Organically modified Pd-silica catalysts applied in Heck coupling

Article abstract of DOI:10.1039/b309307g

Pd-silica catalysts prepared by depositing Pd onto silica precursors modified by surface methyl or phenyl groups through the reduction of Pd ²⁺ ions with surface Si-H functions exhibit high activity and selectivity in Heck coupling.

Full text of DOI:10.1039/b309307g

Organically modified Pd–silica catalysts applied in Heck coupling
Árpád Molnár,* Attila Papp, Krisztina Miklós and Péter Forgo
Dóm tér 8, Szeged, H-6720, Hungary. E-mail: amolnar@chem.u-szeged.hu; Fax: 36 62 544200; Tel: 36 62
544277
Received (in Cambridge, UK) 6th August 2003, Accepted 29th August 2003
First published as an Advance Article on the web 17th September 2003
Pd–silica catalysts prepared by depositing Pd onto silica
precursors modified by surface methyl or phenyl groups
through the reduction of Pd2+ ions with surface Si–H
functions exhibit high activity and selectivity in Heck
coupling.
For a better comparison of the catalytic performance of these
new catalyst materials, a second set of catalysts was prepared by
reacting 1.994 g of the modified silica samples with a 1.25 3
1023 M PdCl2 solution (10 mg PdCl2 in 45 ml methanol) to get
five catalysts with equal Pd loading of 0.3 wt%. The resulting
silica precursors and the samples loaded with Pd were
characterised by physical methods (Table 2). Reaction with
silanes results in some decrease in the BET surface area of the
parent silica (455 m2 g21), whereas deposition of Pd has the
opposite effect in most cases. According to TEM character-
isation the samples have a narrow range of particle sizes and are
of low dispersion.
Coupling reactions to form carbon–carbon bonds are important
transformations in organic synthesis. One of these, Heck
coupling, allows the olefination of aryl, vinyl, benzyl, allyl
halides, acetates or triflates through their reaction with various
alkenes in the presence of palladium in a single step.1 Most
studies focused on the use of soluble Pd complexes with
phosphine ligands.2,3 It was also shown, however, that hetero-
geneous Pd catalyst systems, namely stabilised colloidal
palladium3–6 and supported palladium catalysts,7–12 are also
active in the reaction. Most recent findings show that even Heck
coupling of chloroarenes can be induced by heterogeneous Pd
catalysts to give the corresponding vinylarenes in high yields
under appropriate reaction conditions.11,13
For economic reasons development of heterogeneous, that is
reusable, catalysts for industrial applications remains an
important goal. This is a particular challenge for Heck coupling
since leaching of Pd from heterogeneous catalyst systems is
known to occur under certain reaction conditions.7,8,10,14 The
main goal of the present study, therefore, is the preparation,
characterisation, and evaluation in Heck coupling of new,
organically modified heterogeneous Pd–silica catalysts.
A novel method for the controlled deposition of metals onto
surfaces was disclosed by Fry in the early 1990’s.15,16 It was
shown that the treatment of silica with trichlorosilane allows the
