Aqueous-phase Suzuki-Miyaura cross-coupling reactions catalyzed by Pd-NHC complexes

Cleavage reactions of [PdBr2(NHC)](2) with two equiv. of pyridine derivatives (L), having one or two carboxylic acid groups [L = NC5H4-2-COOH, NC5H4-3-COOH, NC5H4-4-COOH, or NC5H3-2,6-(COOH)(2) in chloroform or DMSO, respectively, afforded the monomeric mixed ligand complexes trans-[PdBr2(NHC)L] (1-4) which, due to deprotonation of the carboxylic acid functionality, are water soluble in KOH. The catalytic activity of complexes 1-4 in aqueous Suzuki-Miyaura cross-coupling reactions were evaluated and compared. The dicarboxylic functionality enhances the catalyst reactivity and stability and the carboxylate derived from 4 could be easily recovered and reused for several cycles under the mild reaction conditions.