Regioselective Functionalization of Quinolines through C-H Activation: A Comprehensive Review

Recent synthetic strategies for the functionalization of fused bicyclic heteroaromatics using organo-Li, -Mg and -Zn reagents

This review highlights the use of functionalized organo-Li, -Mg and -Zn reagents for the construction and selective functionalization of 5- and 6-membered fused bicyclic heteroaromatics. Special attention is given to the discussion of advanced syntheses for the preparation of highly functionalized heteroaromatic scaffolds, including quinolines, naphthyridines, indoles, benzofurans, benzothiophenes, benzoxazoles, benzothiazoles, benzopyrimidines, anthranils, thienothiophenes, purine coumarins, chromones, quinolones and phthalazines and their fused heterocyclic derivatives. The organometallic reagents used for the desired functionalizations of these scaffolds are generally prepared in situ using the following methods: (i) through directed selective metalation reactions (DoM), (ii) by means of halogen/metal exchange reactions, (iii) through oxidative metal insertions (Li, Mg, Zn), and (iv) by transmetalation reactions (organo-Li and Mg transmetalations with ZnCl2 or ZnO(Piv)2). The resulting reactive organometallic reagents allow a wide range of C-C, C-N and C-X cross-coupling reactions with different electrophiles, employing in particular Kumada or Negishi protocols among other transition metal (Pd, Ni, Co, Cu, Cr, Fe, etc.)-catalyzed processes. In addition, key developments concerning selective metalation techniques will be presented, which rely on the use of RLi, LDA and TMP metal bases. These methods are now widely employed in organic synthetic chemistry and have proven to be particularly valuable for drug development programs in the pharmaceutical industry. New and improved protocols have resulted in many Li, Mg and Zn organyls now being compatible with functionalized aryl, heteroaryl, alkenyl, alkynyl and alkyl compounds even in the presence of labile functional groups, making these reagents well-suited for C(sp2)-C(sp2), C(sp2)-C(sp) and C(sp2)-C(sp3) cross-coupling reactions with fused heteroaryl halides. In addition, the use of some transition metal-catalyzed processes occasionally allows a reversed role of the reactants in cross-coupling reactions, providing alternative synthetic routes for the preparation of fused heteroaromatic-based bioactive drugs and natural products. In line with this, this article points to novel methods for the functionalization of bicyclic heteroaromatic scaffolds by organometallic reagents that have been published in the period 2010-2023.

Dual C(sp3)-H and C(sp2)-H Activation of 8-Methylquinoline N-Oxides: A Route to Access C7-H Bond

A Pd(II)-catalyzed regioselective dual C(sp3)-H/C7(sp2)-H activation and annulation of 8-methylquinoline N-oxides with maleimide has been accomplished. The use of N-oxide as a weak directing group under Pd(II)-complex catalysis activates the initial C(sp3)-H and triggers a relayed, second C7(sp2)-H activation. The dual C-H bond activation, [3 + 2]-annulation, facile introduction and removal of the directing group, substrate scope, and functional group diversity are the important practical features.

The Versatile and Strategic O-Carbamate Directed Metalation Group in the Synthesis of Aromatic Molecules: An Update

The aryl O-carbamate (ArOAm) group is among the strongest of the directed metalation groups (DMGs) in directed ortho metalation (DoM) chemistry, especially in the form Ar-OCONEt2. Since the last comprehensive review of metalation chemistry involving ArOAms (published more than 30 years ago), the field has expanded significantly. For example, it now encompasses new substrates, solvent systems, and metalating agents, while conditions have been developed enabling metalation of ArOAm to be conducted in a green and sustainable manner. The ArOAm group has also proven to be effective in the anionic ortho-Fries (AoF) rearrangement, Directed remote metalation (DreM), iterative DoM sequences, and DoM-halogen dance (HalD) synthetic strategies and has been transformed into a diverse range of functionalities and coupled with various groups through a range of cross-coupling (CC) strategies. Of ultimate value, the ArOAm group has demonstrated utility in the synthesis of a diverse range of bioactive and polycyclic aromatic compounds for various applications.

Rh(iii)-catalyzed sp3/sp2-C-H heteroarylations via cascade C-H activation and cyclization

The development of an efficient strategy for facile access to quinoline-based bis-heterocycles holds paramount importance in medicinal chemistry. Herein, we describe a unified approach for accessing 8-(indol-3-yl)methyl-quinolines by integrating Cp*Rh(iii)-catalyzed C(sp3)-H bond activation of 8-methylquinolines followed by nucleophilic cyclization with o-ethynylaniline derivatives. Remarkably, methoxybiaryl ynones under similar catalytic conditions delivered quinoline tethered spiro[5.5]enone scaffolds via a dearomative 6-endo-dig C-cyclization. Moreover, leveraging this method for C8(sp2)-H bond activation of quinoline-N-oxide furnished biologically relevant oxindolyl-quinolines. This reaction proceeds via C(sp2)-H bond activation, regioselective alkyne insertion, oxygen-atom-transfer (OAT) and intramolecular nucleophilic cyclization in a cascade manner. One C-C, one C-N and one C[double bond, length as m-dash]O bond were created with concomitant formation of a quaternary center.

