Electrochemical Oxidative Difunctionalization of Alkenes to Access α-Oxygenated Ketones

Electrochemical Aerobic Oxygenation and Nitrogenation of Cyclic Alkenes via C═C Bond Cleavage or Oxygenation and Azidation of Open-Chain Alkenes

An efficient strategy involving electrochemical C═C double-bond cleavage and functionalization of cyclic alkenes for the synthesis of ketonitriles is described. This transformation features environmentally friendly conditions and utilizes relatively safe TMSN₃ as the nitrogenation reagent and molecular oxygen as the oxidant. For the open-chain alkenes, the reaction gave 1,2-difunctionalized products. A wide range of cyclic alkenes and open-chain alkenes were found to be compatible, providing the corresponding ketonitriles and α-azido aromatic ketones in moderate to good yields.

Direct Cyanation of Thiophenols or Thiols to Access Thiocyanates under Electrochemical Conditions

A novel electrochemical cross-coupling method for the synthesis of thiocyanates via the direct cyanation of readily available thiophenols or thiols with trimethylsilyl cyanide (TMSCN) was developed. This approach was also suitable for selenols. External oxidant-free, transition-metal-free and mild operating conditions were the main advantages of this protocol. A series of thiocyanates and selenocyanates could be obtained in moderate to high yields.

Scalable Electrochemical Aerobic Oxygenation of Indoles to Isatins without Electron Transfer Mediators by Merging with an Oxygen Reduction Reaction

An approach to electrochemical oxygenation of indoles leading to isatins was developed by merging with a complementary cathode oxygen reduction reaction. The features of this green protocol include the use of molecular oxygen as the sole oxidant, it being free of an electron transfer mediator, and gram-scale preparation. Mechanistic studies suggested a radical process, and the two oxygen atoms in the isatins were both most likely from molecular oxygen. A detailed mechanism of the reaction utilizing density functional theory calculations was elucidated.

Multicomponent reactions and photo/electrochemistry join forces: atom economy meets energy efficiency

Visible-light photoredox catalysis has been regarded as an extremely powerful tool in organic chemistry, bringing the spotlight back to radical processes. The versatility of photocatalyzed reactions has already been demonstrated to be effective in providing alternative routes for cross-coupling as well as multicomponent reactions. The photocatalyst allows the generation of high-energy intermediates through light irradiation rather than using highly reactive reagents or harsh reaction conditions. In a similar vein, organic electrochemistry has experienced a fruitful renaissance as a tool for generating reactive intermediates without the need for any catalyst. Such milder approaches pose the basis toward higher selectivity and broader applicability. In photocatalyzed and electrochemical multicomponent reactions, the generation of the radical species acts as a starter of the cascade of events. This allows for diverse reactivity and the use of reagents is usually not covered by classical methods. Owing to the availability of cheaper and more standardized photo- and electrochemical reactors, as well as easily scalable flow-setups, it is not surprising that these two fields have become areas of increased research interest. Keeping these in view, this review is aimed at providing an overview of the synthetic approaches in the design of MCRs involving photoredox catalysis and/or electrochemical activation as a crucial step with particular focus on the choice of the difunctionalized reagent.

Visible-Light-Mediated α-Ketoacylations of NH-Sulfoximines with gem-Difluoroalkenes

A photochemical approach for the preparation of α-keto-N-acyl sulfoximines from NH sulfoximines and gem-difluoroalkenes has been developed. In the presence of NBS, the reactions proceed in air without the need of a photocatalyst or additional oxidant. Results of mechanistic studies suggest that the two oxygens in the products stem from water and dioxygen.

Electrifying Phthalimide-N-Oxyl (PINO) Radical Chemistry: Anodically Induced Dioxygenation of Vinyl Arenes with N-Hydroxyphthalimide

An electrochemical process of free-radical difunctionalization of vinyl arenes with N-hydroxyphthalimide resulting in vicinal dioxyphthalimides was discovered. The reaction proceeds with the use of pyridinium perchlorate and pyridine as a supporting electrolyte and a base, respectively. The present approach involves the anodic generation of stabilized phthalimide-N-oxyl (PINO) radical, which adds to the carbon-carbon double bond of vinyl arenes and recombines with the subsequently formed benzylic radical. A wide range of dioxyphthalimides were obtained in yields up to 81%.

Oxoammonium Salt-Mediated Regioselective Vicinal Dioxidation of Alkenes: Relying on Transient and Persistent Nitroxides

A novel, easy-to-handle, and regioselective vicinal dioxidation of alkenes under transition metal and organic peroxide free conditions has been developed. This approach uses N-hydroxyphthalimide and its analogues as the transient nitroxyl-radical precursors and 2,2,6,6-tetramethylpiperidine-1-oxoammonium tetrafluoroborate (TEMPO⁺BF₄^-) as the oxidant as well as the source of persistent nitroxide. By employing this method, multifarious structurally important dioxidation products were efficiently synthesized from simple alkenes and complex bioactive molecule derivatives.