Synthetically Relevant Post-Transition State Bifurcation Leading to Diradical and Zwitterionic Intermediates: Controlling Nonstatistical Kinetic Selectivity through Solvent Effects

A post-transition state surface intersection (PTSSI) between radical and zwitterionic states that causes a bifurcation in the reaction pathway was discovered through density functional theory calculations on potential energy surfaces and ab initio molecular dynamics simulations of cycloadditions between a bicyclobutane and a triazolinedione (BCB-TAD). It was predicted that changes to the solvent polarity would enable control over the dynamic selectivity in this system; indeed, experimental evidence supported this prediction. This work not only provides new insights into an unusual type of post-transition state bifurcation, but also demonstrates how the nonstatistical dynamic effects that control selectivity for such reactions can be manipulated rationally to increase the yields of synthetically useful reactions.