Some Factors that Influence Anchimeric Assistance

Abstract

It is obvious that a number of requirements are necessary for anchimeric assistance to occur. First, the neighboring group must be configurationally and conformationally situated to provide proper orbital overlap in the transition state. Second, the reaction that involves neighboring group participation must not be very much slower than any competing reactions. Thus, solvolyses of tertiary halides and esters usually occur with greatly reduced anchimeric assistance (as measured by k Δ/k c ) even when a good neighboring group is appropriately situated,1 because the introduction of the extra α substituent lowers the free energy of the transition state for forming a carbocation while hardly affecting the free energy of the transition state of the neighboring group process. In fact the transition state for the pathway that involves neighboring group participation may be of higher energy with a tertiary than with a secondary halide owing to steric crowding.