Cyclobutene photochemistry. Nonstereospecific photochemical ring opening of simple cyclobutenes

Abstract

The photochemistry of bicyclo(3.2.0)hept-6-ene, bicyclo(4.2.0)oct-7-ene, and cis- and trans-3,4-dimethylcyclobutene has been investigated in hydrocarbon solution with monochromatic far-ultraviolet (185 and 193 nm) light sources. All of these simple cyclobutene derivatives undergo ring opening to yield the isomeric 1,3-dienes, and the latter three open nonstereospecifically to yield mixtures of the possible geometric isomers. The isomeric 3,4-dimethylcyclobutenes yield different mixtures of the three 2,4-hexadiene isomers, and in each case the mixtures are weighted in favor of the orbital symmetry forbidden isomer(s). Attempts have been made to analyze the relative isomeric diene yields from ring opening of bicyclo(4.2.0)octene and the isomeric 3,4-dimethylcyclobutenes within the context of the purely disrotatory, adiabatic ring-opening mechanism that recent ab initio calculations suggest should be possible. While the results for the former compounds are consistent with this mechanism, analyses of the relative yields of the isomeric 2,4-hexadienes from photolysis of the latter two compounds indicates that photochemical ring opening by the formally forbidden, conrotatory pathway may compete to some extent with disrotatory ring opening.