An effective procedure for the synthesis of acid-sensitive epoxides: Use of 1-methylimidazole as the additive on methyltrioxorhenium-catalyzed epoxidation of alkenes with hydrogen peroxide

Article abstract of DOI:10.1039/b926575a

An effective method for suppression of ring opening and rearrangement of acid-sensitive epoxides during methyltrioxorhenium(MTO)-catalyzed epoxidation of alkenes with H₂O₂ by using 1-methylimidazole as a co-additive has been found. The combined use of 3-methylpyrazole and 1-methylimidazole as the additives has been found to be an effective procedure that affords excellent yields of acid-sensitive epoxides for MTO-catalyzed epoxidation.

Full text of DOI:10.1039/b926575a

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An effective procedure for the synthesis of acid-sensitive epoxides: Use of
1-methylimidazole as the additive on methyltrioxorhenium-catalyzed
epoxidation of alkenes with hydrogen peroxide†
Shigekazu Yamazaki*
Received 22nd December 2009, Accepted 19th February 2010
First published as an Advance Article on the web 17th March 2010
DOI: 10.1039/b926575a
An effective method for suppression of ring opening and rearrangement of acid-sensitive epoxides
during methyltrioxorhenium(MTO)-catalyzed epoxidation of alkenes with H2O2 by using
1-methylimidazole as a co-additive has been found. The combined use of 3-methylpyrazole and
1-methylimidazole as the additives has been found to be an effective procedure that affords excellent
yields of acid-sensitive epoxides for MTO-catalyzed epoxidation.
while the selectivity toward epoxides increases, the activity of the
Introduction
catalytic system decreases.
Methyltrioxorhenium(VII) (CH3ReO3, MTO) as an oxidation
A major improvement in the MTO-catalyzed epoxidation was
achieved by Sharpless and co-workers.20a They reported that
biphasic system and addition of a significant excess of pyridine
relative to the catalyst not only suppresses the ring-opening but
also accelerates the alkene epoxidation.
catalyst was first reported by Herrmann and co-workers in
1991 for the epoxidation of alkenes with hydrogen peroxide
(H2O2) as the terminal oxidant.1 Since then, MTO-catalyzed H2O2
oxidations have been explored with a variety of substrates,2 such
as alkanes,3 alkynes,4 arenes,5 phenols,6 cyclic ketones (Baeyer–
Villiger oxidation),7 benzaldehydes,8 organonitrogen compounds,9
organosulfur compounds,10 alcohols,11 and so on.12 The important
features of MTO as a catalyst are its availability (ease of synthesis,13
now commercially available14), its stability in air (dioxygen and
humidity), and its solubility in various solvents, including water.15
Furthermore, H2O2 is an environmentally advantageous oxidizing
agent, since its only waste by-product is water.16
Epoxidation is an important transformation of alkenes, because
epoxides are versatile intermediates in organic synthesis.17 The use
of transition metal complexes as the epoxidation catalyst is of
particular interest, due to their ability to activate environmentally
benign oxidants such as molecular oxygen18 and H2O2.16,17,19 MTO
has emerged as one of the most active catalysts for alkene
epoxidation in the presence of H2O2 as the terminal oxidant.20–29
At a first stage, MTO-catalyzed epoxidation was investigated in
a homogeneous medium using t-BuOH as solvent with anhydrous
H2O2.1 While high yields of certain epoxides may be obtained
by this procedure, the main disadvantage, however, was the low
selectivity in the cases of acid-sensitive epoxides were formed.
The Lewis acidity of the rhenium center caused hydrolysis and
concomitant cleavage of the epoxide ring leading to the formation
of 1,2-diols in the presence of water.1
Shortly afterward further improvements were reported by the
groups of Sharpless and Herrmann. It was found that the use
of 3-cyanopyridine20b and pyrazole22b as the Lewis base additives
is more effective and less problematic than the use of pyridine,
since pyridine is easily oxidized to pyridine N-oxide that is a
less efficient additive.22a Recently we reported 3-methylpyrazole
as a superior additive to pyridines and pyrazole because MTO/3-
methylpyrazole system has higher catalytic activity and longer cat-
alyst lifetime than MTO/pyridines and MTO/pyrazole systems.29
However, even under these improved conditions, the hydrolysis
or rearrangement of particular acid-sensitive epoxides proceeds
and produces a significant amount of diols or rearrangement
products.26c–d,28a–b Therefore, there still is a strong need for further
improvement of MTO-catalyzed epoxidation to produce acid-
sensitive epoxides in high yield.
