A versatile method of epoxide formation with the support of peroxy ionic liquids

Article abstract of DOI:10.1039/c5nj00644a

The application of the peroxy ionic liquid, 1-butyl-3-methylimidazolium peroxymonosulphate, as an oxidation agent and a solvent for the synthesis of epoxides was described. The 2.5-molar excess of the peroxy ionic liquid to olefin was applied. The reaction system consisted of 1,1,1-trifluoroacetone as an oxirane precursor, which was used with the molar ratio of 1:3 relative to olefin and water solution of NaHCO₃. Under these conditions the epoxidation of 4-bromocinnamic acid led to the epoxide formation at the ambient temperature in 30 minutes. Dioxiranes, generated from the peroxy ionic liquid and 1,1,1-trifluoroacetone, demonstrated encouraging potential for the epoxidation of a variety of other olefins: styrene, limonene, stilbene, linalyl acetate and a complex steroid molecule with high yields of final epoxides from 65-98%.

Full text of DOI:10.1039/c5nj00644a

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A versatile method of epoxide formation with the
support of peroxy ionic liquids
Cite this:DOI: 10.1039/c5nj00644a
Przemysław Zawadzki,a Karolina Matuszek,b Wojciech Czardybona and
Anna Chrobok*b
The application of the peroxy ionic liquid, 1-butyl-3-methylimidazolium peroxymonosulphate, as an oxidation
agent and a solvent for the synthesis of epoxides was described. The 2.5-molar excess of the peroxy ionic
liquid to olefin was applied. The reaction system consisted of 1,1,1-trifluoroacetone as an oxirane precursor,
which was used with the molar ratio of 1 : 3 relative to olefin and water solution of NaHCO3. Under these
conditions the epoxidation of 4-bromocinnamic acid led to the epoxide formation at the ambient
temperature in 30 minutes. Dioxiranes, generated from the peroxy ionic liquid and 1,1,1-trifluoroacetone,
demonstrated encouraging potential for the epoxidation of a variety of other olefins: styrene, limonene,
stilbene, linalyl acetate and a complex steroid molecule with high yields of final epoxides from 65–98%.
Received (in Porto Alegre, Brazil)
14th March 2015,
Accepted 30th April 2015
DOI: 10.1039/c5nj00644a
www.rsc.org/njc
Introduction
Epoxides due to their high reactivity, which is caused by a highly
strained tricyclic ring, under certain conditions and in the
presence of a selected nucleophile, can be converted to a wide
range of valuable derivatives.1 Epoxides are important synthetic Scheme 1 Catalytic cycle of oxidation by the ketone/Oxonet method.
building blocks widely used in the chemical industry in the
production of pharmaceutical products, flavours, fragrances,
resins, adhesives or paints.2–6
In our previous studies, alternative methods of the oxidation
Most epoxidation systems use catalysts based on transition of ketones or alcohols with KHSO5 were presented. The first
metals (V, Mn, W, Ti, Re, etc.) which activate oxidants, such as method was based on the use of an alternative solvent, such as
H2O2 or hydroperoxides.1 However, non-metal epoxidation using an ionic liquid, to dissolve KHSO5 and eliminate water from the
dioxiranes can also be performed. Dioxiranes are usually gene- reaction system.11 The work referred to above demonstrated that
rated from the potassium peroxymonosulphate salt KHSO5 and the use of homogenous conditions is critical for the oxidation of
ketones as an efficient and remarkably versatile class of oxidants.7 ketones with KHSO5 to lactones, which was provided by ionic
The commercial sources of KHSO5 are low-cost industrial bulk liquids. The second method used a phase transfer catalysis to
chemicals, e.g., the triple salt Oxonet (2KHSO5ÁKHSO4ÁK2SO4). avoid hydrolysis of lactones.12
These products are stable oxidizing agents commonly used in the
Unique physical properties of ionic liquids, such as low volatility,
fine chemical synthesis; they are easy to handle, non-toxic and thermal stability, make them attractive alternatives to organic
