Oriented Electric Fields Accelerate Diels-Alder Reactions and Control the endo/exo Selectivity

Herein we demonstrate that an external electric field (EEF) acts as an accessory catalyst/inhibitor for Diels=Alder (DA) reactions. When the EEF is oriented along the "reaction axis'' (the coordinate of approach of the reactants in the: reaction,path), the barrier of the DA reactions is lowered by a significant amount, equivalent to rate enhancements by 4-6 orders of magnitude. Simply flipping the EEF direction has the,opposite effect, and the EEF acts as an inhibitor. Additionally, an EEF;oriented perpendicular to the "reaction axis" in the direction of the individual molecule dipoles can change the endo/exo selectivity, favouring one or the other depending on the positive/negative directions of the EEF vis-a-vis the individual molecular dipole. At some critical value of the EEF along the "reaction axis", there is a crossover to a stepwise mechanism that involves a zwitterionic intermediate. The valence bond diagram model is used to comprehend these trends and to derive a selection rule for EEF effects on chemical reactions: an EEF aligned in the direction of the electron flow between the reactants will lower the reaction barrier. It is shown that the exo/endo control by the EEF is not associated with changes in secondary orbital interactions.