Novel polymer-supported ruthenium and iron complexes that catalyze the conversion of epoxides into diols or diol mono-ethers: Clean and recyclable catalysts

Article abstract of DOI:10.1039/b707028d

Polymer-supported metal (Fe or Ru) complexes for epoxide ring opening reactions were successfully prepared by anchoring the bis(2-picolyl)amine ligand onto the polymer poly(chloromethylstyrene-co-divinylbenzene) (PCD); the catalysts showed heterogeneous catalytic activity and easy recyclability in the ring opening reactions of various epoxide substrates with methanol or H ₂O at room temperature under mild and neutral conditions. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b707028d

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Novel polymer-supported ruthenium and iron complexes that catalyze the
conversion of epoxides into diols or diol mono-ethers: clean and
recyclable catalystsw
Sun Hwa Lee,a Eun Yong Lee,a Dong-Woo Yoo,b Sung Jin Hong,a
Jung Hwan Lee,a Han Kwak,a Young Min Lee,a Jinheung Kim,c Cheal Kim*a and
Jin-Kyu Lee*b
Received (in Montpellier, France) 9th May 2007, Accepted 11th July 2007
First published as an Advance Article on the web 20th July 2007
DOI: 10.1039/b707028d
Polymer-supported metal (Fe or Ru) complexes for epoxide ring
opening reactions were successfully prepared by anchoring the
bis(2-picolyl)amine ligand onto the polymer poly(chloromethyl-
styrene-co-divinylbenzene) (PCD); the catalysts showed hetero-
geneous catalytic activity and easy recyclability in the ring
opening reactions of various epoxide substrates with methanol
or H2O at room temperature under mild and neutral conditions.
(PCD = poly(chloromethylstyrene-co-divinylbenzene, tpy =
terpyridine).3m Moreover, Ru-loaded catalyst 2b showed a
reactivity comparable to iron catalyst 2a, although Ru is well
known as an inert metal.5
The insoluble polymer backbone employed in the present
study is PCD, a cross-linked polystyrene, in which all the
styryl moieties contain chloromethyl groups that can be easily
modified to other useful functional groups by simple SN2-type
reactions (see ESIw).3m 2a and 2b were synthesized as shown in
Scheme 1. PCD, potassium carbonate and BPA were reacted
in DMF at 50 1C for 12 h. After washing the crude PCD–BPA
(1), it was transferred to an MeOH solution of Fe(ClO4)3 or
RuCl3. The mixture was stirred for 12 h at room temperature.
Yellow 1 turned greenish-yellow due to the formation of the
PCD–BPA–Fe(ClO4)3 complex 2a, while RuCl3 showed a
color change from yellow to black. Next, 2a and 2b were
thoroughly washed with MeOH several times to remove
residual metal species.
Epoxides are important intermediates in organic synthesis,
and the reaction of epoxides with nucleophiles under mild and
neutral conditions has been the subject of extensive studies.1
The 1,2-diol and 1,2-diol mono-ether synthetic intermediates
that result from the solvolytic reactions of epoxides have been
synthesized by the ring opening methodology of epoxides with
various nucleophiles in the presence of metal complexes.2
More conveniently, the covalent attachment of homogeneous
catalysts to insoluble polymer supports has been studied
recently as an attractive strategy for extending the practical
advantages of heterogeneous catalysis to homogeneous sys-
tems,3 since the catalysts are easily separated from the reagents
and products, and can possibly be reused. In contrast, im-
mobilization often results in catalysts with lower efficiencies
than their solution phase counterparts.4 Such approaches
frequently lead to partial loss of reactivity. In order to further
develop efficient polymer-bound metal complexes, we have
synthesized new polymer-bound BPA–metal complexes (BPA
= bis(2-picolyl)amine) and studied their reactivity in the ring
opening of epoxides. Herein, we report the successful synthesis
of novel polymer-bound BPA–metal complexes (metal = Fe
(2a) or Ru (2b)) and their application to the mild ring opening
reactions of epoxides by methanol or H2O at ambient tem-
perature. In addition, iron catalyst 2a showed about 10 times
better activity than our previous PCD–tpy–Fe(ClO4)3 complex
The attachment of BPA and iron to the PCD was identified
by EPMA (Electron Probe Micro Analysis) and elemental
analysis. With EPMA, the iron atom coordinating to the ligand
showed 2 peaks (Ka, Kb) by X-ray emission, and the nitrogen
atoms of BPA were detected using elemental analysis (see ESIw).
