Mechanism of the Photoepoxidation with and Photodecarboxylation of α-Keto Acids

Article abstract of DOI:10.1021/ja00411a033

The photooxidation of benzoylformic acid (1a) in benzene gave peroxybenzoic acid, hydrogen peroxide, and phenyl benzoate.The addition of α-methylstyrene to the oxidation system afforded the epoxide together with acetophenone as a C-C cleaved product, and the ester yield was significantly increased at the expense of the peracid.The photooxidation of 1a was not sensitized by methylene blue or other sensitizers, but was efficiently accelerated by pyridine or other weakly basic solvents such as ethers.Pyridine effectively catalyzed the photoepoxidation as well as the photodecarboxylation of 1a to benzaldehyde.The photoepoxidation gave predominantly trans epoxides, and the relative reactivities of olefins were similar to the photoepoxidation with benzoin (i.e., PhCO3^.) and quite different from the peracid epoxidation.Similar results were obtained by other α-keto acids or the corresponding esters.These facts suggest that the photoepoxidation proceeds via radical epoxidation by acylperoxy radical, affording trans epoxide predominantly.Contrary to previous reports, the photooxidation of α-keto acids via an 1O2 reaction was not substantiated.The photodecarboxylation of 1a to afford benzaldehyde was selectively catalyzed by water, and its undissociated form was about tenfold more reactive than the corresponding carboxylate ion.