Re-engineering potential energy surfaces: trapezoidally distorted π2s + π2s thermal cycloaddition/elimination reactions

Abstract

Calculations at the PM3 or B3LYP/6-31G(d) SCF-MO levels of theory indicate that electron-withdrawing substituents can modify the potential energy surface for a simple thermal _π2_s + _π2_s pericyclic cycloaddition–elimination via a trapezoidal geometric distortion. We propose this thermally allowed synchronous reaction mode as an alternative to the Woodward–Hoffmann antarafacial mode or the stepwise biradical pathways normally invoked for such thermally activated cycloadditions. Reaction of the substituted syn tricyclic system 1 via the trapezoidal and synchronous transition state 6 is predicted as a potential experimental test of this new reaction mode.