generate non-polluting by-products.8 Epoxidations of olefins pro- solvents. Excellent solubility of organic and inorganic compounds
ceeding via in situ generated dioxiranes (Scheme 1) often require a in ionic liquids and the possibility of easy recycling of these salts
range of pH 7–8,9 higher pHs lead to rapid autodecomposition of have evoked wide interest in their application in the organic
Oxonet.10 The in situ oxidation with dioxiranes presents many synthesis as solvents. The last decade brought much interest in
advantages, such as e.g. the possibility of regeneration of the task-specific ionic liquids, which are terms that refer to the potential
parent ketones after the oxygen transfer. In early studies, a two- design capacity of ionic liquids for chemical tasks.13 Building on
phase solvent system was employed, in which the solubility this, we demonstrated a new class of task-specific ionic liquids with
properties of alkene and ketone played a crucial role.9
peroxymonosulphate anions, e.g. 1-butyl-3-methylimidazolium salt
(Scheme 2). The resulting salts were liquids at the room temperature
and served as oxidants and solvents in the model oxidation of
cyclohexanol to e-caprolactone.14
Ionic liquids used as solvents in epoxidation reactions of a
broad substrate scope with various oxidation agents were used
a
´
´
Selvita S.A., ul. Bobrzynskiego 14, 30-348 Krakow, Poland
b Silesian University of Technology, Faculty of Chemistry, Department of Chemical
Organic Technology and Petrochemistry, ul. Krzywoustego 4, Gliwice 44-100,
Poland. E-mail: anna.chrobok@polsl.pl; Fax: +48 322371032; Tel: +48 322372917
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follows: olefin and NaHCO3 were suspended in the acetone–
water mixture, then a water solution of EDTA was added. The
molar ratio of olefin to NaHCO3 was 1 : 15. Oxonet was the last
reagent which was introduced under vigorous stirring and
allowed to react at 20–25 1C for an appropriate time. The reaction
progress was monitored with ultra performance liquid chromato-
graphy (UPLC) (Table 1).
For the presented experiments, even if acetone was used as a
solvent and the dioxirane precursor in a big excess (the molar
ratio of olefin to acetone was 1: 50) together with Oxonet (the
molar ratio of olefin to Oxonet was 7), only 60% yield of epoxide
was achieved in 4 h (Table 1, entry 1). In the next step, attempts to
eliminate acetone using a more reactive dioxirane precursor, TFA
with acetonitrile as a solvent, were performed. The molar ratio of
olefin to TFA was 1: 5 and even when Oxonet was used in big
excess, still the yield reached only 43% in 1 h (Table 1, entry 2).
The system was strengthened by the addition of the ionic
liquid [bmim][HSO5]. It is worth noting that in the first experi-
ment the ionic liquid [bmim][HSO5] (Table 1, entry 3) was
added in the first step, together with water. Oxonet, as applied
above, was the last component. The yield of epoxide was higher,
but the reaction stopped after 4 h and a full conversion was not
reached, probably because of the creation of a highly viscous
Scheme 2 1-Butyl-3-methylimidazolium peroxymonosulphate [bmim]-
[HSO5]À.
to improve yield, selectivity or the rate of the reaction.15,16 The
epoxidation of alkenes by Oxonet was also investigated in ionic
liquids. To this aim 2-alkyl-3,4-dihydroisoquinolinium salts were
used as catalysts.17,18 The effectiveness of the reaction systems
depended on the miscibility of the reagents with the ionic liquids.
The application of water miscible ionic liquids gave similar
results to the conventional acetonitrile based systems.17
As presented above, new methods of epoxidation of alkenes
are an ongoing area of research. This work is focused on the
development of a versatile method of the epoxide ring formation
using novel task-specific ionic liquids based on the peroxysulfate
anion as both the oxidant and the solvent.
