Glucopyranoside-Incorporated N-Heterocyclic Carbene Complexes of Silver(I) and Palladium(II): Efficient Water-Soluble Suzuki−Miyaura Coupling Palladium(II) Catalysts

Abstract

Glucopyranoside-incorporated silver(I) bis(carbene) complexes have been prepared through the reactions of silver oxide with related imidazolium salts. The palladium(II) bis(carbene) complexes were synthesized from their corresponding silver(I) complexes via the technique of transmetalation. In solution, both trans-anti and trans-syn rotamers are observed for these square-planar Pd(II) complexes. In the solid state, the complex [1-methyl-3-(methyl 2,3,4-tri-O-benzoyl-6-deoxy-α-d-glucopyrano-6-yl)imidazol-2-ylidene]2PdBr2 adopts a trans-anti conformation, while the complexes [1-ethyl-3-(methyl 2,3,4-tri-O-benzoyl-6-deoxy-α-d-glucopyrano-6-yl)imidazol-2-ylidene]2PdBr2 and [1-n-butyl-3-(methyl 2,3,4-tri-O-benzoyl-6-deoxy-α-d-glucopyrano-6-yl)imidazol-2-ylidene]2PdBr2 adopt a trans-syn conformation. The steric congestion between the two glucopyranoside groups in the last two trans-syn compounds are perhaps relaxed through tilting of the imidazole rings and rotation of the flexible CH2 spacer, which connects the imidazole ring and the glucopyranoside group. The palladium(II) complexes were further hydrolyzed to produce water-soluble hydrolysates. Preliminary catalytic studies show that these hydrolyzed palladium precatalysts are efficient in the Suzuki−Miyaura coupling reaction between phenylboronic acid and a hydrophilic substrate, 2-chlorobenzoic acid, in aqua medium. In the presence of tetra-n-butylammonium bromide (TBAB), these precatalysts also display efficient catalytic properties in the reaction of phenylboronic acid with both the hydrophilic substrate 1-chloro-3-nitrobenzene and the hydrophobic substrate 2-chlorophenol. In all cases, the catalytic systems are stable, which allow them to be reused for at least three cycles. This catalytic system also facilitates the separation of products from the catalyst and, therefore, is environmentally benign.