Origin of Stereoselectivity in a Mechanochemical Reaction of Diphenylfulvene and Maleimide.

Mechanochemical reactions sometimes yield unexpected products or product ratios in comparison to conventional reaction conditions. In the present study, we theoretically reveal the origin of the mechanochemical selectivity by considering the Diels-Alder reaction of diphenylfulvene and maleimide as an example. The application of an external force is equivalent to the production of a structural deformation. Here, we show that a mechanical force applied in a direction orthogonal to the reaction mode can lower the activation barrier by varying the potential energy curvature in the transition state. In the case of the Diels-Alder reaction, the -type pathway was found to be more mechanochemically favorable than the -type pathway, which is consistent with the experimental observations.