Results and discussion
For the initial study a model epoxidation of 4-bromocinnamic mixture, very difficult to stir.
acid was chosen to determine crucial parameters for the epoxide
The use of the ionic liquids [bmim][HSO5] with Oxonet gave
ring formation. Among dioxiranes the most useful dimethyl- a much better conversion of 4-bromocinnamic acid compared
dioxirane and 1,1,1-trifluoroacetone (TFA) were used, as in their to the reaction with Oxonet alone in conventional solvents
case high electronegativity of fluorine makes them a stronger (Table 1, entry 3). These observations drive us to a conclusion
epoxidising agent. From amongst the peroxymonosulphate ionic that maybe there is a possibility of eliminating Oxonet comple-
liquids 1-butyl-3-methylimidazolium salt [bmim][HSO5] was chosen tely from the reaction mixture and performing this synthesis only
as a model oxidation agent. The use of [bmim][HSO5] eliminated in the presence of an ionic liquid. Additionally, we have observed
the addition of other solvents to the reaction system, while typical that the order of the reagent addition may have a crucial role in
solvents like water–acetone or water–acetonitrile were used for the outcome of the reaction. Theoretically, an ionic liquid added
comparative tests with Oxonet. Due to the short life of the as a thirst system component can react with the basic NaHCO3
in situ forming reactive dioxiranes, the reaction system required before it starts to react with ketone to form dioxirane. On the
stabilization by the introduction of EDTA and maintaining the other hand, dioxiranes are very reactive species, and thus they are
pH around 7–8 by the addition of NaHCO3 (Table 1).
highly unstable. Therefore, it is necessary to avoid a situation
For comparison reasons the experiments were carried out where all of the formed dioxirane decomposes before it starts to
with Oxonet. The order of the addition of the reagents was as react with olefin.
Table 1 Influence of reaction conditions on epoxidation of 4-bromocinnamic acid with Oxonet or [bmim][HSO5]
No.
1
Solvent
Source of dioxirane
Molar ratio olefin/[HSO5]À
Time
Yield of epoxidec [%]
H2O–acetonea
Acetone
Acetone
Acetone
Acetone
TFA
TFA
TFA
TFA
TFA
TFA
TFA
2.5 (Oxonet)
2.5 (Oxonet)
2.5 (Oxonet)
7 (Oxonet)
5 (Oxonet)
5 (Oxonet)
7 (Oxonet)
7 (Oxonet)
5 (Oxonet) + 7 [bmim][HSO5]
5 (Oxonet) + 7 [bmim][HSO5]
7 [bmim][HSO5]
7 [bmim][HSO5]
5 min
1 h
24 h
4 h
5 min
24 h
5 min
1 h
5 min
4 h
5 min
30 min
17
50
50
60
20
20
20
43
60
60
88
99
H2O–acetonea
H2O–acetonea
H2O–acetonea
2
H2O–acetonitrileb
H2O–acetonitrileb
H2O–acetonitrileb
H2O–acetonitrileb
H2O–[bmim][HSO5]b
H2O–[bmim][HSO5]b
H2O–[bmim][HSO5]b
H2O–[bmim][HSO5]b
3
4
TFA
a
Reaction conditions: olefin (0.10 g, 0.44 mmol), NaHCO3 (0.50 g, 6.60 mmol), 2 ml acetone/water (1:1 v/v), 1 ml water solution of EDTA (0.40 mol dmÀ3),
b
RT. Reaction conditions: olefin (0.10 g, 0.44 mmol), NaHCO3 (0.50 g, 6.60 mmol), 1 ml water, 1 ml acetonitrile, 1 ml water solution of EDTA
(0.40 mol dmÀ3), TFA (0.24 g, 2.20 mmol), RT. Yield was determined by UPLC, no by-products were detected, only unreacted olefin was found.
c
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Table
3
Influence of the amount of TFA on the epoxidation of
4-bromocinnamic acid in the presence of NaHCO3 and [bmim][HSO5]a
Molar ratio
olefin/TFA
Reaction
time [h]
Conversion
of olefin [%]
Yield of
No.
epoxideb [%]
1
2
3
4
5
3
2
1
0.5
0.5
0.5
0.5
24
100
100
90
60
80
0
99
99
91
60
80
0
c
5
—
24
a
Olefin (0.10 g, 0.44 mol), NaHCO3 (0.50 g, 6.60 mmol), 1 ml water,
b
[bmim][HSO5] (0.25 g, 1.10 mmol) RT. Yield was determined by UPLC.
c
The reaction without TFA.
Fig. 1 Influence of the amount of the ionic liquid [bmim][HSO5] on the
conversion of 4-bromocinnamic acid and yield of epoxide in the presence
of TFA and EDTA at RT.
