Pd(II)-Schiff base complexes heterogenised on MCM-41 and delaminated zeolites as efficient and recyclable catalysts for the heck reaction

Article abstract of DOI:10.1002/adsc.200404119

We present a widely applicable approach for the preparation of Pd-complexes supported on silica-based mesoporous and laminar inorganic solids (silica gel, purely siliceous and acidic MCM-41, and delaminated zeolites ITQ-2 and ITQ-6) which involves the use of the Schiff bases 2-tert-butyl-4-methyl-6{(E)-[(2S)-1- (1-arylmethyl)pyrrolidinyl]imino}methylphenol (aryl = phenyl, 1-naphthyl, 2-naphthyl) as anchored ligands. As an extension of our research in recyclable catalytic systems we report results demonstrating the reactivity and recyclability of these systems for the Heck reaction (in a biphasic mode using ethylene glycol and toluene) and Suzuki coupling under phosphine-free conditions. The complexes are insensitive to oxygen or moisture, and no change of their activity was observed when exposed to an open system during the usual operation. No palladium black was observed after an extended reaction time and no residual palladium was detected from the filtrate at the end of the reaction.

Full text of DOI:10.1002/adsc.200404119

FULL PAPERS
Pd(II)-Schiff Base Complexes Heterogenised on MCM-41 and
Delaminated Zeolites as Efficient and Recyclable Catalysts for
the Heck Reaction
a, c
b
c,
a,
´
´
C. Gonzalez-Arellano, A. Corma, M. Iglesias, * F. Sanchez *
a
´
Instituto de Química Organica General, CSIC, C/Juan de la Cierva, 3, 28006 Madrid, Spain
Fax: (þ34)-915-644-872, e-mail: felix-iqo@iqog.csic.es
b
c
Instituto de Tecnología Química, UPV-CSIC, Avda. de los Naranjos, s/n, 46022 Valencia, Spain
Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain
Fax: (þ34)-913-720-623, e-mail: marta.iglesias@icmm.csic.es,
Received: April 23, 2004; Accepted: September 5, 2004
Abstract: We present a widely applicable approach ethylene glycol and toluene) and Suzuki coupling un-
for the preparation of Pd-complexes supported on der phosphine-free conditions. The complexes are in-
silica-based mesoporous and laminar inorganic solids sensitive to oxygen or moisture, and no change of
(silica gel, purely siliceous and acidic MCM-41, and their activity was observed when exposed to an
delaminated zeolites ITQ-2 and ITQ-6) which in- open system during the usual operation. No palladi-
volves the use of the Schiff bases 2-tert-butyl-4-meth- um black was observed after an extended reaction
yl-6{(E)-[(2S)-1-(1-arylmethyl)pyrrolidinyl]imino}me- time and no residual palladium was detected from
thylphenol (aryl¼phenyl, 1-naphthyl, 2-naphthyl) as the filtrate at the end of the reaction.
anchored ligands. As an extension of our research in
“recyclable catalytic systems” we report results dem-
onstrating the reactivity and recyclability of these sys- Keywords: Heck reaction; heterogenisation; immobi-
tems for the Heck reaction (in a biphasic mode using lised; palladium; Schiff bases; Suzuki coupling
Introduction
phines which are commonly used as ligands in the cata-
lyst, and which are toxic and unpleasant to work with.
The commonly used palladium species in Heckꢀs reac-
The heterogenisation of homogeneous catalysts and
their use in the production of fine chemical synthesis tions are palladium acetate, palladium chloride, or pre-
has become a major area of research, since the potential formed triarylphosphine palladium complexes.[3] How-
advantages of these materials (simplified recovery and ever, all homogeneous catalysts present separation, re-
reusability, the potential for incorporation in continuous generation and reuse problems. These problems are of
reactors and microreactors) over homogeneous systems environmental and economic concern in large scale-syn-
can have positive environmental consequences.[1,2] The thesis where large volumes of hazardous wastes and high
majority of the novel heterogenised catalysts are based production costs can be limiting factors. These problems
on silica supports, primarily because silica displays can be minimised by carrying out the reactions using
some advantageous properties, such as excellent stabili- heterogeneous catalysts which can be more easily sepa-
ty (chemical and thermal), good accessibility, and the rated from the reaction mixture and recycled in succes-
fact that organic groups can be robustly anchored to sive batch operations.[7–10] We are focusing on new chiral
the surface to provide catalytic centres.
