Suzuki reaction | |
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Named after | Akira Suzuki |
Reaction type | Coupling reaction |
Identifiers | |
Organic Chemistry Portal | suzuki-coupling |
RSC ontology ID | RXNO:0000140 |
The Suzuki reaction or Suzuki coupling is an organic reaction that uses a palladium complex catalyst to cross-couple a boronic acid to an organohalide. [1] [2] [3] 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 noble metal catalysis in organic synthesis. [4] This reaction is sometimes telescoped with the related Miyaura borylation; the combination is the Suzuki–Miyaura reaction. It is widely used to synthesize polyolefins, styrenes, and substituted biphenyls.
The general scheme for the Suzuki reaction is shown below, where a carbon-carbon single bond is formed by coupling a halide (R1-X) with an organoboron species (R2-BY2) using a palladium catalyst and a base. The organoboron species is usually synthesized by hydroboration or carboboration, allowing for rapid generation of molecular complexity.
Several reviews have been published describing advancements and the development of the Suzuki reaction. [5] [6] [7]
The mechanism of the Suzuki reaction is best viewed from the perspective of the palladium catalyst. The catalytic cycle is initiated by the formation of an active Pd0 catalytic species, A. This participates in the oxidative addition of palladium to the halide reagent 1 to form the organopalladium intermediate B. Reaction (metathesis) with base gives intermediate C, which via transmetalation [8] with the boron-ate complex D (produced by reaction of the boronic acid reagent 2 with base) forms the transient organopalladium species E. Reductive elimination step leads to the formation of the desired product 3 and restores the original palladium catalyst A which completes the catalytic cycle.
The Suzuki coupling takes place in the presence of a base and for a long time the role of the base was not fully understood. The base was first believed to form a trialkyl borate (R3B-OR), in the case of a reaction of a trialkylborane (BR3) and alkoxide (−OR); this species could be considered as being more nucleophilic and then more reactive towards the palladium complex present in the transmetalation step. [9] [10] [11] Duc and coworkers investigated the role of the base in the reaction mechanism for the Suzuki coupling and they found that the base has three roles: Formation of the palladium complex [ArPd(OR)L2], formation of the trialkyl borate and the acceleration of the reductive elimination step by reaction of the alkoxide with the palladium complex. [9]
In most cases the oxidative addition is the rate determining step of the catalytic cycle. [12] During this step, the palladium catalyst is oxidized from palladium(0) to palladium(II). The catalytically active palladium species A is coupled with the aryl halide substrate 1 to yield an organopalladium complex B. As seen in the diagram below, the oxidative addition step breaks the carbon-halogen bond where the palladium is now bound to both the halogen (X) as well as the R1 group.
Oxidative addition proceeds with retention of stereochemistry with vinyl halides, while giving inversion of stereochemistry with allylic and benzylic halides. [13] The oxidative addition initially forms the cis–palladium complex, which rapidly isomerizes to the trans-complex. [14]
The Suzuki coupling occurs with retention of configuration on the double bonds for both the organoboron reagent or the halide. [15] However, the configuration of that double bond, cis or trans is determined by the cis-to-trans isomerization of the palladium complex in the oxidative addition step where the trans palladium complex is the predominant form. When the organoboron is attached to a double bond and it is coupled to an alkenyl halide the product is a diene as shown below.
