New Hybrid Inorganic-Organic Polymers as Supports for Heterogeneous Catalysis: A Novel Pd(O) Metalated Cyclophosphazene-Containing Polymer as an Efficient Heterogeneous Catalyst for the Heck Reaction

Article abstract of DOI:10.1021/ol026098z

(equation presented) A new Pd(0) complex of a pendant cyclophosphazene-containing cross-linked polymer is found to be an effective heterogeneous catalyst for the Heck arylation reaction. The catalyst is robust and can be recycled without significant loss of activity.

Full text of DOI:10.1021/ol026098z

ORGANIC
LETTERS
2002
Vol. 4, No. 12
2113-2116
New Hybrid Inorganic−Organic Polymers
as Supports for Heterogeneous
Catalysis: A Novel Pd(0) Metalated
Cyclophosphazene-Containing Polymer
as an Efficient Heterogeneous Catalyst
for the Heck Reaction
Vadapalli Chandrasekhar* and Arunachalampillai Athimoolam
Department of Chemistry, Indian Institute of Technology, Kanpur-208 016, India
Vc@iitk.ac.in
Received April 29, 2002
ABSTRACT
A new Pd(0) complex of a pendant cyclophosphazene-containing cross-linked polymer is found to be an effective heterogeneous catalyst for
the Heck arylation reaction. The catalyst is robust and can be recycled without significant loss of activity.
Polyphosphazenes, [NdPR2]n, constitute the largest and most
diverse class of inorganic polymers.1 The macromolecular
substitution reaction involving the replacement of chlorine
atoms2 on polydichlorophosphazene [NdPCl2]n by an ap-
propriate nucleophile (such as amino,2a-c alkoxy,2d-e or
aryloxy2f-h) is the principal synthetic method that has been
employed to prepare a large number of examples of this
family. The recent discovery of an ambient temperature route
to polydichlorophosphazene via the liVing cationic poly-
merization of the phosphoranimine Cl3PdNSiMe3 provides
an advanced method with molecular weight control and
narrow polydispersities.3 Despite their versatility, applications
of polyphosphazenes as heterogeneous catalyst supports are
fraught with difficulties. Thus, although polydichlorophos-
phazene can be prepared in the cross-linked form, substitution
of chlorine atoms on the latter is a complex task. Conse-
quently, designing a polymeric ligand with well-controlled
(2) (a) Allcock, H. R.; Cook, W. J.; Mark, D. P. Inorg. Chem. 1972, 11,
2584. (b) Allcock, H. R.; Kugel, R. L. Inorg. Chem. 1966, 5, 1716. (c)
White, J. E.; Singler, R. E.; Leone, S. A. J. Polym. Sci., Polym. Chem. Ed.
1975, 13, 2531. (d) Allcock, H. R.; Kugel, R. L.; Valan, K. J. Inorg. Chem.
1966, 5, 1709. (e) Allcock, H. R.; Connolly, M. S.; Sisko, J. T.; Al-Shali,
S. Macromolecules 1988, 21, 323. (f) Allen, G.; Lewis, C. J.; Todd, S. M.
Polymer 1970, 11, 31. (g) Singler, R. E.; Hagnauer, G. L.; Schneider, N.
S.; LaLiberte, B. R.; Sacher, R. E.; Matton, R. W. J. Polym. Sci., Polym.
Chem. Ed. 1974, 12, 433. (h) Thompson, J. E.; Reynard, K. A. J. Appl.
Polym. Sci. 1977, 21, 2575.
(1) For book and some additional reviews, see: (a) Mark, J. E.; Allcock,
H. R.; West, R. Inorganic Polymers; Prentice Hall: NJ, 1992. (b) De Jaeger,
R.; Gleria, M. Prog. Polym. Sci. 1998, 23, 179. (c) Manners, I. Angew.
Chem., Int. Ed. Engl. 1996, 35, 1602. (d) Neilson, R. H.; Wisian-Neilson,
P. Chem. ReV. 1988, 88, 541. (e) Allcock, H. R. Chem. Eng. News 1985,
22. (f) Allcock, H. R. Polymer 1980, 21, 673. (g) Allcock, H. R. Acc. Chem.
Res. 1979, 12, 351. (h) Allcock, H. R. Angew. Chem., Int. Ed. Engl. 1977,
16, 147.
(3) Honeyman, C. H.; Manners, I.; Morrissey, C. T.; Allcock, H. R. J.
Am. Chem. Soc. 1995, 117, 7035.
10.1021/ol026098z CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/22/2002

