Sonogashira coupling

History

The Sonogashira cross-coupling reaction was first reported by Kenkichi Sonogashira, Yasuo Tohda, and Nobue Hagihara in their 1975 publication. It is an extension to the Cassar and Dieck and Heck reactions, which afford the same reaction products, but use harsh reaction conditions, such as high temperature, to do so. Both of these reactions make use of a palladium catalyst to carry out the coupling, while Sonogashira uses both palladium and copper catalysts simultaneously. This results in the increased reactivity of the reagents and the ability of the reaction to be carried out at room temperature, making the Sonogashira cross-coupling reaction a highly useful reaction, particularly in the alkynylation of aryl and alkenyl halides. The reaction's remarkable utility can be evidenced by the amount of research still being done on understanding and optimizing its synthetic capabilities. A search for the term "Sonogashira" in Scifinder provides over 1500 references for journal publications between 2007 and 2010. It has become so well known that often, all reactions that use a palladium(0) catalyst to couple a sp² and even sp³ halide or triflate with a terminal alkyne, regardless of whether or not a copper co-catalyst is used, are termed "Sonogashira reactions," despite the fact that these reactions are not carried out under true Sonogashira reaction conditions.

Mechanism

The reaction mechanism is not clearly understood but the textbook mechanism revolves around a palladium cycle and a copper cycle that is less well known.

The palladium cycle

The copper cycle

Mechanistic studies suggest that these catalytic cycles represent the preferred reaction pathway, however there is debate about the exact identity of some intermediates, which may depend upon reaction conditions. For example, it has been shown that monoligated Pd⁰(PR₃) complexes (B) can be formed when dealing with bulky phosphanes and have been suggested as possible catalytic species in coupling reactions. In contrast, some results point to the formation of anionic palladium species, which would be the real catalysts instead of the coordinatively unsaturated Pd⁰L₂. Generally seen in the presence of anions and halides, it is known that Pd⁰(PPh₃)₂ does not exist in solution when generated in the presence of halide anions because they coordinate the Pd⁰ center to form anionic species of the type [L₂Pd⁰Cl]⁻ which can participate in cross-coupling reactions.

Catalysts

Typically, two catalysts are needed for this reaction: a zerovalent palladium complex and a halide salt of copper(I). Examples of such palladium catalysts include compounds in which palladium is ligated to phosphines (Pd(PPh₃)₄). A common derivative is Pd(PPh₃)₂Cl₂, but bidentate ligand catalysts, such as Pd(dppe)Cl, Pd(dppp)Cl₂, and Pd(dppf)Cl₂ have also been used. The drawback to such catalysts is the need for high loadings of palladium (up to 5 mol %), along with a larger amount of a copper co-catalyst. Pd^II is often employed as a pre-catalyst since it exhibits greater stability than Pd⁰ over an extended period of time and can be stored under normal laboratory conditions for months. The Pd ^II catalyst is reduced to Pd⁰ in the reaction mixture by either an amine, a phosphine ligand, or a reactant, allowing the reaction to proceed. The oxidation of triphenylphosphine to triphenylphosphine oxide can also lead to the formation of Pd⁰ in situ when catalysts such as bis(triphenylphosphine)palladium(II) chloride are used.

Copper(I) salts, such as copper iodide, react with the terminal alkyne and produce a copper(I) acetylide, which acts as an activated species for the coupling reactions. Cu(I) is a co-catalyst in the reaction, and is used to increase the rate of the reaction.

Reaction conditions

The Sonogashira reaction is typically run under mild conditions. The cross-coupling is carried out at room temperature with a base, typically an amine, such as diethylamine, that also acts as the solvent. The reaction medium must be basic to neutralize the hydrogen halide produced as the byproduct of this coupling reaction, so alkylamine compounds such as triethylamine and diethylamine are sometimes used as solvents, but also DMF or ether can be used as solvent. Other bases such as potassium carbonate or cesium carbonate are occasionally used. In addition, deaerated conditions are formally needed for Sonogashira coupling reactions because the palladium(0) complexes are unstable in the air, and oxygen promotes the formation of homocoupled acetylenes. Recently, development of air-stable organopalladium catalysts enable this reaction to be conducted in the ambient atmosphere.

Depending on the sp²-carbon halide-or triflate used, these reaction conditions have varying results.

Complications

Due to the crucial role of base, specific amines must be added in excess or as solvent for the reaction to proceed. It has been discovered that secondary amines such as piperidine, morpholine, or diisopropylamine in particular can react efficiently and reversibly with trans-RPdX(PPh₃)₂ complexes by substituting one PPh₃ ligand. The equilibirium constant of this reaction is dependent on R, X, a factor for basicity, and the amine's steric hindrance. The result is competition between the amine and the alkyne group for this ligand exchange, which is why the amine is generally added in excess to promote preferential substitution.

