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

Radicals\ngenerated by the photocatalytic reaction of carboxylic\nacids with a platinum-loaded TiO<sub>2</sub> photocatalyst were trapped\nby a DBNBS (3,5-dibromo-4-nitrosobenzenesulfonate sodium salt) spin\ntrapping agent, and the radical adducts were observed using electron\nspin resonance spectroscopy. In the case of ethanoic acid and propanoic\nacid in mixed solvents of water and acetonitrile, only adducts of\nmethyl and ethyl radicals were observed as a result of decarboxylation\nreactions. The longer the alkyl chain of the linear alkyl carboxylic\nacid, the greater the percentage of adducts of secondary radicals\nand the higher the concentration of the total adduct. The results\nshow that as the alkyl chain length increases, the diffusion rate\nof the generated radicals slows down. Additionally, during diffusion,\nintramolecular hydrogen transfer reactions take place. These reactions\noccur at a rate higher than that of the reaction with the spin-trapping\nreagent. However, not only the adducts of undecyl radicals but also\nthose of dodecanoic acid radicals were detected by electrospray ionization\nmass spectrometry (ESI-MS), indicating that competitive reactions\nof radical cations ([HRCOOH]<sup>•+</sup>) of longer alkyl\ncarboxylic acids to decarboxylation to produce an alkyl radical (HR<sup>•</sup>) or proton elimination to produce an alkyl carboxylic\nacid radical (<sup>•</sup>RCOOH) occur due to stabilization\nof radical cations by hyperconjugation with longer alkyl chain and\nthe presence of water in the solvent as a proton acceptor. The same\nis true for branched alkylcarboxylic acids with tertiary carbon, which\nstabilizes the radical cations. Although this result also implies\nan increase in product diversity due to coupling between product radicals\nand a lack of product selectivity, we found that only the decarboxylation\nreaction proceeds when the percentage of water in the solvent is reduced\nto less than 5% to reach an increase in the product selectivity. Although\nOH radicals, which can produce multiple alkyl carboxylic acid radicals,\nwere produced to a small extent in our systems, they made a very limited\ncontribution to the overall product by increasing the ratio of acetonitrile\nin the solvent.