Efficient Palladium-Catalyzed Direct C−H Phenylselenylation of (Hetero)Arenes in Water

Recent advances in selective mono-/dichalcogenation and exclusive dichalcogenation of C(sp2)-H and C(sp3)-H bonds

Organochalcogen compounds are prevalent in numerous natural products, pharmaceuticals, agrochemicals, polymers, biological molecules and synthetic intermediates. Direct chalcogenation of C-H bonds has evolved as a step- and atom-economical method for the synthesis of chalcogen-bearing compounds. Nevertheless, direct C-H chalcogenation severely lags behind C-C, C-N and C-O bond formations. Moreover, compared with the C-H monochalcogenation, reports of selective mono-/dichalcogenation and exclusive dichalcogenation of C-H bonds are relatively scarce. The past decade has witnessed significant advancements in selective mono-/dichalcogenation and exclusive dichalcogenation of various C(sp2)-H and C(sp3)-H bonds via transition-metal-catalyzed/mediated, photocatalytic, electrochemical or metal-free approaches. In light of the significance of both mono- and dichalcogen-containing compounds in various fields of chemical science and the critical issue of chemoselectivity in organic synthesis, the present review systematically summarizes the advances in these research fields, with a special focus on elucidating scopes and mechanistic aspects. Moreover, the synthetic limitations, applications of some of these processes, the current challenges and our own perspectives on these highly active research fields are also discussed. Based on the substrate types and C-H bonds being chalcogenated, the present review is organized into four sections: (1) transition-metal-catalyzed/mediated chelation-assisted selective C-H mono-/dichalcogenation or exclusive dichalcogenation of (hetero)arenes; (2) directing group-free selective C-H mono-/dichalcogenation or exclusive dichalcogenation of electron-rich (hetero)arenes; (3) C(sp3)-H dichalcogenation; (4) dichalcogenation of both C(sp2)-H and C(sp3)-H bonds. We believe the present review will serve as an invaluable resource for future innovations and drug discovery.

Transition-Metal-Catalyzed Directed C-H Functionalization in/on Water

Directing group assisted C-H bond functionalization using transition-metal-catalysis has emerged as a reliable synthetic tool for the construction of regioselective carbon-carbon/heteroatom bonds. Off late, "in/on water directed transition-metal-catalysis", though still underdeveloped, has appeared as one of the prominent themes in sustainable organic chemistry. This article covers the advancements, mechanistic insights and application of the sustainable directed C-H bond functionalization of (hetero)arenes in/on water in the presence of transition-metal-catalysis.

Palladium-Catalyzed Direct Selanylation of Chalcogenophenes and Arenes Assisted by 2-(Methylthio)amide

The direct and selective conversion of a C-H bond into a C-Se bond remains a significant challenge, which is even more intricate with substrates having an innate regioselectivity under several reaction conditions, such as chalcogenophenes. We overrode their selectivity toward selanylation using palladium, copper, and the 2-(methylthio)amide directing group. This chelation-assisted direct selanylation was also suitable for mono and double ortho functionalization of arenes. The mechanistic studies indicate high-valent Pd(IV) species in the catalytic cycle, a reversible C-H activation step, and Cu(II) as a sequestering agent for organoselenide byproducts.

Transition-metal-catalyzed one-pot selenylation of electrophilic arylating agents using triphenyltin chloride/Se as a phenylselenating agent

Three novel and efficient protocols for the synthesis of phenyl aryl selenides through a three-component coupling reaction of triphenyltin chloride with aryl halides, phenolic esters or nitroarenes, and Se powder catalyzed by CuI or Cu(OAc)₂ in the presence of a base in PEG200 at 90-100 °C have been developed. Also, NiFe₂O₄ as a magnetically reusable nanocatalyst was applied in these reactions under similar reaction conditions. The present methods are superior to other currently available methods due to the use of triphenyltin chloride/Se as a phenylselenating agent and phenolic esters and nitroarenes as a coupling partner for C-Se-C bond formation for the first time, a green solvent and inexpensive and reusable catalysts, and avoidance of any toxic and expensive arylselenating reagents.