Herein we wish to report an improved method of MTO-
catalyzed epoxidation of alkenes to produce acid-sensitive epox-
ides in high yields. During our research on the effect of additives for
the MTO-catalyzed epoxidation,29 we found 1-methylimidazole to
be an excellent additive that strongly suppresses the ring opening
andrearrangement ofacid-sensitive epoxides. The combined useof
3-methylpyrazole and 1-methylimidazole afforded acid-sensitive
epoxides in excellent yields within reasonable reaction times.
Several methods have been investigated to overcome this
problem. Herrmann and co-workers have reported that addition
of Lewis base-ligand suppresses the epoxide ring-opening by
reducing the Lewis acidity of the rhenium center.1 However,
Results and discussion
Indene epoxidation
Toyama Industrial Technology Center, 150 Futagami, Takaoka, Toyama
933-0981, Japan. E-mail: s-yamazk@itc.pref.toyama.jp; Fax: +81-766-21-
2402; Tel: +81-766-21-2121
† Electronic supplementary information (ESI) available: GC analysis
conditions for all epoxidations, characterization of epoxide 20, NMR
spectra of isosafrole oxide, and a table of NMR references for the known
epoxides. See DOI: 10.1039/b926575a
We first examined MTO-catalyzed epoxidation of indene 1.
Indene oxide 2 is known to be very prone to ring-opening
by acid-catalyzed hydrolysis, and it is difficult to obtain high
yield of indene oxide by catalytic epoxidations including MTO-
catalyzed epoxidation.20a,28a–b,30 In order to evaluate the effect of
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various additives for indene epoxidation, we compared pyridine,20a
pyrazole,22b 3-methylpyrazole,29 and 1- methylimidazole in MTO-
catalyzed epoxidation of indene 1.
The time courses of indene 1 consumption and epoxide 2
formation by 0.2 mol% MTO-catalyzed epoxidation are indicated
in Fig. 1. Although indene 1 was consumed rapidly by the
oxidation without additive, only a small amount of indene oxide 2
was detected. The main product is corresponding diol 3 with minor
amount of oxidatively C–C bond cleaved product 431 (Scheme 1).
The epoxidation of indene 1 with pyrazole or 3-methylpyrazole
as the additive proceeded smoothly and produce indene oxide 2
over 70% yield within 1 h. However, the epoxide decomposed
under the reaction conditions to diol 3 and oxidatively C–C
cleaved product 4, and the amount of indene oxide 2 decreased
rapidly by time course. The epoxidation of indene 1 with pyridine
additive stopped within 1 h at 62% conversion, probably because
pyridine was rapidly oxidized to pyridine oxide under the reaction
conditions.9f ,22a Interestingly, the epoxide 2 was not affected under
the reaction conditions. On the other hand, the epoxidation
of indene 1 with 1-methylimidazole as the additive indicated
different results. The initial rate of epoxidation of indene 1 with 1-
methylimidazole was slower than that of the reaction without any
additives. However, indene 1 was consumed steadily, and indene
oxide 2 was produced correspondingly and survived under the
reaction conditions. These results indicate that the addition of 1-
methylimidazole retards the rate of the epoxidation, while the ring
opening reaction of epoxide was strongly suppressed.
From the above results, we have expected to obtain a high
yield of indene oxide 2 within a reasonable reaction time by the
combined use of 3-methylpyrazole and 1-methylimidazole as the
additives for MTO-catalyzed epoxidation. Fig. 2 showed the time
profile of conversion of indene 1 and the yield of indene oxide 2 in
the cases of single use of 10 mol% 3-methylpyrazole and combined
use of 10 mol% 3-methylpyrazole and 1 mol% 1-methylimidazole.
