What is the Sonogashira Coupling Reaction Mechanism and Applications
Sonogashira cross-coupling is when a potential reaction takes place during the fabrication of carbon materials. It was first found during the investigation of the reaction between iodobenzene and phenylacetylene on a flat Au(111) atomic surface. After home coupling, the products, diphenylacetylene, a Sonogashira coupling is formed. This Sonogashira coupling reaction is further proved to be operative on flat surfaces like Ag(100) as well as roughened surfaces like Au(100). It is important to note that Palladium doesn't participate in Sonogashira coupling, and therefore its underlying reaction shall be uncovered.
History of Sonogashira Reaction
The Sonogashira coupling was initially proposed in 1975 by Kenkichi Sonogashira, Nobue Hagihara and Yasuo Tohda in their publications. The reaction is termed as an extension to the original Cassar, Dieck and Heck reactions which carry out the coupling of compounds using palladium as a catalyst. But Sonogashira uses both copper and palladium together, for an enhanced coupling.
Sonogashira Cross-Coupling Reaction
It is a cross-coupling reaction that is mostly used in the coupling of vinyl or aryl halides - during the organic synthesis of carbon-carbon bonds. It is used in a variety of fields and has become an essential step in the synthesis of compounds in product chemistry, nanomaterials and material science pharmaceuticals. It also can be implemented in mild room temperature to synthesize even the most complex molecules.
Sonogashira Coupling Reaction Mechanism
Songashira mechanism is not entirely comprehended as it is difficult to analyse and isolate the organometallic compounds, precisely. These compounds are usually present in the reactions as intermediates. The mechanism is predicted around the palladium cycle and copper cycle. The formation of complex E and pi-alkyne complex is dependent on the presence of the base. The base acidifies the proton on the alkyne. The compound F reacts with the palladium to generate the copper halide.
Steps of Sonogashira Coupling Reaction Mechanism
Palladium and copper involved Sonogashira coupling happens in two independent catalytic cycles.
Catalytic Cycle of Palladium and Copper Sonogashira Reaction
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Before the actual oxidative addition, the 14-electron PdL2 complex is formed in a reductive process known as a σ-complexation-dehydropalladation-reductive reaction. During this process, palladium is reduced to form a complex with electron donors as they act either as solvents or ligands during the reaction.
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σ-Complexation-Dehydropalladation-Reductive Reaction With Amide
The Pd0 complex is subjected to oxidative addition of R1-X forming a coordinated palladium complex. At this point, the palladium cycle is intersected with the copper cycle. For a co-catalyzed reaction, amide shall be used due to its low basicity. As a result, a π-alkyne-copper complex is formed to increase the acidity of the alkyne to undergo the process of deprotonation. Following the deprotonation step, copper acetylide is formed. The formation of the palladium acetylide from copper acetylide and palladium complex is the rate-determining step in this cycle, known as transmetalation. In this step, Trans/cis isomerization happens to arrive at the final product.
Copper-free Catalytic Cycle
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For copper-free Sonogashira coupling, the first step is an oxidative addition to form palladium complexes. The basicity of amine is not sufficient for the process of deprotonation. In such cases, the dissociation of the neutral ligand and the formation of the π-alkyne-palladium complex occurs. After the formation of the complex, the deprotonation of the alkyne occurs for the formation of palladium acetylide. The trans/cis isomerization, along with reductive elimination occurs to generate the Pd0L2 catalyst and the final product.
Sonogashira Cross-Coupling Reaction Mechanistic Studies
Without no knowledge of the exact mechanism, it is difficult to characterize and isolate palladium intermediates. However, some transient species are identified when used with a multinuclear NMR spectroscopy. Many alternative methods have considered using heterogeneous catalysts to analyze the transient organometallic intermediates using gas chromatography to corroborate with the mechanisms. The real catalysts that are involved in the cycle are still debatable.
Mono-ligated palladium is seen when the neutral ligand is bulky, suggesting that the catalyst is subjected to dissociation before oxidative addition. It has also been shown that Pd0L2 will form an anionic palladium complex if the solution contains halides in the place of anions. For example, [L2Pd0Cl] can be formed from the Pd0L2 in the presence of chloride ions. It is possible for anionic palladium to act as a real-time catalyst in the cycle.
Example of Sonogashira Cross-Coupling Reaction
A basic example of the reaction is the synthesis of Tazarotene in the treatment of psoriasis and acne. It is also mostly known as Altinicline.
