PHOTOCHEMISTRY OF 1-ALKOXY-9,10-ANTHRAQUINONES AND 1-(BENZYLOXY)-9,10-ANTHRAQUINONES IN METHANOL - A DELTA-HYDROGEN ATOM ABSTRACTION PROCESS THAT EXHIBITS A CAPTODATIVE EFFECT

The photochemistry of a variety of 1-alkoxy- and 1-(benzyloxy)-9,10-anthraquinones in methanol has been examined. In the absence of oxygen the primary photoproducts of these reactions are 9,10-anthrahydroquinones with MeOCH2O and ArCH(OMe)O groups at the 1-position, respectively. Quenching studies established the multiplicity of the excited state to be triplet. Deuterium isotope effects, substituent effects, and solvent polarity studies support a mechanism in which a biradical intermediate is formed by an intramolecular delta-hydrogen atom transfer that occurs in one step but not a reaction pathway in which the hydrogen atom transfer is accomplished in two discrete steps via separate electron and proton transfers. Apparent rate constants for the disappearance of 1-(p-XPhCH2O)-2-methyl-9,10-anthraquinone (X = H, Cl, CH3, NO2, OCH3, CF3, and CN) were measured and found to vary by a factor of 3.4 from the largest (X = NO2) to the smallest (X = OCH3). Captodative interactions account for the small differences in these rate constants.