An oxime-carbapalladacycle complex covalently anchored to silica as an active and reusable heterogeneous catalyst for Suzuki cross-coupling in water

Article abstract of DOI:10.1039/b211742h

A preformed oxime-carbapalladacycle complex covalently anchored onto mercaptopropyl modified silica is highly active (> 99%) for the Suzuki reaction of p-chloroacetophenone and phenylboronic acid in water; no leaching occurs and the same catalyst sample was reused eight times without decreased activity.

Full text of DOI:10.1039/b211742h

An oxime–carbapalladacycle complex covalently anchored to silica as an
active and reusable heterogeneous catalyst for Suzuki cross-coupling in
water
ab
a
a
a
Carlos Baleizão, Avelino Corma,* Hermenegildo García* and Antonio Leyva
a
Instituto de Tecnología Química CSIC-UPV, Avda. de Los Naranjos s/n, 46022 Valencia, Spain.
E-mail: hgarcia@quim.upv.es
INETI-DTIQ, Estrada do Paço do Lumiar, 22,1649-038 Lisboa, Portugal; Tel: 351217165141
b
Received (in Cambridge, UK) 27th November 2002, Accepted 20th January 2003
First published as an Advance Article on the web 3rd February 2003
1
A preformed oxime–carbapalladacycle complex covalently
anchored onto mercaptopropyl modified silica is highly
active ( > 99%) for the Suzuki reaction of p-chloroacetophe-
none and phenylboronic acid in water; no leaching occurs
and the same catalyst sample was reused eight times without
decreased activity.
two intense bands at 2950 and 2900 cm2 corresponding to the
stretching vibration of CH of the tether were recorded together
2
with the most characteristic absorption bands in the aromatic
region. The covalent binding between the complex and the
modified silica through the –SCH
inferred by the fact that exhaustive CH
PdL@SiO solid does not lead to a decrease of the signal
2
– linkage is indirectly
2
2
Cl washings of the
2
In recent years there has been an increasing interest in
developing green processes in aqueous media.1 While a
considerable progress is being achieved, the vast majority of
examples of catalytic reactions in aqueous medium still use
homogeneous catalysis. It is clear that green chemistry not only
requires the use of friendly solvents but also it is very
convenient to convert homogeneous catalysis into heteroge-
neous catalysis in order to recover and reuse the catalyst. The
Suzuki reaction is one of the most important Pd-catalysed C–C
bond forming reactions and has a large potential commercial
applicability.4 One general strategy to transform a homoge-
neous into a heterogeneous process is to anchor the active site
onto a large surface solid carrier provided that the anchoring
methodology maintains the intrinsic activity and selectivity of
intensity either of the DR-UV-Vis or the IR spectra. Also the
leaching experiments (vide infra) indicate that the complex
remains anchored on the solid surface.
–3
2
To test the activity of PdL@SiO as a heterogeneous Suzuki
catalyst in water we selected the cross-coupling arylation of
,5
6
the catalytic center. In the present work we report the results
obtained with an oxime–carbapalladacycle covalently anchored
on a high surface silica as a heterogeneous catalyst for the
Suzuki coupling of arylboronic acids and aryl halides.
Recently, it has been reported that a Pd complex analogous to
7
1
effects the Suzuki cross-coupling of aryl chlorides in water as
well as Ullman type, Heck, Sonogashira and other related C–C
forming reactions in organic solvents.7 Herein we report that
by anchoring this complex on a high surface silica it is possible
to perform the Suzuki reaction in water or water–organic
solvent mixtures, and that the catalyst can be recovered and
reused without noticeable loss of activity or leaching of the Pd
from the solid to the liquid phase.
–10
The preparation procedure followed to obtain the PdL@SiO
2
catalyst is indicated in Scheme 1.† Basically, it consists in
preparing a Pd oxime–carbacycle complex having an w-
terminated CNC alkyl chain as substituent of the aromatic ring.
The covalent bond between the mercaptopropyl functionalised
silica and the Pd-complex was formed through a radical chain
mechanism using AIBN as initiator in the absence of oxygen.
Our strategy has the advantage of allowing a complete
characterization (chemical analysis, UV, FAB-MS, IR and
NMR spectroscopy) of each step of the complex preparation
before the final binding of the solid, thus avoiding the excess of
uncomplexed palladium or palladium oxide species on the solid
surface.
Scheme 1 Anchoring procedure of the oxime carbapalladacycle onto the
mercaptopropyl modified high surface silica.
2
The PdL@SiO catalyst was characterised by chemical
analysis and UV and IR spectroscopy. From the Pd, C and N
analysis it is calculated that the loading of active complex in the
2
1
solid was 0.34 mmol PdL g . In the DR-UV-Vis spectrum of
PdL@SiO the most characteristic band is the absorption at 310
2
nm specific of the metal–ligand interaction that is present also
in the complex in solution but is absent in the ligand (Fig. 1). IR
spectroscopy also confirms the presence of the intact Pd-
carbacycle complex on the silica surface (Fig. 2). In particular,
Fig. 1 UV-Vis spectra of the oxime ligand (a), oxime–carbapalladacycle
compound 1 (b) and diffuse reflectance UV-Vis spectrum of PdL@SiO2
(c).
6
06
CHEM. COMMUN., 2003, 606–607
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Fig. 3 Time conversion plot for the Suzuki reaction of p-bromoacetophe-
none and phenylboronic acid in water/TBAB in the presence of PdL@SiO