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FAQs on Sonogashira Coupling in Organic Synthesis
1. What is Sonogashira coupling?
The Sonogashira coupling is a palladium-catalyzed cross-coupling reaction that forms a carbon–carbon (C–C) bond between a terminal alkyne and an aryl or vinyl halide. It is widely used in organic synthesis to prepare substituted alkynes.
- General reaction: Ar–X + HC≡CR → Ar–C≡CR
- X = I, Br (sometimes Cl)
- Catalyst: Pd(0) complex, often with a Cu(I) co-catalyst
- Base: amine such as triethylamine
2. What is the general reaction mechanism of the Sonogashira coupling?
The Sonogashira mechanism proceeds through oxidative addition, transmetalation, and reductive elimination steps catalyzed by palladium. The catalytic cycle involves:
- 1. Oxidative addition: Ar–X reacts with Pd(0) to form Ar–Pd(II)–X.
- 2. Formation of copper acetylide: HC≡CR is deprotonated by base and reacts with CuI to form RC≡C–Cu.
- 3. Transmetalation: The alkynyl group transfers from Cu to Pd.
- 4. Reductive elimination: Ar–C≡CR is formed and Pd(0) is regenerated.
3. What reagents are used in a Sonogashira coupling reaction?
A typical Sonogashira reaction requires an aryl or vinyl halide, a terminal alkyne, a palladium catalyst, a copper(I) co-catalyst, and a base. The common components are:
- Aryl/vinyl halide: Ar–I or Ar–Br
- Terminal alkyne: HC≡CR
- Palladium catalyst: Pd(PPh3)4 or PdCl2(PPh3)2
- Copper(I) iodide (CuI)
- Base: triethylamine (Et3N) or diisopropylamine
4. Why is copper used in Sonogashira coupling?
Copper(I) iodide is used as a co-catalyst in Sonogashira coupling to form a reactive copper acetylide intermediate that accelerates transmetalation. Its role includes:
- Deprotonation of the terminal alkyne (with base assistance)
- Formation of RC≡C–Cu species
- Faster transfer of the alkynyl group to palladium
5. What is the difference between Sonogashira and Suzuki coupling?
The key difference is that Sonogashira coupling forms C–C bonds with terminal alkynes, while Suzuki coupling forms C–C bonds using organoboron compounds. The main distinctions are:
- Sonogashira: Ar–X + HC≡CR → Ar–C≡CR
- Suzuki: Ar–X + Ar′–B(OH)2 → Ar–Ar′
- Sonogashira often uses CuI; Suzuki does not require copper
- Suzuki commonly uses a base like K2CO3
6. Can you give an example of a Sonogashira coupling reaction?
A common example of Sonogashira coupling is the reaction of iodobenzene with phenylacetylene to form diphenylacetylene. The reaction is:
- C6H5I + C6H5C≡CH → C6H5C≡CC6H5 + HI
7. What are copper-free Sonogashira conditions?
Copper-free Sonogashira conditions are modified protocols that omit CuI to prevent side reactions like oxidative homocoupling of alkynes. In these conditions:
- Only a Pd catalyst is used
- A stronger base activates the terminal alkyne
- Glaser-type byproducts are minimized
8. What are the limitations of the Sonogashira coupling?
The main limitations of Sonogashira coupling include sensitivity to air, side reactions, and reduced reactivity of aryl chlorides. Common challenges are:
- Glaser homocoupling of terminal alkynes
- Lower reactivity of Ar–Cl compared to Ar–I or Ar–Br
- Requirement for dry and oxygen-free conditions
- Catalyst deactivation by sulfur- or nitrogen-containing impurities
9. What types of halides can be used in Sonogashira coupling?
Sonogashira coupling works best with aryl iodides and aryl bromides, while aryl chlorides are less reactive. The typical reactivity order is:
- Ar–I > Ar–Br > Ar–Cl
10. What are the applications of Sonogashira coupling in chemistry?
Sonogashira coupling is widely used to synthesize substituted alkynes for pharmaceuticals, natural products, and advanced materials. Key applications include:
- Preparation of conjugated enynes and polyynes
- Synthesis of drug intermediates and bioactive molecules
- Formation of π-conjugated systems in organic electronics
- Construction of complex molecular frameworks in total synthesis