Fig. 1 Time course of MTO-catalyzed epoxidation of indene 1 with
various additives. (a) Consumption of indene 1. (b) Production of indene
oxide 2. Conditions: indene 1 (10 mmol), 35% H2O2 (20 mmol), MTO
(0.02 mmol), additive (1.0 mmol), in CH2Cl2 (5 mL) at rt. Analysis by
GC. ( ) Curve A: 3-Methylpyrazole. ( ) Curve B: Pyrazole. ( ) Curve C:
Pyridine. ( ) Curve D: 1-Methylimidazole. ( ) Curve E: No additive.
Fig. 2 Time course of MTO-catalyzed epoxidation of indene 1 in the
presence of ( ) 10 mol% 3-methylpyrazole and ( ) 10 mol% 3-methylpyra-
zol + 1 mol% 1-methylimidazole. Conditions: indene 1 (10 mmol), 35%
H2O2 (20 mmol), MTO (0.02 mmol), and additive(s) in CH2Cl2 (5 mL) at
rt. Analysis by GC.
Scheme 1 MTO catalyzed epoxidation of indene.
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As expected, indene oxide 2 was obtained quantitatively within
4 h by combined use of 3-methylpyrazole and 1-methylimidazole.
The necessary amount of 1-methylimidazole as the additive for
the combined use with 3-methylpyrazole was examined. As shown
in Fig. 3, the amount of 1-methylimidazole added could be reduced
to 0.2 mol% (equal amount to MTO) without large reduction of
the epoxide yield.
Fig. 4 Time course of MTO-catalyzed epoxidation of indene 1 in the
presence of various imidazoles. Conditions: indene 1 (10 mmol), 35%
H2O2 (20 mmol), MTO (0.02 mmol), and additive (1 mmol) in CH2Cl2
(5 mL) at rt. Analysis by GC. ( ) Curve A: 1-methylimidazole. ( ) Curve B:
1-butylimidazole. ( ) Curve C: imidazole. ( ) Curve D: 4-methylimidazole.
(
) Curve E: 1,2-dimethylimidazole. ( ) Curve F: 2-methylimidazole.
a-methylstyrene 6, afforded corresponding epoxides quantita-
tively (Entries 3 and 5). Although the epoxidation of isosafrole
5 and a-methylstyrene 6 in the presence of 3-methylpyrazole as
the sole additive afforded corresponding epoxides in high yields at
first, prolonged reaction caused the decomposition of the epoxides
formed (Entries 4 and 6).
Fig. 3 Time course of MTO-catalyzed epoxidation of indene 1 in
the presence of 10 mol% 3-methylpyrazole and varying amount of
1-methylimidazole. Conditions: indene 1 (10 mmol), 35% H2O2 (20 mmol),
MTO (0.02 mmol), 3-methylpyrazole (1 mmol) and 1-methylimidazole
(0.1–0.01 mmol) in CH2Cl2 (5 mL) at rt. Analysis by GC. ( ) Curve A:
1 mol% (0.1 mmol) 1-methylimidazole. ( ) Curve B: 0.5 mol% (0.05 mmol)
1-methylimidazole. ( ) Curve C: 0.2 mol% (0.02 mmol) 1-methylimidazole.
(
) Curve D: 0.1 mol% (0.01 mmol) 1-methylimidazole.
2,2-Dimethyl-2H-chromene epoxidation
Various imidazoles were examined as the additive for MTO-
Chromenes
(2H-1-benzopyran
derivatives)
and
3,4-
catalyzed epoxidation (Fig. 4). Imidazole, 4-methylimidazole, 1,2-
dimethylimidazole and 2-methylimidazole were inferior additives
than 1-methylimidazole. 1-Butylimidazole showed comparable
effect with 1-methylimidazole.
The role of 1-methylimidazole is to protect indene oxide from
acid-catalyzed ring-opening reaction. Heterocycles having higher
basicity coordinate stronger to rhenium metal of MTO, thereby
decreasing the Lewis acidity of the rhenium more effectively.20d
1-Methylimidazole (pKa 7.3)32 has higher basicity than pyrazole
(pKa 2.5), 3-methylpyrazole (pKa 3.6), and pyridine (pKa 5.2).
Furthermore, 1-methylimidazole does not undergo N-oxidation
by MTO/H2O2 oxidation system.
