Mechanism asymmetric hydrosilylation

Metal complexes of a C2-symmetric bisproline catalyse the asymmetric hydrosilylation of ketones 29Si NMR spectra provide evidence of the mechanism. 265 ... 

Rhodium-phosphine complexes are usually active and effective in the asymmetric hydrosilylation of olefins, ketones, and aldehydes, allowing for the virtual synthesis of optically active alkoxysilanes and organic compounds of high purity. Chiral rhodium-phosphine catalysts predominate in the hydrosilylation of pro-chiral ketones. This subject has been comprehensively reviewed by several authors who have made major contributions to this field [52-54]. A mechanism for the hydrosilylation of carbonyl groups involving the introduction of asymmetry is shown in Scheme 3 [55]. 

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. " ... 

Asymmetric reduction of ketones by hydrosilylation in the presence of a chiral catalyst followed by hydrolysis has been studied by several research groups independently. In this Section, results so far obtained are properly compiled and plausible mechanisms of the asymmetric hydrosilylation of prochiral ketones are discussed. 

More recently, an alternative mechanism has been proposed [45] for asymmetric hydrosilylation of prochiral ketones using ( f)-DIOP-rhodium(I) complex (6) and a-naphthylpenylsilane, the latter undergoing concomitant conversion into an optically active, bifunctional alkoxysilane, which will be discussed separately (see Section 7.1). According to this proposed mechanism, diastereomeric silylhydrido-rhodium(III) complexes having trigonal bipyramidal structure are assumed as intermediates, which distinguish enantiotopic faces of a prochiral ketone in terms of steric approach control . 

Results of simple and double asymmetric alcoholysis of prochiral dihydrosilanes catalyzed by the chiral rhodium(I) complex, (DIOP)Rh(S)Cl (6), are also shown in Table 23. Optical yields of the simple asymmetric alcoholysis using the chiral rhodium(I) complex were rather low, while the double asymmetric one provided good results [72]. The mechanism of the asymmetric induction may be closely similar to that of the asymmetric hydrosilylation. 

A review on the industrial applications of homogeneous catalysts is particularly welcome. The proceedings of recent symposia and a general text have been published in addition to reviews on metal cluster catalysts, activation of saturated hydrocarbons by metal complexes in solution, catalysis by arene Group-VIB tricarbonyls, titanocene-catalysed reactions ofalkenes, transition-metal hydrides in catalysis, the mechanisms of the catalytic cyclization of aliphatic, hydrocarbons, asymmetric hydrosilylation and asymmetric synthesis. A n.m.r. study of the conformations of chelated Diop and a MO study of organo-metallic migration reactions are also of interest. Polymer supported catalysts have been reviewed and the relationship between cross-linking of the polymer and catalytic activity has been discussed. ... 

Both nickel(0)- and nickel(n)-phosphine complexes, e.g. [Ni(C2H4)(diphos)], [NiCl2(diphos)], can catalyse hydrosilylation of alkenes. An induction period is observed with the nickel(ii) system and it appears that an important step with these catalysts is reduction of nickel(ii) to nickel(0). The reduction does not occur below 100 °C. A possible mechanism for the reaction is given in Scheme 27. Asymmetric hydrosilylation of styrene and cyclic dienes with HSiCls can... 

Asymmetric cyclization-hydrosilylation of 1,6-enyne 91 has been reported with a cationic rhodium catalyst of chiral bisphosphine ligand, biphemp (Scheme 30).85 The reaction gave silylated alkylidenecyclopentanes with up to 92% ee. A mechanism involving silylrhodation of alkyne followed by insertion of alkene into the resulting alkenyl-rhodium bond was proposed for this cyclization. 

The reaction is catalyzed by lanthanide complexes CpjfLnR,41 although noble metal catalysts, notably rhodium, are most widely applied, particularly in asymmetric hydroboration,42 The mechanism is likely to be similar to hydrosilylation. The products may be oxidized with H202 and converted to alcohols or amines. 

Review R. E. Merrill, Asymmetric synthesis using chiral Phosphine ligands, Reaction Design Corp., Hillside, N.J., 1979. This review covers the literature to mid-1979 (234 references). It discusses mechanisms and applications to asymmetric hydrogenation, hydrosilylation, hydroformylation and alkylation. 

Ravlov, V. A Mechanism of Asymmetric Induction in Catalytic Hydrogenation, Hydrosilylation, and Cross-Coupling Reactions on Metal Complexes, Russ. Chem. Rev. 2002, 71, 39-56. 

Enantioselective reduction of simple ketone carbonyls is possible, but catalysts that deliver consistently high selectivities in such reactions have been elusive [61-64]. More success has been recorded in the asymmetric reduction of functionalized ketones and imines (reviews [65,66]). Two types of stoichiometric reductants are used dihydrogen and dihydrosilanes (reviews ref. [67,68]), but as the mechanism of hydrosilylation is highly controversial [68], we will discuss only the former. 

An enantioselective version of the hydrosilylation reaction would greatly extend its synthetic utility. The reaction mechanism of this catalytic asymmetric process has been investigated in great detail and shown to be extremely complicated [132]. Nevertheless,... 

Especially noteworthy is the field of asymmetric catalysis. Asymmetric catalytic reactions with transition metal complexes are used advantageously for hydrogenation, cyclization, codimerization, alkylation, epoxidation, hydroformylation, hydroesterification, hydrosilylation, hydrocyanation, and isomerization. In many cases, even higher regio- and stereoselectivities are required. Fundamental investigations of the mechanism of chirality transfer are also of interest. New chiral ligands that are suitable for catalytic processes are needed. 

First, for reasons of clarity, the currently-accepted mechanism of transition-metal complex catalyzed-hydrosilylation reactions will be described briefly. Furthermore, consideration of selective, if not asymmetric, reduction of certain carbonyl compounds by way of rhodium(I)-catalyzed hydrosilylation (Section 4) is included in this review because the catalytic process and stereochemical course of this reaction correlate closely with those of their asymmetric reduction under similar conditions that will be described in the succeeding section. 

---