**

**Table 1.**

*Other examples of C-C bond formation and allylic alkylation reactions [41–44].*

In a seminal work by Plietker group [19], allylic alkylation by the catalyst **14** was shown through the reaction of allyl carbonate and a Michael donor resulting into two isomeric products, i.e. (i) Product **X**, through the *ipso* substitution, and (ii) Product **Y**, via a σ−π−σ isomerization (**Figure 14A**). Mechanistic investigation suggests that the product ratio is greatly influenced by the steric crowd around the metal center, created due to the substituents on the nitrogen atoms of NHC moiety. Increased steric crowd hinders the isomerization process and thus favoring *ipso* substitution product **X**. For example, if *tert-*butyl group is present on the N atom of NHC, *ipso* substitution is favored, on the other hand, mesitylene group, which creates less steric hindrance around the metal center, favors isomerized product **Y**. In addition, stronger nucleophilicity of Michael donor favors the *ipso*-substitution. A plausible mechanism is outlined in **Figure 14B**. Few more allylic alkylation reactions are presented in **Table 1**.

## **3.3 C-X (heteroatom) bond formations**

Catalytic C-H bond activation has been one of the major tools to perform effective chemical transformations. Applicability of Fe-NHC complex as the catalyst for C-H bond activation has gained momentum since it can produce the formation of a range of different C-X bonds such as C-N, C-B, C-Mg, and C-S bond. Fe-NHC complex catalyzed C-N bond formation is important because of the three very basic

**Figure 14.** *(A) (TBA)Fe/NHC catalyzed allylic alkylation [19]; (B) proposed mechanism.*

reasons, (a) aziridine based compounds are of medicinal importance and therefore essential for pharmaceutical industry, (b) demand of aziridine derivatives in polymer chemistry as cross-linker agents for two-component resins, and (c) relative to well-known synthesis of *O*-epoxidation analogs, it is hard to synthesize the designer *N*-building blocks. Catalytic aziridination of alkenes by using Fe-NHC complex **15**

**195**

*Catalytic Activity of Iron N-Heterocyclic Carbene Complexes*

(0.1–1 mol%) as the catalyst was published by Jenkins et al. [24] to form respective aryl-substituted aziridines by treating aryl azides with various substituted alkene (**Figure 15A**). As proposed, the reaction involved the formation of a key and highly reactive intermediate Fe(IV) imido complex (**Figure 15B**). Few more C-X bond

There are several reports on the reduction of alkenes via silylation using iron-NHC complexes. Royo group was first to show such conversion using piano stool type complex **16** (**Figure 16A**) [25]. The reaction is sensitive to the type of substituent present at para-position in the aromatic ring of the reactant, e.g. quantitative yields for reactions of *p*-aryl-substituted aldehydes and alkyl-substituted aldehydes or ketones remained unreactive. Another piano stool type complex **17** reduces ketones and aldehydes into the corresponding alcohols very efficiently (**Figure 16B**) [26]. Same catalyst **17** can reduce the carbonyl group of various amides in moderate to excellent yields (**Figure 16C** and **D**) [27]. In both cases, irradiation of visible light is crucial for the reported effective conversions, where PhSiH3 works as the hydride source. Catalyst shows differential reactivity with the primary, secondary and tertiary amides. Secondary and tertiary amides give usual conversion of carbonyl group into alcohol, while primary amide converts into nitrile compound. Cyclic amides have to be protected before reduction; otherwise a mixture of products forms.

formation reactions are presented in **Table 2**.

*Other examples of C-X bond formations [23, 45–50].*

**3.4 Reduction reactions**

**Table 2.**

*DOI: http://dx.doi.org/10.5772/intechopen.90640*

*Catalytic Activity of Iron N-Heterocyclic Carbene Complexes DOI: http://dx.doi.org/10.5772/intechopen.90640*

*Organic Synthesis - A Nascent Relook*

reasons, (a) aziridine based compounds are of medicinal importance and therefore essential for pharmaceutical industry, (b) demand of aziridine derivatives in polymer chemistry as cross-linker agents for two-component resins, and (c) relative to well-known synthesis of *O*-epoxidation analogs, it is hard to synthesize the designer *N*-building blocks. Catalytic aziridination of alkenes by using Fe-NHC complex **15**

*(A) Fe-NHC catalyzed aziridination of alkenes [24]; (B) proposed mechanism.*

**194**

**Figure 15.**

#### **Table 2.** *Other examples of C-X bond formations [23, 45–50].*

(0.1–1 mol%) as the catalyst was published by Jenkins et al. [24] to form respective aryl-substituted aziridines by treating aryl azides with various substituted alkene (**Figure 15A**). As proposed, the reaction involved the formation of a key and highly reactive intermediate Fe(IV) imido complex (**Figure 15B**). Few more C-X bond formation reactions are presented in **Table 2**.

### **3.4 Reduction reactions**

There are several reports on the reduction of alkenes via silylation using iron-NHC complexes. Royo group was first to show such conversion using piano stool type complex **16** (**Figure 16A**) [25]. The reaction is sensitive to the type of substituent present at para-position in the aromatic ring of the reactant, e.g. quantitative yields for reactions of *p*-aryl-substituted aldehydes and alkyl-substituted aldehydes or ketones remained unreactive. Another piano stool type complex **17** reduces ketones and aldehydes into the corresponding alcohols very efficiently (**Figure 16B**) [26]. Same catalyst **17** can reduce the carbonyl group of various amides in moderate to excellent yields (**Figure 16C** and **D**) [27]. In both cases, irradiation of visible light is crucial for the reported effective conversions, where PhSiH3 works as the hydride source. Catalyst shows differential reactivity with the primary, secondary and tertiary amides. Secondary and tertiary amides give usual conversion of carbonyl group into alcohol, while primary amide converts into nitrile compound. Cyclic amides have to be protected before reduction; otherwise a mixture of products forms.

#### **Figure 16.**

*Hydrosilylative reductions of (A) benzaldehyde derivatives [25]; (B and C) substituted and primary amides, respectively [27].*

Various recently reported iron-NHC complex catalyzed reduction reactions are summarized in **Table 3**.

#### **3.5 Cyclization reactions**

Fe-NHC catalyzed ring expansion of the epoxides with functionalized alkenes presents a very intriguing case because cyclic structures are of great importance in various fields such as the pharmaceutical industry, fine chemicals, agriculture, etc. Fe-NHC catalyzed such reactions not only have shown functional group tolerance but also high chemo- and regioselectivity.

**197**

*Catalytic Activity of Iron N-Heterocyclic Carbene Complexes*

Hilt et al. [51] used a mixture of FeCl2, phosphine ligands and *in situ* generated free NHCs, **18** and performed reaction under reductive conditions using Zn and NEt3 (**Figure 17A**). The reaction mechanism demonstrates the first step as a SET (single-electron transfer) in epoxide ring-opening, the second step as the formation of an elongated alkoxy radical via reaction between formed radical intermediate and added alkene, and the final step as a BET (back-electron transfer), which gave the desired expanded cyclic product via a zwitterionic intermediate cyclization

So far, the application of Fe-NHC complexes have not been much explored in the area of polymerization [52]. Grubbs has first reported the use of Fe-NHC complex **19** as the catalyst in atom transfer radical polymerization (ATRP) reaction of styrene and methyl methacrylate (**Figure 18**) [20]. The reaction shows pseudo first-order kinetics, a decent control of radical concentration, and polydispersity

Shen and co-workers have reported the ring-opening polymerization (ROP) reaction of ε-caprolactone by using Fe-NHC complex **20** as the catalyst [22]. Even though reaction suffers some side reaction of transesterification, polymerization progresses with quantitative conversion and moderate number average molecular

*DOI: http://dx.doi.org/10.5772/intechopen.90640*

(**Figure 17B**).

**3.6 Polymerization**

index (PDI) near 1.1.

weight distribution (**Figure 19**).

#### *Catalytic Activity of Iron N-Heterocyclic Carbene Complexes DOI: http://dx.doi.org/10.5772/intechopen.90640*

Hilt et al. [51] used a mixture of FeCl2, phosphine ligands and *in situ* generated free NHCs, **18** and performed reaction under reductive conditions using Zn and NEt3 (**Figure 17A**). The reaction mechanism demonstrates the first step as a SET (single-electron transfer) in epoxide ring-opening, the second step as the formation of an elongated alkoxy radical via reaction between formed radical intermediate and added alkene, and the final step as a BET (back-electron transfer), which gave the desired expanded cyclic product via a zwitterionic intermediate cyclization (**Figure 17B**).

## **3.6 Polymerization**

*Organic Synthesis - A Nascent Relook*

**196**

summarized in **Table 3**.

**Figure 16.**

*respectively [27].*

**3.5 Cyclization reactions**

but also high chemo- and regioselectivity.

Various recently reported iron-NHC complex catalyzed reduction reactions are

*Hydrosilylative reductions of (A) benzaldehyde derivatives [25]; (B and C) substituted and primary amides,* 

Fe-NHC catalyzed ring expansion of the epoxides with functionalized alkenes presents a very intriguing case because cyclic structures are of great importance in various fields such as the pharmaceutical industry, fine chemicals, agriculture, etc. Fe-NHC catalyzed such reactions not only have shown functional group tolerance

So far, the application of Fe-NHC complexes have not been much explored in the area of polymerization [52]. Grubbs has first reported the use of Fe-NHC complex **19** as the catalyst in atom transfer radical polymerization (ATRP) reaction of styrene and methyl methacrylate (**Figure 18**) [20]. The reaction shows pseudo first-order kinetics, a decent control of radical concentration, and polydispersity index (PDI) near 1.1.

