THEORETICAL-STUDIES OF ALDOL STEREOSELECTIVITY - THE DEVELOPMENT OF A FORCE-FIELD MODEL FOR ENOL BORINATES AND THE INVESTIGATION OF CHIRAL ENOLATE PI-FACE SELECTIVITY

Ab initio calculations on enol borinate structures have been used to generate empirical force field parameters for the title systems. The optimized structures of enol borinates are nonplanar with the BR2 group twisted out of the enolate plane, rendering the enolate 7r-faces nonequivalent. The resulting force field model has been used for a preliminary investigation of the aldol stereoselectivity of enol borinates with chiral groups attached either to boron or to the carbonyl carbon. This suggests that the π-facial selectivity of Z-enol diisopinocampheyl borinates derived from ethyl ketones may be explained by the conformational bias of the enolate and a Zimmerman-Traxler chair transition state with the aldehyde. The reversed aldehyde enantioface selectivity observed for enol diisopinocampheyl borinates derived from methyl ketones does not fit this pathway, and alternative boat transition states are required. © 1990, American Chemical Society. All rights reserved.