We found, using the same calculation method as shown in the
previous study,3m that 50% of the Cl in the PCD was substituted
by BPA (2.14 mmol in 1 g PCD; see ESIw). The quantity of
metal ions loaded into the catalytic sites of 1 g of the polymer
support was determined to be 0.565 mmol for Fe (26% of BPA)
a Department of Fine Chemistry and the Eco-Product and Materials
Education Center, Seoul National University of Technology, Seoul,
139-743, Korea. E-mail: chealkim@snut.ac.kr; Fax: +82 2 973
9149; Tel: +82 2 970 6693
b School of Chemistry, Seoul National University, Seoul, 151-747,
Korea. E-mail: jinklee@snu.ac.kr; Fax: +82 2 882 1080; Tel: +82
2 880 8830
c Department of Chemistry and Division of Nano Sciences, Ewha
Womans University, Seoul, 120-750, Korea
w Electronic supplementary information (ESI) available: Further
synthesis and characterisation details. See DOI: 10.1039/b707028d
Scheme 1 Synthesis of PCD-supported BPA–metal catalysts.
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Table 1 Ring opening reactions of epoxides using catalysts 2a and 2b in methanol at room temperature
2aa
2ba
Time/hb Conversion (%) (yield (%))c
Time/hb Conversion (%) (yield (%))c
Entry Substrate
1
2
3
4
5
6
7
8
a
Styrene oxide
1
2.5
35
8
100 (88.1 ꢁ 1.3; primary alcohol only)
1.5
4.5
24
12
48
100 (90.5 ꢁ 1.1; primary alcohol only)
Cyclohexene oxide
Cyclopentene oxide
cis-2,3-Epoxybutane
100 (86.3 ꢁ 2.5)
100 (89.4 ꢁ 3.0)
100
100
100
100
100
100
trans-2,3-Epoxybutane 22
trans-Stilbene oxide
cis-Stilbene oxide
1,2-Epoxyhexane
12
30
200
100 (anti-stereomer : syn-stereomer = 84 : 16)d 10
100 (anti-stereomer : syn-stereomer = 93 : 7)e 28
100 (anti-stereomer : syn-stereomer = 83 : 17)d
100 (anti-stereomer : syn-stereomer = 93 : 7)e
100 (1-ol : 2-ol = 57 : 43)f
100 (1-ol : 2-ol = 57 : 43)f
190
Catalyst 1 mg (0.565 mmol catalytic sites for 2a and 0.260 mmol for 2b), substrate 0.5 mmol. All reactions were run at least in triplicate. The data
b
reported represents the average of these reactions. The internal standard was dodecane. Time needed for the complete conversion of
c
d
epoxide. The yield was based on the starting epoxide and monitored by GC/mass spectroscopy. Anti-stereomer means 2-methoxy-1,2-
e
diphenylethanol and syn-stereomer means (1R,2R)-2-methoxy-1,2-diphenylethanol. Anti-stereomer means (1R)-2-methoxy-1,2-diphenylethanol
f
and syn-stereomer means (2S)-2-methoxy-1,2-diphenylethanol. 1-ol and 2-ol represent the primary and secondary alcohols, respectively.
and 0.260 mmol for Ru (12% of BPA), respectively, by ICP
(inductively coupled plasma) spectrometry.
react with another aliquot of styrene oxide. We observed that the
filtrates showed little catalytic activity (less than 5%) within the
same time interval, while the ring opening reactions with the
filtered catalysts showed similar reactivities to the initial catalysts.
These results strongly suggest that the dominant reactive species
are the heterogeneous catalysts 2a and 2b, not the other species,
such as leached-out metal.
