Palladium containing nanostructured silica functionalized with pyridine sites: a versatile heterogeneous catalyst for Heck, Sonogashira, and cyanation reactions

Article abstract of DOI:10.1016/j.tet.2007.04.073

Nanostructured hybrid silica bearing pyridine binding sites was prepared in a template assisted hydrolysis-polycondensation of tetraethylorthosilicate (TEOS) and the ionic precursor N,N-dimethyl-pyridin-4-yl-(3-triethoxysilyl-propyl)-ammonium iodide using N-dodecyl-N′-methyl-imidazolium bromide as structure directing agent. After treatment with palladium acetate, the material appeared as a versatile heterogeneous catalyst for Heck, Sonogashira, and cyanation reactions. In Heck and Sonogashira cross-coupling reactions, unchanged catalytic activity was observed in at least five reaction cycles.

Full text of DOI:10.1016/j.tet.2007.04.073

Tetrahedron 63 (2007) 6784–6790
Palladium containing nanostructured silica functionalized with
pyridine sites: a versatile heterogeneous catalyst for Heck,
Sonogashira, and cyanation reactions
*
Vivek Polshettiwar, Peter Hesemann and Joel J. E. Moreau
*
€
´
UMR 5253, Institut Charles Gerhardt, Ecole Nationale Superieure de Chimie de Montpellier, 8, rue de l’Ecole Normale,
34296 Montpellier Cedex 5, France
Received 29 January 2007; revised 17 April 2007; accepted 23 April 2007
Available online 4 May 2007
Abstract—Nanostructured hybrid silica bearing pyridine binding sites was prepared in a template assisted hydrolysis–polycondensation of
tetraethylorthosilicate (TEOS) and the ionic precursor N,N-dimethyl-pyridin-4-yl-(3-triethoxysilyl-propyl)-ammonium iodide using N-dode-
cyl-N0-methyl-imidazolium bromide as structure directing agent. After treatment with palladium acetate, the material appeared as a versatile
heterogeneous catalyst for Heck, Sonogashira, and cyanation reactions. In Heck and Sonogashira cross-coupling reactions, unchanged
catalytic activity was observed in at least five reaction cycles.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
by significant Pd-metal agglomeration. A modified system
reported by Hagiwara et al. consisting of Pd(OAc)2 immo-
bilized in a silica supported ionic liquid phase showed
improved properties in terms of metal agglomeration as the
ionic liquid acts as a protective shell for the catalyst.9 How-
ever, a small part of the ionic liquid and the metal catalyst
may dissolve in the reaction medium during the reaction
especially when higher temperatures are required, and may
lead to the formation of inactive Pd black.10 Crudden’s mer-
captopropyl-modified mesoporous silica containing molecu-
lar Pd species elegantly combines high chemical stability,
excellent catalytic activity, and negligible metal leaching.11
The formation of carbon–carbon bonds via palladium-cata-
lyzed cross-coupling reactions plays a crucial role in syn-
thetic organic chemistry. Particular attention has been paid
to the coupling reaction of aryl halides with alkenes and al-
kynes, commonly called Heck1 and Sonogashira2 reaction,
respectively. These reactions are generally catalyzed by sol-
uble Pd complexes. However, the efficient separation and
subsequent recycling of homogeneous transition metal cata-
lysts remains a scientific challenge and an aspect of econom-
ical and ecological relevance. Several strategies for catalyst
recycling have been explored, including aqueous phase ca-
talysis,3 fluorous phase catalysis,4 the use of ionic liquids,5
and colloidal dispersions,6 but in particular solid supported
catalysts,7 both on organic7f and inorganic support.7g Unfor-
tunately, the utility of some of these concepts is often limited
by metal leaching or catalyst deterioration.
We are currently investigating functionalized silica contain-
ing ionic substructures.12 In an earlier communication,12b
we showed that the use of various ionic trialkoxysilylated
precursor molecules led to modified structural features of
the obtained functionalized silica materials obtained by
nanocasting, particularly depending on the constitution of
the used organic precursor. In continuation of our work on
silica based materials for catalysis,13 we report here nano-
structured hybrid silica bearing pyridine binding sites, which
can be used as heterogeneous ligands for the immobilization
of palladium(II) species through metal–ligand interaction.
