Hydrosilane, enantiomeric

Although it is not a catalytic asymmetric hydrosilylation, chirality transfer was reported in the palladium-catalyzed addition of an enantiomerically enriched hydrosilane to norbornene.76... 

Enantiomeric silylmercury compounds are obtained from reaction of diorganomercurys with an optically active hydrosilane [Eq. (6)] (76). The configuration is retained predominantly at silicon. These compounds, however, racemized readily when sublimed during purification. 

Reaction of an enantiomeric hydrosilane with KH yields a racemic silyl anion (77). Thus, treatment of (-l-)-(l-Np)PhMeSiH with KH at 50°C in DME for 24 hours and subsequent addition of n-BuBr give racemic ( )-(l-Np)PhMeSi-n-Bu, via the formation of the corresponding silyl anion with loss of optical activity. Hydrolysis or deuterolysis also gives a racemic product. These observations clearly rule out a mechanism in which the silyl anion is formed by proton abstraction from the hydrosilane, because retention of configuration would be expected. A possible mechanism involves a pentacoordinate dihydrosilyl anion formed via coordination of H in an initial fast, reversible process, and its decomposition to the racemic silyl anion with loss of molecular hydrogen [Eq. (7)]. A gas-... 

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. 

In the preceding Sections it was described that chiral phosphine-rhodium complexes are effective in causing stereoselective addition of a hydrosilane to a variety of prochiral carbonyl compounds to give silyl ethers of the corresponding alkanols with fairly high enantiomeric bias at the carbon atom. The present section describes an application of the catalytic asymmetric hydrosilylation of ketones to the preparation of some new asymmetric bifunctional organosilanes. 

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