Recent Progress in Decarboxylative Oxidative Cross-Coupling for Biaryl Synthesis

Electrochemical Decarboxylation Coupling Reactions

Carboxylic acids are attractive synthetic feedstocks with stable, non-toxic, and inexpensive properties that can be easily obtained from natural sources or through synthesis. Carboxylic acids have long been considered environmentally friendly coupling agents in various organic transformations. In recent years, electrochemically mediated decarboxylation reactions of decarboxylic acids and their derivatives (NHPI) have emerged as effective new methods for constructing carbon-carbon or carbon-heterocarbon chemical bonds. Compared with transition metal and photochemistry-mediated catalytic reactions, which do not require the addition of oxidants and strong bases, electrochemically-mediated decarboxylative transformations are considered a sustainable strategy. In addition, various functional groups tolerate the electrochemical decarboxylation conversion strategy well. Here, we summarize the recent electrochemical decarboxylation reactions to better elucidate the advantages of electrochemical decarboxylation reactions.

Photocatalytic decarboxylation of free carboxylic acids and their functionalization

Visible light mediated decarboxylative functionalization of carboxylic acids and their derivatives has recently emerged as a novel and powerful toolkit for small molecule activation in diverse carbon-carbon and carbon-hetero bond forming reactions. Naturally abundant highly functionalized bench-stable carboxylic acid analogs have been employed as promising alternatives to non-trivial organometallic reagents for mild and eco-benign synthetic transformation with traceless CO2 by-products. In this highlight article, we focus on the development of various photodecarboxylative functionalization strategies along with intra/inter-molecular cyclization via concerted single electron transfer (SET) or energy transfer (ET) pathways. Moreover, widely explored carboxylic acids are systematically classified here into four categories; i.e., α-keto, aliphatic, α,β-unsaturated, and aromatic analogs for a concise overview to the readership. The association of decarboxylative radical species with coupling partners to construct C-C and C-N/O/S/P/X bonds for each analogous acid has been presented in brief.

Unimolecular Fragment Coupling: A New Bond-Forming Methodology via the Deletion of Atom(s)

Unimolecular fragment coupling (UFC) is defined as a reaction format, wherein atom(s) located in the middle of a molecule are extruded, and the remaining fragments are coupled. UFC is a potentially powerful strategy that is an alternative to transition-metal-catalyzed cross-coupling because the target chemical bond is formed in an intramolecular fashion, which is inherently beneficial for chemoselectivity and stereoselectivity issues. In this Perspective, we will present an overview of the recent advances in UFC reactions, which encompass those proceeding through the elimination of CO2, CO, SO2, isocyanates, N2, or single atoms primarily via transition metal catalysis.

Switchable Decarboxylation by Energy- or Electron-Transfer Photocatalysis

Kolbe dimerization and Hofer-Moest reactions are well-investigated carboxylic acid transformations, wherein new carbon-carbon and carbon-heteroatom bonds are constructed via electrochemical decarboxylation. These transformations can be switched by choosing an electrode that allows control of the reactive intermediate, such as carbon radical or carbocation. However, the requirement of a high current density diminishes the functional group compatibility with these electrochemical reactions. Here, we demonstrate the photocatalytic decarboxylative transformation of activated carboxylic acids in a switchable and functional group-compatible manner. We discovered that switching between Kolbe-type or Hofer-Moest-type reactions can be accomplished with suitable photocatalysts by controlling the reaction pathways: energy transfer (EnT) and single-electron transfer (SET). The EnT pathway promoted by an organo-photocatalyst yielded 1,2-diarylethane from arylacetic acids, whereas the ruthenium photoredox catalyst allows the construction of an ester scaffold with two arylmethyl moieties via the SET pathway. The resulting radical intermediates were coupled to olefins to realize multicomponent reactions. Consequently, four different products were selectively obtained from a simple carboxylic acid. This discovery offers new opportunities for selectively synthesizing multiple products via switchable reactions using identical substrates with minimal cost and effort.

