Photoinduced electron-transfer, pyrolyses, and direct irradiation reactions of 2-methylenecyclobutanones: Regioselective bond cleavage depends on the substituents and the method of activation

The regioselectivity of bond cleavage and the subsequent reactivity of 4-methylene-3,3-dimethyl-2,2-diphenylcyclobutan-1-one (5) and 4-isopropylidene-2,2-diphenylcyclobutan-1-one (6) in photoinduced electron transfer, pyrolysis, and direct irradiation reactions were investigated. Novel photoinduced electron-transfer rearrangements of 5 and 6 were found to occur through 1,4-diyl radical cations formed by C1-C2 cleavage of 5^.+ and 6^.+; that is, similarly to radical-cation Norrish type I cleavage, but in contrast to the diansyl derivative 1^.+ corresponding to 5 ^.+, which gives an oxatetramethyleneethane (OTME)-type radical cation through C2-C3 cleavage. In contrast, solvent effects on the thermal rearrangement of 5 to 6 and theoretical calculations suggested the intervention of an OTME-type diradical intermediate 16^.. formed by homolytic C2-C3 cleavage of 5. In direct photoreactions, 5 and 6 undergo [2+2] cycloreversion to give 1,1-diphenylethylenes and methyleneallenes, which are explained by both C1-C2 and C3-C4 cleavage. Consequently, the regioselectivity of 5 and 6 in bond cleavage and their reactivity strongly depend on the substituents and the method of activation.