Oxidative Tandem Cyclization of Aromatic Acids with (Benzo)thiophenes: One-Pot Access to Planar Sulfur-Containing Polycyclic Heteroarenes for Lipid-Droplet-Targeted Probes

The efficient construction of π-conjugated polycyclic heteroarenes represents a significant task in the field of functional materials. A one-step oxidative tandem cyclization of aromatic acids with (benzo)thiophenes was developed to access planar sulfur-containing polycyclic heteroarenes. This protocol undergoes intermolecular cross-dehydrogenative coupling followed by intramolecular Friedel-Crafts acylation and provides a facile pathway to planar polycyclic compounds from inexpensive reactants. The synthesized heteroarenes serving as lipid-droplet-targeted probes exhibit outstanding performance with favorable biocompatibility and photostability.

Mono and Dinuclear Palladium Pincer Complexes of NNSe Ligand as a Catalyst for Decarboxylative Direct C-H Heteroarylation of (Hetero)arenes

This report describes the synthesis of a new NNSe pincer ligand and its mono- and dinuclear palladium(II) pincer complexes. In the absence of a base, a dinuclear palladium pincer complex (C1) was isolated, while in the presence of Et3 N base a mononuclear palladium pincer complex (C2) was obtained. The new ligand and complexes were characterized using techniques like 1 H, 13 C{1 H} nuclear magnetic resonance (NMR), fourier transform infrared (FTIR), high-resolution mass spectrometry (HRMS), ultraviolet-visible (UV-Visible), and cyclic voltammetry. Both the complexes showed pincer coordination mode with a distorted square planar geometry. The complex C1 has two pincer ligands attached through a Pd-Pd bond in a dinuclear pincer fashion. The air and moisture-insensitive, thermally robust palladium pincer complexes were used as the catalyst for decarboxylative direct C-H heteroarylation of (hetero)arenes. Among the complexes, dinuclear pincer complex C1 showed better catalytic activity. A variety of (hetero)arenes were successfully activated (43-87 % yield) using only 2.5 mol % of catalyst loading under mild reaction conditions. The PPh3 and Hg poisoning experiments suggested a homogeneous nature of catalysis. A plausible reaction pathway was proposed for the dinuclear palladium pincer complex catalyzed decarboxylative C-H bond activation reaction of (hetero)arenes.

Cationic Iridium-Catalyzed Decarboxylation of Aromatic Carboxylic Acids

Practical synthetic applications of catalytic decarboxylation in producing useful molecules are limited. We report herein the cationic Ir-catalyzed decarboxylations of various electron-rich and -poor aromatic carboxylic acids to produce hydrocarbons in good yield (up to >99%). Additionally, this reaction is applicable in decarboxylative hydroarylation of bicyclic alkenes and decarboxylative fluorination, indicating the potential utility of this catalytic decarboxylation in synthetic chemistry.

Palladium-Catalyzed Cascade Carbonylation to α,β-Unsaturated Piperidones via Selective Cleavage of Carbon-Carbon Triple Bonds

A direct and selective synthesis of α,β-unsaturated piperidones by a new palladium-catalyzed cascade carbonylation is described. In the presented protocol, easily available propargylic alcohols react with aliphatic amines to provide a broad variety of interesting heterocycles. Key to the success of this transformation is a remarkable catalytic cleavage of the present carbon-carbon triple bond by using a specific catalyst with 2-diphenylphosphinopyridine as ligand and appropriate reaction conditions. Mechanistic studies and control experiments revealed branched unsaturated acid 11 as crucial intermediate.

Domino Decarboxylative Arylation and C-O Selective Bond Formation toward Chromeno[2,3-b]pyridine-2-one Skeletons

Practical Pd-catalyzed 2-pyridones were designed to achieve chromeno[2,3-b]pyridine-2-ones. The reaction proceeds through domino nucleophilic addition and decarboxylative arylation, respectively. This methodology offers a moderately efficient approach to construct the bioactive, fused-heterocyclic skeletons via selective C-O bond formation and decarboxylative arylation in a single step with high selectivity and good yields.

Copper-Catalyzed C-3 Functionalization of Imidazo[1,2-a]pyridines with 3-Indoleacetic Acids

A copper-catalyzed C-3 functionalization of imidazo[1,2-a]pyridines with 3-indoleacetic acids through an aerobic oxidative decarboxylative process has been developed. The protocol provided a series of 3-(1H-indol-3-ylmethyl)-imidazo[1,2-a]pyridines in moderate to good yields under simple reaction conditions. Importantly, some products exhibited potent antiproliferative activity in cancer cell lines.

Cross-Coupling Reactions of Monosubstituted Tetrazines

A Ag-mediated Pd-catalyzed cross-coupling method for 3-bromo-1,2,4,5-tetrazine with boronic acids is presented. Electronic modification of the 1,1'-bis(diphenylphosphine)ferrocene (dppf) ligand was found to be crucial for good turnover. Using this fast method, a variety of alkyl-, heteroatom-, and halide-substituted aryl- and heteroaryl-tetrazines were prepared (29 examples, up to 87% yield).