Schiff base ligands containing pyrrolidinamino func-
The palladium-catalysed coupling of olefins with aryl tionalities in place of one phenolic group on salen li-
or vinyl halides, known as the Heck reaction, is a stan- gands [salen¼(R,R)-N’,N’-bis(3,5-di-tert-butylsalicyli-
dard method for carbon-carbon formation in organic dene)-1,2-cyclohexanediamine]. Themethodwehavede-
synthesis.[3] Academic and industrial interest in this re- veloped should allow the easy synthesis of a wide variety
action has increased in recent years, including the ad- of chiral Schiff base ligands.[11] The iminophenoxy ligands
vent of an enantioselective variant,[4] the development were designed to coordinate with Pd(II) and the addition-
of more active catalyst systems, the discovery of waste- al amino group could be varied in order to improve the
free versions,[1,5] and the desire to put the vast empirical performance of the complexes in catalytic reactions.
data on a sound mechanistic basis.[6] Moreover, if would
be interesting to find good alternatives to the phos- approach for the preparation of Pd complexes support-
Herein we present a very easy and widely applicable
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DOI: 10.1002/adsc.200404119
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Pd(II)-Schiff Base Complexes
FULL PAPERS
ed on silica-based MCM-41 and laminar inorganic solids mixture of the precursor 2 and the respective support,
with very high specific surface (>600 m2/g), which in- in toluene, for 16 hours. These anchored aldehydes re-
volves the use of the Schiff bases 2-tert-butyl-4-methyl- acted with an equimolar amount of (S)-(1-benzyl-2-
6{(E)-[(2S)-1-(1-arylmethyl)pyrrolidinyl]imino}methyl- pyrrolidinyl)methylamine, (S)-[1-(1-naphthylmethyl)-
phenol (aryl¼phenyl, 1-naphthyl, 2-naphthyl) as an- 2-pyrrolidinyl]methylamine and (S)-[1-(2-naphthyl-
chored ligands. One relevant feature of our catalyst is methyl)-2-pyrrolidinyl]methylamine to afford the sup-
that it does not require addition of phosphines (com- ported chiral Schiff ligands, 3–5-(support), as fine pow-
monly introduced into palladium-catalysed reactions) dered solids. In the 13C NMR spectra, the C-OH carbon
thus also improving the atom economy of the reaction.
signals appear at d¼ ꢀ153 ppm, and the imine carbons
appear at d¼165–166 ppm.
The heterogenised ligands reacted with palladium(II)
acetate to furnish the corresponding anchored Pd com-
plexes. The catalysts prepared present metal loading of
Results and Discussion
In the last years we have developed a modular system 0.25–0.80 mmol metal/g support as determined by
combining functionalised ligands with different sup- atomic absorption analysis (see Table 1). The spectra
ports and linkers in order to have a systematic access of diamagnetic palladium complexes show the simulta-
to a variety of immobilised chiral catalysts.[12] We have neous occurrence of two set of signals, which are related
applied here this methodology to immobilise Schiff with the substituted benzaldimine entity and with the
base ligands on
a
mesoporous silica supports aliphatic part of the ligand. The 13C NMR spectra
¼
(Scheme 1), selected to encompass a wide range of top- showed the signals assigned to the C N carbon high-
ologies, such as MCM-41, ITQ-2, and ITQ-6 delaminat- field shifted and C-1 at d¼ca. 162 down-field shifted
ed zeolites, and amorphous silica. With respect to the confirming that metallation had occurred.
