Theoretical Evaluation of Alternative Pathways in the Stevens Rearrangement

Abstract

Ab initio and semi-empirical molecular orbital theory has been used to explore the ion-pair and lithium-catalysed pathways in the Stevens rearrangement of an alkylammonium ylide to an amine. These pathways represent alternatives to the concerted and free radical mechanisms which we have previously studied. Solvation effects have been included via a polarizable continuum model. Finally, the calculations have been extended to a set of experimentally observable compounds. For the nine systems studied here, the stepwise free radical pathway is predicted in every case to be the favourable mechanism for the Stevens rearrangement. Single-point MP2/6-31G(d) calculations on fully optimized PM3 geometries are shown to provide a useful method of including electron correlation at low computational cost.