PI-FACIAL SELECTIVITY IN CATALYTIC OSMYLATION REACTIONS OF CHIRAL C(1)-OXYGENATED ALLYLIC SILANES

Oxygen-substituted allylic and crotyl silanes undergo vicinal hydroxylations with unprecedented levels of diastereofacial selectivities in the presence of catalytic amounts of osmium tetroxide to predominantly form 1,2-anti-1,2,3-triol units. The magnitude of face selectivity was influenced by two factors: the steric bulk of the silicon group and the type of substituent on the Cl-oxygen. As the size of the silicon group increased [Me3Si- → Et3Si- → PhMe2Si- → tBuMe2Si- → Ph2MeSi-] the anti selectivity improved. Allylic silanes containing ethers and esters gave moderate to good anti selectivity. (E)-crotylsilanes gave higher selectivities than the corresponding Z stereoisomers. When a hydroxyl group is present the vicinal hydroxylation proceeded with virtual face specificity for the allylic and (E)-crotylsilanes producing the triol products with anti/syn selectivities reaching 147:1. Lower but still significant selectivities were obtained for the Z stereoisomers. These experiments present evidence supporting the notion that the steric effects of trialkyl silyl group are as important as σ donor effects in these electrophilic addition reactions. © 1990, American Chemical Society. All rights reserved.