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

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.

New methods and strategies for the direct functionalization of C-H bonds are beginning to reshape the field of retrosynthetic analysis, affecting the synthesis of natural products, medicines and materials. The oxidation of allylic systems has played a prominent role in this context as possibly the most widely applied C-H functionalization, owing to the utility of enones and allylic alcohols as versatile intermediates, and their prevalence in natural and unnatural materials. Allylic oxidations have featured in hundreds of syntheses, including some natural product syntheses regarded as "classics". Despite many attempts to improve the efficiency and practicality of this transformation, the majority of conditions still use highly toxic reagents (based around toxic elements such as chromium or selenium) or expensive catalysts (such as palladium or rhodium). These requirements are problematic in industrial settings; currently, no scalable and sustainable solution to allylic oxidation exists. This oxidation strategy is therefore rarely used for large-scale synthetic applications, limiting the adoption of this retrosynthetic strategy by industrial scientists. Here we describe an electrochemical C-H oxidation strategy that exhibits broad substrate scope, operational simplicity and high chemoselectivity. It uses inexpensive and readily available materials, and represents a scalable allylic C-H oxidation (demonstrated on 100 grams), enabling the adoption of this C-H oxidation strategy in large-scale industrial settings without substantial environmental impact.

http://europepmc.org/articles/PMC4860034?pdf=render

Scalable and sustainable electrochemical allylic C–H oxidation

Oriented toward Process Optimization and Development Rogeŕio A. F. Tomaś,† Joaõ C. M. Bordado,‡ and Joaõ F. P. Gomes*,‡,§ †ARTLANT, Zona Industrial e Logística de Sines, Zona 2, Lote 2E1, Monte Feio, 7520-064 Sines, Portugal ‡Instituto de Biotecnologia e Bioengenharia/Instituto Superior Tećnico (IBB), Universidade Tećnica de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Instituto Superior de Engenharia de Lisboa (ISEL), Área Departamental de Engenharia Química, R. Conselheiro Emídio Navarro, 1959-007 Lisboa, Portugal

p-Xylene oxidation to terephthalic acid: a literature review oriented toward process optimization and development.

The phthalimide N-oxyl (PINO) radical was generated by the oxidation of N-hydroxyphthalimide (NHPI) with Pb(OAc)4 in acetic acid. The molar absorptivity of PINO• is 1.36 × 103 L mol-1 cm-1 at λmax 382 nm. The PINO radical decomposes slowly with a second-order rate constant of 0.6 ± 0.1 L mol-1 s-1 at 25 °C. The reactions of PINO• with substituted toluenes, benzaldehydes, and benzyl alcohols were investigated under an argon atmosphere. The second-order rate constants were correlated by means of a Hammett analysis. The reactions with toluenes and benzyl alcohols have better correlations with σ+ (ρ = −1.3 and −0.41), and the reaction with benzaldehydes correlates better with σ (ρ = −0.91). The kinetic isotope effect was also studied and significantly large values of kH/kD were obtained:  25.0 (p-xylene), 27.1 (toluene), 27.5 (benzaldehyde), and 16.9 (benzyl alcohol) at 25 °C. From the Arrhenius plot for the reactions with p-xylene and p-xylene-d10, the difference of the activation energies, EaD − EaH, was 12...

Kinetic study of the phthalimide N-oxyl radical in acetic acid. Hydrogen abstraction from substituted toluenes, benzaldehydes, and benzyl alcohols

The reactions of the phthalimide N-oxyl (PINO) radical with several hydrocarbons having different C−H bond dissociation energies (BDEs) were investigated in HOAc at 25 °C. The slope of the Evans−Polanyi plot is 0.38, which indicates that the reactions are mildly exothermic or almost thermoneutral. This finding is supported by the O−H BDE of N-hydroxyphthalimide (NHPI), 375 ± 10 kJ mol-1, obtained by means of a thermodynamic cycle. The observed kinetic isotope effects (KIEs) are related to the reaction free-energy changes, which can be explained by a Marcus-type equation. The comparison of the PINO radical reactions with analogous hydrogen atom abstraction reactions of the dibromide radical implies that HBr2• reactions are more exothermic than the PINO radical reactions. This factor is put forth to explain the different KIEs.

