Photochemical C2-C6 Cyclization of Enyne-Allenes: Detection of a Fulvene Triplet Diradical in the Laser Flash Photolysis

A series of enyne-allenes, with and without benzannulation at the ene moiety and equipped with aromatic and carbonyl groups as internal triplet sensitizer units at the allene terminus, was synthesized. Both sets, the cyclohexenyne-allenes and benzenyne-allenes, underwent thermal C-2-C-6 cyclization exclusively to formal ene products. In contrast, the photochemical C-2-C-6 cyclization of enyne-allenes provided formal Diels-Alder and/or ene products, with higher yields for the benzannulated systems. A raise of the temperature in the photochemical cyclization of enyne-allene 1b' led to increasing amounts of the ene product in relation to that of the formal Diels-Alder product. Laser flash photolysis at 266 and 355 nm as well as triplet quenching studies for 1b,b' indicated that the C-2-C-6 cyclization proceeds via the triplet manifold. On the basis of a density functional theory (DFT) study, a short-lived transient (tau = 30 ns) was assigned as a triplet allene, while a long-lived transient (tau = 33 mu s) insensitive to oxygen was assigned as fulvene triplet diradical. An elucidation of the reaction mechanism using extensive DFT computations allowed rationalization of the experimental product ratio and its temperature dependence.