Decarboxylative cross-coupling

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Decarboxylative cross coupling reactions are chemical reactions in which a carboxylic acid is reacted with an organic halide to form a new carbon-carbon bond, concomitant with loss of CO2. Aryl and alkyl halides participate. Metal catalyst, base, and oxidant are required.

Contents

Decarboxylative cross-coupling general reaction scheme Decarboxylative Cross Coupling reaction scheme.png
Decarboxylative cross-coupling general reaction scheme

A significant advantage of this reaction is that it uses relatively inexpensive carboxylic acids (or their salts) and is far less air and moisture sensitive in comparison to typical cross-coupling organometallic reagents. Furthermore, the carboxylic acid moiety is a common feature of natural products and can also be prepared by relatively benign air oxidations. Additional benefits include the broad tolerance of functional groups, as well as the capacity to avoid the use of strong bases. An important elementary step in this reaction is protodecarboxylation or metalation to first convert the C–COOH bond to a C–H or C–M bond respectively. [1]

History and catalyst development

Copper monometallic systems

The first reported decarboxylative cross coupling reaction was an Ullmann reaction, in 1966 by Nilsson et al. Thermal decarboxylation of copper benzoates, in the presence of an aryl halide, was found to produce (both symmetric and unsymmetric) biaryls through aryl-Cu intermediates. [2]

First reported decarboxylative Ullmann coupling (Nilsson, 2005) Decarboxlyative Ullmann Coupling Reaction Scheme.png
First reported decarboxylative Ullmann coupling (Nilsson, 2005)

This monometallic copper system required drastic conditions for complete cross-coupling, and had various intrinsic limitations, both of which prevented development of a catalytic, preparatory version of this reaction. [3] It was not until 2009 that Liu and Shang et al. found that decarboxylative cross-coupling of aryl bromides and iodides with potassium polyfluorobenzoates could be achieved using monometallic copper iodide as a catalyst. The oxidative addition step was determined to be the rate-limiting step in the copper-only catalyst cycle (a contrast with Pd-catalyzed decarboxylative cross-coupling). [4]

Copper catalyzed decarboxylative biaryl synthesis reported by Goossen et al. Copper catalyzed decarboxylative biaryl synthesis reported by Goossen et al.png
Copper catalyzed decarboxylative biaryl synthesis reported by Goossen et al.

Cu(I)-only systems have also been found to promote coupling of alkynyl carboxylic acids with aryl halides (see aryl alkynes below), as well as decarboxylative dehydrogenative cross-coupling of amino acids with alkynes (or similar nucleophiles). [5] [6]

Cu-catalyzed decarboxylative coupling of amino acids, reported by Jiang et al. Cu-catalyzed decarboxylative coupling of amino acids, reported by Jiang et al.png
Cu-catalyzed decarboxylative coupling of amino acids, reported by Jiang et al.

Catalysts for decarboxylative cross-coupling are of the general form ML2, with a wide variety of ligand types optimized for different substrates. Copper (and silver) centers are often complexed with phenanthrolines, and activity is reported to increase with electron-rich substituents on the ligands. [1]

Palladium monometallic systems

In 2000, Steglich et al. reported an intramolecular Pd(II)-mediated decarboxylative cross-coupling reaction in their synthesis of lamellarin L. [7] Myers et al. reported decarboxylative olefination of ortho-substituted arene carboxylates in the presence of an oxidant (Ag2CO3) in 2002. [8]

Pd-catalyzed Heck olefination, reported by Myers et al. Pd-catalyzed Heck olefination, reported by Myers et al.png
Pd-catalyzed Heck olefination, reported by Myers et al.

Subsequent studies showed that homogeneous Pd catalysts were able to decarboxylate acids at lower temperatures than their Cu and Ag counterparts, but were limited to electron rich ortho-substituted aromatic carboxylic acids. [9] [10] Despite this, palladium catalysts are able to promote a wide variety of cross-coupling reactions including biaryl formation and aryl alkyne formation, along with a variety of cross-coupling reactions in which the carboxylic acid is not bonded to an aromatic. [5] [11] [12] Other Pd-catalyzed decarboxylation cross-coupling reactions include conjugated diene preparation (see dienes and trienes below) and dehydrogenative reactions (with a variety of substrate and catalyst combinations). [1] [13]

Pd-catalyzed decarboxylative cross-coupling of aryl halides with potassium cyanoacetate, reported by Yeung et al. Pd-catalyzed decarboxylative cross-coupling of aryl halides with potassium cyanoacetate, reported by Yeung et al.png
Pd-catalyzed decarboxylative cross-coupling of aryl halides with potassium cyanoacetate, reported by Yeung et al.