Scheme 1. Preparation of 5
coordination sites in a cross-linked polyphosphazene remains
The composition of complex 5 is confirmed from its
analytical data. An exact structure of this compound must
await an X-ray diffraction study. We were thus far unable
to obtain single crystals of 5 that are suitable for X-ray
diffraction. However, the 31P{1H} NMR of 5 shows two
resonances at -6.46 and -5.16 ppm. These correspond to
the uncoordinated phosphines. The phosphorus chemical shift
for the cyclophosphazene ring occurs at 9.10 ppm. The
coordinated phosphines resonate at 25.10 ppm.6 On this basis
we propose that two nongeminally substituted phosphines
are involved in coordination to a dicoordinate palladium.
Weakly bound solvent molecules and alternative coordination
schemes involving for example geminally substituted phos-
phines cannot be ruled out. However, it is clear that the
triphenylphosphine added during the reaction is not incor-
porated in the complex.
a very difficult proposition. In this regard hybrid polymers4
that contain the cyclophosphazene ring as a pendant group
attached at regular intervals to the backbone of an organic
polymer can be more versatile because of the following
reasons: (a) cyclophosphazenes with appropriate coordinat-
ing groups can be assembled to afford ligands with definite
coordinating capabilities and which can include a polymer-
izable vinyl group and (b) such suitably designed cyclo-
phosphazene monomers can be readily cross-linked by
copolymerization with divinylbenzene to afford cross-linked
polymeric ligands with definite knowledge on the orientation
and number of coordinating groups. Also, since the nature
of the coordinating group can be varied quite readily in the
pre-polymerization step, this methodology can be quite
general and polymeric ligand engineering can be ac-
complished in a facile manner leading to a library of ligands.5
In view of the importance of the Heck reaction in C-C bond
formation, we have designed a cyclophosphazene-containing
cross-linked polymeric ligand and have demonstrated its
applicability in the standard Heck arylation reaction. These
results are discussed herein.
The catalytic activity of 5 has been tested under the
standard Heck reaction conditions (Table 1). Compound 5
Table 1. Heck Arylation Reaction Catalyzed by 5a
The reaction of N3P3Cl6 (1) with 4-hydroxy-4′-vinyl-
biphenyl afforded N3P3Cl5(O-C6H4-p-C6H4-p-CHdCH2) (2).
Compound 2 is the primary starting material for the assembly
of the polymeric ligand as it possesses five reactive chlorines
and also the polymerizable vinyl group. Reaction of 2 with
OH-C6H4-p-PPh2 3 affords the fully substituted product 4
in a very good yield. The choice of 3 to introduce the
phosphino groups in the cyclophosphazene is deliberate. This
serves to keep the coordinating phosphino groups away from
the cyclophosphazene ring, and the immediate environment
around the phosphorus is analogous to a triphenylphosphine
as found in the Merrifield-Wang resin. Compound 4 shows
a molecular ion peak at 1717 in its FAB-MS. The 31P{1H}
NMR of 4 shows that the phosphorus chemical shifts
corresponding to the cyclophosphazene skeleton are iso-
chronous and resonate at 9.25 ppm. Two phosphine reso-
nances are seen at -6.54 and -6.59 ppm. Compound 4 was
used as a ligand for the preparation of Pd(0) complex 5 as
shown in Scheme 1.
time
(h)
productd
yield (%)
no.
X
catalyst
PdCl2
Pd(PPh3)4
5
5
5
5b
5b
5b
R
R1
1
2
3
4
5
6
7
8
I
I
I
I
I
I
I
Br
Ph
Ph
Ph
H
H
H
H
H
Me
H
5
5
2
3
3
20
20
60
67c
61c
98
99
96
72c
69c
99
b
COOMe
COOEt
COOMe
CN
Ph
H
a Reaction conditions: aryl halide (5 mmol), olefin (5 mmol), (n-Bu)3N
(5 mmol), and catalyst (0.01 mol %) are dissolved in acetonitrile (5 mL)
and kept at a bath temperature 90 °C in an argon atmosphere. The reaction
was monitored by thin-layer chromatography. b Aryl halide (5 mmol), olefin
(6.25 mmol), (n-Bu)3N (6.25 mmol), and catalyst (0.1 mol %). c Unreacted
iodobenzene was removed by column chromatography. d A single product
corresponding to the trans-arylation was obtained in each case. Yield
represents isolated yield.
2114
Org. Lett., Vol. 4, No. 12, 2002