Modern Strategies for Carbon Isotope Exchange

In contrast to stable and natural abundant carbon-12, the synthesis of organic molecules with carbon (radio)isotopes must be conceived and optimized in order to navigate through the hurdles of radiochemical requirements, such as high costs of the starting materials, harsh conditions and radioactive waste generation. In addition, it must initiate from the small cohort of available C-labeled building blocks. For long time, multi-step approaches have represented the sole available patterns. On the other side, the development of chemical reactions based on the reversible cleavage of C-C bonds might offer new opportunities and reshape retrosynthetic analysis in radiosynthesis. This review aims to provide a short survey on the recently emerged carbon isotope exchange technologies that provide effective opportunity for late-stage labeling. At present, such strategies have relied on the use of primary and easily accessible radiolabeled C1-building blocks, such as carbon dioxide, carbon monoxide and cyanides, while the activation principles have been based on thermal, photocatalytic, metal-catalyzed and biocatalytic processes.

Carboxylic Acid Salts as Dual-Function Reagents for Carboxylation and Carbon Isotope Labeling

The potassium salts of carboxylic acids are developed as efficient carboxylating agents through CO₂ exchange. We describe these carboxylates as dual-function reagents because they function as a combined source of CO₂ and base/metalating agent. By using the salt of a commercially available carboxylic acid, this protocol overcomes difficulties when using CO₂ gas or organometallic reagents, such as pressurized containers or strictly inert conditions. The reaction proceeds under mild conditions, does not require transition metals or other additives, and shows broad substrate scope. Through the preparation of several biologically important molecules, we show how this strategy provides an opportunity for isotope labeling with low equivalents of labeled CO₂ .

Deaminative Arylation and Alkenyaltion of Aliphatic Tertiary Amines with Aryl and Alkenylboronic Acids via Nitrogen Ylides

Transition-metal-catalyzed Suzuki-Miyaura coupling has significantly advanced C-C bond formation and has been well recognized in organic synthesis, pharmaceuticals, materials science and other fields. In this rapid development, cross coupling without transition metal catalyst is a big challenge in this field, and using widely existing tertiary amines as electrophiles to directly couple with boronic acids has great hurdles yet significant application prospects. Herein, we report an efficient and general deaminative arylation and alkenylation of tertiary amines (propargyl amines, allyl amines and 1H-indol-3-yl methane amines) with ary and alkenylboronic acids enabled by difluorocarbene under transition-metal-free conditions. Preliminary mechanism experiments suggest that in situ formed difluoromethyl quaternary amine salt, nitrogen ylide and tetracoordinate boron species are the key intermediates, the subsequent 1,2-metallate shift and protodeboronation complete the new coupling reaction.

Concise Syntheses of Alternariol, Alternariol-9-monomethyl Ether and Their D3-Isotopologues

Alternariol (AOH) and alternariol-9-monomethyl ether (AME) are two secondary metabolites of Alternaria fungi which can be found in various foodstuffs like tomatoes, nuts, and grains. Due to their toxicity and potential mutagenic activity the need for the development of high-throughput methods for the supervision of AOH and AME levels is of increasing interest. As the availability of both native and labeled AOH and AME analytical standards is very limited, we herein present a novel and concise approach towards their synthesis by employing a ruthenium-catalyzed ortho-arylation as the key step. Finally, we demonstrate their suitability as internal standards in stable-isotope dilution assay (SIDA)-HPLC-MS/MS analysis, a technique commonly used for the quantification of natural products in food and feed.

