Acidity Ionic solvents

In cases in which the ionic liquid is not directly involved in creating the active catalytic species, a co-catalytic interaction between the ionic liquid solvent and the dissolved transition metal complex still often takes place and can result in significant catalyst activation. When a catalyst complex is, for example, dissolved in a slightly acidic ionic liquid, some electron-rich parts of the complex (e.g., lone pairs of electrons in the ligand) will interact with the solvent in a way that will usually result in a lower electron density at the catalytic center (for more details see Section 5.2.3). 

A number of new conditions and catalysts have been used for the synthesis of quinazolinones 50 from anthranilic acids, amines and ortho esters, including bismuth trifluoroacetate with an ionic liquid <06TL3561>, lanthanum nitrate or tosic acid under solvent-free conditions at room temperature <06TL4381> and Nafion-H <06SL2507>. 

Gui JZ, Liu D, Wang C, Lu F, Lian JZ, Jiang H, Sun ZL (2009) One-Pot Synthesis of 3,4-Dihydropyrimidin-2(lH)-ones Catalyzed by Acidic Ionic Liquids Under Solvent-Free Conditions. Synth Commun 39 3436-3443... 

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. 

The acylative cleavage of the cyclic ether tetrahydrofuran showed an excellent yield (95%) to 4-iodobutylbenzoate when fully acidic ionic liquid [EMIM]Al2Cl7 was used, but the yield to di-functionalized product (61%) suffered when the mildly acidic halogenoaluminate ionic liquid (which has a mole fraction of AICI3 (x) at a value of 0.52) was used as the solvent. In contrast, for 1,5-dimethyltetrahydrofuran and tetrahydropyran, a good yield to the cleavage products was obtained when the mildly acidic chloroaluminate was used. 

The polymerization of styrene by trichloroacetic acid without solvent and in 1,2-dichloro-ethane and nitroethane solutions illustrates the situation where the initiator solvates ionic propagating species [Brown and Mathieson, 1958]. The kinetic order in the concentration of trichloroacetic acid increases from 1 in the highly polar nitroethane to 2 in the less polar 1,2-dichloroethane to 3 in neat styrene. 

Cole, A.C. et al.. Novel Bronsted acid ionic liquids and their use as dual solvent-catalysts, /. Am. Chem. Soc., 124,5962,2002. 

The equilibrium constants for these reactions are 8.9 x 10 8 (Eq. 17.11), 1 x 10-7 (Eq. 17.12), and 1 x 10"14 (Eq. 17.13) at 175°C [29]. As a consequence of Equation 17.13, there is a significant vapor pressure of A12C16 over acidic AlCl3-NaCl melt, making experimentation with this ionic solvent difficult. This is not a problem for the equimolar and NaCl(satd) melts. 

In this chapter alternative acidic ionic liquids systems will be briefly presented that have been recently developed as alternatives to chloroaluminate ionic liquids. In spite of this selection, it should be noted that chloroaluminate ionic liquids may still be attractive catalyst phases in reactions where their tuneable acidity and solubility properties offer advantage over AICI3 in organic solvents. 

Abbott et al. [98-103] reported the synthesis and characterization of new moisture-stable, Lewis acidic ionic liquids made from metal chlorides and commercially available quaternary ammonium salts (see Chapter 2.3). They showed that mixtures of choline chloride (2-hydroxyethyltrimethylammonium chloride, [Me3NC2H40H]Cl and MCU (M=Zn, Sn) give conducting and viscous liquids at or around room temperature. These deep eutectic solvents/ionic liquids are easy to prepare, are water-and air-stable, and their low cost enables their use in large-scale applications. Furthermore, they reported [104] that a dark green, viscous liquid can be formed by mixing choline chloride with chromium(III) chloride hexahydrate and that the... 

Our primary objective was to develop a computational technique which would correlate the ionization constant of a weak electrolyte (e.g., weak acid, ionic complexes) in water and the ionization constant of the same electrolyte in a mixed-aqueous solvent. Consideration of Equations 8, 22, and 28 suggested that plots of experimental pKa vs. some linear combination of the reciprocals of bulk dielectric constants of the two solvents might yield the desirable functions. However, an acceptable plot should have the following properties it should be continuous without any maximum or minimum the plot should include the pKa values of an acid for as many systems as possible and the plot should be preferably linear. The empirical equation that fits this plot would be the function sought. Furthermore, the function should be analogous to some theoretical model so that a physical interpretation of the ionization process is still possible. 

It should be noted that RTILs do not always act as inert reaction media, and in fact many RTILs have been developed with reactivity in mind. For example, moisture-sensitive chloroaluminate ionic liquids can be used as Lewis acids and solvents simultaneously. 

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