Scope and limitations

The Sonogashira coupling is applied in the synthesis of cross-conjugated oligo(phenylene enynylene)s and phenanthroline derivatives.

Inverse Sonogashira Coupling

In an inverse Sonogashira coupling the reactants are an aryl or vinyl compound and an alkynyl halide.

Copper-free reaction

While a copper co-catalyst is added to the reaction to increase reactivity, the presence of copper can result in the formation of alkyne dimers. This leads to what is known as the Glaser coupling reaction, which is an undesired formation of homocoupling products of acetylene derivatives upon oxidation. As a result, when running a Sonogashira reaction with a copper co-catalyst, it is necessary to run the reaction in an inert atmosphere to avoid the unwanted dimerization. Copper-free variations to the Sonogashira reaction have been developed to avoid the formation of the homocoupling products. The exact mechanism by which the copper-free reaction occurs is still under debate. One mechanism seems to indicate the following:

Due to mounting evidence that amines may also be involved in various steps exclusive of (via a new mode of reactivity) and/or preceding deprotonation events, an alternate mechanism suggests the following:

The crucial difference between the two mechanisms is that the former would be preferred if the amine is a weaker ligand than the reacting alkyne, while the latter mechanism would be preferred if the amine were a better ligand than the alkyne.

Catalyst variations

Recently, a nickel-catalyzed Sonogashira coupling has been developed which allows for the coupling of non-activated alkyl halides to acetylene without the use of palladium, although a copper co-catalyst is still needed. It has also been reported that gold can be used as a heterogeneous catalyst, which was demonstrated in the coupling of phenylacetylene and iodobenzene with an Au/CeO₂ catalyst. In this case, catalysis occurs heterogeneously on the Au nanoparticles, with Au(0) as the active site. Selectivity to the desirable cross coupling product was also found to be enhanced by supports such as CeO₂ and La₂O₃. Additionally, iron-catalyzed Sonogashira couplings have been investigated as relatively cheap and non-toxic alternatives to palladium. Here, FeCl₃ is proposed to act as the transition-metal catalyst and Cs₂CO₃ as the base, thus theoretically proceeding through a palladium-free and copper-free mechanism.

While the copper-free mechanism has been shown to be viable, attempts to incorporate the various transition metals mentioned above as less expensive alternatives to palladium catalysts have shown a poor track record of success due to contamination of the reagants with trace amounts of palladium, suggesting that these theorized pathways are extremely unlikely, if not impossible, to achieve.

Studies shown that organic and inorganic starting materials can also contain enough (ppb level) palladium for the coupling.

Gold and Palladium Combined

A highly efficient gold and palladium combined methodology for the Sonogashira coupling of a wide array of electronically and structurally diverse aryl and heteroaryl halides have been reported. The orthogonal reactivity of the two metals shows high selectivity and extreme functional group tolerance in Sonogashira coupling. A brief mechanistic study reveals that the gold-acetylide intermediate enters into palladium catalytic cycle at the transmetalation step.

Use of arenediazonium

Arenediazonium salts have been reported as an alternative to aryl halides for the Sonogashira coupling reaction. Gold(I) chloride has been used as co-catalyst combined with palladium(II) chloride in the coupling of arenediazonium salts with terminal alkynes, a process carried out in the presence of bis-2,6-diisopropylphenyl dihydroimidazolium chloride (IPr NHC) (5 mol%) to in situ generate a NHC–palladium complex, and 2,6-di-tert-butyl-4-methylpyridine (DBMP) as base in acetonitrile as solvent at room temperature. This coupling can be carried out starting from anilines by formation of the diazonium salt followed by in situ Sonogashira coupling, where anilines are transformed into diazonium salt and furtherly converted into alkyne by coupling with phenylacetylene.

Palladium-phosphorus complexes

The issues dealing with recovery of the often expensive catalyst after product formation poses a serious drawback for large-scale applications of homogeneous catalysis. Structures known as metalodendrimers combine the advantages of homogeneous and heterogeneous catalysts, as they are soluble and well defined on the molecular level, and yet they can be recovered by precipitation, ultrafiltration, or ultracentrifugation. Some recent examples can be found about the use of dendritic palladium complex catalysts for the copper-free Sonogashira reaction. Thus, several generations of bidentate phosphanated palladium(II) polyamino dendritic catalysts have been used solubilized in triethylamine for the coupling of aryl iodides and bromides at 25-120 °C, and of aryl chlorides, but in very low yields. The dendrimeric catalysts could usually be recovered by simple precipitation and filtration and reused up to five times, with diminished activity produced by dendrimer decomposition and not by palladium leaching being observed. These dendrimeric catalysts showed a negative dendritic effect; that is, the catalyst efficiency decreases as the dendrimer generation increases. The recyclable polymeric phosphane ligand shown below is obtained from ring-opening metathesis polymerization of a norbornene derivative, and has been used in the copper cocatalyzed Sonogashira reaction of methyl piodobenzoate and phenylacetylene using Pd(dba)₂•CHCl₃ as a palladium source. Despite recovery by filtration, polymer catalytic activity decreased by approximately 4-8% in each recycle experiment.

