Photoreactions of substituted 4-methylenebicyclo[3.2.0]hept-2-enes

Abstract

Photolysis of several 7-aryl-substituted derivatives of 4-methylenebicyclo[3.2.0]hept-2-ene leads to fragmentation to fulvenes and styrenes in a process which shows high stereoselectivity and proceeds exclusively from the excited singlet state. In certain cases rearrangement to derivatives of 7-methylenebicyclo[2.2.1]hept-2-ene may also occur from both excited singlet and excited triplet states but with different stereoselectivity from the two states. Quantum yield measurements show that the latter rearrangement proceeds far more efficiently from the triplet state than the singlet and the relevance of this to current views on the free rotor effect of the exo-methylene group is considered.