Handbook of Solvents - George Wypych - ChemTech - Ventech!

1470 D.W. Rooney, K.R. Seddon
Chauvin et al. reported the asymmetric hydrogenation of acetamidocinnamic acid 83 to
(S)-phenylalanine with a cationic chiral rhodium catalyst in [C 4-mim][SbF 6] ionic liquid,
more recently the 2-arylacrylic acid has been produced with a reasonable 64% yield 84 using
a chiral ruthenium catalysts in [C 4-mim][BF 4] ionic liquids. Palladium catalysts 85 immobilized
in an ionic liquid-polymer gel membrane 86 containing either [C 2-mim][CF 3SO 3]or
[C 2-mim][BF 4] have also been reported as catalysts for heterogeneous hydrogenation reactions.
Fuller et al have also reported the hydroformylation 86 of pent-1-ene in [C 4-mim][PF 6]
using a rhodium catalysts showing both a high catalytic activity and product separation as a
second organic phase. However it was observed that a small quantity of the neutral catalyst
leached into the organic phase.
Other commercially important reactions include the hydrodimerization of 1,3-butadiene
to octa-2,7-dien-1-ol 87,88 carried out using palladium catalysts in [C 4-mim][BF 4]. The
catalyst precursor [Pd(mim) 2Cl 2] was prepared in situ from an imidazolium
tetrachloropalladate(II) salt, [C 4-mim] 2[PdCl 4], dissolved in the ionic liquid solvent. The reaction
proceeds in a liquid-liquid two-phase system, where the products separate from the
catalytic reaction mixture as a separate layer on cooling.
Ionic liquids have been demonstrated as effective solvents for Diels-Alder
reactions 89-91 where they have shown significant rate enhancements as well as high yields
and selectivities when compared with the best results obtained in conventional solvents. To
date, the biggest developments in Diels-Alder chemistry have come through reactions in
Li[ClO 4]-Et 2O, where the high electrolyte concentrations are cited as beneficial through
“salt-effects” and the high internal pressure of the solvent. However the use of such mixtures
of perchlorate salts with organic molecules could cause a number of hazards when
used on an industrial scale. Hence there is considerable potential for ionic liquids in this
area.
The stability of the neutral ionic liquids allows them to be used in environments unsuitable
to the tetrachloroaluminate(III)
based ionic liquids. As such they offer considerable
advantages allowing them to be
used with “wet” process streams. The use of
the fluorinated anions as shown in Figures
21.2.6 and 21.2.7, i.e., [PF 6] and [BF 4], introduces
the concept of hydrophilic ionic liquids
which are partially immiscible with
water. In fact all [PF 6] liquids are found to be
immiscible with water and all [BF 4] liquids
with chain lengths greater than C 4 will form
two separate phases with water at sufficient
concentrations. This property has prompted
investigation into the application of these
solvents as extraction solvents for a number
of materials. Rogers et al. 92 has studied the
partition of benzene with water and has re-
Figure 21.2.10. The phase preference of the three forms
of thymol blue in [C 4-mim][PF6]. 93
cently studied the relationship between pH
and extraction efficiency. 93 Figure 21.2.10 il-