Cyclobutene Photochemistry. The Photochemistry of cis- and trans-Bicyclonon-8-ene

Article abstract of DOI:10.1021/jo00004a043

Direct photolysis of cis- and trans-bicyclo<5.2.0.>non-8-ene in hydrocarbon solution with monochromatic far-UV (185-214 nm) light sources affords cis,cis- and cis,trans-1,3-cyclononadiene via formal electrocyclic ring opening, cycloheptene (and acetylene) via formal (?2s + ?2s) cycloreversion, and minor amounts of molecular rearrangement products.Product quantum yields have been determined for the 185-nm photolysis.The two isomers lead to similar distributions of the isomeric 1,3-cyclononadienes, with similar quantum yields at 185 nm; the diene mixtures are in bothcases 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 cis,trans-1,3-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.