Ullmann Reaction

Introduction

Ullmann reaction is a coupling reaction between aryl halide molecules. Fritz Ullmann was a German chemist who first noticed this type of reaction and this reaction is called the Ullmann reaction after his name. Copper plays an important role throughout the entire coupling mechanism.

Meaning of Ullmann Reaction

Ullmann coupling or Ullmann reaction means a reaction is carried out for coupling two aryl halide molecules for the formation of biaryl compounds using some thermal conditions and copper catalyst. There are different reaction mechanisms for Ullmann reactions and two of them are very popular. In the radical mechanism process, an aryl radical is formed by transferring of one electron from the metal copper to aryl halide. Then biaryl product is formed ultimately by the reaction of two aryl radicals.

In the second coupling mechanism, organocuprate reagent is formed by the exchange of single electron and addition of copper metal by oxidation with aryl halide. An oxidative addition is again performed for the second time on an aryl halide by the organocuprate. This leads to reductive elimination and the ultimate biaryl product is formed.

Mechanism of Ullmann Reaction

Ullmann coupling mechanisms as per their reaction natures are divided into two types −

Radical Mechanism and Aryl Copper Intermediate Mechanism.

Figure 1: Ullman reaction in ionic liquid

Step 1

The mechanism of Ullmann coupling is mentioned in the steps below. The aryl halide is over-exposed or metallic copper becomes abundant at a high relative temperature at above 200°C to produce active copper species (I). The copper species is the base part of the Ullmann reaction. In the initial step, aryl halide reacts to form active copper.

Step 2

The oxidative addition of active copper species joins the twin molecules to other haloarene molecule. The copper that is formed in the initial process, proceeds with oxidative addition and forms a molecule link while joining other haloarene molecule.

Step 3

The two molecules of aryl halide are created from the previous steps experience elimination at reduced rates, these results in a carbon-carbon bond between two aryl compounds. The two individual copper compounds in the final step react with each other to create a carbon-carbon bond.

Role of Copper in Ullmann Reaction

Copper plays as a main element in the reaction. It forms a couple of other molecules. An atom of hydrogen is expelled from an aromatic compound and creates an aryl group. The aryl consists of the phenyl group of the aromatic compound. The metal-catalyzed coupling represents the benzene-halogen derivatives.

Carbon-based frameworks result in larger amounts and biaryls are produced from these derivatives. A couple of factors are crucial for the organic-based transistor development with the enhancement of electron conduction and mechanical stability. This reaction has high potential for reaching molecular capabilities. Copper is also used in the Ullmann reaction in other ways.

Figure 2: Ullmann Reaction

• Copper metal initiates a pathway to carry forward the Ullmann reaction.

• The metal takes the responsibility for creating a couple of new molecules. This becomes a reaction of cross-coupling as part of the Ullmann reaction in classic form.

• Copper plays its catalytic role by participating in the chemical reaction without changing its form but with some physical changes.

• In the traditional Ullmann mechanism, the cross-coupling happens first. It is followed by oxidation of copper. Cuprate intermediate molecules are generated. The copper halides are formed as alternative by products.

• Copper plays its dominating role in the entire reaction process. It is vital for oxidation and ultimate production of two molecules of aryl halide.

Ullmann Reaction – Applications

Ullmann coupling reaction is used from creating different products, especially byproducts. The Ullmann Reaction is implemented in various sectors. These are mentioned below.

• 2, 2-diiodo biphenyl is used to produce biphenylenes using the mechanism of the Ullmann reaction.

• If one reactant is present as excess, the reaction can occur in an unsymmetrical pattern.

• The coupling of chiral substances by the Ullmann coupling reaction presents a chiral result.

• The five-membered rings can be completed using the procedure of the Ullmann reaction.

Importance of Ullmann Reaction

The Ullmann reaction has importance in organic chemistry. Ullmann coupling mechanism is highly potential and it has an instrumental role in synthesizing organic compounds. The reaction is used for developing drugs. Ullmann processes using copper as a catalyst are significantly developed with the support of synthetic tools and with the utilization of novel ligands.

Green synthetic methodologies including ligand, metal, renewable heterogeneous catalysts, admixtures conditions, and synthesis assisted by microwaves will provide a direct impact in this sector as expected. These are some quick and intriguing developments of Ullmann reactions.

Conclusion

Ullmann coupling or Ullmann reaction is a type of organic name reaction. It is the coupling reaction of two aryl halides. Ullmann coupling is occurred in presence of copper. Biaryl is produced as a final product in this process.

Fritz Ullmann is the chemist who mainly contributed to the discovery of this reaction and after his name the reaction is called the Ullmann reaction. The reaction mechanism is done through three steps. The carbon-carbon bond is created as the outcome of this reaction.

FAQs

Q1. What is the need for copper in the Ullmann reaction?

Ans. Copper plays its catalytic role in Ullmann reaction. It is a pathway alternatively provided to the reaction. The activation energy for the reaction gets lowered in presence of copper. The equilibrium point is easily reachable due to the increased reaction rate.

Q2. Who are the reagents of Ullmann reaction?

Ans. Ullmann biaryl synthesis or Ullmann reaction offer a reaction with organic type. It makes fractions of two aryl halide molecules to create biaryl with help of copper in thermal conditions.

Q3. What is dimethyl formamide?

Ans. Dimethyl formamide or DMF is a non-aqueous and highly polar solvent. It is used in the Ullmann process.