A ruthenium porphyrin catalyst immobilized in a highly cross-linked polymer

Article abstract of DOI:10.1021/ol016754f

MATRIX PRESENTED An immobilized Ru(meso-tetraarylporphyrin) complex was shown to be a very efficient catalyst for the epoxidation of olefins. In the presence of HBr, alcohols and even alkanes could be converted to the corresponding ketones. For the latter reactions the polymeric catalyst displayed a significantly enhanced activity when compared to that of the homogeneous version.

Full text of DOI:10.1021/ol016754f

ORGANIC
LETTERS
2
001
Vol. 3, No. 24
907-3909
A Ruthenium Porphyrin Catalyst
Immobilized in a Highly Cross-linked
Polymer
3
Oliver Nestler and Kay Severin*
Institut de Chimie Min e´ rale et Analytique, EÄ cole Polytechnique F e´ d e´ rale de Lausanne,
1015 Lausanne, Switzerland
kay.seVerin@icma.unil.ch
Received September 14, 2001
ABSTRACT
An immobilized Ru(meso-tetraarylporphyrin) complex was shown to be a very efficient catalyst for the epoxidation of olefins. In the presence
of HBr, alcohols and even alkanes could be converted to the corresponding ketones. For the latter reactions the polymeric catalyst displayed
a significantly enhanced activity when compared to that of the homogeneous version.
Macroporous, highly cross-linked polymers are excellent
matrices for immobilized transition metal catalysts. As a
result of the permanent pore structure and the high surface
disubstituted pyridine N-oxides as the oxidant, the efficient
conversion of olefins and sulfides to epoxides and sulfoxides
is possible. In the presence of mineral acids even inactivated
alkanes can be oxidized.
2
-1 1
area (typically between 100 and 600 m g ), the acces-
sibility of the catalytically active centers is very good.
Furthermore, a variety of different solvents can be used for
catalysis, whereas for classical Merrifield-type resins solvents
with good swelling abilities are needed. An additional
advantage is that the catalysts are fixed within a very rigid
matrix. As a result, catalyst deactivation by intermolecular
reactions is strongly suppressed. Moreover, the microenvi-
ronment of the catalyst and thus the activity and selectivity
Immobilized ruthenium porphyrin catalysts for alkene
epoxidation have been prepared by Che et al. They have
employed a standard Merrifield resin with covalently attached
6
Ru-porphyrins or a surface modified mesoporous molecular
7
sieve with coordinative bonding to a Ru-porphyrin. In both
(3) For some selected examples see: (a) Clapham, B.; Reger, T. S.; Janda,
K. D. Tetrahedron 2001, 57, 4637-4662 and references therein. (b) Taylor,
R. A.; Santora, B. P.; Gagn e´ , M. R. Org. Lett. 2000, 2, 1781-1783. (c)
Canali, L.; Cowan, E.; Deleuze, H.; Gibson, C. L.; Sherrington, D. C. Chem.
Commun. 1998, 2561-2562. (d) Nozaki, K.; Itoi, Y.; Shibahara, F.;
Shirakawa, E.; Ohta, T.; Takaya, H.; Hiyama, T. J. Am. Chem. Soc. 1998,
2
can be modified using the technique of molecular imprinting.
Given these advantages, it is surprising that polymeric
supports of this kind have rarely been employed compared
to other organic matrices.3,4
1
20, 4051-4052. (e) Breysse, E.; Pinel, C.; Lemaire, M. Tetrahedron:
Asymmetry 1998, 9, 897-900. (f) Santora, B. P.; Larsen, A. O.; Gagn e´ , M.
R. Organometallics 1998, 17, 3138-3140. (g) Salvadori, P.; Pini, D.; Petri,
A. J. Am. Chem. Soc. 1997, 119, 6929-6930.
Mimicking natural metalloenzymes, manganese- and iron-
containing porphyrin catalysts have mainly been used for
oxidation reactions, but in recent years Ru(CO)(porphyrin)
(4) For molecularly imprinted polymers with transition metal catalysts
see: (a) Polborn, K.; Severin, K. Chem. Eur. J. 2000, 6, 4604-4611. (b)
Polborn, K.; Severin, K. Eur. J. Inorg. Chem. 2000, 1687-1692. (c) Polborn,
K.; Severin, K. Chem. Commun. 1999, 2481-2482. (d) Locatelli, F.; Gamez,
P.; Lemaire, M. J. Mol. Catal. A 1998, 135, 89-98. (e) Matsui, J.; Nicholls,
I. A.; Karube, I.; Mosbach, K. J. Org. Chem. 1996, 61, 5414-5417.
(5) (a) Gross, Z.; Ini, S. Inorg. Chem. 1999, 38, 1446-1449. (b) Liu,
C.-J., Yu, W.-Y.; Che, C.-M.; Yeung, C.-H. J. Org. Chem. 1999, 64, 7365-
7374. (c) Ohtake, H.; Higushi, T.; Hirobe, M. Heterocycles 1995, 40, 867-
903 and references therein.
5
complexes have received increasing attention. With 2,6-
(
1) Santora, B. P.; Gagn e´ , M. R.; Moloy, K. G.; Radu, N. S. Macro-
molecules 2001, 34, 658-661.
2) Molecularly Imprinted Polymers; Sellergren, B, Ed.; Elsevier: Am-
sterdam, 2001.
(
1
0.1021/ol016754f CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/03/2001