Synthesis of Imidazoles and Oxazoles via a Palladium-Catalyzed Decarboxylative Addition/Cyclization Reaction Sequence of Aromatic Carboxylic Acids with Functionalized Aliphatic Nitriles

We herein report an efficient approach for the assembly of multiply substituted imidazoles and oxazoles in a single-step manner. These transformations are based on a decarboxylation addition and annulation of readily accessible aromatic carboxylic acids and aliphatic nitriles and exhibit good functional group compatibility and a high step economy. The reaction is scalable, and as-prepared products could be transformed into practical skeletons. Importantly, the late-stage derivatization of Momelotinib highlights the potential utility of this methodology.

Activation of Aryl Carboxylic Acids by Diboron Reagents towards Nickel-Catalyzed Direct Decarbonylative Borylation

The Ni-catalyzed decarbonylative borylation of (hetero)aryl carboxylic acids with B₂ cat₂ has been achieved without recourse to any additives. This Ni-catalyzed method exhibits a broad substrate scope covering poorly reactive non-ortho-substituted (hetero)aryl carboxylic acids, and tolerates diverse functional groups including some of the groups active to Ni⁰ catalysts. The key to achieve this decarbonylative borylation reaction is the choice of B₂ cat₂ as a coupling partner that not only acts as a borylating reagent, but also chemoselectively activates aryl carboxylic acids towards oxidative addition of their C(acyl)-O bond to Ni⁰ catalyst via the formation of acyloxyboron compounds. A combination of experimental and computational studies reveals a detailed plausible mechanism for this reaction system, which involves a hitherto unknown concerted decarbonylation and reductive elimination step that generates the aryl boronic ester product. This mode of boron-promoted carboxylic acid activation is also applicable to other types of reactions.

Rh-Catalyzed General Method for Directed C-H Functionalization via Decarbonylation of in-Situ-Generated Acid Fluorides from Carboxylic Acids

A Rh-catalyzed decarbonylative C-H coupling of in-situ-generated acid fluorides with amide substrates bearing ortho-Csp²-H bonds has been developed. This method enables alkyl, aryl, and alkenyl carboxylic acids to undergo decarbonylative coupling with C-H bonds of (hetero)aromatic or alkenyl amides in generally good yields via the in situ conversion of carboxylic acids into acid fluorides and also allows for the functionalization of a series of structurally complex carboxyl-containing natural products and pharmaceuticals as well as pharmaceutical amide derivatives.

Bimetallic Cooperative Catalysis for Decarbonylative Heteroarylation of Carboxylic Acids via C-O/C-H Coupling

Cooperative bimetallic catalysis is a fundamental approach in modern synthetic chemistry. We report bimetallic cooperative catalysis for the direct decarbonylative heteroarylation of ubiquitous carboxylic acids via acyl C-O/C-H coupling. This novel catalytic system exploits the cooperative action of a copper catalyst and a palladium catalyst in decarbonylation, which enables highly chemoselective synthesis of important heterobiaryl motifs through the coupling of carboxylic acids with heteroarenes in the absence of prefunctionalization or directing groups. This cooperative decarbonylative method uses common carboxylic acids and shows a remarkably broad substrate scope (>70 examples), including late-stage modification of pharmaceuticals and streamlined synthesis of bioactive agents. Extensive mechanistic and computational studies were conducted to gain insight into the mechanism of the reaction. The key step involves intersection of the two catalytic cycles via transmetallation of the copper-aryl species with the palladium(II) intermediate generated by oxidative addition/decarbonylation.

Tunable C–H arylation and acylation of azoles with carboxylic acids by Pd/Cu cooperative catalysis

Direct C–H arylation and acylation of azoles with carboxylic acids are achieved selectively through Pd/Cu cooperative catalysis: biaryls are generated selectively with dppp as ligand, while biaryl ketones are obtained with high selectivity using dpph or Ph₂PCy as ligand.

Oxalic Acid Monothioester for Palladium-Catalyzed Decarboxylative Thiocarbonylation and Hydrothiocarbonylation

Oxalic acid monothioester (OAM), an easily accessible and storable reagent, was reported herein as a thioester synthetic equivalent for palladium-catalyzed decarboxylative thiocarbonylation of organohalides and hydrothiocarbonylation of unsaturated carbon-carbon bonds at room temperature with high chemo- and regioselectivity. The reaction is applicable to the synthesis of cysteine-derived thioesters, thus allowing chemical modification of cysteine-containing peptides. Decarboxylation of OAM proceeds through oxidative addition of Pd(0) to the acyl-S bond, which accounts for the very mild reaction conditions.

The decarboxylative C–H heteroarylation of azoles catalysed by nickel catalysts to access unsymmetrical biheteroaryls

The direct decarboxylative C–H heteroarylation of azoles with heteroaryl carboxylic acids through nickel catalysts has been developed.

Direct C-3 alkylation of coumarins via decarboxylative coupling with carboxylic acids

A metal-free direct alkylation of coumarins using carboxylic acids in aqueous media with a broad substrate scope is devised.

