**2.1. Mechanism of zirconocene catalysis in alkene hydroalumination**

The catalytic alkene hydroalumination has found wide application as an efficient regio‐ and stereoselective method for the double and triple bond reduction providing important syn‐ thons for organic and organometallic chemistry [4, 13–15]. Various transition metal complexes can be used as the catalysts of the reaction, however, the compounds based on the metals with no vacant d orbital show much less activity in the reaction (e.g., Cu, Zn vs. Ti, Zr, Co, Ni) [16–21]. Moreover, the significant effect of the OAC nature and ligand structure on the hydrometalation product yield has been shown [22, 23].

Studies on the catalytic activity of the systems L<sup>2</sup> ZrCl<sup>2</sup> ‐XAlBui 2 (L = C<sup>5</sup> H5 , C<sup>5</sup> H4 Me, Ind, C<sup>5</sup> Me5 ; L2 = *rac‐*Mе<sup>2</sup> С(2‐Me‐4‐But ‐C<sup>5</sup> H2 )2 , *meso‐*Mе<sup>2</sup> С(2‐Me‐4‐But ‐C<sup>5</sup> H2 )2 , *rac‐*Mе<sup>2</sup> С(3‐But ‐C<sup>5</sup> H3 )2 , *rac‐*Me2 C(Ind)<sup>2</sup> , *rac‐*Me2 Si(Ind)<sup>2</sup> and *rac‐*C<sup>2</sup> H4 (Ind)<sup>2</sup> ); X = H, Cl, Bui ) in the alkene hydroalu‐ mination [22, 23] showed that the most active catalytic systems are those based on the Zr complexes with sterically hindered ligands in combination with HAlBui 2 (**Figure 1**). Catalysts with less bulky ligands are most active in the reaction of alkenes with AlBui 3 or ClAlBui 2 .

The reaction mechanism (e.g., see [16–21]) implies the generation of transition metal hydride LnMH formed upon σ‐ligand exchange; then this species coordinates alkene to give an alkyl derivative. In the last step, as a result of the transmetalation of alkyl fragment from M to Al, the organoaluminum product is formed and the transition metal hydride is recovered (**Scheme 2**).

Furthermore, a large number of various bimetallic hydride complexes were identified in reac‐ tions of metal chlorides, hydrides and alkyl derivatives with OAC (see, e.g., reviews [24, 25]) that gave an idea on the involvement of such a type of complexes as key intermediates in the hydrometalation reaction. The structural types of the hydride Zr, Al‐complexes, which could

‐ligand and OAC structure on octane yield in the reaction of 1‐octene hydroalumination

+ HAlBui 2

 + HAlBui 2 (▲).

, *t* = 20°С). (a) L = Cp; (b) L = Ind; (c) L = CpMe<sup>5</sup>

Alkene and Olefin Functionalization by Organoaluminum Compounds, Catalyzed...

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47

(♦), *rac‐*Me2

(●), *rac‐*C<sup>2</sup>

H4 (Ind)<sup>2</sup> ZrCl<sup>2</sup>

> ‐C<sup>5</sup> H2 )2 ZrCl<sup>2</sup>

C(2‐Me‐4‐But

; (d) *rac‐*Me2

 + AlBui 3 (□),

 + HAlBui 2 (▼),

C(2‐

**Figure 1.** The effect of η<sup>5</sup>

C(Ind)<sup>2</sup>

ZrCl<sup>2</sup>

ZrCl<sup>2</sup>

:АОС:alkene 1:60:50, C<sup>6</sup>

(\*), *meso‐*Me2

**Scheme 2.** Generalized scheme of catalytic alkene hydrometalation.

(■), *rac‐*Me2

(►), *rac‐*Me2

 + HAlBui 2

 + HAlBui 2

 + AlBui 3 H6

Si(Ind)<sup>2</sup>

C(2‐Me‐4‐But

C(3‐But ‐C<sup>5</sup> H3 )2 ZrCl<sup>2</sup>

ZrCl<sup>2</sup>

‐C<sup>5</sup> H2 )2 ZrCl<sup>2</sup>

 +HAlBui 2

(molar ratio L<sup>2</sup>

Me‐4‐But ‐C<sup>5</sup> H2 )2 ZrCl<sup>2</sup>

*rac‐*Me2

*rac‐*C<sup>2</sup> H4 (Ind)<sup>2</sup> ZrCl<sup>2</sup>

Alkene and Olefin Functionalization by Organoaluminum Compounds, Catalyzed... http://dx.doi.org/10.5772/intechopen.69319 47

**Figure 1.** The effect of η<sup>5</sup> ‐ligand and OAC structure on octane yield in the reaction of 1‐octene hydroalumination (molar ratio L<sup>2</sup> ZrCl<sup>2</sup> :АОС:alkene 1:60:50, C<sup>6</sup> H6 , *t* = 20°С). (a) L = Cp; (b) L = Ind; (c) L = CpMe<sup>5</sup> ; (d) *rac‐*Me2 C(2‐ Me‐4‐But ‐C<sup>5</sup> H2 )2 ZrCl<sup>2</sup> + HAlBui 2 (■), *rac‐*Me2 C(3‐But ‐C<sup>5</sup> H3 )2 ZrCl<sup>2</sup> + HAlBui 2 (●), *rac‐*C<sup>2</sup> H4 (Ind)<sup>2</sup> ZrCl<sup>2</sup> + HAlBui 2 (▼), *rac‐*Me2 C(Ind)<sup>2</sup> ZrCl<sup>2</sup> + HAlBui 2 (►), *rac‐*Me2 Si(Ind)<sup>2</sup> ZrCl<sup>2</sup> +HAlBui 2 (♦), *rac‐*Me2 C(2‐Me‐4‐But ‐C<sup>5</sup> H2 )2 ZrCl<sup>2</sup> + AlBui 3 (□), *rac‐*C<sup>2</sup> H4 (Ind)<sup>2</sup> ZrCl<sup>2</sup> + AlBui 3 (\*), *meso‐*Me2 C(2‐Me‐4‐But ‐C<sup>5</sup> H2 )2 ZrCl<sup>2</sup> + HAlBui 2 (▲).