Contrarily to Cu-only systems, decarboxylative palladation is the rate-limiting step in the palladium catalytic cycle. [4]

Decarboxylative cross-coupling of potassium polyfluorobenzoates, reported by Shang et al. Decarboxylative cross-coupling of potassium polyfluorobenzoates, reported by Shang et al.png
Decarboxylative cross-coupling of potassium polyfluorobenzoates, reported by Shang et al.

Palladium-–copper bimetallic systems

A Pd–Cu bimetallic system was not discovered until 2006 when Goossen et al. reported a decarboxylative cross-coupling of aryl halides with ortho-substituted aromatic carboxylic acids. [14] Through subsequent studies it was found that the use of aryl triflates allowed substrate scope for cross-coupling to be extended to some aromatic carboxylates lacking any ortho-substitution (less reactive). This was a result of the fact that any halide anion generated in the reaction inhibited the Cu-catalyzed decarboxylation process. [15] Further optimization of the system and catalyst conditions has made decarboxylative cross-coupling using bimetallic Pd–Cu systems applicable to organic synthesis, most predominantly in the formation of biaryls. [3] As well, the variability of this combined catalytic system allows for promotion of a large spectrum of reactions, including aryl ketone formation, c-heteroatom cross-coupling, and many others. [1]

Biaryl synthesis using a Cu-Pd catalyst system, reported by Shang et al. Biaryl synthesis using a Cu-Pd catalyst system.png
Biaryl synthesis using a Cu–Pd catalyst system, reported by Shang et al.

Palladium–silver bimetallic systems

Silver being in the same group as copper, Pd–Ag(I) bimetallic systems are inherently similar to Pd–Cu catalytic systems. However, silver salts are better suited for protodecarboxylation of carboxylic acids than their copper equivalents, allowing milder reaction conditions in Pd–Ag cycles relative to Pd–Cu cycles. [16] Ag(I) catalyzed monometallic systems have also been reported. Their proficiency (relative to copper) is likely attributed to lower electronegativity and greater expansion of d-orbitals, which promote decarboxylation of the substrate. [17] One limitation of this catalyst combination is that the silver salts will form insoluble silver halides, forcing the reaction to require a stoichiometric amount of Ag if halides are present. This obstacle was overcome by Goossen et al. in 2010 by using aryl triflates, and catalytic reaction with aryl sulfonates has also been reported. [3] [18]

Decarboxylative cross-coupling of aryl triflates with aryl carboxylates using a Pd-Ag catalyst system, reported by Goossen et al. Decarboxylative cross-coupling of aryl triflates with aryl carboxylates using a Pd-Ag catalyst system, reported by Goossen et al.png
Decarboxylative cross-coupling of aryl triflates with aryl carboxylates using a Pd–Ag catalyst system, reported by Goossen et al.

Product scope via variation of substrates

The product scope of this reaction is extremely broad with the use of different substrates; however development of different functionalities has required accompanied studies to determine the proper catalyst system. The most typical class of reactions involves coupling between C–COOH and C–X bonds, however C–COOH and C–M cross-coupling, homo-coupling of carboxylic acids, heck coupling, and dehydrogenative cross-coupling can also be including in this class as they release CO2. Heteroatom cross coupling reactions involving formation of C–N, C–S, C–P, and C–X bonds have also been demonstrated. [1]

Biaryl formation

Per IUPAC, the term biaryl refers to an assembly of two aromatic rings joined by a single bond, [19] starting with the simplest, biphenyl. Biaryls constitute an important structural motif of physical organic, synthetic, and catalytic interest—for instance, underlying the area of atropisomers in enantioselective synthesis—and they appear in many pharmaceutical, agrochemical, and materials (e.g. LCD) applications.[ citation needed ] The example of a coupling reaction reaction used in their preparation is an alternative to the traditional Suzuki and Stille cross-coupling reactions, and various catalysts have been employed for this transformation; Goossen et al. reported the formation of biaryls from palladium and copper-catalzyed cross-coupling reactions of an aryl or heteroaryl carboxylic acid and an aryl halide (I, Br, or Cl) in the presence of a base. [20]

