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

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 (pi)2(s) + (pi)2(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.