Table 2 Influence of additives on the epoxidation of 4-bromocinnamic
acid in the presence of TFA and [bmim][HSO5]a
EDTA Water Molar ratio
No. [ml] [ml]
Time Conversion Yield of
olefin/NaHCO3 [min] of olefin [%] epoxideb [%]
1
2
3
—
1
—
1
15
—
15
5
30
5
30
5
3
6
0
0
45
100
4
6
0
0
45
99
—
1
30
a
Reaction conditions: olefin (0.10 g, 0.44 mol), NaHCO3 (0.50 g, 6.60 mmol),
1 ml water, 1 ml water solution of EDTA (0.40 mol dmÀ3), TFA (0.24 g,
2.20 mmol), [bmim][HSO5] (0.25 g, 1.10 mmol), RT. b Yield and conversion
were determined by UPLC.
Scheme 3 Proposed mechanism for epoxidation of olefin with [bmim][HSO5].
Taking the observation made in the last experiment into olefin after 30 minutes. It was proved by Shi et al. that TFA forms
account (Table 1, entry 4) Oxonet was eliminated from the much more stable dioxirane species compared to dimethyldioxirane.
system and [bmim][HSO5] was added as the last reagent. This Even catalytic amounts of ketone were enough to obtain a satis-
time we achieved our goal, the reaction was fast and a full factory conversion, although the reaction requires several hours
conversion was observed after 30 minutes.
to be completed.19 The reaction without the addition of TFA
In order to determine the influence of crucial factors on the does not occur, which can be the proof of the necessity of
course of the model reaction several experiments under the formation of dioxirane during the reaction.
conditions determined above (Table 1, entry 4) were performed.
The proposed mechanism for the epoxidation of 4-bromo-
At first a sufficient amount of oxidizing agent was established. cinnamic acid with [bmim][HSO5] as an oxidant is presented in
To this aim [bmim][HSO5] was added to the reaction system in Scheme 3. In the first step the carbonyl group from TFA 1 is
portions. After the addition of each portion the conversion of attacked by [HSO5]À peracid anions to form intermediate 2. In
olefin and yield of the product were analyzed with UPLC (Fig. 1). the next steps the formation of dioxirane 4 occurs. In the end
The results lead to a conclusion that the necessary molar ratio the resulted dioxirane oxidises 4-bromocinnamic acid 5 to its
of olefin to the ionic liquid to obtain almost 100% of the product epoxide 6.
is only 2.5.
The new method has already shown a significant improve-
In the next stage the impact of the presence of stabilizing ment in comparison with the conventional process of epoxidation
agents: EDTA and NaHCO3 as well as water on the reactivity of it terms of yield and stabilization requirements. Another advan-
olefin was checked (Table 2). It was confirmed that the base tage of this method would be easy isolation of the product. After a
and water are necessary to keep the desirable pH of the reaction simple aqueous work-up and extraction with ethyl acetate it is
system (Table 2, entry 3). However, when the reaction is carried possible to isolate desired 3-(4-bromophenyl)oxirane-2-carboxylic
out in the presence of [bmim][HSO5] there is no need for further acid with 98% yield and 99% purity (determined by NMR).
stabilization of dioxiranes by the EDTA complex.
Finally, the most active reaction system was examined in the
In the experiments presented above, the molar ratio of olefin epoxidation of various olefins to determine its practical potential.
to TFA was 1 : 5. The possibility of lowering the amount of ketone A few derivatives which are commonly used for this type of reaction
was also checked (Table 3). It was found that 3 equivalents of were selected: styrene, stilbene, limonene, linalyl acetate and also
1,1,1-trifluoroacetone are enough to reach a full conversion of steroid structure (IMDA). The yields of epoxides obtained in the
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Table 4 Epoxidation of selected olefins in the presence of TFA and [bmim][HSO5]a
Conversion
Yield of
No.
