Enantioselective functionalization of radical intermediates in redox catalysis: copper-catalyzed asymmetric oxytrifluoromethylation of alkenes

Enantioselective radical process for synthesis of chiral indolines by metalloradical alkylation of diverse C(sp3)-H bonds

A new C–C bond formation strategy based on enantioselective radical alkylation of C(sp³)–H bonds via Co(ii)-based metalloradical catalysis has been demonstrated for stereoselective synthesis of chiral indolines.

A new C–C bond formation strategy based on enantioselective radical alkylation of C(sp³)–H bonds via Co(ii)-based metalloradical catalysis has been demonstrated for stereoselective synthesis of chiral indolines. The Co(ii)-based system enables activation of aryldiazomethanes as radical precursors at room temperature for enantioselective intramolecular radical alkylation of broad types of C–H bonds, constructing 2-substituted indolines in high yields with excellent enantioselectivities. In addition to chemoselectivity and regioselectivity, this Co(ii)-catalyzed alkylation features tolerance to functional groups and compatibility with heteroaryl substrates. Detailed mechanistic studies provide insight into the underlying stepwise radical pathway.

A redox-economical synthesis of trifluoromethylated enamides with the Langlois reagent

A redox-economical strategy for the synthesis of trifluoromethylated enamides using copper catalysis is reported. The reaction employs the inexpensive Langlois reagent (CF₃SO₂Na) and takes place without the need of an external oxidant. The trifluoromethylated enamide products can easily be converted into the corresponding ketone, saturated amide or oxazole.

Organic base-catalysed solvent-tuned chemoselective carbotrifluoromethylation and oxytrifluoromethylation of unactivated alkenes

An unprecedented and efficient organic base-catalysed highly chemoselective carbo- and oxytrifluoromethylation of unactivated alkenes with Togni's reagent was developed. The switchable chemoselectivity was tuned by simply changing the organic base catalyst and solvent. Mechanistic studies indicated that a radical cyclization pathway for carbotrifluoromethylation in DMSO and a carbocation pathway for oxytrifluoromethylation in DCE were probably involved.

Copper catalyzed one-pot difluoroalkylation and lactonization of unsaturated carboxylic acids

A new cost-effective method for one-pot bifunctionalization of unsaturated carboxylic acids to afford difluoroalkylated γ-lactones has been reported using a Cu(i) catalyst.

Visible-light-induced oxidant and metal-free dehydrogenative cascade trifluoromethylation and oxidation of 1,6-enynes with water

Generally, oxy-trifluoromethylation in olefins is achieved using oxidants and transition metal catalysts. However, labile olefins remain unexplored due to their incompatibility with harsh reaction conditions. Here, unprecedented light-induced oxidant and metal-free tandem radical cyclization-trifluoromethylation and dehydrogenative oxygenation of 1,6-enynes have been achieved using a photoredox catalyst, CF3SO2Na, and phenanthrene-9,10-dione (PQ), Langlois' reagent (CF3SO2Na) and water as the oxygen source. This benign protocol allows for access to various CF3-containing C3-aryloyl/acylated benzofurans, benzothiophenes, and indoles. Moreover, the oxidized undesired products, which are inherently formed by the cleavage of the vinylic carbon and heteroatom bond, have been circumvented under oxidant free conditions. The mechanistic investigations by UV-visible and ESR spectroscopy, electrochemical studies, isotope labelling and density functional theory (DFT) suggest that light induced PQ produced a CF3 radical from CF3SO2Na. The generated CF3 radical adds to the alkene, followed by cyclization, to provide a vinylic radical that transfers an electron to PQ and generates a vinylic cation. Alternatively, electron transfer may occur from the CF3-added alkene moiety, forming a carbocation, which would undergo cationic cyclization to generate a vinylic carbocation. The subsequent addition of water to the vinylic cation, followed by the elimination of hydrogen gas, led to the formation of trifluoromethylated C3-aryloyl/acylated heterocycles.

Catalytic Asymmetric Synthesis of Butenolides and Butyrolactones

γ-Butenolides, γ-butyrolactones, and derivatives, especially in enantiomerically pure form, constitute the structural core of numerous natural products which display an impressive range of biological activities which are important for the development of novel physiological and therapeutic agents. Furthermore, optically active γ-butenolides and γ-butyrolactones serve also as a prominent class of chiral building blocks for the synthesis of diverse biological active compounds and complex molecules. Taking into account the varying biological activity profiles and wide-ranging structural diversity of the optically active γ-butenolide or γ-butyrolactone structure, the development of asymmetric synthetic strategies for assembling such challenging scaffolds has attracted major attention from synthetic chemists in the past decade. This review offers an overview of the different enantioselective synthesis of γ-butenolides and γ-butyrolactones which employ catalytic amounts of metal complexes or organocatalysts, with emphasis focused on the mechanistic issues that account for the observed stereocontrol of the representative reactions, as well as practical applications and synthetic potentials.

Asymmetric Copper-Catalyzed Intermolecular Aminoarylation of Styrenes: Efficient Access to Optical 2,2-Diarylethylamines

We have developed a copper-catalyzed enantioselective intermolecular aminoarylation of alkenes using a novel N-fluoro-N-alkylsulfonamide as the amine reagent, which could react with the Cu(I) catalyst to release a related amino radical. After addition to styrene, the generated benzylic radical could couple with a chiral L*Cu^IIAr complex to achieve enantioselective arylation. Various optical 2,2-diarylethylamines were efficiently synthesized from simple styrenes with high enantioselectivity, and these products can serve as valuable synthons toward bioactive molecules' synthesis.