supports, both silica and MCM-41 are short-range amor-
Catalysis in liquid-liquid two phase systems is general-
phous materials containing a large number of silanol ly referred to as biphasic catalysis, the catalyst is present
groups available for grafting. In the case of MCM-41 in one phase while reactants and products are present in
however, the material presents a long-range ordering the other organic phase. In our case the term “biphasic”
with hexagonal symmetry with regular monodirectional refers to the mixture of two immiscible solvents while
channels of 3.5 nm diameter. On the other hand, ITQ-6 the catalyst exists in a third phase. In this work we
and ITQ-2 delaminated zeolites present both short- and have studied the performance of various supported Pd
long-range order, together with a very large, well struc- complexes for Heck reactions of iodobenzene using
tured external surface on which the silanol groups act as the biphasic mixture toluene and ethylene glycol as sol-
grafting centres. However, there are structural differen- vent in the presence of potassium acetate.[15]
ces between the two delaminated zeolites. Indeed, in the
We now show the reactivity and recyclability of these
case of the ITQ-2 there are “cup-like” apertures to the systems for the Heck reaction of aryl halides (chloro-,
external surface with ꢀ0.8ꢁ0.8 nm dimensions, while bromo-, and iodobenzene) with olefins (styrene, diethyl
in the case of the ITQ-6 these “cups” are less deep (ꢀ itaconate and n-butyl acrylate) under phosphine-free
0.3 nm). In a previous work and by a molecular dynam- conditions (Table 2). The complexes are insensitive to
ics study, we have shown that there is a minimum of po- oxygen or moisture and no change in their activity was
tential energy in the void space of the “cups” in the ITQ- observed when the reaction was carried out in an open
2, in which the molecules that can fit have a high tenden- system. As expected, the bromo and iodo compounds
cy to adsorb.[13] Taking this into account and the fact that are both active while chlorobenzene was virtually inert
the structured silanols are located at the borders of these in our experimental conditions.
“cups”, one can envisage an structural model analogous
to that presented by Breslow et al.[14] for cyclodextrins in
which the grafted complex site is located at the border of
the cyclodextrin “cup”. In our case, the metal complex
will be located at the border of the inorganic silicate
cups. Then, one may expect that the reactant is adsorbed
in the cavity of the delaminated zeolite and reacts there
with the catalyst located at the border. If this model is
Table 1. Metal loading in the support.
Catalyst
mmol-metal/g support
3Pd-(MCM-41)
3Pd-(ITQ-2)
0.27
0.31
0.25
0.40
0.46
0.39
0.32
0.51
0.80
correct, we should expect an increase in the concentra- 3Pd-(ITQ-6)
3Pd-(MCM-41) (Si/Alꢂ15)
tion of reactants close to the catalytic active site and,
consequently, an increase in the rate of the reaction
should occur.
All solids were functionalised in the same manner ac-
cording to the procedure shown in Scheme 1. Supported
precursors, 2-(support), were obtained by refluxing a
4Pd-(Sil)
4Pd-(MCM-41)
4Pd-(ITQ-2)
5Pd-(MCM-41)
5Pd-(ITQ-6)
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Scheme 1. Synthesis of ligands and complexes.
Among the various bases examined, 1.14 mmol of was observed after more than 72 hours. As shown in Ta-
NaOAc or KOAc proved to be most suitable in combi- ble 2, under the optimised conditions, the Heck-type
nation with catalyst (<5 mol %), and use of other bases olefination reactions of aryl halides were readily ach-
resulted in lower yields: Et3N (<10%). More polar sol- ieved with a variety of olefins.
