ORIGINS OF STEREOSELECTIVITY IN THE ADDITION OF ALLYL AND CROTYLBORONATES 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 crotyl boronates to aldehydes was developed, based on ab initio calculations and on a process of vial and error optimization. The optimized force field reproduces the experimental syn-anti stereoselectivity for the intermolecular addition of E and Z crotylboronates to aldehydes. The force field is used to analyse the stereoselectivity of various synthetically interesting reactions. In particular, the force field is able to reproduce with excellent quantitative agreement the experimental results of intramolecular boronate reactions leading to methyl or benzyloxy substituted cyclohexanol or cyclopentanol derivatives (R.W. Hoffmann, et al.). In addition the force field is able to reproduce the experimental results of variously substituted allylboronates (gamma,gamma-alkyl substituted, alpha,gamma-alkyl substituted, alpha,gamma,gamma-alkyl substituted, gamma-alkoxy substituted) and the Felkin-antiFelkin ratios of allyl, E-crotyl and Z-crotyl addition to chiral aldehydes.