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 C2–C6 (Schmittel) cyclization pathway in a concerted fashion although the Myers–Saito diradical formation is kinetically preferred. The experimentally verified preference of the C2–C6 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 C2–C6 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.
应用DFT方法对enyne-carbodiimides 7a-c的热环化机制进行了计算研究。结果表明,enyne-carbodiimides优先以协同的C2-C6(Schmittel)环化途径进行,尽管Myers-Saito二重态形成在动力学上更为优先。实验证实,C2-C6途径优先于Myers-Saito途径,是由于Myers-Saito二重态在速率决定的捕获反应(无论是内分子还是间分子)中无法与协同的C2-C6环化竞争。如使用enyne-carbodiimide 11所示,如果通过氢转移的捕获反应不再是速率决定步骤,则可以使Myers-Saito通道成为优先路径。