**Scheme 2.** Generalized scheme of catalytic alkene hydrometalation.

based on zirconocenes due to several reasons. First, a broad range of catalytic reactions can be implemented in these systems, from hydro‐, carbo‐ and cyclometalation to polymerization of unsaturated compounds. Second, these systems are convenient for fundamental investi‐

electronic state of the transition metal atom and reflecting the molecule symmetry. Third, the reaction times and intermediate lifetimes appear to be convenient for nuclear magnetic resonance (NMR) monitoring, which is the most informative method for the studies of homo‐ geneous catalytic reactions. Moreover, the systems are substantially free of paramagnetic species, which, for example, in the case of titanium complexes, preclude observation of the

Thus, the chapter presents the results on the experimental and theoretical studies of the mechanisms of alkene hydro‐, carbo‐ and cyclometalation by organoaluminum compounds

intermediate reactivity and, consequently, the activity of the catalytic systems, reaction pathway and enantioselectivity are considered. The prospects of the development of stere‐ oselective methods using these catalytic systems for the alkene and olefin transformations

The catalytic alkene hydroalumination has found wide application as an efficient regio‐ and stereoselective method for the double and triple bond reduction providing important syn‐ thons for organic and organometallic chemistry [4, 13–15]. Various transition metal complexes can be used as the catalysts of the reaction, however, the compounds based on the metals with no vacant d orbital show much less activity in the reaction (e.g., Cu, Zn vs. Ti, Zr, Co, Ni) [16–21]. Moreover, the significant effect of the OAC nature and ligand structure on the

ZrCl<sup>2</sup>

С(2‐Me‐4‐But

‐XAlBui 2 (L = C<sup>5</sup> H5 , C<sup>5</sup> H4

); X = H, Cl, Bui

‐C<sup>5</sup> H2 )2

, *rac‐*Mе<sup>2</sup>

**2. Mechanisms of alkene functionalization, catalyzed by zirconium** 

, *meso‐*Mе<sup>2</sup>

Zr complexes with sterically hindered ligands in combination with HAlBui

H4 (Ind)<sup>2</sup>

Catalysts with less bulky ligands are most active in the reaction of alkenes with AlBui

mination [22, 23] showed that the most active catalytic systems are those based on the

The reaction mechanism (e.g., see [16–21]) implies the generation of transition metal hydride LnMH formed upon σ‐ligand exchange; then this species coordinates alkene to give an alkyl derivative. In the last step, as a result of the transmetalation of alkyl fragment from M to Al, the organoaluminum product is formed and the transition metal hydride is recovered (**Scheme 2**).

and *rac‐*C<sup>2</sup>

genesis of intermediates due to pronounced NMR signal broadening.

), catalyzed with zirconium η<sup>5</sup>

**2.1. Mechanism of zirconocene catalysis in alkene hydroalumination**

hydrometalation product yield has been shown [22, 23].

‐C<sup>5</sup> H2 )2

Si(Ind)<sup>2</sup>

Studies on the catalytic activity of the systems L<sup>2</sup>

С(2‐Me‐4‐But

, *rac‐*Me2

‐ligands bound to zirconium atoms act like magnetic probes indicating the

‐complexes. The factors that determine the

Me, Ind, C<sup>5</sup>

С(3‐But

) in the alkene hydroalu‐

2

Me5 ;

‐C<sup>5</sup> H3 )2 ,

(**Figure 1**).

3 or

gations, since η<sup>5</sup>

46 Alkenes

(AlR<sup>3</sup>

**η5**

L2

*rac‐*Me2

ClAlBui 2 .

= *rac‐*Mе<sup>2</sup>

C(Ind)<sup>2</sup>

are discussed.

**‐complexes**

and XAlBui

2

Furthermore, a large number of various bimetallic hydride complexes were identified in reac‐ tions of metal chlorides, hydrides and alkyl derivatives with OAC (see, e.g., reviews [24, 25]) that gave an idea on the involvement of such a type of complexes as key intermediates in the hydrometalation reaction. The structural types of the hydride Zr, Al‐complexes, which could be observed in the reactions of zirconocenes with aluminum hydrides or alkylaluminums, are presented in **Scheme 3**.

