Hydrosilylation Chalk-Harrod mechanism

Hydrosilylation turned out to be a unique method in organosilicone chemistry, but in some cases it suffers from severe side reactions. An explanation is provided by the generally accepted reaction mechanism known as "Chalk-Harrod mechanism" described elsewhere [7]. Included in this series of reaction steps is an insertion of olefmic ligands into a platinum-hydrogen bond. Since the metal may be bonded to either of the unsaturated carbon atoms and the reaction is also an equilibrium, alkenes may result which are in fact isomerized starting material. Isomeric silanes are to be expected as well (Eq. 1), along with 1-hexylsilane, which is by far, the main compound produced. 

Using quantum chemical molecular modelling tools we have examined the reaction mechanism of palladium catalyzed hydrosilylation of styrene by the precatalyst system, 1, developed by Togni and co-workers. One fundamental question that we have focused on is whether the reaction proceeds by the classical Chalk-Harrod mechanism or by an alternative mechanism such as the modified-Chalk-Harrod mechanism. In this section, the general features of the catalytic cycle are examined. 

The fact that isomers 8a, 8b and 8c are the lowest energy styrene coordinated complexes have potentially important ramifications that concern the modified-Chalk-Harrod mechanism and the regioselectivity observed in the hydrosilylation. With the modified-Chalk-Harrod mechanism olefin insertion into the M-Si bond follows styrene coordination. However, in all three isomers depicted in Figure 8, the coordinated hydride lies between the... 

If we were to assume that the reaction followed the Chalk-Harrod mechanism, then insertion of the olefin into the Pd-hydride bond in all three isomers 8a-c would lead to the correct regioisomer product. Thus, to some degree the regioselectivity of the hydrosilylation in this catalyst system is determined in the styrene coordination step. We will discuss the origin of the regioselectivity in more detail in Section 4. 

According to our simulations, hydrosilylation reaction proceeds through the classic Chalk-Harrod mechanism as depicted on the right-hand-side of Figure 5. The modified or anti-Chalk-Harrod mechanism is hindered by a rather large silylmetallation barrier which is calculated to be 46 kcal/mol for the minimal QM model A. 

Detailed mechanistic studies with respect to the application of Speier s catalyst on the hydrosilylation of ethylene showed that the process proceeds according to the Chalk-Harrod mechanism and the rate-determining step is the isomerization of Pt(silyl)(alkyl) complex formed by the ethylene insertion into the Pt—H bond.613 In contrast to the platinum-catalyzed hydrosilylation, the complexes of the iron and cobalt triads (iron, ruthenium, osmium and cobalt, rhodium, iridium, respectively) catalyze dehydrogenative silylation competitively with hydrosilylation. Dehydrogenative silylation occurs via the formation of a complex with cr-alkyl and a-silylalkyl ligands ... 

When the hydrosilylation reactions were reviewed in this series and published in 19893, the Chalk-Harrod mechanism 69 (Scheme 5) was apparently the most widely accepted mechanism for the alkene hydrosilylation, with minor exceptions of photocatalyzed... 

The first mechanistic proposal for the hydrosilylation reaction where mononuclear homogeneous catalytic intermediates are assumed is known as the Chalk-Harrod mechanism. The catalytic cycle in a slightly modified form is shown in Fig. 7.16. All steps of this catalytic cycle belong to organometallic reaction types that we have encountered many times before. Thus conversions of 7.60 to 7.61, 7.61 to 7.62, and 7.62 to 7.59 are examples of oxidative addition of HSiR3, insertion of alkene into an M-Si bond, and reductive elimination, respectively. 

A widely accepted cycle for olefin hydrosilylation is the so-called Chalk-Harrod mechanism, where it is assumed that H migration to the alkene is faster than silyl migration, followed by reductive elimination from a silyl-alkyl intermediate,... 

A recent detailed theoretical study of the platinum-catalyzed hydrosilylation of ethylene [15] led to a conclusion that this process proceeds through the Chalk-Harrod mechanism. The rate-determining step in this mechanism is the isomerization of the Pt(silyl)(alkyl) complex formed by ethylene insertion into the Pt-H bond, and the activation barrier of this step is 23 kcal moP for R = Me and -26 kcal mol for R = Cl). In the modified Chalk-Harrod mechanism, however, the rate-determining step is ethylene insertion into the Pt-SiRa bond and its barrier is 44 kcal moP for R = Me and 60 kcal moP for R = Cl. 

