A Convenient Synthesis ofN-Aryl Benzamides by Rhodium-Catalyzedortho-Amidation and Decarboxylation of Benzoic Acids

Manganese(I)-Catalyzed Direct Addition of C(sp3)-H Bonds to Aryl Isocyanates

The addition of C-H bonds to isocyanates catalyzed by transition metals is a highly auspicious methodology for providing synthetically and biologically important amides. However, the substrates are limited to C(sp2)-H bonds. In this work, an efficient manganese(I)-catalyzed direct addition reaction of C(sp3)-H bonds of 8-methylquinolines to aryl isocyanates has been developed, leading to the synthesis of various α-quinolinyl amide compounds in moderate to high yields. The reaction has a broad range of substrates and a good functional group tolerance. A possible mechanism is proposed based on the experimental results.

A Fujiwara-Moritani-Type Alkenylation Using a Traceless Directing Group Strategy: A Rare Example of C-C Bond Formation towards the C2-Carbon of Terminal Alkenes

Herein we report, a rhodium-catalyzed Fujiwara-Moritani-type reaction of unactivated terminal alkenes and benzoic acid derivatives bearing electron donating residues under mild conditions. The acid functionality acts as a traceless directing group delivering products alkenylated in meta-position to the electron donating substituent in contrast to the usually obtained ortho- and para-substitution in Friedel-Crafts-type reactions. Remarkably, the new C-C bond is formed to the C2 of the terminal olefin, in contrast to similar reported transformations. Initially formed mixtures of exo- and endo-double bond isomers can be efficiently isomerized to the more stable endo-products.

Metal -free PhI(OAc)2-oxidized decarboxylation of propiolic acids towards synthesis of α-acetoxy ketones and insights into general decarboxylation with DFT calculations

A metal-free oxidative decarboxylation reaction of propiolic acids mediated by hypervalent iodine(III) reagents is described. This decarboxylative C-O bond-forming reaction used a combination of (diacetoxyiodo)benzene and aromatic, heteroaromatic or aliphatic propiolic acids to give the corresponding α-acetoxy ketones. Preliminary mechanistic studies based on both DFT calculations and high-resolution mass spectroscopy (HRMS) suggested that the reaction proceeded through decarboxylation to form a propargyl iodide intermediate. This reaction provides an attractive alternative to existing methods for the exclusive synthesis of α-acyloxy ketones.

Manganese-catalyzed hydroarylation of multiple bonds

Transition metal-catalyzed C-H activation has become a promising strategy in organic synthesis due to its improved atom-, step- and resource economy. Considering the Earth's abundance, economic benefits, and low toxicity, 3d metal catalysts for C-H activation have received a significant focus. In particular, organometallic manganese-catalyzed C-H activation has proven to be versatile and suitable for a wide range of transformations such as C-H addition to π-components, arylation, alkylation, alkynylation, amination, and many more. Among them, manganese-catalyzed C-H addition to C-C and C-heteroatom multiple bonds exhibited unique and promising reactivity to construct a wide range of complex organic molecules. In this review, we highlight the developments in the field of manganese-catalyzed hydroarylation of multiple bonds via C-H activation with a range of applications until August 2022.

Access to quinolinones via DMAP-catalysed cascade reaction of 2-substituted benzoic acids with organic azides

Herein, we report a DMAP-catalysed Curtius rearrangement and intramolecular cyclisation cascade reaction of 2-substituted aryl carboxylic acids with organic azides for the first time. This protocol features simple operation, broad scope and metal-free conditions, furnishing a broad spectrum of biologically attractive heterocycles. The synthetic virtue of this reaction was demonstrated by gram-scale synthesis and applicability toward drug-like molecules.

Pd(ii)-catalyzed meta-C-H bromination and chlorination of aniline and benzoic acid derivatives

The classic electrophilic bromination leads to ortho- and para-bromination of anilines due to their electron-rich properties. Herein we report the development of an unprecedented Pd-catalyzed meta-C-H bromination of aniline derivatives using commercially available N-bromophthalimide (NBP), which overcomes the competing ortho/para-selectivity of electrophilic bromination of anilines. The addition of acid additives is crucial for the success of this reaction. A broad range of substrates with various substitution patterns can be tolerated in this reaction. Moreover, benzoic acid derivatives bearing complex substitution patterns are also viable with this mild bromination reaction, and meta-C-H chlorination is also feasible under similar reaction conditions. The ease of the directing group removal and subsequent diverse transformations of the brominated products demonstrate the application potential of this method and promise new opportunities for drug discovery.

