Conventional methods for carrying out carbon–hydrogen functionalization and carbon–nitrogen bond formation are typically conducted at elevated temperatures, and rely on expensive catalysts as well as the use of stoichiometric, and perhaps toxic, oxidants. In this regard, electrochemical synthesis has recently been recognized as a sustainable and scalable strategy for the construction of challenging carbon–carbon and carbon–heteroatom bonds. Here, electrosynthesis has proven to be an environmentally benign, highly effective and versatile platform for achieving a wide range of nonclassical bond disconnections via generation of radical intermediates under mild reaction conditions. This review provides an overview on the use of anodic electrochemical methods for expediting the development of carbon–hydrogen functionalization and carbon–nitrogen bond formation strategies. Emphasis is placed on methodology development and mechanistic insight and aims to provide inspiration for future synthetic applications in the field of electrosynthesis.

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Electrochemical strategies for C-H functionalization and C-N bond formation.

The coupling between electron and proton transfers has a long experimental and theoretical history in chemistry and biochemistry. To take just one example, the fact that acceptance of an electron triggers the addition of an acid or the removal of a base and vice versa for oxidations towers over all understanding of organic electrochemistry. Protoncoupled electron transfer (PCET) reactions also play a critical role in a wide range of biological processes, including enzyme reactions, photosynthesis, and respiration. A recent impressive review describes PCET reactions and phenomena. PCET is employed here as a general term for reactions in which both an electron and a proton are transferred, either in two separate steps or in a single step. Reactions in which the electron and proton transfer between the same donor and acceptor, that is, hydrogen atom transfer, are, of course, not considered here because we consider electrochemical PCET reactions in which electrons are flowing into or from an electrode while protons are transferred between acid and base. Molecular electrochemistry, through nondestructive techniques such as cyclic voltammetry, has proved to be very useful in characterizing electron transfers and deciphering mechanisms in which chemical reactions are associated with electron transfer. Therefore, it has been a convenient tool for the mechanistic study of reactions in which electron transfer is coupled to proton transfer, that is, in which an electron leaves or enters an electrode while a proton is transferred from or to the redox species. Until recently, PCET has been mostly thought of as stepwise electron and proton transfer (ET-PT or PT-ET). We thus review in an initial section (section 2) the analysis of such stepwise mechanisms both in aprotic media and in water. In aprotic media, * E-mail address: cyrille.costentin@univ-paris-diderot.fr. Cyrille Costentin was born in Normandy, France, in 1972. He received his undergraduate education at Ecole Normale Superieure (Cachan, France) and pursued his graduate studies under the guidance of Prof. Jean-Michel Saveant and Dr. Philippe Hapiot at the University of ParisDiderot (Paris 7), where he received his Ph.D. in 2000. After a year as a postdoctoral fellow at the University of Rochester, working with Prof. J. P. Dinnocenzo, he joined the faculty at the University of Paris-Diderot as an associate professor. He was promoted to professor in 2007. His interests include mechanisms and reactivity in electron transfer chemistry with particular recent emphasis on electrochemical and theoretical approaches to proton-coupled electron transfer processes. Chem. Rev. 2008, 108, 2145–2179 2145

Electrochemical Approach to the Mechanistic Study of Proton-Coupled Electron Transfer

Unter den klassischen Mesmethoden der elektroanalytischen Chemie geniest die Polarographie noch immer den hochsten Bekanntheitsgrad. In den letzten Jahren wurde ihr diese Position allerdings zunehmend von der Cyclovoltammetrie streitig gemacht. Einfach anzuwendende diagnostische Kriterien und eine relativ leicht zugangliche Mestechnik haben diese Entwicklung begunstigt. Die Attraktivitat der Methode resultiert weiterhin daraus, das sich aus Cyclovoltammogrammen neben Informationen uber die Thermodynamik von Redoxprozessen auch Kenntnisse uber die Kinetik von heterogenen Elektronentransferreaktionen sowie von angekoppelten chemischen Reaktionen erhalten lassen. Die charakteristischen Formen der Cyclovoltammogramme und die eindeutigen Potentiallagen der Peaks, die nahezu „fingerprint”-artig die elektrochemischen Eigenschaften von Redoxsystemen widerspiegeln, haben der Methode den Beinamen „elektrochemische Spektroskopie” eingetragen.

Cyclovoltammetrie — die „Spektroskopie”︁ des Elektrochemikers

3.1.5. Spectroelectrochemistry 2189 3.2. Photochemical Initiation 2190 3.2.1. Laser Flash Photolysis 2190 3.3. Initiation by Solvated Electrons 2190 3.3.1. Pulse Radiolysis 2191 3.4. Thermal Homogeneous Electron Transfer 2191 3.4.1. Self-Exchange Electron Transfer 2191 3.4.2. Stop Flow 2191 3.4.3. Continuous Flow 2191 4. Electron Transfer Reactions: Fundamental Aspects and Experimental Examples 2192

Jeffrey A. Richards

Papers

Electrochemical oxidation of 2,4,6-tri-tert-butylphenol

Flow cell for electrolysis within the probe of a nuclear magnetic resonance spectrometer

Electrochemical oxidation of 2,6-di-tert-butyl-4-isopropylphenol

Electrochemical reduction of .alpha.,.beta.-unsaturated ketones