Scheme 2. Preparation of the Cross-Linked Polymeric Catalyst CPPLP-Pd
is more active than PdCl2 or Pd(PPh3)4 as well as other
Although CPPLP-Pd is not effective with substrates such
as acrylonitrile or bromobenzene, it is found to be quite
efficient in catalyzing the Heck reaction in other situations.
The isolated yields of the products are quite high.
Although slightly longer reaction times are involved with
CPPLP-Pd in comparison to 5, this is understandable in
view of the heterogeneous nature of the former. It must be
noted that hitherto the application of polymer-supported
catalysts for the Heck reaction have not been very successful,
and only a few examples have been published.8 In some
instances the actual catalytic species was shown to be some
palladium species, leaching out into solution.9
To confirm that the activity of CPPLP-Pd arises from
the polymer-bound metal, the following experiments were
carried out. After the completion of the first cycle, the
catalyst was removed by filtration, washed sequentially with
dichloromethane, methanol, and acetone, and dried in a
vacuum for 3 h at 40 °C and analyzed by elemental analysis
recently reported homogeneous catalysts.7 In all the cases,
only the trans isomer is formed exclusively. Bromobenzene
can also be used in the reaction with styrene although the
reaction time is longer. Similarly a disubstituted olefin such
as methyl methacrylate could also be arylated.
Encouraged by the results of 5, we incorporated the
structural motif of 4 in the cross-linked polymer CPPLP as
shown in Scheme 2 by the copolymerization of 4 with
divinylbenzene. A Pd(0) complex of CPPLP was prepared
by adopting the procedure used for the preparation of the
complex 5. To ensure that the palladium incorporation in
CPPLP-Pd is a result of coordination and not due to
occlusion, the latter was washed thoroughly with dichloro-
methane, acetonitrile, methanol, and acetone and dried. The
catalyst CPPLP-Pd was characterized by elemental analysis
and infrared spectroscopy. The amount of palladium in
CPPLP-Pd was determined by atomic absorption spec-
troscopy to be 18.9 mg/g of the polymer.
The catalytic activity of CPPLP-Pd has been tested by
the same method as described in the Heck arylation reaction
by 5. The results obtained are summarized in Table 2.
Table 2. CPPLP-Pd Catalyzed Heck Arylation Reactiona
(4) (a) Allcock, H. R.; Laredo, W. R.; Kellam, E. C., III; Morford, R.
V. Macromolecules 2001, 34, 787. (b) Allen, C. W.; Brown, E. D.; Worley,
S. D. Inorg. Chem. 2000, 39, 810. (c) Allcock, H. R.; McIntosh, M. B.;
Klingenberg, E. H.; Napierala, M. E. Macromolecules 1998, 31, 5255. (d)
Allen, C. W. Main Group Chem. News 1998, 6(4), 6. (e) Inoue, K.; Sasaki,
Y.; Itaya, T.; Tanigaki, T. Eur. Polym. J. 1997, 33, 841. (f) Selvaraj I. I.;
Chandrasekhar, V. Polymer 1997, 38, 3617. (g) Allen, C. W. Trends. Polym.
Sci. 1994, 2, 342. (h) McNally, L.; Allen, C. W. Heteroatom Chem. 1993,
4, 159. (i) Allen, C. W. Stud. Inorg. Chem. 1992, 14, 171. (j) Inoue, K.;
Nitta, H.; Tanigaki, T. Makromol. Chem., Rapid Commun. 1990, 11, 467.
(k) Inoue, K.; Nakano, M.; Takagi, M.; Tanigaki, T. Macromolecules 1989,
22, 1530. (l) Inoue, K.; Takagi, M.; Nakano, M.; Nakamura, H.; Tanigaki,
T. Makromol. Chem., Rapid Commun. 1988, 9, 345. (m) Allen, C. W.;
Bright, R. P. Macromolecules 1986, 19, 571. (n) Allen, C. W. J. Polym.
Sci. Polym. Symp. 1983, 70, 79.
(5) Chandrasekhar, V.; Nagendran, S. Chem. Soc. ReV. 2001, 30, 193.
(6) Mann, B. E.; Musco, A. J. Chem. Soc., Dalton Trans. 1975, 1673.
(7) (a) Wu, Y.; Hou, J.; Yun, H.; Cui, X.; Yuan, R. J. Organomet. Chem.
2001, 637-639, 793. (b) Sjo¨vall, S.; Johansson, M. H.; Andersson, C. Eur.
J. Inorg. Chem. 2001, 2907.
(8) (a) Beletskaya I. P.; Cheprakov, A. V. Chem. ReV. 2000, 100, 3009.
(b) Terasawa, M.; Kaneda, K.; Imanaka, T.; Teranishi, S. J. Organomet.
Chem. 1978, 162, 403. (c) Andersson, C.-M.; Karabelas, K.; Hallberg, A.
J. Org. Chem. 1985, 50, 3891. (d) Wang, P.-W.; Fox, M. A. J. Org. Chem.
1994, 59, 5358.
time
(h)
product
yieldd (%)
no.
X
catalyst
R
R1
1
2
3
4
5
6
7
8
9
I
I
I
I
I
Br
I
I
I
CP P LP -P d
CP P LP -P d
CP P LP -P d
CP P LP -P d b COOMe
CP P LP -P d b CN
CP P LP -P d b Ph
cycle-2
cycle-3
cycle-4
Ph
H
H
H
Me
H
H
H
H
H
4
5
5
30
20
70
4
93
99
98
79c
-
COOMe
COOEt
-
Ph
Ph
Ph
89
86
82
4
4
a The cross-linked polymer CPPLP was prepared by copolymerizing 4
with divinylbenzene in a 2.5:1 weight ratio. Reaction conditions of catalysis:
aryl halide (5 mmol), olefin (6.25 mmol), (n-Bu)3N (6.25 mmol), and catalyst
(0.036 mol %) are dissolved in acetonitrile (5 mL) and heated at 90 °C in
an argon atmosphere. The catalyst concentration is calculated with respect
to the amount of palladium contained in the polymer. b Aryl halide (5 mmol),
olefin (6.25 mmol), (n-Bu)3N (6.25 mmol), and catalyst (0.1 mol %).
c Unreacted iodobenzene was removed by column chromatography. d Yield
represents isolated yield.
(9) (a) Zhao, F.; Shirai, M.; Arai, M. J. Mol. Catal. A: Chem. 2000,
154, 39. (b) Biffis, A.; Zeccz, M.; Basatom, M. Eur. J. Inorg. Chem. 2001,
1131.
Org. Lett., Vol. 4, No. 12, 2002
2115