The basicity of substituted imidazoles examined in Fig. 4 is
not so different to 1-methylimidazole (pKa of imidazole 7.0,
1-butylimidazole 7.1, 4-methylimidazole 7.6, 2-methylimidazole
7.9, 1,2-dimethylimidazole 8.0).32,33 The reason why only the
imidazoles substituted by alkyl group at 1-position are effective
additives for MTO-catalyzed epoxidation is not clear at this stage,
and further study on the substituent effect of imidazole is in
progress.
The MTO-catalyzed epoxidation of styrenes that produce
acid-sensitive epoxides in the presence of both 3-methylpyrazole
and 1-methylimidazole was examined. The results are summa-
rized in Table 1. Substituted styrenes such as isosafrole 5 and
epoxychroman derivatives have attracted much attention
because of their biological activity and their importance as
synthetic intermediates for biologically active compounds.34–36
The conventional epoxidation using m-chloroperbenzoic
acid (mCPBA) was not suitable for the preparation of 3,4-
epoxychromans. The epoxidation of 2,2-dimethyl-2H-chromene
7 by mCPBA under buffered conditions was reported to give the
corresponding epoxide 8 only 46% yield along with 33% ketone 9
(Scheme 2).34a,b
MTO-catalyzed epoxidation of 2,2-dimethyl-2H-chromene 7
with 3-methylpyrazole as the additive resulted in 93% epoxide 8
along with 4% diol 10 by 2 h reaction at 97% conversion (Table 2,
Entry 1). The ketone 9 was not observed under the reaction
conditions. Prolonged reaction time did not increase the yield
of epoxide 8 and the yield of diol 10 increased (Entry 2). The
epoxidation of the chromene 7 with 10 mol% 3-methylpyrazole
and 1 mol% 1-methylimidazole afforded 99% epoxide 8 with trace
amount (<1%) of diol 10 (Entry 3).
Epoxidation of bishomoallylic alcohols
The epoxidation of bishomoallylic alcohols often resulted in the
formation of cyclic ethers (furan and pyran compounds). This
is because the epoxides initially formed are unstable under the
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Table 1 MTO-catalyzed epoxidation of indene and styrenes in the presence of 3-methylpyrazole and 1-methylimidazolea
Entry
Alkene
Additive
MTO (mol%)
Time/h
Conversion (%)b
Epoxide (%)b
1
2
A
B
0.2
0.2
4
>99
98
>99
99
87 (89)
72
1c
2d
3
4
A
B
0.1
0.1
3
>99
97
>99
99
79 (81)
59
1c
2d
5
6
A
B
0.2
0.2
5
>99
97
>99
>99
96 (99)
87
1c
2d
a General conditions: alkene (10 mmol), 35% H2O2 (20 mmol), MTO, and additive(s) in CH2Cl2 (5 mL) at rt. Additive A: 3-methylpyrazole (1.0 mmol)
and 1-methylimidazole (0.1 mmol). Additive B: 3-methylpyrazole (1.0 mmol). b Determined by GC analysis. Yields based on alkenes used. The numbers
in parentheses are the yields based on alkenes consumed. c Time at maximum epoxide yield. d Time at conversion of alkene over 99%.
Scheme 2 Epoxidation of 2,2-dimethyl-2H-chromene
Table 2 MTO-catalyzed epoxidation of 2,2-dimethyl-2H-chromene 7a
Espenson’s group reported the formation of furans exclusively
by MTO-catalyzed oxidation of bishomoallylic alcohols without
Additive
Entry (mol%)
Conversion Epoxide 8 Diol 10
any amine additives.23h Rudler and co-workers reported that
they observed the formation of epoxide 12 by MTO-catalyzed
epoxidation of linalool 11 with bipyridine as the additive but they
obtained the epoxide 12 only 10% yield.26d On the other hand,
Jacobs and co-workers reported that the use of large amount of
pyridine additives (combined use of pyridine and 3-cyanopyridine,
40 mol% each) afforded the 6,7-epoxide of linalool 12 in good yield
(82%) with small amount of the cyclic ethers 13 (6%) and 14.26c
The combined use of 10 mol% 3-methylpyrazole and 1 mol% 1-
methylimidazole as the additives on MTO-catalyzed epoxidation
of linalool 11 afforded the corresponding epoxide 12 in 95% yield
with very small amount (<2% total) of the cyclic ethers 13 and 14
(Scheme 3).
