Re-engineering potential energy surfaces: trapezoidally distorted (pi)2(s)+(pi)2(s) 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 surf ace for a simple thermal (pi)2(s) + (pi)2(s) pericyclic cycloaddition-elimi nation via a trapezoidal geometric distortion. We propose this thermally al lowed synchronous reaction mode as an alternative to the Woodward-Hoffmann antarafacial mode or the stepwise biradical pathways normally invoked for s uch thermally activated cycloadditions. Reaction of the substituted syn tri cyclic system 1 via the trapezoidal and synchronous transition state 6 is p redicted as a potential experimental test of this new reaction mode.