One-Electron Oxidation of a Hydrogen-Bonded Phenol Occurs by Concerted Proton-Coupled Electron Transfer

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Abstract
The hydrogen-bonded phenol 2-(aminodiphenylmethyl)-4,6-di-tert-butylphenol (HOAr - NH 2) was prepared and oxidized in MeCN by a series of one-electron oxidants. The product is the phenoxyl radical in which the phenolic proton has transferred to the amine, •OAr - NH 3 +. The reaction of HOAr - NH 2 and tris(p-tolyl)aminium ([N(tol)3]•+) to give •OAr - NH 3 + + N(tol)3 has Keq = 2.0 ± 0.5, follows second-order kinetics with k = (1.1 ± 0.2) × 105 M-1 s-1 (ΔG⧧ = 11 kcal mol-1), and has a primary isotope effect kH/kD = 2.4 ± 0.4. Oxidation of HOAr - NH 2 with [N(C6H4Br)3]•+ is faster, with k ≅ 4 × 107 M-1 s-1. The isotope effect, thermochemical arguments, and the dependence of the rate on driving force (ΔΔG⧧/ΔΔG° = 0.53) all indicate that electron transfer from HOAr - NH 2 must occur concerted with intramolecular proton transfer from the phenol to the amine (proton-coupled electron transfer, PCET). The data rule out stepwise paths that involve initial electron transfer to form the phenol radical cation •+HOAr - NH 2 or that involve initial proton transfer to give the zwitterion -OAr - NH 3 +. The dependence of the electron-transfer rate constants on driving force can be fit with the adiabatic Marcus equation, yielding a large intrinsic barrier: λ = 34 kcal mol-1 for reactions of HOAr - NH 2 with NAr3•+.