Radical Chain Reduction via Carbon Dioxide Radical Anion (CO₂^•–)

Abstract

We developed an effective method for reductive radical formation that utilizes the radical anion of carbon dioxide (CO 2•− ) as a powerful single electron reductant. Through a polarity matched hydrogen atom transfer (HAT) between an electrophilic radical and a formate salt, CO 2•− formation occurs as a key element in a new radical chain reaction. Here, radical chain initiation can be performed through photochemical or thermal means, and we illustrate the ability of this approach to accomplish reductive activation of a range of substrate classes. Specifically, we employed this strategy in the intermolecular hydroarylation of unactivated alkenes with (hetero)aryl chlorides/bromides, radical deamination of arylammonium salts, aliphatic ketyl radical formation, and sulfonamide cleavage. We show that the reactivity of CO 2•− with electron-poor olefins results in either single electron reduction or alkene hydrocarboxylation, where substrate reduction potentials can be utilized to predict reaction outcome.

Selective Electrosynthetic Hydrocarboxylation of α,β‐Unsaturated Esters with Carbon Dioxide**

“…CO 2 − and that Michael acceptors that possess a more negative reduction potential undergo hydrocarboxylation while those with more positive reduction potentials undergo reduction. [21] This does not fully explain our electrochemical carboxylation but there are some similarities, for example, trans ‐cinnamate esters undergo carboxylation in low yield ( 1 m , 1 n ) and under Jui's conditions are reduced to the alkane, thus perhaps indicating that two mechanisms could be in operation (reduction of CO 2 and/or reduction of the olefin).…”

Selective Electrosynthetic Hydrocarboxylation of α,β‐Unsaturated Esters with Carbon Dioxide**

“…However,J ui has recently described the photochemical generation of CCO 2 À and that Michael acceptors that possess am ore negative reduction potential undergo hydrocarboxylation while those with more positive reduction potentials undergo reduction. [21] This does not fully explain our electrochemical carboxylation but there are some similarities,f or example, trans-cinnamate esters undergo carboxylation in low yield (1m, 1n)a nd under Juis conditions are reduced to the alkane,thus perhaps indicating that two mechanisms could be in operation (reduction of CO 2 and/or reduction of the olefin).…”

An organophotocatalytic late-stage N–CH3oxidation of trialkylamines toN-formamides with O2in continuous flow

“…Especially, given the aforementioned putative presence of DCAS˙ − via reductive quenching of 1 DCAS* by trialkylamines (well known for DCA's case) 50,51 and given that photoexcited radical anions are known to reductively cleave aryl halides and other strong bonds. 50–53 C–F bonds and N–Ts groups are also prone to reductive cleavage under reductive photocatalysis 54 or by photoexcited super electron donors. 55 …”