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.