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  • Title: Ambimodal Transition States in Diels-Alder Cycloadditions of Tropolone and Tropolonate with N-Methylmaleimide*.
    Author: Zhang H, Thøgersen MK, Jamieson CS, Xue XS, Jørgensen KA, Houk KN.
    Journal: Angew Chem Int Ed Engl; 2021 Nov 15; 60(47):24991-24996. PubMed ID: 34472178.
    Abstract:
    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.
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