These functionalized nanostructured silica materials bearing
both ammonium substructures and pyridine binding sites
may be particularly suitable systems for the immobilization
of transition metal catalysts, as the stability of the immobi-
lized metallic species can be enhanced by synergetic effects
involving (i) metal–ligand interactions and (ii) ionic
In the area of heterogeneous catalysts for Heck reactions,
Ying et al. recently reported palladium nanoparticles sup-
ported on mesoporous silica, which showed good activity
for the coupling reaction of various aryl halides with styrene
or butylacrylate.8 Recycling of this catalyst was hampered
Keywords: Nanostructured hybrid silica; Heck reaction; Sonogashira reac-
tion; Cyanation; Imidazolium salts.
* Corresponding authors. Tel.: +33 467147217; fax: +33 467147212 (P.H.);
tel.: +33 467147211; fax: +33 467147212 (J.J.E.M.); e-mail addresses:
peter.hesemann@enscm.fr; joel.moreau@enscm.fr
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.04.073

V. Polshettiwar et al. / Tetrahedron 63 (2007) 6784–6790
6785
interactions involving the ionic species covalently grafted on
the surface of the silica support and the ionic metal species.
In our study, we used the Pd-containing silica based material
as catalyst in Heck and Sonogashira reaction as well as in
cyanation of aryl iodides.
2. Results and discussion
2.1. Precursor synthesis
The trialkoxysilylated precursor N,N-dimethyl-pyridin-4-
yl-(3-triethoxysilyl-propyl)-ammonium iodide was easily
synthesized by heating 4-(N,N-dimethylamino)pyridine
(DMAP) and 3-iodopropyltriethoxysilane in acetonitrile in
1
a Schenk tube under inert atmosphere. The H NMR spec-
trum of the resulting product shows a singlet at d¼3.24 ppm
characteristic for methyl groups linked to quaternary nitro-
gen atoms. This result indicates that the reaction exclusively
yields the ammonium salt and keeps the pyridine site intact,
which was essential for subsequent complexation with
Pd(OAc)2 (Scheme 1).
Figure 1. SEM image of hybrid silica catalyst I-Pd.
Electronic microscopy gave information about the morphol-
ogy of the synthesized Pd containing nanostructured hybrid
silica I-Pd. The SEM image shows agglomerates of particles
with diameters in the range of several hundreds of nanometer
up to a micrometer (Fig. 1). X-ray fluorescence experiment
with the material gave a Si/Pd molar ratio of 95.2:4.8. As the
materials were synthesized using a 9:1 ratio of TEOS and the
ionic precursor, this result indicates a 2:1 ratio of the immo-
bilized pyridine substructures and supported Pd species. The
complete incorporation of the organic substructure in the
material was also confirmed by elemental analysis.
2.2. Materials’ synthesis
The functionalized nanostructured silica was prepared by
nanocasting14 using N,N0-dodecyl-methyl-imidazolium bro-
mide as structure directing agent. Imidazolium based ionic
liquids recently appeared as efficient templates for nanocast-
ing techniques.15 We chose an imidazolium salt as structure
directing agent as the interaction between the ammonium
precursor bearing pyridine substructures and imidazolium
salts may be more favorable than that between the ionic pre-
cursor and surfactants based on ammonium substructures
such as CTAB.16 Hydrolysis–polycondensation of the pre-
cursor with tetraethylorthosilicate (TEOS) led to the forma-
tion of nanostructured silica I functionalized with ionic
pyridine binding sites. After removal of the template by
washing, the heterogeneous catalyst was obtained by subse-
quent treatment with Pd(OAc)2. The schematic illustration
of the synthesis of the catalytic material I-Pd is given in
Scheme 2.
The TEM micrograph of some smaller particles of the func-
tionalized silica catalyst I-Pd (Fig. 2) clearly indicates nano-
structured morphology of the material. This material appears
of high regularity, as all of the observed particles show high
degree of structuration.
Nitrogen adsorption–desorption experiment with the
palladium containing material I-Pd resulted in a typical
type IV isotherm. Nitrogen sorption experiments of the
catalyst showed a BET surface area of 154 m2/g and
Acetonitrile
80 °C, 12 h
N
N
N
N
Si(OEt)3
I
Si(OEt)3
I
Scheme 1. Synthesis of N,N-dimethyl-pyridin-4-yl-(3-triethoxysilyl-propyl)-ammonium iodide.
