Question

Bi-aryls, compounds containing two aromatic rings joined by a C-C bond, can often be efficiently made by two different Suzuki couplings; that is, either aromatic ring can be used to form the organoborane needed for coupling. In some cases, however, only one route is possible. With this in mind, synthesize each of the following bi-aryls using benzene as the starting material for each aromatic ring. When more than one route is possible, draw both of them. You may use any required organic or inorganic reagents.

Short answer

Answer

a.

b.

c.

Step-by-step solution

Suzuki Coupling reaction

Suzuki Coupling is an organic reaction in which an organohalide is reacted with an organoboronic acid or boronic ester in the presence of palladium (0) complex in a basic medium. The cross-coupling reaction would create a new C-C bond. The organohalide can be an aryl ,a vinyl, or an alkyne group.

Let us consider the synthesis of the given bi-phenyls by using the starting material as benzene,

Synthesis of Bi-aryls

a. Benzene is treated with the nitrating mixture to form nitrobenzene. This is reduced using sodium borohydride followed by treatment with sodium nitrate in presence of HCl to form benzene diazotization salt. This is hydrated to form phenol. Phenol is treated with a base followed by methyl iodide to form methyl phenyl ether, which is an activating substituent. Bromination of the compound followed by reaction with toluene boronic acid in the presence of palladium (0) complex in basic medium would give the required biphenyl.

Formation of methyl diphenyl ether.

b. Benzene is brominated to form bromobenzene having a deactivating substituent. This is treated with nitrating mixture followed by reduction to form aniline. m-bromoaniline is treated with sodium nitrate in the presence of HCl to form the corresponding diazotization salt. This is hydrated to form m-bromo phenol which when treated with phenyl boronic acid in the presence of palladium (0) complex in a basic medium would give the required biphenyl.

Formation of substituted biphenyl

c. Benzene is chlorinated to form chlorobenzene. This is treated with aqueous sodium hydroxide to form phenol. Phenol is brominated to form p-bromo phenol. The compound is treated with ethanol in an acidic medium to form the corresponding ester. This compound is treated with phenyl boronic acid in presence of palladium (0) complex in a basic medium to give the required biphenyl.

Formation of substituted biphenyl