Transmetalation is an organometallic reaction where ligands are transferred from one species to another. In the case of the Suzuki coupling the ligands are transferred from the organoboron species D to the palladium(II) complex C where the base that was added in the prior step is exchanged with the R2 substituent on the organoboron species to give the new palladium(II) complex E. The exact mechanism of transmetalation for the Suzuki coupling remains to be discovered. The organoboron compounds do not undergo transmetalation in the absence of base and it is therefore widely believed that the role of the base is to activate the organoboron compound as well as facilitate the formation of R1-Pdll-OtBu intermediate (C) from oxidative addition product R1-Pdll-X (B). [12]
The final step is the reductive elimination step where the palladium(II) complex (E) eliminates the product (3) and regenerates the palladium(0) catalyst (A). Using deuterium labelling, Ridgway et al. have shown the reductive elimination proceeds with retention of stereochemistry. [16]
The ligand plays an important role in the Suzuki reaction. Typically, the phosphine ligand is used in the Suzuki reaction. Phosphine ligand increases the electron density at the metal center of the complex and therefore helps in the oxidative addition step. In addition, the bulkiness of substitution of the phosphine ligand helps in the reductive elimination step. However, N-heterocyclic carbene ligands have recently been used in this cross coupling, due to the instability of the phosphine ligand under Suzuki reaction conditions. [17] N-Heterocyclic carbenes are more electron rich and bulky than the phosphine ligand. Therefore, both the steric and electronic factors of the N-heterocyclic carbene ligand help to stabilize active Pd(0) catalyst. [18]
The advantages of Suzuki coupling over other similar reactions include availability of common boronic acids, mild reaction conditions, and its less toxic nature. Boronic acids are less toxic and safer for the environment than organotin and organozinc compounds. It is easy to remove the inorganic by-products from the reaction mixture. Further, this reaction is preferable because it uses relatively cheap and easily prepared reagents. Being able to use water as a solvent [19] makes this reaction more economical, eco-friendly, and practical to use with a variety of water-soluble reagents. A wide variety of reagents can be used for the Suzuki coupling, e.g., aryl or vinyl boronic acids and aryl or vinyl halides. Work has also extended the scope of the reaction to incorporate alkyl bromides. [20] In addition to many different type of halides being possible for the Suzuki coupling reaction, the reaction also works with pseudohalides such as triflates (OTf), as replacements for halides. The relative reactivity for the coupling partner with the halide or pseudohalide is: R2–I > R2–OTf > R2–Br >> R2–Cl. Boronic esters and organotrifluoroborate salts may be used instead of boronic acids. The catalyst can also be a palladium nanomaterial-based catalyst. [21] With a novel organophosphine ligand (SPhos), a catalyst loading of down to 0.001 mol% has been reported. [22] These advances and the overall flexibility of the process have made the Suzuki coupling widely accepted for chemical synthesis.
The Suzuki coupling reaction is scalable and cost-effective for use in the synthesis of intermediates for pharmaceuticals or fine chemicals. [23] The Suzuki reaction was once limited by high levels of catalyst and the limited availability of boronic acids. Replacements for halides were also found, increasing the number of coupling partners for the halide or pseudohalide as well. Scaled up reactions have been carried out in the synthesis of a number of important biological compounds such as CI-1034 which used triflate and boronic acid coupling partners which was run on an 80 kilogram scale with a 95% yield. [24]
Another example is the coupling of 3-pyridylborane and 1-bromo-3-(methylsulfonyl)benzene that formed an intermediate that was used in the synthesis of a potential central nervous system agent. The coupling reaction to form the intermediate produced 278 kilograms in a 92.5% yield. [15] [23]
Significant efforts have been put into the development of heterogeneous catalysts for the Suzuki CC reaction, motivated by the performance gains in the industrial process (eliminating the catalyst separation from the substrate), and recently a Pd single atom heterogeneous catalyst has been shown to outperform the industry default homogeneous Pd(PPh3)4 catalyst. [25]
The Suzuki coupling has been frequently used in syntheses of complex compounds. [26] [27] The Suzuki coupling has been used on a citronellal derivative for the synthesis of caparratriene, a natural product that is highly active against leukemia: [28]
Various catalytic uses of metals other than palladium (especially nickel) have been developed. [29] The first nickel catalyzed cross-coupling reaction was reported by Percec and co-workers in 1995 using aryl mesylates and boronic acids. [30] Even though a higher amount of nickel catalyst was needed for the reaction, around 5 mol %, nickel is not as expensive or as precious a metal as palladium. The nickel catalyzed Suzuki coupling reaction also allowed a number of compounds that did not work or worked worse for the palladium catalyzed system than the nickel-catalyzed system. [29] The use of nickel catalysts has allowed for electrophiles that proved challenging for the original Suzuki coupling using palladium, including substrates such as phenols, aryl ethers, esters, phosphates, and fluorides. [29]
Investigation into the nickel catalyzed cross-coupling continued and increased the scope of the reaction after these first examples were shown and the research interest grew. Miyaura and Inada reported in 2000 that a cheaper nickel catalyst could be utilized for the cross-coupling, using triphenylphosphine (PPh3) instead of the more expensive ligands previously used. [31] However, the nickel-catalyzed cross-coupling still required high catalyst loadings (3-10%), required excess ligand (1-5 equivalents) and remained sensitive to air and moisture. [29] Advancements by Han and co-workers have tried to address that problem by developing a method using low amounts of nickel catalyst (<1 mol%) and no additional equivalents of ligand. [32]
It was also reported by Wu and co-workers in 2011 that a highly active nickel catalyst for the cross-coupling of aryl chlorides could be used that only required 0.01-0.1 mol% of nickel catalyst. They also showed that the catalyst could be recycled up to six times with virtually no loss in catalytic activity. [33] The catalyst was recyclable because it was a phosphine nickel nanoparticle catalyst (G3DenP-Ni) that was made from dendrimers.
