Palladium Supported on Cross-Linked Imidazolium Network on Silica as Highly Sustainable Catalysts for the Suzuki Reaction under Flow Conditions

Abstract

Highly cross-linked imidazolium-based materials,obtained by radical oligomerization of bisvinylimidazolium salts in the presence of 3-mercaptopropyl-modified silica gel, were used as supports for palladium catalysts. Thanks to the high imidazolium loading these materials were able to support a high amount of the metal (10 wt%). Such materials were characterized by several techniques (13C magic angle spinning nuclear magnetic resonance, the Brunauer– Emmett–Teller technique, X-ray photoelectron spectroscopy, and transmission electron microscopy). The palladium catalysts displayed good activity allowing the synthesis of several biphenyl compounds in high yields working with only 0.1 mol% of palladium loading at 50 8C in ethanol/water under batch condition. Moreover, a flow apparatus, to optimize the efficiency of the isolation of the pure products and minimize waste (E-factor), was investigated. For the first time the palladium catalyst and base(K2CO3) were placed in two separate columns allowing an easy recovery of the products with very low E-factor values (<4). Waste production was reduced by over 99% compared to classic batch conditions. Because of the high Pd loading only 42 mg of catalysts were employed in the Suzuki reaction between 160 mmol of 4-bromotoluene and 180 mmol of phenylboronic acid. No loss in activity was observed.