Time/h
(%)b
(%)b,c
(%)b,c
1
2
3
3MePz (10)
2
3
7
97
>99
>99
93
90
99
4
9
<1
3MePz (10)
3MePz (10) +
1MeIm (1)
a General conditions: 2,2-dimethyl-2H-chromene 7 (10 mmol), 35% H2O2
(20 mmol), MTO (0.02 mmol), and additive(s) in CH2Cl2 (5 mL) at
20 ◦C. Additive: 3-methylpyrazole (3MePz), 1-methylimidazole (1MeIm).
b Determined by GC analysis. c Yields based on alkene used.
reaction conditions (generally acidic) and easily rearrange to a
mixture of furans and pyrans by an intramolecular reaction.
The mainly obtained products by epoxidation of bishomoallylic
alcohols with peracids and most of catalytic epoxidation systems
are such cyclic ethers.37,38,40a,c,e
MTO-catalyzed epoxidation of bishomoallylic alcohols has
been reported to produce epoxides and/or cyclic ethers according
to the reaction conditions. Sharpless and co-workers reported
that a bishomoallylic alcohol (4-penten-1-ol) gave only furan in
high yield in the presence of 3-cyanopyridine as the additive.20b
This procedure was successfully applied to various bishomo-
allylic alcohols. As shown in Table 3, acid-sensitive epoxides were
obtained in excellent yields. Only a trace amount of cyclized
products via acid-catalyzed intramolecular cyclization of initially
produced epoxides were detected. These epoxidation could be
performed also under organic solvent-free conditions29b without
significant reduction of the epoxide yields (Entries 2, 4, and 6).
In every bishomoallylic alcohol examined, a considerable amount
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Table 3 MTO-catalyzed epoxidation of bishomoallylic alcohols in the presence of 3-methylpyrazole and 1-methylimidazolea
Entry
Alkene
Solvent
CH2Cl2
T/◦C
Time/h
Epoxide
Yield (%)b
1
2
10
10
2
1
95
94
c
—
3
4
CH2Cl2
—
15
10
2
2
99
95
c
5
6
CH2Cl2
—
15
10
2
1
99
97
c
7d
CH2Cl2
20
4
99e
a Conditions: alkene (10 mmol), 35% H2O2 (12 mmol), MTO (0.01 mmol), 3-methylpyrazole (1.0 mmol), 1-methylimidazole (0.1 mmol), CH2Cl2 (5 mL)
or without organic solvent. b Analysis by GC. c Reaction without organic solvent. d Reaction with 0.02 mmol MTO. e Isolated yield of crude epoxide. See
experimental and ESI for detail.†
Scheme 3 MTO-catalyzed epoxidation of linalool
of cyclization products were produced without the addition of
1-methylimidazole.
conditions and in CH2Cl2 (Entries 1, 3, 5, and 6). The epoxidation
2-carene 21 without 1-methylimidazole resulted in lower yield
of the epoxide 22, that decomposed during prolonged reaction
(Entry 2). This procedure can easily be carried out on a 10 g scale
of 3-carene epoxidation (Entry 6). a-Pinene 25 and b-pinene 27
also provided excellent yield of corresponding epoxides 26 and
28 (Entries 7–10). Although Saladino and co-workers reported
excellent results of catalytic epoxidation of (+)-3-carene 23 and a-
pinene 25 by using microencapsulated Lewis base adduct of MTO,
low reaction temperature (-10 ◦C) and use of CH3CN–CH2Cl2 as
the reaction solvent were required.25b–d
Limonene 29 afforded 1,2-epoxide 30 as the main product
along with diepoxide 31 (Entry 12). The use of excess H2O2 and
prolonged reaction time resulted in the selective formation of
diepoxide 31 (Entry 13). Geraniol 32 afforded the mixture of 6,7-
epoxide 33 and diepoxide 34 under organic solvent-free conditions
(Entry 14). The epoxides 30, 31, 33, and 34 were stable enough
under the reaction conditions without 1-methylimidazole.
