Pd-Cu-Exchanged montmorillonite K10 clay: An efficient and reusable heterogeneous catalyst for vinylation of aryl halides

Article abstract of DOI:10.1039/a705870e

Palladium and copper exchanged montmorillonite K10 clay has been found to catalyse very efficiently the reaction between aryl halides (X = Br, I) and acrylates, and styrenes affording alkyl (E)-cinnamates and (E)-stilbenes respectively in high yields.

Full text of DOI:10.1039/a705870e

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Pd–Cu–Exchanged montmorillonite K10 clay: an efficient and reusable
heterogeneous catalyst for vinylation of aryl halides
R. K. Ramchandani, B. S. Uphade, M. P. Vinod, R. D. Wakharkar, V. R. Choudhary and A. Sudalai*
National Chemical Laboratory, Pune 411 008, India
Palladium and copper exchanged montmorillonite K10 clay
has been found to catalyse very efficiently the reaction
between aryl halides (X = Br, I) and acrylates, and styrenes
affording alkyl (E)-cinnamates and (E)-stilbenes respec-
tively in high yields.
at 110 °C for 12 h. The metal content of the catalysts was
determined using an electron disperse X-ray microscope (EDX)
(Kevex, US) connected to a JEOL (JSM-5200) scanning
electron microscope (SEM). The surface area of Pd (0.29 wt%)–
Cu (0.36 wt%)–Mont.10 and Cu (0.39 wt%)–Mont.K10 being
262.9 and 238.9 m2 g21 respectively (BET method). The XRD
pattern and FT-IR spectra of these samples show that they are
crystalline, however no significant differences were observed
among their structures. Definite information about the nature of
the metal species present in the catalyst could be obtained from
cyclic voltammetry (CV). Fig. 1 shows the CV of Cu–Pd
exchanged Mont.K10 clay at a scan rate of 50 mV s21. It clearly
indicates two reversible peaks with E1/2 (vs. saturated calomel
electrode) at 20.119 and +0.117 V. We attribute, after
comparing the potential values from literature,6 the peak at
+0.117 to a Pd/Pd2+ process and the one at 20.119 V to a Cu/
Cu2+ process. Although two one-step electron processes (i.e.
Cu2+/Cu+ and Cu+/Cu) are possible, separate experiments with
Cu and Pd exchanged Mont.K10 clay have shown that the CV
behaviour corresponds to Cu2+/Cu and Pd/Pd2+ redox proc-
esses. Hence the presence of Pd is mediating the Cu redox
process (Fig. 2). This explains the high activity of the Pd–Cu
exchanged catalyst compared to the Cu or Pd exchanged
catalyst.
The palladium-catalysed vinylation of aryl halides (X = Br, I)
(Heck reaction) provides a very convenient and powerful
method for forming carbon–carbon bonds at unsubstituted
vinylic positions.1 Major improvements in Heck-type reactions
over the last decade have been brought about by the introduction
of tetraalkylammonium salts, silver(i) or thallium(i) salts as
promoting additives or by use of organotrifluoro-
methanesulfonates.2 Two recent promising approaches involve
reactions in aqueous (or water–organic solvent) media3 and the
use of vinyl iodonium salts.4 For the Heck reaction, a PdII salt
or complex is employed which is reduced in situ to an active
zero valent palladium species under homogeneous conditions.
The PdII species that is subsequently regenerated in the
reaction2 is very difficult to separate and reuse for the reaction
directly without further processing. In view of the practical and
industrial applications, improvements in catalyst efficiency and
catalyst recycling are essential. To overcome these problems,
heterogeneous catalysts such as polymer-supported palladium
catalysts and Pd–Ph2P–Si supported on clay have been reported
for the arylation of alkenes.5 However, these catalysts suffer
from drawbacks such as (i) their preparation involves many
steps and the use of expensive phosphorus and silicon
compounds and (ii) deactivation of the catalyst on its reuse. We
report here our preliminary results on the use of Pd and Cu
exchanged montmorillonite K10 (Mont.K10) clay catalysts for
the vinylation of aryl halides (X = Br, I) (Scheme 1).
Results of the reaction of iodobenzene with acrylates, over
different catalysts and over the best catalyst Pd (0.29 wt%)–Cu
(0.36 wt%)-exchanged Mont.K10 clay catalyst are presented in
Tables 1 and 2 respectively.† A comparison of the results in
1400
1000
600
Pd, Cu and Pd–Cu exchanged clay catalysts were prepared by
exchanging the clay with dilute aqueous PdCl2, Cu(NO3)2 or a
mixture of two respectively. For example, Pd (0.29 wt%)–Cu
(0.36 wt%)-exchanged Mont.K10 clay catalyst was prepared by
treating Mont.K10 clay (25 g) with 600 ml aqueous PdCl2 and
Cu(NO3)2 under vigorous stirring at room temperature for 24 h,
centrifuging and washing the Pd–Cu exchanged clay with
distilled deionised water repeatedly until the discarded filtrate
was free from Cl2 and NO32 ions, and drying the resulting mass
200
I
–200
–600
–1000
–1400
1200
800
400
0
–400
–800 –1200
E / mV
O
Fig. 1 Cyclic voltammogram of Pd–Cu exchanged Mont.K10 clay at a scan
rate of 50 mV s21
X
OR2
i
R1
O
R1
+
+
OR2
1
Pd2+
Cu
32–88%
2e–
X
ii
R3
R3
Pd
Cu2+
2
51–93%
Scheme 1 Reagents and conditions: i, Pd–Cu–Mont.K10 (10% m/m),
K2CO3, DMF, reflux, 2 h; ii, Pd–Cu–Mont.K10 (15% m/m), K2CO3, DMF,
reflux, 2–3 h
Electrode
Fig. 2 Pd mediates the redox behaviour of Cu
Chem. Commun., 1997
2071