Our studies on the olefin hydroalumination by XAlBui 2 (X = H, Cl, Bui ), catalyzed with Zr η<sup>5</sup> ‐complexes, using the quantum chemical methods [31, 32], chemical kinetics [33] and NMR [22, 23], showed that the reaction is a complex multi‐step process (**Scheme 4**). The use of zirconocenes with less electron‐donating and sterically hindered ligands provides the stable Zr, Al‐hydride clusters L<sup>2</sup> Zr(μ‐H)<sup>3</sup> (AlBui 2 ) 2 (μ‐Cl) (**6**) (L = Cp, CpMe, Ind; L<sup>2</sup> = *rac‐*Me2 C(Ind)<sup>2</sup> , *rac‐* Me2 Si(Ind)<sup>2</sup> , *rac‐*C<sup>2</sup> H4 (Ind)<sup>2</sup> ), L<sup>2</sup> Zr(μ‐H)<sup>3</sup> (AlBui 2 ) 3 (μ‐Cl)(μ‐H) (**7**), L<sup>2</sup> Zr(μ‐H)<sup>3</sup> (AlBui 2 ) 3 (μ‐Cl)<sup>2</sup> (**8**) (L = Cp, CpMe), which tend to form bridging Zr─H─Al bonds, and, hence, these complexes have low activity in the reaction with alkene. Moreover, intra‐ and intermolecular exchange between the hydride atoms in these clusters and [HAlBui 2 ] <sup>n</sup> oligomers were found. Thus, the complexes exist in equilibrium with each other and HAlBui 2 self‐associates, while the intermolecular exchange involves the OAC monomer and occurs via dissociation of bimetallic complexes

(**Figure 2**). Increasing of the [HAlBui

ZrCl<sup>2</sup>

which give the intermediates Cp<sup>2</sup>

catalytic system toward alkene [23].

**Figure 2.** EXSY spectra of (a) HAlBui

‐toluene at 275 K (*τ* = 0.3 s); (c) system (CpMe<sup>5</sup>

d8

complexes with bulky ligands (L = CpMe<sup>5</sup>

with AlBui

**Scheme 4.** Mechanism of alkene hydroalumination by XAlBui

3

2 in С<sup>6</sup> D6

cross‐peaks of the same phase demonstrate the existence of chemical exchange.

Reaction of Cp<sup>2</sup>

2 ] *n*

hampered due to their competing intermolecular exchange with OAC oligomers.

Zr(μ‐H)<sup>3</sup>

sites with free Zr─H bond, and this is responsible for the high activity of the Cp<sup>2</sup>

High yields of hydroalumination products in the reactions of alkenes with HAlBui

are caused by the formation of Zr, Al‐bimetallic active sites (**4**) containing a [L<sup>2</sup>

)2 ZrCl<sup>2</sup>

tions, shifts the equilibrium toward low active large clusters into which the alkene insertion is

2

(X = H, Cl, Bui

(AlBui 2 )(AlBui 3

The absence of fast exchange between these hydride clusters increases the lifetime of the active

, *rac‐*Me2

with a free Zr‐H bond, in which the steric hindrance in the ligand prevents the formation of

─HAlBui 2 (1:26) in d<sup>8</sup>

C(2‐Me‐4‐But

(3.3 mol/L, 300 K, *τ* = 0.3 s); (b) system Ind<sup>2</sup>

concentration, that is, realization the catalytic condi‐

), catalyzed with Zr η<sup>5</sup>

goes via alkyl chloride exchange and isobutylene elimination,

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49

) and Cp<sup>2</sup>

‐C<sup>5</sup> H2 )2

Zr(μ‐H)<sup>3</sup>

, *rac‐*Me2

2

ZrCl<sup>2</sup>

‐toluene at 265 K (*τ* = 0.3 s). Diagonal and

─HAlBui 2 (1:12) in

(AlBui 2 )2 (μ‐Cl).

‐complexes.

C(3‐But

ZrH<sup>3</sup>

ZrCl<sup>2</sup>

, catalyzed by Zr

‐C<sup>5</sup> H3 ) 2 )

‐AlBui 3

] moiety

**R= Bui , L=Me2Si(2-Me3Si-4-But C5H2)2, Me2Si(3-But C5H3)2**

**Scheme 3.** Structural types of some hydride Zr, Al‐complexes [22, 23, 26–30].

Alkene and Olefin Functionalization by Organoaluminum Compounds, Catalyzed... http://dx.doi.org/10.5772/intechopen.69319 49

be observed in the reactions of zirconocenes with aluminum hydrides or alkylaluminums, are

= Cp, CpMe), which tend to form bridging Zr─H─Al bonds, and, hence, these complexes have low activity in the reaction with alkene. Moreover, intra‐ and intermolecular exchange between

> 2 ]

exchange involves the OAC monomer and occurs via dissociation of bimetallic complexes

2

‐complexes, using the quantum chemical methods [31, 32], chemical kinetics [33] and NMR [22, 23], showed that the reaction is a complex multi‐step process (**Scheme 4**). The use of zirconocenes with less electron‐donating and sterically hindered ligands provides the stable Zr,

2

(μ‐Cl) (**6**) (L = Cp, CpMe, Ind; L<sup>2</sup>

(μ‐Cl)(μ‐H) (**7**), L<sup>2</sup>

(X = H, Cl, Bui

Zr(μ‐H)<sup>3</sup>

<sup>n</sup> oligomers were found. Thus, the complexes

self‐associates, while the intermolecular

), catalyzed with

C(Ind)<sup>2</sup>

, *rac‐*

(**8**) (L

[26,22,29]

[28]

**L=Cp; X=H, R= AlBui**

**L=Cp**

**X=Cl, R= Et, Bui**

**X=R= Me, Et, Bui**

**;**

= *rac‐*Me2

**XAlR2**

(AlBui 2 ) 3 (μ‐Cl)<sup>2</sup>

presented in **Scheme 3**.