MTO-catalyzed epoxidations of a-terpineol 35 with 3-
methylpyrazole as the sole additive afforded 94% yield of epoxide
36 within 1 h (Entry 17). After leaving the reaction mixture at
room temperature for 20 h, the epoxide 36 decreased to 37%
and triol 37 and rearrangement product 2-hydroxy-1,8-cineol 38
increased (Scheme 4). The epoxidation of a-terpineol 35 with
combined use of 10 mol% 3-methylpyrazole and 1 mol% 1-
methylimidazole afforded 97% epoxide 36 (Entry 16). After 20 h,
Epoxidation of terpenes
Terpenes are widely available from nature, and their epoxides
are important starting materials for the synthesis of fragrances,
flavors, therapeutically active substances, and pharmaceuticals.39
Organic peroxides, particularly peracetic acid and mCPBA, are
still the most widely used for the epoxidation of terpenes.
Although a number of metal-catalyzed epoxidations with H2O2
as oxidant have been reported,40 the epoxidation of terpenes is
still a challenging task due to the sensitivity of epoxy terpenes
under acidic conditions.41 Terpenes and their epoxides easily
undergo ring opening, rearrangement, double-bond migration,
and hydrolysis by acid catalysis or by heating. Though some MTO-
catalyzed epoxidation of terpenes have been reported,25b–d,26a,c,d the
results reported are not fully satisfactory, because of the use of
CH2Cl2 as solvent and/or the complicated multistep preparation
of immobilized catalysts.
The application of above mentioned MTO-catalyzed epox-
idation with combined use of 3-methylpyrazole and 1-
methylimidazole as the additives successfully afforded terpene
epoxides in excellent yields. Some examples are summarized in
Table 4. 2-Carene 21 and 3-carene 23 provided excellent yield of
corresponding epoxides 22 and 24 both under organic solvent-free
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Table 4 MTO-catalyzed epoxidation of terpenesa
Entry
Alkene
MTO (mol%)
Additiveb
Solvent
Time/h
Conv (%)c
Epoxide
Yield (%)c
d
1
2
0.2
0.2
A
B
—
—
3
1
2
3
>99
75
94
>99
d
63 (84)
66 (70)
35 (36)
98
3
4
5
6
0.1
0.1
0.3e
0.2
A
B
A
A
CH2Cl2
CH2Cl2
—
2
2
6
2.5
>99
>99
>99
>99
>99
99
>99
>99
94f
d
CH2Cl2
d
7
8
0.3e
0.2
A
A
—
5
4
>99
>99
95
91
CH2Cl2
d
9
10
11
0.2
0.2
0.2
A
A
B
—
6
2.5
1.5
>99
>99
>99
82
94
92
CH2Cl2
CH2Cl2
d
12g
0.2
0.2
B
B
—
3
8
97
100
83h, 14i
1h, 98i
13g,j
CH2Cl2
d
14g
0.2
B
—
0.5
93
74j, 14i
(80k, 15i)
d
15
16
17
0.05
0.1
0.1
A
A
B
—
2
1.5
1
>99
>99
>99
97
CH2Cl2
CH2Cl2
97l
94m
18
0.1n
A
CH2Cl2
1
>99
>99
a General conditions: alkene (10 mmol), 35% H2O2 (20 mmol or 12 mmol), at 10 ◦C. b Additives A: 3-methylpyrazole (1.0 mmol) and 1-methylimidazole
(0.1 mmol). Additives B: 3-methylpyrazole (1.0 mmol). c Analysis by GC. Yields based on alkenes used. The numbers in parentheses are the yields based
on alkenes consumed. d Reaction without organic solvent. e Addition of 0.2 mol% MTO at start, and additional MTO (0.1 mol%) at 1 h. f Isolated yield
after distillation. g Reaction at 15 ◦C. h 1,2-Epoxide. i Diepoxide. j 25 mmol H2O2 was used. k 6,7-Epoxide. l 90% of epoxide remaining in the reaction
mixture after leaving for 20 h at rt. m 37% of epoxide remaining in the reaction mixture after leaving for 20 h at rt. n Addition of 0.06 mol% MTO at start,
and additional MTO (0.04 mol%) at 30 min.
