Electroreductive Cross-Coupling Reactions: Carboxylation, Deuteration, and Alkylation.

Abstract

ConspectusElectrochemistry has been used as a tool to drive chemical reactions for more than two centuries. With the help of an electrode and a power source, chemists are provided with a system whose potential can be precisely dialed in. The theoretically infinite redox range renders electrochemistry capable of oxidizing or reducing some of the most tenacious compounds. Indeed, electroreduction offers an alternative to generating highly active intermediates from electrophiles (e.g., halides, alkenes, etc.) in organic synthesis, which can be untouchable with traditional reduction methods. Meanwhile, the reductive coupling reactions are extensively utilized in both industrial and academic settings due to their ability to swiftly, accurately, and effectively construct C-C and C-X bonds, which present innovative approaches for synthesizing complex molecules. Nonetheless, its application is constrained by several inherent limitations: (a) the requirement for stoichiometric quantities of reducing agents, (b) scarce activation strategies for inert substrates with high reduction potentials, (c) incomplete mechanistic elucidation, and (d) challenges in the isolation of intermediates. The merging of electrochemistry and reductive coupling represents an attractive approach to address the above limitations in organic synthesis and has seen increasing use in the synthetic community over the past few years.Since 2020, our group has been dedicated to developing electroreductive cross-coupling reactions using readily available organic substrates with small molecules, such as organic halides, alkenes, arenes, CO₂, and D₂O, to construct high value-added organic products. Electroreductive chemistry is highly versatile and offers powerful reducing capacity and precise selectivity control, which has allowed us to develop three electrochemical modes in our lab: (1) An economically advantageous electrochemical direct reduction (EDR) strategy that emphasizes efficiency, achieves high atom utilization, and minimizes unnecessary atomic waste. (2) A class of electrochemical organo-mediated reduction (EOMR) methods that are capable of effectively controlling reaction intermediates and reaction pathways. This allows for precise modulation of reaction processes to enhance efficiency and selectivity. (3) The electrochemical metal-catalyzed reduction (EMCR) method that enables selective activation and functionalization of specific chemical bonds or functional groups under mild conditions, thereby reducing the occurrence of side reactions. We commenced our studies by establishing an organic-mediator-promoted electroreductive carboxylation of aryl and alkyl halides. This strategy was then employed for the arylcarboxylation of simple styrenes with aryl halides in a highly selective manner. Meanwhile, under direct electrolysis conditions, the carboxylation of arenes and epoxides with CO₂ as the carboxyl source was achieved. Moreover, through the precise adjustment of the electroreductive conditions, we successfully accomplished the electroreductive deuteration of arenes, olefins, and unactivated alkyl halides, enabling the efficient and selective formation of D-labeled products. Finally, building on our previous understanding of alkyl halides, we developed a series of electrochemical alkylation reactions that enable the efficient formation of C(sp³)-C(sp³) bonds using alkyl halides.