N. Clousier et al. / C. R. Chimie 14 (2011) 680–684
681
chitosan supported ionic liquid [4] phase and demon-
2.4. General procedure for the Pd-catalysed allylic
strated their high activity and efficient recycling in the
benchmark Tsuji-Trost allylation reaction, one of the most
important reactions for carbon-carbon and carbon-het-
eroatom bonds formation [5,6]. In order to evaluate the
influence of the chemical structure of the support on the
catalytic properties of the SILC, we have extended our
work to another biopolymer: the alginates. In this article,
the preparation of new supported ionic liquid phase
catalysts based on alginate is reported and the activity of
these new catalytic materials is compared with that
obtained with chitosan-supported ionic liquid phase
catalysts.
substitution of (E)-1,3-diphenyl-3-acetoxyprop-1-ene with
dimethyl malonate
(E)-1,3-diphenyl-3-acetoxyprop-1-ene (100 mg, 0.40
mmol, 1 equiv.), AcOK (0.02 mmol, 0.05 equiv.), N,O-
Bis(trimethylsilyl)acetamide (BSA) (1.19 mmol, 3 equiv.)
and dimethyl malonate (1.19 mmol, 3 equiv.) were added
to the biopolymer-SILC previously prepared. The mixture
was stirred at room temperature under inert atmosphere
and the reaction monitored by TLC. Products were
extracted with 10 ꢁ 1 mL of ether for chitosan and
15 ꢁ 1 mL of ether for alginate (monitored by TLC). The
combined organic layers were washed with water, dried
over MgSO4, filtered and concentrated under reduced
pressure. Then, the obtained residue was purified by
column chromatography on silica gel (petroleum ether/
ether: 9/1). After the extraction of the product, the
biopolymer-SILC was dried under vacuum and reused
for another catalytic cycle.
2. Experimental
2.1. General comments
The ionic liquid [bmim][BF4] was supplied by
Solvionic (Toulouse, France). All commercially available
compounds were used as received. Thin layer chroma-
tography was carried out on silica gel 60 F254 (1.1 mm,
Merck) with spot detection under UV light or through
KMnO4 oxidation. Chromatographic separations were
achieved on Merck silica gel column (Geduran Si 60,
0.040–0.063 nm).
3. Results and discussion
Alginates are produced by brown algae and mainly
consist of (1! 4) linked
b-D-mannuronic (M) and a-L-
guluronic residues (G) (Fig. 1a). Alginates differ one from
another only by their M/G ratio. The use of alginates as
catalytic support is recent and mainly lies in their ability to
form heat-stable strong gel with divalent cations, espe-
cially Ca2+ ([7] and references cited therein). They have
been largely used for the entrapment of biologically active
materials ([8] and references cited therein).
2.2. Preparation of biopolymer beads
2.2.1. Alginate beads
Sodium alginate (Acros) was dissolved in distilled water
at a concentration of 1% (w/w). The polymer solution was
added dropwise at room temperature to the stirred CaCl2
solution (0.25 M) using a syringe. The beads were cured in
the gelation solution for 3 h. They were rinsed with water
and freeze-dried.
Chitosan, which consists of 2-amino-2-deoxy-(1-4)-a-
D-glucopyranose residues (D-glucosamine units) with no
or a small amount of N-acetyl-D-glucosamine units, is
characterised by its strong affinity toward transition metal
(Fig. 1b) [9]. This biopolymer, which is derived mainly from
the shells of crustaceans is produced in considerable
amounts each year. Chitosan has already been exploited in
heterogeneous catalysis for the reduction of nitro-aromat-
ic derivatives [10], cyclopropanation of olefins [11], suzuki
and heck reactions [12].
Supported ionic liquid phase catalysts (SILC) based on
alginate and chitosan supports were prepared as follow: a
thin film of ionic liquid containing the homogeneous
catalyst was immobilized on the surface of the biopolymer
support (Fig. 2).
2.2.2. Chitosan beads
1 g of chitosan (Fluka) characterised by an average
molar mass of 330,000 g.molꢀ1 (determined by viscosi-
metry) and a degree of deacetylation determined by 1H
NMR of 80%, was dissolved in 100 mL of a 0.2% HCl solution.
After complete dissolution, the solution was filtered on a
Bu¨chner, and dropped into a NaOH solution (0.25 mol.Lꢀ1
)
through a syringe needle. Chitosan beads were rinsed with
water until reaching water conductivity. Beads were then
freeze-dried.
Typically, the catalytic materials were prepared by
impregnation (physisorption) of lyophilized biopolymer
beads by a [bmim][BF4] phase containing 0.05 equivalent
of the palladium catalyst (Pd(OAc)2) and 0.2 equivalent of
2.3. Preparation of biopolymer-SILC
Predegassed [bmim][BF4] was added to Pd(OAc)2
(0.02 mmol, 0.05 equiv.), trisulfonated triphenyl phos-
phine trisodium salt (TPPTS ligand) (0.079 mmol, 0.2
equiv.) and the biopolymer beads (100 mg of chitosan or
50 mg of alginate). An optimum amount of 0.5 mL of
[bmim][BF4], which corresponds to the maximum quantity
of IL which can be absorbed by the beads, was used for
chitosan-SILC and 0.3 mL for alginate-SILC. The resulting
mixture was stirred 30 min at room temperature under
inert atmosphere.
phosphine ligand with a ratio biopolymer weight/IL
volume of 50 mg/0.3 mL and 100 mg/0.5 mL for alginate
and chitosan, respectively. The amphiphilic nature of the
polysaccharides presumably accounts for the great immo-
bilization of the ionic liquid phase. The ionic ligand TPPTS
(P(m-C6H4SO3Na)3) was selected in order to favour the
anchoring of the catalyst in the ionic liquid phase. After
stirring at room temperature for 30 minutes, the biopoly-
mer-SILC was ready to be used in the palladium catalysed
allylic substitution. Alginate and chitosan based catalytic