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Scheme 4 MTO-catalyzed epoxidation of a-terpineol
90% of the epoxide 36 has survived in the reaction mixture. These
results also clearly indicate the effect of 1-methylimidazole for
the suppression of epoxide decomposition. This epoxidation also
proceeded smoothly under organic solvent-free conditions with
0.05 mol% MTO (Entry 15). Epoxidation of b-caryophyllene 39
was successfully produced the corresponding monoepoxide 40
(Entry 18).
ported previously. 3,4-Dihydroxy-2,2-dimethylchroman 10 and
p-menthane-1,2,8-triol 37 were identified by GC-MS analysis
(without isolation of the products).
Typical procedures for epoxidations
Indene (1) epoxidation (Table 1, Entry 1). A 50-mL flask
equipped with a stirrer bar was charged with CH2Cl2, indene
1 (1.16 mL, 10 mmol), 3-methylpyrazole (81 mL, 1.0 mmol,
10 mol%), 1-methylimidazole (8 mL, 0.1 mmol, 1 mol%), and MTO
(5 mg, 0.020 mmol, 0.2 mol%). H2O2 (35%, 1.68 mL, 20 mmol) was
added all at once to the stirring solution. The resulted two-phase
mixture was stirred vigorously (1000 rpm) at room temperature.
The progress of the reaction was monitored at appropriate interval
by GC analysis of small aliquots of the organic phase. The
conversion of indene and yield of indene oxide were determined by
GC internal standard method. The GC internal standard material
(n-undecane) was added just before the first analysis.
Conclusions
We have found that addition of 1-methylimidazole strongly inhibits
hydrolysis and rearrangement of acid-sensitive epoxides produced
by MTO-catalyzed epoxidation, and that the combined use of
3-methylpyrazole and 1-methylimidazole could produce variety
of acid-sensitive epoxides in excellent yields within reasonable
reaction times. We demonstrated the efficiency of the procedure
by producing variety of acid-sensitive epoxides from styrenes,
a chromene, bishomoallylic alcohols, and terpenes in excellent
yields.
NMR data of isosafrole oxide (Table 1. Entries 3 and 4, mixture
of isomers). Ratio of trans:cis is approximately 4 : 1 by 1H NMR.
1H NMR (90 MHz, CDCl3) dH 1.10 (d, J = 5.4 Hz, 3H, cis), 1.41
(d, J = 5.4 Hz, 3H, trans), 2.98 (dq, J = 5.4, 1.8 Hz, 1H, trans),
3.28 (dq, J = 5.4, 4.3 Hz, 1H, cis), 3.50 (d, J = 1.8 Hz, 1H, trans),
3.98 (d, J = 4.3 Hz, 1H, cis), 5.94 (s, 2H, cis and trans), 6.6–6.9
(m, 3H, cis and trans); 13C NMR (22 MHz, CDCl3) dC 12.4 (cis),
17.7 (trans), 55.1 (cis), 57.4 (cis), 58.7 (trans), 59.5 (trans), 101.0
(cis and trans), 105.6 (trans), 107.1 (cis), 108.0 (cis), 108.1 (trans),
119.5 (trans), 119.9 (cis), 131.7 (trans), 147.5 (trans), 148.0 (trans).
The signals of quaternary carbons of cis isomer were not observed
on the spectrum.
Experimental
General remarks
All reagents obtained were from commercial sources unless
otherwise noted and were used without further purification.
Methyltrioxorhenium was prepared according to the reported
procedure.13b 2,2-Dimethyl-2H-chromene was prepared according
to the reported procedure.42 The concentration of H2O2 was
determined by iodometric titration before use. The progress of
the reaction was monitored by GC analysis. The conversion of
alkenes and yield of epoxides were determined by GC internal
standard technique. GC analyses were performed on Shimadzu
GC-2010 (FID detector) equipped with GL Sciences InertCap
1 column (30 m length x 0.25 mm ID x 0.25 mm film thickness).
