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

The series of water-soluble palladacyclic aqua complexes [(tBu-SCS)PdII(H2O)]+ ([1]+, tBu-SCS =
C6H3-2,6-(CH2StBu)2), [(iPr-SCS)PdII(H2O)]+ ([2]+, iPr-SCS = C6H3-2,6-(CH2SiPr)2), [(PCP)PdII(H2O)]+
([3]+, PCP = C6H3-2,6-(OPiPr2)2), and [(PC)PdII(H2O)2]+ ([4]+, PC = 4-MeC6H3-2-(OPiPr2)) 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.