Advantages and disadvantages apply to both the palladium and nickel-catalyzed Suzuki coupling reactions. Apart from Pd and Ni catalyst system, cheap and non-toxic metal sources like iron and copper [34] have been used in Suzuki coupling reaction. The Bedford research group [35] and the Nakamura research group [36] have extensively worked on developing the methodology of iron catalyzed Suzuki coupling reaction. Ruthenium is another metal source that has been used in Suzuki coupling reaction. [37]
Nickel catalysis can construct C-C bonds from amides. Despite the inherently inert nature of amides as synthons, the following methodology can be used to prepare C-C bonds. The coupling procedure is mild and tolerant of myriad functional groups, including: amines, ketones, heterocycles, groups with acidic protons. This technique can also be used to prepare bioactive molecules and to unite heterocycles in controlled ways through shrewd sequential cross-couplings. A general review of the reaction scheme is given below. [38]
The synthesis of a tubulin-binding compound (antiproliferative agent) was carried out using a trimethoxybenzamide and an indolyl pinacol atoboron coupling partner on a gram scale. [38]
Aryl boronic acids are comparatively cheaper than other organoboranes and a wide variety of aryl boronic acids are commercially available. Hence, it has been widely used in Suzuki reaction as an organoborane partner. Aryltrifluoroborate salts are another class of organoboranes that are frequently used because they are less prone to protodeboronation compared to aryl boronic acids. They are easy to synthesize and can be easily purified. [39] Aryltrifluoroborate salts can be formed from boronic acids by the treatment with potassium hydrogen fluoride which can then be used in the Suzuki coupling reaction. [40]
The Suzuki coupling reaction is different from other coupling reactions in that it can be run in biphasic organic-water, [41] water-only, [19] or no solvent. [42] This increased the scope of coupling reactions, as a variety of water-soluble bases, catalyst systems, and reagents could be used without concern over their solubility in organic solvent. Use of water as a solvent system is also attractive because of the economic and safety advantages. Frequently used in solvent systems for Suzuki coupling are toluene, [43] THF, [44] dioxane, [44] and DMF. [45] The most frequently used bases are K2CO3, [41] KOtBu, [46] Cs2CO3, [47] K3PO4, [48] NaOH, [49] and NEt3. [50]
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 (also known as organostannanes). A variety of organic electrophiles provide the other coupling partner. The Stille reaction is one of many palladium-catalyzed coupling reactions.
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.
Organoboron chemistry or organoborane chemistry studies organoboron compounds, also called organoboranes. These chemical compounds combine boron and carbon; typically, they are organic derivatives of borane (BH3), as in the trialkyl boranes.
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 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 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.
In organic chemistry, the Buchwald–Hartwig amination is a chemical reaction 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 formations.
Phenylboronic acid or benzeneboronic acid, abbreviated as PhB(OH)2 where Ph is the phenyl group C6H5-, is a boronic acid containing a phenyl substituent and two hydroxyl groups attached to boron. Phenylboronic acid is a white powder and is commonly used in organic synthesis. Boronic acids are mild Lewis acids which are generally stable and easy to handle, making them important to organic synthesis.
1,1′-Bis(diphenylphosphino)ferrocene, commonly abbreviated dppf, is an organophosphorus compound commonly used as a ligand in homogeneous catalysis. It contains a ferrocene moiety in its backbone, and is related to other bridged diphosphines such as 1,2-bis(diphenylphosphino)ethane (dppe).
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.
The Liebeskind–Srogl coupling reaction is an organic reaction forming a new carbon–carbon bond from a thioester and a boronic acid using a metal catalyst. It is a cross-coupling reaction. This reaction was invented by and named after Jiri Srogl from the Academy of Sciences, Czech Republic, and Lanny S. Liebeskind from Emory University, Atlanta, Georgia, USA. There are three generations of this reaction, with the first generation shown below. The original transformation used catalytic Pd(0), TFP = tris(2-furyl)phosphine as an additional ligand and stoichiometric CuTC = copper(I) thiophene-2-carboxylate as a co-metal catalyst. The overall reaction scheme is shown below.
