Photocyclization of iminyl radicals: theoretical study and photochemical aspects

The irradiation of acyloximes was studied by theoretical methods. CASPT2/6-31G*//CASSCF/6-31G* calculations, using an active space of 14 electrons in 11 orbitals, indicate that S2 should be the spectroscopic state, and its relaxation leads directly to N-O bond breakage due to coupling between the imine pi* and the sigma* N-O orbitals. Subsequent calculations at the B3PW91/6-31+G* level suggest that the resulting iminyl radicals are able to cyclize to the five- or six-membered ring, depending on the presence of a phenyl group as a spacer, a process that has been verified experimentally. The photochemical aspects of the more common five-membered ring formation, such as excited-state quenching, quantum yield, excited-state sensitizers, laser flash photolysis experiments, Stern-Volmer plot, and luminescence measurements, were investigated. These studies indicate that singlet and triplet excited states undergo the same reaction. Emission lifetimes of ca. tau = 10.6 micros for compound 11 are suggestive of triplet parentage, while no fluorescence was detected, in agreement with the computed MEP energy profile.