Suzuki coupling reaction is an organic coupling reaction wherein the coupling partners include a boronic acid and an organohalide. Palladium (0) complex is used to catalyze this reaction. This reaction is named after Nobel Laureate Japanese chemist Akira Suzuki, who first published work on this reaction in 1979. This reaction is also called the Suzuki-Miyaura reaction or the Suzuki coupling. In the Suzuki Couplings general scheme, the coupling of the organoboron specimen with the halide over palladium(0) catalyst and a base leads to the formation of a carbon-carbon single bond. This general scheme for the Suzuki coupling can be illustrated as follows.
In the example given above – R1 and R3 = aryl, alkene(vinyl) or alkyne
R2= H(boronic acid) or alkyl(boronic ester)
Merits of the Suzuki Coupling Reaction
This reaction is commonly used in the synthesis of substituted biphenyls, poly-olefins, and styrenes. Boronic acids are commonly available. They are also less toxic and environment-friendly when compared to organostannane and organozinc compounds. The reaction conditions of the Suzuki coupling reaction are relatively mild. Due to these reasons, the Suzuki coupling method is preferred over other similar coupling reactions. Furthermore, the reagents used in this reaction can be easily prepared and are relatively cheap.
Suzuki Coupling Reaction Mechanism
The Suzuki coupling mechanism follows a catalytic cycle involving three primary steps, namely – oxidative addition, transmetalation, and reductive elimination. Each of these steps have been elaborated below.
The Oxidative Addition Step
In this step, the oxidation of the catalytic palladium takes place. It is oxidized form palladium(0) to palladium(II). This step is generally the rate determining step of the cycle. The reaction in this step is the coupling of the alkyl halide with the palladium catalyst. The product formed in this step is an organopalladium complex where the carbon-halogen bond is broken and the palladium inserts itself in between the R group and the halogen. This step can be illustrated as.
The Transmetalation Step
In this step, an organometallic reaction occurs and a ligand transfer takes place from the organoboron specimen to the palladium(II) complex. The organoborane compound reacts with the first intermediate upon the addition of a base. This gives another palladium(II) complex as shown below.
The Reductive Elimination Step
Now, the palladium(II) complex proceeds to eliminate the required product and is converted into the palladium(0) form. Thus, the catalyst is regenerated and the catalytic cycle is ended. This step can be illustrated as:
Combining the three steps in the given order, the complete catalytic cycle can be represented as:
Thus, the required coupling of the boronic acid with the organohalide is achieved and the carbon-carbon single bond is formed.