The thermal C2–C6/[2 + 2] cyclisation of enyne-allenes: Reversible diradical formation

Abstract

New enyne-allenes, structurally designed toward the thermal C²–C⁶/[2 + 2] cyclisation mode, were prepared and characterised, one of them even by X-ray crystallography. The mechanism of their transformation to formal [2 + 2] cycloadducts was interrogated by trapping experiments and DFT computations. The results support a stepwise mechanism that involves the reversible formation of the C²–C⁶ diradical intermediate.