Shen and co-workers have reported the ring-opening polymerization (ROP) reaction of ε-caprolactone by using Fe-NHC complex **20** as the catalyst [22]. Even though reaction suffers some side reaction of transesterification, polymerization progresses with quantitative conversion and moderate number average molecular weight distribution (**Figure 19**).

#### **Table 3.** *Other examples of reduction reactions [53–60].*

#### **Figure 17.**

*(A) Fe-NHC catalyzed epoxide ring expansions [51]; (B) proposed mechanism.*

## **4. Conclusion**

Iron will remain a metal of choice for the replacement of all the heavy metal ions currently being used for the application of catalytic processes for the obvious reason

**199**

**Figure 19.**

**Figure 18.**

*Catalytic Activity of Iron N-Heterocyclic Carbene Complexes*

of it being economical, very high natural abundance, environmentally benign and more importantly biologically compatible. Earlier, several iron-based complexes have enjoyed their success in many processes like Fischer−Tropsch and the Haber− Bosch processes, but the progress of iron-NHC complexes has gained momentum only after the success of Grubb's catalyst at the onset of this century and now the number of published articles is growing with every passing year. The importance of Fe-NHC complexes can be evaluated from the aforementioned fact that they have found applicability in diverse fields from academia (e.g. biomimetic studies, various intriguing chemical transformations) to industries (e.g. pharmaceutical industry). The existing and ever possible versatility of (i) various structural motifs with different oxidation states, (ii) their flexible coordination geometries before and after the reaction, and (iii) substitution patterns in the iron N-heterocyclic carbene complexes along with their potential economic and toxicity benefits present an

Badri Nath Jha is grateful to the SERB-DST (Project No. YSS/000699/2015), India, for the financial support to carry out research in the area of catalysis and cathodic materials of LIBs. B.N. Jha is also thankful to Pradeep Mathur for his

exciting scenario for the upcoming generation.

*Ring-opening polymerization of ε-caprolactone [22].*

*Atom transfer radical polymerization (ATRP) of olefins [20].*

**Acknowledgements**

*DOI: http://dx.doi.org/10.5772/intechopen.90640*

*Catalytic Activity of Iron N-Heterocyclic Carbene Complexes DOI: http://dx.doi.org/10.5772/intechopen.90640*

#### **Figure 18.**

*Organic Synthesis - A Nascent Relook*

**198**

**4. Conclusion**

**Figure 17.**

**Table 3.**

*Other examples of reduction reactions [53–60].*

Iron will remain a metal of choice for the replacement of all the heavy metal ions currently being used for the application of catalytic processes for the obvious reason

*(A) Fe-NHC catalyzed epoxide ring expansions [51]; (B) proposed mechanism.*

*Atom transfer radical polymerization (ATRP) of olefins [20].*

**Figure 19.** *Ring-opening polymerization of ε-caprolactone [22].*

of it being economical, very high natural abundance, environmentally benign and more importantly biologically compatible. Earlier, several iron-based complexes have enjoyed their success in many processes like Fischer−Tropsch and the Haber− Bosch processes, but the progress of iron-NHC complexes has gained momentum only after the success of Grubb's catalyst at the onset of this century and now the number of published articles is growing with every passing year. The importance of Fe-NHC complexes can be evaluated from the aforementioned fact that they have found applicability in diverse fields from academia (e.g. biomimetic studies, various intriguing chemical transformations) to industries (e.g. pharmaceutical industry). The existing and ever possible versatility of (i) various structural motifs with different oxidation states, (ii) their flexible coordination geometries before and after the reaction, and (iii) substitution patterns in the iron N-heterocyclic carbene complexes along with their potential economic and toxicity benefits present an exciting scenario for the upcoming generation.

### **Acknowledgements**

Badri Nath Jha is grateful to the SERB-DST (Project No. YSS/000699/2015), India, for the financial support to carry out research in the area of catalysis and cathodic materials of LIBs. B.N. Jha is also thankful to Pradeep Mathur for his

continuous motivation to write book chapters/books and pursue research. Abhinav Raghuvanshi is thankful to the SERB for NPDF fellowship file no. PDF/2016/001786 for the financial support to carry out the research.

## **Conflict of interest**

Authors have no conflict of interests to declare.

## **Author details**

Badri Nath Jha1 \*, Nishant Singh1 and Abhinav Raghuvanshi<sup>2</sup>

1 University Department of Chemistry, T. M. Bhagalpur University, Bhagalpur, India

2 Department of Chemistry, Indian Institute of Technology Indore, Indore, India

\*Address all correspondence to: bnjha06@gmail.com

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**201**

cr940472u

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**References**

## **References**

*Organic Synthesis - A Nascent Relook*

**Conflict of interest**

for the financial support to carry out the research.

Authors have no conflict of interests to declare.

**200**

**Author details**

Badri Nath Jha1

India

\*, Nishant Singh1

\*Address all correspondence to: bnjha06@gmail.com

provided the original work is properly cited.

and Abhinav Raghuvanshi<sup>2</sup>

1 University Department of Chemistry, T. M. Bhagalpur University, Bhagalpur,

continuous motivation to write book chapters/books and pursue research. Abhinav Raghuvanshi is thankful to the SERB for NPDF fellowship file no. PDF/2016/001786

2 Department of Chemistry, Indian Institute of Technology Indore, Indore, India

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

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[22] Chen M-Z, Sun H-M, Li W-F, Wang Z-G, Shen Q, Zhang Y.

Synthesis, structure of functionalized N-heterocyclic carbene complexes of Fe (II) and their catalytic activity for ring-opening polymerization of ε-caprolactone. Journal of Organometallic Chemistry. 2006;**691**:2489-2494. DOI: 10.1016/j. jorganchem.2006.01.031

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[42] Hatakeyama T, Nakamura M. Iron-catalyzed selective biaryl coupling:Remarkable suppression of homocoupling by the fluoride anion. Journal of the American Chemical Society. 2007;**129**:9844-9845. DOI:

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978-3-527-61940-5

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978-8-177-58130-0

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978-3-527-32349-4

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Germany: Wiley-VCH; 1998. ISBN: 978-3-527-61940-5

*Organic Synthesis - A Nascent Relook*

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ccr.2006.10.004

Stereoelectronic parameters associated with N-heterocyclic carbene (NHC) ligands: A quest for understanding. Coordination Chemistry Reviews. 2007;**251**:874-883. DOI: 10.1016/j.

Synthesis, structure of functionalized N-heterocyclic carbene complexes of Fe (II) and their catalytic activity for ring-opening polymerization of ε-caprolactone. Journal of Organometallic Chemistry.

2006;**691**:2489-2494. DOI: 10.1016/j.

2010;**5**:1657-1666. DOI: 10.1002/

[24] Cramer SA, Jenkins DM. Synthesis of aziridines from alkenes and aryl azides with a reusable macrocyclic tetracarbene iron catalyst. Journal of the American Chemical Society. 2011;**133**:19342-19345. DOI: 10.1021/

[25] Kandepi VVKM, Cardoso JMS, Peris E, Royo B. Iron(II) complexes bearing chelating cyclopentadienyl-Nheterocyclic carbene ligands as catalysts for hydrosilylation and hydrogen transfer reactions. Organometallics. 2010;**29**:2777-2782. DOI: 10.1021/

[26] Jiang F, Bézier D, Sortais J-B, Darcel C. N-heterocyclic carbene piano-stool iron complexes as efficient catalysts for hydrosilylation of carbonyl derivatives. Advanced Synthesis and Catalysis. 2011;**353**:239-244. DOI:

[27] Bézier D, Venkanna GT, Sortais J-B, Darcel C. Well-defined cyclopentadienyl NHC iron complex as the catalyst for efficient hydrosilylation of amides to amines and nitriles. ChemCatChem. 2011;**3**:1747-1750. DOI: 10.1002/

[28] Beller M, Bolm C. Transition Metals for Organic Synthesis: Building Blocks and Fine Chemicals. Weinheim,

10.1002/adsc.201000781

cctc.201100202

asia.201000140

ja2090965

om100246j

[23] Hatanaka T, Ohki Y, Tatsumi K. C-H bond activation/borylation of furans and thiophenes catalyzed by a halfsandwich iron N-heterocyclic carbene complex. Chemistry - An Asian Journal.

jorganchem.2006.01.031

[16] Jacobsen H, Correa A, Poater A, Costabile C, Cavallo L. Understanding the M-(NHC) (NHC = N-heterocyclic carbene) bond. Coordination Chemistry Reviews. 2009;**253**:687-703. DOI:

[17] Crudden CM, Allen DP. Stability and reactivity of N-heterocyclic carbene complexes. Coordination Chemistry Reviews. 2004;**248**:2247-2273. DOI:

[18] Riener K, Haslinger S, Raba A, Högerl MP, Cokoja M, Herrmann WA, et al. Chemistry of iron N-heterocyclic carbene complexes: Syntheses, structures, reactivities, and catalytic applications. Chemical Reviews. 2014;**114**:5215-5272.

10.1016/j.ccr.2008.06.006

10.1016/j.ccr.2004.05.013

DOI: 10.1021/cr4006439

[19] Plietker B, Dieskau A,

Chemie, International Edition. 2008;**47**:198-201. DOI: 10.1002/

anie.200703874

DOI: 10.1039/B003957H

Möws K, Jatsch A. Ligand-Dependent mechanistic dichotomy in ironcatalyzed allylic substitutions: σ-allyl versus π-allyl mechanism. Angewandte

[20] Louie J, Grubbs RH. Highly active iron imidazolylidene catalysts for atom transfer radical polymerization. Chemical Communications. 2000;**16**:1479-1480.