Acyl Radical Chemistry via Visible-Light Photoredox Catalysis

Visible light photoredox catalysis has enabled easy access to acyl radicals under mild reaction conditions. Reactive acyl radicals, generated from various acyl precursors such as aldehydes, α-ketoacids, carboxylic acids, anhydrides, acyl thioesters, acyl chlorides, or acyl silanes, can undergo a diverse range of synthetically useful transformations, which were previously difficult or inaccessible. This review summarizes such recent progress of visible-light-driven acyl radical generation using transition metal photoredox catalysts, metallaphotocatalysts, hypervalent iodine catalysts or organic photocatalysts.

A Predictive Model for the Decarboxylation of Silver Benzoate Complexes Relevant to Decarboxylative Coupling Reactions

Decarboxylative coupling reactions offer an attractive route to generate functionalized arenes from simple and readily available carboxylic acid coupling partners, yet they are underutilized due to limitations in the scope of carboxylic acid coupling partner. Here we report that the field effect parameter (F) has a substantial influence on the rate of decarboxylation of well-defined silver benzoate complexes. This finding provides the opportunity to surpass current substrate limitations associated with decarboxylation and to enable widespread utilization of decarboxylative coupling reactions.

Palladium-catalyzed decarboxylative, decarbonylative and dehydrogenative C(sp2)-H acylation at room temperature

Over the past few decades, an impressive array of C-H activation methodology has been developed for organic synthesis. However, due to the inherent inertness of the C-H bonds (e.g. ∼110 kcal mol^-1 for the cleavage of C(aryl)-H bonds) harsh reaction conditions have been realized to overcome high energetic transition states resulting in a limited substrate scope and functional group tolerance. Therefore, the development of mild C-H functionalization protocols is in high demand to exploit the full potential of the C-H activation strategy in the synthesis of a complex molecular framework. Although, electron-rich substrates undergo electrophilic metalation under relatively mild conditions, electron-deficient substrates proceed through a rate-limiting C-H insertion under forcing conditions at high temperature. In addition, a stoichiometric amount of toxic silver salt is frequently used in palladium catalysis to facilitate the C-H activation process which is not acceptable from the environmental and industrial standpoint. We report herein, a Pd(ii)-catalyzed decarboxylative C-H acylation of 2-arylpyridines with α-ketocarboxylic acids under mild conditions. The present protocol does not require stoichiometric silver(i) salts as additives and proceeds smoothly at ambient temperature. A novel decarbonylative C-H acylation reaction has also been accomplished using aryl glyoxals as acyl surrogates. Finally, a practical C-H acylation via a dehydrogenative pathway has been demonstrated using commercially available benzaldehydes and aqueous hydroperoxides. We also disclose that acetonitrile solvent is optimal for the acylation reaction at room temperature and has a prominent role in the reaction outcome. Control experiments suggest that the acylation reaction via decarboxylative, decarbonylative and dehydrogenative proceeds through a radical pathway. Thus we disclose a practical protocol for the sp² C-H acylation reaction.

Recent developments in palladium-catalysed non-directed coupling of (hetero)arene C–H bonds with C–Z (Z = B, Si, Sn, S, N, C, H) bonds in bi(hetero)aryl synthesis

This review article describes the palladium-catalysed non-directed coupling of (hetero)arene C–H bonds with C_(Ar)–Z (Z = B, Si, Sn, S, N, C, H) bonds for facile bi(hetero)aryl synthesis in the past 10 years.

Transition-Metal-Free Decarboxylative Iodination: New Routes for Decarboxylative Oxidative Cross-Couplings

Constructing products of high synthetic value from inexpensive and abundant starting materials is of great importance. Aryl iodides are essential building blocks for the synthesis of functional molecules, and efficient methods for their synthesis from chemical feedstocks are highly sought after. Here we report a low-cost decarboxylative iodination that occurs simply from readily available benzoic acids and I₂. The reaction is scalable and the scope and robustness of the reaction is thoroughly examined. Mechanistic studies suggest that this reaction does not proceed via a radical mechanism, which is in contrast to classical Hunsdiecker-type decarboxylative halogenations. In addition, DFT studies allow comparisons to be made between our procedure and current transition-metal-catalyzed decarboxylations. The utility of this procedure is demonstrated in its application to oxidative cross-couplings of aromatics via decarboxylative/C–H or double decarboxylative activations that use I₂ as the terminal oxidant. This strategy allows the preparation of biaryls previously inaccessible via decarboxylative methods and holds other advantages over existing decarboxylative oxidative couplings, as stoichiometric transition metals are avoided.

Recent Progress in Decarboxylative Oxidative Cross-Coupling for Biaryl Synthesis

The beginning of the 21st century has seen tremendous growth in the field of decarboxylative activation. Benzoic acid derivatives are now recognised as atom-economic alternatives to traditional cross-coupling partners, and they also benefit from being inexpensive, readily available and shelf-stable reagents. In this microreview we discuss recent developments in the coupling of benzoic acid derivatives either with an arene or with a second benzoic acid derivative, a process often termed decarboxylative oxidative cross-coupling. These procedures offer great promise for the development of highly selective and atom-economic cross-couplings.