Synthesis of 2,2′-biimidazolium-based ionic liquids: Use as a new reaction medium and ligand for palladium-catalyzed Suzuki cross-coupling reactions

Abstract

(Chemical Equation Presented) Neat reactions of 2,2′-biimidazole with an excess of alkyl or polyfluoroalkyl iodides at 140°C, followed by anion exchange with LiN(SO2CF3)2 or KPF6, gave the diquaternary salts 3a-k in >80% yields. However, by controlling the reaction stoichiometry, 2,2′-biimidazole can also be monoquaternized with the same electrophiles at 100°C under similar conditions. Subsequent metathesis reactions with LiN(SO2CF3)2 or KPF6 resulted in the ionic liquids 4a-m in high yields. Thermal properties were determined with a differential scanning calorimeter (DSC) and a thermogravimetric analyzer (TGA). Most of the monoquaternary salts are room-temperature ionic liquids. 1,3,1′-Tributyl-2,2′-biimidazolium hexafluorophosphate was demonstrated to be an excellent solvent and ligand for palladium-catalyzed Suzuki cross-coupling reactions. The catalytic ionic liquid system may be recycled at least 14 times without a significant decrease in catalytic performance. © 2005 American Chemical Society.