The Chalk-Harrod mechanism has been widely accepted with various modifications to account for the hydrosilylation of other multiple bonds (C=C), C=0, C=N), homogeneously catalyzed by various metal complexes. 

Cyclopentadienyl complexes of Rh —> Rh appeared as valuable examples in the mechanistic study of ethylene hydrosilylation [14, 47]. GC/MS and NMR studies as well as deuterium labeling of the substrates allowed an alternative proposal to the Chalk-Harrod mechanism, initiated by a hydrogen shift to form... 

This Chalk-Harrod mechanism includes the insertion of an olefin into a hydrogen-metal bond (step iii). However, it is also conceivable that an olefin can insert into a silicon-metal bond, and this type of mechanism was found to be operative when Fe(CO)5, M3(CO)12 (M = Fe, Ru, Os) and R3SiCo(CO)4 were used as catalysts for the photocatalyzed hydrosilylation of alkenes104. 

Rhodium complexes also catalyze the hydrosilylation of olefins, and one of the earliest soluble catalysts used for hydrosilylation was Wilkinson s catalyst. - As is described in more detail below, the mechanisms for hydrosilylation catalyzed by rhodium complexes differ from those catalyzed by platinum complexes. Olefin insertion occurs into different ligands in the two mechanisms. The rhodium-catalyzed processes occiu by a so-called modified Chalk-Harrod mechanism. " Rhodium complexes were also among tlie first complexes used for the asymmetric hydrosilylation of ketones. " ... 

The mechanism of hydrosilylation involves a sequence of elementary reactions described in the earlier chapters of the book. The most commonly cited mechanism for hydrosilylation was first described by Chalk and Harrod and involves oxidative addition of the silane, insertion of an olefin into the metal-hydride bond, and reductive elimination to form the silicon-carbon bond in the organosilane product. More recently, a related but distinct mechanism involving insertion of the olefin into the silyl group has been recognized, and this mechanism is often called the modified Chalk-Harrod mechanism. Before these steps are described, some of the mechanistic issues regarding the specific systems of Speier s catalyst and Karstedt s catalyst are described briefly. 

Trans hydrosilylation by a combination of Chalk-Harrod and modified Chalk-Harrod mechanisms for hydrosilylation and isomerization via T -vinyl intermediates. 

The mechanistic scheme presents the conventional oxidative addition— reductive elimination steps to explain the hydrosilylation. The oxidative addition of trisubstituted silanes HSiRs to a metal alkene complex (usually with d and d ° configuration) is followed by migratory insertion of alkene into the M—H bond, and the resulting metal(silyl)-(alkyl) complex undergoes reductive elimination by the Si—C bond formation and regeneration of metal alkene complex in excess of alkene. As the facile reductive elimination of silylalkane from [alkenyl-M]-SiRs species has not been well established in stoichiometric reaction, a modified Chalk-Harrod mechanism has been proposed to explain the formation of unsaturated (vinylsilane) organosilicon product, involving the alkene insertion into the metal-silyl bond followed by C—H reductive elimination (Scheme 2) (38). 

The theoretical study of [RhCl(PH3)3]-catalyzed hydrosilylation of ethylene by the DFT, MP4 (SDQ), and CCSD(T) methods shows that the rate-determining step in the Chalk-Harrod mechanism is the Si—C reductive elimination (43). 

Internal alkenes are difficult to hydrosilylate with NHC-M catalysts. In the case of terminal alkenes, the silyl group is usually added to the terminal carbon, although a-isomers and dehydrogenative products might be also formed (Equation (13.4)). Most of these catalysts operate according to the Chalk-Harrod mechanism (CHM) or one of its variations. However, for PhSiHa an alternative mechanism, involving a silylene intermediate, was proposed. ... 

A proposed mechanism for hydrosilylation catalyzed by nickel-phosphine complexes, based on the Chalk-Harrod mechanism for platinum catalysis, incorporates an equilibrium between a o and a n complex. It is possible that four intermediates are formed a terminal or inner o or n complex (Scheme 22.7). It was observed that the internal-terminal ratio was consistent over a reaction period of 45 h, which suggests a similar stability of the formed inner and terminal o complexes (Table 22.17). In these o complexes the involved organic group can be regarded as carbanionic in... 

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