Reaction strategies for the meta-selective functionalization of pyridine through dearomatization

The pyridine moiety is a crucial structural component in various pharmaceuticals. While the direct ortho- and para-functionalization of pyridines is relatively straightforward, the meta-selective C-H functionalization remains a significant challenge. This review highlights dearomatization strategies as a key area of interest in expanding the application of meta-C-H functionalization of pyridines. Dearomatization enables the meta-functionalization through various catalytic methods that directly generate dearomatization products, and some products can be rearomatized back to pyridine derivatives. Furthermore, this article also covers the dearomatization of multiple positions of pyridine in the synthesis of polycyclic compounds. It offers a comprehensive overview of the latest advancements in dearomatization at different positions of pyridine, aiming to provide a valuable resource for researchers in this field. It also highlights the advantages and limitations of existing technologies, aiming to inform a broader audience about this important field and foster its future development.

Rh(iii)-catalyzed highly site- and regio-selective alkenyl C-H activation/annulation of 4-amino-2-quinolones with alkynes via reversible alkyne insertion

3,4-Fused 2-quinolone frameworks are important structural motifs found in natural products and biologically active compounds. Intermolecular alkenyl C-H activation/annulation of 4-amino-2-quinolone substrates with alkynes is one of the most efficient methods for accessing such structural motifs. However, this is a formidable challenge because 4-amino-2-quinolones have two cleavable C-H bonds: an alkenyl C-H bond at the C3-position and an aromatic C-H bond at the C5-position. Herein, we report the Rh(iii)-catalyzed highly site-selective alkenyl C-H functionalization of 4-amino-2-quinolones to afford 3,4-fused 2-quinolones. This method has a wide substrate scope, including unsymmetrical internal alkynes, with complete regioselectivity. Several control experiments using an isolated key intermediate analog suggested that the annulation reaction proceeds via reversible alkyne insertion involving a binuclear Rh complex although alkyne insertion is generally recognized as an irreversible process due to the high activation barrier of the reverse process.

Construction of unsymmetrical heterobiaryls via the Cp*Rh(III)-catalysed C-H/C-H coupling of heteroarenes

Herein, a concise method for the Rh(III)-catalyzed, directing-group-assisted C-H/C-H cross-coupling of N-heterocycles (quinolines, indolines, indoles, pyridines, pyrimidines, pyrazoles) with other heteroarenes (benzoxazoles, benzofurans, and thiophenes) is disclosed for the synthesis of unsymmetrical heterobiaryl compounds in good to excellent yields. A plausible catalytic cycle has been delineated based on experimental and computational mechanistic studies.

Directed C8-H allylation of quinoline N-oxides with vinylcyclopropanes via sequential C-H/C-C activation

The Rh(III)-catalyzed C8-allylation of quinoline N-oxides has been accomplished using vinylcyclopropanes as an allyl source with excellent diastereoselectivity at room temperature. The C-H/C-C activation, substrate scope and natural product mutation are the important practical features.

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.

Rhodium-Promoted C-H Bond Activation of Quinoline, Methylquinolines, and Related Mono-Substituted Quinolines

The C-H bond activation of methylquinolines, quinoline, 3-methoxyquinoline, and 3-(trifluoromethyl)quinoline promoted by the square-planar rhodium(I) complex RhH{κ³-P,O,P-[xant(PⁱPr₂)₂]} [1; xant(PⁱPr₂)₂ = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene] has been systematically studied. Results reveal that the activation of the heteroring is preferred over the activation of the carbocycle, and the activated position depends upon the position of the substituent in the substrate. Thus, 3-, 4-, and 5-methylquinoline reacts with 1 to quantitatively form square-planar rhodium(I)-(2-quinolinyl) derivatives, whereas 2-, 6-, and 7-methylquinoline quantitatively leads to rhodium(I)-(4-quinolinyl) species. By contrast, quinoline and 8-methylquinoline afford mixtures of the respective rhodium(I)-(2-quinolinyl) and -(4-quinolinyl) complexes. 3-Methoxyquinoline displays the same behavior as that of 3-methylquinoline, while 3-(trifluoromethyl)quinoline yields a mixture of rhodium(I)-(2-quinolinyl), -(4-quinolinyl), -(6-quinolinyl), and -(7-quinolinyl) isomers.

N-Oxide-Induced Ugi Reaction: A Rapid Access to Quinoline-C2-amino Amides via Deoxygenative C(sp2)-H Functionalization

A logic-based replacement of the carboxylic acid component of the Ugi reaction by quinoline N-oxides has been developed. In this approach, the carboxylic isostere, quinoline N-oxide, plays a vital role by shifting the equilibria toward the product side with irreversible addition onto the C2-position of the N-oxide. Thus, aldehydes react with amines, isocyanides, and quinoline N-oxides to furnish quinoline four-component Ugi adducts. The unique reactivity of N-oxides with Ugi components opens an efficient synthetic route for the preparation of biologically active compounds.

Thioether-Assisted Cu-Catalyzed C5-H Arylation of Imidazo[1,5-a]pyridines

A Cu-catalyzed regioselective C5-H arylation of imidazo[1,5-a]pyridines with aryl iodides was achieved with the assistance of an ethylthio group at the C3 position. This directing group could be easily removed to furnish a range of 5-(hetero)arylimidazo[1,5-a]pyridine derivatives. The reaction tolerates a variety of functionalities and is compatible with sterically hindered substrates.

Organic-acid catalysed Minisci-type arylation of heterocycles with aryl acyl peroxides

A metal-free method for the Minisci-type arylation of heterocycles with aryl acyl peroxides has been reported. This strategy enables the rapid and simple synthesis of a series of Minisci-type adducts...