Electrochemically Induced Intermolecular Cross-Dehydrogenative C–O Coupling of β-Diketones and β-Ketoesters with Carboxylic Acids

1O2 and Base Assisted Oxidative Conversion of β-Enaminoesters to α-Acyloxy-β-ketoesters under Visible Light Irradiation

Singlet oxygen (1O2) and base assisted conversion of β-enaminoesters to α-acyloxy-β-ketoesters is demonstrated under visible light irradiation. The reaction involves formation of an imine intermediate via ene-type pathway initiated by 1O2 followed by base promoted dimerization and hydrolysis steps. The method is mild, environmentally friendly, requires air as the oxidant, and gives the products in moderate to high yields.

Progress in Electrochemically Empowered C-O Bond Formation: Unveiling the Pathway of Efficient Green Synthesis

(C-X) bonds (X=C, N, O) are the main backbone for making different skeleton in the organic synthetic transformations. Among all the sustainable techniques, electro-organic synthesis for C-X bond formation is the advanced tool as it offers a greener and more cost-effective approach to chemical reactions by utilizing electrons as reagents. In this review, we want to explore the recent advancements in electrochemical C-O bond formation. The electrochemically driven C-O bond formation represents an emerging and exciting area of research. In this context, electrochemical techniques offers numerous advantages, including higher yields, cost-efficient production, and simplified work-up procedures. This method enables the continuous and consistent formation of C-O bonds in molecules, significantly enhancing overall reaction yields. Furthermore, both intramolecular and intermolecular C-O bond forming reaction provided valuable products of O-containing acyclic/cyclic analogue. Hence, carbonyl (C=O), ether -O-), and ester (-COOR) functionalization in both cyclic/acyclic analogues have been prepared continuously via this innovative pathway. In this context, we want to discuss one-decade electrochemical synthetic pathways of various C-O bond contains functional group in chronological manner. This review focused on all the synthetic aspects including mechanistic path and has also mentioned overall critical finding regarding the C-O bond formation via electrochemical pathways.

Electrochemical thiocyanation of barbituric acids

The electrochemical thiocyanation of barbituric acids with NH₄SCN was disclosed in an undivided cell under constant current conditions. The electrosynthesis is the most efficient at a record high current density (j_anode ≈50-70 mA cm^-2). NH₄SCN has a dual role as the source of the SCN group and as the electrolyte. Electrochemical thiocyanation of barbituric acids starts with the generation of (SCN)₂ from the thiocyanate anion. The addition of thiocyanogen to the double bond of the enol tautomer of barbituric acid gives thiocyanated barbituric acid. A variety of thiocyanated barbituric acids bearing different functional groups were obtained in 18-95% yields and were shown to exhibit promising antifungal activity.

Anodic C(sp3)-H Acyloxylation of Indolin-3-ones Enabled by Oxidant-Free Cross-Dehydrogenative C(sp3)-O Coupling

An efficient anodic C(sp³)-H acyloxylation protocol has been established via intermolecular cross-dehydrogenative C(sp³)-O coupling. The protocol provides various C2-acyloxy indolin-3-ones without the addition of metal catalysts and external oxidants because indolin-3-ones can be directly oxidized at the anode. The effective application of several medical drugs and the realization of the gram-scale experiment have proven the practicality of this protocol.

From methylarenes to Esters: Efficient oxidative Csp3-H activation promoted by CuO decorated magnetic reduced graphene oxide

Magnetic reduced graphene oxide supported CuO (rGO/Fe3O4-CuO) as the heterogeneous catalyst in cross dehydrogenative coupling (CDC) reactions has been demonstrated for the synthesis of esters using methyl aromatics, aldehydes/benzyl alcohols...

Oxidative α-acyloxylation of acetals with cyclic diacyl peroxides

Selective functionalization of the non-activated acetal α-position with formal retaining of the acetal fragment was realized using cyclic diacyl peroxides.

Recent progress on electrochemical synthesis involving carboxylic acids

Carboxylic acids are not only essential sections of medicinal molecules, natural products and agrochemicals but also basic building blocks for organic synthesis. However, high temperature, expensive catalysts and excess oxidants are normally required for carboxylic acid group transformations. Therefore, more eco-friendly and efficient methods are urgently needed. Organic electrochemistry, as an environmentally friendly and sustainable synthetic method, can potentially avoid the above problems and is favored by more and more organic chemists. This review summarized the recent progress on the electrochemical synthesis of carboxylic acids to construct more complex compounds, emphasizing the development of electrosynthesis methodologies and mechanisms in order to attract more chemists to recognize the importance and applications of electrochemical synthesis.

