Key words
ketyl radicals - umpolung - electrosynthesis - electroreduction - carbonyl
Ketyl radicals have been widely used in modern organic synthesis to construct value-added alcohols from carbonyl compounds. In contrast to the intrinsic electrophilicity of carbonyls, the nucleophilic ketyl radicals display complementary reactivities with respect to the reaction scope.[1] For this reason, the generation of ketyl radicals under mild conditions is of high synthetic value. Traditionally, the generation of ketyl radicals from carbonyl compounds has relied on the use of SmI2 or active metals such as K, Sn, and Ti, but the requirement for stoichiometric quantities of metals or metal salts diminishes the synthetic utility of this approach. Recently, the rapid development of photoredox chemistry has stimulated a resurgence of interest in the chemistry of ketyl radicals since it represents a milder strategy for obtaining such radicals. However, due to the high reduction potential of carbonyls, the range of accessible photocatalysts that meet the redox properties that match with the corresponding carbonyls is limited.
Organic electrosynthesis has emerged as a unique and irreplaceable tool for sustainable synthesis by employing electrons to circumvent the need for stoichiometric amounts of chemical redox agents.[2] Moreover, the direct electroreduction of carbonyls to the corresponding ketyl radicals obviates the use of expensive photocatalysts. As such, significant achievements toward the electrochemical generation of ketyl radicals have been made in the past decade. Since ketyl radicals are prone to homocoupling to afford pinacols, their employment in couplings with polarity-matched partners or other coupling partners in large molar excess are common strategies. In this graphical review, these electrochemical advancements are classified into three major categories: cross-pinacol couplings, coupling of carbonyls with alkyl radical precursors, and coupling of carbonyls with unsaturated systems (alkenes, alkynes, cyanoarenes, and heterocycles).
Figure 1 Electrochemical cross-pinacol coupling[3a]
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Figure 2 Electrochemical coupling of carbonyls with alkyl radical precursors[4`]
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Figure 3 Electrochemical coupling of ketones with alkenes and alkynes[5`]
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Figure 4 Electrochemical coupling of carbonyls with cyanoarenes[6a]
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Figure 5 Electrochemical coupling of carbonyls with N-heterocycles[7`]
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