Recent progress in cathodic reduction-enabled organic electrosynthesis: Trends, challenges, and opportunities

Compared with general redox chemistry, electrochemistry using the electron as a potent, controllable, yet traceless alternative to chemical oxidants/reductants usually offers more sustainable options for achieving selective organic synthesis. With its environmentally benign features gradually being uncovered and studied, organic electrosynthesis is currently undergoing a revival and becoming a rapidly growing area within the synthetic community. Among the electrochemical transformations, the anodically enabled ones have been far more extensively exploited than those driven by cathodic reduction, although both approaches are conceptually attractive. To stimulate the development of cathodically enabled organic reactions, this review summarizes the recently developed reductive electrosynthetic protocols, discussing and highlighting reaction features, substrate scopes, applications, and plausible mechanisms to reveal the recent trends in this area. Herein, cathodic reduction-enabled preparative organic transformations are categorized into four types: reduction of (1) unsaturated hydrocarbons, (2) heteroatom-containing carbon-based unsaturated systems, (3) saturated C-hetero or C–C polar/strained bonds, and (4) hetero-hetero linkages. Apart from net electroreductive reactions, a few examples of reductive photo-electrosynthesis as well as paired electrolysis are also introduced, which offer opportunities to overcome certain limitations and improve synthetic versatility. The electrochemically driven, transition metal-catalyzed reductive cross-couplings that have been comprehensively discussed in several other recent reviews are not included here.