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 (ωB97X‐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. [ABSTRACT FROM AUTHOR]