Structure and mechanism in the photo-retro-aldol type reactions of nitrobenzyl derivatives. Photochemical heterolytic cleavage of C-C bonds

Journal Article · Wed Jun 22 00:00:00 EDT 1988 · J. Am. Chem. Soc.; (United States)

DOI:https://doi.org/10.1021/ja00221a038· OSTI ID:5992705

Wan, P; Muralidharan, S

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 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 CO/sub 2/ 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 ..cap alpha..-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 /approximately/ 1 /times/ 10/sup 9/ M/sup /minus/1/ s/sup /minus/1/. The results show that photochemical C-C bonds 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.

• Research Organization: Univ. of Victoria, British Columbia (Canada)
• OSTI ID: 5992705
• Journal Information: J. Am. Chem. Soc.; (United States), Journal Name: J. Am. Chem. Soc.; (United States) Vol. 110:13; ISSN JACSA
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400500* -- Photochemistry
AQUEOUS SOLUTIONS
AROMATICS
CARBON
CATALYSIS
CATALYSTS
CATALYTIC EFFECTS
CHEMICAL BONDS
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
CHEMISTRY
DATA
DISPERSIONS
ELEMENTS
ENERGY LEVELS
EXCITED STATES
EXPERIMENTAL DATA
HYDROGEN COMPOUNDS
HYDROXIDES
INFORMATION
KINETICS
MIXTURES
NITRO COMPOUNDS
NONMETALS
NUMERICAL DATA
ORGANIC COMPOUNDS
ORGANIC NITROGEN COMPOUNDS
OXYGEN COMPOUNDS
PH VALUE
PHOTOCHEMICAL REACTIONS
PHOTOCHEMISTRY
REACTION KINETICS
SOLUTIONS