O
O
O
N
N
O
N
X-
N
(EtO)3Si
1
Si
Si
HO
HO
MeImC12 Br
H2O
I-
X-
Pd(OAc)2
N
N
N
N
+
: [PdX2]
X-
N
X-
HO
O
HO
O
Si
Si
9 (EtO)4Si
N
O
O
I
I-Pd
Scheme 2. Synthesis of the catalytic material I-Pd by nanocasting using N,N-dodecyl-methyl-imidazolium bromide.

6786
V. Polshettiwar et al. / Tetrahedron 63 (2007) 6784–6790
covalently linked to the silica support. The material appears
of high textural homogeneity, which is an important feature
in the view of the elaboration of materials showing high sta-
bility and long-lasting catalytic activity.
2.3. Catalytic tests
In order to explore the catalytic properties of this nanostruc-
tured silica I-Pd, we studied the utilization of this material as
heterogeneous catalyst in Heck, Sonogashira, and cyanation
reactions.
2.3.1. Heck reactions. The Heck coupling reaction (Scheme
3) was performed using a variety of substrates. Firstly, the re-
action conditions were optimized using iodobenzene and
methyl acrylate as a substrate. After screening a range of
usual inorganic and organic bases and exploring the scope
of various solvents, we found that the catalyst is most effi-
cient for Heck reaction using triethylamine as a base and
acetonitrile as a solvent. The efficiency of this catalyst was
studied in the Heck reaction of various aryl halides and
alkenes under optimized reaction conditions. The results
are summarized in Table 1.
Figure 2. TEM image of hybrid silica catalyst I-Pd.
a pore volume of 0.366 cm3/g. The average pore size can be
estimated to be 3.5 nm.
The material was also characterized by solid state MAS
NMR spectroscopy. The 29Si CP-MAS spectra (Fig. 4)
show two characteristic signals. The signal at ꢀ100 and
ꢀ109 ppm can be assigned to Q3 and Q4 sites corresponding
to SiO4 substructures of different condensation degree. The
signal at ꢀ66 ppm can be assigned to major T3 sites for
RSiO3 units possessing three siloxane bridges. In order to
identify organic functionalities attached to the silica support,
X
Catalyst I-Pd
R
a
13C CP-MAS experiment was performed. The spectrum
Y
R
shows signals at 24, 40, 60, 108, 142 and 157 ppm, similar
to the chemical shifts found in the liquid 13C NMR spectrum
of the silylated organic precursor molecule (Fig. 3).
Y
R - H, alkyl, aryl
X - Halide
Y - CO2CH3, C6H5
Scheme 3. Heck coupling reaction using nanostructured hybrid silica cata-
lyst I-Pd.
In consequence, the synthesized material incorporating pal-
ladium species is a mesoporous silica with nanostructured
morphology functionalized with pyridine binding sites
We found that aryl iodides and -bromides were efficiently
coupled with methyl acrylate and styrene to give the corre-
sponding 1,2 substituted alkenes in good to excellent yields.
Unfortunately, reactivity toward chloroarenes, which was
observed by others,17 was not achieved with our catalyst sys-
tem (entries 3 and 6). The reactions with 2-iodothiophene
were successfully achieved (entries 7 and 8), providing a
useful way for introduction of unsaturated group on the thio-
phene ring. Our efforts for employing 2-bromopyridine
(entries 11 and 12) produced low product yields.
39.8
107.9
156.5
142.2
140
23.8
20
60.1
60
*
*
*
180
160
120
100
80
40
0
2.3.2. Sonogashira reactions. Similarly, the nanostructured
material I-Pd was used as catalyst in the Sonogashira cou-
pling reaction (Scheme 4). Here, the reaction conditions
were optimized for the cross-coupling of iodobenzene with
phenyl acetylene as the model reaction. Using acetonitrile
as a solvent and triethylamine as a base, the material showed
good catalytic activity and led to various disubstituted
alkynes in 65–80% yield (Table 2).
ppm
Figure 3. 13C CP-MAS NMR spectrum of material I-Pd (spinning side
bands are marked with asterisks).
-108.8
-99.8
I
-66.4
Catalyst I-Pd
Z
Z
R
R
R - H, aryl
Z - C6H5, 4-Me-C6H5
100 80 60 40 20
0
-20 -40 -60 -80 -100-120-140-160-180-200
ppm
Scheme 4. Sonogashira coupling reaction using nanostructured hybrid silica
catalyst I-Pd.