Formation of Biaryls (Goossen et al. (2007)) Formation of Biaryls by Goossen et al.jpg
Formation of Biaryls (Goossen et al. (2007))

Aryl alkynes

Aryl alkynes are typically made utilizing the Sonogashira reaction which is the palladium catalyzed cross-coupling reaction of terminal alkynes and aryl halides. Instead of the terminal alkynes, alkyne carboxylic acids has advantages, easy handling and storage. The first decarboxylative coupling of alkyne carboxylic acids was reported in 2008 by S. Lee. They employed propiolic acid as an alkyne source. One year later, S. Lee applied the decarboxylative coupling reactions toward 2-octynoic acid and phenylpropiolic acid. In 2010, Xue et al. reported the coupling of an aryl halide and alkynyl carboxylic acid under mild reactions conditions and a copper-only catalyst to obtain aryl alkynes.

[21] [22] [23] [24]

Formation of Aryl Alkynes (Zhao et al. (2010)) Formation of Aryl Alkynes by Zhao et al.jpg
Formation of Aryl Alkynes (Zhao et al. (2010))

Aryl ketones

Further work by Goossen et al. described the synthesis of ketones from α-oxocarboxylic acids with aryl or heteroaryl bromides through an acyl anion intermediate. [25]

Formation of Aryl Ketones (Goossen et al. (2008)) Formation of Aryl Ketones by Goossen et al.jpg
Formation of Aryl Ketones (Goossen et al. (2008))

Aryl esters

Shang et al. discovered the decarboxylative coupling of potassium oxalate monoesters with aryl halides to obtain aryl or alkenyl esters. [5]

Formation of Aryl Esters (Shang et al. (2009)) Formation of Aryl Esters by Shang et al.jpg
Formation of Aryl Esters (Shang et al. (2009))

sp3C carboxylic acids

Many decarboxylative cross coupling reactions involve the breaking of sp2C–COOH and spC–COOH bonds, therefore subsequent studies have attempted to enable cross coupling with sp3C carboxylic acids. One such reaction by Shang et al. described a palladium catalyzed cross coupling that enables the formation of functionalized pyridines, pyrazines, quinolines, benzothiazoles, and benzoxazoles. The position of the nitrogen atom in the '2' position relative to the linkage is found to be required, therefore implying its binding to Pd in a transition state. [26]

Formation of sp3C-heteroaromatics by Shang et al. 2010 Formation of sp3C-heteroaromatics by Shang et al.jpg
Formation of sp3C-heteroaromatics by Shang et al. 2010

Dienes and trienes

Miura et al. reported the cross coupling of vinyl bromides with an alkenyl carboxylic acid using a palladium catalyst. Some of the conjugated dienes prepared were reported to exhibit solid state fluorescence. [27]

Formation of conjugated dienes (Miura et al. (2010)) Conjugated dienes by Miura et al.jpg
Formation of conjugated dienes (Miura et al. (2010))

Olefins via Heck-type

A decarboxylative Heck coupling by Su et al. can be used to obtain an aryl olefin using benzoquinone as the oxidant. [28]

Formation of olefins by Hu et al. (Hu et al. (2009)) Formation of Olefins via Heck-type reaction by Hu et al.jpg
Formation of olefins by Hu et al. (Hu et al. (2009))

Phenanthrene derivatives

Wang et al. proposed a novel method for [4+2] annulation via a palladium catalyzed intermolecular pathway. Derivatives are formed in moderate to good yield; acridine is essential for high reaction efficiency. [29]

Formation of phenanthrene derivatives by Wang et al. (Wang et al. (2010)) Formation of Phenanthrene derivatives by Wang et al.jpg
Formation of phenanthrene derivatives by Wang et al. (Wang et al. (2010))

C–N coupling

Jiao et al. enabled the formation of a C–N bond via cross-coupling using air as an oxidant and a copper catalyst. No conditions are known for a C–N cross-coupling that breaks a sp3 or sp2 C–COOH bond. [30]

