Sructure an Mechanism in the Photo-Retro-Aldol Type Reactions of Nitrobenzyl Derivatives. Photochemical Heterolytic Cleavage of C-C Bonds

Article abstract of DOI:10.1021/ja00221a038

The photo-retro-aldol type reactions of several nitroaromatic compounds have been studied in aqueous solution over the pH 1-14 range.These reactions are observed only in aqueous or in predominantly aqueous solution.Catalysis of reaction due to hydroxide ion is observed for several derivatives.Quantum yields of reaction and product ratios (of nitrotoluene vs dinitrobibenzyl) are reported as a function of pH.The proposed mechanism of reaction involves heterolytic cleavage of the benzylic C-C bond from the triplet excited state in the primary photochemical step to generate a nitrobenzyl carbanion and a carbocation-equivalent fragment, except for the nitrophenylacetates 24-26, which eliminate CO2 in place of such a fragment.Photogenerated nitrobenzyl carbanions are efficiently trapped by molecular oxygen to give isolable hydroperoxides at pH < 12.The pH dependence of quantum yield of reaction along with α-deuterium isotope effects indicates that different transition states for benzylic C-C bond heterolysis are operative, depending on the substrate as well as the pH of the solution.Hydroxide ion catalyzed rate constants for the primary photochemical step are estimated for alcohols 8 and 9 to be ca. 1 x 10⁹ M^-1 s^-1.The results show that photochemical C-C bond heterolysis requires favorable stabilization of both the carbanion and carbocation-derived fragments.Hydroxide ion catalysis may also facilitate the process.The use of the nitrobenzyl moiety as the carbanion-stabilizing group appears to be generally applicable, as demonstrated by the systems studied.