Asymmetric hydrosilylation of olefins

As noted earlier in this chapter, the enantioselective hydrosilylation of olefins could be a useful method to prepare chiral, non-racemic alcohols. A.lthough the scope of highly enantioselective hydrosilylations is limited, high enantioselectivities have been obtained for the asymmetric hydrosilylation of alkenes and vinylarenes. A majority of the most selective chemistry has been conducted using a palladium catalyst containing an axially chiral monophosphine ligand. 

The formation of chiral products from hydrosilylation depends on the substitution pattern of the olefin and the regioselectivity of the hydrosilylation process. The products of the hydrosilylation of 1,1-disubstituted olefins are cliiral if the two substituents on the alk-ene are different. Hydrosilylation of terminal olefins can generate chiral products if the regioselectivity of the hydrosilylation is reversed from that typically observed, and the hydrosilylation process forms branched products. Asymmetric hydrosilylation of gemi-nally disubstituted alkenes has not generated products with high enantiomeric excess, but asymmetric hydrosilylation by additions to terminal olefins to form branched alkylsilanes has occurred with high ee. 

Reactions of alkenes and vinylarenes catalyzed by palladium complexes of axially chiral biaryhnonophosphines generate branched products, and these products are formed with high ee. As shown in Equation 16.36, the hydrosilylation of hexene, 4-pheny 1-1-butene, and 

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ASYMMETRIC HYDROSILYLATION OF OLEFINS WITH TRANSITION-METAL CATALYSTS 127... 

As discussed above, the cross-coupling reaction of organosilicon compounds proceeds stereospecifically, depending on the reaction conditions. Thus, the transformation C—Si C —C is demonstrated to be accompanied by chirality transfer. Now, the question arises of how to prepare organosilicon compounds whose chiral allylic carbon is substituted by a silyl group. The most accessible solution is asymmetric hydrosilylation of olefins [35]. We studied asymmetric hydrosilylation of 1-substituted 1,3-butadienes using... 

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]. 

This review deals with recent advances in catalytic asymmetric hydrosilylation of olefins, carbonyl and imino compounds in the presence of transition metal complexes of chiral phosphine ligands with particular emphasis on the asymmetric reduction of prochiral carbonyl compounds, which has been extensively studied in the last few years by several research groups and proved to provide an effective reduction method for organic syntheses. 

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