Photochemistry of 1-alkoxy- and 1-(benzyloxy)-9,10-anthraquinones in methanol: a .delta.-hydrogen atom abstraction process that exhibits a captodative effect

Abstract

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 MeOCH 2 O and ArCH(OMe)O groups at the 1-position, respectively. Quenching studiens 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 δ-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-XPhCH 2 O)-2-methyl-9,10-anthraquinone (X=H, Cl, CH 3 , NO 2 , OCH 3 , CF 3 , and CN) were measured and found to vary by a factor of 3.4 from the largest (X=NO 2 ) to the smallest (X=OCH 3 ). Captodative interactions account for the small differences in these rate constants