Mechanisms and Kinetics of Proton-Coupled Electron-Transfer Oxidation of Phenols

Abstract

The kinetics of the aqueous oxidation of phenol by a deficiency of [IrCl6]2– has been investigated. The reaction initially produces [IrCl6]3– and phenoxyl radicals. The inhibition caused by [IrCl6]3– can be prevented by use of dibromonitroso-benzenesulfonate (DBNBS) as a phenoxyl radical scavenger. The phenoxyl radicals primarily couple to form 4,4'-biphenol, 2,2'-biphenol, 2,4'-biphenol, and 4-phenoxyphenol. Further oxidation of these coupling products leads to a rather complex mixture of final products. The rate laws for the oxidation of the four coupling products by [IrCl6]2– have the same form as those for the oxidation of phenol itself: . Values for kArOH and kArO– have been determined at 25 °C and are assigned to H2O-CPET (water associated concerted proton coupled electron transfer) and electron-transfer mechanisms respectively. Kinetic simulations of a combined mechanism that includes the oxidation of phenol as well as the subsequent reactions show that the degree of overoxidation is rather limited at high pH but quite extensive at low pH. This pH-dependent overoxidation leads to a pH-dependent stoichiometric factor in the rate law for oxidation of phenol, and causes some minor deviations in the rate law for oxidation of phenol. Empirically, these minor deviations can be accommodated by introduction of a third term in the rate law that includes a "pH-dependent rate constant", but this approach masks the mechanistic origins of the effect. One-electron oxidation of alkyl- and alkoxy-substituted phenols (2-methylphenol, 2,6-dimethylphenol, 2,4,6-trimethylphenol, 4-tert-butylphenol and 4-methoxyphenol) has been studied. pH-dependent stoichiometric factors corresponding to overoxidation are found with all substituted phenols except for 2,4,6-trimethylphenol and 4-methoxyphenol. In the 2,4,6-trimethylphenol reaction, the identification of product, 4-hydroxymethyl-2,6-dimethylphenol, rules out the overoxidation steps and there is no need to include an “overoxidation pH-dependent” rate constant. The solvent H/D KIE’s for the phenols pathway provide further evidence for a H2O-CPET mechanism of oxidation of phenols by [IrCl6]2–. Overoxidation is also observed in the reaction between N-acetyl-L-tyrosinamide (a protected tyrosine derivative) and [IrCl6]2–. Fitting the data to a two-term rate law yields second-order rate constants of kArOH = 5.4 ± 0.6 M–1 s–1 and kArO– = (4.5 ± 0.3) × 107 M–1 s–1. Analysis of the kinetic data of the oxidation of phenol by [Os(phen)3]3+ yields the rate law: where the reaction rate is second-order in both [Os(phen)3]3+ and phenol. KArOH = 1.1 × 10–10 M and KArO– = 7.0 are obtained from thermodynamics. kArO– is calculated to be 2.1 × 109 M–1 s–1 according to Marcus theory and this value is also supported by kinetic simulations.