Mild and efficient ring opening of epoxides catalyzed by potassium dodecatungstocobaltate(III)

Article abstract of DOI:10.1007/s00706-005-0415-7

Efficient ring opening of epoxides under mild conditions is reported. Potassium dodecatungstocobaltate(III) trihydrate was used as an efficient catalyst for the alcoholysis and acetolysis of epoxides. Conversion of epoxides to thiiranes was also performed efficiently in the presence of this catalyst. Springer-Verlag 2005.

Full text of DOI:10.1007/s00706-005-0415-7

Monatshefte f €u r Chemie 137, 235–242 (2006)
DOI 10.1007/s00706-005-0415-7
Mild and Efficient Ring Opening
of Epoxides Catalyzed by Potassium
Dodecatungstocobaltate(III)
1
;ꢀ
Bahram Yadollahi , and S. Mohammad R. Mirmohammadi
2
1
Shahram Tangestaninejad , Majid Moghadam , Valiollah Mirkhani ,
3
1
1
Chemistry Department, Isfahan University, Isfahan 81746-73441, Iran
2
Chemistry Department, Yasouj University, Yasouj 75918-74831, Iran
3
Institute for Advanced Studies in Basic Sciences, Gava-Zang, Zanjan 45138-79368, Iran
Received August 23, 2005; accepted October 6, 2005
Published online December 13, 2005 # Springer-Verlag 2005
Summary. Efficient ring opening of epoxides under mild conditions is reported. Potassium dodeca-
tungstocobaltate(III) trihydrate was used as an efficient catalyst for the alcoholysis and acetolysis of
epoxides. Conversion of epoxides to thiiranes was also performed efficiently in the presence of this
catalyst.
Keywords. Epoxides; Polyoxometalate; Alcoholysis; Acetolysis; Thiirane.
Introduction
Epoxides have become very popular in organic synthesis not only as building
blocks but also as synthesis intermediates [1–4]. They can be easily prepared from
a variety of substrates and are easily opened under a broad range of conditions
giving a wide spectrum of regio- and stereoselective ring-opened products. Epox-
ide ring-opening reactions to give ꢀ-substituted alcohols with carbon- and hetero-
aromatic nucleophiles are useful tools in organic synthesis, and most of these
reactions generally proceed under basic or acidic conditions [5]. In most of the
epoxides’ ring-opening reactions under acidic conditions, the formation of a mix-
ture of regio isomers and polymerization is observed. Some of the reported cata-
lysts suffer from disadvantages such as high acidity, the non-catalytic nature of the
reagents, long reaction times, and inconvenient handling [6–8]. Therefore, the
introduction of new methods for the nucleophilic ring-opening of epoxides, which
work under mild conditions, is still in demand and is important in synthetic organic
ꢀ Corresponding author. E-mail: stanges@sci.ui.ac.ir

2
36
S. Tangestaninejad et al.
Scheme 1
Scheme 2
chemistry. A number of methods using Lewis acids and one-electron transfer cat-
alysts have been also reported for the ring-opening reactions of epoxides with
different nucleophiles [9–25]. Recently, we have reported the ability of electron
deficient metalloporphryins in epoxides ring opening [26–29].
Heteropolyoxometalates as stable inorganic porphyrins have attracted much
attention in the last two decades [30, 31]. The versatility of these catalysts has
been demonstrated in various oxidation reactions, but there has been very little
exploitation of their catalytic activity in nucleophilic ring-opening of epoxides
[
32, 33]. The high catalytic activity of potassium dodecatungstocobaltate(III) tri-
hydrate, K [CoW O ] ꢁ 3H O, in various organic syntheses [34–38] and our inter-
5
12 40
2
est in the application of polyoxometalates in nucleophilic ring opening of epoxides
39–41] prompted us to explore the potential of this catalyst for the nucleophilic
[
ring-opening of epoxides. We found that K [CoW O ] ꢁ 3H O can be used as a
5
12 40
2
mild and highly efficient catalyst for alcoholysis, acetolysis, and preparation of
thiiranes with ammonium thiocyanate (Schemes 1 and 2).
Results and Discussion
Ring-Opening of Epoxides by Various Heteropolyanions
At first, in order to show the ability of various heteropolyanions in the ring-opening
of epoxides, the methanolysis of styrene oxide was investigated. The summarized
results in Table 1 show that K [CoW O ] ꢁ 3H O, Na [W O ] ꢁ 4H O, and
5
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2
4
10 32
2
[
(C H ) N][W O ] are more reactive than other catalysts and among these
4 9 4 10 32
K [CoW O ] ꢁ 3H O is the best catalyst for the methanolysis of styrene oxide.
5
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2
Therefore, K [CoW O ] ꢁ 3H O was chosen as the catalyst for the ring-opening
5
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2
of epoxides.
Alcoholysis and Acetolysis of Epoxides Catalyzed by K [CoW O ] ꢁ 3H O
5
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2
Alcoholysis of various epoxides such as cyclohexene and styrene oxides, (chloro-
methyl)oxirane, allyl oxiranylmethyl ether, isopropyl oxiranylmethyl ether, and

