Transition‐Metal‐Free Direct Trifluoromethylation and Perfluoroalkylation of Imidazopyridines under Mild Conditions

Recent Progress in Synthetic Applications of Hypervalent Iodine(III) Reagents

Hypervalent iodine(III) compounds have found wide application in modern organic chemistry as environmentally friendly reagents and catalysts. Hypervalent iodine reagents are commonly used in synthetically important halogenations, oxidations, aminations, heterocyclizations, and various oxidative functionalizations of organic substrates. Iodonium salts are important arylating reagents, while iodonium ylides and imides are excellent carbene and nitrene precursors. Various derivatives of benziodoxoles, such as azidobenziodoxoles, trifluoromethylbenziodoxoles, alkynylbenziodoxoles, and alkenylbenziodoxoles have found wide application as group transfer reagents in the presence of transition metal catalysts, under metal-free conditions, or using photocatalysts under photoirradiation conditions. Development of hypervalent iodine catalytic systems and discovery of highly enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important recent achievement in the field of hypervalent iodine chemistry. Chemical transformations promoted by hypervalent iodine in many cases are unique and cannot be performed by using any other common, non-iodine-based reagent. This review covers literature published mainly in the last 7-8 years, between 2016 and 2024.

Programmed Heterocycle Synthesis Using Halomucononitriles as Pyridinimine Precursors

Herein we report a method to convert primary amines, ubiquitous motifs found in pharmaceutical libraries, to either imidazo[1,2-a]pyridines or 7-alkyl azaindoles in two steps from known compounds. Using halomucononitrile reagents, we can directly access 5-bromo-6-imino-1-alkyl-1,6-dihydropyridine-2-carbonitriles (pyridinimines) in a single step from primary amines (25-93% yield) through the cyclization of transient aminomucononitrile intermediates. We then demonstrate that these compounds can be readily converted to 7-alkylazaindoles using Sonogashira cross-coupling conditions (13 examples, up to 91% yield). Under oxidative conditions, the pyridinimines serve as directing groups for C-H functionalization reactions to afford imidazo[1,2-a]pyridines. We also studied the mechanism of the cyclization event using DFT calculations and propose that this takes place via sequential base-mediated E/Z isomerization and cyclization steps.

Iodine(III)-promoted oxidative carbotrifluoromethylation of maleimides with imidazopyridines and Langlois' reagent

A metal-free protocol for oxidative carbotrifluoromethylation of maleimides with imidazopyridines and Langlois' reagent has been developed using (diacetoxyiodo)benzene (PIDA) as an oxidant. This three-component strategy enables one-step construction of 3,4-disubstituted maleimides in good yields with high functional group tolerance. Both experimental and theoretical studies support the proposed radical reaction mechanism.

Combining Donor Strength and Oxidative Stability in Scorpionates: A Strongly Donating Fluorinated Mesoionic Tris(imidazol-5-ylidene)borate Ligand

Strongly donating scorpionate ligands support the study of high-valent transition metal chemistry; however, their use is frequently limited by oxidative degradation. To address this concern, we report the synthesis of a tris(imidazol-5-ylidene)borate ligand featuring trifluoromethyl groups surrounding its coordination pocket. This ligand represents the first example of a chelating poly(imidazol-5-ylidene) mesoionic carbene ligand, a scaffold that is expected to be extremely donating. The {NiNO}10 complex of this ligand, as well as that of a previously reported strongly donating tris(imidazol-2-ylidene)borate, has been synthesized and characterized. This new ligand's strong donor properties, as measured by the υNO of its {NiNO}10 complex and natural bonding orbital second-order perturbative energy analysis, are at par with those of the well-studied alkyl-substituted tris(imidazol-2-ylidene)borates, which are known to effectively stabilize high-valent intermediates. The good donor properties of this ligand, despite the electron-withdrawing trifluoromethyl substituents, arise from the strongly donating imidazol-5-ylidene mesoionic carbene arms. These donor properties, when combined with the robustness of trifluoromethyl groups toward oxidative decomposition, suggest this ligand scaffold will be a useful platform in the study of oxidizing high-valent transition-metal species.

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.

Copper-assisted trifluoromethylthiolation/radical cascade cyclization of alkynes to construct SCF3-containing dioxodibenzothiazepines

A mild and efficient Cu-assisted trifluoromethylthiolation/radical cascade cyclization of alkynes with readily available and stable AgSCF₃ as the trifluoromethylthiolating reagent has been disclosed. This transformation provides an opportunity to construct a series of potential medicinally valuable trifluoromethylthio-substituted dioxodibenzothiazepines with wide functional group compatibility. This protocol opens up a new avenue for the construction of useful trifluoromethylthiolated seven-membered N-heterocycles.