Al‐hydride clusters L<sup>2</sup>

[28,23,30]

**, L=Me2Si(2-Me3Si-4-But**

**L=Cp, BunC5H4, 1,2-Me2C5H3, Me3SiC5H4;**

**, L= Cp, CpMe, Ind; L2=** *rac-***Me2C(Ind)2,**  *rac-***Me2Si(Ind)2,** *rac-***C2H4(Ind)2**

> **R= Bui , Oct n**

**R= Bui**

**R= Bui**

, *rac‐*C<sup>2</sup> H4 (Ind)<sup>2</sup> ), L<sup>2</sup>

Si(Ind)<sup>2</sup>

Zr η<sup>5</sup>

48 Alkenes

Me2

Our studies on the olefin hydroalumination by XAlBui

(AlBui 2 ) 2

Zr(μ‐H)<sup>3</sup>

(AlBui 2 ) 3

Zr(μ‐H)<sup>3</sup>

the hydride atoms in these clusters and [HAlBui

exist in equilibrium with each other and HAlBui

**HAlR2**

[27,28,23] [23]

**R= Bui**

[28]

**C5H2)2, Me2Si(3-But**

**Scheme 3.** Structural types of some hydride Zr, Al‐complexes [22, 23, 26–30].

**C5H3)2**

[28]

**L= Me2Si(Ind)2, C2H4(Ind)2, C2H4(THInd)2, Me2C(C5H4)2, Me2Si(C5H4)2, etc.**

**, L=Cp, CpMe**

**Scheme 4.** Mechanism of alkene hydroalumination by XAlBui 2 (X = H, Cl, Bui ), catalyzed with Zr η<sup>5</sup> ‐complexes.

(**Figure 2**). Increasing of the [HAlBui 2 ] *n* concentration, that is, realization the catalytic condi‐ tions, shifts the equilibrium toward low active large clusters into which the alkene insertion is hampered due to their competing intermolecular exchange with OAC oligomers.

Reaction of Cp<sup>2</sup> ZrCl<sup>2</sup> with AlBui 3 goes via alkyl chloride exchange and isobutylene elimination, which give the intermediates Cp<sup>2</sup> Zr(μ‐H)<sup>3</sup> (AlBui 2 )(AlBui 3 ) and Cp<sup>2</sup> Zr(μ‐H)<sup>3</sup> (AlBui 2 )2 (μ‐Cl). The absence of fast exchange between these hydride clusters increases the lifetime of the active sites with free Zr─H bond, and this is responsible for the high activity of the Cp<sup>2</sup> ZrCl<sup>2</sup> ‐AlBui 3 catalytic system toward alkene [23].

High yields of hydroalumination products in the reactions of alkenes with HAlBui 2 , catalyzed by Zr complexes with bulky ligands (L = CpMe<sup>5</sup> , *rac‐*Me2 C(2‐Me‐4‐But ‐C<sup>5</sup> H2 )2 , *rac‐*Me2 C(3‐But ‐C<sup>5</sup> H3 ) 2 ) are caused by the formation of Zr, Al‐bimetallic active sites (**4**) containing a [L<sup>2</sup> ZrH<sup>3</sup> ] moiety with a free Zr‐H bond, in which the steric hindrance in the ligand prevents the formation of

**Figure 2.** EXSY spectra of (a) HAlBui 2 in С<sup>6</sup> D6 (3.3 mol/L, 300 K, *τ* = 0.3 s); (b) system Ind<sup>2</sup> ZrCl<sup>2</sup> ─HAlBui 2 (1:12) in d8 ‐toluene at 275 K (*τ* = 0.3 s); (c) system (CpMe<sup>5</sup> )2 ZrCl<sup>2</sup> ─HAlBui 2 (1:26) in d<sup>8</sup> ‐toluene at 265 K (*τ* = 0.3 s). Diagonal and cross‐peaks of the same phase demonstrate the existence of chemical exchange.

low‐active intermediates. The *meso*‐isomer of the sterically hindered cyclopentadienyl com‐ plex Me<sup>2</sup> C(2‐Me‐4‐But ‐C<sup>5</sup> H2 ) 2 ZrCl<sup>2</sup> gives the intermediate with the shielded free Zr─H bond, which makes the catalytic system inactive [23].

Thus, the L<sup>2</sup> ZrCl<sup>2</sup> ‐XAlBui 2 systems provide Zr, Al‐hydride complexes with Zr─H*─*Zr and Zr─H─Al‐bridged bonds in which intra‐ and intermolecular hydride exchange between Zr and Al, controlled by the steric factor of the η<sup>5</sup> ‐ligand, OAC nature and by the reaction condi‐ tions (reactant ratio), plays the key role in the catalytic process. The energy of cleavage of these bridging bonds and the ability of the complex to have initially a free Zr─H bond are the fac‐ tors determining the activity of Zr, Al‐hydride intermediates in the alkene hydroalumination.

### **2.2. Mechanisms of zirconocene catalysis in alkene carbo‐ and cycloalumination**

Catalytic alkene and acetylene carbo‐ and cycloalumination are convenient one‐pot synthetic routes to the acyclic and cyclic OACs that could be converted into alcohols, halides, hetero‐ cycles, carbocycles and others [2–11]. The using of enantiomerically pure complexes as the catalysts affords the asymmetric induction in the reactions. Thus, the method of Zr‐catalyzed asymmetric carboalumination of alkenes‐ZACA‐reaction has been developed [7–11, 34], which was applied to the synthesis of a number of biologically active compounds. The involvement of methylaluminoxane (MAO) [35–38] or other Lewis acidic cocatalysts [39] substantially increases the activity of the catalytic systems providing alkene dimers, oligomers and polymers.

due to the generation of highly active cationic species, which are formed as a result of the ionic

= 60:50:1, reaction time 24 h, 22°C).

