Origin of Stereoselectivity in a Mechanochemical Reaction of Diphenylfulvene and Maleimide

Mechanochemicalreactions sometimes yield unexpected products orproduct ratios in comparison to conventional reaction conditions.In the present study, we theoretically reveal the origin of the mechanochemicalselectivity by considering the Diels-Alder reaction of diphenylfulveneand maleimide as an example. The application of an external forceis equivalent to the production of a structural deformation. Here,we show that a mechanical force applied in a direction orthogonalto the reaction mode can lower the activation barrier by varying thepotential energy curvature in the transition state. In the case ofthe Diels-Alder reaction, the endo-type pathwaywas found to be more mechanochemically favorable than the exo-type pathway, which is consistent with the experimentalobservations.