Catalytic difunctionalization of alkenes with CO₂ via visible light photoredox catalysis is mainly limited to the addition of free radicals¹ or CO₂ radical cation² to the alkenes to generate new carbon radicals intermediate. However, the visible light-driven difunctionalization of alkenes with CO₂ via radical anions of alkenes still remains unknown. Herein, we report a successive single electron transfer (SSET) strategy for the dicarboxylation of alkenes, allenes with two molecules of CO₂ via radical anion intermediate, providing a simple and sustainable approach to valuable succinic acids. Moreover, diverse (hetero)arenes also are suitable substrates to undergo the dearomative dicarboxylation with high regio- and diastereoselectivities.

The research group of Prof. Da-Gang Yu at Sichuan University has been focusing on developing new strategies for CO₂ utilization and visible-light photoredox catalysis in the past few years. In the area of carboxylation with CO₂ via SSET process, we have developed the selective and catalytic hydrocarboxylation of enamides and imines with CO₂ to generate α,α-disubstituted ɑ-amino acids (ACIE 2018, 57, 13897), cross-electrophile couplings of tetraalkyl ammonium salts with CO₂ without external reductant (JACS 2018, 140, 17338), ring-opening carboxylation of cyclic oxime esters with CO₂ (ChemSusChem 2020, 13, 6312), as well as reductive dearomative arylcarboxylation of indoles with CO₂ (Nat. Commun. 2020, 11, 3263). Very recently, we have developed a novel strategy for dicarboxylation of alkenes, allenes, and (hetero)arenes with CO₂ via radical anions of such unsaturated substrates with SSET process, as shown below (Nat. Catal. 2021, DOI : 10.1038/s41929-021-00594-1).

Electrochemical dicarboxylation of alkenes have been reported, but unfortunately, the high electrochemical potentials (-2 to -2.5 V vs SHE) for practical current densities leading to narrow substrate socpe and low selectivity³. In the area of visible-light photoredox-catalyzed carboxylations with CO₂, our group and others⁴ have reported many examples to the synthesis of industrially relevant carboxylic acids, however, these carboxylation protocols remain currently confined to single CO₂ insertion. This situation might arise from the high reduction potential of alkenes and (hetero)arenes (for styrene, E_red = -2.91 V vs Ag/Ag⁺)⁵ and competitive side reactions, including reduction⁶, reductive homocoupling⁷, hydrocarboxylation⁸. As we developed highly reductive system for reductive carboxylation of imines and indoles with CO₂, we considered that whether alkenes also could be reduced and then react with two CO₂ through SSET process. With these challenges and the strategy in mind, we further realized dicarboxylation of alkenes, allenes, and (hetero)arenes with CO₂ via radical anion intermediate with SSET process. Notably, this method realized multiple C-C bond formations with high chemo- and diastereoselectivity under mild conditions. Moreover, this transition-metal-free protocol exhibits low catalyst loading, good functional group tolerance, broad substrate scope, facile scalability and easy product derivatizations to drug and material molecules. The Stern-Volmer quenching studies, UV−Vis monitoring studies and control experiments all suggested alkene radical anion was the key intermediate. Deuterium-labeling control experiments also suggested the generation of a benzylic carbanion. These studies indicated a pathway by SSET via radical anions of such unsaturated substrates, broadening the repertoire of strategies for CO₂ fixation.

More details of this work could be found here: “Dicarboxylation of alkenes, allenes, and (hetero)arenes with CO₂ via visible-light photoredox catalysis” in Nature Catalysis (https://www.nature.com/articles/s41929-021-00594-1).

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