**Table 2.** Effect of catalyst structure and solvent on the product yields in the reaction of hexene‐1 with AlMe<sup>3</sup>

Further transformations of the neutral alkyl bimetallic complexes via α‐C‐H (Ti) or β‐C‐H

has been observed by the means of dynamic 2D NMR spectroscopy [58, 59] (**Figure 3a**). Moreover, the exchange between the magnetically nonequivalent hydrogens, which belong

(**Figure 3b**). This dynamic picture could be explained by the intermolecular exchange

intermediate that is responsible for the cycloalumination pathway [50, 56, 57].

**conversion, %**

The Me‐group exchange between Zr and Al atoms in the complexes L<sup>2</sup>

**conversion, %**

**Table 3.** Effect of catalyst structure and solvent on the product yields in the reaction of hexene‐1 with AlEt<sup>3</sup>

= 60:50:1, reaction time 24 h, 22°C).

ZrCH<sup>2</sup>

**Product yield, %**

Cl<sup>2</sup> 92 3 14 7 68

CH<sup>3</sup> 69 3 21 7 38

Cl<sup>2</sup> 84 11 14 7 52

CH<sup>3</sup> 39 9 9 9 12

Cl<sup>2</sup> 68 53 8 7 ‐

CH<sup>3</sup> 44 15 14 14 1

Cl<sup>2</sup> 87 28 18 8 33

CH<sup>3</sup> 70 38 14 10 8

‐Al, M‐CH<sup>2</sup>

SiInd<sup>2</sup>

**Product yield, %**

Cl<sup>2</sup> 96 16 16 13 <1 51

Cl<sup>2</sup> 98 16 9 10 – 62

Cl<sup>2</sup> 99 48 13 10 7 21

Cl<sup>2</sup> 93 36 3 7 2 45

H6 91 24 2 2 – 63

H6 97 6 10 12 – 69

H6 96 15 2 5 – 74

H6 99 25 – <1 <1 74

(μ‐Cl)AlEt<sup>2</sup>

**15 16 17 18 19**

**15 16 17 18**

http://dx.doi.org/10.5772/intechopen.69319

Alkene and Olefin Functionalization by Organoaluminum Compounds, Catalyzed...

CH<sup>2</sup>

CHR‐Al or M‐CH<sup>2</sup>

ZrMe(μ‐Cl)AlMe<sup>3</sup>

CH<sup>2</sup> ‐M

, catalyzed

51

, was found

, catalyzed by

was found to be the

ZrMe(μ‐Cl)AlMe<sup>3</sup>

pair dissociation [39, 54, 55].

(mole ratio AlMe<sup>3</sup>

**L2**

Cp<sup>2</sup>

(CpMe)<sup>2</sup>

(CpMe<sup>5</sup> )2

Ind2

by L<sup>2</sup> ZrCl<sup>2</sup>

**L2**

Cp<sup>2</sup>

(CpMe)<sup>2</sup>

(CpMe<sup>5</sup> )2

Ind2

L2 ZrCl<sup>2</sup>

ZrCl<sup>2</sup> CH<sup>2</sup>

ZrCl<sup>2</sup> CH<sup>2</sup>

ZrCl<sup>2</sup> CH<sup>2</sup>

ZrCl<sup>2</sup> CH<sup>2</sup>

(Zr) activation gives the stable structures with M‐CH<sup>2</sup>

:alkene:L<sup>2</sup>

ZrCl<sup>2</sup>

to the opposite parts of *ansa*‐ligand in the complex Me<sup>2</sup>

bridges. Five‐membered bimetallic complex L<sup>2</sup>

**ZrCl2 Solvent Hexene‐1** 

C6

C6

C6

C6

:alkene:L<sup>2</sup>

ZrCl<sup>2</sup>

ZrCl<sup>2</sup> CH<sup>2</sup>

ZrCl<sup>2</sup> CH<sup>2</sup>

ZrCl<sup>2</sup> CH<sup>2</sup>

ZrCl<sup>2</sup> CH<sup>2</sup>

(mole ratio AlEt<sup>3</sup>

**ZrCl2 Solvent Hexene‐1** 

C6 H5

C6 H5

C6 H5

C6 H5

Summarizing the information on the study of the reaction of alkenes with alkylaluminums (R = Me, Et) in the presence of metalcomplexes [40–46], it should be noted that the process can give various products depending on the reagent nature, catalyst structure and reaction conditions (**Scheme 5**): saturated and unsaturated alkylated products (**15** and **16**), hydrometa‐ lation products (**17**), dimers (**18**) and cyclic OACs (**19**). As shown in **Tables 2** and **3** [46], the use of chlorinated solvent altogether with Zr catalysts, which contain bulky ligand (CpMe<sup>5</sup> ), increases the yield of carbometalation products **15**. The dimers **18** predominate in the reaction of alkenes with AlMe<sup>3</sup> , catalyzed by zirconocenes with Cp and CpMe ligands. The maximal yields of cyclic OAC **19** were observed in the reaction that runs in hydrocarbon solvent and in the presence of Zr complexes substituted with Cp, CpMe, CpMe<sup>5</sup> and Ind ligands.

