Sonogashira Coupling: Reaction, Mechanism, Questions

Sonogashira Coupling is a cross-coupling reaction and it's one of the most used for coupling aryl or vinyl halides, as well as producing carbon-carbon bonds in organic synthesis. When a potential reaction occurs during the manufacturing of carbon materials, it is known as Sonogashira cross-coupling. It was discovered through research into the interaction of iodobenzene with phenylacetylene on a flat Au(111) atomic surface. 

A Sonogashira coupling is formed after the home coupling of the products, diphenylacetylene. This Sonogashira coupling process has also been shown to work on both flat and roughened surfaces, such as Ag(100). It's worth noting that Palladium doesn't engage in Sonogashira coupling, thus its underlying reaction must be revealed.

It has application in material science, medicines, nanomaterials, and natural product chemistry, and it has become vital in the synthesis of chemicals. To manufacture complicated compounds, it can be done in a benign environment like room temperature. 

Key Terms: Sonogashira Cross-Coupling, Catalytic Cycle of Palladium, Catalytic Cycle without Copper

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The sonogashira reaction's history

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Kenkichi Sonogashira, Nobue Hagihara, and Yasuo Tohda proposed the Sonogashira coupling in their writing in 1975. This reaction is referred to as an extension of the classic Cassar, Dieck, and Heck reactions, which use palladium as a catalyst to couple compounds. Sonogashira, on the other hand, combines copper and palladium for a stronger connection.

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The cross-coupling reaction of Sonogashira

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It's a cross-coupling reaction that's commonly utilized in the chemical synthesis of carbon-carbon bonds to couple vinyl or aryl halides. It has become a crucial stage in the synthesis of molecules in product chemistry, nanomaterials, and material science pharmaceutical, and is utilised in a range of applications. It may also be used to create even the most complex molecules in a benign room temperature environment.

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Mechanism of the Sonogashira Coupling Reaction

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Because it is difficult to analyse and separate the organometallic compounds exactly, the Sonogashira mechanism is not fully understood. These chemicals are commonly found as intermediate in processes. The process is thought to revolve around the palladium and copper cycles. The presence of the base is required for the development of complex E and the pi-alkyne complex. The proton on the alkyne is acidified by the base. The p interacts with the chemical F.

The steps for the Sonogashira coupling reaction mechanism are Palladium and copper are both used. In two separate catalytic cycles, sonogashira coupling occurs.

Sonogashira Reaction Catalytic Cycle of Palladium and Copper.

The 14- electron PdLâ‚ complex is produced in a reductive process known as É‘ complexation- dehydropalladation- reductive reaction before the actual oxidative addition. Palladium is reduced to create a complex with electron donors during this reaction, which acts as either solvents or ligands.

With Amide, Complexation Dehydropalladation- Reductive Reaction

The oxidative addition of R1-X to the Pd0 complex results in the formation of a coordinated palladium complex. The palladium cycle crosses over into the copper cycle at this point. Because of its low basicity, amide should be utilised in a co- catalysed reaction. As a result, a π alkyne-copper complex is produced, which raises the alkyne's acidity and allows it to undergo deprotonation. Copper acetylide is generated after the deprotonation stage. The rate-determining phase in this cycle, known as transmetalation, is the production of palladium acetylide from copper acetylide and palladium complexes. Trans/cis isomerization occurs at this point, resulting in the final product.

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Catalytic cycle without copper

Oxidative addition to producing palladium complexes is the initial step in copper-free Sonogashira coupling. The basicity of amine is insufficient for the deprotonation process. In such instances, the neutral ligand dissociates, resulting in the formation of the alkyne palladium complex. Following the formation of the complex, the alkyne is deprotonated, resulting in palladium acetylide. The Pd0Lâ‚‚ catalyst and the end product are made through trans/cis isomerization and reductive elimination.

Mechanistic Studies of the Sonogashira Cross-Coupling Reaction

It's challenging to describe and separate palladium intermediate without knowing the specific mechanism. However, by using multinuclear NMR spectroscopy, several transitory species can be discovered. Many more ways have been proposed, such as employing heterogeneous catalysts to investigate transitory organometallic intermediate using gas chromatography to confirm the mechanisms. The true catalysts involved in the cycle are still up for debate.

When the neutral ligand is bulky, monoligated palladium is seen, implying that the catalyst has been dissociated before oxidative addition. It has also been demonstrated that if the solution contains halides instead of anions, will form an anionic palladium complex.

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Things to remember:

• The sonogashira coupling reaction is cross- reaction, and it's one of the most used for coupling aryl or vinyl halides, as well as producing carbon-carbon bonds in organic synthesis.
•  It has application in material science, medicines, nanomaterials, and natural product chemistry and it has become vital in the synthesis of chemicals.
• To manufacture complicated compounds, it can be done in a benign environment like room temperature.
• The sonogashira reaction is usually carried out in a moderate environment. The cross-coupling is done at room temperature with a base, usually an amine-like diethylamine, which also serves as a solvent.

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