The Importance of the Ene Reaction for the C2−C6 Cyclization of Enyne−Allenes

Abstract
The present study establishes the ene reaction as a competing reaction mechanism to the diradical mechanism for the thermal C2−C6 cyclization of enyne−allenes which possess bulky substituents at the alkyne terminus. Both reaction routes are found to possess nearly equal free energies of activation. As shown by our computations, primary H/D isotope effects could be used for a definite decision about the mechanism. Concerning the regioselectivity of the cyclization reactions of enyne−allenes our study resolves a long-standing deviation between theoretical results and experimental findings.

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