A new polyaniline-based catalyst for the oxidation of alkenes

Article abstract of DOI:10.1055/s-2002-35566

Polymer-supported heterogeneous catalysts based on cobalt acetate and Co(II)Salen were synthesized. As a support, conjugated polymers were used: polyaniline, poly-o-toluidine or poly-o-anisidine. Oxidation reactions of alkenes were carried out on synthesized polymer-supported catalysts at atmospheric pressure in the presence of molecular oxygen. Epoxides or ketones were obtained as main products. Oxidation reactions were carried out under very mild conditions, at room temperature, and the desired products were obtained in very high selectivity and relatively high yields.

Full text of DOI:10.1055/s-2002-35566

LETTER
2107
A New Polyaniline-Based Catalyst for the Oxidation of Alkenes
A
G
N
ew Polyaniline
r
-Based
z
C
atalyst for
e
the
O
xidat
g
ion of
A
lken
o
es rz Kowalski, Jan Pielichowski*
Department of Polymer Science and Technology, Cracow University of Technology, ul. Warszawska 24, 31-155 Krakow, Poland
Fax +48(12)6282038; E-mail: pielich@usk.pk.edu.pl
Received 7 October 2002
6
We chose cobalt acetate and Co(II)Salen complex as cat-
alytically active species, which were immobilized into the
polymer matrix. In our experiments the catalysts were
prepared by a two-step method: (i) preparation of the
Abstract: Polymer-supported heterogeneous catalysts based on co-
balt acetate and Co(II)Salen were synthesized. As a support, conju-
gated polymers were used: polyaniline, poly-o-toluidine or poly-o-
anisidine. Oxidation reactions of alkenes were carried out on syn-
thesized polymer-supported catalysts at atmospheric pressure in the polymers by chemical oxidative polymerization of mono-
presence of molecular oxygen. Epoxides or ketones were obtained mers in HCl solution at –5 °C by ammonium persulfate
as main products. Oxidation reactions were carried out under very
mild conditions, at room temperature, and the desired products were
obtained in very high selectivity and relatively high yields.
7
and then deprotonation in aqueous ammonia solution and
(
ii) supporting of cobalt-based catalysts on polymers.8
Oxidation reactions of alkenes were carried out in aceto-
nitrile under very mild conditions. The experimental pro-
cedure involves a simple mixing of alkene, polymer
supported catalyst and 2-methylpropanal in a flask. We
used molecular oxygen at atmospheric pressure as an ox-
idizing agent which was bubbled through the reaction me-
dium. 2-Methylpropanal plays the role of reductant as
well as co-catalyst. Then the reaction mixture was mixed
until completion of the reaction (Table 1).9
Key words: oxidation, polyaniline based-catalysts, conjugated-
polymers, catalysts, alkenes
The use of polymer-supported catalysts offers several ad-
vantages in preparative procedures. The main advantage
of this type of catalyst lies in the much simpler purifica-
tion of products and the possibility of isolation as well as
1
reuse of a catalyst. In 1988, a new direction in conjugated
polymers research evolved. They were used as convenient After completion of the oxidation of alkenes the solvent
catalytic supports, on which catalytic active species could was evaporated to yield a residue, which was dissolved in
be deposited. In the first application of conjugated poly- ethyl acetate. Then the mixture was extracted with aque-
mers, the authors used catalytically active heteropolyan- ous NaHCO and water. Finally, the organic layer was
3
2
ions. Hirao et al. has used another approach. They dried and the solvent was evaporated to give the desired
inserted transition metals (Pd, Fe) into a polyaniline ma- product (Scheme 1).
3
trix. Especially, polyaniline and its derivatives have an
advantage over other polymers due to the presence of ni-
trogen atoms. Therefore, polymer and catalytically active
metals can form polymer-metal complexes which are sta-
ble in the reaction medium and can be recycled. Poly-
aniline Co(II) supported catalysts were introduced by
4
Pielichowski et al. in 1997.