Obviously, the reaction pathways are determined by the catalytically active sites of a definite type. Thus, bimetallic Zr, Al‐alkyl complexes L<sup>2</sup> ZrR(μ‐Cl)AlRnCl3‐*<sup>n</sup>* in the reaction of zircono‐ cenes with alkylaluminums were found [47–52] and the complexes were suggested as key intermediates of olefin β‐alkylation (**Scheme 6**). Using of strong Lewis acids, for example, MAO or perfluoroaryl boranes enhances the catalytic system productivity by several orders

**Scheme 5.** Reaction of alkenes with AlR<sup>3</sup> (R = Me, Et) in the presence of metal complexes.


low‐active intermediates. The *meso*‐isomer of the sterically hindered cyclopentadienyl com‐

Zr─H─Al‐bridged bonds in which intra‐ and intermolecular hydride exchange between Zr

tions (reactant ratio), plays the key role in the catalytic process. The energy of cleavage of these bridging bonds and the ability of the complex to have initially a free Zr─H bond are the fac‐ tors determining the activity of Zr, Al‐hydride intermediates in the alkene hydroalumination.

Catalytic alkene and acetylene carbo‐ and cycloalumination are convenient one‐pot synthetic routes to the acyclic and cyclic OACs that could be converted into alcohols, halides, hetero‐ cycles, carbocycles and others [2–11]. The using of enantiomerically pure complexes as the catalysts affords the asymmetric induction in the reactions. Thus, the method of Zr‐catalyzed asymmetric carboalumination of alkenes‐ZACA‐reaction has been developed [7–11, 34], which was applied to the synthesis of a number of biologically active compounds. The involvement of methylaluminoxane (MAO) [35–38] or other Lewis acidic cocatalysts [39] substantially increases

**2.2. Mechanisms of zirconocene catalysis in alkene carbo‐ and cycloalumination**

the activity of the catalytic systems providing alkene dimers, oligomers and polymers.

Summarizing the information on the study of the reaction of alkenes with alkylaluminums (R = Me, Et) in the presence of metalcomplexes [40–46], it should be noted that the process can give various products depending on the reagent nature, catalyst structure and reaction conditions (**Scheme 5**): saturated and unsaturated alkylated products (**15** and **16**), hydrometa‐ lation products (**17**), dimers (**18**) and cyclic OACs (**19**). As shown in **Tables 2** and **3** [46], the use of chlorinated solvent altogether with Zr catalysts, which contain bulky ligand (CpMe<sup>5</sup>

increases the yield of carbometalation products **15**. The dimers **18** predominate in the reaction

yields of cyclic OAC **19** were observed in the reaction that runs in hydrocarbon solvent and in

Obviously, the reaction pathways are determined by the catalytically active sites of a definite

cenes with alkylaluminums were found [47–52] and the complexes were suggested as key intermediates of olefin β‐alkylation (**Scheme 6**). Using of strong Lewis acids, for example, MAO or perfluoroaryl boranes enhances the catalytic system productivity by several orders

**15 16 17**

(R = Me, Et) in the presence of metal complexes.

the presence of Zr complexes substituted with Cp, CpMe, CpMe<sup>5</sup>

type. Thus, bimetallic Zr, Al‐alkyl complexes L<sup>2</sup>

, catalyzed by zirconocenes with Cp and CpMe ligands. The maximal

ZrR(μ‐Cl)AlRnCl3‐*<sup>n</sup>*

and Ind ligands.

**18**

in the reaction of zircono‐

observed in the reaction with AlEt3

**19**

gives the intermediate with the shielded free Zr─H bond,

‐ligand, OAC nature and by the reaction condi‐

),

systems provide Zr, Al‐hydride complexes with Zr─H*─*Zr and

plex Me<sup>2</sup>

50 Alkenes

Thus, the L<sup>2</sup>

C(2‐Me‐4‐But

ZrCl<sup>2</sup>

of alkenes with AlMe<sup>3</sup>

[M], AlR'3 (XnAlR'3-n) M= Ti, Zr, Hf R'= Me, Et; X= Cl

**Scheme 5.** Reaction of alkenes with AlR<sup>3</sup>

‐C<sup>5</sup> H2 ) 2 ZrCl<sup>2</sup>

which makes the catalytic system inactive [23].

‐XAlBui 2

and Al, controlled by the steric factor of the η<sup>5</sup>

**Table 2.** Effect of catalyst structure and solvent on the product yields in the reaction of hexene‐1 with AlMe<sup>3</sup> , catalyzed by L<sup>2</sup> ZrCl<sup>2</sup> (mole ratio AlMe<sup>3</sup> :alkene:L<sup>2</sup> ZrCl<sup>2</sup> = 60:50:1, reaction time 24 h, 22°C).

due to the generation of highly active cationic species, which are formed as a result of the ionic pair dissociation [39, 54, 55].

Further transformations of the neutral alkyl bimetallic complexes via α‐C‐H (Ti) or β‐C‐H (Zr) activation gives the stable structures with M‐CH<sup>2</sup> ‐Al, M‐CH<sup>2</sup> CHR‐Al or M‐CH<sup>2</sup> CH<sup>2</sup> ‐M bridges. Five‐membered bimetallic complex L<sup>2</sup> ZrCH<sup>2</sup> CH<sup>2</sup> (μ‐Cl)AlEt<sup>2</sup> was found to be the intermediate that is responsible for the cycloalumination pathway [50, 56, 57].

