Efficient heterogeneous palladium-montmorillonite catalysts for heck coupling of aryl bromides and chlorides

Article abstract of DOI:10.1055/s-2006-951493

New palladium catalysts were prepared using ion exchange or intercalation of Pd species into montmorillonite. The catalysts promote Heck reaction of various aromatic halides including aryl chlorides to give coupling products in high yields at low catalyst ratios down to 0.001 mol%. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-951493

3130
LETTER
Efficient Heterogeneous Palladium–Montmorillonite Catalysts for Heck
Coupling of Aryl Bromides and Chlorides
P
alladium–Montm
r
orillonit
p
e
Catalysts fo
á
r
H
eck Cou
d
pling Molnár,* Attila Papp
Department of Organic Chemistry, University of Szeged, Dóm tér 8, Szeged, 6720, Hungary
Fax +36(62)544200; E-mail: amolnar@chem.u-szeged.hu
Received 17 April 2006
acrylate.12 In all these latter studies10–14 K-10 montmoril-
lonite, an acid-treated clay, was applied.
Abstract: New palladium catalysts were prepared using ion ex-
change or intercalation of Pd species into montmorillonite. The cat-
alysts promote Heck reaction of various aromatic halides including
aryl chlorides to give coupling products in high yields at low cata-
lyst ratios down to 0.001 mol%.
On the basis of this information and our recent successful
efforts to develop new, highly stable and active heteroge-
neous Pd catalysts for the Heck coupling13 and the Suzuki
reaction,14 which also exhibit unprecedented stability in
recycling studies15 we have decided to prepare and use
montmorillonite-based Pd catalysts in the Heck reaction.
Key words: C–C coupling, Heck reaction, palladium, montmoril-
lonite, heterogeneous catalysts
Two catalysts were prepared using a high-sodium-content
montmorillonite. Treatment with a slightly acidic solution
of Pd(NO3)2 resulted in the exchange of Pd2+ ions for Na+
ions (Pd content = 2.16%).16 The intercalation of
[Pd(OH2)4]2+ ions can be achieved by treating montmoril-
lonite with an aqueous solution of Pd(NO3)2 (Pd content =
1.29%).16,17 The two catalyst samples thus prepared will
be referred to as Pd–montm1 and Pd–montm2, respective-
ly. The metal loadings correspond to 25% (Pd–montm1)
and 38% (Pd–montm2) of the ion-exchange capacity of
the montmorillonite sample used here (1.05 mmol g–1). As
shown earlier17 the ions are homogeneously dispersed
throughout the support and found between the silicate
sheets.
The Pd-catalyzed reactions of substituted aromatics, char-
acteristically aryl halides, with unsaturated compounds
(activated alkenes and terminal alkynes) or aryl boronates
are referred to as Heck-type arylation processes. These
cross-coupling reactions have become invaluable synthet-
ic methodologies to produce aromatics with vinyl or ethy-
nyl substituents, and biaryls. The original processes
known today as Heck coupling,1 Suzuki–Miyaura reac-
tion,2 and Sonogashira coupling3 have broadened signifi-
cantly because the scope of the reactants widened
tremendously.
In recent years new Pd complexes of high activities have
been developed in ever-increasing numbers, which are
able to promote the reaction of aryl chlorides.4 Palladacy-
cles also proved to be useful and effective.5 In addition,
efforts have been made to search for new, efficient, and
recyclable heterogeneous or heterogenized Pd catalysts6
and Pd nanoparticles.7 In this respect, recent observations
by Köhler et al. are of particular interest.8 High dispersion
of Pd species in the +2 oxidation state was shown to be a
crucial requirement allowing Pd leaching and re-deposi-
tion and resulting in catalytic activities comparable to
those of the best homogeneous systems.
First, the catalysts were tested in Heck coupling of bro-
mobenzene and other, activated and deactivated aryl ha-
lides (Scheme 1) under reaction conditions applied in our
previous studies using 0.1 mol% of Pd [150 °C, NMP (N-
methyl-2-pyrrolidinone) as solvent, no special precaution
to exclude air and moisture].13,15,18 As seen in Table 1 both
catalysts showed good catalyst performance under appro-
priately selected reaction conditions. Deactivated com-
pounds (4-iodoanisole, 4-bromoanisole) and 3-
bromopyridine, however, required the use of an increased
amount of the catalysts (0.3 mol%) to have satisfactory re-
sults. Tetrabutylammonium bromide (TBAB), which acts
as supporting ligand and phase-transfer agent also assist-
ing in the decomposition of HPdX species,19 was also nec-
essary to be used in two cases. In all reactions complete E
selectivity is observed, whereas products of a coupling
(6–11%) are also formed in the transformation of styrene.