[21] Mo Z, Zhang Q, Deng L. Dinuclear iron complex-catalyzed cross-coupling of primary alkyl fluorides with aryl Grignard reagents. Organometallics. 2012;**31**:6518-6521. DOI: 10.1021/

[22] Chen M-Z, Sun H-M, Li W-F, Wang Z-G, Shen Q, Zhang Y.

**202**

om300722g

[29] Hartwig JF. Organotransition Metal Chemistry: From Bonding to Catalysis. Mill Valley, CA: University Science Books; 2010. ISBN: 978-1-891-38953-5

[30] Crabtree RH. The Organometallic Chemistry of the Transition Metals. Hoboken, NJ: Wiley; 2011. ISBN: 978-0-470-25762-3

[31] Huheey JE, Keiter EA, Keiter RL, Medhi OK. Inorganic Chemistry: Principles of Structure and Reactivity. Upper Saddle River, NJ: Pearson Education; 2006. ISBN: 978-8-177-58130-0

[32] Lippard SJ, Berg JM. Principles of Bioinorganic Chemistry. Mill Valley, CA: University Science Books; 1994. ISBN: 0-935702-73-3

[33] Ochiai EI. Bioinorganic Chemistry: A Survey. Amsterdam: Elsevier Science/Academic Press; 2010. ISBN: 978-0-120-88756-9

[34] Beller M, Renken A, van Santen RA. Catalysis: From Principles to Applications. Weinheim, Germany: Wiley-VCH; 2012. ISBN: 978-3-527-32349-4

[35] Gopalaiah K. Chiral iron catalysts for asymmetric synthesis. Chemical Reviews. 2013;**113**:3248-3296. DOI: 10.1021/cr300236r

[36] Plietker B. Iron Catalysis in Organic Chemistry: Reactions and Applications. Weinheim, Germany: Wiley-VCH; 2008. ISBN: 978-3-527-31927-5

[37] Lavallo V, El-Batta A, Bertrand G, Grubbs RH. Insights into the carbeneinitiated aggregation of [Fe(cot)2]. Angewandte Chemie, International Edition. 2011;**50**:268-271. DOI: 10.1002/ anie.201005212

[38] Bedford RB, Betham M, Bruce DW, Danopoulos AA, Frost RM, Hird M. Iron-phosphine, -phosphite, -arsine, and -carbene catalysts for the coupling of primary and secondary alkyl halides with aryl grignard reagents. The Journal of Organic Chemistry. 2006;**71**:1104- 1110. DOI: 10.1021/jo052250+

[39] Nakamura M, Matsuo K, Ito S, Nakamura E. Iron-catalyzed crosscoupling of primary and secondary alkyl halides with aryl grignard reagents. Journal of the American Chemical Society. 2004;**126**:3686-3687. DOI: 10.1021/ja049744t

[40] Martin R, Fürstner A. Crosscoupling of alkyl halides with aryl Grignard reagents catalyzed by a low-valent iron complex. Angewandte Chemie, International Edition. 2004;**43**:3955-3957. DOI: 10.1002/ anie.200460504

[41] Silberstein AL, Ramgren SD, Garg NK. Iron-catalyzed alkylations of aryl sulfamates and carbamates. Organic Letters. 2012;**14**:3796-3799. DOI: 10.1021/ol301681z

[42] Hatakeyama T, Nakamura M. Iron-catalyzed selective biaryl coupling:Remarkable suppression of homocoupling by the fluoride anion. Journal of the American Chemical Society. 2007;**129**:9844-9845. DOI: 10.1021/ja073084l

[43] Guisán-Ceinos M, Tato F, Buñuel E, Calle P, Cárdenas D. Fe-catalysed Kumada-type alkyl-alkyl cross-coupling. Evidence for the intermediacy of Fe(I) complexes. Journal of Chemical Sciences. 2013;**4**:1098-1104. DOI: 10.1039/C2SC21754F

[44] Holzwarth M, Dieskau A, Tabassam M, Plietker B. The ammosamides: Structures of cell cycle modulators from a marine-derived *Streptomyces* species. Angewandte Chemie, International Edition.

2009;**48**:725-727. DOI: 10.1002/ anie.200804890

[45] Pottabathula S, Royo B. First iron-catalyzed guanylation of amines: A simple and highly efficient protocol to guanidines. Tetrahedron Letters. 2012;**53**:5156-5158. DOI: 10.1016/j. tetlet.2012.07.065

[46] Obligacion JV, Chirik P. Highly selective bis(imino)pyridine ironcatalyzed alkene hydroboration. Journal of Organic Letters. 2013;**15**:2680-2683. DOI: 10.1021/ol400990u

[47] Yamagami T, Shintani R, Shirakawa E, Hayashi T. Iron-catalyzed arylmagnesiation of aryl(alkyl) acetylenes in the presence of an N-heterocyclic carbene ligand. Organic Letters. 2007;**9**:1045-1048. DOI: 10.1021/ol063132r

[48] Holzwarth MS, Frey W, Plietker B. Binuclear Fe-complexes as catalysts for the ligand-free regioselective allylic sulfenylation. Chemical Communications. 2011;**47**:11113-11115. DOI: 10.1039/C1CC14599A

[49] Jegelka M, Plietker B. α-Sulfonyl succinimides: Versatile sulfinate donors in Fe-Catalyzed, salt-free, neutral allylic substitution. Chemistry - A European Journal. 2011;**17**:10417-10430. DOI: 10.1002/chem.201101047

[50] Jegelka M, Plietker B. Dual catalysis: Vinyl sulfones through tandem iron-catalyzed allylic sulfonation amine-catalyzed isomerization. ChemCatChem. 2012;**4**:329-332. DOI: 10.1002/cctc.201100465

[51] Hilt G, Bolze P, Kieltsch I. An ironcatalysed chemo- and regioselective tetrahydrofuran synthesis. Chemical Communications. 2005:1996-1998. DOI: 10.1039/B501100K

[52] Pintauer T, Matyjaszewski K. Atom transfer radical addition and polymerization reactions

catalyzed by PPM amounts of copper complexes. Chemical Society Reviews. 2008;**37**:1087-1097. DOI: 10.1039/ B714578K

[53] Bézier D, Jiang F, Roisnel T, Sortais J-B, Darcel C. Cyclopentadienyl-NHC iron complexes for solvent-free catalytic hydrosilylation of aldehydes and ketones. European Journal of Inorganic Chemistry. 2012;**2012**:1333-1337. DOI: 10.1002/ejic.201100762

[54] Grohmann C, Hashimoto T, Fröhlich R, Ohki Y, Tatsumi K, Glorius F. An Iron(II) complex of a diaminebridged bis-N-heterocyclic carbene. Organometallics. 2012;**31**:8047-8050. DOI: 10.1021/om300888q

[55] Warratz S, Postigo L, Royo B. Direct synthesis of Iron (0) N-heterocyclic carbene complexes by using Fe3(CO)12 and their application in reduction of carbonyl groups. Organometallics. 2013;**32**:893-897. DOI: 10.1021/ om3012085

[56] Bézier D, Venkanna GT, Misal Castro LC, Zheng J, Roisnel T, Sortais J-B, et al. Iron-catalyzed hydrosilylation of esters. Advanced Synthesis and Catalysis. 2012;**354**:1879-1884. DOI: 10.1002/adsc.201200087

[57] Demir S, Gökçe Y, Kaloğlu N, Sortais J-B, Darcel C, Özdemir İ. Synthesis of new Iron-NHC complexes as catalysts for hydrosilylation reactions. Applied Organometallic Chemistry. 2013;**27**:459- 464. DOI: 10.1002/aoc.3006

[58] Li H, Misal Castro LC, Zheng J, Roisnel T, Dorcet V, Sortais J-B, et al. Selective reduction of esters to aldehydes under the catalysis of well-defined NHC-Iron complexes. Angewandte Chemie, International Edition. 2013;**52**:8045-8049. DOI: 10.1002/anie.201303003

[59] Volkov A, Buitrago E, Adolfsson H. Direct hydrosilylation of tertiary amides

**205**

*Catalytic Activity of Iron N-Heterocyclic Carbene Complexes*

*DOI: http://dx.doi.org/10.5772/intechopen.90640*

to amines by an *in situ* formed Iron/N-heterocyclic carbene catalyst. European Journal of Organic Chemistry.

2013:2066-2070. DOI: 10.1002/

[60] Misal Castro LC, Sortais J-B,

2012;**48**:151-153. DOI: 10.1039/

cyclopentadienyl iron based catalyst for a general and efficient hydrosilylation of imines. Chemical Communications.

Darcel C. NHC-carbene

ejoc.201300010

C1CC14403K

*Catalytic Activity of Iron N-Heterocyclic Carbene Complexes DOI: http://dx.doi.org/10.5772/intechopen.90640*

to amines by an *in situ* formed Iron/N-heterocyclic carbene catalyst. European Journal of Organic Chemistry. 2013:2066-2070. DOI: 10.1002/ ejoc.201300010

*Organic Synthesis - A Nascent Relook*

2009;**48**:725-727. DOI: 10.1002/

catalyzed by PPM amounts of copper complexes. Chemical Society Reviews. 2008;**37**:1087-1097. DOI: 10.1039/

[53] Bézier D, Jiang F, Roisnel T, Sortais J-B, Darcel C. Cyclopentadienyl-NHC iron complexes for solvent-free catalytic

hydrosilylation of aldehydes and ketones. European Journal of Inorganic Chemistry. 2012;**2012**:1333-1337. DOI:

[54] Grohmann C, Hashimoto T,

Fröhlich R, Ohki Y, Tatsumi K, Glorius F. An Iron(II) complex of a diaminebridged bis-N-heterocyclic carbene. Organometallics. 2012;**31**:8047-8050.

