Solving the puzzling competition of the thermal C2-C6 vs Myers-Saito cyclization of enyne-carbodiimides

The mechanism of the thermal cyclization of enyne-carbodiimides 7a-c has been studied computationally by applying the DFT method. The results indicate that enyne-carbodiimides preferentially follow the C-2-C-6 (Schmittel) cyclization pathway in a concerted fashion although the Myers-Saito diradical formation is kinetically preferred. The experimentally verified preference of the C-2-C-6 over the Myers-Saito pathway is guided by the inability of the Myers-Saito diradical to kinetically compete in the rate-determining trapping reactions, either inter- or intramolecular, with the concerted C-2-C-6 cyclization. As demonstrated with enyne-carbodiimide 11, the Myers-Saito channel can be made the preferred pathway if the trapping reaction by hydrogen transfer is no more rate determining.