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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)