We have examined the catalytic activity of the newly devel-
oped polymer-supported metal complexes 2a and 2b in
epoxide ring opening reactions (eqn. (1)). Firstly, methanol
was examined as a nucleophile for the epoxide ring opening
reaction. The treatment of styrene oxide (0.5 mmol) with
methanol (1 mL) in the presence of 2a (1 mg, 0.565 mmol for
the quantity of catalytic sites loaded with iron salts) at room
The ring opening reaction of various epoxide substrates
catalysed by 2a and 2b were also examined, and the results are
given in Table 1. Cyclic epoxides, such as cyclohexene oxide
and cyclopentene oxide, were effectively converted to
trans-1,2-diol mono-ethers within 2.5 and 35 h for 2a, and
1.5 and 4.5 h for 2b, respectively (Table 1, entries 2 and 3).
Acyclic epoxides, such as cis- and trans-2-butene oxides, were
also ring opened efficiently to the corresponding products,
respectively. cis- and trans-Stilbene oxides showed complete
conversion to the corresponding product (93 : 7 anti-stereomer :
syn-stereomer for cis-stilbene oxide for both 2a and 2b, and
84 : 16 anti-stereomer : syn-stereomer for trans-stilbene oxide
for both 2a and 2b; Table 1, entries 6 and 7). trans-Stilbene
oxide underwent conversion to the product about three times
faster than cis-stilbene oxide. All the products were
temperature produced exclusively and quantitatively
a
primary alcohol, 2-methoxy-2-phenylethanol, within 1 h under
neutral conditions (see entry 1 of Table 1). No epoxide ring
opening reaction occurred without catalyst 2a over the same
time period. 2b also showed a catalytic reactivity similar to
that of 2a (Table 1, entry 1). Importantly, it is worth pointing
out that 2a showed about 10 times better reactivity than our
previous catalyst (PCD–tpy–Fe(ClO4)3).3m
ð1Þ
Potential benefits of the heterogeneous catalyst include facil-
itating the separation of the catalyst from the reagents and
reaction products, and its recyclability for repeated use.6 It is
critical that catalyst recovery should be simple and efficient,
and that the recovered catalyst should retain its original
reactivity through multiple cycles. Based on our previous
experience,3m,3n,7 we have examined the reusability of 2a and
2b at room temperature. After the ring opening reaction of
styrene oxide was complete, the catalysts were recovered by
filtration and thoroughly washed with methanol for
consecutive runs. The recovered catalysts were used in a new
reaction batch of styrene oxide. Our polymer support-im-
mobilized catalysts 2a and 2b showed excellent 10 cycle
reusability without showing any significant deterioration in
catalytic activity (see Table 2).
Table 2 Catalyst recycling experiments in the ring opening reaction
of styrene oxide catalyzed by polymer-supported catalysts 2a and 2ba
2a
2b
Time/hb Conversion (%)c
Time/hb Conversion (%)c
Cycle
1
2
3
4
5
6
7
8
9
10
1
1
1
1
1
1
1
1
1
1
100
100
100
100
100
100
100
100
100
100
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
100
100
100
100
100
100
100
100
100
100
a
Next, we performed another control reaction, because in the
process of catalysis, the metal species leached from the catalysts
might catalyze the ring opening reaction instead of the hetero-
geneous catalyst.7 We filtered catalysts 2a and 2b after the ring
opening reaction of styrene oxide and allowed the filtrates to
Reaction conditions: Catalyst: 1 mg (0.565 mmol catalytic sites for 2a
and 0.260 mmol for 2b), styrene oxide : 0.5 mmol, and methanol : 1
b
c
mL. Time needed for the whole conversion of styrene oxide. Based
on the starting epoxide and monitored by GC/Mass. Dodecane was
used as an internal standard.
