Copper-Mediated C6-Selective Dehydrogenative Heteroarylation of 2-Pyridones with 1,3-Azoles

A comprehensive overview of directing groups applied in metal-catalysed C-H functionalisation chemistry

The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation. In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate. Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it. Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure. The schemes feature typical substrates used, the products obtained as well as the required reaction conditions. Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness. The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them. Accordingly, this review should be of particular interest also for scientists from industrial R&D sector. Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date.

Rhodium-catalyzed C-H functionalization of heteroarenes using indoleBX hypervalent iodine reagents

The C–H indolation of heteroarenes was realized using the benziodoxolone hypervalent iodine reagents indoleBXs. Functionalization of the C–H bond in bipyridinones and quinoline N-oxides catalyzed by a rhodium complex allowed to incorporate indole rings into aza-heteroaromatic compounds. These new transformations displayed complete regioselectivity for the C-6 position of bipyridinones and the C-8 position of quinoline N-oxides and tolerated a broad range of functionalities, such as halogens, ethers, or trifluoromethyl groups.

Ruthenium-catalyzed site-selective C-H arylation of 2-pyridones and 1-isoquinolinones

An efficient Ru(ii)-catalyzed site-selective C-H arylation of 2-pyridones and 1-isoquinolinones was achieved with boronic acids by using pyridine as a directing group. The developed protocol is general and provides rapid access to an array of C6-arylated 2-pyridones and C3-arylated 1-isoquinolinones in excellent yields. These designed arylated 2-pyridones and 1-isoquinolinones can serve as key structural motifs for accessing functionalized pyridines and isoquinolines.

A lesson for site-selective C-H functionalization on 2-pyridones: radical, organometallic, directing group and steric controls

A 2-pyridone ring is a frequently occurring subunit in natural products, biologically active compounds, and pharmaceutical targets. Thus, the selective synthesis of substituted 2-pyridone derivatives through decoration and/or formation of pyridone rings has been one of the important longstanding subjects in organic synthetic chemistry. This minireview focuses on recent advances in site-selective C-H functionalization on 2-pyridone. The reported procedures are categorized according to the site selectivity that is achieved, and the substrate scope, limitations, mechanism, and controlling factors are briefly summarized.

Rhodium-catalyzed oxidative C–H/C–H cross-coupling of aniline with heteroarene: N-nitroso group enabled mild conditions

A rhodium(iii)-catalyzed oxidative ortho-heteroarylation of N-nitrosoaniline with heteroarene has been developed for the synthesis of (2-aminophenyl)heteroaryls under mild conditions.

Iridium-catalyzed oxidative Ar–H/Ar–H cross-coupling of primary benzamides with thiophenes

Described herein is an iridium(iii)-catalyzed oxidative Ar–H/Ar–H cross-coupling of primary benzamides with thiophenes for the synthesis of (2-thienyl)benzamides.

Cobalt(iii)-catalyzed site-selective C–H amidation of pyridones and isoquinolones

In this study, Cp*Co(iii)-catalyzed site-selective amidation of pyridones and isoquinolones using oxazolones as the amidation reagent is reported.

Oxidative C-H/C-H Coupling Reactions between Two (Hetero)arenes

Transition metal-mediated C-H bond activation and functionalization represent one of the most straightforward and powerful tools in modern organic synthetic chemistry. Bi(hetero)aryls are privileged π-conjugated structural cores in biologically active molecules, organic functional materials, ligands, and organic synthetic intermediates. The oxidative C-H/C-H coupling reactions between two (hetero)arenes through 2-fold C-H activation offer a valuable opportunity for rapid assembly of diverse bi(hetero)aryls and further exploitation of their applications in pharmaceutical and material sciences. This review provides a comprehensive overview of the fundamentals and applications of transition metal-mediated/catalyzed oxidative C-H/C-H coupling reactions between two (hetero)arenes. The substrate scope, limitation, reaction mechanism, regioselectivity, and chemoselectivity, as well as related control strategies of these reactions are discussed. Additionally, the applications of these established methods in the synthesis of natural products and exploitation of new organic functional materials are exemplified. In the last section, a short introduction on oxidant- or Lewis acid-mediated oxidative Ar-H/Ar-H coupling reactions is presented, considering that it is a very powerful method for the construction of biaryl units and polycylic arenes.

