Heck cross coupling

What is Heck cross coupling?

Heck cross coupling reaction is a chemical reaction that allows the synthesis of unsymmetrical biaryls, which are compounds made up of two aromatic rings that are bonded together by a single bond. This reaction was first developed by Richard F. Heck and his colleagues in the 1970s, and has since become a widely used synthetic method in the field of chemistry..

Heck cross coupling reaction involves the use of a palladium catalyst and a coupling partner, which is typically an aryl halide or a triflate. The reaction is typically carried out in the presence of a base, such as potassium carbonate or sodium hydroxide, and a solvent, such as dimethylformamide or acetonitrile..

One of the key benefits of the Heck cross coupling reaction is that it allows for the synthesis of biaryls with a high degree of regioselectivity and stereoselectivity. This means that the reaction can be used to produce biaryls with specific arrangements of substituents on the aromatic rings..

Heck cross coupling reaction has a wide range of applications in the synthesis of a variety of compounds, including pharmaceuticals, dyes, and materials. For example, it has been used to synthesize the anti-cancer drug taxol, as well as other compounds that have potential medicinal applications..

Summary

Heck cross coupling reaction is an important tool in the synthesis of biaryls and has a wide range of applications in the field of chemistry..

Example

The Heck reaction is a chemical reaction in which an organic halide is reacted with an alkene in the presence of a palladium catalyst to form a new carbon-carbon bond. An example of the Heck reaction is the reaction between styrene and bromoethane in the presence of a palladium catalyst:

Styrene and bromoethane are added to a flask containing a palladium catalyst such as palladium on carbon (Pd/C)..

The palladium catalyst activates the bromoethane, making it more reactive towards the alkene..

The bromoethane reacts with the alkene to form a new carbon-carbon bond, and a molecule of HBr is eliminated..

The reaction produces an alkylated alkene, in this case, ethylbenzene..

Mechanism of reaction

The mechanism of the Heck reaction can be described in the following steps:

  • The palladium catalyst, such as palladium on carbon (Pd/C), activates the organic halide, such as bromoethane, by coordinating to the halide atom, which forms a complex called palladium halide complex..
  • The alkene, such as styrene, is then added to the reaction mixture, and it coordinates with the palladium catalyst, forming an intermediate called palladium alkene complex..
  • The palladium alkene complex then undergoes an oxidative addition, where the halide from the palladium halide complex adds across the alkene double bond, forming a new carbon-carbon bond, and a molecule of HX (X = halide) is eliminated..
  • The palladium catalyst is then removed by treatment with a reducing agent, such as zinc metal, to give the final product, an alkylated alkene..
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