Catalytic asymmetric radical aminoperfluoroalkylation and aminodifluoromethylation of alkenes to versatile enantioenriched-fluoroalkyl amines

Although great success has been achieved in asymmetric fluoroalkylation reactions via nucleophilic or electrophilic processes, the development of asymmetric radical versions of this type of reactions remains a formidable challenge because of the involvement of highly reactive radical species. Here we report a catalytic asymmetric radical aminoperfluoroalkylation and aminodifluoromethylation of alkenes with commercially available fluoroalkylsulfonyl chlorides as the radical sources, providing a versatile platform to access four types of enantioenriched α-tertiary pyrrolidines bearing β-perfluorobutanyl, trifluoromethyl, difluoroacetyl and even difluoromethyl groups in excellent yields and with excellent enantioselectivity. The key to success is not only the introduction of the CuBr/chiral phosphoric acid dual-catalytic system but also the use of silver carbonate to suppress strong background and side hydroamination reactions caused by a stoichiometric amount of the in situ generated HCl. Broad substrate scope, excellent functional group tolerance and versatile functionalization of the products make this approach very practical and attractive.

Methods for the asymmetric introduction of organofluorine groups are often limited by the lack of variability in the starting materials. Here the authors report an asymmetric radical process for the introduction of fluoroalkyl groups using readily available fluoroalkylated sulfonyl chlorides.

Asymmetric Cu-Catalyzed Intermolecular Trifluoromethylarylation of Styrenes: Enantioselective Arylation of Benzylic Radicals

A novel asymmetric radical trifluoromethyl-arylation of alkenes has been developed, which provides an efficient approach to access chiral CF₃-containing 1,1-diarylmethane derivatives with good to excellent enantioselectivity. Various vinyl arenes and aryl boronic acids are compatible with these conditions. The utility of the method is demonstrated by accessing modified bioactive molecules.

Copper(ii) catalyzed iodine-promoted oxidative cyclization of 2-amino-1,3,5-triazines and chalcones: synthesis of aroylimidazo[1,2-a][1,3,5]triazines

An efficient method for the synthesis of aroylimidazo[1,2-a][1,3,5]triazine derivatives based on a copper(ii) catalyzed iodine-promoted reaction of 2-amino-1,3,5-triazines and chalcones is presented.

Phosphine-catalyzed remote α-C–H bond activation of alcohols or amines triggered by the radical trifluoromethylation of alkenes: reaction development and mechanistic insights

An efficient radical protocol for concomitant functionalization of both alkenes and remote α-C–H bonds of alcohols or amines via 1,5(6,7)-HAT was realized in a highly controlled site-selective manner.

Metal-free and light-promoted radical iodotrifluoromethylation of alkenes with Togni reagent as the source of CF3 and iodine

Iodotrifluoromethylation of alkenes is effectively conducted by combining benzophenone, “black light”, isopropanol and Togni reagent 1 as the source of both the CF₃ group and iodine atom.

Selective C–H trifluoromethylation of benzimidazoles through photoredox catalysis

This protocol presented a new strategy for visible light induced C–H trifluoromethylation at the C₄ of benzimidazoles using Togni's reagent in the presence of fac-Ir(ppy)₃.

Palladium-Catalysed and Phosphine-Promoted Synthesis of Diaryl Ethers through Self-Coupling

An efficient, palladium acetate-catalysed, tributylphosphine-promoted direct synthesis of symmetrical diaryl ethers through the self-coupling of aryl fluorides has been developed with K2CO3/ZrO2 as a base. This provides an alternative method to prepare aromatic polymers, important synthetic intermediates and natural products for use in the field of pharmaceuticals and industrial materials.

Radical Enantioselective C(sp3 )-H Functionalization

Radical style: Several breakthroughs have recently been achieved in enantioselective C(sp³ )-H functionalization through radical activation. These new strategies show how radical chemistry can be used to convert alkanes into functionalized chiral molecules (see scheme; HAT=hydrogen atom transfer).

Radicals: Reactive Intermediates with Translational Potential

This Perspective illustrates the defining characteristics of free radical chemistry, beginning with its rich and storied history. Studies from our laboratory are discussed along with recent developments emanating from others in this burgeoning area. The practicality and chemoselectivity of radical reactions enable rapid access to molecules of relevance to drug discovery, agrochemistry, material science, and other disciplines. Thus, these reactive intermediates possess inherent translational potential, as they can be widely used to expedite scientific endeavors for the betterment of humankind.

The dual role of thiourea in the thiotrifluoromethylation of alkenes

Alkenes substituted with a thiourea undergo C-CF₃ followed by intramolecular C-S bond formation with the Togni reagent and trifluoroacetic acid (TFA) at room temperature; thiols and thioamides are not suitable S-sources for this reaction. This anti-addition process involves a CF₃ radical, and affords CF₃-substituted thiazolines and thiazines for medicinal applications. A metal or photoredox catalyst is not required as the thiourea acts as a reductant, as well as serving as an S-source capable of adding to a C-centered radical. Mechanistic work comparing the reactivity of thiourea, urea, thioamide and thiol in the context of alkene trifluoromethylation demonstrates that in this series, the thiourea is unique for its ability to release CF₃ radical from the Togni reagent, and to orchestrate trifluoromethylation followed by S-cyclization with both activated and unactivated alkenes.