Cyclobutene photochemistry. Identification of the excited states responsible for the ring-opening and cycloreversion reactions of alkylcyclobutenes

Article abstract of DOI:10.1021/ja00013a039

Two substituted bicyclic cyclobutene derivatives - 7-methyl- and 7-(trifluoromethyl)bicyclo[4.2.0]oct-7-ene - have been prepared. Gas- and solution-phase UV absorption and Hel UV photoelectron spectra have been recorded for the two compounds as well as for the parent hydrocarbon bicyclo[4.2.0]oct-7-ene. The gas-phase spectra suggest that the π,R(3s) state is the lowest energy state in bicyclo[4.2.0]octene and the 7-methyl derivative but is raised to higher energies than the π,π* state in the 7-trifluoromethyl derivative. Direct photolysis of the three compounds in hydrocarbon solution with monochromatic far-UV (193 and 214 nm) light leads to competitive ring opening to the corresponding cis,cis- and cis,trans-1,3-cyclooctadiene derivatives, as well as fragmentation to cyclohexene and alkyne in all three cases. Product quantum yields (193-nm excitation) have been measured for both substituted derivatives relative to those for the parent compound. The quantum yields of fragmentation products are highest for the methyl- and unsubstituted compounds, suggesting that these products arise from a Rydberg-like excited state. In contrast, ring opening is most efficient for the trifluoromethyl-substituted compound, although the diene distributions obtained from the reaction do not vary throughout the series. These results indicate that nonstereospecific ring opening arises largely from the π,π* state of cyclobutene. The contribution to the fragmentation process from a concerted [σ_2s+σ_2s] cycloreversion pathway is minor, at best. The wavelength dependence of the product distributions from photolysis of the three derivatives is in perfect accord with these assignments.