Hydrosilane asymmetric reactions

The catalytic reaction giving allenes by the addition of a hydrosilane twice to 1,3-diynes65 has been applied to the asymmetric synthesis of axially chiral allenylsilanes although the selectivity and scope of this reaction are relatively low. A chiral rhodium complex coordinated with (23, 43 )-PPM is the best catalyst for the addition of phenyldimethyl-silane to diyne 52 giving allene 53 with 22% ee (Scheme 14).66 663... 

Asymmetric reduction of a, /I-unsaturated esters, lactones or lactames can be effected with copper-hydride catalysts and chiral phosphanes such as various BINAP related compounds in excellent yields and enantioselectivities (equation 23). As the hydrosilane component, polymethylhydrosiloxane (PMHS) is frequently used for this reaction. 

Asymmetric hydrosilylation of ketimines in the presence of a chiral titanocene difluoride is improved by a nucleophilic additive (e.g., isobutylamine) which serves to release the chiral amine products and thereby generates less hindered amido complexes which are susceptible to reaction with the hydrosilane. ... 

Palladium-catalyzed hydrosilylation of 1,3-dienes is one of the important synthetic methods for allylic silanes, and considerable attention has been directed to the asymmetric synthesis of the latter by catalytic methods [9]. Optically active allyhc silanes have been used as chiral allylating reagents in S reactions with electrophiles, typically aldehydes [38,39]. In the presence of Pd catalysts the reaction with hydrosilanes containing electron-withdrawing atoms or substituents on sihcon usually proceeds in a 1,4-fashion giving allyHc silanes [40,41]. Asymmetric hydrosilylation of cyclopentadiene (29) forming optically active 3-silylcyclopentene (30) has been most extensively studied (Scheme 13). In the first report, hydrosilylation of cyclopentadiene (29) with methyldichlorosilane in the presence of 0.01 mol % of palladium-(l )-(S)-PPFA (15a) as a catalyst gave... 

A proposed mechanism for the rhodium-catalyzed alcoholysis is represented in Scheme 49 (77). In the first step, activation of the hydrosilane occurs through oxidative addition. Formation of the alkoxysilane then takes place by nucleophilic attack of a noncoordinated alcohol molecule. The dihydro-rhodium complex 143 thus obtained liberates a hydrogen molecule upon reductive elimination. Nucleophilic cleavage of the silicon-rhodium bond, without prior coordination of the alcohol at the rhodium is supported by results obtained in asymmetric alcoholysis (cf. Sect. II-D). Optical yields were shown to be little dependent on the catalyst ligands (in marked contrast with the asymmetric hydro-silylation), indicating but weak interaction between alcohol and catalyst during the reaction. Moreover, inversion of configuration at silicon, which occurs in the particular case of methanol as solvent, is not likely to occur in a reaction between coordinated silane and alcohol. 

More recently, the asymmetric hydrosilylation of aryl ketones and aryl imines has been developed using copper catalysts. " In this case, axially chiral biaryl bisphos-phine ligands boimd to copper generate remarkably active catalysts for tihe hydrosilylation of ketones. These reactions occur with high selectivity using the hydrosilane polymer... 

On the basis of the fact that (R)-BMPP coordinated to the metal center can induce asymmetric addition of methyldichlorosilane across the carbon-carbon double bond of 2-substituted propenes to afford an enantiomeric excess of (R)-2-substituted propylmethyldichlorosilanes, the following processes should be involved in these reactions (a) insertion of the metal center into the silicon-hydrogen bond (oxidative addition of the hydrosilane) (b) addition of the resulting hydridometal moiety to the coordinated olefin preferentially from its re face (in a cis manner) to convert the olefin into an alkyl-metal species and (c) transfer of the silyl group from the metal center to the alkyl carbon to form the product. Since process (b) most likely involves diastereomeric transition states or intermediates, the overall asymmetric bias onto the R configuration at the chiral carbon would have already been determined prior to process (c). A schematic view of such a process is given in Scheme 1. 

Up to now, although we are not in a position to correlate reaction conditions, especially reaction temperatures, with optimal optical yields of the reduction of particular alkyl phenyl ketones, some features observed in the asymmetric hydrosilylation may be pointed out. As regards the catalysis by the two complexes (6) and (8), both configuration and optical yield of the resulting alcohols depend markedly on the structure of hydrosilanes employed. For example, asymmetric reduction of alkyl phenyl ketones using [(5)-BMPP]2Rh(SX l as catalyst and diethylsilane as silane component gave rise to (5)-l-phenylalkanols predominantly, while with phenyldimethylsilane to (/ )-enantiomers uniformly in much higher optical yields. 

The extent of asymmetric hydrosilylation depends strongly upon the structure of hydrosilanes employed in a similar manner to the cases of other chiral rhodium complex-catalyzed reactions with dimethylphenylsilane optical yields are generally more than several times as high as with trimethylsilane. Most remarkable is the fact that the addition of dimethylphenylsilane to pivalophenone gave the silyl ether of (iS)-2,2-dimethyH-phenylpropanol, while that of trimethylsilane led to the (R)-enantiomer. 

The marked effect of hydrosilanes on the stereoselectivity, which is very characteristic of the asymmetric hydrosilylation of ketones as described in the previous Sections, is seen here again. Fairly good optical yields comparable to those obtained in other chiral rhodium complex-catalyzed reactions were attained. For example, the reaction of acetophenone with diphenylsilane catalyzed by (/ )-( S )-MPFA-rhodium complex gave higher optical yield than when (/ )-BMPP or DIOP was used as ligands. 

Very recently, asymmetric synthesis of optically active alkoxyhydrosilanes has been accomplished by way of this type of reaction. Alcoholysis and treatment of the product with an appropriate Grignard reagent led to the known optically active hydrosilane, equation (31). Results are summarized in Table 23. 

Hydrosilanes undergo addition to carbon-carbon multiple bonds under catalysis by transition metal complexes. Nickel, rhodium, palladium, and platinum were used as catalytically active metals. By incorporating chiral ligands into the metal catalyst, the hydrosilylation can be performed analogously to other addition reactions with double bonds, for example, asymmetric hydrogenation to obtain optically active alkylsilanes. 

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