Many, if not most, redox reactions are coupled to proton transfers. This includes most common sources of chemical potential energy, from the bioenergetic processes that power cells to the fossil fuel combustion that powers cars. These proton-coupled electron transfer or PCET processes may involve multiple electrons and multiple protons, as in the 4 e–, 4 H+ reduction of dioxygen (O2) to water (eq 1), or can involve one electron and one proton such as the formation of tyrosyl radicals from tyrosine residues (TyrOH) in enzymatic catalytic cycles (eq 2). In addition, many multi-electron, multi-proton processes proceed in one-electron and one-proton steps. Organic reactions that proceed in one-electron steps involve radical intermediates, which play critical roles in a wide range of chemical, biological, and industrial processes. This broad and diverse class of PCET reactions are central to a great many chemical and biochemical processes, from biological catalysis and energy transduction, to bulk industrial chemical processes, to new approaches to solar energy conversion. PCET is therefore of broad and increasing interest, as illustrated by this issue and a number of other recent reviews.

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Thermochemistry of Proton-Coupled Electron Transfer Reagents and its Implications

Proton-coupled electron transfer (PCET) reactions involve the concerted transfer of an electron and a proton. Such reactions play an important role in many areas of chemistry and biology. Concerted PCET is thermochemically more favorable than the first step in competing consecutive processes involving stepwise electron transfer (ET) and proton transfer (PT), often by >=1 eV. PCET reactions of the form X-H + Y X + H-Y can be termed hydrogen atom transfer (HAT). Another PCET class involves outersphere electron transfer concerted with deprotonation by another reagent, Y+ + XH-B Y + X-HB+. Many PCET/HAT rate constants are predicted well by the Marcus cross relation. The cross-relation calculation uses rate constants for self-exchange reactions to provide information on intrinsic barriers. Intrinsic barriers for PCET can be comparable to or larger than those for ET. These properties are discussed in light of recent theoretical treatments of PCET.

Proton-coupled electron transfer: a reaction chemist's view.

N-Oxyl compounds represent a diverse group of reagents that find widespread use as catalysts for the selective oxidation of organic molecules in both laboratory and industrial applications. While turnover of N-oxyl catalysts in oxidation reactions may be accomplished with a variety of stoichiometric oxidants, N-oxyl reagents have also been extensively used as catalysts under electrochemical conditions in the absence of chemical oxidants. Several classes of N-oxyl compounds undergo facile redox reactions at electrode surfaces, enabling them to mediate a wide range of electrosynthetic reactions. Electrochemical studies also provide insights into the structural properties and mechanisms of chemical and electrochemical catalysis by N-oxyl compounds. This review provides a comprehensive survey of the electrochemical properties and electrocatalytic applications of aminoxyls, imidoxyls, and related reagents, of which the two prototypical and widely used examples are 2,2,6,6-tetramethylpiperidine N-oxyl (TEMPO) a...

Tetramethylpiperidine N-Oxyl (TEMPO), Phthalimide N-Oxyl (PINO), and Related N-Oxyl Species: Electrochemical Properties and Their Use in Electrocatalytic Reactions.

Free radical functionalization of organic compounds catalyzed by N-hydroxyphthalimide.

Tuning the electronic structure of heterogeneous metal catalysts has emerged as an effective strategy to optimize their catalytic activities. By preparing ethylenediamine-coated ultrathin platinum nanowires as a model catalyst, here we demonstrate an interfacial electronic effect induced by simple organic modifications to control the selectivity of metal nanocatalysts during catalytic hydrogenation. This we apply to produce thermodynamically unfavourable but industrially important compounds, with ultrathin platinum nanowires exhibiting an unexpectedly high selectivity for the production of N-hydroxylanilines, through the partial hydrogenation of nitroaromatics. Mechanistic studies reveal that the electron donation from ethylenediamine makes the surface of platinum nanowires highly electron rich. During catalysis, such an interfacial electronic effect makes the catalytic surface favour the adsorption of electron-deficient reactants over electron-rich substrates (that is, N-hydroxylanilines), thus preventing full hydrogenation. More importantly, this interfacial electronic effect, achieved through simple organic modifications, may now be used for the optimization of commercial platinum catalysts.

Interfacial electronic effects control the reaction selectivity of platinum catalysts

The homolytic bond dissociation energies (BDEs) of the O−H bonds in DMSO solution for (a) phenol and a number of its derivatives, (b) three oximes, (c) three alcohols, (d) three hydroxylamines, and (e) two hydroxamic acids have been estimated by eq 1:  BDEHA = 1.37pKHA + 23.1Eox(A-) + 73.3 kcal/mol. For most of these hydroxylic acids, the BDEs of the O−H bonds estimated by eq 1 are within ±2 kcal/mol of the literature values in nonpolar solvents or in the gas phase. There is no reason to believe, therefore, that these BDEs are “seriously in error because of failure to correct for solvent effects” as has been claimed on the basis that BDEs in highly polar solvents estimated for the O−H bond in phenol by photoacoustic calorimetry must be so corrected.

Solvent Effects on Homolytic Bond Dissociation Energies of Hydroxylic Acids

The equilibrium acidities (pKHA values) in DMSO of the following hydroxylamines have been measured:  (1) N-phenylhydroxylamine and its p-bromo and p-cyano derivatives, (2) N-benzyl-N-phenylhydroxylamine and its p-bromo and p-cyano derivatives, (3) O-benzyl-N-phenylhydroxylamine, (4) N-benzoylphenylhydroxylamine and its p-bromo and p-cyano derivatives, and (5) N-hydroxylpiperidine. The BDEs of the O−H and/or N−H bonds in these 11 weak acids have been estimated by combining their pKHA values with the oxidation potentials of their conjugate bases according to the following equation:  BDEHA = 1.37pKHA + 23.1Eox(A-) + 73.3 kcal/mol.

Wei-Zhong Liu

Papers

Solvent Effects on Homolytic Bond Dissociation Energies of Hydroxylic Acids

Equilibrium Acidities and Homolytic Bond Dissociation Energies of N−H and/or O−H Bonds in N-Phenylhydroxylamine and Its Derivatives

Equilibrium Acidities and Homolytic Bond Dissociation Energies of N-H and/or O-H Bonds in N-Phenylhydroxylamine and Its Derivatives.