Handbook of Solvents - George Wypych - ChemTech - Ventech!

1468 D.W. Rooney, K.R. Seddon
other polymerization processes. Polyisobutene, traditionally prepared by the Cosden process,
is a valuable lubricant, and also a route to higher value-added materials. In general it
was observed that the catalytic activity of the ionic liquids increases towards higher degrees
of polymerization from short-chain oligomers as the alkylchain length of the
1-alkyl-3-methylimidazolium or N-alkylpyridinium cation is increased. 63
The ionic liquid process has a number of significant advantages over the industrial
Cosden process. This system uses a supported or liquid phase aluminum(III) chloride catalyst.
63 Using the ionic liquid process, the polymer forms a separate layer, which is substantially
free of catalyst and ionic liquid solvent. This effect greatly enhances the degree of
control available to reduce undesirable secondary reactions (i.e., isomerization) without requiring
alkali quenching of the reaction.
In addition Ziegler-Natta polymerization reactions have also shown some success
when carried out in ionic liquids. 64 The most common production methods for this form of
polymerization involve the use of triethylaluminium catalysts at ca. 100°C and 100 atmospheres
pressure. Advances have been developed through the use of organometallic transition
metal catalysts, typically nickel or titanium. Given the solvent characteristics of ionic
liquids it should be possible to effectively immobilize such catalysts in an ionic liquid solvent.
Indeed, Carlin and Wilkes 64 have reported the Ziegler-Natta polymerization of ethene
in an ionic liquid solvent. In these reactions an acidic [C 2-mim]Cl-AlCl 3 ionic liquid solvent
was used to support dichlorobis(η 5 -cyclopentadienyl)titanium(IV) with an alkylchloroaluminium(III)
co-catalyst.
Electrophilic substitution 65 and other reactions of naphthalenes (alkylation, acylation,
condensation and migration in acidic ionic liquids 66,67 have been reported. Anthracene undergoes
photochemical [4+4] cycloaddition reactions 68,69 in acidic chloroaluminate(III)
ionic liquids. One interesting study included a one-pot synthesis of anthraquinone from
benzene giving a 94% yield. In general a much wider range of redox products are formed
than occur in conventional solvents; the strong Brønsted acidity of the ionic liquid induces
protonation of anthracene, by residual traces of HCl, to form an anthracenium species which
couples readily via photochemically driven electron transfer mechanisms.
Both the Friedel-Crafts alkylations and acylations are of great importance to the fine
chemical and pharmaceutical industries. Typically, these reactions are run in an inert solvent
with suspended or dissolved aluminum(III) chloride as a catalyst, and may take six
hours and go only to 80% completion giving a mixture of isomeric products. In addition,
there are a number of problems, especially with misnamed “catalytic” Friedel-Crafts
acylation reactions, which are actually stoichiometric, consuming 1 mole of AlCl 3 per mole
of reactant. Annually, massive quantities of aluminum(III) chloride are consumed in these
reactions causing a number environmental problems through waste disposal. Both
alkylation and acylation reactions under Friedel-Crafts conditions have been demonstrated
using chloroaluminate(III) ionic liquids as both solvent and catalysts. 66-77 Here it has been
shown that reaction rates are much faster with total reagent conversion and often with surprising
specificity to a single product.
Boon et al. have reported the alkylation of benzene with a wide number of alkyl
halides in acidic chloroaluminate(III) ionic liquids 73 and general organic reactions in low
melting chloroaluminate ionic liquids have also been described, 75,78 which include
chlorinations and nitrations in acidic ionic liquids. 72,73,76