| Crynodeb: | Cation radical Diels–Alder cycloadditions proceed via an acyclic intermediate that exists on a flat region of the potential energy surface. Competition between cyclization and C–C bond rotation results in varying levels of suprafacial stereoselectivity. Quasi-classical trajectories were used to explore reaction dynamics on this surface. Even though there is no discernible energy barrier toward cyclization, a dynamically stepwise process is found, for which the acyclic intermediate is found to reside for several hundreds of femtoseconds. In a small number of cases, exceptionally long lifetimes (>1000 fs) are found, leading to a loss of alkene stereochemistry.
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