Density functional study of the ring effect on the Myers-Saito cyclization and a comparison with the Bergman cyclization

Solving the puzzling competition of the thermal C(2)-C(6) 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 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.

Photochemical generation and reversible cycloaromatization of a nine-membered ring cyclic enediyne

Irradiation of the nine-membered ring enediyne precursor, which has one of its triple bonds masked as cyclopropenone, efficiently (Phi = 0.34) generates the reactive 4,5-benzocyclonona-2,6-diynol. The latter rapidly equilibrates with the corresponding 1,4-didehydronaphthalene diradical and then undergoes rate-limiting hydrogen abstraction to produce the ultimate product of the Bergman cyclization, benz[f]indanol.