pH-Dependent C−C Coupling Reactions Catalyzed by Water-Soluble Palladacyclic Aqua Catalysts in Water

The series of water-soluble palladacyclic aqua complexes [( t Bu-SCS)PdII(H2O)]+ ([1]+, t Bu-SCS = C6H3-2,6-(CH2S t Bu)2), [( i Pr-SCS)PdII(H2O)]+ ([2]+, i Pr-SCS = C6H3-2,6-(CH2S i Pr)2), [(PCP)PdII(H2O)]+ ([3]+, PCP = C6H3-2,6-(OP i Pr2)2), and [(PC)PdII(H2O)2]+ ([4]+, PC = 4-MeC6H3-2-(OP i Pr2)) have been synthesized from the reaction of the corresponding palladacyclic chloro complexes with silver salts in water to optimize the catalytic activity for pH-dependent C−C coupling reactions in water by changing the supporting ligands from S-supporting (SCS) to P-supporting (PCP) ligands and also from tridentate (PCP) to bidentate (PC) ligands. It was confirmed that there was no precipitation of palladium black under the present reaction conditions. The PC palladacyclic aqua complex [4]+ exhibits the highest catalytic activity among the water-soluble palladacyclic aqua complexes [1]+−[4]+ for the pH-dependent Suzuki−Miyaura, Mizoroki−Heck, and Stille coupling reactions in water. This is one of the highest catalytic activities ever reported for coupling reactions with water-soluble palladium catalysts in water. The catalytic ability of the palladacyclic aqua catalysts is drastically dependent on the pH of the solution and the structures of supporting ligands. The structures of palladacyclic aqua and chloro complexes were unequivocally determined by X-ray analysis.