Green advancements towards the electrochemical synthesis of heterocycles

Heterocyclic chemistry is a large field with diverse applications in the areas of biological research and pharmaceutical advancement. Numerous initiatives have been proposed to further enhance the reaction conditions to reach these compounds without using harmful compounds. This paper focuses on the recent advances in the eco-friendly and green synthetic procedures to synthesize N-, S-, and O-heterocycles. This approach demonstrates considerable potential in accessing such compounds while circumventing the need for stoichiometric quantities of oxidizing/reducing agents or catalysts containing precious metals. Merely employing catalytic quantities of these substances proves sufficient, thereby offering an optimal means of contributing to resource efficiency. Renewable electricity plays a crucial role in generating environmentally friendly electrons (oxidant/reductant) that serve as catalysts for a series of reactions. These reactions involve the production of reactive intermediates, which in turn allow the synthesis of new chemical bonds, enabling beneficial transformations to occur. Furthermore, the utilization of metals as active catalysts in electrochemical activation has been recognized as an effective approach for achieving selective functionalization. The aim of this review was to summarize the electrochemical synthetic procedures so that the undesirable side reactions can be considerably reduced and the practical potential range of the chemical reactions can be expanded significantly.

Electrochemically Enable N-Sulfenylation/Phosphinylation of Sulfoximines via Oxidative Dehydrocoupling Reaction

An electrochemical oxidative cross-coupling strategy for the synthesis of N-sulfenylsulfoximines from sulfoximines and thiols was accomplished, giving diverse N-sulfenylsulfoximines in moderate to good yields. Moreover, this strategy can be extended to construct the N-P bond of N-phosphinylated sulfoximines. With electrons as reagents, the oxidative dehydrogenation cross-coupling reaction proceeds smoothly in the absence of traditional redox reagents.

Electrocatalytic C-H/S-H Coupling of Amino Pyrazoles and Thiophenols: Synthesis of Amino Pyrazole Thioether Derivatives

A mild method for the C-H/S-H coupling of pyrazol-5-amines and thiophenols was developed via electrochemistry, giving diverse amino pyrazole thioether derivatives in 37-98% yields. This electrochemical reaction is sustainable and an atom-efficient approach with good functional group tolerance and scalability by avoiding metal and external chemical oxidants.

Electrochemical Syntheses of Polycyclic Aromatic Hydrocarbons (PAHs)

Polycyclic aromatic hydrocarbons (PAHs) have surfaced as increasingly viable components in optoelectronics and material sciences. The development of highly efficient and atom-economic tools to prepare PAHs under exceedingly mild conditions constitutes a long-term goal. Traditional syntheses of PAHs have largely relied on multistep approaches or the conventional Scholl reaction. However, Scholl reactions are largely inefficient with electron-deficient substrates, require stoichiometric chemical oxidants, and typically occur in the presence of strong acid. In sharp contrast, electrochemistry has gained considerable momentum during the past decade as an alternative for the facile and straightforward PAHs assembly, generally via electro-oxidative dehydrogenative annulation, releasing molecular hydrogen as the sole stoichiometric byproduct by the hydrogen evolution reaction. This review provides an overview on the recent and significant advances in the field of electrochemical syntheses of various PAHs until January 2023.

Electrochemically Enabled Cascade Cyclization Reaction of Aromatic Aldehydes and Pyrazol-5-amines: Synthesis of Bis-pyrazolo[3,4-b:4',3'-e]pyridines

A facile method for the synthesis of bis-pyrazolo[3,4-b:4',3'-e]pyridines from easily available aromatic aldehydes and pyrazol-5-amines was developed via electrochemistry. The reaction proceeded smoothly under metal and external chemical oxidant-free conditions, giving a variety of bis-pyrazolo[3,4-b:4',3'-e]pyridines in moderate yields.

Metal-Free Synthesis of N-Heterocycles via Intramolecular Electrochemical C-H Aminations

N-heterocycles are key structural units in many drugs, biologically interesting molecules and functional materials. To avoid the residues of metal catalysts, the construction of N-heterocycles under metal-free conditions has attracted much research attention in academia and industry. Among them, the intramolecular electrochemical C-H aminations arguably constitute environmentally friendly methodologies for the metal-free construction of N-heterocycles, mainly due to the direct use of clean electricity as the redox agents. With the recent renaissance of organic electrosynthesis, the intramolecular electrochemical C-H aminations have undergone much progress in recent years. In this article, we would like to summarize the advances in this research field since 2019. The emphasis is placed on the reaction design and mechanistic insight. The challenges and future developments in the intramolecular electrochemical C-H aminations are also discussed.