Mehdi et al.
Although most cerium(IV)-mediated organic reactions require
use of at least stoichiometric amounts of cerium(IV) salts, few
examples of catalytic reactions based on cerium(IV) have been
described. Cerium(IV) can be a redox mediator in electrosyn-
thesis17 or catalytic amounts of cerium(IV) salts can be used in
combination with co-oxidants like sodium bromate18,19 or
potassium persulfate.20 A special type of catalytic reaction is
the cerium-mediated oxidation reaction by molecular oxygen
in which cerium(IV) very likely acts as an intermediate.21-23
The relatively high molecular mass of cerium(IV) salts is a
disadvantage for stoichiometric reactions, especially because
cerium(IV) salts are only one-electron donors. However, these
salts can be regenerated by either chemical oxidation (for
instance, with K2S2O8 or O3) in acidic media7 or electrochemical
oxidation.14 Another problem that hampers the applicability of
cerium(IV) salts in organic transformations is their poor
solubility in organic solvents. In contrast, many cerium(IV) salts
are very soluble in water, but the presence of an acid is required
to avoid hydrolysis of the cerium(IV) ions.24,25 Moreover,
organic substrates are often poorly soluble in water. Micellar
systems can be used to overcome the solubility problems of
the organic reactants, but separation of the products can be
cumbersome.26,27 Another possibility is to use hydrophobic
cerium(IV) reagents28 or work under phase-transfer conditions.29
Many different solvents have been used for cerium(IV)-mediated
reactions, but the most popular solvents besides water are
acetonitrile, dichloromethane, THF, and methanol.7 It is common
practice to choose a solvent in which the organic substrate is
soluble but the cerium(IV) salt is not, so that the reactions are
carried out under heterogeneous conditions. Of course, the
insolubility of the cerium(IV) salts in the reaction medium has
a negative effect on the reactivity.
FIGURE 1. Overview of the ionic liquids studied as solvents for
cerium(IV)-mediated oxidation reactions.
liquids have not been explored for reactions with cerium(IV)
salts yet, although several examples of rare-earth-mediated
organic reactions in ionic liquids have been described.39-44 It
should be noted that ionic liquids combined with an excess of
DMSO have been investigated as a solvent for cerium(IV)-
mediated reactions.45
In this paper we evaluate various ionic liquids as solvents
for cerium(IV)-mediated organic reactions (Figure 1). First, the
solubility of different cerium(IV) salts in ionic liquids has been
tested. Second, we studied the oxidation of benzyl alcohol to
benzaldehyde by in-situ spectroscopic techniques (NMR, FTIR)
to obtain some insight in the mechanisms involved. Finally, the
reaction was extended to other substrates as well.
Results and Discussion
Solubility Tests. In order to find the best combination of a
cerium(IV) salt and an ionic liquid that could provide a
homogeneous reaction system, the solubility of cerium(IV) salts
in different ionic liquids was tested. The following cerium(IV)
salts were selected: ammonium hexanitratocerate(IV), cerium-
(IV) sulfate dihydrate, cerium(IV) ammonium sulfate, cerium-
(IV) ammonium sulfate dihydrate, cerium(IV) hydroxide, cerium-
(IV) triflate, and hydrated cerium(IV) triflate. The ionic liquids
include several 1-alkylimidazolium and 1-alkyl-3-methylimi-
dazolium salts combined with trifluoromethanesulfonate (triflate,
TfO), bis(trifluoromethylsulfonyl)imide (Tf2N), hydrogen sul-
fate, ethylsulfate, and nitrate anions (Figure 1). Hexafluoro-
One can wonder whether ionic liquids are useful solvents
for cerium(IV)-mediated organic transformations, taking into
account the popularity of ionic liquids as alternative solvents
for organic reactions.30-38 To the best of our knowledge, ionic
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