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Table 1 Results of Heck reaction between methyl acrylates and iodo-
benzene or p-methoxyiodobenzene over different catalystsa
Table 3 Results of the reaction of aryl halides with styrene over Pd (0.29
wt%)–Cu (0.36 wt%)–Mont.K10 clay as catalyst
Yield (%) of methyl
Aryl halide
cinnamate 1
Yield (%) of
t/h stilbenea,b
Entry
R3
X
Entry
Catalyst
R1 = H
R1 = OMe
1
2
3
4
5
6
H
H
H
Cl
Cl
OMe
I
2
2
12
3
3
3
93
51
0
81
88
72
1
2
3
4
Cu (0.07 wt%)–Mont.K10
Cu (0.39 wt%)–Mont.K10
Pd (0.33 wt%)–Mont.K10
Pd (0.29 wt%)–Cu (0.36 wt%)–
Mont.K10
0
30
41
0
51
56
Br
Cl
Br
I
83
88
I
5
Pd (1.73 wt%)–Cu (0.16 wt%)–
Mont.K10
Cu (0.32 wt%)–kaolin
56
0
61
0
a Characterized by IR, 1H and 13C NMR and mass spectroscopy. b Isolated
after chromatographic purification.
6
a
Reaction conditions: iodobenzene (3 mmol), methyl acrylate (6 mmol),
anhydr. K2CO3 (8 mmol), catalyst (10% m/m), DMF (5 ml), reflux, 2 h.
b Isolated yield obtained after chromatographic purification.
Pd
Ar
X
Ar Cu
X
Ar Pd
X
O
Table 1 shows that among the catalysts, the Pd (0.29 wt%)–Cu
(0.36 wt%)-exchanged Mont.K10 clay catalyst gives the best
results for the Heck reaction. The higher activity of this catalyst
seems to be due to a synergistic effect produced by the presence
of Pd and Cu in close vicinity. The results also indicate that for
this catalyst to be more active, enough copper must be present
along with the palladium. It is remarkable that Cu (0.39 wt%)–
Mont.K10 clay also catalyses the Heck reaction leading to the
formation of (E)-cinnamate esters. The choice of solvent for the
arylation reaction catalysed by the Pd (0.29 wt%)–Cu (0.36
wt%)–Mont.K10 clay catalyst is also critical as the reaction fails
when carried out in MeCN under reflux. Among the different
bases tried for this reaction, viz. NaHCO3, K or Na acetate,
K2CO3, Et3N and Bu3N, K2CO3 gave the best results (i.e. high
yields with 100% selectivity).
RO
O
H
O
+ HX + Pd0
OR
Ar
Ar
OR
Pd
X
Scheme 2
The catalyst from the reaction mixture was recovered by
simple filtration and was successfully reused three times
without loosing its activity in the alkenylation of 4-methoxy-
iodobenzene with methyl acrylate.
In conclusion, the Pd (0.29 wt%)–Cu (0.36 wt%)–Mont.K10
clay catalyst has high activity for the Heck reaction in which
aryl halides (X = Br, I) are reacted with acrylates and styrenes
to form the corresponding (E)-cinnamates and (E)-stilbenes
respectively in high yields. The high activity of the catalyst is
attributed to a synergistic effect produced by the presence of Pd
and Cu species in the catalyst.
Table 2 lists the results for different types of aryl halides (X
= Br, I) that have been vinylated with methyl or ethyl acrylate
to afford the corresponding cinnamates in high yields using the
Pd (0.29 wt%)–Cu (0.36 wt%)–Mont.K10 clay catalyst. In all
the substrates examined, exclusive formation of the
1
(E)-cinnamate was observed (GC, H and 13C NMR and mass