Interesting results in homogeneous catalysis have been
achieved by using catalytic systems based on cobalt salts
5
or complexes for selected oxidation processes. Our pur-
pose was to combine the advantages of immobilized catal-
ysis and use of molecular oxygen as an oxidizing agent
and to develop polymer-supported metals for the efficient
synthesis of diverse organic compounds. In this paper, we
describe polyaniline (PANI), or its derivatives (poly-o-
toluidine – POT and poly-o-anisidine – POA), supported
cobalt catalysts that proved to be effective in a number of
practically important oxidation reactions of organic com-
pounds. All reactions were carried out in the presence of Scheme 1
molecular oxygen at room temperature under atmospheric
pressure.
Generally, all used polymers (PANI, POT and POA) are
effective as supports for the cobalt-based catalyst. We
found that for the reactions investigated in the course of
this work there were no significant changes between
supports. We observed only minor increases in yield of
Synlett 2002, No. 12, 02 12 2002. Article Identifier:
1
©
437-2096,E;2002,0,12,2107,2109,ftx,en;G29002ST.pdf.
Georg Thieme Verlag Stuttgart · New York
ISSN 0936-5214

2
108
G. Kowalski, J. Pielichowski
LETTER
Table 1 Results of the Polymer-Supported Cobalt Catalyst Oxidation with Molecular Oxygena
Substrate
Yield (%)b
PANI + Co(CH COO)
31 (20 h)
26 (20 h)
18 (20 h)
38 (20 h)
15 (20 h)
36 (20 h)
71 (5 h)
71 (5 h)
70 (5 h)
82 (4 h)
78 (4 h)
74 (4 h)
86 (2 h)
89 (2 h)
87 (2 h)
87 (2 h)
21 (2 h)
88 (2 h)
17 (1 h)
93 (1 h)
92 (1 h)
85 (3 h)
85 (3 h)
91 (3 h)
3
2
POT + Co(CH COO)2
3
POA + Co(CH COO)
3
2
PANI + Co(II)Salen
POT + Co(II)Salen
POA + Co(II)Salen
a
All the products were characterized by analytical and spectroscopic data.
Reaction time is shown in parentheses.
b
Table 2 Total Co(II) Content in Catalysts before and after Oxida-
tion Reaction
oxidation of 1-dodecene and -pinene when we used
PANI as a support both for Co(CH COO)2 and
3
Co(II)Salen.
Co(II) content [wt%]
However, in all tested reactions, the catalysts are charac-
terized by high selectivity and relatively high yields. In
some cases abnormally low yield is observed, what is
caused by steric hindrance between specific catalytically
active centre on polymer particles and the alkene. Interest-
before
1.69
2.6
after
1.24
1.99
1.58
0.30
1.02
0.26
PANI + Co(CH COO)
3
2
POT + Co(CH COO)
3
2
10
POA + Co(CH COO)
2.79
0.38
1.43
0.35
ing results were obtained on homogeneous and
3
2
1
1
Co(II)Salen in zeolites catalysts, but these reactions
were carried out under high pressure of oxygen.
PANI + Co(II)Salen
POT + Co(II)Salen
Alkenes with terminal double bond display low reactivity
therefore they need longer reaction times. Under these POA + Co(II)Salen
conditions, 1-dodecene gives 1,2-decyloxirane 2 with
very high selectivity. No changes in yield were observed
Taking into consideration the Co(II) content in the poly-
mers, which is ranged between 0.35 and 2.79 wt%, and
only insignificant changes in yield of trans-stilbene ep-
oxidation, it could be supposed that only some of the Co
species are active in the oxidation reaction. It is probably
due to the heterogeneous character of the catalyst that in-
volves difficult contact between the active catalytic sites
and substrate taking part in oxidation reaction. It was also
observed that only a small part of the initial amount of cat-
alyst is deposited into the polymer matrix.
when the same reactions were carried out for 120 hours.
Oxidation of double bonds proceeds more easily when the
double bond is activated by the presence of another group.
Oxidation of 3 under the same reaction conditions gives
two products: epoxide 4, which was the main product and
as a by-product small amounts (less then 5%) of ketone 5.
