Photo-Kolbe decarboxylation

Simple carboxylic acids suffer photo-Kolbe decarboxylation under the usual photoelectrochemical conditions. In fact, this reaction type was one of the first characterized examples of oxidative cleavage of an organic molecule induced by long w avelength irradiation of a semiconductor. In the decarboxylation of acetic acid at... 

A large variety of organic oxidations, reductions, and rearrangements show photocatalysis at interfaces, usually of a semiconductor. The subject has been reviewed [326,327] some specific examples are the photo-Kolbe reaction (decarboxylation of acetic acid) using Pt supported on anatase [328], the pho-... 

The photo-Kolbe reaction is the decarboxylation of carboxylic acids at tow voltage under irradiation at semiconductor anodes (TiO ), that are partially doped with metals, e.g. platinum [343, 344]. On semiconductor powders the dominant product is a hydrocarbon by substitution of the carboxylate group for hydrogen (Eq. 41), whereas on an n-TiOj single crystal in the oxidation of acetic acid the formation of ethane besides methane could be observed [345, 346]. Dependent on the kind of semiconductor, the adsorbed metal, and the pH of the solution the extent of alkyl coupling versus reduction to the hydrocarbon can be controlled to some extent [346]. The intermediacy of alkyl radicals has been demonstrated by ESR-spectroscopy [347], that of the alkyl anion by deuterium incorporation [344]. With vicinal diacids the mono- or bisdecarboxylation can be controlled by the light flux [348]. Adipic acid yielded butane [349] with levulinic acid the products of decarboxylation, methyl ethyl-... 

A variety of photocatalyzed decarboxylation reactions on Ti02 powder including the decomposition of acetate to methane and carbon dioxide and the breakdown of benzoic acid yielding predominantly CO2 have been reported by Bard and coworkers (23,24). Evidence for the occurrence of these "photo-Kolbe" reactions has stimulated the search for other organic reactions that might be photochemically initiated by excitation of semiconductors and extensive work in this area is in progress (25). 

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