Novel construction of diversely functionalized N-heteroaryl-2-pyridones via copper(ii)-catalyzed [3+2+1] annulation

A facile synthesis of diversely functionalized N-heteroaryl-2-pyridones is achieved by Cu(OTf)₂-catalyzed [3+2+1] annulation of various 2-aminopyridines via cascade reaction.

Rhodium(III)-Catalyzed Site-Selective C-H Alkylation and Arylation of Pyridones Using Organoboron Reagents

In this study we developed a method for the pyridine-directed, rhodium-catalyzed, site-selective C-H alkylation and arylation of pyridones using commercially available trifluoroborate reagents. This simple and versatile transformation proceeded smoothly under relatively mild conditions with perfect site selectivity. The coupling groups in the boron reagents can be extended to primary alkyl, benzyl, and cycloalkyl. Moreover, direct C-H arylation products could also be obtained under similar conditions.

Copper-promoted site-selective carbonylation of sp3 and sp2 C-H bonds with nitromethane

Copper-promoted direct carbonylation of unactivated sp3 C-H and aromatic sp2 C-H bonds of amides was developed using nitromethane as a novel carbonyl source. The sp3 C-H functionalization showed high site-selectivity by favoring the C-H bonds of α-methyl groups. The sp2 C-H carbonylation featured high regioselectivity and good functional group compatibility. Kinetic isotope effect studies indicated that the sp3 C-H bond breaking step is reversible, whereas the sp2 C-H bond cleavage is an irreversible but not the rate-determining step. Control experiments showed that a nitromethyl intermediate should be involved in the present reaction.

Copper-catalyzed oxidative C-H/C-H cross-coupling of benzamides and thiophenes

A copper-catalyzed oxidative C-H/C-H cross-coupling of benzamides and thiophenes has been developed. This reaction exhibits a broad substrate scope and excellent tolerance of functional groups. The directing group could be readily removed to afford 2-thienylbenzoic acids.

A green synthetic approach to synthesizing diverse 2-pyridones for their exceptional UV shielding functions

An efficient catalyst- and solvent-free multicomponent reaction of 4-oxo-4H-chromene-3-carbaldehydes with malonates and ammonium acetate was developed for diverse N-nonsubstituted 2-pyridones.

Recent advances in copper-mediated chelation-assisted functionalization of unactivated C–H bonds

Recent advances in copper-mediated (both stoichiometric and catalytic) chelation-assisted functionalization of unactivated C–H bonds are reviewed.

Cu(ii)-catalyzed C6-selective C–H thiolation of 2-pyridones using air as the oxidant

Cu(II)-catalyzed C6-selective C–H thiolation of 2-pyridones with aryl and alkyl disulfides to provide functional thioethers has been achieved.

Iridium(iii)- and rhodium(iii)-catalyzed coupling of anilines with α-diazoesters via chelation-assisted C–H activation

Ir(iii) catalysts can catalyze the efficient synthesis of indoles via C–H activation of anilines in the coupling with α-diazo β-ketoesters.

Palladium-catalyzed direct alkenylation of 4-hydroxy-2-pyridones

The first direct 3-alkenylation of N-substituted 4-hydroxy-2-pyridones with unactivated alkenes is reported through a palladium acetate catalyzed, oxidative coupling.

Unprecedented copper-mediated oxidative demethylation of propionamides via bidentate-chelation assistance

The copper-mediated bidentate-chelation directing group assisted oxidative demethylation of substituted propionamides was developed for the first time.

Competitive intramolecular C–C vs. C–O bond coupling reactions toward C6 ring-fused 2-pyridone synthesis

This work described a competitive C–C vs. C–O bond forming reaction at the challenging C₆-position of 2-pyridones through Pd catalysis and silver radical cyclization.

Pyridine-enabled copper-promoted cross dehydrogenative coupling of C(sp2)-H and unactivated C(sp3)-H bonds

The pyridine-enabled cross dehydrogenative coupling of sp2 C-H bonds of polyfluoroarenes and unactivated sp3 C-H bonds of amides was achieved via a copper-promoted process with good functional group compatibility. This reaction showed great site-selectivity by favoring the sp2 C-H bonds ortho to two fluoro atoms of arenes and the sp3 C-H bonds of α-methyl groups over those of the α-methylene, β- or γ-methyl groups of the aliphatic amides. Mechanistic studies revealed that sp3 C-H bond cleavage is an irreversible but not the rate-determining step, and the sp2 C-H functionalization of arenes appears precedent to the sp3 C-H functionalization of amides in this process.