C-N cross-coupling by Jiao et al. (Jiao et al. (2010)) C-N cross-coupling by Jiao et al.jpg
C-N cross-coupling by Jiao et al. (Jiao et al. (2010))

C-S Coupling

Liu et al. reported the C-S coupling of aryl carboxylic acids with disulphides or thiols using a Pd/Cu catalyst system. [31]

C-S cross-coupling by Liu et al. (Liu et al. (2009)) C-S Cross-Coupling by Liu et al.jpg
C-S cross-coupling by Liu et al. (Liu et al. (2009))

C–P coupling

Using either Pd–Cu or Cu catalysts Yang et al. reported the first example of decarboxylative C–P cross-coupling. [32]

C-P cross-coupling by Yang et al. (Yang et al. (2011)) C-P Cross-Coupling by Yang et al.jpg
C-P cross-coupling by Yang et al. (Yang et al. (2011))

C–X coupling

Wu et al. reported a C–X cross coupling using CuX2 (X= Br, Cl) and a silver catalyst to obtain aryl halides. [33]

C-X cross-coupling by Wu et al. 2010 C-X Cross-Coupling by Wu et al.jpg
C-X cross-coupling by Wu et al. 2010

Mechanistic studies

Decarboxylative Heck type

In 2005, Meyers et al. Proposed the following mechanism for the decarboxylative cross-coupling reaction. [10] The initial and rate determining step is the decarboxylation. The ipso carbon of the arene ring is thought to coordinate to the palladium centre initially and is followed by the expulsion of carbon dioxide, forming an aryl–palladium intermediate. The olefin then inserts between the arene and palladium center, which then undergoes beta elimination to form the desired vinyl halide, as well as a palladium hydride. This proton is abstracted by silver carbonate, which acts as both a base and an oxidant to regenerate the starting palladium complex completing the catalytic cycle.

by Meyers et al. 2005 Decarboxylative Cross coupling Heck Type Mechanism.png
by Meyers et al. 2005

Biaryl synthesis via redox-neutral decarboxylative cross-coupling

In 2006 Goossen et al. proposed a reaction to synthesize biaryl compounds via catalytic decarboxylative cross coupling. [34] The mechanism involves two overlapping cycles, one using a copper halide and the other using palladium. The decarboxylation step occurs between the substituted benzoic acid and copper halide to form the intermediate aryl copper species. The palladium initially undergoes oxidative addition from the aryl halide to form a Pd(II) aryl complex. After both of these initial steps, the substituted aryl copper undergoes trans-metalation with the palladium complex. This step forms the copper halide, which then undergoes anion exchange with the substituted benzoic acid to reform the aryl copper intermediate, continuing the catalytic cycle. The other complex formed in the trans-metalation step is a bis-aryl palladium(II), which then undergoes reductive elimination to form the desired bis-aryl species as well as the starting Pd(0) complex, thus completing the catalytic cycle.

Decarboxylative biaryl synthesis mechanism, Goossen et al. 2006 Decarboxylative Biaryl Synthesis Mechanism.png
Decarboxylative biaryl synthesis mechanism, Goossen et al. 2006

Heteroaromatic acid coupling

Forgione, P., Bilodeau, F. et al. reported that heteroatoms containing a carboxylic acid also are tolerated by palladium monometallic systems and undergo decarboxylative cross coupling with aryl halides. [35] In the proposed mechanism the initial step is oxidative addition of the aryl halide forming an aryl–palladium intermediate. Electrophilic palladation then occurs at carbon-3 of the heteroatom. From this intermediate there are two possible pathways for the cycle to continue on. The first is palladium migration from carbon-3 to carbon-2 along with the expulsion of carbon dioxide. This forms the aryl–palladium–heteroatom intermediate, which undergoes reductive elimination to form the final heteroaromatic compound. The second pathway only occurs when R is a proton. If this is the case, deprotonation occurs to regain aromaticity of the heteroatom. This intermediate then undergoes reductive elimination, coupling the aryl to the carbon-3 position of the heteroatom. As this compound still contains the carboxylic acid it is then free to re enter the catalytic cycle where it undergoes coupling at the carbon 2 position, along with the expulsion of carbon dioxide to form a biaryl heteroatom. As this pathway competes with the decarboxylation step, two products are formed making this reaction less selective. As a result, heteroatoms, which are substituted at the carbon 3 position and are more favored due to the higher level of control they provide.

