TL;DR: A review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis can be found in this paper.
Abstract: We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.
TL;DR: The intermolecular alkylation of pyridine units with simple alkenes has been achieved via a photoredox radical mechanism, which is mild, tolerant of many functional groups, and effective for the preparation of a wide range of complex alkylpyridines.
Abstract: The intermolecular alkylation of pyridine units with simple alkenes has been achieved via a photoredox radical mechanism. This process occurs with complete regiocontrol, where single-electron reduction of halogenated pyridines regiospecifically yields the corresponding radicals in a programmed fashion, and radical addition to alkene substrates occurs with exclusive anti-Markovnikov selectivity. This system is mild, tolerant of many functional groups, and effective for the preparation of a wide range of complex alkylpyridines.
TL;DR: A combination of computational and experimental studies support a mechanism involving proton-coupled electron transfer followed by medium-dependent alkene addition and rapid hydrogen atom transfer mediated by a polarity-reversal catalyst.
Abstract: We report the photoredox alkylation of halopyridines using functionalized alkene and alkyne building blocks. Selective single-electron reduction of the halogenated pyridines provides the corresponding heteroaryl radicals, which undergo anti-Markovnikov addition to the alkene substrates. The system is shown to be mild and tolerant of a variety of alkene and alkyne subtypes. A combination of computational and experimental studies support a mechanism involving proton-coupled electron transfer followed by medium-dependent alkene addition and rapid hydrogen atom transfer mediated by a polarity-reversal catalyst.
TL;DR: A catalytic redox system for the direct conjugate addition of pyridines and diazines to Michael acceptors has been developed.
Abstract: The direct addition of pyridine and diazine units to electron-poor alkenes has been achieved via a redox radical mechanism that is enabled by limiting the effective concentration of the hydrogen-atom source. The described method is tolerant of acidic functional groups and is generally applicable to the union of a wide range of Michael acceptors and 6-membered heterocyclic halides.
TL;DR: In this article, the reduction of chalcone at the dropping mercury electrode is expressed by scheme (A)−(U) with an approximate value p K 6 =10.2 and p K 9 =8.75.
Abstract: Summary Individual steps in the reduction of chalcone at the dropping mercury electrode are expressed by scheme (A)−(U). The product of the first two-electron step is dihydrochalcone; in the second, alcohol is formed. Reduction of dihydrochalcone is governed by the rate of its general acid-base catalysed formation from the carbanion-enolate which is the primary electrolytic product. In the first one-electron step, an organomercury compound is formed. Reduction processes are accompanied by antecedent and interposed proton, transfers. For the protonation of the radical anion, [ArCO-CHCHAr] (•) , resulting in the chalcone reduction, an approximate value, p K 6 =10.2 was found and for that of the radical anion, [ArCOCH 2 CH 2 Ar] (•) , resulting in the dihydrochalcone reduction, an approximate value, p K 9 =8.75 was found. Radical anions react with alkali metal cations, but the waves of ketyls formed are not separated from those of radicals, resulting in analogous reactions with dydronium ions. The importance of using results obtained with controlled-potential electrolysis by means of a dropping mercury electrode rather than with a mercury pool electrode, for elucidation of polarographic processes, was stressed.