Syntheses and Catalytic Ability of Sugar-Incorporated N-Heterocyclic Carbene Pincer Pd Complexes Possessing Various N-Substituents

Exploration of catalysts for organic reactions in water, which is a nontoxic and nonexplosive liquid, has been studied to achieve environmentally friendly chemical processes. For such a purpose, we utilized click chemistry to incorporate sugar units into transition-metal complexes affording water-soluble complexes. In this study, a series of palladium complexes with C–C–N pincer ligands, each of which consisted of two N-heterocyclic carbenes (NHCs) and a triazole moiety with a D-glucopyranosyl unit were synthesized. These complexes possess various N-substituents, namely, methyl, isopropyl, benzyl, or D-glucopyranosyl units on the terminal NHC moieties. The diastereoselectivity in the formation of the complexes and their catalytic ability toward the Suzuki–Miyaura cross-coupling reactions in water exhibited clear N-substituent effects. The tridentate ligand with an acetylated D-glucopyranosyl unit on the terminal NHC moiety led to high diastereoselectivity in the formation of its complex, while each of the other ligands with achiral substituents on the terminal NHC units afforded both of the diastereomers with low selectivity. Catalytic reactions using the complexes were examined for the Suzuki–Miyaura cross-coupling reactions of phenylboronic acid and 4′-bromoacetophenone in water. The complex possessing an isopropyl group on the terminal NHC moiety exhibited excellent catalytic activity with TON (800000), which is comparable to the best value reported for a similar reaction.