In organic chemistry, a cross-coupling reaction is a reaction where two different fragments are joined. Cross-couplings are a subset of the more general coupling reactions. Often cross-coupling reactions require metal catalysts. One important reaction type is this:
Bis(triphenylphosphine)palladium chloride is a coordination compound of palladium containing two triphenylphosphine and two chloride ligands. It is a yellow solid that is soluble in some organic solvents. It is used for palladium-catalyzed coupling reactions, e.g. the Sonogashira–Hagihara reaction. The complex is square planar. Many analogous complexes are known with different phosphine ligands.
A metal-phosphine complex is a coordination complex containing one or more phosphine ligands. Almost always, the phosphine is an organophosphine of the type R3P (R = alkyl, aryl). Metal phosphine complexes are useful in homogeneous catalysis. Prominent examples of metal phosphine complexes include Wilkinson's catalyst (Rh(PPh3)3Cl), Grubbs' catalyst, and tetrakis(triphenylphosphine)palladium(0).
Metal-catalyzed C–H borylation reactions are transition metal catalyzed organic reactions that produce an organoboron compound through functionalization of aliphatic and aromatic C–H bonds and are therefore useful reactions for carbon–hydrogen bond activation. Metal-catalyzed C–H borylation reactions utilize transition metals to directly convert a C–H bond into a C–B bond. This route can be advantageous compared to traditional borylation reactions by making use of cheap and abundant hydrocarbon starting material, limiting prefunctionalized organic compounds, reducing toxic byproducts, and streamlining the synthesis of biologically important molecules. Boronic acids, and boronic esters are common boryl groups incorporated into organic molecules through borylation reactions. Boronic acids are trivalent boron-containing organic compounds that possess one alkyl substituent and two hydroxyl groups. Similarly, boronic esters possess one alkyl substituent and two ester groups. Boronic acids and esters are classified depending on the type of carbon group (R) directly bonded to boron, for example alkyl-, alkenyl-, alkynyl-, and aryl-boronic esters. The most common type of starting materials that incorporate boronic esters into organic compounds for transition metal catalyzed borylation reactions have the general formula (RO)2B-B(OR)2. For example, bis(pinacolato)diboron (B2Pin2), and bis(catecholato)diborane (B2Cat2) are common boron sources of this general formula.
In organometallic chemistry, palladium-NHC complexes are a family of organopalladium compounds in which palladium forms a coordination complex with N-heterocyclic carbenes (NHCs). They have been investigated for applications in homogeneous catalysis, particularly cross-coupling reactions.
Dialkylbiaryl phosphine ligands are phosphine ligands that are used in homogeneous catalysis. They have proved useful in Buchwald-Hartwig amination and etherification reactions as well as Negishi cross-coupling, Suzuki-Miyaura cross-coupling, and related reactions. In addition to these Pd-based processes, their use has also been extended to transformations catalyzed by nickel, gold, silver, copper, rhodium, and ruthenium, among other transition metals.
Miyaura borylation, also known as the Miyaura borylation reaction, is a named reaction in organic chemistry that allows for the generation of boronates from vinyl or aryl halides with the cross-coupling of bis(pinacolato)diboron in basic conditions with a catalyst such as PdCl2(dppf). The resulting borylated products can be used as coupling partners for the Suzuki reaction.
Norio Miyaura was a Japanese organic chemist. He was a professor of graduate chemical engineering at Hokkaido University. His major accomplishments surrounded his work in cross-coupling reactions / conjugate addition reactions of organoboronic acids and addition / coupling reactions of diborons and boranes. He is also the co-author of Cross-Coupling Reactions: A Practical Guide with M. Nomura E. S.. Miyaura was a world-known and accomplished researcher by the time he retired and so, in 2007, he won the Japan Chemical Society Award.
Palladium forms a variety of ionic, coordination, and organopalladium compounds, typically with oxidation state Pd0 or Pd2+. Palladium(III) compounds have also been reported. Palladium compounds are frequently used as catalysts in cross-coupling reactions such as the Sonogashira coupling and Suzuki reaction.