Abstract
The first example of triflometallate ionic liquids, named in analogy to chlorometallate ionic liquids, is reported. Trifloaluminate ionic liquids, synthesized from 1-alkyl-3-methylimidazolium triflates and aluminum triflate, were characterized by multinuclear NMR spectroscopy and FT-IR spectroscopy, revealing the existence of oligonuclear, multiply-charged trifloaluminate anions, with multiple bridging triflate modes. Acceptor numbers were determined to quantify their Lewis acidity, rendering trifloaluminate ionic liquids as medium-strength Lewis acids (AN = ca. 65). Used as acidic catalysts in the cycloaddition of 2,4-dimethylphenol and isoprene (molar ratio 2:1) to prepare chromane, trifloaluminate systems outperformed literature systems, showing high activity (conversions 94–99%, selectivities 80–89%) and at low loadings (0.2 mol%) at 35°C. Using these new systems as supported ionic liquid phase (SILP) on multi-walled carbon nanotubes (ionic liquid loading 16 wt%) delivered a recyclable catalytic system, with activity enhanced with respect to the homogenous regime.
Highlights
Lewis acid catalysis is widely used, from multi-ton industrial processes to asymmetric synthesis of fine chemicals
Ionic liquids have been used to address some of these challenges: typically inflammable, they offer enhanced separation and recycling strategies, from liquid biphasic systems to supported ionic liquid phases (SILPs) (Plechkova and Seddon, 2008)
In chemoenzymatic Baeyer-Villiger reaction, the performance of lipase was significantly improved by immobilization on multi-walled carbon nanotubes (MWCNTs) (Markiton et al, 2017)
Summary
Lewis acid catalysis is widely used, from multi-ton industrial processes to asymmetric synthesis of fine chemicals. Traditional Lewis acids (AlCl3 and BF3) suffer from several drawbacks, in particular high corrosivity and propensity to hydrolysis. Many Lewis-acid catalyzed reactions must be carried out under strictly anhydrous conditions and using corrosion-resistant vessels. Low stability toward water limits application of strong Lewis acids in reactions where water is eliminated, e.g., condensation. Lewis acid catalysis in organic synthesis is typically carried out in an organic solvent, which comes with a set of challenges in terms of process sustainability, from flammability and issues of recycling/disposal, to difficulties in dissolving both the catalyst and the reactants at sufficient concentrations.
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