Evolution of Earth-Abundant 3 d-Metallaelectro-Catalyzed C-H Activation: From Chelation-Assistance to C-H Functionalization without Directing Groups

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.

Palladium-Catalyzed Electrochemical Iodination of 1-Arylpyridine N-Oxides

The palladium-catalyzed C-H iodination of 1-arylpyridine N-oxides proceeded under electrochemical oxidation conditions using I2 as an iodine source. The reaction of isoquinoline N-oxides possessing various para- or meta-substituted aryl groups at the 1-position proceeded to give the corresponding iodination products. Electron-donating groups on the aryl group facilitated the reaction to give relatively high yields of the product. The reaction was also found to be applicable to 2-aryl-3-picoline N-oxides.

Polymer up-cycling by mangana-electrocatalytic C(sp3)-H azidation without directing groups

The chemical up-cycling of polymers into value-added materials offers a unique opportunity to place plastic waste in a new value chain towards a circular economy. Herein, we report the selective up-cycling of polystyrenes and polyolefins to C(sp3)-H azidated materials under electrocatalytic conditions. The functionalized polymers were obtained with high retention of mass average molecular mass and high functionalization through chemo-selective mangana-electrocatalysis. Our strategy proved to be broadly applicable to a variety of homo- and copolymers. Polyethylene, polypropylene as well as post-consumer polystyrene materials were functionalized by this approach, thereby avoiding the use of hypervalent-iodine reagents in stoichiometric quantities by means of electrocatalysis. This study, hence, represents a chemical oxidant-free polymer functionalization by electro-oxidation. The electrocatalysis proved to be scalable, which highlights its unique feature for a green hydrogen economy by means of the hydrogen evolution reaction (HER).

Heterocyclic Electrochemistry: Renewable Electricity in the Construction of Heterocycles

Numerous applications in the realm of biological exploration and drug synthesis can be found in heterocyclic chemistry, which is a vast subject. Many efforts have been developed to further improve the reaction conditions to access this interesting family to prevent employing hazardous ingredients. In this instance, it has been stated that green and environmentally friendly manufacturing methodologies have been introduced to create N-, S-, and O-heterocycles. It appears to be one of the most promising methods to access these types of compounds avoiding use of stoichiometric amounts of oxidizing/reducing species or precious metal catalysts, in which only catalytic amounts are sufficient, and it represent an ideal way of contributing toward the resource economy. Thus, renewable electricity provides clean electrons (oxidant/reductant) that initiate a reaction cascade via producing reactive intermediates that facilitate in building new bonds for valuable chemical transformations. Moreover, electrochemical activation using metals as catalytic mediators has been identified as a more efficient strategy toward selective functionalization. Thus, indirect electrolysis makes the potential range more practical, and less side reactions can occur. The latest developments in using an electrolytic strategy to create N-, S-, and O-heterocycles are the main topic of this mini review, which was documented over the last five years.

Nickel-Catalyzed Thermal Redox Functionalization of C(sp3 )-H Bonds with Carbon Electrophiles

C(sp³ )-H bond coupling with carbon electrophiles remains rarely explored under thermo-driven hydrogen atom transfer (HAT) conditions due to the challenge of integrating oxidation and reduction in a single operation. We report here a Ni-catalyzed arylation and alkylation of C(sp³ )-H bonds with organohalides to forge C(sp³ )-C bonds by merging economical Zn and tBuOOtBu (DTBP) as the external reductant and oxidant. The mild and easy-to-operate protocol enables facile carbofunctionalization of N-/O-α- and cyclohexane C-H bonds, and preparation of a few intermediates of bioactive compounds and drug derivatives. Preliminary mechanistic studies implied addition of an alkyl radical to a Ni^II salt.

Synthesis of active pharmaceutical ingredients using electrochemical methods: keys to improve sustainability

Organic electrochemistry is receiving renewed attention as a green and cost-efficient synthetic technology. Electrochemical methods promote redox transformations by electron exchange between electrodes and species in solution, thus avoiding the use of stoichiometric amounts of oxidizing or reducing agents. The rapid development of electroorganic synthesis over the past decades has enabled the preparation of molecules of increasing complexity. Redox steps that involve hazardous or waste-generating reagents during the synthesis of active pharmaceutical ingredients or their intermediates can be substituted by electrochemical procedures. In addition to enhance sustainability, increased selectivity toward the target compound has been achieved in some cases. Electroorganic synthesis can be safely and readily scaled up to production quantities. For this pupose, utilization of flow electrolysis cells is fundamental. Despite these advantages, the application of electrochemical methods does not guarantee superior sustainability when compared with conventional protocols. The utilization of large amounts of supporting electrolytes, enviromentally unfriendly solvents or sacrificial electrodes may turn electrochemistry unfavorable in some cases. It is therefore crucial to carefully select and optimize the electrolysis conditions and carry out green metrics analysis of the process to ensure that turning a process electrochemical is advantageous.

Hydrogen Evolution Enabled Rhodaelectro-Catalyzed [4+2] Annulations of Purines and 7-Deazapurines with Alkynes

A rhodium-catalyzed electrochemical regioselective annulation of 6-phenylpurines and 6-phenyl-7-deazapurines with alkynes has been developed. Electricity is used to recycle the active rhodium-based catalyst, promote the evolution of H2 and help...

Rhoda-Electrocatalyzed C-H Methylation and Paired Electrocatalyzed C-H Ethylation and Propylation

The use of electricity over traditional stoichiometric oxidants is a promising strategy for sustainable molecular assembly. Herein, we describe the rhoda‐electrocatalyzed C−H activation/alkylation of several N‐heteroarenes. This catalytic approach has been successfully applied to several arenes, including biologically relevant purines, diazepam, and amino acids. The versatile C−H alkylation featured water as a co‐solvent and user‐friendly trifluoroborates as alkylating agents. Finally, the rhoda‐electrocatalysis with unsaturated organotrifluoroborates proceeded by paired electrolysis.