Sugar-Incorporated Chelating Bis-N-Heterocyclic Carbene Palladium Complexes. Synthesis, Structures, and Catalytic Ability for Suzuki-Miyaura Cross-Coupling Reactions in Water

Abstract

Abstract Sugar-incorporated chelating bis-N-heterocyclic carbene ligand precursors 1,1′-bis(2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl)-3,3′-ethylene-diimidazolyl ([(bisNHC-C2)H2]2+) and 1,1′-bis(2,3,4,6-tetra-O-acetyl-β-d-glucopyranosyl)-3,3′-propylene-diimidazolyl ([(bisNHC-C3)H2]2+) were synthesized and used to prepare palladium complexes [Pd(bisNHC-C2)Cl2] and [Pd(bisNHC-C3)Cl2], respectively, by direct metallation reaction using dichlorobis(acetonitrile)palladium as a metal source. Catalytic abilities of the complexes for Suzuki-Miyaura coupling in aqueous media were examined and moderate TON (85,000) and TOF (170,000 h−1) were obtained. Because the terminal chloro ligands in the complexes were substituted with solvent molecules or chloride anion and hence 1H NMR spectra of the chloro complexes were complicated, hydrosulfido complexes [Pd(bisNHC-Cn)(SH)2] (n = 2 and 3) were prepared to confirm structures in solution. Dynamic behavior of flapping-wing motion of the bisNHC ligand in [Pd(bisNHC-C3)(SH)2] in solution was evaluated using the signals of the SH protons, which are inequivalent in 1H NMR spectra attributed to the chirality of the glucopyranosyl units.