[55] Warratz S, Postigo L, Royo B. Direct synthesis of Iron (0) N-heterocyclic carbene complexes by using Fe3(CO)12 and their application in reduction of carbonyl groups. Organometallics. 2013;**32**:893-897. DOI: 10.1021/

[56] Bézier D, Venkanna GT, Misal Castro LC, Zheng J, Roisnel T, Sortais J-B, et al. Iron-catalyzed hydrosilylation of esters. Advanced Synthesis and Catalysis. 2012;**354**:1879-1884. DOI:

[57] Demir S, Gökçe Y, Kaloğlu N, Sortais J-B, Darcel C, Özdemir İ. Synthesis of new Iron-NHC complexes as catalysts for hydrosilylation reactions. Applied Organometallic Chemistry. 2013;**27**:459-

10.1002/adsc.201200087

464. DOI: 10.1002/aoc.3006

10.1002/anie.201303003

[58] Li H, Misal Castro LC, Zheng J, Roisnel T, Dorcet V, Sortais J-B, et al. Selective reduction of esters to aldehydes under the catalysis of well-defined NHC-Iron complexes. Angewandte Chemie, International Edition. 2013;**52**:8045-8049. DOI:

[59] Volkov A, Buitrago E, Adolfsson H. Direct hydrosilylation of tertiary amides

10.1002/ejic.201100762

DOI: 10.1021/om300888q

om3012085

B714578K

[45] Pottabathula S, Royo B. First iron-catalyzed guanylation of amines: A simple and highly efficient protocol to guanidines. Tetrahedron Letters. 2012;**53**:5156-5158. DOI: 10.1016/j.

[46] Obligacion JV, Chirik P. Highly selective bis(imino)pyridine ironcatalyzed alkene hydroboration. Journal of Organic Letters. 2013;**15**:2680-2683.

anie.200804890

tetlet.2012.07.065

DOI: 10.1021/ol400990u

10.1021/ol063132r

[47] Yamagami T, Shintani R,

arylmagnesiation of aryl(alkyl) acetylenes in the presence of an

Shirakawa E, Hayashi T. Iron-catalyzed

N-heterocyclic carbene ligand. Organic Letters. 2007;**9**:1045-1048. DOI:

[48] Holzwarth MS, Frey W, Plietker B. Binuclear Fe-complexes as catalysts for the ligand-free regioselective allylic sulfenylation. Chemical

Communications. 2011;**47**:11113-11115.

[49] Jegelka M, Plietker B. α-Sulfonyl succinimides: Versatile sulfinate donors in Fe-Catalyzed, salt-free, neutral allylic substitution. Chemistry - A European Journal. 2011;**17**:10417-10430. DOI:

[50] Jegelka M, Plietker B. Dual catalysis:

[51] Hilt G, Bolze P, Kieltsch I. An ironcatalysed chemo- and regioselective tetrahydrofuran synthesis. Chemical Communications. 2005:1996-1998. DOI:

[52] Pintauer T, Matyjaszewski K. Atom transfer radical addition and polymerization reactions

DOI: 10.1039/C1CC14599A

10.1002/chem.201101047

10.1002/cctc.201100465

10.1039/B501100K

Vinyl sulfones through tandem iron-catalyzed allylic sulfonation amine-catalyzed isomerization. ChemCatChem. 2012;**4**:329-332. DOI:

**204**

[60] Misal Castro LC, Sortais J-B, Darcel C. NHC-carbene cyclopentadienyl iron based catalyst for a general and efficient hydrosilylation of imines. Chemical Communications. 2012;**48**:151-153. DOI: 10.1039/ C1CC14403K

**207**

**Chapter 10**

**Abstract**

**1. Introduction**

Torrefaction of Sunflower Seed:

Effect on Extracted Oil Quality

*Jamel Mejri, Youkabed Zarrouk and Majdi Hammami*

The aim of this work is to study the effect of heat treatment on the lipidic profile of sunflower seed oil. It determined and compared the contents of bioactive components in seed oils extracted with n-hexane (Soxhlet method) from raw and roasted sunflower. The influence of torrefaction on fatty acid composition, triglyceride composition, and peroxide value (PV) has been studied. Thermal oxidation assays were carried out, and samples were evaluated by measuring induction time. Oleic acid was the main unsaturated fatty acid. Concerning triglyceride composition, OOL + LnOO, OOO + PoPP, POP and OOO + PoPP, OOL + LnOO, POP were the main, respectively, for raw and roasted samples. The seed oil samples extracted from the roasted sample exhibited a higher peroxide value (213.68 meq.O2/kg) than the raw sample (5.79 meq.O2/kg). The acid values were, respectively, 3.24 and 1.81 mg of KOH/g of oil for roasted and raw samples. On the other hand, induction time for raw sample was higher (16.23 h) than the roasted sample one (2.67 h).

Lipids are major components of a man's diet. Their high quantities may be found in plant seeds distributed in many regions of the world. They can provide oils with a high concentration of monounsaturated fatty acids that prevent cardiovascular diseases by several mechanisms [1]. Several oleaginous seeds exist in the world. Some seeds are eaten as they are, such as sunflower seeds; others are used in the extraction of oil [2]. Sunflower (*Helianthus annuus* L.) is cultivated for its seeds' high oil content. Oil represents up to 80% of its economic value [3]. Abd EL-Satar et al. [4] concluded from their works that wider plant spacing and increasing nitrogen fertilization levels in addition to cultivars with high yield potential increase the plant's ability to take the needs of nutrients and solar radiation; this leads to an increase in photosynthesis, which reflected the increasing economic yield. Solvent extraction is one of the traditional techniques of extracting vegetable oil from oil seeds. Oil seeds are put in contact with a suitable solvent, in its pure form, for extracting the oil from the solid matrix to the liquid phase [5]. In many cases, chemical studies that employ a series of chemical compounds and/or sensory descriptors are used to characterize edible oil and fats [6]. In Tunisia roasted sunflower seeds, called "glibettes," are frequently consumed. Roasting enhances the organoleptic characteristics of seeds and gives them a taste and a pleasant smell. A huge number of papers on studies of different oils and fats are published every year. However, the effect of this heat treatment on the composition and nutritional qualities has not been studied.

**Keywords:** torrefaction, sunflower, seed oil, oxidation

## **Chapter 10**

## Torrefaction of Sunflower Seed: Effect on Extracted Oil Quality

*Jamel Mejri, Youkabed Zarrouk and Majdi Hammami*

## **Abstract**

The aim of this work is to study the effect of heat treatment on the lipidic profile of sunflower seed oil. It determined and compared the contents of bioactive components in seed oils extracted with n-hexane (Soxhlet method) from raw and roasted sunflower. The influence of torrefaction on fatty acid composition, triglyceride composition, and peroxide value (PV) has been studied. Thermal oxidation assays were carried out, and samples were evaluated by measuring induction time. Oleic acid was the main unsaturated fatty acid. Concerning triglyceride composition, OOL + LnOO, OOO + PoPP, POP and OOO + PoPP, OOL + LnOO, POP were the main, respectively, for raw and roasted samples. The seed oil samples extracted from the roasted sample exhibited a higher peroxide value (213.68 meq.O2/kg) than the raw sample (5.79 meq.O2/kg). The acid values were, respectively, 3.24 and 1.81 mg of KOH/g of oil for roasted and raw samples. On the other hand, induction time for raw sample was higher (16.23 h) than the roasted sample one (2.67 h).

**Keywords:** torrefaction, sunflower, seed oil, oxidation

## **1. Introduction**

Lipids are major components of a man's diet. Their high quantities may be found in plant seeds distributed in many regions of the world. They can provide oils with a high concentration of monounsaturated fatty acids that prevent cardiovascular diseases by several mechanisms [1]. Several oleaginous seeds exist in the world. Some seeds are eaten as they are, such as sunflower seeds; others are used in the extraction of oil [2]. Sunflower (*Helianthus annuus* L.) is cultivated for its seeds' high oil content. Oil represents up to 80% of its economic value [3]. Abd EL-Satar et al. [4] concluded from their works that wider plant spacing and increasing nitrogen fertilization levels in addition to cultivars with high yield potential increase the plant's ability to take the needs of nutrients and solar radiation; this leads to an increase in photosynthesis, which reflected the increasing economic yield. Solvent extraction is one of the traditional techniques of extracting vegetable oil from oil seeds. Oil seeds are put in contact with a suitable solvent, in its pure form, for extracting the oil from the solid matrix to the liquid phase [5]. In many cases, chemical studies that employ a series of chemical compounds and/or sensory descriptors are used to characterize edible oil and fats [6]. In Tunisia roasted sunflower seeds, called "glibettes," are frequently consumed. Roasting enhances the organoleptic characteristics of seeds and gives them a taste and a pleasant smell. A huge number of papers on studies of different oils and fats are published every year. However, the effect of this heat treatment on the composition and nutritional qualities has not been studied.

There is no published work. The main objective of this study was to determine the TG, total FA composition, peroxide value (PV), acid value, and oxidative stability of the sunflower seed oil before and after torrefying. This study can be used to understand the causes of certain diseases related to the consumption of oxidized fat.