ORGANOMETALLIC GAS-PHASE ION CHEMISTRY AND CATALYSIS: INSIGHTS INTO THE USE OF METAL CATALYSTS TO PROMOTE SELECTIVITY IN THE REACTIONS OF CARBOXYLIC ACIDS AND THEIR DERIVATIVES

Carboxylic acids are valuable organic substrates as they are widely available, easy to handle, and exhibit structural and functional variety. While they are used in many standard synthetic protocols, over the past two decades numerous studies have explored new modes of metal-mediated reactivity of carboxylic acids and their derivatives. Mass spectrometry-based studies can provide fundamental mechanistic insights into these new modes of reactivity. Here gas-phase models for the following catalytic transformations of carboxylic acids and their derivatives are reviewed: protodecarboxylation; dehydration; decarbonylation; reaction as coordinated bases in C-H bond activation; remote functionalization and decarboxylative C-C bond coupling. In each case the catalytic problem is defined, insights from gas-phase studies are highlighted, comparisons with condensed-phase systems are made and perspectives are reached. Finally, the potential role for mechanistic studies that integrate both gas- and condensed-phase studies is highlighted by recent studies on the discovery of new catalysts for the selective decomposition of formic acid and the invention of the new extrusion-insertion class of reactions for the synthesis of amides, thioamides, and amidines. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Copper-Mediated Decarboxylative Coupling between Arylacetic Acids and 1,3-Dicarbonyl Compounds

A copper-mediated decarboxylative coupling reaction between arylacetic acids and 1,3-dicarbonyl compounds was described. Significantly, methanocycloocta[b]indoles were also obtained by sequential intramolecular dehydrocyclization process in some cases. This protocol featured a broad substrate scope, simple operations, and good yields. Moreover, the products exhibited potent antiproliferative activity against the human cancer cell lines by a MTT assay.

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.

Wonderful fusion of organofluorine chemistry and decarboxylation strategy

Decarboxylation strategy has been emerging as a powerful tool for the synthesis of fluorine-containing organic compounds that play important roles in various fields such as pharmaceuticals, agrochemicals, and materials science. Considerable progress in decarboxylation has been made over the past decade towards the construction of diverse valuable fluorinated fine chemicals for which the fluorinated part can be brought in two ways. The first way is described as the reaction of non-fluorinated carboxylic acids (and their derivatives) with fluorinating reagents, as well as fluorine-containing building blocks. The second way is dedicated to the exploration and the use of fluorine-containing carboxylic acids (and their derivatives) in decarboxylative transformations. This review aims to provide a comprehensive summary of the development and applications of decarboxylative radical, nucleophilic and cross-coupling strategies in organofluorine chemistry.

Oxidative cross-coupling processes inspired by the Chan-Lam reaction

The Cu-catalyzed oxidative cross-coupling of N- and O-nucleophiles with aryl boronic acids (the Chan-Lam reaction) remains among the most useful approaches to prepare aniline and phenol derivatives. The combination of high chemoselectivity, mild reaction conditions, and the ability to use simple Cu-salts as catalysts makes this process a valuable alternative to aromatic substitutions and Pd-catalyzed reactions of aryl electrophiles (Buchwald-Hartwig coupling). Despite the widespread use of Chan-Lam reactions in synthesis, the analogous carbon-carbon bond forming variant of this process had not been developed prior to our work. This feature article describes our discovery and application of Cu-catalyzed oxidative coupling reactions of activated methylene derivatives or carboxylic acids with nucleophiles including aryl boronic esters and amines.

Photocatalyzed Transition-Metal-Free Oxidative Cross-Coupling Reactions of Tetraorganoborates*

Readily accessible tetraorganoborate salts undergo selective coupling reactions under blue light irradiation in the presence of catalytic amounts of transition‐metal‐free acridinium photocatalysts to furnish unsymmetrical biaryls, heterobiaryls and arylated olefins. This represents an interesting conceptual approach to forge C−C bonds between aryl, heteroaryl and alkenyl groups under smooth photochemical conditions. Computational studies were conducted to investigate the mechanism of the transformation.