vents than 3, such as dimethylformamide (DMF), di-
Comparative reactions have been carried out with a
methyl sulfoxide (DMSO) or acetonitrile were tested homogeneous equivalent of the supported catalyst in ex-
for this reaction, but only low conversion of the aromatic actly the same conditions. The initial green solution of
halide was achieved. The use of N-methylpyrrolidone homogeneous catalysts in N-methylpyrrolidone turned
(NMP) produces a clear increase in conversion and se- to red upon heating after addition of base and particles
lectivity, probably due to the high solubility of the base of palladium black could be seen in the flask. These ob-
and the intermediate salts formed in the reaction media. servations led us to the hypothesis that palladium col-
The problem with this solvent and the supported catalyst loids will be generated upon addition of base to a solu-
is the leaching of the metal (>50%) to the solution un- tion of the homogeneous precursor when heating the re-
der the usual reaction conditions. On the contrary, action mixture.[16] Indeed, upon centrifugation of the re-
when Heck vinylation reactions were performed in a bi- acted mixture we could isolate palladium particles.
phasic mode using ethylene glycol and toluene (Fig- These isolated particles were used in a new reaction
ure 1) with 5 mol % Pd(II), and KOAc as an efficient and they were catalytically inactive. The absence of re-
base, high yields were achieved without the need for activity is presumably due to a significant aggregation
an extra ligand such as triphenylphosphine. This catalyt- of the particles under the reaction conditions. When
ic system is stable and recyclable and reaction products, the filtered solution was used as catalyst, it was observed
catalyst and base can be easily separated. In the biphasic that transformation of iodobenzene does not occur.
vinylation system, the toluene phase contains the reac-
In Figure 2 the catalytic performance of various sup-
tants and products, while metal complex and potassium ported Pd-complexes is shown for the reaction of iodo-
acetate are in the ethylene glycol phase. The inorganic benzene and butyl acrylate under biphasic conditions.
base added is partly soluble in the ethylene glycol phase. The similar rates observed for homogeneous and sup-
The reaction should occur in the ethylene glycol phase ported catalysts indicate that diffusion is not a major
and the ethylene glycol-toluene interface in which the problem under the conditions used here, and the extend-
base is partly soluble.
ed lifetime may be a feature of the Pd being tethered to
In Table 2 we show the catalytic performance of vari- the support at a low density, stopping deactivation
ous supported complexes for the Heck reaction. The re- through clustering. It is interesting to notice that while
action was successful using low Pd catalyst loadings (2– the reaction selectivity is practically not influenced by
5 mol %), but the reactions did not proceed at concen- the catalyst, the activity is dominated by the support.
trations below 2 mol %. Analogous nickel complexes In a first approximation, the following order of activity
do not react under the same reaction conditions. When for the supported catalyst can be inferred from the re-
using the palladium complexes, no palladium black sults (after 24 hours): 4Pd-(ITQ-2)>5Pd-(MCM-
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Pd(II)-Schiff Base Complexes
FULL PAPERS
Figure 1. Heck vinylation reactions performed in a biphasic
mode using ethylene glycol and toluene.
41)>4Pd-(MCM-41)>3Pd-(MCM-41)>3Pd-(MCM-
41, Si/Al¼15)>4Pd-(Sil). The aluminium content of
MCM-41 shows only a minor influence on the conver-
sion in the Heck model reaction investigated, indicating
Figure 2. Catalytic performance of the Heck reaction.
that there is no effective charge separation along the re- al and the better resistance to leaching by the reaction
action.[17] From a preparative point of view, conversions solution. Moreover, the recycling procedure for our cat-
higher than 95% were achieved after longer reaction alysts is significantly easier than the standard Pd/C cata-
times and 100% selectivity to trans-butyl cinnamate lyst.
were observed irrespectively of the precursor used.
The activities for diethyl itaconate were similar to those
for n-butyl acrylate, whilst the activity for styrene was Separation, Recycling and Catalyst Reuse
lower, and longer reaction times are needed for high
conversions (5 days, 85%).
