Mechanistic aspects of 1,4-dicyanonaphthalene-sensitized phototransformation of aryl glycopyranosides

Steady-state irradiation of phenyl glycopyranosides under conditions of photosensitization (electron transfer) by 1,4-dicyanonaphthalene (DCN) leads to the formation of the corresponding monosaccharides (or their methyl derivatives, if methanol is present in the solvent). Mechanistic aspects of this photoreaction have been examined by steady-state fluorescence quenching measurements and 337.1-nm laser flash photolysis. Formation of the DCN radical anion (with yields of 0.18 to 0.35 delta/sub ion/) as well as the DCN triplet (with yields of 0.14 to 0.27 delta/sub T/) is observed as a result of the electron-transfer quenching of the DCN singlet by phenyl glycopyranosides in acetonitrile and a methanol-acetonitrile (1:10) mixture. Phenoxy radical is also shown to be a transient photoproduct in the latter solvent. Various features of the reaction scheme involving the fragmentation of electron-transfer-derived radical cations of phenyl glycopyranosides into the phenoxy radical and carbonium ions and possible roles of oxygen and methanol in enhancing or inhibiting the yields are discussed.