Synthesis of the chelated axially chiral 1,2,3-Triazol-5-ylidene-Pd(II), Ni(II) and Ru(II) complexes and exploration of Bis-MIC-Pd(II) catalyst's application in coupling reaction

Abstract

A series of chelated (R)-binaphthyl-2,2-diamine (BINAM) based chiral mesoionic Pd(II), Ni(II) and Ru(II) complexes, including three chelated bis-1,2,3-triazol-5-ylidene Pd(II) complexes, one chelated bis-1,2,3-triazol-5-ylidene Ni(II) complex and one chelated mono-1,2,3-triazol-5-ylidene Ru(II) complex, have been developed through either direct metalation (Ni(II) complex) or silver(I) transmetallation (Pd(II) and Ru(II) complexes) methods. These MIC-Pd(II), Ni(II) and Ru(II) complexes have been fully characterized by NMR, ESI-MS data, in which, the structures of (R)-5a, (R)-7, and (R)-9 have been further determined by X-ray diffraction studies of their single crystals. The catalytic activities of the chiral bis-MIC-Pd(II) complexes were explored in Pd(II)-catalyzed Suzuki–Miyaura coupling reaction, in which, bisaryl coupling products could be synthesized in good to excellent yields from the reactions of various aryl boronic acids with aryl iodides. Asymmetric coupling reaction was also tested, but no enantioselectivity was obtained. Scale–up reaction and mercury poisoning experiment were then performed. The catalytic activity of (R)-5a was partly inhibited by a large excess of elemental mercury, indicating the presence of heterogeneous Pd nanoparticles in the reaction mixture.