Steric Inhibition of Photochemical Reactions: The -Cycloaddition Reaction

Article abstract of DOI:10.1021/jo00264a018

Photochemical <2 + 2>-cycloaddition reactions of a number of diphenylcycloalkenes to tetrachloroethylene (TCE) have been examined. 1,2-Diphenylcyclobutene (1) reacts efficiently with TCE to yield the cyclobutane anticipated for a <2 + 2>-photocycloaddition reaction.In contrast, 1,2-diphenyl-3,3,4,4-tetramethylcyclobutene (1TM), 1,2-diphenylcyclopentene (2), and 1,2-diphenylcyclohexene (3) yield only phenanthrene products.Photochemical quantum yields have been determined for all four reactions.For 1 and 1TM, excited-state lifetimes have been measured as a function ofthe concentration of added TCE and 2,5-dimethyl-2,4-hexadiene (DMHD).The observed data indicate that the methyl groups in 1TM effectively inhibit the interaction of possible <2 + 2>-reactants with the 1TM^* excited state.