Advances in Electrochemical Decarboxylative Transformation Reactions

Owing to their non-toxic, stable, inexpensive properties, carboxylic acids are considered as environmentally benign alternatives as coupling partners in various organic transformations. Electrochemical mediated decarboxylation of carboxylic acid has emerged as a new and efficient methodology for the construction of carbon-carbon or carbon-heteroatom bonds. Compared with transition-metal catalysis and photoredox catalysis, electro-organic decarboxylative transformations are considered as a green and sustainable protocol due to the absence of chemical oxidants and strong bases. Further, it exhibits good tolerance with various functional groups. In this Minireview, we summarize the recent advances and discoveries on the electrochemical decarboxylative transformations on C-C and C-heteroatoms bond formations.

Chichibabin pyridinium synthesis via oxidative decarboxylation of photoexcited α-enamine acids

A visible light-induced decarboxylative Chichibabin pyridinium synthesis between α-amino acids and aldehydes was developed. When the in situ generated α-enamine acids were photoexcited, they were oxidized by aerobic oxygen to give radical cation species. After decarboxylation and further oxidation, the generated iminium undergoes Chichibabin cyclization to afford pyridiniums. This photochemical protocol enables the synthesis of various tetra-substituted pyridiniums and related natural products in one-step.

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.

Transition-metal-free decarboxylative ipso amination of aryl carboxylic acids

An unprecedented DMAP-catalysed decarboxylative amination of carboxylic acids has been achieved under metal free conditions, enabling the convenient synthesis of structurally diverse aryl and alkyl amines.

Mechanistic Insight into Palladium-Catalyzed γ-C(sp3)-H Arylation of Alkylamines with 2-Iodobenzoic Acid: Role of the o-Carboxylate Group

Density functional theory calculations were performed to understand the distinctly different reactivities of o-carboxylate-substituted aryl halides and pristine aryl halides toward the Pd^II-catalyzed γ-C(sp³)-H arylation of secondary alkylamines. It is found that, when 2-iodobenzoic acid (a representative of o-carboxylate-substituted aryl halides) is used as an aryl transfer agent, the arylation reaction is energetically favorable, while when the pristine aryl halide iodobenzene is used as the aryl transfer reagent, the reaction is kinetically difficult. Our calculations showed an operative Pd^II/Pd^IV/Pd^II redox cycle, which differs in the mechanistic details from the cycle proposed by the experimental authors. The improved mechanism emphasizes that (i) the intrinsic role of the o-carboxylate group is facilitating the C(sp³)-C(sp²) bond reductive elimination from Pd^IV rather than facilitating the oxidative addition of the aryl iodide on Pd^II, (ii) the decarboxylation occurs at the Pd^II species instead of the Pd^IV species, and (iii) the 1,2-arylpalladium migration proceeds via a stepwise mechanism where the reductive elimination occurs before decarboxylation, not via a concerted mechanism that merges the three processes decarboxylation, 1,2-arylpalladium migration, and C(sp³)-C(sp²) reductive elimination into one. The experimentally observed exclusive site selectivity of the reaction was also rationalized well.

Conversions of aryl carboxylic acids into aryl nitriles using multiple types of Cu-mediated decarboxylative cyanation under aerobic conditions

Here, we used malononitrile or AMBN as a cyanating agent to develop efficient and practical protocols for Cu-mediated decarboxylative cyanations, under aerobic conditions, of aryl carboxylic acids bearing nitro and methoxyl substituents at the ortho position as well as of heteroaromatic carboxylic acids. These protocols involved economical methods to synthesize value-added aryl nitriles from simple and inexpensive raw materials. Further diversification of the 2-nitrobenzonitrile product was performed to highlight the practicality of the protocols.

