Chemoselective Suzuki Coupling of Bromoarenes Catalysed by Palladium(II)-Complexing Macrocycles in Aqueous Media

Palladium-complexing macrocyclic catalysts efficiently and chemoselectively catalysed the Suzuki coupling reaction of bromoarenes. Bromobenzene and 2-bromopyridine were efficiently monoarylated with 4-methoxyphenylboronic acid when catalysed by macrocyclic catalysts comprising cyclic pyridine-2,6-bisamide pincer-type ligands with palladium (II) located in the inner cavity (PdMCs). The Suzuki coupling of 2,6-dibromopyridine to aryl boronic acids catalysed by PdMCs afforded the corresponding monoarylpyridines with high selectivity and yield, whereas a mixture of mono- and di-arylpyridines was obtained when palladium chloride was used as the catalyst. Interestingly, the chemoselectivity increased as the cavity size of the PdMCs decreased. The high chemoselectivity seemed to arise from the steric effect of the macrocyclic ligand. Moreover, an unsymmetrical 2,6-diarylpyridine was successfully synthesised in high yield by two sequential couplings employing PdMCs followed by palladium chloride.