Concerted Proton−Electron Transfer in the Oxidation of Hydrogen-Bonded Phenols

Abstract

Three phenols with pendant, hydrogen-bonded bases (HOAr-B) have been oxidized in MeCN with various one-electron oxidants. The bases are a primary amine (-CPh(2)NH(2)), an imidazole, and a pyridine. The product of chemical and quasi-reversible electrochemical oxidations in each case is the phenoxyl radical in which the phenolic proton has transferred to the base, (*)OAr-BH(+), a proton-coupled electron transfer (PCET) process. The redox potentials for these oxidations are lower than for other phenols, predominately from the driving force for proton movement. One-electron oxidation of the phenols occurs by a concerted proton-electron transfer (CPET) mechanism, based on thermochemical arguments, isotope effects, and DeltaDeltaG(++)/DeltaDeltaG degrees . The data rule out stepwise paths involving initial electron transfer to form the phenol radical cations [(*)(+)HOAr-B] or initial proton transfer to give the zwitterions [(-)OAr-BH(+)]. The rate constant for heterogeneous electron transfer from HOAr-NH(2) to a platinum electrode has been derived from electrochemical measurements. For oxidations of HOAr-NH(2), the dependence of the solution rate constants on driving force, on temperature, and on the nature of the oxidant, and the correspondence between the homogeneous and heterogeneous rate constants, are all consistent with the application of adiabatic Marcus theory. The CPET reorganization energies, lambda = 23-56 kcal mol(-)(1), are large in comparison with those for electron transfer reactions of aromatic compounds. The reactions are not highly non-adiabatic, based on minimum values of H(rp) derived from the temperature dependence of the rate constants. These are among the first detailed analyses of CPET reactions where the proton and electron move to different sites.

Concerted heavy-atom bond cleavage and proton and electron transfers illustrated by proton-assisted reductive cleavage of an O–O bond

“…At first sight, the answer is expected to be positive by reference to CPET reactions with no heavy-atom bond breaking where small but significant (of the order of 2) H/D kinetic isotope effects are commonly observed (8)(9)(10)28). There is, however, an important difference between these reactions and the present ones, namely the large irreversibility caused by the breaking of the heavy-atom bond, as pictured in Fig.…”

“…In this framework, uncovering the mechanisms and modeling the kinetic characteristics of these reactions has attracted and continues to attract a great deal of attention concerning both heavy-atoms bonds (2-4) and proton transfers (5)(6)(7)(8)(9)(10)(11). As pictured in Fig.…”

Using coherence to enhance function in chemical and biophysical systems

“…An opportunity for coherence is in the coordination of the electron and proton transfer, where quantum effects are relevant because these are light particles. Evidence suggests they can be concerted 99 , but to what extent are they quantum-mechanically coherent? Interestingly, many of the principles discussed in this Review are relevant to the mechanistic details of proton-coupled electron transfer reactions 100 .…”

Antimicrobial and Spectral Studies of Zinc Metal Complexes Derived From Imidazo [4, 5-F] [1, 10] – Phenanthroline Derivative

“…[15] The structure of ligands and their metal complexes were established using elemental analyses, 1 H-NMR, 13 C-NMR and Mass spectra. Physical data of newly synthesized compounds are summarized in the table 1.…”