Electrochemically Initiated Radical Reactions

Abstract

This article reports on the applications of electrochemistry for the generation of radicals as neutral, cationic, and anionic intermediates and their use in preparative reactions for the formation of carbon–carbon and carbon–heteroatom bonds. The focus of this article is on electrochemical oxidation reactions of enol ethers, ketene acetals, and electron‐rich aromatic compounds. Furthermore, recent advances in the electrochemically induced synthesis of four‐membered ring systems and biaryl compounds are covered. The oxidative carbon–carbon bond formation by Kolbe electrolysis concludes the section dealing with oxidative transformations. The section describing reductive processes consists of indirect electrochemical reduction of alkylhalides and thioesters, as well as the reductive interconversion of functional groups described recently.

Controlled Single-Electron Transfer via Metal–Ligand Cooperativity Drives Divergent Nickel-Electrocatalyzed Radical Pathways

“…While it is well-established that these open-shell intermediates are readily accessed via halogen atom transfer from alkyl halide (RX) feedstocks, − the toxicity of tin, silicon, or trialkylborane reagents traditionally employed to initiate this reactivity motivates the development of complementary synthetic methods with improved sustainability. Among the numerous alternatives pursued, − strategies that enable RX activation via single electron transfer (SET) present an opportunity for radical generation to be interfaced with SET catalysts, where energy input from visible light or electricity , enables turnover under mild conditions. RX reduction under photoredox conditions typically employs coordinatively saturated Rh or Ir polypyridyl catalysts. − ,,, In these systems, it is hypothesized that the initial light sensitization steps form reductants that activate substrates via outer-sphere electron transfer (OSET). , Thus, the interaction of the starting material with the metal center, promoting an inner-sphere electron transfer (ISET) event or coordination of the ensuing carbon-centered radical, is not invoked.…”

Sustainable radical approaches for cross electrophile coupling to synthesize trifluoromethyl- and allyl-substituted tert-alcohols

“…Based on the above observations and literature precedent (Bohn et al, 2012;Liu et al, 2018;Pe ´ter et al, 2021;Sharma et al, 2016;Tang et al, 2020;Zeng et al, 2019), plausible mechanisms for the photoredox (a) and electrochemical (b) reaction between 1a and 2 are proposed in Figure 4. Upon visible-light irradiation, the excited photocatalyst [Ir (III) ]* (E (Ir (IV) /Ir (III) *) = À0.96 V, E (Ir (III) */Ir (II) ) = +0.66 V) (Prier et al, 2013) is formed and is reductively quenched by single-electron transfer from DCHMA (E 1/2 = +0.67) (Figure S2), producing the [Ir (II) ] complex and radical cation A.…”

“…Recently, electrochemistry has emerged as a sustainable approach for efficient generation of radical intermediates by utilizing eco-friendly electric energy (Bohn et al, 2012;Lennox et al, 2018;Monroe and Heien, 2013;Nu ´n ˜ez-Vergara et al, 1997;Xiong and Xu, 2019), without the use of stoichiometric amounts of external oxidizing and reducing agents (Bohn et al, 2012;Xiong and Xu, 2019). We developed a sustainable electrochemical approach for the generation of ketyl and allyl radical to synthesize trifluoromethyl-and allyl-substituted tert-alcohols (Figure 3).…”

Organic Chromophores Designed for Hole Injection into Wide-Band-Gap Metal Oxides for Solar Fuel Applications

“…Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed in an acetonitrile solution (0.1 M tetrabutylammonium hexafluorophosphate electrolyte) to obtain the ground-state redox potentials, E °(GS +/0 ) and E °(GS 0/– ), and to characterize the electrochemical properties of the DAD triads, as shown in Figures and S6–S10. The electrochemical measurements were conducted on the ester form of the dyes due to their solubility in organic solvents and because they are less susceptible to radical reactions compared to their carboxylic acid forms . The three-electrode cell setup consisted of a Pt counter electrode, a Ag/AgNO 3 reference electrode, and a glassy carbon working electrode.…”

“…The electrochemical measurements were conducted on the ester form of the dyes due to their solubility in organic solvents and because they are less susceptible to radical reactions compared to their carboxylic acid forms. 67 The three-electrode cell setup consisted of a Pt counter electrode, a Ag/AgNO 3 reference electrode, and a glassy carbon working electrode. The electrochemical results are summarized in Table 2.…”