Mechanistic investigations into the photochemistry of 4-allyl-tetrazolones in solution: a new approach to the synthesis of 3,4-dihydro-pyrimidinones.

Photolysis (lambda = 254 nm) of 4-allyl-tetrazolones 2a-c was carried out in methanol, 1-propanol, 1-hexanol, acetonitrile, and cyclohexane. The sole primary photochemical process identified was molecular nitrogen elimination, with formation of pyrimidinones 6a-c. Following the primary photocleavage, secondary reactions were observed in acetonitrile and cyclohexane, leading to phenyl-isocyanate (7), aniline (9), and 1-phenylprop-1-enyl-isocyanate (10a). In alcoholic solutions, the primary products, 6a-c, remained photostable even under extended irradiation, making possible the isolation of 3,4-dihydro-pyrimidinones as stable compounds in very high yields. The observed photostability of pyrimidinones 6a-c in alcohols is ascribed to the excited state quenching via reversible proton transfer, facilitated by the solvent cage stabilization due to formation of hydrogen bonds. The viscosity of alcohols is directly related to the cage effects observed. The photocleavage of 4-allyl-tetrazolones leads probably to a caged triplet radical pair. This hypothesis is confirmed by the solvent viscosity effect on the photolysis quantum yields. Additionally, dissolved molecular oxygen sensitizes the formation of pyrimidinones, as should be expected for a triplet intermediate that can only form the product molecule after T-S conversion, which is accelerated by oxygen.