The photochemical reactions of benzene with 1,2-,1,3-, and 1,4-dienes

Abstract

The photocycloaddition reactions of benzene with 1,2-, 1,3-, and 1,4-dienes are described. Propa-1,2-diene (allene) and cyclonona-1,2-diene are unique among hydrocarbon ethylenic systems since 1,2-para-cycloaddition to the benzene ring is the major mode of reaction. The meta-cycloaddition of 1,2-dienes is regiospecific and this is accounted for by a mechanism involving prior meta-bonding in the arene leading to a polarised prefulvene species which undergoes 1,2-addition to the diene. The conformationally fixed cis-diene 1,2-dimethylenecyclohexane undergoes selective 1,4-para- and 1,4-meta-photocycloaddition to benzene whereas the reaction with 3-methylenecyclohexene is of low yield and non-selective giving diene dimers, substitution products, and adducts. Penta-1, 4-diene and cyclohexa-1,4-diene yield 1,2-meta-cyclo-adducts with benzene, and evidence is presented for formation of 1,2-para-cycloadducts from each system : diene dimers are also produced in the latter case but solutions of bicyclo[2.2.1]hepta-2,5-diene in benzene are essentially inert photochemically.