Radical Intermediates in Photocatalytic Activation of Carboxylic Acids: A Spin Trapping Study

Abstract

Radicals generated by the photocatalytic reaction of carboxylic acids with a platinum-loaded TiO₂ photocatalyst were trapped by a DBNBS (3,5-dibromo-4-nitrosobenzenesulfonate sodium salt) spin trapping agent, and the radical adducts were observed using electron spin resonance spectroscopy. In the case of ethanoic acid and propanoic acid in mixed solvents of water and acetonitrile, only adducts of methyl and ethyl radicals were observed as a result of decarboxylation reactions. The longer the alkyl chain of the linear alkyl carboxylic acid, the greater the percentage of adducts of secondary radicals and the higher the concentration of the total adduct. The results show that as the alkyl chain length increases, the diffusion rate of the generated radicals slows down. Additionally, during diffusion, intramolecular hydrogen transfer reactions take place. These reactions occur at a rate higher than that of the reaction with the spin-trapping reagent. However, not only the adducts of undecyl radicals but also those of dodecanoic acid radicals were detected by electrospray ionization mass spectrometry (ESI-MS), indicating that competitive reactions of radical cations ([HRCOOH]^•+) of longer alkyl carboxylic acids to decarboxylation to produce an alkyl radical (HR^•) or proton elimination to produce an alkyl carboxylic acid radical (^•RCOOH) occur due to stabilization of radical cations by hyperconjugation with longer alkyl chain and the presence of water in the solvent as a proton acceptor. The same is true for branched alkylcarboxylic acids with tertiary carbon, which stabilizes the radical cations. Although this result also implies an increase in product diversity due to coupling between product radicals and a lack of product selectivity, we found that only the decarboxylation reaction proceeds when the percentage of water in the solvent is reduced to less than 5% to reach an increase in the product selectivity. Although OH radicals, which can produce multiple alkyl carboxylic acid radicals, were produced to a small extent in our systems, they made a very limited contribution to the overall product by increasing the ratio of acetonitrile in the solvent.