Figure 4. 29Si CP-MAS NMR spectrum of material I-Pd.

V. Polshettiwar et al. / Tetrahedron 63 (2007) 6784–6790
Table 1. Heck reaction of aryl halides and alkenes using catalyst I-Pda Table 2. Sonogashira reaction using catalyst I-Pda
6787
Entry
1
Aryl halide
Product
Yield (%)b
Entry
1
Aryl halide
Product
Yield (%)b
80
I
90
I
CO2Me
2
3
82
65
I
2
82
Br
CO2Me
CO2Me
S
I
S
No reactivity
detected
3
4
5
Cl
I
S
I
S
4
5
70
70
77
70
I
Br
I
No reactivity
detected
6
7
Cl
6
71
S
S
S
I
93
84
CO2Me
CO2Me
a
Reaction was carried out using 1 mmol of aryl halide, 1.5 mmol of alkene,
2 mmol of triethylamine, and 500 mg (0.1 mmol as Pd(OAc)2) catalyst in
refluxed acetonitrile.
S
I
8
9
b
Yield was determined by GC with dodecane as internal standard with
respect to aryl halide, and was confirmed by 1H NMR spectroscopy.
I
80
79
the cyanation reaction under homogeneous condition.20
However, homogeneous catalysts suffer from the problems
associated with the difficulty of catalyst recovery, separa-
tion, and deactivation via the formation of Pd aggregates
in situ. In this field, the use of heterogeneous catalyst sys-
tems can be advantageous.
I
10
11
12
13
12
10
87
N
CO2Me
CO2Me
The cyanation reaction (Scheme 5) was performed using a
variety of substrates. Firstly, the reaction conditions were op-
timized using iodobenzene as a substrate. We found that the
heterogeneous Pd-catalyst is most efficient for cyanation re-
action in presence of triethylamine as a base and DMF as
a solvent. Using optimized reaction conditions, the efficiency
of this catalyst was studied for cyanation reaction of various
aryl halides. The results are summarized in Table 3.
Br
Br
N
N
N
I
14
87
I
I
CN
Catalyst I-Pd
R
K4[Fe(CN)6], DMF
R
a
Reaction was carried out using 1 mmol of aryl halide, 1.5 mmol of alkene,
2 mmol of triethylamine, and 500 mg (0.1 mmol as Pd(OAc)2) catalyst in
refluxed acetonitrile.
Where, R-H, Alkyl, Aryl
b
Yield was determined by GC with dodecane as internal standard with
Scheme 5. Cyanation reaction using nanostructured hybrid silica catalyst
I-Pd.
respect to aryl halide, and was confirmed by 1H NMR spectroscopy.
2.3.3. Cyanation reactions. We finally explored the utility
of this nanostructured silica I-Pd for cyanation of aryl
iodides. For many years, the only method for cyanation of
an aryl halide required stoichiometric CuCN and harsh reac-
tion conditions.18 Since the discovery of Pd-catalyzed cyan-
ation reactions allows the conversion of aryl halides to
nitriles,19 more selective methods have been reported. Palla-
dium-catalyzed cyanation reactions are particularly attrac-
tive due to their functional group tolerance, air stability,
and high catalytic activity. K4[Fe(CN)6], recently reported
as a cyanide source by Beller et al., is particularly intriguing
because it is inexpensive, easily handled, and nontoxic. Fur-
thermore, all cyano groups of the complex are available for
It appeared that aromatic iodo compounds were efficiently
reacted to give cyanated products in good yields (entries 1,
5, and 8). Reactions with 2-iodothiophene were also success-
fully achieved (entry 4), providing a useful way for introduc-
tion of cyanide groups on thiophene ring. However, the
catalyst shows no activity toward aryl bromides or chlorides
(entries 2 and 3), which can be used for selective cyanation
of iodo groups keeping bromo or chloro functionalities intact
(entries 6 and 7).
