Dynamic effects in the didehydro-Diels-Alder (DDDA) reaction of enyne-ketoenes: 50% stepwise bond formation in spite of concerted transition state

Abstract

The C2─C6/Diels-Alder cyclization of enyne-ketoene 1 was studied by experiments, theoretical calculations, and dynamic trajectory computations. The failure to trap possible intermediate(s), indicates a concerted reaction mechanism. A detailed search for stationary points revealed a concerted mechanism and surprisingly a diradical intermediate with no direct connection to the enyne-ketoene 1. To probe the accessibility of this intermediate quasiclassical trajectories were initiated from the concerted transition state structure. Notably, 36% of the trajectories reach the product zone directly and 5% arrive at the product via the intermediate diradical. Additionally, 31% of the trajectories go to the intermediate zone and stay there within the simulation time limit.