ORIGINS OF STEREOSELECTIVITY IN THE ADDITION OF CHIRAL ALLYLBORANES AND CROTYLBORANES TO ALDEHYDES - THE DEVELOPMENT AND APPLICATION OF A FORCE-FIELD MODEL OF THE TRANSITION-STATE

A molecular mechanics model of the transition state for the addition of allyl- and crotylboranes to aldehydes was developed based on ab initio calculations. A force field was created (FF1), based on a process of trial and error optimization, which reproduces the relative energies and geometries of the chairlike ab initio transition structures. An automated procedure for the optimization of the torsional parameters was developed and used to improve the way the force field reproduces the ab initio data (geometries and relative energies). The optimized force field (FF2) reproduces the synanti selectivity of E and Z crotylborane addition to aldehydes. The force field is used to analyze the stereoselectivity of various synthetically interesting reactions. The model reproduces the sense and degree of stereoselectivity in a number of cases.