Transition-state modeling of the aldol reaction of boron enolates: a force field approach

Abstract

A force field model for the aldol reactions of ketone-derived enol borinates with aldehydes has been developed, based on MM2 and ab initio calculations, which can reproduce the geometries and energies of the ab initio transition structures. It reproduces the experimental syn:anti stereoselectivity for the aldol additions of simple Z and E substituted enol borinates derived from ethyl ketones, and it shows that unsubstituted enol borinates (methyl ketone derived) have a greater degree of flexibility with a larger number of accessible chair and boat transition structures found. The force field also reproduces the aldehyde si/re face selectivity in the aldol reactions of a range of chiral Z enol borinates for which experimental data is available, and it may be a useful predictive tool for assessing stereoselection in new reactions. New ab initio calculations (RHF/3-21G) are described, which indicate the existence, not only of the chair and the twist-boat (boat A) reported by Houk, but also of an additional boat transition structure (boat B) found by the force field