Recent advances on non-precious metal-catalysed fluorination, difluoromethylation, trifluoromethylation, and perfluoroalkylation of N-heteroarenes

This review highlights the recent advances, from 2015 to 2023, on the introduction of organo-fluorine derivatives at the N-heteroarene core. Notable features considering new technologies based on organofluorine compounds such as: (i) approaches based on non-precious metal catalysis (Fe, Co, Mn, Ni, etc.), (ii) the development of new strategies using non-precious metal-catalysts for the introduction of organo-fluorinine derivatives using N-heterocycles with one or more heteroatoms, (iii) newer reagents for fluorination, difluoromethylation, trifluoromethylation, or perfluoroalkylation of N-heteroarenes using different approaches, (iv) mechanistic studies on various catalytic transformations, as and when required, and (v) the synthetic applications of various bio-active organo-fluorine compounds, including post-synthetic drug derivatization, are discussed.

Copper-catalyzed selective C5-H bromination and difluoromethylation of 8-aminoquinoline amides using ethyl bromodifluoroacetate as the bifunctional reagent

A simple and effective method for the copper-catalyzed selective C5-H bromination and difluoromethylation of 8-aminoquinoline amides with ethyl bromodifluoroacetate as the bifunctional reagent was developed. The combination of cupric catalyst and alkaline additive results in a C5-bromination reaction, whereas cuprous catalyst combined with silver additive results in the C5-difluoromethylation reaction. This method has a broad substrate scope and allows for easy and convenient access to desired C5-functionalized quinolones with good to excellent yields.

FeIII/TBHP mediated remote C–O bond construction of 8-aminoquinolines: access to methoxylation and cyanomethoxylation

The C5 regioselective methoxylation and cyanomethoxylation of 8-aminoquinolines were achieved under FeIII/TBHP system by tuning the temperature and solvent. TBHP was investigated as an “oxygen” source in the ether bond formation for the first time.

Metal-free site-selective C-H cyanoalkylation of 8-aminoquinoline and aniline-derived amides with azobisisobutyronitrile

Using K₂S₂O₈, an efficient and metal-free site-selective C–H cyanoalkylation of 8-aminoquinoline and aniline-derived amides with AIBN (azobisisobutyronitrile) was developed. Without any catalyst, various substrates and functional groups were compatible to afford corresponding products in moderate to high yields. A mechanism study displayed that a radical–radical coupling process was involved via the N-centered radical generation and delocalization of aryl amides.

An efficient metal-free cyanoalkylation of 8-aminoquinoline and aniline-derived amides was achieved in the presence of K₂S₂O₈. The method showed good substrate tolerance and also suitable for bromination and dimerization reactions.

Fluorination of 2H-Indazoles Using N-Fluorobenzenesulfonimide

An efficient, simple, and metal-free fluorination of 2H-indazoles has been developed using N-fluorobenzenesulfonimide (NFSI) in water under ambient air. This transformation provides direct access to fluorinated indazole derivatives with broad functionalities in satisfactory yields. The experimental results suggest a radical mechanistic pathway of this protocol.

Recent Strategies for Carbon-Halogen Bond Formation Using Nickel

Nickel catalysis has demonstrated the capability of performing a broad range of synthetically challenging transformations over the last decade. Though recent literature has focused on the formation of C-C and C-N bonds, a variety of breakthroughs in the field of C-X bond generation have also been reported. A diverse range of strategies using nickel have been developed, in an effort to expand the scope and synthetic utility of these halogenation methods. This Minireview will cover six emerging strategies in this field including: oxidatively induced C-X reductive elimination, triflate-to-halogen exchange reactions, directed C-H halogenation, non-directed electrophilic C-H halogenation of arenes, enantioselective α-fluorination of carbonyl containing compounds, and 1,2-difunctionalization-halogenation reactions. The final section has been split into two parts: nickel-catalyzed hydrohalogenation and nickel-catalyzed carbohalogenation reactions.

Selectfluor-Enabled C(sp3)-H Alkoxylation of 3-Methylfuranocoumarins

A facile and metal-free method for the direct C(sp³)-H bond alkoxylation of 3-methylfuranocoumarins with alcohols has been disclosed. Selectfluor enabled the (hetero)benzylic C-H etherification by tuning the reaction temperature and solvent. Various alcohols were compatible in this transformation with suitable yields. The mechanistic studies revealed that the reaction might undergo the double addition process of alcohols, as well as the departure of a fluoride anion and the formation of an oxonium ion.

Regioselective remote C5 cyanoalkoxylation and cyanoalkylation of 8-aminoquinolines with azobisisobutyronitrile

The efficient regioselective C-H cyanoalkoxylation and cyanoalkylation of 8-aminoquinoline derivatives at the C5 position have been achieved under O2 and N2 atmospheres, respectively. Using 2,2'-azobisisobutyronitrile (AIBN) as a radical precursor, the protocols afforded the corresponding products in moderate to good yields with broad substrate generality through Cu(OAc)2 or NiSO4 catalysis. Furthermore, the single electron transfer (SET) mechanism was proposed via a radical coupling pathway.

N-Fluorobenzenesulfonimide: a useful and versatile reagent for the direct fluorination and amination of (hetero)aromatic C–H bonds

This review updates recent advances and developments in the direct fluorination and amination of (hetero)aromatic C–H bonds utilizing N-fluorobenzenesulfonimide, classified according to the type of catalyst.

This review updates recent advances and developments in the direct fluorination and amination of (hetero)aromatic C–H bonds utilizing N-fluorobenzenesulfonimide, classified according to the type of catalyst.