Trans-stilbene gives epoxide 10 with very high yield and
selectivity in a short time. Catalysts based on cobalt ace-
tate need less time to obtain satisfactory yields of 10. In-
terestingly, indene is transformed into two isomeric
ketones 7 and 8. The molar ratio of isomer 7 to 8 was In conclusion, we have described the synthesis of conju-
5:15 and it was constant for all reactions. Even small gated polymer-supported cobalt based catalysts, which
8
amounts of epoxides were not detected.
are very effective in oxidizing alkenes under mild condi-
tions. The described method allows to oxidize alkenes at
room temperature using molecular oxygen. Insignificant
increases in yield of oxidation of 1-decene and -pinene
were observed when we used PANI as a support. The
main advantage of our catalysts is the simplicity of sepa-
rating heterogeneous catalysts from the reaction medium
by filtration.
We also studied the total Co(II) content in all tested poly-
mers before and after the oxidation reaction. The results
summarized in Table 2 showed that the Co(II) content
changes irregularly on different supports and is decreas-
ing during the oxidation reaction.
Synlett 2002, No. 12, 2107–2109 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
A New Polyaniline-Based Catalyst for the Oxidation of Alkenes
2109
Acknowledgment
solution was then cooled to about –5 °C in a salt–ice
mixture. Ammonium persulfate (0.125 mol, 28.5 g),
dissolved in water, was added dropwise to a vigorously
Authors thank the Polish State Committee for Scientific Research
KBN), grant No. 7 T09B 03721, for financial support.
(
stirred mixture. The rate of addition of the oxidizing agent
was used to control the temperature of the reaction mixture,
which varied between –5 °C and –3 °C. The reaction was
stirred for 4 h as recommended in literature. The greenish-
black precipitate was filtered off, washed with water and
then with methanol until the filtrate became colorless.
Oligomers were extracted from polyaniline with THF in a
Soxhlet apparatus until the extract became colorless. Poly-o-
toluidine and poly-o-anisidine were washed with water and
MeOH only. The polymer was then dried at 60 °C for 24 h
References
(
1) Hartley, F. R. Supported Metal Complexes. A New
Generation of Catalysts; Reidel: Dordrecht, 1985.
(
2) (a) Bidan, G.; Genies, E. M.; Lapkowski, M. J. Chem. Soc.,
Chem. Commun. 1988, 533. (b) Bidan, G.; Genies, E. M.;
Lapkowski, M. J. Electroanal. Chem. 1988, 251, 297.
(
1
c) Bidan, G.; Lapkowski, M.; Travers, J. P. Synth. Met.
989, 28, C113. (d) Shimidzu, T.; Ohtani, A.; Aiba, M.;
and deprotonated with 250 mL of 12% aq NH solution for
3
6
h. Deprotonated polymer was dried at 60 °C for 24 h and
Honda, K. J. Chem. Soc., Faraday Trans. 1 1988, 84, 3491.
3) (a) Hirao, T.; Higuchi, M.; Ohshiro, Y.; Ikeda, I. Chem. Lett.
washed again with water and MeOH. All these purification
steps are important for the entire removal of all oligomeric
species accompanying the polymer.
(
1993, 1889. (b) Hirao, T.; Higuchi, M.; Hatano, B.; Ikeda, I.
Tetrahedron Lett. 1995, 36, 5925. (c) Higuchi, M.; Imoda,
D.; Hirao, T. Macromolecules 1996, 29, 8277. (d) Higuchi,
M.; Ikeda, I.; Hirao, T. J. Org. Chem. 1997, 62, 1072.