Proposed mechanism of heteroaromatic acid coupling, Forgione et al. 2006 Proposed Mechanism of Heteroaromatic Acid Coupling.png
Proposed mechanism of heteroaromatic acid coupling, Forgione et al. 2006

Related Research Articles

The Heck reaction is the chemical reaction of an unsaturated halide with an alkene in the presence of a base and a palladium catalyst to form a substituted alkene. It is named after Tsutomu Mizoroki and Richard F. Heck. Heck was awarded the 2010 Nobel Prize in Chemistry, which he shared with Ei-ichi Negishi and Akira Suzuki, for the discovery and development of this reaction. This reaction was the first example of a carbon-carbon bond-forming reaction that followed a Pd(0)/Pd(II) catalytic cycle, the same catalytic cycle that is seen in other Pd(0)-catalyzed cross-coupling reactions. The Heck reaction is a way to substitute alkenes.

The Stille reaction is a chemical reaction widely used in organic synthesis. The reaction involves the coupling of two organic groups, one of which is carried as an organotin compound. A variety of organic electrophiles provide the other coupling partner. The Stille reaction is one of many palladium-catalyzed coupling reactions.

The Suzuki reaction is an organic reaction, classified as a cross-coupling reaction, where the coupling partners are a boronic acid and an organohalide and the catalyst is a palladium(0) complex. It was first published in 1979 by Akira Suzuki, and he shared the 2010 Nobel Prize in Chemistry with Richard F. Heck and Ei-ichi Negishi for their contribution to the discovery and development of palladium-catalyzed cross-couplings in organic synthesis. This reaction is also known as the Suzuki–Miyaura reaction or simply as the Suzuki coupling. It is widely used to synthesize polyolefins, styrenes, and substituted biphenyls. Several reviews have been published describing advancements and the development of the Suzuki reaction. The general scheme for the Suzuki reaction is shown below, where a carbon-carbon single bond is formed by coupling an organoboron species (R1-BY2) with a halide (R2-X) using a palladium catalyst and a base.

The Sonogashira reaction is a cross-coupling reaction used in organic synthesis to form carbon–carbon bonds. It employs a palladium catalyst as well as copper co-catalyst to form a carbon–carbon bond between a terminal alkyne and an aryl or vinyl halide.

Organopalladium chemistry is a branch of organometallic chemistry that deals with organic palladium compounds and their reactions. Palladium is often used as a catalyst in the reduction of alkenes and alkynes with hydrogen. This process involves the formation of a palladium-carbon covalent bond. Palladium is also prominent in carbon-carbon coupling reactions, as demonstrated in tandem reactions.

Bamford–Stevens reaction

The Bamford–Stevens reaction is a chemical reaction whereby treatment of tosylhydrazones with strong base gives alkenes. It is named for the British chemist William Randall Bamford and the Scottish chemist Thomas Stevens Stevens (1900–2000). The usage of aprotic solvents gives predominantly Z-alkenes, while protic solvent gives a mixture of E- and Z-alkenes. As an alkene-generating transformation, the Bamford–Stevens reaction has broad utility in synthetic methodology and complex molecule synthesis.

The Hiyama coupling is a palladium-catalyzed cross-coupling reaction of organosilanes with organic halides used in organic chemistry to form carbon–carbon bonds. This reaction was discovered in 1988 by Tamejiro Hiyama and Yasuo Hatanaka as a method to form carbon-carbon bonds synthetically with chemo- and regioselectivity. The Hiyama coupling has been applied to the synthesis of various natural products.

The Ullmann reaction or Ullmann coupling is a coupling reaction between aryl halides. Traditionally this reaction is effected by copper, but palladium and nickel are also effective catalysts. The reaction is named after Fritz Ullmann.

In the Ullmann condensation or Ullmann-type reaction is the copper-promoted conversion of aryl halides to aryl ethers, aryl thioethers, aryl nitriles, and aryl amines. These reactions are examples of cross-coupling reactions.

The Negishi coupling is a widely employed transition metal catalyzed cross-coupling reaction. The reaction couples organic halides or triflates with organozinc compounds, forming carbon-carbon bonds (C-C) in the process. A palladium (0) species is generally utilized as the metal catalyst, though nickel is sometimes used. A variety of nickel catalysts in either Ni0 or NiII oxidation state can be employed in Negishi cross couplings such as Ni(PPh3)4, Ni(acac)2, Ni(COD)2 etc.