cases selective catalysts with good activities were obtained.
For the latter system, however, catalyst leaching and/or
deactivation was found to be a problem.
Here we describe an immobilized ruthenium porphyrin
catalyst, the basic matrix of which is a highly cross-linked
organic polymer. With this catalyst we were able to ef-
ficiently oxidize olefins, secondary alcohols, and surprisingly
even alkanes. Especially in the latter reactions the hetero-
geneous catalyst was clearly superior to the corresponding
homogeneous version.
To evaluate the catalytic properties of our polymer we first
investigated the epoxidation of various olefins. The reactions
were performed with 2,6-dichloropyridine N-oxide as the
oxidant. By using only 0.2 mol % catalyst excellent yields
of the corresponding epoxides were obtained after 24 h, the
only exception being trans-stilbene (Table 1). This substrate
is known to be less suited for Ru(porphyrin)-catalyzed
9
epoxidations. A competition experiment with the catalyst
P1 and equal amounts of cis- and trans-stilbene gave 93%
cis- and 31% trans-stilbeneoxide. This is in contrast to the
results obtained with the homogeneous catalyst 1 (94% cis;
5% trans), indicating that the selectivity is affected by the
polymeric support. Interestingly, an increased reactivity
toward trans-stilbene was also observed for the resin-bound
To incorporate the ruthenium complex, we first synthesized
the vinyl-substituted meso-tetraarylporphyrin complex 1 by
8
reaction of the free base porphyrin with [Ru
3
(CO)12] in
boiling toluene. It is important to add a scavenger such as
styrene to the reaction mixture. Otherwise the vinyl groups
of the porphyrin are partially reduced, presumably in a
ruthenium-catalyzed reaction.
6
catalyst described by Che.
Overall the activity of the polymeric catalyst P1 in
epoxidation reactions is comparable or slightly higher than
that of the homogeneous version 1: for styrene an initial
-
1
-1
turnover frequency of 40 h was determined; 27 h was
found for catalyst 1. The stability of the catalyst P1 is good:
using larger amounts of styrene, turnover numbers of more
3
than 5 × 10 have been observed. Nevertheless, a slightly
decreased activity (approximately 15%) was found upon
recycling of the catalyst by filtration.
It has been shown by Hirobe et al. that in the presence of
HCl and HBr, Ru(porphyrin) complexes can be used to
oxidize compounds with very low reactivity such as alkanes.5
The utilization of supported catalysts for this type of reaction
is especially challenging because the substrate will have to
compete with a very high local concentration of a polymeric
matrix having a similar reactivity (autoxidation). Conse-
quently, alkane oxidations with immobilized Ru(porphyrin)
complexes have not been described so far.
Complex 1 was subsequently copolymerized with ethylene
glycol dimethacrylate (1:EGDMA ) 1:99) in the presence
of an equal volume of CHCl
porogen. To initiate the reaction we employed 2,2′-azobis-
4-methoxy-2,4-dimethylvaleronitrile). The dark red polymer
3
/THF (98:2), which acts as a
(
P1 was ground and sieved (particle size, 25-100 µm). The
nearly quantitative incorporation of the Ru-complex in the
polymer was evidenced by the fact that the THF washing
solutions were colorless and by ICP analysis.
With 1 mol % of the polymeric catalyst P1, however,
aromatic alkanes such as ethylbenzene or Indane are oxidized
to acetophenone and indanone in good or excellent yields
(Table 2). Here, the difference between P1 and the homo-
geneous catalyst 1 is striking, particularly for the oxidation
Table 1. Catalytic Epoxidation of Olefins with the Polymeric
Catalyst P1a
Table 2. Catalytic Oxidation of Aromatic Alkanesa
a
a
The reactions were performed in CHCl3 at room temperature with a
The reactions were performed in a solution of HBr in benzene at 55
substrate/Cl2pyNO/catalyst molar ratio of 500:750:1. The conversion was
determined after 24 h by gas chromatography or H NMR spectroscopy.
°C with a substrate/Cl2pyNO/catalyst molar ratio of 100:150:1. The
conversion was determined after 24 h by gas chromatography.
1
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Org. Lett., Vol. 3, No. 24, 2001

