Redirecting a Diels–Alder Reaction toward (2 + 2)-Cycloaddition

Recently, reactions of allylidenhydrazones with tetracyanoethylene were found to lead to cyclobutanesproducts of usually unfavorable (2 + 2) cycloaddition. Herein we computationally demonstrate that the (4 + 2) product of this reaction is severely destabilized by incomplete C–N bond formation, arising from a complex interplay of substituent electronic effects. We show how destabilization of a single bond in the front-runner product averts its formation and redirects chemical reaction toward an uncharacteristic pathway.