Origin of stereofacial selectivity in electrophilic additions to methylenecyclohexanes and methylenedioxanes. A theoretical and experimental study

Abstract

Addition reaction studies and ab initio calculations on methylenecyclohexane and 5-methylene-1,3-dioxane systems suggest that two electronic factors contribute to the stereoselectivity of epoxidation and diimide reduction. These are respectively the spatial anisotropy of the HOMO with respect to the two faces of the double bond, common to both molecules, which is likely to be responsible for the overall axial stereofacial selectivity exhibited, and a similar anisotropy in the electrostatic potential field of the methylenedioxane caused by the oxygens; which also favours attack from an axial direction by polarisable electrophilic species. The anisotropy of the HOMO arises from the important topological difference between the contributions made to the HOMO by the periplanar β C–H σ bonds and opposing β C–O or C–C σ bonds. Catalytic reduction proceeds with equatorial face selectivity for both the cyclohexane and the dioxane systems and appears to be governed largely by steric effects.