Electrochemical Regioselective C(sp2)-H Selenylation and Sulfenylation of Substituted 2-Amino-1,4-naphthoquinones

Design, Synthesis, and Antibacterial Activities of Multi-Functional C2-Functionalized 1,4-Naphthoquinonyl Organoseleniums

A practical and efficient reaction for C2-selenylation of 1,4-naphthoquinones has been explored. This coupling reaction of two redox structural motifs, such as 2-bromo-1,4-naphthoquinone with diaryldiselenide/ebselen has been achieved by using sodium borohydride reducing agent at room temperature. Using this approach, several 2-selenylated-1,4-naphthoquinones were obtained in moderate to good yields and thoroughly characterized by multinuclear (1H, 13C, and 77Se) NMR, cyclic voltammetry, and mass spectrometry. Further, light-irradiated thiolation of the synthesized selenazinone was also performed to show the utility of the synthesized compound for post-functionalization. Several 2-selenylated-1,4-naphthoquinones were studied by SC-XRD in which intramolecular Se⋅⋅⋅N (from quinolinyl ligand) non-bonded interactions were observed. Photophysical studies (UV-visible, emission, solvatochromism, and quantum yield) were also performed on selected C2-selenylated naphthoquinones. The naphthoquinonyl organoseleniums were also screened for their antibacterial properties and quinonyl organoselenium 5 d shows good antibacterial potential against S. aureus ATCC 29213 with MIC 0.5 μg/mL and a Selectivity Index of >200. Moreover, it also exhibited equipotent activity against various strains of S. aureus and Enterococcus faecium, including strains resistant to vancomycin and meropenem. From structure-activity correlation, it seems that nice blend of oxidant properties from quinone and antioxidant properties from selenium moiety makes it better candidate for antibacterial activity.

Visible light-driven and substrate-promoted alkenyltrifluoromethylation of alkenes to synthesize CF3-functionalized 1,4-naphthoquinones

The first example of the visible light-driven and substrate-promoted three-component alkenyltrifluoromethylation of alkenes is developed. This approach uses easily available alkenes, 2-arylamino-1,4-naphthoquinones and Togni reagent as the reactants, and describes good functionality tolerance. The reaction offers a precise synthesis of valuable CF3-functionalized 1,4-naphthoquinones and can be applied in late-stage modification of natural products and pharmaceuticals. Experimental results imply that bifunctional 2-arylamino-1,4-naphthoquinones serve as both substrates and catalysts. In terms of this autocatalytic system, the protocol enables a straightforward intermolecular difunctionalization of alkenes under visible light irradiation without external catalysts.

Electrochemical Regioselective C(sp2)-H Bond Chalcogenation of Pyrazolo[1,5-a]pyrimidines via Radical Cross-Coupling at Room Temperature

Herein, we disclose an electrochemical approach for the C(sp2)-H chalcogenation of pyrazolo[1,5-a]pyrimidines. This technique offers an oxidant and catalyst-free protocol for achieving regioselective chalcogenation of pyrazolo[1,5-a]pyrimidines. The procedure uses only 0.5 equiv. of diaryl chalcogenides which underscores the atom economy of the protocol. Key attributes of this methodology include mild reaction conditions, short reaction time, utilization of cheap electrode materials, and eco-friendly reaction conditions. Cyclic voltammetry studies and radical quenching experiments revealed a radical cross-coupling pathway for the reaction mechanism.

Oxidative Aminotrifluoromethylation of 1,4-Naphthoquinone

A strategy for convenient and precise oxidative aminotrifluoromethylation of 1,4-naphthoquinone with the Togni reagent and amines has been demonstrated via a radical process. This method allows efficient access for the preparation of a wide range of CF3-functionalized 1,4-naphthoquinones under mild conditions, and its application in late-stage modification of drug molecules is achieved. Mechanistic studies indicate that 1,4-naphthoquinone serves as both a substrate and a catalyst and that the Togni reagent plays a dual role of a substrate and an oxidant. As a result, the title reaction can take place in the dark without external catalysts and oxidants.

Electro- and Mechanochemical Strategy as a Dual Synthetic Approach for Biologically Relevant 3-Nitro-imidazo-[1,2-a]pyridines

We herein disclose a dual synthetic approach involving electrochemical and mechanochemical strategies for diversely functionalized 3-nitro-2-aryl-immidazo[1,2-a]pyridines. Both methods offer a practical and straightforward alternative route for accessing this important class of biologically promising nitrogen-containing heterocycles. Significant advantages of the newly developed methods include mild and energy-efficient reaction conditions, avoidance of transition metal catalysts, external heating and additional oxidants, shorter reaction times, good to excellent yields, broad substrate scope, gram-scale applicability, operational simplicity, and eco-friendliness. Furthermore, a synthetic application was extended by successfully reducing synthesized 3-nitro-2-aryl-immidazo[1,2-a]pyridines to their corresponding amino derivatives.