Rhodium(III)-catalysed decarboxylative C-H functionalization of isoxazoles with alkenes and sulfoxonium ylides

Decarboxylative C-H functionalization of isoxazoles with electron-deficient alkenes and sulfoxonium ylides at the C5 position was achieved in the presence of rhodium(iii) catalysts to give the corresponding alkenylation and acylmethylation products, respectively.

Carboxyl-Assisted meta-Selective C-H Functionalizations of Benzylsulfonamides

A protocol of carboxyl-group-assisted, Pd(II)-catalyzed remote meta-C(sp²)-H olefination and arylation of benzylsulfonamides has been developed. It was supposed to proceed through a κ² coordination of the carboxyl group to the Pd center. These findings demonstrated the versatility of carboxyl-assisted remote meta-C-H activation strategy and might stimulate the exploration of novel reactivity and selectivity of other traditional chelating groups in different contexts.

Traceless directing groups: a novel strategy in regiodivergent C-H functionalization

The use of functional groups as internal ligands for assisting C-H functionalization, termed the chelation assisted strategy, is emerging as one of the most powerful tools for construction of C-C and C-X bonds from inert C-H bonds. However, there are various directing groups which cannot be either removed after functionalization or require some additional steps or reagents for their removal, thereby limiting the scope of structural diversity of the products, and the step and atom economy of the system. These limitations are overcome by the use of the traceless directing group (TDG) strategy wherein functionalization of the substrate and removal of the directing group can be carried out in a one pot fashion. Traceless directing groups serve as the most ideal chelation assisted strategy with a high degree of reactivity and selectivity without any requirement for additional steps for their removal. The present review overviews the use of various functional groups such as carboxylic acids, aldehydes, N-oxides, nitrones, N-nitroso amines, amides, sulfoxonium ylides and silicon tethered directing groups for assisting transition metal catalyzed C-H functionalization reactions in the last decade.

The ruthenium-catalyzed C-H functionalization of enamides with isocyanates: easy entry to pyrimidin-4-ones

Ruthenium-catalyzed heteroannulation between enamides and isocyanates has been realized as a complementary approach to conventional strategies for the synthesis of pyrimidin-4-ones. High step- and atom-economy was achieved for the rapid construction of such privileged scaffolds, which are found in a multitude of pharmaceutical compounds. The generality and practicability of this transformation were reflected by the broad scope of substrates with diverse functional groups, large-scale synthesis, and late-stage diversification.

Rhodium(iii)-catalysed carboxylate-directed C-H functionalizations of isoxazoles with alkynes

An efficient oxidative [4+2] annulation of isoxazolyl-4-carboxylic acids with internal alkynes proceeded in the presence of the [Cp*RhCl₂]₂ catalyst. Oxidants controlled the formation of pyranoisoxazolones and isoquinolines. A decarboxylative approach for the hydroarylation of alkynes with isoxazolyl-4-carboxylic acids was also developed in the presence of the [Cp*Rh(CH₃CN)₃][SbF₆]₂ catalyst.

AuBr3 -Catalyzed Chemoselective Annulation of Isocyanates with 2H-Azirine

The chemoselective cyclization of isocyanates with 2H-azirine was achieved with AuBr₃ as catalyst. This transfer sets the stage for the synthesis of aromatic oxazole-ureas in a tandem process. The addition of a catalytic amount of phosphite enhances the process enormously. The reaction can also be performed in a one-pot process using benzoyl azide instead of isocyanate under the same conditions. A detailed study on the role of the phosphite that was applied as an additive revealed that only non-coordinated phosphite can reduce gold(III) and that gold(I) coordinated phosphite is not oxidized. Accompanied by the reduction of gold, HBr is generated in situ, which turned out to be the actual promotor in combination with the remaining AuBr₃ . The positive effect of acid can be explained by a strong N-Au coordination, which tends to break more easily in the presence of small amount of protic acid in the reaction solution.

Rhodium(iii)-catalyzed indole synthesis at room temperature using the transient oxidizing directing group strategy

Rh-Catalyzed reactions of N-alkyl anilines with internal alkynes at room temperature have been developed using an in situ generated N-nitroso group as a transient oxidizing directing group.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

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.