The Me‐group exchange between Zr and Al atoms in the complexes L<sup>2</sup> ZrMe(μ‐Cl)AlMe<sup>3</sup> has been observed by the means of dynamic 2D NMR spectroscopy [58, 59] (**Figure 3a**). Moreover, the exchange between the magnetically nonequivalent hydrogens, which belong to the opposite parts of *ansa*‐ligand in the complex Me<sup>2</sup> SiInd<sup>2</sup> ZrMe(μ‐Cl)AlMe<sup>3</sup> , was found (**Figure 3b**). This dynamic picture could be explained by the intermolecular exchange


**Table 3.** Effect of catalyst structure and solvent on the product yields in the reaction of hexene‐1 with AlEt<sup>3</sup> , catalyzed by L2 ZrCl<sup>2</sup> (mole ratio AlEt<sup>3</sup> :alkene:L<sup>2</sup> ZrCl<sup>2</sup> = 60:50:1, reaction time 24 h, 22°C).

hydrogens in the pairs H<sup>1</sup>

and Al‐CH<sup>2</sup>

**Figure 4.** EXSY spectra of system Me<sup>2</sup>

SiInd<sup>2</sup> ZrCl<sup>2</sup>

─(AlEt<sup>3</sup> )2 in d<sup>8</sup>

‐toluene at 305 K (*τ* = 0.3 s).

ZrHEt with loss of ethylene.

into **22** is accompanied by removal of the ClAlEt<sup>2</sup>

Zr‐CH<sup>2</sup>

AlEt<sup>2</sup>

and L2

pane structure.

(Cp′= η<sup>5</sup>

‐H<sup>4</sup>

and H2

Another evidence of the zirconacyclopropane generation in the systems L<sup>2</sup>

realization of two parallel stages—two types of β‐C‐H activation in L<sup>2</sup>

elimination of ethane to give zirconacyclopropane and (ii) formation of Et<sup>2</sup>

‐H<sup>3</sup>

the observation of diastereomeric five‐membered bimetallic complexes CpCp′ZrCH<sup>2</sup>

five‐membered complex diastereomers, which apparently goes via the zirconacyclopro‐

Moreover, our density functional theory (DFT) calculations showed that equilibrium between zirconacyclopropane (**23**) and bimetallic five‐membered Zr, Al‐complex (**22**) is thermodynam‐ ically probable; however, it is shifted toward the bimetallic intermediate [61]. Analysis of the reactions between the complexes and olefins demonstrated that zirconacyclopropane is more reactive toward the substrate than the intermediate **22** (**Scheme 7**). The insertion of olefin

sphere. The interaction of olefins with zirconacyclopropane and bimetallic five‐membered Zr, Al‐complex provides zirconacyclopentane structures, which is involvement in the cyclometa‐ lation process, has been proposed earlier [62, 63]. Transmetalation of zirconacyclopentane

of *ansa*‐ligand, as well as between the H‐atoms of

ZrCl<sup>2</sup>

http://dx.doi.org/10.5772/intechopen.69319

ZrEt<sup>2</sup>

molecule from the zirconium coordination

‐AlEt<sup>3</sup>

could be

(μ‐H)

Al

53

CH<sup>2</sup>

(**Scheme 7**): (i)

AlH from Et<sup>3</sup>

groups. The exchange may exist due to the equilibrium between the

Alkene and Olefin Functionalization by Organoaluminum Compounds, Catalyzed...

‐(1‐neomenthyl‐4,5,6,7‐tetrahydroindenyl)) [60], the formation is possible due to

**Scheme 6.** Bimetallic Zr, Al‐intermediates in the reaction of zirconocenes with alkylaluminums [39, 47–56].

between the diastereomers of the complex, containing a stereogenic center on the transition metal atom, via Me<sup>2</sup> SiInd<sup>2</sup> ZrCl<sup>2</sup> .

On the basis of these investigations, we proposed the mechanism, where the alkyl chloride bimetallic complex associated with the AlR<sup>3</sup> molecule is the starting point of the several cata‐ lytic cycles, carbo‐, cyclometalation, hydrometalation and dimerization (**Scheme 7**). The zir‐ conocenes with more electron‐deficient η<sup>5</sup> ‐ligands in combination with chlorinated solvents provide a greater concentration of a key intermediate, which speeds up all the pathways, ensuring the high conversion of a substrate. The sterical hindrances in η<sup>5</sup> ‐ligand and solva‐ tion by chlorine containing solvents delay the processes of C─H activation in the methylalkyl substituted intermediate increasing the cabometalation product yield.

As shown in **Figure 4**, dynamic processes are also characteristic to the five‐membered bimetallic complex L<sup>2</sup> ZrCH<sup>2</sup> CH<sup>2</sup> (μ‐Cl)AlEt<sup>2</sup> . Thus, we found intermolecular exchange by

**Figure 3.** EXSY spectra of (a) system Cp<sup>2</sup> ZrCl<sup>2</sup> ─(AlMe<sup>3</sup> ) 2 in CD<sup>2</sup> Cl<sup>2</sup> at 300 K (*τ* = 0.3 s); (b) system Me<sup>2</sup> SiInd<sup>2</sup> ZrCl<sup>2</sup> ─(AlMe<sup>3</sup> )2 in CD<sup>2</sup> Cl<sup>2</sup> at 300 K (*τ* = 0.3 s).

hydrogens in the pairs H<sup>1</sup> ‐H<sup>4</sup> and H2 ‐H<sup>3</sup> of *ansa*‐ligand, as well as between the H‐atoms of Zr‐CH<sup>2</sup> and Al‐CH<sup>2</sup> groups. The exchange may exist due to the equilibrium between the five‐membered complex diastereomers, which apparently goes via the zirconacyclopro‐ pane structure.