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determined by their characteristic GC retention times and
mass spectra, which were obtained and compared with
authentic samples.7
Table 3 Hydrolysis of epoxides using 2b in a mixture of acetone/H2O
(8 : 2) at room temperaturea
Conversion (%)
(yield (%))c
Substrate
Time/hb
To further study the regioselectivity of the epoxide conver-
sion, a non-symmetric epoxide, 1,2-epoxyhexane, was used as
a substrate (Table 1, entry 8). A mixture of primary (1-ol) and
secondary (2-ol) alcohol was obtained (57 : 43 for both 2a and
2b for the 1-ol and 2-ol products of 1-hexene oxide), showing
no regioselectivity in the reaction with alcohol as the nucleo-
phile. Importantly, the exclusive attack at the benzylic position
of styrene oxide (Table 1, entry 1) and the lack of steric
preference in 1-hexene oxide shown by the alcoholic
nucleophile suggest that the regiochemistry of the ring opening
catalysed by 2a and 2b is dependent on the electronic nature of
the substrate rather than on steric hindrance. On the other
hand, it is very surprising that 2b showed a catalytic reactivity
comparable to that of 2a, because Ru metal is well known to
be inert in such catalytic reactions.5
Styrene oxide
1
3
20
10
30
100 (74.5 ꢁ 3.3)
100 (88.7 ꢁ 2.7)
100 (90.2 ꢁ 1.0)
100
Cyclohexene oxide
Cyclopentene oxide
cis-2,3-Epoxybutane
Trans-2,3-Epoxybutane
a
100
Catalyst 1 mg (0.260 mmol catalytic sites for 2b), substrate 0.5 mmol.
All reactions were run at least in triplicate. The data reported represent
b
the average of these reactions. The time needed for the complete
c
conversion of epoxide. Conversion and yield were based on the
starting epoxide and monitored by GC/mass spectroscopy. The inter-
nal standard was dodecane.
reaction (1 h; Table 3, entry 1). The ring opening reactions of
epoxide compounds bearing cyclic and acyclic structures
(Table 3, entries 2–5) with 2b proceeded well at room tem-
perature. All the reactions in Table 3 were performed with the
same recycled catalyst. A further detailed study on the various
solvolysis reactions with 2a and 2b is now in progress.
It deserves to be mentioned that 2a and 2b did not catalyze
transesterification reactions, while our previous Zn-containing
polymer-supported catalysts carried out the reaction efficiently
but did not catalyze epoxide ring opening reactions.3n These
results suggest that the nature of individual metals play an
important role when performing specific reactions. For exam-
ple, transesterification with Zn3n vs. epoxide ring opening with
Fe and Ru.
The epoxide ring opening mechanism catalysed by 2a and 2b
could be inferred from the methanolysis of styrene oxide since the
methoxy group was incorporated exclusively at the benzylic
position
(a-carbon)
instead
of
the
less
hindered
b-carbon to generate the primary alcohol (Scheme 2). Firstly, the
substrate styrene oxide substitutes an anion or solvent (X) to give
the adduct PCD–BPA–MX2(substrate). Next, the more substituted
carbon in the intermediate adduct, generated from the catalyst
complex 2 and the epoxide, might have significant cationic char-
acter. This cationic character could be stabilized by the phenyl
group through resonance, and therefore methanol would attack the
more positive a-carbon to give 2-methoxy-2-phenylethanol.
Detailed mechanistic studies are currently under investigation.
We also attempted the hydrolysis reaction of epoxide com-
pounds with water to produce diol products, since diol com-
pounds are considered to be valuable building blocks for
organic synthesis as intermediates for pharmaceuticals and
agrochemicals,8 and the hydrolysis of epoxide rings is much
more difficult than methanolysis.9 Reactions in the presence of
2b were carried out under similar conditions in a mixture of
H2O/acetone (2 : 8) instead of methanol as the solvent. In case
of 2a, Fe metal ions leached-out during the reactions. All the
epoxide substrates underwent ring opening by H2O to produce
the corresponding diol compounds, as shown in Table 3.
Styrene oxide proved to be the best substrate for the hydrolysis
In summary, we have developed new polymer-supported
catalysts 2a and 2b by deliberately anchoring the tridentate
BPA ligand onto a solid support. 2a and 2b appear to be
efficient, mild and easily recyclable catalysts for the ring
opening of epoxides, and these catalysts showed about 10
times better activity than our previous catalyst PCD–tpy–Fe-
(ClO4)3. Interestingly, Ru catalyst 2b showed comparable
reactivities to iron catalyst 2a. This system could also be
applied to the preparation of further substituted ether alco-
hols, which might be exploited immediately for the practical
synthesis of a wide range of interesting 1,2-diol mono-ethers.
Moreover, this catalytic system is applicable to a wide range of
epoxide substrates to prepare various diol compounds. Sig-
nificantly, these findings may constitute a good starting point
for the development of new polymer-supported, efficient and
recyclable catalysts for many important reactions, such as
enantioselective solvolysis, hydrogenation, C–H bond activa-
tion and various environmentally friendly oxidation reactions.
