The Photochemical C2-C6 Cyclization of Enyne-Allenes: Interception of the Fulvene Diradical with a Radical Clock Ring Opening

The mechanism of the photochemical C-2-C-6 cyclization of enyne-allenes has been studied through radical clock, intramolecular kinetic isotope effect, and laser flash photolysis (LFP) experiments as well as density functional theory (DFT) and ab initio computations, While the photochemical cyclization of enyne-allenes 1 and 2 furnished ene and Diels-Alder products without any cyclopropyl ring opening, that of 3 carrying the ultrafast diphenylcyclopropylcarbinyl radical clock, afforded products derived from cyclopropyl ring opening. Laser flash photolysis (LFP) studies oil enyne-allene 3 point to an allene triplet excited state (transient A) as it primarily formed short-lived (tau = 430 ns) intermediate. In addition, we have obtained evidence for the forma(ion of it diphenylmethyl-type diradical (transient C, tau = 1.0 mu s) resulting from ring opening of it diphenylcyclopropane ring. C subsequently undergoes a Surprisingly slow (tau = 1.0 mu s) 1,6-hydrogen shift leading to the stable 1,3-diene 6.