2.3.4. Recycling experiments. For practical applications of
heterogeneous systems, the lifetime of the catalyst and its

6788
V. Polshettiwar et al. / Tetrahedron 63 (2007) 6784–6790
Table 3. Cyanation reaction of aryl iodides using catalyst I-Pda
Table 5. Recyclability of catalyst I-Pd in the cyanation reaction
Entry
1
Aryl halide
Product
CN
Yield (%)b
78
Run
Reaction condition
Conversion (%)
1
2
3
4
5
130 ꢁC/24 h
130 ꢁC/24 h
130 ꢁC/24 h
130 ꢁC/24 h
130 ꢁC/24 h
81
76
74
52
32
I
2
3
4
No reaction
No reaction
76
Br
Cl
CN
CN
CN
CN
for the filtered portion. In another test, after 3 h, the reaction
was split at a conversion of 17%. After an additional 24 h,
the portion containing the suspended catalyst had proceeded
to 90% conversion, while the catalyst free portion reacted
only up to 20%. Pd leaching was also studied by inductively
coupled plasma-atomic emission spectroscopy (ICP-AES)
analysis, indicating that the liquid part of Heck reaction mix-
ture contained 0.9 ppm of palladium accounting for 0.08%
of the initially added amount of Pd.
S
I
S
I
5
78
6
7
76
76
Br
I
I
Br
Cl
CN
Cl
CN
However, the catalyst showed reduced recyclability in the
cyanation of iodobenzene (Table 5). Once again, all runs
were carried out under similar conditions in refluxing
DMF. We found that the catalyst showed good catalytic per-
formance in three reaction cycles, but decreased activity
starting from the fourth run. This result may be due to cata-
lyst deterioration under the harsh conditions of the cyanation
reaction.
8
77
I
CN
a
Reaction was carried out using 1 mmol of aryl iodide, 0.7 mmol of
K4[Fe(CN)6], 2 mmol of triethylamine, and 500 mg nanostructured
hybrid silica catalyst in refluxed DMF.
Yield was determined by GC with dodecane as internal standard with
respect to aryl iodide.
b
We believe that the low Pd leaching observed in Heck and
Sonogashira coupling reactions is due to immobilized pyri-
dine binding site located on the surface of the nanostructured
silica, which acts as a ligand through metal–ligand interac-
tion. The anchoring of Pd species by pyridine sites supported
on nanostructured silica minimizes catalyst deterioration
and metal leaching and therefore allows efficient catalyst
recycling.
level of reusability are very important features. To clarify
this issue, we performed set of experiments using the re-
cycled catalyst. We found that the catalyst showed excellent
recyclability in the Heck coupling reaction of iodobenzene
with methyl acrylate (Table 4). All runs were carried out un-
der similar conditions in refluxing acetonitrile. After the first
reaction giving methyl cinnamate in 90% yield, the catalyst
was recovered by filtration, washed with dichloromethane
and acetone, and finally dried at 100 ꢁC for 1 h. A new reac-
tion was then performed with fresh solvent and reactants
under similar conditions. The nanostructured hybrid silica
catalyst I-Pd shows unchanged catalytic activity after sim-
ple washing and drying in at least five reaction cycles. No
catalyst deterioration was observed, confirming the high
stability of the heterogeneous catalyst under the reaction
conditions.
3. Conclusion
In summary, we report a palladium containing nanostruc-
tured silica bearing trialkyl-(4-pyridyl)-ammonium binding
sites, which was efficiently used as heterogeneous catalyst
for Heck and Sonogashira coupling reactions as well as in
cyanation reactions involving various iodo- and bromo-
arenes. The reported material system shows interesting fea-
tures in heterogeneous catalysis: (i) low Pd leaching due to
(1) non-covalent anchoring and (2) Coulombic interactions
between pyridine binding site and the immobilized Pd spe-
cies; (ii) nanostructured morphology of the heterogeneous
catalyst, which ensures strong interaction between substrates
and the solid, allowing high catalytic activity, and (iii) high
efficiency and economy gain by simple reaction processing
and easy recovery and re-use of the catalyst.