Copper-promoted difunctionalization of unactivated alkenes with silanes

An efficient copper-catalyzed cascade difunctionalization of N-allyl anilines toward the synthesis of silylated indolines using commercially available silanes has been reported. This strategy provides a new avenue for the synthesis of a diverse array of indolines in reasonable yields. Preliminary mechanistic investigations indicate that the reaction probably proceeds via a radical pathway with unactivated alkenes as radical acceptors and simple silanes as radical precursors. This protocol is distinguished by its atom economy, broad substrate scope and readily available starting materials.

Metal-Free Alkylation of Quinoxalinones with Aryl Alkyl ketones

The first metal-free method for alkylation of quinoxalinones using cheap and stable aryl alkyl ketones as nucleophilic alkylation reagents is reported. This strategy greatly broadens the application channels of aryl...

Visible-Light-Induced Radical Difluoromethylation/Cyclization of Unactivated Alkenes: Access to CF2H-Substituted Quinazolinones

A mild and efficient visible-light-induced radical difluoromethylation/cyclization of unactivated alkenes toward the synthesis of substituted quinazolinones with easily accessible difluoromethyltriphenylphosphonium bromide has been developed. The transformation has the advantages of wide functional group compatibility, a broad substrate scope, and operational simplicity. The benign protocol offers a facile access to pharmaceutically valuable difluoromethylated polycyclic quinazolinones.

Visible-light-promoted direct C3-trifluoromethylation and perfluoroalkylation of imidazopyridines

An efficient method for direct trifluoromethylation and perfluoroalkylation at C3 of imidazopyridines through visible light-promoted C-H bond functionalization was developed. Under the irradiation of a blue LED, a series of C3-perfluoroalkyl-substituted imidazopyridines were synthesized from the corresponding imidazopyridines and perfluoroalkyl iodides in moderate to good yields at room temperature. It should be mentioned that this reaction proceeded in the absence of any transition-metal catalyst, oxidant and photocatalyst.

Visible-Light-Induced Direct Csp2-H Radical Trifluoroethylation of Coumarins with 1,1,1-Trifluoro-2-iodoethane (CF3CH2I)

Herein, we developed the first visible-light-induced direct Csp²-H radical 2,2,2-trifluoroethylation of coumarins with commercially available and cheap reagent CF₃CH₂I at room temperature. This transformation proceeded smoothly under mild conditions and showed excellent functional group compatibility. The synthetic value of the protocol was also demonstrated by the successful functionalization of several pharmaceuticals.

Copper-catalyzed radical cascade cyclization for synthesis of CF3-containing tetracyclic benzimidazo[2,1-a]iso-quinolin-6(5H)-ones

Here, a general copper-catalyzed radical cascade carbocyclization reaction with 2-arylbenzoimidazoles and a Togni reagent was realized. Structurally diverse CF3-containing tetracyclic core benzimidazo[2,1-a]isoquinoline-6(5H)-ones were obtained in moderate to good yields. The wide substrate scope, good functional group tolerance, and ease of scale-up of this method are expected to promote its potential applications in pharmacy and biotechnology.

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.

Catalyst-free, visible-light-induced direct radical cross-coupling perfluoroalkylation of the imidazo[1,2-a]pyridines with perfluoroalkyl iodides

A mild and eco-friendly visible-light-induced direct radical cross-coupling perfluoroalkylation of the imidazo[1,2-a]pyridines with perfluoroalkyl iodides was established.

Regioselective Direct C-H Trifluoromethylation of Pyridine

A highly efficient and regioselective direct C-H trifluoromethylation of pyridine based on an N-methylpyridine quaternary ammonium activation strategy has been developed. A variety of trifluoromethylpyridines can be obtained in good yield and excellent regioselectivity by treating the pyridinium iodide salts with trifluoroacetic acid in the presence of silver carbonate in N,N-dimethylformamide. The protocol features good functional group compatibility, easily available starting materials, and operational simplicity. Controlled experiments showed that the reaction may involve a nucleophilic trifluoromethylation mechanism.

Synthesis of naphtho[1′,2′:4,5]imidazo[1,2-a]pyridines via Rh(iii)-catalyzed C–H functionalization of 2-arylimidazo[1,2-a]pyridines with cyclic 2-diazo-1,3-diketones featuring with a ring opening and reannulation

An unprecedented synthesis of functionalized naphtho[1′,2′:4,5]imidazo[1,2-a]pyridines via rhodium-catalyzed cascade reactions of 2-arylimidazo[1,2-a]pyridine-3-carbaldehydes with cyclic α-diazo-1,3-diketones is presented.