Mild and Efficient Ring Opening of Epoxides
237
a
Table 1. Methanolysis of styrene oxide by different heteropolyanions
b
Entry
Heteropolyanion
Yield=%
1
2
3
4
5
6
7
8
9
K [CoW O ] ꢁ 3H O
100
22
15
0
5
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2
Na [W O ] ꢁ 4H O
4
10 32
2
[(C H ) N][W O ]
4
9 4
10 32
Na [NiMo H O ] ꢁ 9H O
4
9
6
24
2
(NH ) [NiMo O ] ꢁ 6H O
0
4
6
9
32
2
K [MnMo O ] ꢁ 6H O
0
6
9
32
2
K [FeW O ] ꢁ 10H O
0
3
12 40
II
(NH ) [Co Co W O ]
2
II
0
4
8
12 42
II
III
(NH ) [Co Co W O ]
4
0
7
12 42
a
3
b
Reaction conditions: styrene oxide (1 mmol), catalyst (0.05 mmol), methanol (4 cm ); GC yields
based on the starting styrene oxide and determined after 15 min at room temperature
1,2-epoxyoctane as examples of aliphatic, alicyclic, activated, and deactivated
epoxides, were performed with primary, secondary, and tertiary alcohols affording
the corresponding ꢀ-alkoxy alcohols in high to excellent yields in the presence of
0
.05 molar equivalents of K [CoW O ] ꢁ 3H O as catalyst (Table 2). The reac-
5
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2
tions in the case of cyclohexene oxide were stereospecific, and only trans products
were obtained (Entries 1–5). In the case of unsymmetrical epoxides, the reactions
are regioselective with an attack of the nucleophile (alcohol) on the less substituted
oxirane carbon to yield the anti-Markovnikov type products (Entries 11–35). The
only exception is given by styrene oxide, in which the reactions occur on the more
substituted carbon; Markovnikov type products were obtained (Entries 6–10). The
K [CoW O ] ꢁ 3H O was used for acetolysis of epoxides and the results (Table 3)
5
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2
show that this catalyst is an efficient catalyst for conversion of epoxides to
ꢀ
-acetoxy alcohols. In the case of allyl oxiranylmethyl ether, isopropyl oxiranyl-
methyl ether, phenyl oxiranylmethyl ether, and 1,2-epoxyoctane, 20–27% of the
minor isomers were produced.
Conversion of Epoxides to Thiiranes Catalyzed by K [CoW O ] ꢁ 3H O
5
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2
Many methods have been already reported for the preparation of thiiranes. The
most general one is based on conversion of oxiranes to thiiranes by an oxygen-
sulfur exchange reaction. Among other reagents NH SCN was already used for this
4
purpose. Conversion of epoxides to thiiranes with NH SCN in the presence of a
4
suitable catalyst [42–53] under nonaqueous conditions different from classical
methods, which are generally carried out in homogeneous or heterogeneous aque-
ous media, is important in organic chemistry. Reaction of different aliphatic and
cyclic epoxides including those with electron-withdrawing substituents with
NH SCN were performed in refluxing acetonitrile in the presence of 0.05 molar
4
equivalent of K [CoW O ] ꢁ 3H O. Table 4 summarizes the results obtained for
5
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2
conversion of different epoxides to their corresponding thiiranes. The effects of
other solvents such as acetone, dichloromethane, chloroform, and carbontetrachlo-
ride were also investigated. Compared to acetonitrile as the solvent, the reaction
times were longer and the yields of thiiranes were lower in all of the other solvents.

2
38
S. Tangestaninejad et al.
a
Table 2. Alcoholysis of various epoxides catalyzed by K [CoW O ] ꢁ 3H O
5
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2
b,c
Entry
Epoxide
R–OH=Condition
Time=min
Product=Yield=%
1
2
3
4
5
R ¼ CH =rt
30
30
30
30
30
100
100
99
3
R ¼ C H =rt
2
5
R ¼ n-Pr=rt
R ¼ i-Pr=rt
97
R ¼ t-Bu=rt
70
6
7
8
9
R ¼ CH =rt
15
30
30
30
30
100
100
100
92
3
R ¼ C H =rt
2
5
R ¼ n-Pr=rt
R ¼ i-Pr=rt
R ¼ t-Bu=rt
1
0
88
11
12
13
14
15
R ¼ CH =reflux
30
30
30
30
30
94
88
86
80
67
3
R ¼ C H =reflux
2
5
R ¼ n-Pr=reflux
R ¼ i-Pr=reflux
R ¼ t-Bu=reflux
16
17
18
19
20
R ¼ CH =reflux
15
30
30
30
30
99
98
97
96
63
3
R ¼ C H =reflux
2
5
R ¼ n-Pr=reflux
R ¼ i-Pr=reflux
R ¼ t-Bu=reflux
21
22
23
24
25
R ¼ CH =rt
30
30
30
30
30
96
100
95
3
R ¼ C H =reflux
2
5
R ¼ n-Pr=reflux
R ¼ i-Pr=reflux
R ¼ t-Bu=reflux
70
42
26
27
28
R ¼ CH =rt
15
30
30
100
100
95
3
R ¼ C H =reflux
2
5
R ¼ n-Pr=reflux
(continued)