immobilisation of the Si–H function. This surface hydrosilane
function is capable of reducing metal ions resulting in the
deposition of a thin metal layer onto the silica surface.
We have prepared two series of catalysts by modifying the
original synthesis protocol. First, we treated silica (Aldrich,
Davisil grade 363, dried at 500 °C for 2 h) with various
chlorohydrosilanes (trichlorosilane, dichloromethyl- and chlor-
odimethyl-silane, dichlorophenyl- and chlorodiphenyl-silane)
to prepare organically modified silica materials. The modified
silicas were reacted with a saturated solution of PdCl2 in
methanol (1 g silica,71 mg PdCl2 in 160 ml methanol) to get five
Pd-on-silica catalysts with various Pd loadings.
The silica precursors analysed by 29Si CP-MAS NMR
spectroscopy showed two major resonances at about 2102 ppm
(Q3) and 2111 ppm (Q4) characteristic of SiO4 units possessing
3 and 4 siloxane bridges, respectively. When organic function-
alities (methyl or phenyl groups) were introduced to the surface
new signals with higher chemical shifts appeared originating
from the polarization transfer of methyl and phenyl protons
(Table 2). 13C CP-MAS NMR spectroscopy showed resonances
at 22.1 ppm (methyl groups) and 127.1, 129.7 and 132.5 ppm
(phenyl groups). The spectra also show a signal at 48.3 ppm,
which is due to methoxy groups attached to silicon. These are
formed when in the final step of the preparation of the silica
precursors, samples are treated with methanol to transform
unreacted Si–Cl functions.
The 0.3% Pd–silica catalysts exhibit lower activity in the
coupling of iodobenzene with styrene (1a) because of the lower
amount of Pd present in the reaction mixture (0.3 mol% with
respect to iodobenzene) but the reaction takes place with
increased selectivities (Table 3).‡ It is significant that even less
Table 1 Catalytic performance of Pd–silica catalysts with various Pd
loading in Heck coupling
1a
1b
Pd content
(wt%)
Catalyst
Conv. (%) Sel. (%)
Conv. (%) Sel. (%)
Pd/SiO2
1.47
0.97
1.28
1.12
0.35
100
93
75
100
65
90
84
84
80
85
65
59
85
85
75
100
100
100
100
100
Pd/SiO2Me
Pd/SiO2Me2
Pd/SiO2Ph
Pd/SiO2Ph2
The catalysts were found to exhibit high activity in the Heck
coupling of iodobenzene with styrene (1a) or methyl acrylate
(1b) (Scheme 1) using equal amounts of catalyst to yield the
corresponding E isomers with good or excellent selectivities
(Table 1)† as evidenced by GC, GC-MS and liquid NMR of the
isolated products.
Table 2 Characterisation of silica precursors and Pd–silica catalysts
Silica precursors 0.3% Pd–silica catalysts
BET/m2 29Si NMR
BET/m2 Particle
Silanea
g21
(ppm)
Catalyst
g21
sizeb/nm
Cl3SiH
390
416
398
417
379
Pd/SiO2
Pd/SiO2Me
Pd/SiO2Me2 451
Pd/SiO2Ph
Pd/SiO2Ph2
419
430
9–11
12–15
21–24
12–15
9–13
Cl2MeSiH
ClMe2SiH
Cl2PhSiH
PhPh2SiH
235.5
22.2
247.1
371
430
218.7
a Silanes used in treatment of silica. b Pd particle sizes, determined from
TEM as the average diameter of about 100 particles.
Scheme 1 Heck coupling.
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This journal is © The Royal Society of Chemistry 2003

