Photochemical Reactivity of 1,6-Methano[10]annulene.

Abstract

1,6-Methano[10]annulene solutions in cyclohexane have been subjected to continuous and pulsed UV irradiation. Photolysis occurs in both cases, giving naphthalene as a minor and major product, respectively. The wavelength dependence of the reaction in solution indicates that the photochemical process occurs, exciting 1,6-methano[10]annulene in the second and third singlet electronic excited states. The reaction kinetics has been determined under pulsed irradiation. From the time dependence of concentrations, along with support of density functional theory calculations and early published data, two mechanisms are proposed for naphthalene production. Reaction steps such as direct migration of the bridging methylene of 1,6-methano[10]annulene to cyclohexane and 1,6-methano[10]annulene isomerization to benzotropilidene have been identified. The calculated energy diagrams relative to the ground and lowest excited states allow one to relate these steps to processes such as electrocyclic closure and sigmatropic shift. The norcaradienic form of 1,6-methano[10]annulene results in the critical species for methylene migration and the sigmatropic [1,5] shift. The present results and those arising from photolysis in the gas phase are good examples of the photochemical reactivity of 1,6-methano[10]annulene.