An argument for abandoning the "allowed" and "forbidden" classification of electrocyclic reactions.

The division of electrocyclic reactions into "allowed" and "forbidden" classes carries the implication that reactions of the latter class are so energetically penalised that they will occur only if their "allowed" alternatives are rendered effectively impossible. The present work tests that assumption, using NEVPT2 and DFT calculations on a variety of cyclobutene ring openings and ()-1,3,5-hexatriene ring closures, and their benzannelated congeners. The results show the assumption to be incorrect.