Redox Properties of Radicals

Abstract

The redox properties of organic radicals in solution and many techniques used to evaluate the thermodynamic and kinetic parameters of this important class of reactive intermediate are reviewed. An overview is provided of the many methods used to generate and detect radicals by direct and indirect electrochemical methods including cyclic voltammetry with microelectrodes and ultramicroelectrodes, competitive q method and alternating current voltammetry, and non‐direct photoelectrochemical methods involving intermediate generation by photochemical means and subsequent investigation by electrochemical and other complimentary techniques including photomodulated voltammetry, laser flash photoelectron injection, and pulse radiolysis. The end of the article provides detailed tables summarizing the reduction and oxidation potentials of various classes of organic radicals.

Electroreductive Carbofunctionalization of Alkenes with Alkyl Bromides via a Radical-Polar Crossover Mechanism

“…The subsequent radical addition to 2 is polarity-matched and will occur rapidly to give a new C-centered radical ( Int-6 ). The competing over-reduction of i Pr • and reduction of 2 happen at a potential of −2.3 V and −2.6 V, respectively, and are thus slower. In the next step, the nascent benzylic radical intermediate ( Int-6 ) will be readily reduced to the carbanion with a potential of −1.6 V .…”

“…The competing over-reduction of i Pr • and reduction of 2 happen at a potential of −2.3 V and −2.6 V, respectively, and are thus slower. In the next step, the nascent benzylic radical intermediate ( Int-6 ) will be readily reduced to the carbanion with a potential of −1.6 V . In contrast, the competing radical addition to another molecule of 2 is slow because the polarity matching is unfavorable .…”

Visible Light Uranyl Photocatalysis: Direct C–H to C–C Bond Conversion

“….32 V vs SCE; nonreversible behavior; Figure S8). The process is likewise feasible when the adduct radical is stabilized by only one electron-withdrawing group (to give compounds 29−30), since the redox potential of the succinonitrile radical E(NCCH • CH 2 CN/ NCCH − CH 2 CN taken as a reference was reported to be +0.165 V vs SCE 19 (Figure 1b). Thus, the reaction with dimethyl maleate 2n proceeded satisfactorily in the presence of water, where the uranyl cation showed a more reversible behavior, with a slightly increased reducing power, S8).…”

Radical Hydrodifluoromethylation of Unsaturated C-C Bonds via an Electroreductively Triggered Two-pronged Approach

“…[66][67][68][69] Alternatively, a carbon-centered radical D would directly perform hydrogenatom transfer (HAT) to form F in the presence of H donor such as thiol, when Ered(D/E) is too negative to be reduced on the cathode (path B, if R 1 = EDG or Alkyl). 72 As anticipated, a control experiment without applied current revealed that electrolytic conditions is necessary for desired reactivity (entry 2). In the absence of either water or piperidine additive, the reaction was prematurely terminated with a significant decrease in yield (entries 3−4).…”

“…Interestingly, vinyl cyclopropane 6a with higher ring opening rate constant (kPh = ~10 8 s −1 ) underwent rupture of the three-membered ring (8a), while the cyclopropyl ring in 6b (kH = ~10 5 s −1 ) remained intact after electrolysis under standard conditions (7b). 72 These results imply that the reduction of the benzylic radical intermediate (Int-8) is sufficiently fast to prevent undesired side reactions, constituting radical-polar crossover mechanism (Fig 2 , path A). Having identified the optimized reaction parameters, we next explored the substrate scope of conjugated alkenes (Table 1).…”