Photorearrangement of α-Azoxy Ketones and Triplet Sensitization of Azoxy Compounds

Although some aspects of azoxy group radical chemistry have been investigated,1 unhindered α-azoxy radicals remain poorly understood. Here we report the generation of α-azoxy radicals under mild conditions by irradiation of α-azoxy ketones 4a,b. These compounds undergo α-cleavage to yield radicals 5a,b, whose oxygen atom then recombines with benzoyl radicals to produce presumed intermediate 15. Formal Claisen rearrangement gives α-benzoyloxyazo compounds 8a,b, which are themselves photolabile, leading to both radical and ionic decomposition. The ESR spectrum of 5a was simulated to extract the isotropic hyperfine splitting constants, which showed its resonance stabilization energy to be exceptionally large. Azoxy compounds have been found for the first time to be good quenchers of triplet excited acetophenone, the main sensitized photoreaction of 7Z in benzene being deoxygenation. While this reaction has been reported previously, it was always in hydrogen atom donating solvents, where chemical sensitization occurred. The principal direct irradiation product of 4bZ and model azoxyalkane 7Z is the E isomer, whose thermal reversion to Z is much faster than that of previously studied analogues.