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

Highly cross-linked imidazolium-based ma- terials, obtained by radical oligomerization of bis- vinA salts in the presence of 3-mercap- topropyl-modified silica gel, were used as supports for palladium catalysts. Thanks to the high imidazoli- um loading these materials were able to support a high amount of the metal (10 wt%). Such materials were characterized by several techniques ( 13 C magic angle spinning nuclear magnetic resonance, the Bru- nauer-Emmett-Teller technique, X-ray photoelec- tron spectroscopy, and transmission electron micros- copy). The palladium catalysts displayed good activi- ty allowing the synthesis of several biphenyl com- pounds in high yields working with only 0.1 mol% of palladium loading at 508C in ethanol/water under batch condition. Moreover, a flow apparatus, to opti- mize the efficiency of the isolation of the pure prod- ucts and minimize waste (E-factor), was investigated. For the first time the palladium catalyst and base (K2CO3) were placed in two separate columns allow- ing 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 cata- lysts were employed in the Suzuki reaction between 160 mmol of 4-bromotoluene and 180 mmol of phe- nylboronic acid. No loss in activity was observed.