2
(
(
/) and after removal of the catalyst by filtration at reaction temperature
-).
Fig. 2 Infrared spectra of oxime–carbapalladacycle (a) and PdL@SiO
2
(
b).
catalysis in the results shown in Table 1, one reaction was
carried out in the presence of the solid until the conversion was
4
-haloacetophenones and phenylboronic acid as the model
2
2% and at that point the solid was filtered off at the reaction
reaction. Both reagents are soluble in water at the required
concentrations and reaction temperature. To increase the
solubility of the reaction product some assays were carried out
using tetrabutylammonium bromide (TBAB, 0.5 equivalents) or
the reaction was conducted in a mixture of dioxane–water (2+3).
The results achieved are collected in Table 1.
temperature. The liquid phase was then allowed to react, but no
further conversion was observed (Fig. 3). This indicates that no
active species were present in the supernatant. The second point
was the deactivation and reusability of the PdL@SiO
this, a series of eight consecutive runs were carried out with the
same PdL@SiO sample, without a noticeable decrease in the
2
. To test
2
Of note is that the presence of TBAB that has been considered
crucial in homogeneous phase not only is not necessary when
using PdL@SiO
decreases the activity of the catalyst (compare runs 1 and 3). A
likely explanation of this negative influence of TBAB would be
activity ( > 99%, entry 5). This reusability demonstrates the
high stability of heterogeneous catalyst. After using the catalyst,
the solid was simply filtered off, washed with ethanol and ether
and reused.
In summary, the results presented above show that a silica
supported Pd catalyst is an active and stable heterogeneous
catalyst for the Suzuki reaction in water, making the whole
process more efficient and environmentally friendly.
Financial support by the Spanish D.G.E.S. (MAT2000-
2
as catalyst but it actually significantly
2
2
that Pd atom could exchange Cl by Br , the higher donicity of
the latter increasing the softness of the active noble metal atom.
Another feature from Table 1 is the fact that pure water is a more
convenient solvent than dioxane–water mixtures. This observa-
tion is in agreement with the reported beneficial influence of
1
768-C02-01) is gratefully acknowledged. A. L. thanks the
7
solvent polarity on the activity of the Pd-carbacycle complex.
Spanish Ministry of Education for a post-graduated scholar-
ship.
In fact, only when dioxane–water mixture is used as solvent was
the presence of significant amounts of biphenyl arising from the
phenylboronic homocoupling observed.
Notes and references
Although the reactions in Table 1 occur faster when p-
bromoacetophenone is used as reagent, essentially complete
conversions are also achieved with p-chloroacetophenone
†
Compound 1 was synthetized adding a methanolic solution (4 ml) of
modified oxime (1.21 g, 4 mmol) and sodium acetate (0.33 g, 4 mmol) to a
solution of Li PdCl (1.05 g, 4 mmol) in methanol (8 ml). The mixture was
(
compare runs 3 and 4).
When using a supported catalyst two points become crucial
2
4
stirred at room temperature for 72 h, filtered and after adding water (10 mL)
the palladium carbacycle 1 precipitated as a yellow solid. To a solution of
issues. The first is the possibility that some active metal
migrates from the solid to the liquid phase and that this leached
Pd would become responsible for a significant extent of the
catalytic activity. To rule out the contribution of homogeneous
1
in degassed chloroform, mercaptopropyl modified solid and AIBN were
added under nitrogen atmosphere. The suspension was stirred magnetically
at 80 °C under N for 20 h. The solid was filtered off and Soxhlet extracted
with dichloromethane for 24 h. After drying the solids (at 45 °C under 10
2
21
Torr for 2 h), the quantity of palladium was determined by quantitative
atomic absorption spectroscopy. Suzuki reactions were carried out by
stirring magnetically 4-bromo- or 4-chloroacetophenone (39.8 or 31.0 mg)
and phenylboronic acid (36.6 mg. 1.5 equivalents) in water or water–
Table 1 Results of the Suzuki cross-coupling reaction of halobenzenes with
phenylboronic acid in aqueous media
2
dioxane in the presence of PdL@SiO (38 mg) at reflux temperature. The
course of the reaction was followed periodically by extracting aliquots of
the aqueous phase with ethyl acetate and analysing the extract with GC
using nitrobenzene as external standard.
Conversion
(%)
1 S. Kobayashi and K. Manabe, Pure Appl. Chem., 2000, 72, 1373.
2 E. Kuntz, Info Chim. Mag., 2000, 421, 51.
bRun
Conditions
X
t/h
3
N. A. Bumagin, Tetrahedron, 1997, 53, 14437.
1
2
3
4
5
6
7
a
Water/TBAB
Water/TBAB
Water
Br
Br
Br
Cl
Cl
Cl
Cl
1
48
< 0.1
0.25
2
0.25
48
62
4 M. Beller and A. Zapf, Top. Catal., 2002, 19, 101.
5 J. G. De Vries and C. E. Tucker, Top. Catal., 2002, 19, 111.
6 A. Corma and H. Garcia, Chem. Rev., 2002, 102, 3879.
7 C. Najera, D. A. Alonso and M. C. Pacheco, J. Org. Chem., 2002, 67,
5588.
8 C. Najera and L. Botella, Angew. Chem., Int. Ed, 2002, 41, 179.
9 C. Najera, M. C. Pacheco and D. A. Alonso, Org. Lett., 2000, 2,
1823.
a
67
> 99
91
> 99
55
Water
Water
b
c
Water–dioxane (3+2)
Water–dioxane (3+2)
a,c
24
.65% of Pd was used. b Catalyst reused eight times. c Biphenyl was
0
1
0 C. Najera, D. A. Alonso and M. C. Pacheco, Adv. Synth. Catal., 2002,
detected as byproduct.
2
, 344.
CHEM. COMMUN., 2003, 606–607
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