Another evidence of the zirconacyclopropane generation in the systems L<sup>2</sup> ZrCl<sup>2</sup> ‐AlEt<sup>3</sup> could be the observation of diastereomeric five‐membered bimetallic complexes CpCp′ZrCH<sup>2</sup> CH<sup>2</sup> (μ‐H) AlEt<sup>2</sup> (Cp′= η<sup>5</sup> ‐(1‐neomenthyl‐4,5,6,7‐tetrahydroindenyl)) [60], the formation is possible due to realization of two parallel stages—two types of β‐C‐H activation in L<sup>2</sup> ZrEt<sup>2</sup> (**Scheme 7**): (i) elimination of ethane to give zirconacyclopropane and (ii) formation of Et<sup>2</sup> AlH from Et<sup>3</sup> Al and L2 ZrHEt with loss of ethylene.

Moreover, our density functional theory (DFT) calculations showed that equilibrium between zirconacyclopropane (**23**) and bimetallic five‐membered Zr, Al‐complex (**22**) is thermodynam‐ ically probable; however, it is shifted toward the bimetallic intermediate [61]. Analysis of the reactions between the complexes and olefins demonstrated that zirconacyclopropane is more reactive toward the substrate than the intermediate **22** (**Scheme 7**). The insertion of olefin into **22** is accompanied by removal of the ClAlEt<sup>2</sup> molecule from the zirconium coordination sphere. The interaction of olefins with zirconacyclopropane and bimetallic five‐membered Zr, Al‐complex provides zirconacyclopentane structures, which is involvement in the cyclometa‐ lation process, has been proposed earlier [62, 63]. Transmetalation of zirconacyclopentane

between the diastereomers of the complex, containing a stereogenic center on the transition

[39, 54, 55]

**-AlR3**

**-**

**Scheme 6.** Bimetallic Zr, Al‐intermediates in the reaction of zirconocenes with alkylaluminums [39, 47–56].

[39, 54, 55]

M=Ti, R= Me

M= Zr

**AlEt3**

On the basis of these investigations, we proposed the mechanism, where the alkyl chloride

lytic cycles, carbo‐, cyclometalation, hydrometalation and dimerization (**Scheme 7**). The zir‐

provide a greater concentration of a key intermediate, which speeds up all the pathways,

tion by chlorine containing solvents delay the processes of C─H activation in the methylalkyl

As shown in **Figure 4**, dynamic processes are also characteristic to the five‐membered

ensuring the high conversion of a substrate. The sterical hindrances in η<sup>5</sup>

(μ‐Cl)AlEt<sup>2</sup>

substituted intermediate increasing the cabometalation product yield.

CH<sup>2</sup>

ZrCl<sup>2</sup>

─(AlMe<sup>3</sup> )2 in CD<sup>2</sup> Cl<sup>2</sup>

molecule is the starting point of the several cata‐

[47,48,50,56] [60]

inactive species

**- +AlR3**

X= Cl, Me

‐ligands in combination with chlorinated solvents

. Thus, we found intermolecular exchange by

at 300 K (*τ* = 0.3 s); (b) system Me<sup>2</sup>

SiInd<sup>2</sup> ZrCl<sup>2</sup>

─(AlMe<sup>3</sup> )2

‐ligand and solva‐

[47,48,50,56]

[47,48,56]

[53]

Tebbe reagent

metal atom, via Me<sup>2</sup>

bimetallic complex L<sup>2</sup>

**Figure 3.** EXSY spectra of (a) system Cp<sup>2</sup>

at 300 K (*τ* = 0.3 s).

in CD<sup>2</sup> Cl<sup>2</sup> SiInd<sup>2</sup>

M= Ti, Zr, Hf

MAO or [Ph3C]+[B(C6F5)4-

strong Lewis acids (SLA):

/AlMe3

**AlR3 (R= Me, Et)**

52 Alkenes

M= Ti, Zr, Hf

**AlRnCl3-n**

bimetallic complex associated with the AlR<sup>3</sup>

conocenes with more electron‐deficient η<sup>5</sup>

ZrCH<sup>2</sup>

ZrCl<sup>2</sup> .

[47-52, 56]

M= Zr, R= Me, Et, Bui

M= Ti, Zr, R=Me M=Hf, R= Me, Et

M= Ti, Zr, Hf -CH4

M= Zr, R=Me M=Hf, R= Me, Et

, Oct i

**Figure 4.** EXSY spectra of system Me<sup>2</sup> SiInd<sup>2</sup> ZrCl<sup>2</sup> ─(AlEt<sup>3</sup> )2 in d<sup>8</sup> ‐toluene at 305 K (*τ* = 0.3 s).

**Scheme 7.** Mechanisms of reactions of alkenes with AlR<sup>3</sup> (R = Me, Et), catalyzed with Zr η<sup>5</sup> ‐complexes.

by OACs goes via several stages and gives alumolanes. The probability of this process was shown experimentally using low temperature NMR spectroscopy [56].
