TRANSITION-STATE MODELING OF THE ALDOL REACTION OF BORON ENOLATES - A FORCE-FIELD APPROACH

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 1-14 (±0.2 kcal mol-1). 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. In the aldol reactions of unsubstituted and E substituted enol borinates, boat B is likely to be involved as well as the chair and boat A; its omission from the development of the force field may explain the poor treatment of methyl ketone reactions.