The recycle experiments were made by separating the
When compared with commercial 5% Pd/C we found ethylene glycol phase and reusing this together with
that anchored palladium complexes were more active fresh toluene and KOAc (Figure 1). Since the stability
and more stable, due to the higher dispersion of the met- of the supported complexes towards dissolution during
Table 2. The Heck reaction of aryl halides with olefins using homogeneous and supported palladium catalysts.[a]
R1
R2
Base
Solvent
Catalyst
Conv. [%][b]
Ph
Ph
Ph
Ph
H
H
H
Me
CO2Et
CO2Et
H
H
H
NaOAc
NaOAc
NaOAc
KOAc
NaOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
NMP
NMP
3Pd-(ITQ-2)
3Pd-(ITQ-6)
5Pd-(ITQ-6)
4Pd-(MCM-41)
5Pd-(ITQ-6)
4Pd-(Sil)
4Pd-(Sil)
3Pd-(MCM-41)
3Pd-(MCM-41) (Si/Alꢂ15)
4Pd-(MCM-41)
5Pd-(MCM-41)
4Pd-(ITQ-2)
45
13 (30)[d]
NMP
44
Biphasic[c]
NMP
10 (35)[d]
CH2CO2Et
CH2CO2Et
CO2Bu
CO2Bu
CO2Bu
CO2Bu
CO2Bu
CO2Bu
39
Biphasic[c]
Biphasic[c]
Biphasic[c]
Biphasic[c]
Biphasic[c]
Biphasic[c]
Biphasic[c]
14 (30)[d]
42
55 (70)[d]
40
58
64
74
H
H
H
[a]
Reactions were carried using 1 mmol of substrate and supported palladium catalyst (5 mmol %) with base (1.14 mmol) and
6 mL of solvent at 1408C.
Conversion after 24 h.
Ethylene glycol/toluene.
In parenthesis conversion after one time recycled.
[b]
[c]
[d]
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Figure 4. Effect of removing the supported palladium catalyst
from the Heck reaction.
Figure 3. Recycling experiments for catalyst 3Pd-(MCM-41).
the reaction mixture was filtered through a micropore
the reaction is a problem of great concern for most an- filter, no residual Pd was detected from the filtrate by
chored catalysts,[18] and considering the leaching prob- ICP analysis (<1 ppm) implying that leaching of Pd
lems observed with palladium supported on polymer from the support is negligible.
beads, quantitative analysis using atomic absorption
Moreover, to rule out the contribution of homogene-
spectroscopy (AAS) was employed to determine the ous catalysis in the results shown in Table 2, one reaction
amount of metal in the final reaction solution. We was carried out in the presence of the solid until the con-
have observed (Figure 3) that the catalyst could be recy- version was ca. 20% and at that point the solid was fil-
cled, at least, six times without significant loss of activity. tered off at the reaction temperature. The liquid phase
In order to check the stability of metal complexes sup- was transferred to another flask and the again allowed
ported on the solid matrix, we have characterised the to react, but no further significant conversion was ob-
solid before and after reaction. As can be deduced served (Figure 4, 23% after 24 h).[21]
from IR, 13C NMR and UV-VIS spectra the nature of
An important point concerning the use of a heteroge-
supported species is very similar and the most important neous catalyst is its lifetime, particularly for industrial
signals for Schiff base ligands appear in the same posi- and pharmaceutical applications of the Heck reaction.
tion after reaction.
After separation and washing, the heterogeneous cata-
Although extensive studies have been carried out to lysts were used several times for the same reaction under
elucidate the true catalytic species in the heterogeneous the same conditions as for the initial run of the catalyst.
Pd-catalysed Heck reaction, it is still unclear in many We usually see an increase of activity after the first run,
cases whether the reaction takes place on the surfaces followed by a slight decrease of the rate due to the small
of the solid Pd catalyst[19] or whether the active catalysts amount of catalyst lost by manipulation (Figure 3).