An atom-economical and regioselective metal-free C-5 chalcogenation of 8-aminoquinolines under mild conditions

A general and simple metal-free protocol for expedient C-H functionalization leading to the regioselective generation of C-5 chalcogenated 8-aminoquinoline analogues in up to 90% yield at room temperature (25 °C) has been established. This methodology is an eco-friendly approach to the atom-economical utilization of diaryl/dialkyl chalcogenides for direct access to chalcogenated quinolines and is scalable to the gram scale without considerable decrease in the yield of the product. It represents a practical alternative to the existing metal-catalyzed functionalization of 8-aminoquinoline derivatives with broad functional group tolerance. The controlled experiments suggest that the reaction possibly proceeds through an ionic pathway at room temperature. Furthermore, the potentiality for the functionalization of free amines in chalcogenated-8-aminoquinolines provides an attractive perspective for further elaboration of the amine substituent through chemical manipulations. The applicability of the standardized method has been augmented through late-stage antimalarial drug diversification of primaquine analogues.

Chemoselective Mono- and Difluorination of 1,3-Dicarbonyl Compounds

By altering the amount of Selectfluor, the highly selective mono- and difluorination of 1,3-dicarbonyl compounds has been achieved, affording a variety of 2-fluoro- and 2,2-difluoro-1,3-dicarbonyl compounds in good to excellent yields. The reaction can be readily performed in aqueous media without any catalyst and base, which features practical and convenient fluorination. Importantly, a gram-scale reaction, transformation of 2-fluoro-1,3-diphenylpropane-1,3-dione to 4-fluoro-1,3,5-triphenyl-1H-pyrazole, and chlorination and bromination of 1,3-dicarbonyl compounds are realized to further exhibit its synthetic utility.

C5-Regioselective C-H fluorination of 8-aminoquinoline amides and sulfonamides with Selectfluor under metal-free conditions

A novel and efficient regioselective C-H fluorination of 8-aminoquinoline amides and sulfonamides at the C5 position was achieved. Using Selectfluor as a "F" reagent and HOAc as an additive, the reaction proceeds smoothly via a radical pathway. This method features metal-free conditions, a broad substrate scope and operational simplicity.

Copper-Mediated Aminoquinoline-Directed Radiofluorination of Aromatic C-H Bonds with K18 F

A Cu-mediated ortho-C-H radiofluorination of aromatic carboxylic acids that are protected as 8-aminoquinoline benzamides is described. The method uses K¹⁸ F and is compatible with a wide range of functional groups. The reaction is showcased in the high specific activity automated synthesis of the RARβ2 agonist [¹⁸ F]AC261066.

Copper-Catalyzed Direct C-5 Fluorination of 8-Aminoquinolines by Remote C–H Activation

A convenient method was developed for direct regioselective fluorination of 8-aminoquinolines at the C-5 position by copper-catalyzed remote C–H activation using Selectfluor as the electrophile fluorinating reagent. With this method, diverse fluorinated quinoline derivatives were facilely obtained under mild conditions with moderate yields.

A general method for the metal-free, regioselective, remote C-H halogenation of 8-substituted quinolines

An operationally simple, metal-free protocol for regioselective halogenation of a range of 8-substituted quinolines has been established using recyclable trihaloisocyanuric acids.

An operationally simple and metal-free protocol for geometrically inaccessible C5–H halogenation of a range of 8-substituted quinoline derivatives has been established. The reaction proceeds under air, with inexpensive and atom economical trihaloisocyanuric acid as a halogen source (only 0.36 equiv.), at room temperature. Exceptionally high generality with respect to quinoline is observed, and in most instances, the reaction proceeded with complete regioselectivity. Quinoline with a variety of substituents at the 8-position gave, exclusively, the C5-halogenated product in good to excellent yields. Phosphoramidates, tertiary amides, N-alkyl/N,N-dialkyl, and urea derivatives of quinolin-8-amine as well as alkoxy quinolines were halogenated at the C5-position via remote functionalization for the first time. This methodology provides a highly economical route to halogenated quinolines with excellent functional group tolerance, thus providing a good complement to existing remote functionalization methods of quinolin-8-amide derivatives and broadening the field of remote functionalization. The utility of the method is further showcased through the synthesis of several compounds of biological and pharmaceutical interest.

An efficient nickel/silver co-catalyzed remote C–H amination of 8-aminoquinolines with azodicarboxylates at room temperature

A highly efficient nickel/silver co-catalyzed C–H amination at the C5 position of 8-aminoquinolines with azodicarboxylates at room temperature is reported. The reaction undergoes a self-redox process without the necessity of external oxidant. It proceeded under simple and mild conditions without any additional ligand, base or oxidant and provided the desired products in good to excellent yields. This method also possessed the merits of good functional group compatibility and air and moisture tolerance. It provides an efficient strategy for the synthesis of useful quinoline derivatives.

C–H amination at the C5 position of 8-aminoquinolines with azodicarboxylates proceeded efficiently using a nickel/silver co-catalyst at room temperature without any additional ligand, base or oxidant.

Radical fluorination reactions by thermal and photoinduced methods

Radical fluorination reactions of aliphatic Csp³ and Csp² atoms, decarboxylative fluorination, and fluorination of (hetero)aromatics can be accomplished with electrophilic fluorinating reagents such as Selectfluor and NFSI.

Cobalt(ii)-catalyzed remote C5-selective C–H sulfonylation of quinolines via insertion of sulfur dioxide

A cobalt(ii)-catalyzed reaction for highly selective C5-sulfonylation of quinolines via insertion of sulfur dioxide is developed, leading to diverse sulfonated quinolines in moderate to good yields.