and infrared spectroscopy. The results of these analyses
showed that there is no change in the structural integrity of
the polymeric catalyst. This can be attributed to the effective
multisite coordinating capability of the polymeric cyclo-
phosphazene ligand. To find whether palladium leaching
occurs during the reaction as reported earlier,10 we have
estimated the amount of palladium of the recovered catalyst
by atomic absorption spectroscopy. No significant change
in the palladium content was found. Further, the recovered
catalyst was tested in three successive cycles; the catalytic
activity remains essentially undiminished (Table 2; entries
7, 8, and 9).
styrene and allowed the reaction to continue. At this stage
the conversion was about 53%. The reaction did not proceed
to any significant extent even after 4 h as found by TLC.
In summary, we have synthesized and demonstrated the
catalytic application of a new cross-linked Pd(0)-metalated
polymeric catalyst. The major strength of this approach is
the flexibility in obtaining a desired polymeric ligand whose
coordination properties can be reliably determined by a
simple model compound study. In our view this approach
would considerably augment the existing methodologies
available for polymer supports.
As a further test to ascertain that the reaction occurs due
to the polymeric catalyst CPPLP-Pd and not due to any
adventitious leached out palladium species, we filtered the
catalyst after 2 h of the reaction between iodobenzene and
Acknowledgment. We thank the Department of Science
and Technology, New Delhi for financial support. A.A.
thanks the Council of Scientific and Industrial Research for
the award of a Senior Research Fellowship.
(10) Schwarz, J.; Bo¨hm, V. P. W.; Gardiner, M. G.; Grosche, M.;
Herrmann, W. A.; Hieringer, W.; Raudaschl-Sieber, G. Chem. Eur. J. 2000,
6, 1773.
OL026098Z
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Org. Lett., Vol. 4, No. 12, 2002