Handbook of Solvents - George Wypych - ChemTech - Ventech!

21.2 Ionic liquids 1469
One specific example is the alkylation of benzene with chloroethane which gives a
mixture of mono- to hexa-substituted products. The ionic liquid solvent/catalyst activates
the reaction and the alkylation can be performed even at temperatures as low as -20°C in the
ionic liquid solvent. Again it was shown that the products have a low solubility in the ionic
liquid leading to facile separation.
Many acylation reactions have been demonstrated in acidic chloroaluminate(III) ionic
liquids liquids. 70,74 However as described above these processes are essentially non-catalytic
in aluminum(III) chloride which necessitates destroying the ionic liquid catalyst by
quenching with water to extract the products. However, regioselectivity and reaction rates
observed from acylation reactions in ionic liquids are equal to the best published results.
The Friedel-Crafts acylation of benzene has been shown to be promoted by acidic
chloroaluminate(III) ionic liquids. 74 It was observed that the acylated products of these reactions
have high selectivities to a single isomer for example toluene, chlorobenzene and
anisole are acylated in the 4-position with 98% specificity. Naphthalene is acylated in the
1-position which is the thermodynamically unfavored product. In addition to benzene and
other simple aromatic rings, a range of organic and organometallic substrates (e.g.,
ferrocene) 79,80 have been acylated in acidic chloroaluminate(III) ionic liquids.
21.2.3.2 Reactions in neutral or second generation ionic liquids
Acidic [C2-mim]Cl-AlCl3 mixtures have been shown above to be very useful catalytic solvents
for a number of industrially relevant reactions. However, reactions catalyzed by aluminum
trichloride, whether used in an ionic liquid or as a solid phase reactions have one
significant disadvantage, they are air and moisture sensitive. Using such liquids within an
industrial process will therefore require that reactants are kept as dry as possible, adding to
the overall process cost. We have seen how the activity and properties of the liquid can be
readily controlled by both changing the anion or the cations present. By switching to neutral
ionic liquids containing for example [BF4] - , [PF6] - and [SbF6] - anions, the reactive polymerization
and oligomerization reactions of olefins catalyzed by acidic anions is not observed
and more controlled, specific reactions can be catalyzed.
Since these ionic liquids cannot support the existence of reactive Lewis acid conjugate
anions (such as [Al2Cl7] - i.e. there is no analogous mechanism to support 2[BF4] - ↔ [B2F7] -
+F - ), they are much less reactive when used as solvents. Under these conditions, many conventional
transition metal catalysts can be utilized. Again modification of the ionic liquid
solvents allows the potential to immobilize catalysts, stabilizing the active species and to
optimize reactant/product solubilities and to permit facile extraction of products. In general
it has been found that charged, especially cationic transition metal complexes are most effectively
“immobilized” in the ionic liquid solvents. In addition ionic liquids can immobilize
less complex catalysts than two-phase aqueous-organic systems, where expensive,
unstable, synthetically challenging ligands are often required.
Hydrogenation of olefins catalyzed by transition metal complexes dissolved in ionic
liquid solvents have been reported using rhodium-, 81 and ruthenium- and cobalt-containing
catalysts. 82 In these studies it was shown that hydrogenation rates where up to five times
higher than the comparable reactions when carried out in propanone. The solubilities of the
alkene reagents, TOFs, and product distributions where all strongly influenced by the nature
of the anion in the ionic liquid solvent.