Ullmann-Type Coupling and Related Reactions

It is worth noting that the Ullmann-Goldberg condensation of aryl halides with phenols and anilines worked efficiently in the presence of copper in water. For example, the coupling of 2-chlorobenzoic acid with 4-chlorophenol (K2C03/pyridine/copper powder) gave 2-(4-chlorophenoxy)carboxyMc acid (Eq. 6.23). The Cu(I)-catalyzed transformation of 2-bromobenzoic acid into salicylic acid has also been studied in aqueous media (Eq. 6.24).  

The homocoupling of aryl halide to diaryl compounds, known as Ull-mann coupling, is a synthetically useful reaction and has wide applications in material research. Such couplings have been studied in aqueous conditions. In 1970, arylsulflnic acids were coupled with Pd(II) in aqueous solvents to biaryls (Eq. 6.25). However, the reaction required the use of a stoichiometric amount of palladium. In the presence of hydrogen gas, aryl halides homocoupled to give biaryl compounds in moderate yields (30-50%) in an aqueous/organic microemulsion (Eq. 6.26). 4 

---