A review on the role of transition metals in selenylation reactions

Organoselenium chemistry has emerged as a distinctive area of research with tremendous utility in the synthesis of biologically and pharmaceutically active molecules. Significant synthetic approaches have been made for the construction of C-Se bonds which find use in other organic transformations. This review focuses on the versatility of transition metal-mediated selenylation reactions, providing insights into various synthetic pathways and mechanistic details. Further, this review aims to offer a broad perspective for designing efficient and novel catalysts to incorporate organoselenium moiety into the inert C-H bonds.

Chelation-assisted transition metal-catalysed C–H chalcogenylations

This review summarizes recent advances in C–S and C–Se formationsviatransition metal-catalyzed C–H functionalization utilizing directing groups to control the site-selectivity.

Palladium-Catalyzed Direct Ortho-C-H Selenylation of Benzaldehydes Using Benzidine as a Transient Directing Group

Benzidine was found to be a novel transient directing group to enable Pd-catalyzed direct selenylation of inert C(sp²)-H bonds of benzaldehydes. Diverse diarylselenides were readily constructed in high efficiency and satisfactory yields with good functional group tolerance. The practical usage of the method was further demonstrated by enlarged reaction to gram scale and application in the facile access to two selenoxanthenes and one fluorescent probe.

Transition metal catalysed direct selanylation of arenes and heteroarenes

Transition metal catalysed C-H functionalization has reached an exciting level of sophistication, and, today, it represents a paradigm shift from the standard logic of synthetic chemistry. The direct conversion of C-H bonds into C-heteroatoms remains, however, a critical challenge. Nowadays, there is a great demand in general synthetic chemistry in, for example, the materials science for the development of straightforward C-Se bond formation, in order to fulfil the practical requirements. In this sense, this review summarizes recent outstanding advances in the C-Se bond formation through transition metal-catalysed direct selanylation, providing new insights into their mechanistic aspects and disclosing effective synthetic routes with high atom economy. In addition, this review intends to show the growing opportunities to construct complex chemical scaffolds containing selenium atoms.

Selenium and Tellurium Electrophiles in Organic Synthesis

This chapter highlights the utility of electrophilic achiral and chiral organoselenium reagents in organic synthesis. A range of reactions from alkene functionalizations, the functionalization of aliphatic and aromatic C–H bonds using stoichiometric and catalytic approaches as well as rearrangement reactions are described. In addition, the utility of organotellurium reagents in organic synthesis is covered in this chapter.

Nanocatalysts for C–Se cross-coupling reactions

This mini review is an attempt to highlight the most important contributions toward the applications of nanocatalysts in carbon–selenium cross-coupling reactions with the emphasis on the mechanistic aspects of the reactions.

Copper-catalyzed diarylation of Se with aryl iodides and heterocycles

The site-selective copper-catalyzed three-component coupling reaction of electron-deficient heterocycles, Se powder and aryl iodides is disclosed, providing an efficient and practical pathway to access 2-arylselenated heterocycles.

Copper-Catalyzed Synthesis of Unsymmetrical Diorganyl Chalcogenides (Te/Se/S) from Boronic Acids under Solvent-Free Conditions

The efficient and mild copper-catalyzed synthesis of unsymmetrical diorganyl chalcogenides under ligand- and solvent-free conditions is described. The cross-coupling reaction was performed using aryl boric acids and 0.5 equiv. of diorganyl dichalcogenides (Te/Se/S) in the presence of 3 mol % of CuI and 3 equiv. of DMSO, under microwave irradiation. This new protocol allowed the preparation of several unsymmetrical diorganyl chalcogenides in good to excellent yields.

Copper-catalyzed ipso-selenation of aromatic carboxylic acids

Study of the synthesis of diaryl and methyl aryl selenides in good to excellent yields, including mechanistic studies and demonstration of the utility of the products in the facile synthesis of 10H-phenoselenazine and 11-methyldibenzo-(b,f)-1,4-selenazepine.