Mechanism of 2-iodophenol photolysis in aqueous solution

The photolysis of 2-iodophenol (2-IPhOH) was investigated by means of laser ash photolysis and product studies. Two major heterolytic dehalogenation pathways could be evidenced upon irradiation of anionic 2-IPhO-: ring contraction leading to cyclopentadienic acids via a Wolff rearrangement (phi(a) = 0.11 +/- 0.02), and alpha-ketocarbene formation (phi(c) = 0.03 +/- 0.01) yielding products characteristic for triplet carbene reactivity. In contrast, the irradiation of neutral 2-IPhOH leads mainly to homolytic cleavage of the carbon-halogen bond (phi = 0.08 +/- 0.01) with subsequent formation of biphenyls in deoxygenated solution. This specific reaction, which is not observed with other halogenated phenols, is explained by the low energy of the C I bond. The relative efficiencies of the heterolytic pathways in the halogenophenol series are discussed in terms of the multiplicity of the excited states involved and of the internal heavy atom effect.