We argued that montmorillonite, a smectite clay mineral
with a layered lattice silicate structure, could be a suitable
support fulfilling the above requirement. Good catalyst
performance of reduced Pd–montmorillonite catalysts in
selective hydrogenations was recently disclosed.9 More
importantly, studies for the application in the Heck cou-
pling of montmorillonite-based Pd catalysts prepared by
ion exchange10 or using immobilized complexes11 are also
known. In addition, Pd–Cu–montmorillonite was shown
to be active in cross-coupling of aryl amines with methyl
Next, studies were performed by running reactions with
decreased molar ratios of catalysts in the transformation
of methyl acrylate and styrene with bromobenzene. Re-
sults summarized in Tables 2 and 3 show unequivocally
that these new Pd–montmorillonite catalysts are efficient
in catalyzing Heck couplings. TON and TOF data ac-
quired at the lowest substrate to catalyst ratios (Table 3,
entries 3, 4, 7, 8, and 9) are in the range of 50,000–70,000
SYNLETT 2006, No. 18, pp 3130–3134
0
3
.1
1
.2
0
0
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Advanced online publication: 25.10.2006
DOI: 10.1055/s-2006-951493; Art ID: S07906ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Palladium–Montmorillonite Catalysts for Heck Coupling
3131
Table 1 Transformation of Aromatic Iodo and Bromo Derivatives in the Presence of 0.1 mol% of Pd–Montmorillonite Catalysts (Scheme 1)a
Aryl halide
Alkene (Scheme 1) Time (h) Pd–montm1
Conversion (%)
Pd–montm2
Yield (%)
Conversion (%)
Yield (%)
4-Iodoanisolea,b
R2 = COOMe
R2 = Ph
2
2
97
96
92
92
87
90
80
82
4-Bromobenzenec
R2 = COOMed
R2 = Ph
3
3
85
80
81
77
87
85
82
80
4-Bromoanisolea,c
R2 = COOMed
R2 = Ph
3
6
83
84
80
91
76
79
71
77
4-Bromacetophenonec
1-Bromo-4-chlorobenzenec
4-Bromopyridinee
R2 = COOMe
R2 = Ph
2
2
100
90
97
95
99
91
94
95
R2 = COOMe
R2 = Ph
2
2
100
100
99
99
100
100
99
99
R2 = COOMe
R2 = Ph
2
2
73
95
68
90
78
88
74
84
a Catalyst ratio in the transformation of 4-iodoanisole, 4-bromoanisole, and 3-bromopyridine: 0.3 mol%. Reaction temperature: 150 °C.
b Reactant ratios: aryl halide–alkene–NaOAc = 1:1:1.
c Reactant ratios: aryl halide–alkene–Na2CO3 = 1:1.2:1.2.
d TBAB (0.2 equiv).
e Reactant ratios: aryl halide–alkene–NaOAc = 1:1.2:1.2.
ide supported nanopalladium catalyst,6c Pd-exchanged
R2
NaY zeolite,20b and Pd on carbon applied in poly(ethylene
gylcol) as phase-transfer catalyst.20c One of the most suc-
R1
X
Pd–montmorillonite
cessful examples is the recent result by Köhler’s group
+
R1
using solid supported catalysts with soluble Pd species.8
Because of the importance of this issue, we have also at-
tempted to transform aryl chlorides over our newly devel-
oped catalysts.
R2
R2
NMP, base,
150–160 °C
X = I, Br, Cl
R
R
R1
1 = H, Cl, MeO, Ac
2 = Ph, COOMe
Scheme 1
Our first tests reacting 4-chloroacetophenone with methyl
acrylate and styrene under the usual reaction conditions
gave encouraging results: both catalysts showed satisfac-
tory activities (Table 4). Moreover, our catalysts worked
at lower temperature than the catalyst samples developed
and applied in Köhler’s study (150 °C vs. 160 °C).8b In
contrast, the reactivity of the neutral chlorobenzene was
much inferior. In fact, conversion values were lower even
under the reaction conditions used by Köhler [catalysts
(0.1 mol%), TBAB (60 mol%), Ca(OH)2 as base, 160 °C].
and above 17,000 h–1, respectively. These values are close
to those observed with homogeneous Pd complexes and
are similar to those found by Köhler et al.8b
Since aryl chlorides are, in general, unreactive in the pres-
ence of heterogeneous Pd catalysts,4a it is an important is-
sue to test the activity of new catalysts in the
transformation of aryl chlorides. In fact, only a handful of
successful attempts are known.6c,d,h,8,20 The best recent re-
sults were achieved by the use of a layered double hydrox-
Table 2 Transformation of Bromobenzene with Methyl Acrylatea
Catalyst
Catalyst concentration (mol%)
Conversion (%)
Yield (%)
TONb
TOFc (h–1)
Pd–montm1
0.012
0.005
97
94
92
87
8,083
18,800
2,694
6,267
Pd–montm2
0.01
0.005
93
85
90
79
9,300
17,000
3,100
5,667
a Reaction conditions: bromobenzene (10 mmol), methyl acrylate (15 mmol), Na2CO3 (12 mmol), NMP (10 mol), 150 °C, 3 h.
b Moles of aryl halide converted/moles of Pd.
c TON/h.
