Synthesis of 2,2‘-Biimidazolium-Based Ionic Liquids:  Use as a New Reaction Medium and Ligand for Palladium-Catalyzed Suzuki Cross-Coupling Reactions

Abstract

Neat reactions of 2,2'-biimidazole with an excess of alkyl or polyfluoroalkyl iodides at 140 degrees C, followed by anion exchange with LiN(SO(2)CF(3))(2) or KPF(6), 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 degrees C under similar conditions. Subsequent metathesis reactions with LiN(SO(2)CF(3))(2) or KPF(6) 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.

Tuning Photophysical and Electrochemical Properties of Heteroleptic Cationic Iridium(III) Complexes Containing Substituted 2-Phenylquinoxaline and Biimidazole

“…Other ligands 2-(2,4-dimethoxyphenyl)quinoxaline (dmpqx) and 2-(2,4-difluorophenyl)quinoxaline (dfpqx) were prepared from 2-chloroquinoxaline with 2,4-dimethoxyphenylboronic acid and 2,4-difluorophenylboronic acid, respectively, by a similar procedure described for pqx ligand. The 2-benzo[b]thiophen-2-yl-pyridine (btp) main ligand 8 and H2biim ancillary ligand 9 was synthesized in accordance with the published procedure. …”

Ionic liquids: a versatile medium for palladium-catalyzed reactions

“…There are several reports on palladium-catalyzed reactions in ILs 50,51 such as Stille, 52 Suzuki, 53 SonogashiraHigihara, 54 Heck arylations 55,56 etc.…”

“…53 Majority of the monoquaternary products are RTILs. The 1,3,1`-tributyl-2,2`-biimidazolium hexafluorophosphate, a monoquaternary RTIL, was found to be very efficient and recyclable solvent for the cross coupling reactions.…”

“…Due to its coordination ability, the monoquaternary IL markedly avoids palladium leaching in recycling experiments. 53 Organic solvent nanofiltration have been used to separate the product from both catalyst and ILs in some cases. 120 All the ILs screened showed positive effects on the catalytic stability, significantly reducing the formation of palladium black and providing high reaction yields over consecutive recycles.…”

Exploring the Use of Ionic Liquid Mixtures to Enhance the Performance of Dicationic Ionic Liquids

“…15 Geminal DILs have also been proposed as advantageous when used as solvents in high temperature reactions, 16,17 as high temperature lubricants, 18 and as stationary phases for gas chromatography capillary columns. [19][20][21] Despite the particular advantages of DILs over their monocationic counterparts, they suffer from higher melting points and viscosities.…”