## **2. Experimental**

## **2.1 Sunflower seed samples**

Sunflower seeds (*Helianthus annuus* L.) are grown in Beja region (latitude 36°43′32″; longitude 9°10′54″; elevation 248 m), located in the northwest of Tunisia. After harvesting the seeds are stored in a dry place at room temperature, protected from light. Then the seeds were roasted at an artisan (called Hammas). The temperature and processing time are, respectively, 180°C and 10 min. Sunflower seeds were placed in a bowl and covered with salted water. Thus, they will absorb some of the water and will not dry too much during cooking. Seeds were drained and salted water was emptied. The oven was preheated to about 180°C. The seeds were arranged in a thin layer on the plate for better cooking. Seeds were baked and broiled for about 10 min. Occasionally, seeds were stirred in order to grill them evenly. Seeds may develop a slight crack in the middle during torrefaction. The still hot seeds were cooled and stored in an airtight box.

## **2.2 Seed oil extraction**

The fat content was measured with a Soxhlet extractor apparatus with 250 ml of hexane at 60°C for 6 h, and then the solvent was removed by evaporation. The seed oil obtained was drained under a nitrogen stream (N2) and was then stored in a freezer at −20°C until analysis.

## **2.3 Fatty acid composition**

Fatty acid composition was determined by the analytical methods described by the European Parliament and the European Council in EEC regulation 2568/91 (1991) [7]. Fatty acids were converted to fatty acid ethyl esters (FAMEs) before being analyzed by shaking off a solution of 0.2 g of oil and 3 ml of hexane with 0.4 ml of 2 N methanolic potassium hydroxide. The FAMEs were then analyzed in a Hewlett-Packard model 4890D gas chromatograph furnished with an HP-INNOWAX-fused silica capillary column (cross-linked PEG), 30 m × 0.25 mm × 0.25 μm, and a flame ionization detector (FID). Inlet and detector temperatures were held at 230 and 250°C, respectively. The initial oven temperature was held at 120°C for 1 min, and then it was raised to 240°C at a rate of 4.0°C/min for 4 min. The FAME-injected volume was 1 μl, and nitrogen (N2) was used as the carrier gas at 1 ml/min with a split inlet flow system at a 1:100 split ratio. Next, heptadecanoic acid C17:0 was added as an internal standard before methylation in order to measure the amount of fatty acids. Eventually, fatty acid contents were calculated using a 4890A Hewlett-Packard integrator.

### **2.4 Triacylglycerol composition**

Triacylglycerol in different samples were determined according the International Olive Council [8]. The chromatographic separation of TAGs was

**209**

*Torrefaction of Sunflower Seed: Effect on Extracted Oil Quality*

performed using an Agilent 1100-reverse phase high-performance liquid chromatography (HPLC) system (Agilent Technologies, Waldbronn, Germany) equipped with an Inertsil ODS-C18 (5 μm, 4.5 × 250 mm) column. Elution was performed by using the mixture of acetonitrile/acetone (50:50, v/v) at a flow rate of 1 mL/min at 30°C. The working solutions of triacylglycerols (1%, w/v) were prepared in the elution mixture and injected into the column to determine their specific retention times. Identification of the peaks was carried out using a soybean oil chromatogram as reference. The mean of the data was calculated from three biological repeats

Official methods of the American Oil Chemists' Society [9] were used for the determination of the peroxide value (method Cd 8-53) and the acid value (method Cd 8-53). The oxidative stability of the oils was determined using a Rancimat 743 Metrohm apparatus (Metrohm Co., Basel, Switzerland). This instrument was used for automatic determination of the oxidation stability of oils and fats. The level of stabilization was measured by the oxidative-induction time using 3.5 ± 0.01 g samples of oils. The temperature was set at 100°C, the purified airflow passing through at a rate of 10 l/h. During the oxidation process, volatile acids were formed in the deionized water and were measured conductometrically [10]. Samples of oils were placed in the apparatus and analyzed simultaneously. The samples were placed at random. The induction times were recorded automatically by the apparatus'

The extraction yields are 43 and 52%, respectively, for raw and roasted seeds. Thus, we get a gain in yield of 9%. This gain is due to the roasting. Hydrolytic and proteolytic enzymes disrupt the structure of the cell and improve extraction yields. Oil yield depends on the cell disruption during the extraction process. Oil was located inside the cell. Various factors can influence the efficiency of the extraction process such as size of the solid particles, agitation, ratio of liquid/solid, extraction duration, pH, and temperature. Since the optimal temperature value coincides with the optimum protein degradation value, extraction of oil can be considered as a process aimed at degrading proteins which results in the release of the oil. However, the quality of the oil obtained depends on the operating conditions of extraction [12]. The yield extraction can be improved using other methods such as the Folch method. Hence, oils extracted using polar solvents such as a combination of chloroform and methanol may cause extraction of polar materials (phospholipids). In addition, neutral triacylglycerols can affect the oil yield extraction [1]. The effect of extraction time and temperature can also be significant for oil yield. However, several researchers have studied aqueous extraction of oil from sunflower. Evon et al. [3] have studied the feasibility of an aqueous process to extract sunflower seed oil using a corotating twin-screw extruder. The best oil extraction yield obtained was approximately 55%.

**Table 1** shows fatty acid composition of sunflower seed oil compared to those of literature. Oleic, linoleic, palmitic, and stearic acids were found as major fatty

*DOI: http://dx.doi.org/10.5772/intechopen.90645*

obtained from three independent experiments.

**2.5 Peroxide value, acid value, and thermal oxidation**

software and taken as the break point of the plotted curves [11].

**3. Results and discussions**

**3.2 Fatty acid composition**

**3.1 Yield oil**

#### *Torrefaction of Sunflower Seed: Effect on Extracted Oil Quality DOI: http://dx.doi.org/10.5772/intechopen.90645*

performed using an Agilent 1100-reverse phase high-performance liquid chromatography (HPLC) system (Agilent Technologies, Waldbronn, Germany) equipped with an Inertsil ODS-C18 (5 μm, 4.5 × 250 mm) column. Elution was performed by using the mixture of acetonitrile/acetone (50:50, v/v) at a flow rate of 1 mL/min at 30°C. The working solutions of triacylglycerols (1%, w/v) were prepared in the elution mixture and injected into the column to determine their specific retention times. Identification of the peaks was carried out using a soybean oil chromatogram as reference. The mean of the data was calculated from three biological repeats obtained from three independent experiments.

## **2.5 Peroxide value, acid value, and thermal oxidation**

Official methods of the American Oil Chemists' Society [9] were used for the determination of the peroxide value (method Cd 8-53) and the acid value (method Cd 8-53). The oxidative stability of the oils was determined using a Rancimat 743 Metrohm apparatus (Metrohm Co., Basel, Switzerland). This instrument was used for automatic determination of the oxidation stability of oils and fats. The level of stabilization was measured by the oxidative-induction time using 3.5 ± 0.01 g samples of oils. The temperature was set at 100°C, the purified airflow passing through at a rate of 10 l/h. During the oxidation process, volatile acids were formed in the deionized water and were measured conductometrically [10]. Samples of oils were placed in the apparatus and analyzed simultaneously. The samples were placed at random. The induction times were recorded automatically by the apparatus' software and taken as the break point of the plotted curves [11].

## **3. Results and discussions**

## **3.1 Yield oil**

*Organic Synthesis - A Nascent Relook*

fat.

**2. Experimental**

**2.1 Sunflower seed samples**

**2.2 Seed oil extraction**

freezer at −20°C until analysis.

a 4890A Hewlett-Packard integrator.

**2.4 Triacylglycerol composition**

**2.3 Fatty acid composition**

hot seeds were cooled and stored in an airtight box.

There is no published work. The main objective of this study was to determine the TG, total FA composition, peroxide value (PV), acid value, and oxidative stability of the sunflower seed oil before and after torrefying. This study can be used to understand the causes of certain diseases related to the consumption of oxidized

Sunflower seeds (*Helianthus annuus* L.) are grown in Beja region (latitude 36°43′32″; longitude 9°10′54″; elevation 248 m), located in the northwest of Tunisia. After harvesting the seeds are stored in a dry place at room temperature, protected from light. Then the seeds were roasted at an artisan (called Hammas). The temperature and processing time are, respectively, 180°C and 10 min. Sunflower seeds were placed in a bowl and covered with salted water. Thus, they will absorb some of the water and will not dry too much during cooking. Seeds were drained and salted water was emptied. The oven was preheated to about 180°C. The seeds were arranged in a thin layer on the plate for better cooking. Seeds were baked and broiled for about 10 min. Occasionally, seeds were stirred in order to grill them evenly. Seeds may develop a slight crack in the middle during torrefaction. The still

The fat content was measured with a Soxhlet extractor apparatus with 250 ml of hexane at 60°C for 6 h, and then the solvent was removed by evaporation. The seed oil obtained was drained under a nitrogen stream (N2) and was then stored in a

Fatty acid composition was determined by the analytical methods described by the European Parliament and the European Council in EEC regulation 2568/91 (1991) [7]. Fatty acids were converted to fatty acid ethyl esters (FAMEs) before being analyzed by shaking off a solution of 0.2 g of oil and 3 ml of hexane with 0.4 ml of 2 N methanolic potassium hydroxide. The FAMEs were then analyzed in a Hewlett-Packard model 4890D gas chromatograph furnished with an

HP-INNOWAX-fused silica capillary column (cross-linked PEG), 30 m × 0.25 mm × 0.25 μm, and a flame ionization detector (FID). Inlet and detector temperatures were held at 230 and 250°C, respectively. The initial oven temperature was held at 120°C for 1 min, and then it was raised to 240°C at a rate of 4.0°C/min for 4 min. The FAME-injected volume was 1 μl, and nitrogen (N2) was used as the carrier gas at 1 ml/min with a split inlet flow system at a 1:100 split ratio. Next, heptadecanoic acid C17:0 was added as an internal standard before methylation in order to measure the amount of fatty acids. Eventually, fatty acid contents were calculated using

Triacylglycerol in different samples were determined according the International Olive Council [8]. The chromatographic separation of TAGs was

**208**

The extraction yields are 43 and 52%, respectively, for raw and roasted seeds. Thus, we get a gain in yield of 9%. This gain is due to the roasting. Hydrolytic and proteolytic enzymes disrupt the structure of the cell and improve extraction yields. Oil yield depends on the cell disruption during the extraction process. Oil was located inside the cell. Various factors can influence the efficiency of the extraction process such as size of the solid particles, agitation, ratio of liquid/solid, extraction duration, pH, and temperature. Since the optimal temperature value coincides with the optimum protein degradation value, extraction of oil can be considered as a process aimed at degrading proteins which results in the release of the oil. However, the quality of the oil obtained depends on the operating conditions of extraction [12]. The yield extraction can be improved using other methods such as the Folch method. Hence, oils extracted using polar solvents such as a combination of chloroform and methanol may cause extraction of polar materials (phospholipids). In addition, neutral triacylglycerols can affect the oil yield extraction [1]. The effect of extraction time and temperature can also be significant for oil yield. However, several researchers have studied aqueous extraction of oil from sunflower. Evon et al. [3] have studied the feasibility of an aqueous process to extract sunflower seed oil using a corotating twin-screw extruder. The best oil extraction yield obtained was approximately 55%.

