The oxidative coupling of aromatic compounds with palladium salts

Reaction of benzene, palladium chloride and sodium acetate in acetic acid solvent produces biphenyl in a high yield. In the absence of sodium acetate no reaction occurs. The reaction is first‐order with respect to benzene and PdCl 2 . With monosubstituted benzenes in general a mixture of isomeric biphenyls (X‐C 6 H 4 ‐C 6 H 4 ‐X,X = alkyl, aryl, OR, Cl, COOR) is obtained. The substitution pattern corresponds with that observed in electrophilic aromatic substitution. The substituents exert a weak polar effect: electron‐donating groups increase the rate, while the opposite holds for electron‐attracting groups. With disubstituted benzenes steric effects play an important role. From o ‐xylene 3,4,3′,4′‐ and 2,3,3′,4′‐tetramethylbiphenyl have been obtained as the only products in a molar ratio of 2.7 : 1. The results are most satisfactorily described by a mechanism involving complex formation between the aromatic compound and PdCl 2 as the rate‐determining step. The complex formed is assumed to yield a π‐cyclohexadienyl‐PdCl complex by a fast reaction with acetate anions. The complex formed, being unstable, decomposes to yield biphenyl and palladium metal.