Electrorearranged Difunctionalization of 4-Hydroxy-α-benzopyrones

We herein report the exploration of an electrosynthetic strategy as a highly efficient and straightforward alternative protocol for accessing diversely substituted and biologically promising alkyl 2-hydroxy-3-oxo-2,3-dihydrobenzofuran-2-carboxylates through an electrorearranged difunctionalization of 4-hydroxycoumarins, involving the singlet oxygen insertion from molecular oxygen, at ambient temperature. The present method is notably more advantageous than the previously reported photochemical conversion regarding yields and reaction times, substrate scope and functional group tolerability, operational simplicity, and scalability.

Practice of green chemistry strategies in synthetic organic chemistry: a glimpse of our sincere efforts in green chemistry research

This feature article summarises our recent contributions (2019-2023) in designing and developing a handful of promising organic transformations for accessing several diversely functionalised biologically relevant organic scaffolds, following the green chemistry principles, particularly focusing on the application of low-energy visible light, electrochemistry, ball-milling, ultrasound, and catalyst- and additive-free synthetic strategies.

Pushing Boundaries: What's Next in Metal-Free C-H Functionalization for Sulfenylation?

The synthesis of thioether derivatives has been explored widely due to the potential application of these derivatives in medicinal chemistry, pharmaceutical industry and material chemistry. Within this context, there has been an increasing demand for the environmentally benign construction of C-S bonds via C-H functionalization under metal-free conditions. In the present article, we highlight recent developments in metal-free sulfenylation that have occurred in the past three years. The synthesis of organosulfur compounds via a metal-free approach using a variety of sulfur sources, including thiophenols, disulfides, sulfonyl hydrazides, sulfonyl chlorides, elemental sulfur and sulfinates, is discussed. Non-conventional strategies, which refer to the development of thioether derivatives under visible light and electrochemically mediated conditions, are also discussed. The key advantages of the reviewed methodologies include broad substrate scope and high reaction yields under environmentally benign conditions. This comprehensive review will provide chemists with a synthetic tool that will facilitate further development in this field.

Regio- and Stereoselective Synthesis of 3-Selenylazaflavanones and 3-Selenylflavanones via Electrochemically Facilitated Selenylation Cascade

Herein, an oxidant- and metal-free electrochemical selenylation reaction of chalcones with diselenides for the synthesis of 3-selenylazaflavanones and 3-selenylflavanones at room temperature was reported. The method proceeded under mild conditions, exhibited a broad substrate scope, and provided the selenylated products in moderate to excellent yields with high regio- and stereoselectivity. The reaction could also be readily scaled up with high efficiency. Detailed mechanistic studies through control experiments disclosed that a selenium-based radical might participate in this electrochemical transformation.

Mechanochemical Solvent-Free One-Pot Synthesis of Poly-Functionalized 5-(Arylselanyl)-1H-1,2,3-triazoles Through a Copper(I)-Catalyzed Click Reaction

A mechanochemistry-driven practical and efficient synthetic protocol for accessing diverse series of biologically relevant poly-functionalized 5-(arylselanyl)-1H-1,2,3-triazoles through copper(I)-catalyzed click reaction between aryl/heteroaryl acetylenes, diaryl diselenides, benzyl bromides, and sodium azide has been accomplished under high-speed ball-milling. Advantages of this method include operational simplicity, avoidance of using solvent and external heating, one-pot synthesis, short reaction time in minutes, good to excellent yields, broad substrate scope, and gram-scale applications. Furthermore, synthesized organoselenium compounds were synthetically diversified to biologically promising selenones.

Blue Light Irradiated Metal-, Oxidant-, and Base-Free Cross-Dehydrogenative Coupling of C(sp2)-H and N-H Bonds: Amination of Naphthoquinones with Amines

Herein, we report a blue-light-driven amination of C(sp²)-H bond of naphthoquinones and quinones with the N-H bond of primary and secondary amines for the synthesis of 2-amino-naphthoquinones and 2-amino-quinones. The coupling of naphthoquinones with a wide array of aliphatic, aromatic, chiral, primary, and secondary amines having electron donating (-CH₃, -OCH₃, -SCH₃), withdrawing (-F, -Cl, -Br, -I), and CO₂H, -OH, -NH₂ groups with acidic protons selectively occurred to afford C-N coupled 2-amino-naphthoquinones in 60-99% yields and hydrogen gas as a byproduct in methanol solvent without using any additional reagents, additives, and oxidant under the blue light irradiation. Mechanistic insight by DFT computation, controlled experiments, kinetic isotopic effect, and substitution effect of the substrates suggest that the reaction proceeds by radical pathway in which naphthoquinone forms a highly oxidizing naphthoquinonyl biradical upon irradiation of blue light (457 nm). Consequently, electron transfer from electron-rich amine to an oxidizing naphthoquinonyl biradical leads to a naphthoquinonyl radical anion and aminyl radical cation, followed by proton transfer and delocalization leading to a carbon-centered naphthoquinonyl radical. The cross-coupling of naphthoquinonyl carbon-centered and aminyl nitrogen radicals forms a C-N bond, with subsequent elimination of hydrogen gas (which was also confirmed by GC-TCD), affording 2-amino-1,4-naphthoquinone under metal-, reagent-, base-, and oxidant-free conditions.