Mild and Efficient Ring Opening of Epoxides
239
Table 2 (continued)
b,c
Entry
Epoxide
R–OH=Condition
Time=min
Product=Yield=%
2
9
0
R ¼ i-Pr=reflux
R ¼ t-Bu=reflux
30
30
90
60
3
3
3
3
3
3
1
2
3
4
5
R ¼ CH =rt
15
30
30
30
30
99
99
96
89
61
3
R ¼ C H =reflux
2
5
R ¼ n-Pr=reflux
R ¼ i-Pr=reflux
R ¼ t-Bu=reflux
a
b
Catalyst 0.05 molar amount; all products were identified by comparison of their physical and
c
spectral data with those of authentic samples; yields refer to GLC yields
a
Table 3. Acetolysis of various epoxides catalyzed by K [CoW O ] ꢁ 3H O
5
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2
b
c
Entry
Epoxide
Time=min=Condition
Product
Yield
100
1
30=rt
2
15=reflux
100
3
4
30=reflux
30=reflux
100
72
2
2
5
30=reflux
74
2
4
6
7
30=rt
66
2
7
30=reflux
64
2
1
a
b
Catalyst 0.05 molar amount; all products were identified by comparison of their physical and
c
spectral data with those of authentic samples; yields refer to GC yields

2
40
S. Tangestaninejad et al.
Table 4. Conversion of epoxides to thiiranes catalyzed by K [CoW O ] ꢁ 3H O in refluxing ace-
5
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2
a
tonitrile
Entry
1
b
c
Epoxide
Time=min
Yield=%
Product
30
100
2
60
100
3
4
60
60
100
100
5
6
60
60
100
100
7
60
100
a
b
c
Catalyst 0.05 molar amount; yields refer to GC yields; all products were identified by compar-
ison of their physical and spectral data with those of authentic samples
In conclusion, we suggest that selective ring opening of different epoxides with
alcohols and acetic acid is potentially performed through an electron transfer
mechanism. The possibility of an electron transfer mechanism was strongly sup-
ported by the following experiment: when small amounts of acrylonitrile were
added to the reaction mixture as radical scavenger, a large decrease in the reaction
rates was noticed.
Experimental
All chemicals used were of reagent grade. The heteropolyanions were prepared according to Refs.
[
54–60]. Products were characterised by comparison of their physical, IR, and NMR data with those of
1
known samples. H NMR spectra were obtained with a Brucker AW 80 (80 MHz) spectrometer. GLC
analyses were performed on a Shimadzu GC-16A instrument with a flame ionization detector using a
Silicon DC-200 or Carbowax 20M column. Infrared spectra were recorded on Philips PU-9716 or
Shimadzu IR-4350 spectrophotometers.
Alcoholysis of Epoxides by K [CoW O ] ꢁ 3H O (General Procedure)
5
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2
3
To a solution of 1 mmol epoxide in 4 cm of the appropriate alcohol was added 157mg
K [CoW O ] ꢁ 3H O (0.05 mmol). The mixture was stirred for the specified time and at the appro-
5
12 40
2
priate temperature according to Table 2. The progress of the reaction was monitored by GLC. The
3
3
solvent was evaporated and 20cm H O were added and extracted with Et O (3ꢂ10cm ). The organic
2
2
layer was separated and dried (Na SO ). The etheral solution was concentrated under reduced pressure
4
2
and chromatographed on a silica gel column to give the pure product.

Mild and Efficient Ring Opening of Epoxides
241
Acetolysis of Epoxides by K [CoW O ] ꢁ 3H O (General Procedure)
5
12 40
2
3
K [CoW O ] ꢁ 3H O (0.05 mmol) was added to a solution of 1 mmol epoxide in 4 cm acetic acid.
5
12 40
2
3
The progress of reaction was monitored by GLC. The solvent was evaporated and 20cm H O were
2
3
added. The product was extracted with ether (3ꢂ10cm ). The organic layer was washed with a 5%
solution of NaHCO and dried (Na SO ). Evaporation of the solvent followed by TLC gave the
3
2
4
product.
Conversion of Oxiranes to Thiiranes by K [CoW O ] ꢁ 3H O (General Procedure)
5
12 40
2
3
To a mixture of 1 mmol oxirane and 2 mmol ammonium thiocyanate in 4 cm acetonitrile, was added
0
.05mmol K [CoW O ] ꢁ 3H O. The reaction mixture was refluxed for 0.5–1 h, the reaction progress
5
12 40
2
was monitored by GLC. After completion of the reaction, the solvent was evaporated, the product was
3
extracted with CCl (3ꢂ10 cm ), and the solid residue was filtered off. Concentration of the solution
4
led to the desired product in quantitative yield.
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