Bronsted Acid Catalysis in Ionic Liquids

Because ionic liquids are in general weakly coordinating, they display low nu-cleophilicity. Such an environment favors the stabilization of electron-deficient intermediates (7). This unique property allows ionic liquids to be used as nonsolvating media for the stabilization of strongly acidic species. It is this property that has given rise to the superacidity of non-solvated protons in acidic [AMIM]Al2Clv 103). 

Several authors reported the use of ionic liquids containing protonic acid in catalysis (118-120). For example, strong Bronsted acidity in ionic liquids has been reported to successfully catalyze tetrahydropyranylation of alcohols (120). Tetra-hydropyranylation is one of the most widely used processes for the protection of alcohols and phenols in multi-step syntheses. Although the control experiments with the ionic liquids showed negligible activity in the absence of the added acids, high yields of product were obtained with the ionic liquid catalysts TPPTS or TPP.HBr-[BMIM]PF6. By rapid extraction of the product from the acidic ionic liquid phase by diethyl ether, the reaction medium was successfully reused for 22 cycles without an appreciable activity loss. A gradual loss of the catalyst and a reduced volume of the ionic liquid were noted, however, as a consequence of transfer to the extraction solvent. 

As the process was carried out at a low temperature (18°C), decomposition of the ionic liquids caused by the presence of the strong acids did not appear to be a serious problem. 

In another example, a catalytic amount of a Bronsted acid such as H3PO4 was dissolved in [NRR 3][NTf2] (R = hexyl, R = butyl) 119). The catalyst was applied for the condensation reaction of alcohols, which usually requires strongly acidic media and dehydrating conditions. The condensation of veratryl alcohol was facilitated because the water that formed was continuously removed as vapor, which assisted in driving the reaction to high yields. The product (cyclotriveratrylene) separation, however, required the addition of a co-solvent. 

Chloride-free ionic liquid was essential for the high activity the addition of only 5 mol% [BMIM]C1 led to a significant decrease in the yield because of the basicity of [BMIM]C1 (722). Remarkably, this acid-ionic liquid combination could be reused many times for the glycosidations without any loss in efficiency. 

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