## **3.2 Fatty acid composition**

**Table 1** shows fatty acid composition of sunflower seed oil compared to those of literature. Oleic, linoleic, palmitic, and stearic acids were found as major fatty

acids of sunflower seed oils. Their contents are 46.64, 38.11, 8.81, and 5.48%, respectively, for the raw sunflower seed. According to the work of [14], this composition depends on the environmental conditions during grain filling. The main environmental factors driving oil fatty acid composition are temperature and solar radiation. For oil quality purposes, oleic and linoleic are the most important fatty acids because they constitute almost 85% of the total fatty acids in sunflower oil. Sunflower fatty acid composition has been modified by breeding and mutagenesis parameters for minimum and maximum oleic acid percentage [15]. The roasted sunflower seed fatty acid contents were found to be 44.91, 36.95, 9.13, and 7.26%, respectively, for oleic, linoleic, palmitic, and stearic acids. Linoleic acid is the fatty acid most susceptible to degradation in sunflower oils [16]. The high amount of linoleic acid present in sunflower seed oil can make it more susceptible to oxidation and consequently cause higher cytotoxicity due to the production of free radicals. Diminution of unsaturated fatty acid was detected, caused by thermal treatment. Two news fatty acids appear: arachidic (0.91%) and behenic acid (0.83%). These fatty acids were detected in sunflower seeds in low amount [12]. They were 0.23 and 1.35%, respectively, for arachidic and behenic acid. Authors confirmed that the amount of arachidic and behenic acid were, respectively, 0.33 and 0.52% [17].

Sunflower seed oil is very nutritional because of its oleic acid content. The oleic acid content is varied: 46.64% in our study, 85.8% in [12], and 24.86% in [13]. It showed that fatty acid composition is highly variable [16, 18]. The palmitic acid, oleic acid, and linoleic acid contents ranged, respectively, from 5.3 to 27.9%, 31.6 to 84%, and 2.4 to 56.8%. Sunflower seed oil was fully liquid at the ambient temperature, as it is very rich in monounsaturated (oleic) and polyunsaturated (linoleic) fatty acids. Sunflower seed oil gives better functional properties such as good spreadability at refrigeration temperatures because of its high content of PUFA [19].


**211**

**Table 2.**

*Triacylglycerol composition of sunflower seed oil.*

*Torrefaction of Sunflower Seed: Effect on Extracted Oil Quality*

The compositions of triglycerides (TGs) expressed as the equivalent carbon number (ECN) found in sunflower seed oil samples are reported in **Table 2**. The main triglycerides found in the sunflower seed oil samples analyzed were OOL + LnOO, OOO + PoPP, POP and OOO + PoPP, OOL + LnOO, POP, respectively, for raw and roasted samples. These accounted for more than 62 and 66% of the total area of peaks in the chromatogram, respectively, for raw and roasted

The level of OOL + LnOO, OOO + PoPP, the main TG in sunflower seed oil samples, was remarkably high, with a concentration of 25.90, 24.50 and 21.30, and 26.90%, respectively, for raw and roasted samples. The OOL + LnOO content of raw sunflower seed oil is greater than that in the roasted sample. However, the OOO + PoPP content is lower in the raw sunflower seed oil one. The next three TG fractions are POP, OOLn + PLL, and SOL with contents of 11.91, 10.80, and 10.34%

Peroxide value is a measure of the concentration of peroxides and hydroperoxides formed in the initial stages of lipid oxidation. Peroxide value is one of the most widely used tests for the measurement of oxidative rancidity in oils and fats [20]. The quality parameters of a crude oil included (i) the acid value, expressed in mg of KOH/g of oil, which is an indication of the free fatty acid content of the oil, and (ii) the peroxide value, expressed in terms of meq.O2/kg of oil [21]. The results of peroxide value, acid value, and Rancimat test are shown in **Table 3**. Peroxide value increases considerably from 5.79 to 213.68 meq.O2/kg, respectively, for raw and roasted oil samples. This is due to the high linoleic acid content, which is the fatty acid most susceptible to degradation in sunflower oils. Thermal oxidation assays of

**TAG ECN Raw Roasted** LLL ECN 42 0.30 0.98 PoLL + OLLn + PoOLn ECN 42 0.28 0.27 PLLn ECN 42 0.51 0 OLL + PoOL ECN 44 0.15 0.17 OOLn + PLL ECN 44 10.80 7.00 PPLn + PPoPo ECN 44 0.20 0 OOL + LnOO ECN 46 25.90 21.30 PoOO ECN 46 5.00 4.12 OOO + PoPP ECN 48 24.50 26.90 SOL ECN 48 10.34 9.62 POO ECN 48 0.64 0.67 POP ECN 50 11.91 18.17 SOO ECN 50 4.21 3.00 POS + SLS ECN 50 4.26 7.77

*P, palmitic; Po, palmitoleic; S, stearic; O, oleic; L, linoleic; Ln, linolenic; and A, arachidic acids.*

and 18.17, 7, and 9.62%, respectively, for raw and roasted samples.

**3.4 Peroxide value, acid value, and thermal oxidation**

*DOI: http://dx.doi.org/10.5772/intechopen.90645*

**3.3 Triglyceride composition**

samples.

*SFA, saturated fatty acid; MUFA, monounsaturated fatty acid; PUFA, polyunsaturated fatty acid. Bold entries are to express the sum.*

#### **Table 1.**

*Fatty acid composition of sunflower seed oil.*

## **3.3 Triglyceride composition**

*Organic Synthesis - A Nascent Relook*

acids of sunflower seed oils. Their contents are 46.64, 38.11, 8.81, and 5.48%, respectively, for the raw sunflower seed. According to the work of [14], this composition depends on the environmental conditions during grain filling. The main environmental factors driving oil fatty acid composition are temperature and solar radiation. For oil quality purposes, oleic and linoleic are the most important fatty acids because they constitute almost 85% of the total fatty acids in sunflower oil. Sunflower fatty acid composition has been modified by breeding and mutagenesis parameters for minimum and maximum oleic acid percentage [15]. The roasted sunflower seed fatty acid contents were found to be 44.91, 36.95, 9.13, and 7.26%, respectively, for oleic, linoleic, palmitic, and stearic acids. Linoleic acid is the fatty acid most susceptible to degradation in sunflower oils [16]. The high amount of linoleic acid present in sunflower seed oil can make it more susceptible to oxidation and consequently cause higher cytotoxicity due to the production of free radicals. Diminution of unsaturated fatty acid was detected, caused by thermal treatment. Two news fatty acids appear: arachidic (0.91%) and behenic acid (0.83%). These fatty acids were detected in sunflower seeds in low amount [12]. They were 0.23 and 1.35%, respectively, for arachidic and behenic acid. Authors confirmed that the amount of arachidic and behenic acid were, respectively, 0.33 and 0.52% [17].

Sunflower seed oil is very nutritional because of its oleic acid content. The oleic acid content is varied: 46.64% in our study, 85.8% in [12], and 24.86% in [13]. It showed that fatty acid composition is highly variable [16, 18]. The palmitic acid, oleic acid, and linoleic acid contents ranged, respectively, from 5.3 to 27.9%, 31.6 to 84%, and 2.4 to 56.8%. Sunflower seed oil was fully liquid at the ambient temperature, as it is very rich in monounsaturated (oleic) and polyunsaturated (linoleic) fatty acids. Sunflower seed oil gives better functional properties such as good spreadability at refrigeration temperatures because of its high content of PUFA [19].

Myristic acid C14:0 0.05 — Palmitic acid C16:0 8.81 9.13 3.48 0.068 Palmitoleic acid C16:1 0.45 — — 6.12 Stearic acid C18:0 5.48 7.26 3.65 3.41 Oleic acid C18:1 46.64 44.91 85.8 24.86 Linoleic acid C18:2 38.11 36.95 4.96 63.18 Linolenic acid C18:3 0.51 — — 0.082 Arachidic acid C20:0 — 0.91 0.23 — Behenic acid C22:0 — 0.83 1.46 — Lignoceric acid C24:0 — — 0.30 — **SFA 14.29 18.13 9.17 3.478 MUFA 47.09 44.91 85.80 30.98 PUFA 38.60 36.65 4.96 63.262 PUFA/SFA 2.70 2.03 0.54 18.18**

*SFA, saturated fatty acid; MUFA, monounsaturated fatty acid; PUFA, polyunsaturated fatty acid.*

**Fatty acid content (%) Symbol Raw Roasted**

**The present study [12] [13]**

**210**

**Table 1.**

*Bold entries are to express the sum.*

*Fatty acid composition of sunflower seed oil.*

The compositions of triglycerides (TGs) expressed as the equivalent carbon number (ECN) found in sunflower seed oil samples are reported in **Table 2**. The main triglycerides found in the sunflower seed oil samples analyzed were OOL + LnOO, OOO + PoPP, POP and OOO + PoPP, OOL + LnOO, POP, respectively, for raw and roasted samples. These accounted for more than 62 and 66% of the total area of peaks in the chromatogram, respectively, for raw and roasted samples.

