Mechanistic interpretation of photochemical thallimetric oxidations catalysed by chloride and bromide ions

The thallimetric oxidation of carboxylic acids appears to proceed through free radical and intermediate activated complex mechanisms. The thermal and photochemical uncatalysed oxidation reactions appear to proceed through the formation of an intermediate metal-substrate complex that eventually decomposes to give the products. However, photochemical oxidation in the presence of chloride and bromide ions appears to proceed through a two-electron step via a halo bridge mechanism. In the presence of bromide at 2–3 times the concentration of thallium(III), the photochemical reduction mainly proceeds through a free radical mechanism involving a one-electron step via the formation of thallium(II) species. The nature and concentration of halide ions appear to be critical in deciding the path of the reaction.