Mechanisms and Dynamics of Reactions Involving Entropic Intermediates

An entropic intermediate is defined as a free energy minimum that is not a potential energy minimum; the favorable entropy in this region of the potential surface is responsible for the increased lifetime of this species. Enabled by molecular dynamics simulations, entropic intermediates have been identified for several types of reactions, particularly pericyclic reactions. This review highlights recent advances in the mechanistic and dynamic investigations of organic and biosynthetic pericyclic reactions that involve entropic intermediates. We have proposed timing criteria to differentiate dynamically concerted from dynamically stepwise mechanisms. These criteria complement the usual definitions of concertedness based on energetics and bonding changes along the potential surface.