Analysis of the conformational composition of the 4,5- and 5,6-dimethyl-1,3,2-oxazaborinane stereoisomers

Abstract

An energy for the molecules of cis and trans 4,5- and 5,6-dimethyl-1,3,2-oxazaborinanes, and of model 1,3,2-oxazaborinanes and tetrahydro-1,3-oxazines with full geometric optimization has been calculated using empirical (MM2) and semiempirical (AM1) methods. From a comparison of experimental and calculated coupling constants and the relative energies of individual conformers it follows that molecules of the cyclic boron esters form a multicomponent equilibrium system, comprizing sofa and half-chair forms with an equatorial orientation of the N−H bond.