Heterogeneity and Pd leaching of this catalyst were exam-
ined by the ‘hot filtration’ test for the Heck reaction of iodo-
benzene and methyl acrylate. We examined the leaching of
Pd at two different points in the reaction. Filtration using
Whatman filter paper after 1 h (7% conversion) followed
by an additional 24 h of reaction gave a final conversion of
90% for the catalyst-containing portion, and 8% conversion
Table 4. Recyclability of catalyst I-Pd in the Heck coupling reaction of
iodobenzene with methyl acrylate
4. Experimental section
Run
Reaction condition
Selectivity (%)
Conversion (%)
80 ꢁC/12 h
80 ꢁC/12 h
80 ꢁC/14 h
80 ꢁC/14 h
80 ꢁC/15 h
100
100
100
100
100
90
88
89
90
89
4.1. Synthesis of N,N-dimethyl-pyridin-4-yl-(3-triethoxy-
silyl-propyl)-ammonium iodide
1
2
3
4
5
4-(N,N-Dimethyl)aminopyridine 1.22 g (0.01 mol) and
(3-iodopropyl)triethoxysilane 3.98 g (0.012 mol) were

V. Polshettiwar et al. / Tetrahedron 63 (2007) 6784–6790
6789
dissolved in 10 mL of acetonitrile in a Schlenk tube. The re-
action mixture was stirred under inert atmosphere at 80 ꢁC
for 24 h. After completion of reaction, solvent was evapo-
rated and product was washed with ether to remove excess
of (3-iodopropyl)triethoxysilane, to yield 4.2 g (92%) of
pure product. FTIR (KBr): 3157, 2975, 2930, 1650, 1564,
1077, 959 cmꢀ1; 1H NMR (CDCl3): d 0.55 (t, 2H), 1.16 (t,
9H), 1.94 (m, 2H), 3.24 (s, 6H), 3.71 (q, 6H), 4.26 (t, 2H),
6.97 (d, 2H), 8.27 (d, 2H) ppm; 13C NMR (CDCl3):
d 6.87, 18.35, 25.06, 40.80, 58.69, 59.90, 108.52, 142.29,
156.24 ppm; 29Si NMR (CDCl3): d ꢀ46.19 ppm; HRMS
(FAB+): calculated for C16H31O2N2Si (M+) 227.2104, found
227.2108.
All the prepared compounds are known and compared
with authentic sample.
4.5. General experimental procedure for cyanation
reactions
To a suspension of nanostructured hybrid silica catalyst
I-Pd (300 mg) in DMF (5 mL), aryl halide (1 mmol),
dry K4[Fe(CN)6] (0.7 mmol), triethylamine (2 mmol), and
dodecane as internal standard were added. The mixture
was refluxed for 24 h and was monitored by GLC. After
completion, the reaction mixture was filtered using What-
man filter paper. The crude product was dissolved in ethyl
acetate. The organic phase was washed with 10% HCl and
dried over sodium sulfate. Evaporation of solvent gives
crude product, which was confirmed by GC–MS. All the pre-
pared compounds are known and compared with authentic
sample.
4.2. Typical preparation of catalyst I-Pd
TEOS/N,N-dimethyl-pyridin-4-yl-(3-triethoxysilyl-propyl)-
ammonium iodide/H2O/NH4OH/N-methyl dodecyl imid-
azolium bromide were mixed in the ratio of 0.9:0.1:114:
8:0.12.12a In typical experimental procedure, template
N-methyl-N0-dodecyl-imidazolium bromide (0.32 g) was
dissolved in water (14 g) and ammonium hydroxide (7 g).
To this stirred solution, an organic sol–gel precursor N,N-di-
methyl-pyridin-4-yl-(3-triethoxysilyl-propyl)-ammonium
iodide (0.36 g) was added and stirred for 15 min, in which an
inorganic sol–gel precursor TEOS (1.6 mL) was added and
the mixture is stirred at 80 ꢁC for 2 days. The removal of
template was carried out by Soxhlet extraction of material
using ethanol (with 10% HCl) for 24 h. After template re-
moval, material was washed with ethanol and acetone, and
then dried. Two grams of this material was then stirred
with Pd(OAc)2 (0.14 g) in ethanol overnight. Pd(OAc)2 en-
capsulated catalyst was filtered and washed with ethanol,
acetone, and finally refluxed with acetonitrile to remove
traces of physically adsorbed Pd(OAc)2. Found elemental
analysis/%: C: 12.19, H: 2.12; N: 2.72; Si: 33.61; Pd: 3.62.
Si/Pd ratio found by scanning electron microscopy:
95.2:4.8. The amount of Pd(OAc)2 encapsulated was
0.34 mmol gꢀ1 based on elemental analysis.
Acknowledgements
V.P. thanks the ‘Direction de la recherche’, French Govern-
ment, for a postdoctoral fellowship.
Supplementary data
Full characterization of the organic precursor molecule and
the nanostructured heterogeneous catalyst are provided.
Supplementary data associated with this article can be found
in the online version, at doi:10.1016/j.tet.2007.04.073.
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