Financial support from the Korean Science & Engineering
Foundation (R01-2005-000-10490-0(2005)), the Korea Re-
search Foundation (2006-312-C00569), Seoul R&BD Program
and the Ministry of Environment (grant no 022-061-023) are
gratefully acknowledged.
Experimental
Chemicals
PCD (poly(chloromethylstyrene-co-divinylbenzene)) was
obtained as by a previous study (see ESIw).3m Methanol,
Scheme 2 A plausible epoxide ring opening mechanism.
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acetone, acetonitrile, DMF, K2CO3, Fe(ClO4)3 and RuCl3
were purchased from Aldrich and used as received. The
epoxides and their products, diol mono-ethers and diols, were
also obtained from Aldrich and used without further
purification.
5.65 mmol catalytic sites for 2a and 2.60 mmol for 2b) was added
and shaken at room temperature. Product analysis and char-
acterization were undertaken by the GC/GC mass spectro-
scopy of 20 mL aliquots withdrawn periodically from the
reaction mixture. All the products were determined by their
characteristic GC retention times and mass spectra, which were
obtained and compared with authentic samples.7
Instrumentation
Elemental analysis for carbon, nitrogen and hydrogen was
carried out using an EA1108 (Carlo Erba Instruments, Italy)
in the Organic Chemistry Research Center of Sogang Uni-
versity, Korea. Product analyses for the ring opening of
epoxides were performed on either a Hewlett-Packard 5890
II Plus gas chromatograph interfaced with a Hewlett-Packard
model 5989B mass spectrometer or a Donam Systems 6200 gas
chromatograph equipped with an FID detector using a 30 m
capillary column (Hewlett-Packard HP-1, HP-5 and Ultra 2).
Detection of metals was carried out using an electron probe
micro analyzer (JEOL JXA-8900R). The amount of metals
loaded in 1 was determined by an Inductively Coupled Plasma
Spectrometer (IRIS XDL Duo).
Hydrolysis. The reaction conditions were the same as de-
scribed above, except that a mixture of H2O/acetone (2 : 8)
was used as the solvent instead of methanol.
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Preparation of catalysts
Attachment of bis(2-picolyl)amine (BPA) onto PCD:
PCD–BPA (1). A mixture of PCD (0.540 g), BPA (0.917 g,
4.6 mmol) and K2CO3 (0.829 g, 6 mmol) in DMF (20 mL) was
shaken at 50 1C for 12 h. The solid product was filtered and
washed with DMF (10 mL for 2 h), water (10 mL for 2 h) and
acetone (10 mL for 2 h). It was then further washed with
acetonitrile (40 mL) for 1 d by shaking to give a yellow solid.
Based on elemental analysis, 50% of the Cl in the PCD was
substituted with BPA. The quantity of BPA sites in 1 g of PCD
was calculated to be 2.14 mmol (see ESIw).
Loading of Fe or Ru ions onto BPA ligand: PCD–BPA–M
(2). PCD–BPA (100 mg, 0.214 mmol of BPA sites) and a
10-fold excess of iron(III) perchlorate (779 mg, 2.2 mmol) or
RuCl3 (456 mg, 2.2 mmol) were mixed in MeOH (30 mL) and
shaken for 12 h at room temperature. The solid product was
filtered and washed with MeOH (30 mL, twice, for 12 h), H2O
(30 mL for 1 h) and acetone (30 mL for 2 h), and then air dried
to give a greenish-yellow solid of PCD–BPA–Fe(ClO4)3 (2a)
and a black solid of PCD–BPA–RuCl3 (2b), respectively.
Determination of the amount of metal in 2. 10 mg of 2 was
dissolved in a mixture of HNO3 (3 mL) and HCl (9 mL), and
heated to 100 1C for 12 h. The solution was cooled to room
temperature and filtered to remove the undissolved material.
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added to make the volume up to 100 mL. Based on standard
solutions (0.01, 0.1, 1 and 10 ppm) and using an ICP
spectrometer, the amount of metal in 2 was determined to be
0.0565 mmol for Fe (2a; 26% of BPA) and 0.0260 mmol for Ru
(2b; 12% of BPA), respectively.
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