Cyclobutene photochemistry. 4. The photochemistry of cis- and trans-bicyclo[5.2.0]non-8-ene

Direct photolysis of cis- and trans-bicyclo[5.2.0]non-e the diene mixtures are in both cases weighted in favor of the less thermodynamically stable cis,trans isomer. The product distributions vary only slightly with excitation wavelength over the 185-214-nm range. Quantum yields for direct photoisomerization of cis,cis- and ck,trans-l,3-cyclononadiene have also been determined. Two mechanisms are considered to explain the nonstereospecificity associated with the ring-opening process: a nonconcerted pathway involving initial cyclobutene bond homolysis and subsequent relaxation of common biradical intermediates, and a pericyclic pathway involving adiabatic, disrotatory ring opening to yield dienes in the first excited singlet state. While the results do not allow a definitive distinction between the two mechanisms, the nonconcerted pathway is suggested to be the more reasonable on the basis of the photobehavior of other cyclobutene derivatives that have been studied. The quantum yield of cycloreversion product is ca. 4 times higher from the cis isomer compared to that from the trans isomer, due to the stereochemical requirements of the process.