Palladium-Catalyzed Decarboxylative ortho-Amidation of Indole-3-carboxylic Acids with Isothiocyanates Using Carboxyl as a Deciduous Directing Group

Palladium-catalyzed ortho-amidation of indole-3-carboxylic acids with isothiocyanates by using the deciduous directing group nature of carboxyl functionality to afford indole-2-amides is demonstrated. Both C-H functionalization and decarboxylation took place in one pot, and hence, this carboxyl group served as a unique, deciduous (or traceless) directing group. This reaction offers a broad substrate scope as demonstrated for several other heterocyclic carboxylic acids like chromene-3-carboxylic acid, imidazo[1,2- a]pyridine-2-carboxylic acid, benzofuran-2-carboxylic acid, pyrrole-2-carboxylic acid, and thiophene-2-carboxylic acid. In the reaction using 2-naphthoic acid, of the two possible isomers, only one isomer of the amide was exclusively formed. The indole-2-amide product underwent palladium-catalyzed C-H functionalization to afford the diindole-fused 2-pyridones by combining two molecules of the indole moiety, with the elimination of an amide group from one of them, attached at the C3-position for the C-C/C-N bond formation. The structures of key products are confirmed by X-ray crystallography.

Ru(II)-Catalyzed C-H Aminocarbonylation of N-(Hetero)aryl-7-azaindoles with Isocyanates

The ruthenium(II)-catalyzed C-H aminocarbonylation of N-(hetero)aryl-7-azaindoles with isocyanates is described. The excellent site selectivity at the ortho-position within the N-(hetero)aryl ring was observed to provide ortho-amidated N-(hetero)aryl-7-azaindoles under the mild reaction conditions. The resulting 7-azaindole derivatives can be readily transformed into 7-azaindoles containing carboxylic acid and alkyl amine functional groups.

Mechanisms and Origins of Chemo- and Regioselectivities of Ru(II)-Catalyzed Decarboxylative C-H Alkenylation of Aryl Carboxylic Acids with Alkynes: A Computational Study

The mechanisms and chemo- and regioselectivities of Ru(II)-catalyzed decarboxylative C-H alkenylation of aryl carboxylic acids with alkynes were investigated with density functional theory (DFT) calculations. The catalytic cycle involves sequential carboxylate-directed C-H activation, alkyne insertion, decarboxylation and protonation. The facile tether-assisted decarboxylation step directs the intermediate toward the desired decarboxylative alkenylation, instead of typical annulation and double alkenylation pathways. The decarboxylation barrier is very sensitive to the tether length, and only the seven-membered ring intermediate can selectively undergo the designed decarboxylation, suggesting a tether-dependent chemoselectivity. This tether-dependent chemoselectivity also applies to the alkyl tethers. In addition, the polarity of solvent is found to control the chemoselectivity between the decarboxylative alkenylation and [4 + 2] annulation. Solvent with low polarity (toluene) favors the decarboxylation pathway, leading to the decarboxylative alkenylation. Solvent with high polarity (methanol) favors the ionic stepwise C-O reductive elimination pathway, leading to the [4 + 2] annulation. To understand the origins of regioselectivity with asymmetric alkynes, the distortion/interaction analysis was applied to the alkyne insertion transition states, and led to a predictive frontier molecular orbital model. The asymmetric alkynes selectively use the terminal with the larger HOMO orbital coefficient to form the C-C bond in the insertion step.

Ruthenium-Catalyzed C-H Arylation of Benzoic Acids and Indole Carboxylic Acids with Aryl Halides

Herein we report the first Ru-catalyzed C-H arylation of benzoic acids with readily available aryl (pseudo)halides. The reaction, which does not require the use of silver salt additives, allows the arylation of previously challenging hindered benzoic acids and the use of generally unreactive ortho-substituted halorarenes. Furthermore, our new protocol can efficiently be applied to indole carboxylic acids, thus allowing access to C7-, C6-, C5- and C4-arylated indole compounds, a departure from the classical enhanced reactivity of the C2 and C3 positions of indole.

Rhodium(iii)-catalyzed and MeOH-involved regioselective mono-alkenylation of N-arylureas with acrylates

A new and versatile Rh(iii)-catalyzed C–H activation/alkenylation/MeOH-involved ammonolysis/esterification cascade reaction has been disclosed.