In Situ Laser Scattering Electrospray Ionization Mass Spectrometry and Its Application in the Mechanism Study of Photoinduced Direct C-H Arylation of Heteroarenes

An in situ laser scattering electrospray ionization mass spectrometry (LS-ESI-MS) was developed, where the laser scattering was simply achieved through the laser radiation of the "media" modified on the capillary. The laser scattering extended the reaction window and powerfully promoted the reaction yield of the photoinduced organic reaction, which enables the trace intermediates to be efficiently tracked in real time. For instance, the key radical cation in the photoinduced direct C-H arylation of heteroarenes was captured inventively, which provided direct experimental evidence for the verification of the reaction mechanism. Together with the characterization of oxidative photocatalytic Ru(III) intermediate, the integral insight into the process of visible-light-mediated direct C-H arylation of heteroarenes was confirmed. This approach is facile, powerful, and promising in the mechanism study of organic reaction.

Transition-Metal-Free Oxidative Cross-Coupling of Tetraarylborates to Biaryls Using Organic Oxidants

Readily prepared tetraarylborates undergo selective (cross)-coupling through oxidation with Bobbitt's salt to give symmetric and unsymmetric biaryls. The organic oxoammonium salt can be used either as a stoichiometric oxidant or as a catalyst in combination with in situ generated NO2 and molecular oxygen as the terminal oxidant. For selected cases, oxidative coupling is also possible with NO2 /O2 without any additional nitroxide-based cocatalyst. Transition-metal-free catalytic oxidative ligand cross-coupling of tetraarylborates is unprecedented and the introduced method provides access to various biaryl and heterobiaryl systems.

PIDA-Mediated Rearrangement for the Synthesis of Enantiopure Triazolopyridinones

A tandem oxidative cyclization/1,2-carbon migration of hydrazides for the synthesis of otherwise inaccessible hindered or enantiopure triazolopyridinones has been developed. This protocol exhibits broad substrate scope and can be easily scaled up by continuous flow synthesis under mild conditions. Most importantly, this method demonstrates a rearrangement with retention of configuration and can be readily applied for the late-stage modification of carboxylic-acid-containing pharmaceuticals, amino acids, and natural products to access enantiopure triazolopyridinones.

Enantiospecific electrochemical rearrangement for the synthesis of hindered triazolopyridinone derivatives

Triazolopyridinone derivatives are of high value in both medicinal and material chemistry. However, the chiral or hindered triazolopyridinone derivatives remain an underexplored area of chemical space because they are difficult to prepare via conventional methods. Here we report an electrochemical rearrangement for the efficient synthesis of otherwise inaccessible triazolopyridinones with diverse alkyl carboxylic acids as starting materials. This enables the efficient preparation of more than 60 functionalized triazolopyridinones under mild conditions in a sustainable manner. This method is evaluated for the late stage modification of bioactive natural products, amino acids and pharmaceuticals, and it is further applied to the decagram scale preparation of enantiopure triazolopyridinones. The control experiments support a mechanism involving an oxidative cyclization and 1,2-carbon migration. This facile and scalable rearrangement demonstrates the power of electrochemical synthesis to access otherwise-inaccessible triazolopyridinones and may find wide application in organic, material and medicinal chemistry.

Chiral and hindered triazolopyridinone derivatives are an underexplored area of chemical space mainly due to their challenging synthesis via classical methods. Here, the authors report an electrochemical rearrangement for the synthesis of triazolopyridinones using diverse, available alkyl carboxylic acids as starting materials.

Cu-Photoredox-catalyzed C(sp)-C(sp3) coupling of redox-active esters with terminal alkynes

Visible-light-induced C(sp)-C(sp3) coupling of redox-active esters with terminal alkynes has been developed. The activation of carboxylic acids as their redox-active ester derivatives was important for this decarboxylative alkynylation. The strategy established here facilitates the straightforward introduction of triple-bonded functional groups and avoids additional photocatalysts. A wide range of primary, secondary and tertiary acids can be converted into the target products; so this reaction exhibits a broad substrate scope and tolerance of functional groups. Mechanistic experiments suggested that this reaction may undergo a radical process. Under mild reaction conditions, a copper acetylide ligand as a photocatalyst delivered an electron to redox-active ester derivatives, and generated alkyl radicals. The radicals reacted with Cu(ii) to deliver a Cu(iii) complex, and then reductive elimination gave the products.