2
Cl pyNO as the oxidant, and 1 mol % P1 as the catalyst,
Table 3. Catalytic Oxidation of Secondary Alcoholsa
the corresponding ketones were obtained in mostly quantita-
tive yields in less than 3 h (Table 3). Again, significantly
lower yields were obtained with the homogeneous catalyst
1
. In some cases the initial turnover frequencies differ by
more than 1 order of magnitude!
The results described above highlight the potential of
highly cross-linked polymers as supports for site-isolated11
porphyrin catalysts. Because the complex is fixed in a very
rigid matrix, destructive autoxidation of the catalyst and the
support is reduced. This is of special significance if low
reactivity substrates such as alkanes are employed.
Currently, we are investigating whether the selectivity of
polymeric catalysts such as P1 can be manipulated in a
controlled fashion using the technique of molecular imprint-
ing.
Supporting Information Available: Methods for the
preparation of the ruthenium complex 1 and the polymer P1
1
13
together with spectroscopic and physical data ( H and
C
NMR, IR, elemental analysis, apparent dry density, swelling)
and detailed procedures for the catalytic oxidations. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL016754F
(
6) Yu, X.-Q.; Huang, J.-S.; Yu, W.-Y.; Che, C.-M. J. Am. Chem. Soc.
000, 122, 5337-5342.
7) (a) Liu, C.-J.; Yu, W.-Y.; Li, S.-G.; Che, C.-M. J. Org. Chem. 1998,
3, 7364-7369. (b) Liu, C.-J.; Li, S.-G.; Pang, W.-Q.; Che, C.-M. Chem.
a
The reactions were performed in a solution of HBr in benzene at 55
2
°
C with a substrate/Cl2pyNO/catalyst molar ratio of 100:150:1. The
(
conversion was determined after the times given by gas chromatography
6
1
or H NMR spectroscopy.
Commun. 1997, 65-66.
(8) The porphyrin was synthesized from pyrrole and p-vinyl-benzalde-
hyde following a standard procedure: Adler, A. D.; Longo, F. R.; Finarelli,
J. D.; Goldmacher, J.; Assour, J.; Korsakoff, L. J. Org. Chem. 1967, 32,
of ethylbenzene: only small amounts of acetophenone are
found in reactions with the homogeneous catalyst 1. Plausible
intermediates for this type of reaction are alcohols. In the
case of diphenylmethane, this intermediate can be detected
in the reaction mixture.
4
76-477.
(
9) (a) Berkessel, A.; Frauenkorn, M. J. Chem. Soc., Perkin Trans. 1
1
997, 2265-2266. (b) Ohtake, H.; Higuchi, T.; Hirobe, M. Tetrahedon Lett.
1992, 33, 2521-2524.
(10) For the catalytic oxidation of alcohols with Ru(porphyrin) complexes
see: (a) Gross, Z.; Ini, S. Org. Lett. 1999, 1, 2077-2080. (b) Ohtake, H.;
Higuchi, T.; Hirobe, M. J. Am. Chem. Soc. 1992, 114, 10660-10662.
(11) For site-isolated cobalt complexes within a cross-linked polymer
see: (a) Padden, K. M.; Krebs, J. F.; MacBeth, C. E.; Scarrow, R. C.;
Borovik, A. S. J. Am. Chem. Soc. 2001, 123, 1072-1079. (b) Sharma, A.
C.; Borovik, A. S. J. Am. Chem. Soc. 2000, 122, 8946-8955.
The results described above prompted us to investigate
the catalytic oxidation of secondary alcohols.10 By using
different aromatic and aliphatic (cyclic and linear) substrates,
Org. Lett., Vol. 3, No. 24, 2001
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