are Pd species leached out from the support, which sim-
The palladium catalysts bound on MCM-41 and de-
ply acts asa reservoir of Pd.[20] Taking this into account, it laminated zeolites have been also tested in the Suzuki
is mandatory to find out if some Pd has passed into the reaction between phenylboronic acid and bromoben-
solution. To do this, we have investigated the residual ac- zene in xylene in the presence of potassium carbonate
tivity of the supernatant solution after separation of the as the base. The optimal reaction conditions were ap-
catalyst. The potential leaching was studied as follows: plied to a series of catalysts and conversions>80%
the organic phase of a first run was separated from the were obtained at 24 h. Better yields have been obtained
solid (Pd-supported). New reagents were added to the with catalysts 5Pd-(Sil) and 5Pd-(MCM-41). This ex-
clear filtrate, and the composition of the reaction mix- periment show that these palladium supported com-
ꢄ
ture was determined by GC. This homogeneous reaction plexes could be applied for other C C couplings.
mixture was treated as a standard catalytic experiment
(1408C, 24 h). After 24 h, the composition was deter-
mined and no reaction was observed, which excludes Conclusion
the presence of active species in solution. Leaching of
the palladium complex can also be discarded because In conclusion, a novel series of mesoporous silica-sup-
the amount of Pd in the solid before (0.27ꢃ ported palladium catalysts has been successfully pre-
0.02 mmol/g) and after (0.25ꢃ0.02 mmol/g) the consec- pared through sequential grafting of organic functional
utive catalytic reactions was mainly unchanged. When groups. The catalysts are active in Heck reactions and,
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Pd(II)-Schiff Base Complexes
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and the deep yellow solid was filtered off and washed thor-
oughly with ethanol and then with ether. The solid was dried
under vacuum to afford the corresponding anchored ligands
[ligand-(support)]. Only spectroscopic data for one represen-
tative member of the series are presented.[25]
more importantly, are stable to the reaction conditions
and can be recycled without loss of activity. The facts
that no inert atmosphere and no additional ligands are
necessary, the easy separation, and the easy availability
make the supported palladium catalysts an interesting
alternative to homogeneous catalysts.
3-(MCM-41): Anal. found: C 16.3, H 3.0, N 2.5%;
(0.44 mmol/g). IR (KBr): n¼1630, 1557 cmꢄ1 (C N, C O,
¼
¼
C C); UV-VIS: l¼224, 264, 347, 419 nm; 13C NMR (solid):
¼
¼
d¼166.60 (CH N), 160.53 (NHCONH), 153.62 (C-OH),
136.99 (C-6, Carom, C-4, C-2), 128.74 (Carom, C-3, C-5), 66.50
Experimental Section
¼
(C-2’, CH NCH2), 58.98 (CH2Ph), 53.64 (C-5ꢀ), 42.48 (CH2
CH2CH2Si, CCH2NH), 28.40 [C(CH3)3, C-3’], 22.09 (CH2CH2
Si, C-4’), 8.02 (CH2Si).
General Remarks
Metal complexes were prepared under dinitrogen by conven-
tional Schlenk tube techniques. Solvents were carefully de-
gassed before use. The silylating agent OCN(CH2)3Si(OEt3)3
was obtained from ABCR. C, H and N analysis were carried
out by the analytical department of the Institute of Materials
Science (C.S.I.C.) with a Perkin-Elmer 240C apparatus. Metal
contents were analysed by atomic absorption using a Perkin El-
mer AAnalyst 300 atomic absorption apparatus and plasma
ICP Perkin Elmer 40. IR spectra were recorded on a Bruker
IFS 66v/S spectrophotometer (range 4000–200 cmꢄ1) in KBr
pellets. High resolution 13C MAS or CP/MAS NMR spectra
of powdered samples, in some cases also with a Toss sequence,
in order to eliminate the spinning side bands, were recorded at
100.63 MHz, 6 ms, 900 pulse width, 2 ms contact time and 5–10
recycle delay, using a Bruker MSL 400 spectrometer equipped
with an FT unit. The spinning frequency at the magic angle
(54044’) was 4 KHz.