Synlett 2006, No. 18, 3130–3134 © Thieme Stuttgart · New York

3132
Á. Molnár, A. Papp
LETTER
Table 3 Transformation of Bromobenzene with Styrenea
Entry
Catalyst
Catalyst concentra- Time (h)/Temp (°C)
tion (mol%)
Conversion (%)
Yield (%)
TON
TOF (h–1)
11,042
1
2
3
4
Pd–montm1
0.005
0.001
3/150
3/150
6/150
6/160
93
45
68
53
91
42
64
50
18,600
45,000
68,000
66,250
0.0008
5
6
7
8
9
Pd–montm1
0.005
0.0014
0.001
0.001
3/150
3/150
6/150
3/160
6/160
97
72
53
53
73
90
68
50
51
71
19,400
51,429
53,000
53,000
73,000
6,467
17,142
8,833
17,667
12,167
a Reaction conditions: bromobenzene (10 mmol), styrene (15 mmol), Na2CO3 (12 mmol), NMP (10 mL).
Table 4 Transformation of Aromatic Chloro Derivativesa
Aryl halide
Alkene (Scheme 1)
Pd–montm1
Pd–montm2
Conversion (%)
Yield (%)
Conversion (%)
Yield (%)
4-Chloro acetophenoneb
Chlorobenzenec
R2 = COOMe
R2 = Ph
73
73
69
70
90
87
85
83
R2 = Ph
17
16
19
17
a Reactant ratios: aryl halide–alkene–base–TBAB = 1:1.2:1.2:0.6.
b Catalyst (0.1 mol%), Na2CO3 as base, 150 °C, 3 h.
c Catalyst (0.1 mol%), Ca(OH)2 as base, 160 °C, 6 h.
The transformation of styrene and 4-chloroacetophenone do-homogeneous systems.21 Leaching of soluble Pd spe-
in the presence of lower amounts of catalysts down to cies from heterogeneous Pd catalysts into solution is well-
about 0.005 mol% gave still reasonable results (Table 5, documented.13b,20d,22 A related interesting observation was
entries 1, 2 and 5–8). Reactions with even lower catalyst made by Arai et al. who showed that the active palladium
quantities but performed at higher temperature and longer species could be deposited onto a suitable support and re-
reaction time resulted in reasonably high TON values. used in repeated reactions.22b,23 Furthermore, it was found
Data also show that the use of Ca(OH)2 as the base gave that very small amounts of Pd species in solution can pro-
only marginal improvements as compared with sodium mote the coupling reaction. For homogeneous systems
carbonate (Table 5, entries 4 and 11).
high activities were observed with Pd(OAc)2 called ‘ho-
meopathic ligand-free palladium’.24 In this respect, obser-
vations by Leadbeater about performing related Suzuki
The results disclosed here strongly support the suggestion
that heterogeneous palladium catalysts, in fact, are pseu-
Table 5 Transformation of 4-Chloroacetophenone with Styrenea
Entry
Catalyst
Catalyst concentra- Time (h)/Temp
Conversion (%)
Yield (%)
TON
TOF (h–1)
tion (mol%)
(° C)
1
2
3
4
Pd–montm1
0.0045
0.0045
0.0027
0.005
3/150
6/160
6/160
3/160
79
86
71
93a
70
81
65
88
17,555
19,111
26,296
18,600
5,852
3,185
4,383
6,200
5
6
7
8
9
Pd–montm1
0.0048
0.0045
3/150
6/150
3/160
6/160
6/160
6/150
3/160
88
95
91
93
81
52
91b
83
90
88
88
77
49
85
18,300
19,792
20,222
20,667
32,400
32,500
18,200
6,111
3,299
6,741
3,444
5,400
5,417
6,067
0.0025
0.0016
0.005
10
11
a Reaction conditions: 4-chloroacetophenone (10 mmol), styrene (12 mmol), Na2CO3 (12 mmol), TBAB (0.6 mmol), NMP (10 mL).
b Ca(OH)2 as base.