Kinetic Study of the Phthalimide N-Oxyl (PINO) Radical in Acetic Acid. Hydrogen Abstraction from C−H Bonds and Evaluation of O−H Bond Dissociation Energy of N-Hydroxyphthalimide

N-Hydroxyphthalimide (NHPI) and its derivatives, such as 3-F-NHPI, 4-Me-NHPI, N-acetoxyphthalimide, and N,N-dihydroxypyromelitimide, were used as promoters with Co(OAc)2 catalyst for the autoxidation of p-xylene (pX) and other methyl arenes. All the promoters gave acceptable rates and yields of terephthalic acid. The initial reaction rates, measured by the rate of oxygen uptake, were analyzed by a rate equation in terms of [pX], [Co(II)], and [NHPI]. The metal cocatalysts Mn(II) and Ce(III) accelerated the reaction significantly at millimolar concentrations. The reaction occurs by a chain mechanism that involves formation of the phthalimide N-oxyl radical, PINO• (that is, R2NO•), which abstracts a hydrogen atom from the methyl group of p-xylene to form the carbon-centered radical ArCH2•. In a stepwise fashion, the sequence progresses through alcohol, aldehyde, and carboxylic acid; at each stage, C−H abstraction by PINO• is involved. A significant kinetic isotope effect on the overall oxidation of p-xylene...

N-Hydroxyphthalimides and Metal Cocatalysts for the Autoxidation of p-Xylene to Terephthalic Acid

Kinetic data have been obtained for three distinct types of reactions of phthalimide N-oxyl radicals (PINO•) and N-hydroxyphthalimide (NHPI) derivatives. The first is the self-decomposition of PINO• which was found to follow second-order kinetics. In the self-decomposition of 4-methyl-N-hydroxyphthalimide (4-Me-NHPI), H-atom abstraction competes with self-decomposition in the presence of excess 4-Me-NHPI. The second set of reactions studied is hydrogen atom transfer from NHPI to PINO•, e.g., PINO• + 4-Me-NHPI ⇄ NHPI + 4-Me-PINO•. The substantial KIE, kH/kD = 11 for both forward and reverse reactions, supports the assignment of H-atom transfer rather than stepwise electron−proton transfer. These data were correlated with the Marcus cross relation for hydrogen-atom transfer, and good agreement between the experimental and the calculated rate constants was obtained. The third reaction studied is hydrogen abstraction by PINO• from p-xylene and toluene. The reaction becomes regularly slower as the ring substit...

Kinetics of Self-Decomposition and Hydrogen Atom Transfer Reactions of Substituted Phthalimide N-Oxyl Radicals in Acetic Acid

An oxorhenium(V) dimer, {MeReO(mtp)}2, D, where mtpH2 is 2-(mercaptomethyl)thiophenol, catalyzes oxygen atom transfer reaction from methyl phenyl sulfoxide to triarylphosphines. Kinetic studies in benzene-d6 at 23 °C indicate that the reaction takes place through the formation of an adduct between D and sulfoxide. The equilibrium constants, KDL, for adduct formation were determined by spectrophotometric titration, and the values of KDL for MeS(O)C6H4-4-R were obtained as 14.1(2), 5.7(1), and 2.1(1) for R = Me, H, and Br, respectively. Following sulfoxide binding, oxygen atom transfer occurs with either internal or external nucleophilic assistance. Because {MeReO(mtp)}2 is a much more reactive catalyst than its monomerized form, MeReO(mtp)PPh3, loss of the active catalyst during the time course of the reaction must be taken into account as a part of the kinetic analysis. As it happens, sulfoxide catalyzes monomerization. Monomerization by triarylphosphines was also studied in the presence of sulfoxide, and...

Nobuyoshi Koshino

Papers

Kinetic study of the phthalimide N-oxyl radical in acetic acid. Hydrogen abstraction from substituted toluenes, benzaldehydes, and benzyl alcohols

Kinetic Study of the Phthalimide N-Oxyl (PINO) Radical in Acetic Acid. Hydrogen Abstraction from C−H Bonds and Evaluation of O−H Bond Dissociation Energy of N-Hydroxyphthalimide

N-Hydroxyphthalimides and Metal Cocatalysts for the Autoxidation of p-Xylene to Terephthalic Acid

Kinetics of Self-Decomposition and Hydrogen Atom Transfer Reactions of Substituted Phthalimide N-Oxyl Radicals in Acetic Acid

Kinetics and mechanism of oxygen atom transfer from methyl phenyl sulfoxide to triarylphosphines catalyzed by an oxorhenium(V) dimer.