Palladium-catalyzed interannular meta-C–H arylation

The interannularmeta-selective C–H arylation of biaryl-2-trifluoroacetamides using Pd(ii)/norbornene catalysis is described.

The use of carboxylic acids as traceless directing groups for regioselective C–H bond functionalisation

This feature article focuses on the use of carboxylic acids as traceless directing groups and the application of this concept to achieve regiochemical control in organic chemistry.

Ruthenium(II)-Catalyzed Regioselective Ortho Amidation of Imidazo Heterocycles with Isocyanates

Direct ortho amidation at the phenyl ring of 2-phenylimidazo heterocycles with aryl isocyanates has been achieved via a chelation-assisted cationic ruthenium(II) complex catalyzed mechanism. The methodology provides a straightforward, high-yielding regioselective approach toward the synthesis of an array of ortho-amidated phenylimidazo heterocycles without prior activation of C(sp2)-H. This also reports the first method for coupling of aryl isocyanates with the imidazo[1,2-a]pyridine system via a pentacyclometalated intermediate. The methodology is found to be easily scalable and could be applied toward the selective ortho amidation of 2-heteroarylimidazo[1,2-a]pyridine frameworks.

Transition-Metal-Catalyzed C-H Bond Addition to Carbonyls, Imines, and Related Polarized π Bonds

The transition-metal-catalyzed addition of C-H bonds to carbonyls, imines, and related polarized π bonds has emerged as a particularly efficient and powerful approach for the construction of an incredibly diverse array of heteroatom-substituted products. Readily available and stable inputs are typically employed, and reactions often proceed with very high functional group compatibility and without the production of waste byproducts. Additionally, many transition-metal-catalyzed C-H bond additions to polarized π bonds occur within cascade reaction sequences to provide rapid access to a diverse array of different heterocyclic as well as carbocyclic products. This review highlights the diversity of transformations that have been achieved, catalysts that have been used, and types of products that have been prepared through the transition-metal-catalyzed addition of C-H bonds to carbonyls, imines, and related polarized π bonds.

Carboxylic Acids as Directing Groups for C-H Bond Functionalization

The selective transformation of C-H bonds is one of the most desirable approaches to creating complexity from simple building blocks. Several directing groups are efficient in controlling the regioselectivity of catalytic C-H bond functionalizations. Among them, carboxylic acids are particularly advantageous, since they are widely available in great structural diversity and at low cost. The carboxylate directing groups can be tracelessly cleaved or may serve as the anchor point for further functionalization through decarboxylative couplings. This Minireview summarizes the substantial progress made in the last few years in the development of reactions in which carboxylate groups direct C-H bond functionalizations with formation of C-C, C-O, C-N, or C-halogen bonds at specific positions. It is divided into sections on C-C, C-O, C-N, and C-halogen bond formation, each of which is subdivided by reactions and product classes. Particular emphasis is placed on methods that enable multiple derivatizations by combining carboxylate-directed C-H functionalization with decarboxylative couplings.

Nonclassical Routes for Amide Bond Formation

The present review offers an overview of nonclassical (e.g., with no pre- or in situ activation of a carboxylic acid partner) approaches for the construction of amide bonds. The review aims to comprehensively discuss relevant work, which was mainly done in the field in the last 20 years. Organization of the data follows a subdivision according to substrate classes: catalytic direct formation of amides from carboxylic and amines ( section 2 ); the use of carboxylic acid surrogates ( section 3 ); and the use of amine surrogates ( section 4 ). The ligation strategies (NCL, Staudinger, KAHA, KATs, etc.) that could involve both carboxylic acid and amine surrogates are treated separately in section 5 .

Rh(III)-Catalyzed Diastereoselective C-H Bond Addition/Cyclization Cascade of Enone Tethered Aldehydes

The Rh(III)-catalyzed cascade addition of a C-H bond across alkene and carbonyl π-bonds is reported. The reaction proceeds under mild reaction conditions with low catalyst loading. A range of directing groups were shown to be effective as was the functionalization of alkenyl in addition to aromatic C(sp2)-H bonds. When the enone and aldehyde electrophile were tethered together, cyclic β-hydroxy ketones with three contiguous stereocenters were obtained with high diastereoselectivity. The intermolecular three-component cascade reaction was demonstrated for both aldehyde and imine electrophiles. Moreover, the first x-ray structure of a cationic Cp*Rh(III) enolate with interatomic distances consistent with an η3-bound enolate is reported.