Synlett 2006, No. 18, 3130–3134 © Thieme Stuttgart · New York

LETTER
Palladium–Montmorillonite Catalysts for Heck Coupling
3133
coupling in water promoted by microwave irradiation us-
ing Pd in ultra-low concentrations are of particular inter-
est.25
Table 6 Catalyst Recycling Experiments with Coupling of
Bromobenzene and Styrene
Entry
Catalyst
Run 1
Run 2
Run 3
For heterogeneous catalysts, clear evidence of the in-
volvement of soluble palladium species in the catalytic
cycle was provided by showing a correlation between the
concentration of leached Pd and the reaction rate.8a,22b,26 It
is also clear that an appropriate selection of support mate-
rials and preparation techniques allow the synthesis of
highly active heterogeneous catalysts, which are able to
promote the coupling reaction of all types of aryl chlo-
rides with satisfactory activities.
1
2
Pd–montm1a
Pd–montm2a
77
89
65
77
55
64
3
4
Pd–montm1b
Pd–montm2b
71
78
51
55
16
15
5
6
Pd–montm1a
Pd–montm2a
55
59
46
61
22
55
a Reaction conditions: alkene (1.5 equiv), Na2CO3 (1.2 equiv), catalyst
(0.3 mol%), 150 °C, 3 h.
On the basis of the highly successful application of Pd-
MCM-41 catalysts in recycling experiments disclosed in
a previous study,15 these new Pd–montmorillonite cata-
lysts were also reused after recovery in the reaction of bro-
mobenzene with styrene.27 Data in Table 5 show that
under the usual reaction conditions on using 0.3 mol% of
catalysts steadily decreasing conversions were observed
(entries 1 and 2). Changing the base from sodium carbon-
ate to a mixed organic–inorganic base (Et3N + Na2CO3)
and an increase in reaction temperature did not result in
any improvements (Table 5, entries 3 and 4). This, how-
ever, is not surprising considering the fact that Pd ions in
ion-exchanged montmorillonites are mobile and, there-
fore, sensitive species. Pd complexes, for example, tend to
decompose above 120 °C to form palladium metal.17 This
process certainly occurring under reaction conditions ap-
plied here (150–160 °C) combined with the leaching/re-
deposition process, obviously results in restructuring of
the original catalysts yielding a material significantly dif-
ferent from the original one concerning, in particular, dis-
persion of both ionic and metallic active sites.
b Reaction conditions: alkene (1.5 equiv), Et3N (1 equiv), Na2CO3 (0.5
equiv), catalyst (0.3 mol%), 160 °C, 3 h.
In conclusion, we have synthesized two new palladium–
montmorillonite catalysts. The catalysts are easy to pre-
pare and insensitive to air and moisture. Both catalysts
exhibit high activity in Heck coupling of aromatic bromo
and activated chloro derivatives with turnover numbers
close to those found in a recent study for a range of sup-
ported Pd catalysts.8 Further studies including the use of
appropriate catalyst characterization techniques, how-
ever, are certainly needed to improve the performance of
these new montmorillonite-supported palladium catalysts.
The main focus is to increase stabilities and enhance
activities to achieve the more challenging task of the
coupling of deactivated aryl chlorides.
Acknowledgment
We thank the Hungarian National Science Fund (OTKA grants
Leaching phenomena are known to depend on the oxida- T042603 and M041451) for financial support.
tion state of palladium.21,22b Pd(0) metal particles in zeo-
lites, for example, were shown to function as truly
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Synlett 2006, No. 18, 3130–3134 © Thieme Stuttgart · New York

3134
Á. Molnár, A. Papp
LETTER
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(16) Preparation of Catalysts. Before catalyst synthesis Na–
montmorillonite (Bentolite H, Laporte) was suspended in
distilled water overnight with stirring at r.t.
(27) Catalyst Recycling: Bromobenzene (1 mmol), styrene (1.5
mmol), Na2CO3 [1.2 mmol or Et3N (1 mmol)–Na2CO3 (0.5
mmol)], and Pd catalyst (0.3 mol%) were reacted in NMP (2
mL). Catalyst samples after reaction were separated by
centrifugation, washed successively with acetone, water,
acetone and, finally, dried under an infra lamp and reused.
(28) Palczewska, W. Adv. Catal. 1975, 24, 245.
Pd–montm1 (2.16% Pd loading): Pd(NO3)2·3H2O (0.4 g)
dissolved in water and acidified with concd HNO3 (0.4 mL)
was added to Na-montmorillonite (8 g). After stirring for 4 h
the solid was separated by centrifugation and washed
thoroughly with water.
Synlett 2006, No. 18, 3130–3134 © Thieme Stuttgart · New York