The level of OOL + LnOO, OOO + PoPP, the main TG in sunflower seed oil samples, was remarkably high, with a concentration of 25.90, 24.50 and 21.30, and 26.90%, respectively, for raw and roasted samples. The OOL + LnOO content of raw sunflower seed oil is greater than that in the roasted sample. However, the OOO + PoPP content is lower in the raw sunflower seed oil one. The next three TG fractions are POP, OOLn + PLL, and SOL with contents of 11.91, 10.80, and 10.34% and 18.17, 7, and 9.62%, respectively, for raw and roasted samples.

## **3.4 Peroxide value, acid value, and thermal oxidation**

Peroxide value is a measure of the concentration of peroxides and hydroperoxides formed in the initial stages of lipid oxidation. Peroxide value is one of the most widely used tests for the measurement of oxidative rancidity in oils and fats [20]. The quality parameters of a crude oil included (i) the acid value, expressed in mg of KOH/g of oil, which is an indication of the free fatty acid content of the oil, and (ii) the peroxide value, expressed in terms of meq.O2/kg of oil [21]. The results of peroxide value, acid value, and Rancimat test are shown in **Table 3**. Peroxide value increases considerably from 5.79 to 213.68 meq.O2/kg, respectively, for raw and roasted oil samples. This is due to the high linoleic acid content, which is the fatty acid most susceptible to degradation in sunflower oils. Thermal oxidation assays of


#### **Table 2.**

*Triacylglycerol composition of sunflower seed oil.*


#### **Table 3.**

*Peroxide value and oxidative stability of sunflower seed oil.*

sunflower seed oil were carried out. The new compounds formed were evaluated [16]. Results showed that the levels of all the new compounds analyzed strongly depended on the degree of oil unsaturation and unsaturated oils with low content of linoleic acid, and high content of palmitic acid behaved exceptionally well. The linoleic acid is most susceptible to polymerization. The saturated fatty acids show a great importance in delaying oil polymerization [16].

The acid value (AV) expresses the extent of hydrolytic changes in the sunflower oils. The acid values were 1.81 mg of KOH/g of oil for the raw sample and 3.24 mg of KOH/g of oil for the roasted one. This increase of acid value indicates that TG hydrolysis occurred during the heat treatment. However, it can be consider that the operating conditions did not change oil quality significantly. The acid value remained stable at less than 3.5 mg of KOH/g of oil. The characteristic of crude sunflower oil based on specification from the American Fats and Oils Associations shall be pure with free fatty acid of 3% maximum or acid value below 6 mg of KOH/g of oil [21]. It showed that the feedstock sunflower oils possessed high free fatty acid [22]. Hydrolysis reactions of triglyceride with enzymatic and chemical pathways produce the free fatty acid (FFA). FFA is one of the important quality parameters. The formation of free fatty acid chain due to hydrolysis may lead to sensorial characterization [23]. The stability of sunflower seed oil expressed as the oxidation induction time was about 2.67 and 16.23 h, respectively, for raw and roasted seeds. This value may be justified by the high contents of MUFA and PUFA [24, 25]. Induction time values were quite different according to the oil composition (degradation), in proportion to the heat treatment. A high oxidation stability (33–45 h) of date seed oil measured by Rancimat was justified by the relatively low content of PUFA and the high content of natural antioxidants, such as phenolic compounds. Authors indicated that the species containing linoleic acid were oxidized more rapidly than those containing oleic acid [24, 26]. TAG polymers are the most characteristic compounds formed at high temperature, their rate of formation being dependent on the content of polyunsaturated fatty acids [27].

## **4. Conclusion**

From the results and discussion of the study conducted, it can be concluded that the operating condition of torrefaction had an important influence on the oil extraction yield and the quality of oil extracted. Higher oil extraction yield was reached with increased temperature (torrefaction). The oil extraction yield of 52% was obtained under operating conditions of 180°C and 10 min. However, torrefaction process produced oil of bad quality. Changes of fatty acid composition, triglyceride composition peroxide value, acid value, and oxidative stability were observed. During torrefaction process oxide species were produced under the effect of high temperature. Thus, we can understand some diseases appeared to the customer of roasted sunflower seed (glibettes).

**213**

**Author details**

(INRAT), Tunisia

Hammam-Lif, Tunisia

\*, Youkabed Zarrouk<sup>2</sup>

provided the original work is properly cited.

and Majdi Hammami3

1 Higher School of Engineers of Medjez el Bab (ESIM), University of Jendouba,

3 Aromatic and Medicinal Plants Laboratory, Biotechnology Center of Borj-Cedria

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

2 Field Crops Laboratory, National Agronomic Research Institute of Tunisia

\*Address all correspondence to: jamel.mejri.faq@gmail.com

Jamel Mejri1

Tunisia

*Torrefaction of Sunflower Seed: Effect on Extracted Oil Quality*

*DOI: http://dx.doi.org/10.5772/intechopen.90645*

*Torrefaction of Sunflower Seed: Effect on Extracted Oil Quality DOI: http://dx.doi.org/10.5772/intechopen.90645*

*Organic Synthesis - A Nascent Relook*

**Table 3.**

sunflower seed oil were carried out. The new compounds formed were evaluated [16]. Results showed that the levels of all the new compounds analyzed strongly depended on the degree of oil unsaturation and unsaturated oils with low content of linoleic acid, and high content of palmitic acid behaved exceptionally well. The linoleic acid is most susceptible to polymerization. The saturated fatty acids show a

**Sample Peroxide value (meq.O2/kg) Acid value (mg of KOH/g of oil) Induction time (h)** Raw 5.79 1.81 16.23 Roasted 213.68 3.24 2.67

The acid value (AV) expresses the extent of hydrolytic changes in the sunflower oils. The acid values were 1.81 mg of KOH/g of oil for the raw sample and 3.24 mg of KOH/g of oil for the roasted one. This increase of acid value indicates that TG hydrolysis occurred during the heat treatment. However, it can be consider that the operating conditions did not change oil quality significantly. The acid value remained stable at less than 3.5 mg of KOH/g of oil. The characteristic of crude sunflower oil based on specification from the American Fats and Oils Associations shall be pure with free fatty acid of 3% maximum or acid value below 6 mg of KOH/g of oil [21]. It showed that the feedstock sunflower oils possessed high free fatty acid [22]. Hydrolysis reactions of triglyceride with enzymatic and chemical pathways produce the free fatty acid (FFA). FFA is one of the important quality parameters. The formation of free fatty acid chain due to hydrolysis may lead to sensorial characterization [23]. The stability of sunflower seed oil expressed as the oxidation induction time was about 2.67 and 16.23 h, respectively, for raw and roasted seeds. This value may be justified by the high contents of MUFA and PUFA [24, 25]. Induction time values were quite different according to the oil composition (degradation), in proportion to the heat treatment. A high oxidation stability (33–45 h) of date seed oil measured by Rancimat was justified by the relatively low content of PUFA and the high content of natural antioxidants, such as phenolic compounds. Authors indicated that the species containing linoleic acid were oxidized more rapidly than those containing oleic acid [24, 26]. TAG polymers are the most characteristic compounds formed at high temperature, their rate of formation being dependent on the content of polyunsaturated fatty

From the results and discussion of the study conducted, it can be concluded that the operating condition of torrefaction had an important influence on the oil extraction yield and the quality of oil extracted. Higher oil extraction yield was reached with increased temperature (torrefaction). The oil extraction yield of 52% was obtained under operating conditions of 180°C and 10 min. However, torrefaction process produced oil of bad quality. Changes of fatty acid composition, triglyceride composition peroxide value, acid value, and oxidative stability were observed. During torrefaction process oxide species were produced under the effect of high temperature. Thus, we can understand some diseases appeared to the customer of

great importance in delaying oil polymerization [16].

*Peroxide value and oxidative stability of sunflower seed oil.*

**212**

acids [27].

**4. Conclusion**

roasted sunflower seed (glibettes).

## **Author details**

Jamel Mejri1 \*, Youkabed Zarrouk<sup>2</sup> and Majdi Hammami3

1 Higher School of Engineers of Medjez el Bab (ESIM), University of Jendouba, Tunisia

2 Field Crops Laboratory, National Agronomic Research Institute of Tunisia (INRAT), Tunisia

3 Aromatic and Medicinal Plants Laboratory, Biotechnology Center of Borj-Cedria Hammam-Lif, Tunisia

\*Address all correspondence to: jamel.mejri.faq@gmail.com

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Ruiz-Mendez MV, Mancha M. Chemical and physical properties of a sunflower oil with high levels of oleic and palmitic acids. European Journal of Lipid Science and Technology. 2003;**105**:130-137

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sunflower seeds during extraction with hexane. Journal of Food Engineering.

[19] Noor Lida HMD, Sundram K, Siew WL, Aminah A, Mamot S. TAG composition and solid fat content of palm oil, sunflower oil, and palm kernel olein blends before and after chemical interesterification. JAOCS.

