Enantioselective Sakurai-Hosomi reaction

Most desirable, however, would be chiral catalysts for the addition of the more readily available and less toxic allyl silanes, but so far the efforts towards an enantioselective variant of the Sakurai-Hosomi reaction have been less successful. Some time ago Ketter and Herrmann [3a] obtained 24 % ee for the addition of allyl silane to aldehydes catalyzed by the dichlorotitanate 1. 

We have further found that BINOL-Ti (1) catalyzes the Sakurai-Hosomi reaction of methallylsilanes with glyoxylates (Scheme 28) [73]. Surprisingly, however, the products were obtained in the allylic silane (ene product) form with high enantioselectivity. 

Substituted allylsilanes are subject to ene reaction with aldehydes and a,/3-unsaturated carbonyl compounds in the presence of a Lewis acid. The Et2AlCl-promoted reaction of /3-siloxymethyl-substituted allylsilane 27 with aldehydes gives more functionalized allylsilanes (Equation (37)).148 The use of TiCU instead of Et2AlCl leads to the Hosomi-Sakurai allylation. Catalytic enantioselective carbonyl-ene reactions of methallylsilanes have been achieved by using chiral Ti and A1 complexes.149,150... 

Enantioselective Mukaiyama-aldol and Sakurai-Hosomi allylation reactions catalyzed by chiral Lewis acid are currently of great interest because of their utility for the introduction of asymmetric centers and functional groups. 

The chiral Bronsted acids (249-250) have been shown to initiate the Hosomi-Sakurai reaction of imines (246) with allyl- and crotyltrimethyl-silane (247) to give products (248) with excellent enantioselectivities (Scheme 64). ... 

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