Table 3 Catalytic performance of 0.3% Pd–silica catalysts in Heck
Table 5 Catalyst recycling in Heck coupling of iodobenzene and styrene
coupling
0.3% Pd/SiO2Ph
treatmenta Conv. (%) Sel. (%)
10% Pd/C
Catalyst
1a
1b
1c
1d
64a (99)b
100 (99)
68 (99)
100 (99)
66 (99)
Catalyst
Reaction
Conv. (%) Sel. (%)
Pd/SiO2
54a (86)b
58 (86)
35 (85)
80 (83)
57 (86)
81a (99)b
84 (99)
54 (99)
97 (99)
51 (99)
47a (99)b
67 (99)
19 (99)
75 (99)
58 (99)
Pd/SiO2Me
Pd/SiO2Me2
Pd/SiO2Phc
Pd/SiO2Ph2
1
2
3
1
2
3
94
96
94
98
92
99
85
84
83
85
86
86
100
96
97
97
99
94
84
84
84
85
86
84
A
A
T
T
a Conversion (%). b Selectivity (%). c Turnover frequencies for this catalyst
vary between 2 min21 and 72 min21 for the four reactions (Pd dispersion
measured by hydrogen chemisorption is 0.04).
a A: washed with acetone, water and acetone and air-dried (1 h). T: washed
with toluene and dried in vacuo (12 h).
reactive activated bromoarenes (p-bromoacetophenone 1c and
p-bromonitrobenzene 1d) react satisfactorily with styrene.
It is seen from these data that the organophilicity of the
surface has a strong effect on catalytic performance. Namely,
catalysts prepared from precursors with a single organic group
as surface modifier (Pd/SiO2Me and Pd/SiO2Ph) show in-
creased activity whereas activities decrease with the introduc-
tion of two methyl or phenyl groups (Pd/SiO2Me2 and Pd/
SiO2Ph2). This effect is more pronounced in the reaction of the
less reactive bromoaromatics (Table 3, 1c and 1d).
The heterogeneity of the catalyst samples was also evaluated.
As mentioned significant amounts of Pd were shown to leach
out from heterogeneous Pd catalysts and the reaction may be
mainly catalysed by Pd species in the liquid phase.8,9 It is also
known that almost all Pd is redeposited after completion of the
reaction. Therefore, as pointed out by Arai,8 a true evaluation of
heterogeneity can be done by interrupting the reaction at low
conversion and then continuing the process with both the
catalyst and the filtrate. In this way, information about the
presence or absence of active species in solution and the activity
of such species can be acquired.
Tests were carried out accordingly by reacting iodobenzene
with styrene using 0.3% Pd/SiO2Ph, which proved to be the best
catalyst. Conversion values thus determined are presented in
Table 4. Under standard conditions (entries 1–3) there is a small
increase in conversion in the filtrate after catalyst removal in the
first 15 min, and then there are no further changes. Similar
observations can be made when toluene is used as solvent
instead of NMP and with triethylamine as base (entries 4–6).
The amount of Pd present in the solution determined by ICP is
about 1 ppm in this latter case, that is, less than 1% of the Pd is
dissolved from the silica support. These observations indicate
that only a very small fraction of Pd dissolved and this amount
of metal in solution does not appear to be active as a
homogeneous catalyst in the transformation.
ments. These results indicate that properties of our new
organically modified catalysts are identical with those of 10%
Pd/C, the best commercial catalyst for heterogeneous Heck
coupling.
In summary, we have demonstrated the feasibility of a novel
approach for the synthesis of organically modified Pd–silica
catalysts and showed that this new catalyst family has great
potential in Heck coupling under heterogeneous conditions. A
strong effect of the organophilicity of the surface on catalytic
performance was also found.
This work was sponsored by the National Research Founda-
tion of Hungary (Grants OTKA T042603, TS044690 and
M041532).
Notes and references
† General reaction conditions: Equimolar amounts (0.89 mmol) of aromatic
halide, alkene, and NaOAc, 27.3 mg catalyst, 2 ml N-methyl-2-pyrrolidi-
none (NMP) and decane or biphenyl (internal standards) were stirred in a
sealed tube at 150 (± 1) °C for 5 h (styrene) or 2 h (methyl acrylate). No
special precaution was taken to exclude air or moisture. Conversion and
selectivity were determined by GC analysis.
‡ 68 mg of the 0.3% Pd–silica catalysts was used in these studies.
1 R. F. Heck and J. P. Nolley, Jr., J. Org. Chem., 1972, 37, 2320; R. F.
Heck, Acc. Chem. Res., 1972, 12, 142.
2 S. Bräse and A. de Meijere, in Metal-Catalyzed Cross-Coupling
Reactions, ed. F. Diederich and P. J. Stang, Wiley-VCH, Weinheim,
1998, ch. 3, p. 99.
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5 M. T. Reetz and G. Lohmer, Chem. Commun., 1996, 1921.
6 M. T. Reetz and E. Westermann, Angew. Chem., Int. Ed., 2000, 39,
165.
Finally, catalyst recycling studies were performed using
0.3% Pd/SiO2Ph and a 10% Pd-on-C catalyst (Aldrich) for
comparison. Results given in Table 5 show that catalyst
performance practically does not change in successive experi-
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249.
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Table 4 Leaching tests with a 0.3% Pd/SiO2Ph catalyst in Heck coupling of
iodobenzene and styrene
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2002, 1062; M. Dams, L. Drijkoningen, B. Pauwels, G. Van Tendeloo,
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Conv. (%) [time/min]
Conv. (%)
[split time/min]
Pd in solution
(ppm)c
Entry
Suspension
Filtrate
1a
2a
3a
53 [15]
69 [15]
84 [45]
90 [75]
64 [15]
64 [45]
60 [75]
2.8
2.1
2.8
Conv. (%)
[split time/h]
Conv. (%) [time/h]
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Sreedhar, J. Am. Chem. Soc., 2002, 124, 14127.
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997.
4b
5b
6b
40 [6]
57 [6]
46 [6]
0.7
1.45
1.7
67 [12]
73 [18]
45 [12]
49 [18]
a Standard reaction with 68 mg of catalyst. b Reaction in toluene with 68 mg
of catalyst and triethylamine as base. c Determined by ICP.
16 J. J. Reed-Mundell, D. V. Nadkarni, J. M. Kunz, Jr., C. W. Fry and J. L.
Fry, Chem. Mater., 1995, 7, 1655.
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