TEMPO-Mediated Cross-Dehydrogenative Coupling of Indoles and Imidazo[1,2-a]pyridines with Fluorinated Alcohols

An efficient direct electrolysis method for the synthesis of 1,1,1,3,3,3-hexafluoroisopropyxy substituted imidazo[1,2-a]pyridines

Electrochemical oxidative cross-dehydrogenative-coupling (CDC) is an ideal strategy to conduct the C3-alkoxylation of imidazo[1,2-a]pyridine, but it remains a challenge owing to limitation imposed by the use of alkyl alcohols and carboxylic acids. Herein, we report a mild and efficient 2-electrode constant-potential electrolysis of imidazo[1,2-a]pyridine with hexafluoroisopropanol (HFIP) to produce various imidazo[1,2-a]pyridine HFIP ethers. Mechanistic studies indicated that the electrooxidation reaction might involve radical coupling and ionic reaction.

TEMPO-Mediated Direct C(sp2)-H Alkoxylation/Aryloxylation of 1,4-Quinones

A convenient and efficient transition-metal-free method has been developed for the C(sp2)-H alkoxylation/aryloxylation of 1,4-quinones by direct cross-dehydrogenative coupling with readily available alcohols and phenols in the presence of TEMPO under simple and mild conditions. The method allowed the installation of a wide range of alkoxy/aryloxy groups, exhibited high functional group tolerance, showed a broad substrate scope, afforded good to excellent yields of products in a simple one-pot operation, and could be performed on a gram scale. Mechanistic investigation indicated the involvement of the radical pathway.

Visible light induced hydroxyfluoroalkylation of quinoxalin-2(1H)-ones with N-trifluoroethoxyphthalimide under catalyst-free conditions

For the first time, we achieved visible light-induced direct C3-hydroxyfluoroalkylation of quinoxalin-2(1H)-ones using N-trifluoroethoxyphthalimide as the trifluoroethanol radical precursor, without the need for a photocatalyst. The metal-free and catalyst-free nature of this method makes it an efficient and environmentally friendly approach for synthesizing C3-hydroxyfluoroalkylated quinoxalin-2(1H)-ones.

Hydroxytrifluoroethylation and Trifluoroacetylation Reactions via SET Processes

Hydroxytrifluoroethyl and trifluoroacetyl groups are of utmost importance in biologically active compounds, but methods to tether these motifs to organic architectures have been limited. Typically, the preparation of these compounds relied on the use of strong bases or multistep routes. The renaissance of radical chemistry in photocatalytic, transition metal mediated, and hydrogen atom transfer (HAT) processes have allowed the installation of these medicinally relevant fluorinated motifs. This review provides an overview of the methods available for the direct synthesis of hydroxytrifluoroethyl- and trifluoroacetyl-derived compounds governed by single-electron transfer processes.

Divergent synthesis of difluoromethylated indole-3-carbinols, bisindolylmethanes and indole-3-methanamines

Indole-3-carbinol, bisindolylmethanes (BIMs) and indole-3-methanamines exhibit diverse therapeutic activities. Fluorinated molecules are widely used in pharmaceuticals. Herein we report a facile and straightforward method for the successful synthesis of difluoromethylated indole-3-carbinols, bisindolylmethanes and indole-3-methanamines by a Friedel-Crafts reaction. The reaction involves the in situ generation of difluoroacetaldehyde from difluoroacetaldehyde ethyl hemiacetal in the presence of a base or an acid. This protocol is distinguished by its good to excellent yields, broad substrate compatibility, good functional group tolerance and scalability.

1,1,1,3,3,3-Hexafluoro-2-Propanol-Promoted Friedel-Crafts Reaction: Metal-Free Synthesis of C3-Difluoromethyl Carbinol-Containing Imidazo[1,2-a]pyridines at Room Temperature

A facile and efficient method has been developed for the synthesis of C3-difluoromethyl carbinol-containing imidazo[1,2-a]pyridines at room temperature via the HFIP-promoted Friedel-Crafts reaction of difluoroacetaldehyde ethyl hemiacetal and imidazo[1,2-a]pyridines. This strategy could be applied to the direct C(sp2)-H hydroxydifluoromethylation of imidazo[1,2-a]pyridines and afford a series of novel difluoromethylated carbinols in good to satisfactory yields with 29 examples. Furthermore, gram-scale and synthetic transformation experiments have also been achieved, demonstrating its potential applicable value in organic synthesis. This green protocol has several advantages, including being transition metal- and oxidant-free, being carried out at room temperature, having high efficiency, and having a wide substrate scope.

Mn-Mediated Direct Regioselective C-H Trifluoromethylation of Imidazopyridines and Quinoxalines

A simple and highly efficient strategy has been developed for direct C-H trifluoromethylation at C-3 of imidazopyridines and C-8 of quinoxalines with readily available Langlois reagent through KMnO₄/AcOH system. This protocol showed broad substrate scope and afforded moderate-to-excellent yields of both products. It is the first report that the functionalization of quinoxalines occurred regioselectively at the C-8 position of quinoxalines. Mechanistic studies revealed that reaction proceeded via radical pathway.

