Ambimodal Transition States in Diels-Alder Cycloadditions of Tropolone and Tropolonate with N-Methylmaleimide.

The Diels-Alder reactions of tropolone and its conjugate base with N-methylmaleimide have been explored computationally and experimentally. Previous studies of the [4+2] cycloaddition under basic conditions show that both endo- and exo-products are obtained in similar, but variable amounts. Density functional theory ( w B97X-D) explorations of potential energy surfaces, and molecular dynamics trajectories show that the reaction involves an ambimodal transition state for the reaction of the ammonium tropolonate with N-methylmaleimide, and that similar amounts of endo- and exo-products are obtained. The thermal reaction, studied experimentally in detail here for the first time, is predicted to form the endo-adduct through an ambimodal transition state. The exo-adduct can be formed from the same transition state, but requires a hydrogen shift, that hinders this reaction dynamically. Longer reaction times give a small excess of the exo-product, which is thermodynamically more stable.