Synthesis, structures and catalytic activity of Pd(II) saccharinate complexes with monophosphines in direct arylation of five-membered heteroarenes with aryl bromides

Abstract

A number of new Pd(II) saccharinate (sac) complexes bearing a range of phenyl (Ph), cyclohexyl (Cy) and alkyl (Me and Et) substituted tertiary phosphine ligands with systematically changing electronic and steric properties, namely trans-[PdCl(sac)(L)2] (L = PPh3 (1); PPh2Cy (3)), trans-[Pd(sac)2(H2O)(L)] (L = PPh3 (2); PPh2Cy (5)), trans-[Pd(sac)2(L)2] (L = PPh2Cy (4); PPhCy2 (6); PCy3 (8)), [PdCl(sac)(PCy3)(DMSO)] (7), trans-[Pd(sac)2(PPh2Me)(DMSO)] (9) and cis-[M(sac)2(L)2] (L = PPhMe2 (10); PPh2Et (11); PPhEt2 (12)), were synthesized and structurally characterized. The Pd(II) complexes were applied for direct C2/C5 arylation of five-membered heteroarenes such as furan, thiophene and thiazole derivatives with aryl bromides. Notably, arylation products up to 99% yields were obtained in the presence of the Pd(II) complexes. The catalytic mechanism of the direct arylation was proposed to proceed via a Pd(0)/Pd(II) pathway, due to elimination of the sac ligand in the Pd(II) complexes during catalysis.