Palladium-Catalyzed Q-Tube-Assisted Protocol for Synthesizing Diaza-dibenzo[a,e]azulene and Diaza-benzo[a]fluorene Derivatives via O2 Acid-Promoted Cross-Dehydrogenative Coupling

An appropriate and efficient Q-tube-assisted palladium-catalyzed strategy for the synthesis of novel, unparalleled diaza-dibenzo[a,e]azulene and diaza-benzo[a]fluorene derivatives has been sophisticated, which includes oxygen and AcOH-induced oxidative C(sp³)-C(sp²) cross-dehydrogenative coupling reactions of 1-amino-2-imino-4-arylpyridine-3-carbonitriles with benzocyclic ketones such as benzosuberone, tetralone, thiochromone, and chromone, respectively. This Q-tube gas purging kit assisted-protocol features safe due to easy pressing and sealing, a wide substrate scope, easy workup and purifying phases, and the use of O₂ as a benign oxidant, in addition to being scalable and having a high atom economy. The suggested mechanistic pathway includes a formal dehydrative step followed by palladium AcOH-induced CH(sp³)-CH(sp²) oxidative cross-coupling. In this study, X-ray crystallographic analysis has been used to authenticate the targeted products.

Electrocatalytic Oxidative Transformation of Organic Acids for Carbon-Heteroatom and Sulfur-Heteroatom Bond Formation

The electrolysis of organic acids has garnered increasing attention in recent years. In addition to the famous electrochemical decarboxylation known as Kolbe electrolysis, a number of other electrochemical processes have been recently established that allow for the construction of carbon-heteroatom and sulfur-heteroatom bonds from organic acids. Herein, recent advances in electrochemical C-X and S-X (X=N, O, S, Se) bond-forming reactions from five classes of organic acids and their conjugate bases, namely, carboxylic, thiocarboxylic, phosphonic, sulfinic, and sulfonic acids, are surveyed.

Acid-Controlled Access to β-Sulfenyl Ketones and α,β-Disulfonyl Ketones by Pummerer Reaction of β-Keto Sulfones and Sulfoxides

A convenient acid-mediated reaction of β-keto sulfones with sulfoxides under metal-free conditions has been developed, thereby delivering the acid-controlled synthesis of β-sulfenyl ketones and α,β-disulfonyl ketones in good to excellent yields. The mechanism of the transformations has been studied carefully, which suggested the involvement of a radical process in the formation of α,β-disulfonyl ketones.

NBS-activated cross-dehydrogenative esterification of carboxylic acids with DMSO

A convenient and versatile cross-dehydrogenative esterification of carboxylic acids with DMSO for preparing (methylsulfinyl)methyl esters has been developed in the presence of N-bromosuccinimide.

Synthetic Strategy for Pyrazolo[1,5-a]pyridine and Pyrido[1,2-b]indazole Derivatives through AcOH and O2-Promoted Cross-dehydrogenative Coupling Reactions between 1,3-Dicarbonyl Compounds and N-Amino-2-iminopyridines

An efficient method has been developed for the synthesis of uniquely substituted pyrazolo[1,5-a]pyridine and pyrido[1,2-b]indazole derivatives, which involves acetic acid and molecular oxygen promoted cross-dehydrogenative coupling reactions of respective β-ketoesters and β-diketones (like ethyl acetoacetate, ethyl benzoylacetate, methyl propionylacetate, acetylacetone, dimedone, 1,3-cyclohexanedione, and 1,3-cyclopentanedione) with N-amino-2-iminopyridines. The proposed tentative mechanism involves formal acetic acid-promoted oxidative C(sp3)-C(sp2) dehydrogenative coupling followed by dehydrative cyclization under a catalyst-free condition within high atom economy processes.

Electrochemical Approach for Direct C-H Phosphonylation of Unprotected Secondary Amine

Direct α-phosphonylation of an unprotected secondary amine in a single step is of practical importance to amino phophophates. However, this protocol is limited due to the high redox barrier of unprotected amine. In this paper, we report C-H phosphonylation of an unprotected secondary amine via an electrochemical approach in the presence of catalytic carboxylate salt. This metal-free and exogenous oxidant-free method furnishes diverse target molecules with satisfactory yield under mild reaction conditions. Successful application of the protocol in a gram-scale experiment demonstrates the potential utility for further functionalization.

Sustainable Synthetic Approach for (Pyrazol-4-ylidene)pyridines By Metal Catalyst-Free Aerobic C(sp2)-C(sp3) Coupling Reactions between 1-Amino-2-imino-pyridines and 1-Aryl-5-pyrazolones

A novel, metal catalyst-free, and efficient method has been developed for the synthesis of (pyrazol-4-ylidene)pyridine derivatives. The process involves dehydrogenative coupling of 1-amino-2-imino-pyridines with 1-aryl-5-pyrazolone derivatives utilizing O2 as the sole oxidant. The new method benefits from a high atom economy, efficiency, and substrate scope, as well as the simplicity of reaction and product purification procedures.