GC-MS analyses (EI) were performed on ThermoQuest GCQplus
with Trace2000GC equipped with GL Sciences InertCap 1MS
column (30 m length x 0.25 mm ID x 0.25 mm film thickness). 1H
(90 MHz) and 13C NMR (22 MHz) were recorded on JEOL EX-90
spectrometer using CDCl3 as solvent. The oxidation products were
identified by comparison of physical data with commercial sam-
ples (1,2-dihydroxyindane 3 and linalool oxide (furano) 13 from
TCI, linalool oxide (pyrano) 14 from Wako Pure Chemical) or
reported data. Indene oxide 230a,b,31,40b 2-(2-oxoethyl)benzaldehyde
4,31 a-methylstyrene oxide,43 3,4-epoxy-2,2-dimethylchroman
8,34a 6,7-epoxy-3,7-dimethyl-1-octen-3-ol 12,26d,34a 6,7-epoxy-3,7-
dimethyl-3-octanol 16,34a 5,6-epoxy-6-methyl-2-heptanol 18,34a
2-carene oxide 22,44 3-carene oxide 24,25b–d,40b,f ,41a,b,44 a-pinene
oxide 26,25b,c,26d,40b,f ,41b,44 b-pinene oxide 28,45 1,2-epoxylimonene
30,25b–d,40f ,44 bis-epoxylimonene 31,25d,44 6,7-epoxygeraniol 33,25b,d
2,3,6,7-bis-epoxygeraniol 34,25b,d a-terpineol epoxide 36,40f 2-
hydroxy-1,8-cineole 38,46 b-caryophyllene oxide 4040f ,41a are re-
cis-4-Decen-1-ol (19) epoxidation (Table 3, Entry 7). A 50 mL
flask equipped with a stirrer bar was charged with CH2Cl2, cis-
4-decen-1-ol 19 (1.83 mL, 10 mmol), 3-methylpyrazole (81 mL,
1.0 mmol, 10 mol%), 1-methylimidazole (8 mL, 0.1 mmol, 1 mol%)
and MTO (5 mg, 0.020 mmol, 0.2 mol%). The mixture was cooled
to 20 ◦C by immersing in a temperature controlled water bath.
H2O2 (35%, 1.01 mL, 12 mmol) was added all at once to the stirring
solution. The resulted two-phase mixture was stirred vigorously
(1000 rpm) at 20 ◦C. The reaction was completed after 4 h.
(The progress of the reaction was monitored by GC analysis.)
The reaction mixture was poured into brine, and extracted with
CH2Cl2. The organic layer was washed with aqueous Na2S2O3,
and then dried over anhydrous Na2SO4. The solvent of the
organic layer was removed out by evaporator, and the residue
was dried under vacuum. 1-Hydroxy-4,5-epoxydecane 20 (1.71 g,
99%) was obtained as pale yellow oil. An attempt of purification
by vacuum distillation was failed by decomposition of the epoxide.
1
1-Hydroxy-4,5-epoxydecane 20: H NMR (90 MHz, CDCl3) dH
0.7–1.9 (m, 16H), 2.4 (br s, 1H), 2.9 (m, 2H), 2.7 (t, J = 6 Hz, 2H);
13C NMR (22 MHz, CDCl3) dC 13.9, 22.4, 24.3, 26.1, 27.7. 29.7,
31.6, 57.0, 57.5, 62.2.
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Procedure for 10g scale epoxidation of 3-carene (Table 4, Entry
6). A 200-mL flask equipped with a stirbar was charged with
CH2Cl2 (35 mL), 3-carene (10 g, 73.4 mmol, 94% purity by GC), 3-
methylpyrazole (0.59 mL, 7.3 mmol, 10 mol%), 1-methylimidazole
(59 mL, 0.73 mmol, 1 mol%), and MTO (36.6 mg, 0.147 mmol,
0.2 mol%). The flask was cooled to 10 ◦C by applying an external
cooling bath. H2O2 (35%, 7.4 mL, 88 mmol) was added dropwise
to the stirring solution from dropping funnel (ca. 10 min). During
the◦H2O2 addition the temperature of the solution was kept below
17 C. The resulted two-phase mixture was stirred vigorously at
10 ◦C. The reaction was completed after 4 h, and the reaction
mixture was poured into brine. The organic layer of reaction
mixture was washed successively with brine (2 times) and with
aqueous solution of Na2S2O3. The organic layer was dried over
anhydrous Na2SO4, and CH2Cl2 was distilled out by evaporator.
Distillation of the residual oil under reduced pressure (9 Torr, 76–
77 ◦C) afforded 3-carene oxide as colorless oil (10.5 g, 94% yield,
94% purity by GC). The physical data agreed with that previously
reported.25b–d,40b,f ,41a,b,44
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