Amide Synthesis from Thiocarboxylic Acids and Amines by Spontaneous Reaction and Electrosynthesis

Visible-Light-Mediated Synthesis of Thioesters Using Thiocarboxylic Acid as the Dual Reagent

We have developed a visible-light-driven method for thioester synthesis that relies on the unique dual role of thiobenzoic acids as one-electron reducing agents and reactants leading to the formation of sulfur radical species. This synthetic process offers a wide scope, accommodating various thioacid and thiol substrates without the need for a photocatalyst.

Condition-Controlled O-Acylation and N-O Bond Reduction of Hydroximic Acids with Thioacetic Acid

Condition-dependent transformations between hydroximic acids and thioacetic acid were achieved. Using NH₄HCO₃ in the ethanol solvent, efficient N-O bond cleavage of hydroxamic acids occurred to afford primary amides with high functional group compatibility. The reaction was switched to O-acylation when NEt₃ and H₂O were used as the base and solvent, respectively. These facile transformations could be scaled up to the gram level smoothly. Preliminary mechanistic studies suggested that the N-O bond cleavage involves a cascade process of acylation/reduction.

Visible-Light-Promoted Hydrogenation of Azobenzenes to Hydrazobenzenes with Thioacetic Acid as the Reductant

A catalyst- and metal-free hydrogenation of azobenzenes to hydrazobenzenes in the presence of thioacetic acid was achieved under visible light irradiation. The transformation was carried out under mild conditions in an air atmosphere at ambient temperature, generating a variety of hydrazobenzenes with yields up to 99%. The current process is compatible with a variety of substituents and is highly chemoselective for azo reduction when other unsaturated functionalities (carbonyl, alkenyl, alkynyl, etc.) are contained. Preliminary mechanistic study indicated that the transformation could be a radical process.

Ionic Liquid-Mediated One-Pot 3-Acylimino-3H-1,2-dithiole Synthesis from Thiocarboxylic Acids and Alkynylnitriles via In Situ Generation of Disulfide Intermediates

A practical and straightforward strategy for the synthesis of 3-acylimino-3H-1,2-dithiol derivatives via a metal-free annulation reaction of alkynylnitriles with thiocarboxylic acids mediated by ionic liquids [BMIM]Br has been reported. This operationally simple protocol offers an easy and rapid access to a library of dithiol derivatives in moderate to good yields. The mechanistic studies show a benzoyldithio anion addition to alkynylnitriles followed by an annulation reaction through the involvement of a disulfide moiety as the key intermediate.

CO2 -Promoted Direct Acylation of Amines and Phenols by the Activation of Inert Thioacid Salts

Herein a carbon dioxide-promoted synthetic approach for the direct amidation between unactivated thioacid salts and amines under mild conditions was developed for a wide range of substrates. The method afforded amides in good to excellent yields under transition-metal-free and activation-reagent-free conditions, in sharp contrast to early methodologies on amide synthesis based on transition-metal catalysis. The method offered a greener and transition metal-free protocol applicable to pharmaceuticals preparations. Phenolic compounds were also found to be suitable acylation substrates with potassium thiosulfide KHS as the only byproduct. Moreover, this approach was applied to amide synthesis of valuable bio-active molecules such as moclobemide, melatonin, and a fungicide. Insights into the reaction mechanism involving carbon dioxide were provided through NMR spectroscopy and computational calculations. A plausible mechanism was proposed that involves weak interactions between carbon dioxide and potassium thioacetate in a dynamic equilibrium state formation of a six-membered ring.

An Electrochemical Beckmann Rearrangement: Traditional Reaction via Modern Radical Mechanism

Electrosynthesis as a potential means of introducing heteroatoms into the carbon framework is rarely studied. Herein, the electrochemical Beckmann rearrangement, i. e. the direct electrolysis of ketoximes to amides, is presented for the first time. Using a constant current as the driving force, the reaction can be easily carried out under neutral conditions at room temperature. Based on a series of mechanistic studies, a novel radical Beckmann rearrangement mechanism is proposed. This electrochemical Beckmann rearrangement does not follow the trans-migration rule of the classical Beckmann rearrangement.

Visible-Light-Induced Beckmann Rearrangement by Organic Photoredox Catalysis

A facile and general strategy for efficient direct conversion of oximes to amides using an inexpensive organic photocatalyst and visible light is described. This radical Beckmann rearrangement can be performed under mild conditions. Various alkyl aryl ketoximes and diaryl ketoximes can be effectively converted into the corresponding amides in excellent yields.