[20] Zhang Y, Yang L, Zu Y, Chen X, Wang F, Liu F. Oxidative stability of sunflower oil supplemented with carnosic acid compared with synthetic antioxidants during accelerated storage. Food Chemistry. 2010;**118**:656-662

[21] Amalia Kartika I, Pontalieb PY, Rigal L. Twin-screw extruder for oil processing of sunflower seeds: Thermomechanical pressing and solvent extraction in a single step. Industrial Crops and Products. 2010;**32**:297-304

[22] Saydut A, Erdogan S, Kafadar AB, Kaya C, Aydin F, Hamamci C. Process optimization for production of biodiesel from hazelnut oil, sunflower oil and their hybrid feedstock. Fuel.

2016;**3**:733-739

2011;**105**:180-185

2002;**79**:1137-1144

*Torrefaction of Sunflower Seed: Effect on Extracted Oil Quality DOI: http://dx.doi.org/10.5772/intechopen.90645*

changing environment. Field Crops Research. 2017;**202**:146-157

[16] Marmesat S, Morales A, Velasco J, Dobarganes MC. Influence of fatty acid composition on chemical changes in blends of sunflower oils during thermoxidation and frying. Food Chemistry. 2012;**135**:2333-2339

[17] de Mello Silva Oliveiraa N, Resendea MR, Moralesa DA, de ragão Umbuzeiroa G, Boriollo MFG. In vitro mutagenicity assay (Ames test) and phytochemical characterization of seeds oil of *Helianthus annuus* Linné (sunflower). Toxicology Reports. 2016;**3**:733-739

[18] Perez EE, Carelli AA, Crapiste GH. Temperature-dependent diffusion coefficient of oil from different sunflower seeds during extraction with hexane. Journal of Food Engineering. 2011;**105**:180-185

[19] Noor Lida HMD, Sundram K, Siew WL, Aminah A, Mamot S. TAG composition and solid fat content of palm oil, sunflower oil, and palm kernel olein blends before and after chemical interesterification. JAOCS. 2002;**79**:1137-1144

[20] Zhang Y, Yang L, Zu Y, Chen X, Wang F, Liu F. Oxidative stability of sunflower oil supplemented with carnosic acid compared with synthetic antioxidants during accelerated storage. Food Chemistry. 2010;**118**:656-662

[21] Amalia Kartika I, Pontalieb PY, Rigal L. Twin-screw extruder for oil processing of sunflower seeds: Thermomechanical pressing and solvent extraction in a single step. Industrial Crops and Products. 2010;**32**:297-304

[22] Saydut A, Erdogan S, Kafadar AB, Kaya C, Aydin F, Hamamci C. Process optimization for production of biodiesel from hazelnut oil, sunflower oil and their hybrid feedstock. Fuel. 2016;**183**:512-517

[23] Öğütcü M, Yılmaz E. Influence of different antioxidants and pack materials on oxidative stability of cold pressed poppy seed oil. La rivista italiana delle sostanze grasse. 2017;**XCIV**:45-52

[24] Besbes S, Blecker C, Deroanne C, Drira NE, Attia H. Date seeds: Chemical composition and characteristic profiles of the lipid fraction. Food Chemistry. 2004;**84**:577-584

[25] Rezig L, Chouaibi M, Msaada K, Hamdi S. Chemical composition and profile characterization of pumpkin (*Cucurbita maxima*) seed oil. Industrial Crops and Products. 2012;**37**:82-87

[26] Guinda Á, Dobarganes MC, Ruiz-Mendez MV, Mancha M. Chemical and physical properties of a sunflower oil with high levels of oleic and palmitic acids. European Journal of Lipid Science and Technology. 2003;**105**:130-137

[27] Dobarganes MC. Formation and analysis of high molecular-weight compounds in frying fats and oil. OCL. 1998;**5**:41-47

**214**

*Organic Synthesis - A Nascent Relook*

[1] Kozłowska M, Gruczynska E, Scibisz I, Rudzinska M. Fatty acids and sterols composition and

2016;**213**:450-456

**References**

2007;**26**:351-359

2017;**4**:241-252

2014;**116**:794-802

antioxidant activity of oils extracted from plant seeds. Food Chemistry.

[8] Aued-Pimentel S, Takemoto E, Kumagai EE, Cano CB. Calculation of the difference between the actual and theoretical ECN 42 triacylglyceride content to detect adulteration in olive oil samples commercialized in Brazil.

[9] AOCS, editor. Official Methods and Recommended Practices of the American Oil Chemist's Society. 5th ed. Champaign, USA: AOCS Press; 1997

[10] Halbault L, Barbé C, Aroztegui M, De La Torre C. Oxidative stability of semisolid excipient mixtures with corn oil and its implication in the degradation of vitamin A. International Journal of Pharmaceutics. 1997;**147**:31-41

[11] Symoniuk E, Ratusz K, Krygier K. Kinetics parameters of refined and coldpressed rapeseed oils after oxidation by Rancimat. Italian Journal of Food

[12] Amalia Kartika I, Pontalier PY, Rigal L. Extraction of sunflower oil by twin screw extruder: Screw configuration and operating condition effects. Bioresource Technology.

[13] Suria R, Azwani ML, Siti AH. Changes on the solid fat content of palm oil/sunflower oil blends via interesterification. Malaysian Journal of Analytical Sciences.

[14] Echarte MM, Puntel LA, Aguirrezabal LAN. Assessment of the critical period for the effect of intercepted solar radiation on sunflower oil fatty acid composition. Field Crops

Research. 2013;**149**:213-222

Pereyra Irujo G, Izquierdo N,

[15] Angeloni P, Mercedes Echarte M,

Aguirrezábal L. Fatty acid composition of high oleic sunflower hybrids in a

Science. 2017;**29**:276-287

2006;**97**:2302-2310

2013;**17**:164-170

2008;**31**:31-34

[2] Bilgic E, Yaman S, Haykiri-Acma H, Kucukbayrak S. Limits of variations on the structure and the fuel

characteristics of sunflower seed shell through torrefaction. Fuel Processing

Pontalier PY, Rigal L. Direct extraction of oil from sunflower seeds by twinscrew extruder according to an aqueous extraction process: Feasibility study and influence of operating condition. Industrial Crops and Products.

[4] Abd EL-Satar MA, Abd-EL-Halime Ahmed A, Ali Hassan TH. Response of seed yield and fatty acid compositions for some sunflower genotypes to plant spacing and nitrogen fertilization.

[5] Dutta R, Sarkar U, Mukherjee A. Extraction of oil from crotalaria Juncea seeds in a modified Soxhlet apparatus: Physical and chemical characterization

of a prospective bio-fuel. Fuel.

[6] Aranda F, Gomez-Alonso S, Rivera Del Alamo RM, Salvador MD, Fregapane G. Triglyceride, total and 2-position fatty acid composition of Cornicabra virgin olive oil: Comparison with other Spanish cultivars. Food Chemistry. 2004;**86**:485-492

[7] EEC. Characteristics of olive and olive pomace oils and their analytical methods. Regulation EEC/2568/1991. Official Journal of the European Communities. 1991;**L248**:1-82

Technology. 2016;**144**:197-202

[3] Evon P, Vandenbossche V,

**217**

**Chapter 11**

**Abstract**

Therapeutic Significance of

*Tangali Ramanaik Ravikumar Naik*

different spectroscopic techniques.

analysis, ZnO nanoparticle

**1. Introduction**

1,4-Dihydropyridine Compounds

A series of 1,4-dihydropyridines have been prepared from a three-component one-pot condensation reaction of β-diketonates, an aromatic aldehyde, and ammonium acetate under microwave irradiation. The reaction is performed using crystalline nano-ZnO in ethanol under microwave irradiation (CEM discover). A wide range of functional groups was tolerated in the developed protocol. The present methodology offers several advantages such as simple procedure, greener condition, excellent yields and short reaction time. The synthesized compounds were evaluated for DNA photocleavage, SAR analysis and molecular docking studies. The compound (**4b, 4c, 4 h, 4i, 4n** and **4o**) showed potent DNA cleavage activities compared to other derivatives. The molecular interactions of the active compounds within the binding site of B-DNA were studied through molecular docking simulations; the compound (**4b, 4c, 4 h, 4i, 4n** and **4o**) showed good docking interaction with minimum binding energies. All synthetic compounds were characterized by

**Keywords:** 1,4-Dihydropyridines, DNA photocleavage, molecular docking, SAR

Facile and efficient synthesis of biological active molecules is one of the main objectives of organic and medicinal chemistry. In recent years, multicomponent reactions have become one of the important tools in the synthesis of structurally diverse chemical libraries of drug-like polyfunctional organic molecules [1–4]. Furthermore, MCRs offer the advantage of simplicity and synthetic efficiency over conventional chemical reactions in several aspects. MCRs allow the construction of combinatorial libraries of complex organic molecules for an efficient lead structure

In continuation of our ongoing research work on microwave assisted synthesis of nano materials [11, 12] we have found that, nano-crystalline metal oxides have attracted considerable attention of synthetic and medicinal chemists because of their high catalytic activity and reusability [13–25]. Zinc oxide is an inexpensive, moisture stable, reusable, commercially available and is non-toxic, insoluble in polar as well as non-polar solvents [26–31]. A wide range of organic reactions that include Beckmann rearrangements [32], N-benzylation [33], acylation [34], dehydration of oximes [35], nucleophilic ring opening reactions of epoxides [36],

identification and optimization in drug discovery [5–10].

as Potential Anticancer Agents

## **Chapter 11**
