Isotope effects and the distinction between synchronous, asynchronous, and stepwise Diels–Alder reactions

A variety of symmetrical or nearly symmetrical Diels–Alder reactions are studied by a combination of experimental isotope effects, theoretical calculations, and rate observations. Becke3LYP calculations predicted highly asynchronous transition structures for Diels–Alder reactions of bis(boryl)acetylenes, dialkyl acetylenedicarboxylates, triazolinediones, and dialkyl maleates. Rate observations and kinetic isotope effects are consistent with these predictions, though the experimental support for the calculated structures is notably ambiguous in some cases. A concerted mechanism is supported for the retro-Diels–Alder reaction of norbornene. Overall, Diels–Alder reactions appear to be only very weakly biased toward synchronous transition states.