The authors R. K. R. and B. S. U. thank CSIR, New Delhi,
India for the award of Senior Research Fellowships.
spectroscopy). Table 3 shows the results of the reaction of
various aryl halides (X = Cl, Br, I) with styrene producing
(E)-stilbenes in high yields using Pd–Cu–Mont.K10 catalyst
(15% m/m). A remarkable feature of these catalytic systems is
that even less reactive substituted bromobenzenes react with
styrene to give (E)-stilbenes in excellent yields. The reaction
however did not proceed using aryl iodides with substituents
such as nitro, amino, chloro etc. The reaction also failed when
methyl vinyl ketone and cyclohexenone were used as alkeny-
lating agents instead of acrylate. Mechanistically, it is proposed
that aryl halides react with copper presumably to yield an
organocopper species, which then transmetallates to PdII
followed by olefin insertion, b-hydride elimination and dis-
sociation of palladium hydride species2 (Scheme 2).
Footnotes and References
* E-mail: otech@ems.ncl.res.in
† In a typical reaction procedure, iodobenzene [580 mg, 2.8 mmol (1 :1
styrene)], methyl acrylate (500 mg, 5.8 mmol), anhydr. K2CO3 (1 g, 7.2
mmol) and catalyst (58 mg, 10% by weight of PhI) in DMF (5 ml), were
refluxed for 2 h. After completion of the reaction (TLC monitoring), the
catalyst was separated by filtration and the reaction mixture was poured into
water followed by its extraction with ethyl acetate to give the crude product
which was subsequently purified by column chromatography to afford
methyl (E)-cinnamate (380 mg, 83%), dC (50.3 MHz, CDCl3) 51.6, 117.9,
127.9, 128.3, 128.9, 129.2, 130.3, 134.4, 144.8 and 167.3; m/z 162 (M+,
65%), 161 (45), 131 (100), 117 (5), 103 (82) and 73 (91).
1 R. F. Heck, Org. React., 1982, 27, 345; R. F. Heck, Palladium Reagents
in Organic Synthesis, Academic Press, London, 1985; L. S. Hegedus, in
Organometallics in Synthesis, ed. M. Schlosser, Wiley, Chichester, 1994,
p. 383.
Table 2 Results of the reaction of aryl halides with acrylates over Pd (0.29
wt%)–Cu (0.36 wt%)–Mont.K10 clay catalyst
Aryl halide
2 A. de Meijere and F. E. Meyer, Angew. Chem., Int. Ed. Engl., 1994, 33,
2379.
3 J. P. Genet, E. Blart and M. Sarignac, Synlett, 1992, 715 and references
cited therein.
Acrylate
R2
Yield (%) of
cinnamate 1a,b
Entry
R1
X
1
2
3
4
5
6
7
H
H
I
Br
I
I
I
Me
Me
Me
Me
Me
Me
Et
83
32
88
76
69
58
86
4 R. M. Moriarty, W. R. Epa and A. K. Awasthi, J. Am. Chem. Soc., 1991,
113, 6315.
5 C. M. Anderson, K. Korabelas and A. Hallberg, J. Org. Chem., 1985, 50,
3891; B. M. Choudhary, R. M. Sarma and K. K. Rao, Tetrahedron, 1992,
48, 719.
6 V. Bertocci and D. R. Turner, in Encyclopedia of Electrochemistry of the
Elements, ed. A. J. Bard, Marcel Dekker, New York, 1974, vol. 2, p. 383
and vol. 6, p. 253.
4-OMe
4-Me
2-OMe
2-Me
4-OMe
I
I
a Characterised by IR, 1H and 13C NMR and mass spectroscopy. b Isolated
after chromatographic purification.
Received in Cambridge, UK, 11th August 1997; 7/05870E
2072
Chem. Commun., 1997