Recent advances in organic electrosynthesis employing transition metal complexes as electrocatalysts

Organic electrosynthesis has been widely used as an environmentally conscious alternative to conventional methods for redox reactions because it utilizes electric current as a traceless redox agent instead of chemical redox agents. Indirect electrolysis employing a redox catalyst has received tremendous attention, since it provides various advantages compared to direct electrolysis. With indirect electrolysis, overpotential of electron transfer can be avoided, which is inherently milder, thus wide functional group tolerance can be achieved. Additionally, chemoselectivity, regioselectivity, and stereoselectivity can be tuned by the redox catalysts used in indirect electrolysis. Furthermore, electrode passivation can be avoided by preventing the formation of polymer films on the electrode surface. Common redox catalysts include N-oxyl radicals, hypervalent iodine species, halides, amines, benzoquinones(such as DDQ and tetrachlorobenzoquinone), and transition metals. In recent years, great progress has been made in the field of indirect organic electrosynthesis using transition metals as redox catalysts for reaction classes including C–H functionalization, radical cyclization, and cross-coupling of aryl halides-each owing to the diverse reactivity and accessible oxidation states of transition metals. Although various reviews of organic electrosynthesis are available, there is a lack of articles that focus on recent research progress in the area of indirect electrolysis using transition metals, which is the impetus for this review.

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.

Recent advances in organic electrosynthesis using heterogeneous catalysts modified electrodes

Organic electrosynthesis as an emerging green and advantageous alternative to traditional synthetic methods has achieved remarkable progress in recent years because sustainable electricity can be employed as traceless redox agents. To surmount the over-oxidation/reduction issues of direct electrolysis,mediated or indirect electrochemical processes are attaining remarkable significance and promoting the selectivity of products. Molecular electrocatalysts, benefiting from the easily electronic and steric modulation, suffers from readily degradation issue in most cases. Remarkably, heterogeneous catalysts have drawn more attention due to their high activity, stability, and recyclability. Hence, in this review, the most recent growth of heterogeneous catalysts modified electrodes for organic electrosynthesis were summarized, highlighting structural optimization and electrochemical performance of these materials as well as reaction mechanism. Furthermore, key challenges and future directions in this area were also discussed.

SOLID-STATE ELECTROCHROMIC DISPLAY

A novel electrochromic display composed of polymeric solid electrolyte and ferroin electroactive species has been assembled, which presents a reversible variation of the color between light blue and red when a potential of 0.98 and 0.75 V is applied respectively. The results indicate that the response speed is increased by lowering the concentration of ferroin without decreasing color contrast.

Electrophotocatalytic Si-H Activation Governed by Polarity-Matching Effects

Trialkylsilanes are important building blocks in organic synthesis;however,their widespread use in redox chemistry is limited by their high oxidation potentials and comparably high bond dissociation energies(BDEs)of Si-H and α-Si-C-H bonds(>92 kcal mol^(−1)).Herein,we report a new strategy for Si-H bond homolysis enabled by the synergistic combination of electrooxidation,photoinduced ligand-to-metal charge transfer(LMCT),and radical-mediated hydrogen atom transfer(HAT).Governed by the polarity-matching effect,the HAT to electrophilic MeO·or[Cl-OHCH_(3)]·from the more hydridic Si-H instead of a C-H bond allows the selective generation of silyl radicals.This electrophotocatalytic protocol provides rapid access to Si-functionalized benzimidazo-fused isoquinolinones with broad functional-group compatibility.Mechanistic studies have shown that n-Bu_(4)NCl is essential to the electrooxidation of CeCl_(3) to form the Ce(Ⅳ)species.

Electrochemically mediated three-component synthesis of isothioureas using thiols as sulfur source

The simultaneous binding/dissociation of multiple bonds in a one-pot manner by multicomponent reactions provide an important approach for developing novel and sustainable pathway in the drug discovery process.Herein we develop an electrocatalytic three-component reaction to construct multifunctional and valuable isothiourea compounds,which uses thiols,isocyanides and amines as substrates.Compared with the previous work,the organic electrosynthesis technique can avoid the requirement of heavy metal catalysts and stoichiometric oxidants.In addition,using thiol as a substrate to participate in the three-component reaction broadens the source of sulfur,which can also construct more abundant isothiourea derivatives.