Dienedioic acid as a useful diene building block via directed Heck-decarboxylate coupling

The concise construction of diene scaffolds is quite useful in the synthesis of polyenes. Many diene building blocks have been developed based on Suzuki, Still and Hiyama couplings. Herein, the commercially available and environmentally friendly compound dienedioic acid is used as a diene building block. Broad substrate scope, good functional group tolerance, and late-stage derivatization of complex drug molecules are achieved. Different moieties can be conveniently introduced to both sides. Piperine and the methyl ester of azoxymycin C are each prepared in three steps. Additionally, one product shows promising anticancer activities in leukemia K562 and MV-4-11 cells. Mechanistic studies indicate that the reaction proceeds through a Heck-decarboxylate coupling procedure, and the carboxylic group acts as a directing group to promote the reaction and control regioselectivity. Our research suggests that dienedioic acid can serve as a good alternative for diene preparation via a directed Heck-decarboxylate coupling.

Approaching the Integer-Charge Transfer Regime in Molecularly Doped Oligothiophenes by Efficient Decarboxylative Cross-Coupling

A library of symmetrical linear oligothiophene was prepared employing decarboxylative cross-coupling reaction as the key transformation. Thiophene potassium carboxylate salts were used as cross-coupling partners without the need of co-catalyst, base, or additives. This method demonstrates complete chemoselectivity and is a comprehensive greener approach compared to the existing methods. The modularity of this approach is demonstrated with the preparation of discreet oligothiophenes with up to 10 thiophene repeat units. Symmetrical oligothiophenes are prototypical organic semiconductors where their molecular electrical doping as a function of the chain length can be assessed spectroscopically. An oligothiophene critical length for integer charge transfer was observed to be 10 thiophene units, highlighting the potential use of discrete oligothiophenes as doped conduction or injection layers in organic electronics applications.

Palladium catalysed C-H arylation of pyrenes: access to a new class of exfoliating agents for water-based graphene dispersions

A new and diverse family of pyrene derivatives was synthesised via palladium-catalysed C-H ortho-arylation of pyrene-1-carboxylic acid. The strategy affords easy access to a broad scope of 2-substituted and 1,2-disubstituted pyrenes. The C1-substituent can be easily transformed into carboxylic acid, iodide, alkynyl, aryl or alkyl functionalities. This approach gives access to arylated pyrene ammonium salts, which outperformed their non-arylated parent compound during aqueous Liquid Phase Exfoliation (LPE) of graphite and compare favourably to state-of-the-art sodium pyrene-1-sulfonate PS1. This allowed the production of concentrated and stable suspensions of graphene flakes in water.

Rhodium(i)-catalyzed C6-selective C-H alkenylation and polyenylation of 2-pyridones with alkenyl and conjugated polyenyl carboxylic acids

A versatile Rh(i)-catalyzed C6-selective decarbonylative C-H alkenylation of 2-pyridones with readily available, and inexpensive alkenyl carboxylic acids has been developed. This directed dehydrogenative cross-coupling reaction affords 6-alkenylated 2-pyridones that would otherwise be difficult to access using conventional C-H functionalization protocols. The reaction occurs with high efficiency and is tolerant of a broad range of functional groups. A wide scope of alkenyl carboxylic acids, including challenging conjugated polyene carboxylic acids, are amenable to this transformation and no addition of external oxidant is required. Mechanistic studies revealed that (1) Boc2O acts as the activator for the in situ transformation of the carboxylic acids into anhydrides before oxidative addition by the Rh catalyst, (2) a decarbonylation step is involved in the catalytic cycle, and (3) the C-H bond cleavage is likely the turnover-limiting step.