Preparation of Supported Palladium Catalysts
Heterogenised complexes were synthesised following the gen-
eral method: an ethanolic solution of Pd(OAc)2 or Ni(OAc)2
(0.5 mmol/15 mL) was added to a suspension of heterogenised
ligand in toluene (20 mL) at room temperature. The resulting
mixture was stirred under reflux for 12 h, cooled to room tem-
perature and filtered. The solid was washed several times with
ethanol, dried and filtered to afford the corresponding hetero-
genised complexes, complex-(support), in almost quantitative
yields.[25] Spectroscopic data for one representative member
are given:
3Pd-(ITQ-2): Anal. found: C 13.4, H 1.4, N 1.9, Pd 3.3%;
¼
¼
¼
(0.31 mmol/g). IR (KBr): n¼1630–1562 (C N, C O , C C),
560 cmꢄ1 (Pd-O); UV-VIS: l¼225, 258, 289, 335, 412 nm;
13C NMR (solid): d¼186.92 (OOCCH3), 169.09 (NHCONH,
¼
C-O-Pd), 158.65 (CH N), 137.78 (C-2), 126.14 (Carom, C-5, C-
3), 119.22 (C-6), 65.23 (CH NCH2, C-2’), 62.56 (CH2Ph),
57.83 (C-5’), 46.66 (CH2CH2CH2Si, CCH2NH), 34.04
[C(CH3)3], 28.95 [C(CH3)3, C-3’), 24.09 (C-4’, CH2CH2CH2Si),
17.30 (OOCCH3), 12.45 (CH2Si).
The inorganic supports for anchoring were silica gel [Merck
silica, average pore diameter 40 Š], purely siliceous MCM-
41,[22] and delaminated zeolites ITQ-2 and ITQ-6.[23] In addi-
tion, Al-containing MCM-41 samples with Si/Al ratios of 15
and 50 were used as support for loading the complex.
¼
Catalyst Testing
Catalyst Preparation
Heck reaction: In a typical experiment, catalyst (0.03 mmol,
5%) was suspended in a mixture of ethylene glycol and toluene
(1:1, 6 mL). Then iodobenzene (1 mmol), n-butyl acrylate
(1.14 mmol), and potassium acetate (1.14 mmol) were added
with stirring. The mixture was refluxed and the reaction prog-
ress was monitored by gas chromatography.
Preparative experiment: Following the general method,
when the reaction was finished the toluene phase was separat-
ed and the product was obtained by solvent evaporation.
Typical Suzuki reaction: Catalyst (0.3 mmol, 5%) was sus-
pended in o-xylene and benzeneboronic acid (10 mmol), bro-
mobenzene (15 mmol), aqueous potassium carbonate
(20 mmol) were added. The reaction mixture was then heated
at 1438C and its progress followed by GC.
The catalyst preparation (Scheme 1) involves the anchoring of
organosilanes onto mesoporous silica followed by subsequent
modification reactions. The organically modified silica-based
mesoporous solids (silica gel, purely siliceous and acidic
MCM-41, and delaminated zeolites ITQ-2 and ITQ-6) materi-
als were prepared and completely characterised as we describ-
ed previously.[24]
Synthesis of Heterogenised Terdentate Ligands
Typically, a solution of 2 mmol of the aldehyde 2 in toluene
(5 mL) and 100 mL of water were added to a suspension of silica
or the corresponding mesoporous solids MCM-41, ITQ-2 or
ITQ-6 (1 g) in toluene (25 mL), stirred at room temperature
for 30 minutes. The slurry was heated at 1008C for 16 h and
the yellow solid was filtered off and washed successively with
toluene, petroleum ether, ethanol and diethyl ether. Then, a
stirredslurryof1 mmolofthecorresponding anchoredhydroxy-
aldehyde 2-(Sil, MCM-41, ITQ-2, ITQ-6) was prepared in
ethanol (10 mL) under argon at room temperature. After
10 min, 125 mg (1.1 mmol) of the amine were added and the
mixture stirred for 16 hours. The mixture was allowed to cool
Acknowledgements
´
´
Financial support by the Direccion General de Investigacion
´
Científica y Tecnica of Spain (Project MAT2003–07945-C02–
02) is gratefully acknowledged.
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Adv. Synth. Catal. 2004, 346, 1758–1764