Organocopper compound Compound with carbon to copper bonds

Organocopper compounds in organometallic chemistry contain carbon to copper chemical bonds. Organocopper chemistry is the science of organocopper compounds describing their physical properties, synthesis and reactions. They are reagents in organic chemistry.

The Buchwald–Hartwig amination is a chemical reaction used in organic chemistry for the synthesis of carbon–nitrogen bonds via the palladium-catalyzed coupling reactions of amines with aryl halides. Although Pd-catalyzed C-N couplings were reported as early as 1983, Stephen L. Buchwald and John F. Hartwig have been credited, whose publications between 1994 and the late 2000s established the scope of the transformation. The reaction's synthetic utility stems primarily from the shortcomings of typical methods for the synthesis of aromatic C–N bonds, with most methods suffering from limited substrate scope and functional group tolerance. The development of the Buchwald–Hartwig reaction allowed for the facile synthesis of aryl amines, replacing to an extent harsher methods while significantly expanding the repertoire of possible C–N bond formation.

The Castro–Stephens coupling is a cross coupling reaction between a copper(I) acetylide and an aryl halide in pyridine, forming a disubstituted alkyne and a copper(I) halide.

In organic chemistry, the Kumada coupling is a type of cross coupling reaction, useful for generating carbon–carbon bonds by the reaction of a Grignard reagent and an organic halide. The procedure uses transition metal catalysts, typically nickel or palladium, to couple a combination of two alkyl, aryl or vinyl groups. The groups of Robert Corriu and Makoto Kumada reported the reaction independently in 1972.

A cross-coupling reaction in organic chemistry is a reaction where two fragments are joined together with the aid of a metal catalyst. In one important reaction type, a main group organometallic compound of the type R-M reacts with an organic halide of the type R'-X with formation of a new carbon–carbon bond in the product R-R'. Cross-coupling reaction are a subset of coupling reactions. It is often used in arylations.

Metal carbon dioxide complexes are coordination complexes that contain carbon dioxide ligands. Aside from the fundamental interest in the coordination chemistry of simple molecules, studies in this field are motivated by the possibility that transition metals might catalyze useful transformations of CO2. This research is relevant both to organic synthesis and to the production of "solar fuels" that would avoid the use of petroleum-based fuels.

Catellani reaction

The Catellani reaction was discovered by Marta Catellani and co-workers in 1997. The reaction uses aryl iodides to perform bi- or tri-functionalization, including C-H functionalization of the unsubstituted ortho position(s), followed a terminating cross-coupling reaction at the ipso position. This cross-coupling cascade reaction depends on the ortho-directing transient mediator, norbornene.

A3 coupling reaction

The A3 coupling (also known as A3 coupling reaction or the aldehyde-alkyne-amine reaction), coined by Prof. Chao-Jun Li of McGill University, is a type of multicomponent reaction involving an aldehyde, an alkyne and an amine which react to give a propargyl-amine.

In organic and organometallic chemistry, dialkylbiaryl phosphine (or dialkylbiarylphosphine) ligands are phosphorus-containing supporting ligands that are used to modulate the chemical reactivity of palladium and other transition metal based catalysts. They were first described by Stephen L. Buchwald in 1998 for applications in palladium-catalyzed coupling reactions to form carbon-nitrogen and carbon-carbon bonds. Before their development, use of first- or second-generation phosphine ligands for palladium-catalyzed C-N bond-forming cross-coupling (e.g., tris(o-tolyl)phosphine and BINAP, respectively) necessitated harsh conditions, and the scope of the transformation was severely limited. The Suzuki-Miyaura and Negishi cross-coupling reactions were typically performed with Pd(PPh3)4 as catalyst and were mostly limited to aryl bromides and iodides at elevated temperatures, while the widely available aryl chlorides were unreactive. The development of new classes of ligands was needed to address these limitations.

Heterobimetallic catalysis is an approach to catalysis that employs two different metals to promote a chemical reaction. Included in this definition are cases where: 1) each metal activates a different substrate, 2) both metals interact with the same substrate, and 3) only one metal directly interacts with the substrate(s), while the second metal interacts with the first.

References

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