Synthesis of Difluoromethylated Carbinols via a HFIP-Promoted Hydroxydifluoromethylation of Aniline, Indole, and Pyrrole Derivatives with Difluoroacetaldehyde Ethyl Hemiacetal

A hexafluoroisopropanol (HFIP)-promoted hydroxydifluoromethylation of aniline, indole, and pyrrole derivatives with difluoroacetaldehyde ethyl hemiacetal has been developed. This protocol provides a facile and straightforward approach to access diverse difluoromethylated carbinols in good to excellent yields under mild conditions. Furthermore, gram-scale and synthetic derivatization experiments have also been demonstrated.

Photoredox Minisci-Type Hydroxyfluoroalkylation of Isoquinolines with N-Trifluoroethoxyphthalimide

A straightforward photocatalytic approach has been demonstrated to incorporate a trifluoroethanol unit onto the isoquinolines. Herein, we report N-trifluoroethoxyphthalimide as a hydroxyfluoroalkyl radical precursor, enabling efficient synthesis of trifluoroethanol-substituted heteroarenes. Radical quenching experiments confirmed the involvement of a free-radical pathway under developed photocatalytic conditions. The DFT calculations confirmed the intramolecular 1,2-HAT reactivity of the O-centered trifluoroethoxy radical (generated from N-trifluoroethoxyphthalimide under photocatalytic condition) to the C-centered trifluoroethanol radical. Fluorescence quenching studies suggested that isoquinoline was responsible for the quenching of Ir-photocatalyst emission. A catalytic cycle involving trifluoroethanol radical reaction with isoquinolines has been proposed.

Heteroarylation of Congested α-Bromoamides with Imidazo-Heteroarenes and Indolizines via Aza-Oxyallyl Cations: Enroute to Dibenzoazepinone and Zolpidem Analogues

Herein, we report a highly efficient and unprecedented approach for heteroarylation of congested α-bromoamides via electrophilic aromatic substitution of imidazo-heteroarenes and indolizines under mild reaction conditions (room temperature, metal, and oxidant free). The participation of an in situ generated aza-oxyallyl cation as an alkylating agent is the hallmark of this transformation. The method was readily adapted to synthesize novel imidazo-heteroarene-fused dibenzoazepinone architectures of potential medicinal value.

An iron-catalyzed domino reaction of donor-acceptor cyclopropanes: a diastereoselective approach towards diversely functionalized pyrrolo-quinazolines

An expeditious synthetic route to access functionalized pyrrolo[2,1-b]quinazoline scaffolds has been achieved via domino ring opening cyclization (DROC) reactions of donor-acceptor (D-A) cyclopropanes and 2-amino(methyl)aniline derivatives. This novel iron catalyzed transformation is amenable to a wide range of substrates. Three new C-N bonds and two rings were sequentially constructed in this divergent one-pot process. The advantages of simple operation, high yields and general applicability make this procedure highly attractive and practical too.

Copper-catalyzed three-component reaction to synthesize polysubstituted imidazo[1,2-a]pyridines

An efficient three-component one-pot and operationally simple cascade of 2-aminopyridines with sulfonyl azides and terminal ynones is reported, providing a variety of polysubstituted imidazo[1,2-a]pyridine derivatives in moderate to excellent yields. In particular, the reaction goes a through CuAAC/ring-cleavage process and forms a highly active intermediate α-acyl-N-sulfonyl ketenimine with base free.

Three-component one-pot synthesis of polysubstituted imidazo[1,2-a]pyridine derivatives through a base free CuAAC/ring-cleavage process.

TEMPO-Mediated Synthesis of Indolyl/Imidazo[1,2-a]pyridinyl-Substituted para-Quinone Methides from Butylated Hydroxytoluene

A series of indolyl or imidazo[1,2-a]pyridinyl-substituted para-quinone methides (p-QMs) is prepared by a metal-free, TEMPO-mediated cross-dehydrogenative coupling of butylated hydroxytoluene (BHT) with indoles or imidazo[1,2-a]pyridines in good to high yields. Broad substrate scope with respect to indoles and imidazo[1,2-a]pyridines, good functional group tolerance, and acid/base-free conditions are advantageous feature of the developed protocol. The method was amenable for scale-up on the gram scale. Based on control experiments, a reaction mechanism is proposed to describe this transformation.

Functionalization of imidazo[1,2-a]pyridines via radical reactions

The recent advances in radical reactions for the direct functionalization of imidazo[1,2-a]pyridines are reviewed.

I2-Catalyzed oxidative dehydrogenative tandem cyclization of 2-methylquinolines, arylamines and 1,4-dioxane

A novel I2-catalyst oxidative dehydrogenative tandem cyclization of 2-methylquinolines, arylamines and 1,4-dioxane for the construction of 2-([2,2′-biquinolin]-3-yloxy)ethan-1-ol derivatives has been developed under metal-free conditions.