4) (a) Bhaskar, C. D.; Iqbal, J. Tetrahedron Lett. 1997, 38,
(
8) (a) Preparation of Polyaniline Supported Cobalt(II)
Acetate is Representative for the Procedure Employed
for Cobalt(II) Acetate Based Catalysts: A mixture of
polyaniline (500 mg) and cobalt acetate (500 mg) was stirred
in an MeCN (25 mL) and HOAc (25 mL) mixture for 72 h
at r.t. Then the reaction mixture was filtered and the solid
catalyst was washed with MeCN (5 5 mL). The catalyst
was dried at 110 °C for 24 h. (b) Preparation of
(
(
1235. (b) Pielichowski, K. Solid State Ionics 1997, 104,
123. (c) Pielichowski, J.; Iqbal, J.; Polaczek, J. Pol. Appl. PL
323292, 1997.
5) (a) Punniyamurthy, T.; Bhatia, B.; Reddy, M. M.; Maikap,
G. C.; Iqbal, J. Tetrahedron 1997, 53, 7649.
Polyaniline Supported Co(II)Salen is Representative for
the Procedure Employed for Co(II)Salen Based
Catalysts: A mixture of polyaniline (500 mg) and
(
b) Punniyamurthy, T.; Reddy, M. M.; Kalra, S. J. S.; Iqbal,
J. Pure Appl. Chem. 1996, 68, 619. (c) Reddy, M. M.;
Punniyamurthy, T.; Iqbal, J. Tetrahedron Lett. 1995, 36,
Co(II)Salen (500 mg) was stirred in 50 mL of MeCN for
159.
48 h at r.t. The reaction mixture was filtered and the solid
(
6) Method of Synthesis of Co(II)Salen: In 50 mL of 95%
boiling EtOH 4.9 g (40 mmol) of salicylaldehyde was
dissolved. To this solution 1.2 g (20 mmol) of
catalyst was stirred again with 50 mL of HOAc for 1 h at r.t.
The reaction mixture was filtered and the solid filtrate was
first washed with HOAc (3 10 mL) and then MeCN (5
mL). The catalyst was dried at 110 °C for 24 h.
5
ethylenediamine was added dropwise. The reaction was
stirred for 3.5 h. After completion, the reaction mixture was
cooled. N,N -Bis(salicylidene)ethylenediimine yellow
crystals were precipitated, filtered out, washed with 20 mL
of EtOH and finally dried. Subsequently, 2 g (7.45 mmol) of
Schiff base was dissolved in 120 mL of EtOH. The reaction
was carried out under argon atmosphere. The cobalt(II)
acetate 1.85 g (8.37 mmol) solution in 15 mL of warm water
(
9) The Oxidation of 1-Dodecene is Representative for the
General Procedure Employed for the Oxidation of
Alkenes: The mixture of 2-methylpropanal (1.08 g, 15
mmol) and polymer-supported catalyst (30 mg) was
dissolved in MeCN (30 mL). The mixture was bubbled with
oxygen for 15 min at r.t. Then, 1-dodecene (0.84 g, 5 mmol)
was added. The reaction was carried out under oxygen
atmosphere for the time period indicated in Table 1. After
completion of the reaction the catalyst was filtered and the
solvent was evaporated to yield a residue, which was
(60 °C) was added dropwise. A brown-red gelatinous
product was formed, which next changed to red crystals. The
reaction was stirred for 1 h. After cooling, the crystals of
Co(II)Salen complex were filtered and dried at 60 °C (under
vacuum) overnight.
dissolved in ethyl acetate and washed with NaHCO solution
3
and water. The organic phase was dried over MgSO and the
(
7) The Preparation of Polyaniline is Representative for the
General Procedure of Monomer Polymerization:
Before polymerization aniline was distilled under vacuum in
the presence of Zn powder.
4
evaporation of solvent yielded the desired product, which
was purified by Kugelrohr distillation.
(
10) Yamada, T.; Imagawa, K.; Nagata, T.; Mukaiyama, T.
Chem. Lett. 1992, 11, 2231.
11) Schuster, C.; Hölderich, W. F. Catal. Today 2000, 60, 193.
In a typical procedure, freshly distilled aniline (0.1 mol,
(
9.3 g) was dissolved in 150 mL of 1.5 M aq HCl. The
Synlett 2002, No. 12, 2107–2109 ISSN 0936-5214 © Thieme Stuttgart · New York