Palladium-nitrogen complexes

Pyridines and pyrimidines have shown good complexation properties for palladium and have been employed in the formation of catalysts suitable for Sonogashira couplings. The dipyrimidyl-palladium complex shown below has been employed in the copper-free coupling of iodo-, bromo-, and chlorobenzene with phenylacetylene using N-butylamine

as base in THF solvent at 65 °C.

Furthermore, all structural features of this complex have been characterized by extensive X-ray analysis, verifying the observed reactivity.

More recently, the dipyridylpalladium complex has been obtained and has been used in the copper-free Sonogashira coupling reaction of aryl iodides and bromides in N-methylpyrrolidinone (NMP) using tetra-n-butylammonium acetate (TBAA) as base at room temperature. It is interesting to note that this complex has also been used for the coupling of aryl iodides and bromides in refluxing water as solvent and in the presence of air, using pyrrolidine as base and TBAB as additive, although its efficiency was higher in N-methylpyrrolidinone (NMP) as solvent. An example of this complex's use is shown in the double coupling of o-diiodobenzene and phenylacetylene to give dialkynylated benzene.

N-heterocyclic carbene (NHC) palladium complexes

Nucleophilic N-heterocyclic carbenes (NHCs) behave like typical σ-donor ligands that can substitute 2-electron ligands (i.e., amines, phosphanes) in metal coordination chemistry, and at times even more efficiently; therefore, they have found application to numerous areas of organometallic homogeneous catalysis. The most easily available are stable carbenes derived from imidazole, not the least because numerous imidazolium precursor compounds can be made along various reliable routes, with the combination of the imidazolium salt with a palladium source under basic conditions generating the NHC-palladium complex. At 1 mol%, the NHC-derived palladium(II) complex shown below has been shown to promote the coupling of aryl bromides at 80 °C in DMF using triethylamine as base, although requiring the presence of catalytic amounts of copper(I) iodide and triphenylphosphine as well.

Applications in Synthesis

Sonogashira couplings are employed in a wide array of synthetic reactions, primarily due to their success in facilitating the following challenging transformations:

Alkynylation reactions

The coupling of a terminal alkyne and an aromatic ring is the pivotal reaction when talking about applications of the copper-promoted or copper-free Sonogashira reaction. The list of cases where the typical Sonogashira reaction using aryl halides has been employed is large, and choosing illustrative examples is difficult. A recent use of this methodology is shown below for the coupling of iodinated phenylalanine with a terminal alkyne derived from d-biotin using an in situ generated Pd(0) species as catalyst, which allowed the preparation of alkynelinked phenylalanine derivative for bioanalytical applications. There are also examples of the coupling partners both being attached to allyl resins, with the Pd(0) catalyst effecting cleavage of the substrates and subsequent Sonogashira coupling in solution.

Natural products

Many metabolites found in nature contain alkyne or enyne moieties, and therefore, the Sonogashira reaction has found frequent utility in their syntheses. Several of the most recent and promising applications of this coupling methodology toward the total synthesis of natural products exclusively employed the typical copper-cocatalyzed reaction.

An example of the coupling of an aryl iodide to an aryl acetylene can be seen in the reaction of the iodinated alcohol and the tris(isopropyl)silylacetylene, which gave alkyne, an intermediate in the total synthesis of the benzindenoazepine alkaloid bulgaramine.

There are other recent examples of the use of aryl iodides for the preparation of intermediates under typical Sonogashira conditions, which, after cyclization, yield natural products such as benzylisoquinoline or indole alkaloids An example is the synthesis of the benzylisoquinoline alkaloids (+)-(S)-laudanosine and (–)-(S)-xylopinine. The synthesis of these natural products involved the use of Sonogashira cross-coupling to build the carbon backbone of each molecule.

Enynes and enediynes

The 1,3-enyne moiety is an important structural unit for biologically active and natural compounds. It is derived from vinylic systems and terminal acetylenes by using a configuration-retention stereospecific procedure such as the Sonogashira reaction. Vinyl iodides are the most reactive vinyl halides to Pd⁰ oxidative addition, and their use is therefore most frequent for Sonogashira cross-coupling reactions due to the usually milder conditions employed. Some examples include:

Pharmaceuticals

The versatility of the Sonogashira reaction makes it a widely used reaction in the synthesis of a variety of compounds. One such pharmaceutical application is in the synthesis of SIB-1508Y, which is more commonly known as Altinicline. Altinicline is a nicotinic acetylcholine receptor agonist that has shown potential in the treatment of Parkinson’s disease, Alzheimer’s disease, Tourette’s syndrome, Schizophrenia, and attention deficit hyperactivity disorder (ADHD). As of 2008, Altinicline has undergone Phase II clinical trials.

Related reactions