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 o f allyl and crotyl boronates to aldehydes was developed, based on ab i nitio calculations and on a process of vial and error optimization. Th e optimized force field reproduces the experimental syn-anti stereosel ectivity for the intermolecular addition of E and Z crotylboronates to aldehydes. The force field is used to analyse the stereoselectivity o f various synthetically interesting reactions. In particular, the forc e 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 derivati ves (R.W. Hoffmann, et al.). In addition the force field is able to re produce the experimental results of variously substituted allylboronat es (gamma,gamma-alkyl substituted, alpha,gamma-alkyl substituted, alph a,gamma,gamma-alkyl substituted, gamma-alkoxy substituted) and the Fel kin-antiFelkin ratios of allyl, E-crotyl and Z-crotyl addition to chir al aldehydes.