Tetraalkylammonium ionic liquid

The first example of biphasic catalysis was actually described for an ionic liquid system. In 1972, one year before Manassen proposed aqueous-organic biphasic catalysis [1], Par shall reported that the hydrogenation and alkoxycarbonylation of alkenes could be catalysed by PtCh when dissolved in tetraalkylammonium chloride/tin dichloride at temperatures of less than 100 °C [2], It was even noted that the product could be separated by decantation or distillation. Since this nascent study, synthetic chemistry in ionic liquids has developed at an incredible rate. In this chapter, we explore the different types of ionic liquids available and assess the factors that give rise to their low melting points. This is followed by an evaluation of synthetic methods used to prepare ionic liquids and the problems associated with these methods. The physical properties of ionic liquids are then described and a summary of the properties of ionic liquids that are attractive to clean synthesis is then given. The techniques that have been developed to improve catalyst solubility in ionic liquids to prevent leaching into the organic phase are also covered. 

In basic tetraalkylammonium chloride ionic liquids, the active catalyst was suggested to form from the dissociation of the chloride ligand of RuCl2(PPh3)3 in the base. The effect of the cation became evident as the catalyst in tetraalkylammonium chloride was much more active than that in [BMIMJCl. It is known that the bulky tetraalkylammonium cation is weaker in its association with the chloride anion than a planar [BMIM] cation. Therefore, it was concluded that the ionic liquids giving the best catalytic activity appeared to be tetraalkylammonium hydroxide, which melts at approximately room temperature. 

Scheme 11 Synthesis of bls-tetraalkylammonium chiral ionic liquids from isomannide...

The use of ionic liquids as reaction media for the palladium-catalyzed Heck reaction was first described by Kaufmann et al, in 1996 [85]. Treatment of bromoben-zene with butyl acrylate to provide butyl trans-cinnamate succeeded in high yield in molten tetraalkylammonium and tetraalkylphosphonium bromide salts, without addition of phosphine ligands (Scheme 5.2-16). 

Cabovska, B., Kreishman, G. R, Wassell, D. F., and Stalcup, A. M., Capillary electrophoretic and nuclear magnetic resonance studies of interactions between halophenols and ionic liquid or tetraalkylammonium cations,. Chromatogr. A, 1007,179-187, 2003. 

Nishi, N., Kawakami, T., Shigematsu, E, Yamamoto, M., Kakiuchi, T., Fluorine-free and hydrophobic room-temperature ionic liquids, tetraalkylammonium bis(2-ethylhexyl)sulfosuccinates, and their ionic liquid-water two-phase properties, Green Chem., 8, 349-355, 2006. 

The aqueous biphasic hydroformylation concept is ineffective with higher olefins owing to mass transfer limitations posed by their low solubility in water. Several strategies have been employed to circumvent this problem [22], e.g. by conducting the reaction in a monophasic system using a tetraalkylammonium salt of tppts as the ligand, followed by separation of the catalyst by extraction into water. Alternatively, one can employ a different biphasic system such as a fluorous biphasic system or an ionic liquid/scC02 mixture (see later). 

Ionic liquids are quite simply liquids that are composed entirely of ions [96, 97]. They are generally salts of organic cations, e.g. tetraalkylammonium, alkylpyridi-nium, 1,3-dialkylimidazolium, tetraalkylphosphonium (Fig. 7.28). Room temperature ionic liquids exhibit certain properties which make them attractive media for performing green catalytic reactions. They have essentially no vapor pressure and are thermally robust with liquid ranges of e.g. 300 °C, compared to 100 °C for water. Polarity and hydrophilicity/hydrophobicity can be tuned by a suitable combination of cation and anion, which has earned them the accolade, designer solvents . 

A number of different ionic liquids have been screened in the ruthenium-catalysed oxidation of secondary alcohols (see Scheme 5.18). Three different ruthenium compounds, RuC13, RuCl2(PPh3)3 and [RuCFl/i-cymene) were compared and best results were obtained with RuCl2(PPh3)3.[76] While imidazolium-based ionic liquids gave only poor results (anion = Cl) or suppressed the reaction completely (anion = [BF4] or [PF6] ), tetraalkylammonium-based solvents such as Aliquat 336 (tricaprylmethylammonium chloride) or tetramethylammonium hydroxide afforded much better yields. 

Various bismuth(III) salts have also been found to act as catalysts for the acylation of benzoylchloride.1 1 Complete conversion could be achieved within five minutes with 10 mol% Bi203 or Bi(OTf)3, though at a relatively high reaction temperature of 150°C. The product was obtained quantitatively with Bi(OTf)3 even at low catalyst loadings of 1 mol%, but reaction times of several hours were required under such conditions. As previously mentioned, catalytic activity is often highly sensitive to the nature of the ionic liquid. In this case, tetraalkylammonium and phosphonium cations led to much lower reaction rates while hydrophilic anions, explicitly BF4 and OTf. led to poor levels of conversion. Recycling of the catalyst solution was possible after either extraction or distillation of the product in vacuo. 

Most ions constituting ionic liquids can be categorized according to their Lewis acid/base properties (i.e., their capability to accept or to donate an electron pair) nevertheless, some ions may be considered according to the Bronsted definition, i.e., on the basis of their ability to accept or donate a proton. Typical ionic liquids are those based on neutral or very weakly basic anions (BF4, PF, NOf, CHsSO, 4 f2N ) and neutral (tetraalkylammonium, dialkyl-pyrrolidium, trialkylsulfonium) or weakly acidic cations (1,3-dialkylimidazolium and 1,2,3-trialkylimidazolium) (Figure 4.1). 

Abstract This chapter discusses the potential usefulness of ionic liquids with respect to biocatalysis by illustrating the stability and activity of enzymes in ionic liquids in the presence or absence of water. Ionic liquids are a class of coulombic fluids composed of organic cations like alkyl-substituted imidazolium, pyrrolidin-ium, and tetraalkylammonium ions and anions such as halides, tetrafluoroborates, hexafluorophosphates, tosylates, etc. The possibility of tunable solvent properties by alternation of cations and anions has made ionic liquids attractive to study biocatalysis which warrants an understanding of enzyme stability and activity in ionic liquids. This chapter systematically outlines the recent studies on the stability of enzymes and their reactivity toward a wide range of catalytic reactions. A careful approach has been taken toward analysis of relationship between stabil-ity/activity of enzymes versus chaotropic/kosmotropic nature of cations and anions of ionic liquids. 

The important families of cations that are present in ionic liquids are alkylpyTidinium ions (8.19), dialkylimidazolium ions (8.20), tetraalkylammonium ions (8.21) and tetraalkyl-phosphonium ions (8.22). 

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