1
Olefin
Epoxide
Oxidising agent
Reaction time
of olefinb [%]
epoxideb [%]
m-CPBA
[bmim][HSO5]
30 min
30 min
80
100
80
99 (98)
m-CPBA
[bmim][HSO5]
30 min
30 min
60
100
60
99 (85)
2
3
m-CPBA
[bmim][HSO5]
1 h
1 h
100
100
50c
80c (65)
m-CPBA
[bmim][HSO5]
1 h
10 min
90
100
90
99 (90)
4
5
m-CPBA
[bmim][HSO5]
5 min
5 min
20
100
20
99 (72)
m-CPBA
[bmim][HSO5]
1 h
20 min
100
100
99
99 (70)
6
a
b
Olefin (0.20 g, 0.88 mmol), NaHCO3 (1.00 g, 12.12 mmol), 2 ml water, TFA (0.29 g, 2.64 mmol), [bmim][HSO5] (0.50 g, 2.20 mmol), RT. Yield and
c
conversion were determined by UPLC/MS, in the parenthesis isolated yields. 20% of the bisoxidised product was observed (determined by NMR).
reaction with a very strong oxidation agent m-chloroperbenzoic acid are very attractive in terms of biological activity and epoxide can be
(m-CPBA) were also presented for comparison. Organic percarboxylic treated as a versatile reagent for further functionalization.
acids as typical oxidants are fairly expensive, often poorly-stable and
hazardous, and this consequently limits their commercial applica-
tion. Therefore, the new approach with relatively stable peroxy ionic
liquids appears to be a very attractive alternative.
Conclusions
A new and effective method of epoxidation of olefins in the pre-
Styrene was oxidized to epoxide with the conversion of 60% in
sence of 1,1,1-trifluoroacetone and versatile peroxy ionic liquid
the presence of m-CPBA in 30 minutes. In comparison with the
was developed. Peroxy ionic liquid based on the 1-methyl-3-butyl-
use of a liquid a complete conversion was observed using a peroxy
imidazolium cation bears the peroxy function in the ion structure
ionic liquid within the same period of time (Table 4, entry 2).
as peroxymonosulphate [HSO5]À. Peroxymonosulphate ionic liquid
Similar results were observed with the epoxidation of limonene,
does not contain the ballast of inorganic salts (KHSO4 and K2SO4)
where the use of the ionic liquid improved conversion of the
against Oxonet, which results in an easier work-up and product
reaction. In this process conversion was not completed mainly
purification. An ionic liquid acts as both an oxidant and a reaction
due to the fact that bisoxidised products were formed during this
medium. In this work it was demonstrated that using the
reaction (Table 4, entry 3). In the oxidation of linalyl acetate
developed method high yields of epoxides and short reaction
improvements of both the reaction time and conversion in
times can be reached. Forevermore, epoxides can be easily iso-
comparison to the traditional method were observed (Table 4,
lated from the reaction mixture by simple extraction.
entry 4). Stilbene was oxidized very rapidly (Table 4, entry 5).
Just after 5 minutes a complete conversion was observed.
Encouraged with very good results we have tried to perform the
reaction on a more complex system. In the case of a steroid molecule
(iso-masticadienonic acid, Table 4, entry 6), the application of the
Experimental
Materials
peroxy ionic liquid led to a 5-times shorter reaction time compared 1-Butyl-3-methylimidazolium bromide, sulfuric acid, Oxonet,
to peracid. It is also worth mentioning that these types of derivatives acetone, 1,1,1-trifluoroacetone, acetonitrile, 4-bromocinnamic acid,
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styrene, stilbene, limonene, linalyl acetate, NaHCO3, m-chloro- 12.3, 9.9 Hz, 1H), 1.26 (d, J = 7.0 Hz, 3H), 1.13–1.03 (m, 8H), 0.93
perbenzoic acid and EDTA were purchased from Sigma Aldrich. (dd, J = 15.8, 3.3 Hz, 7H), 0.78 (s, 3H).
Iso-masticadienonic acid was purchased from Angene Inter-
national Limited. [bmim][HSO5] was synthesized according to
the literature procedure.14
Acknowledgements
This work was financed by the Polish National Science Centre
(Grant no. UMO-2012/06/M/ST8/00030).
Instrumentation
The structure and purity of all the synthesized substances were
confirmed by the NMR analysis. 1H NMR spectra were recorded
on a Bruker 400 MHz in CDCl3 or DMSO (internal standard
TMS). All epoxides were characterised by comparing their NMR
spectra with those of authentic samples. UPLC analyses were
performed using a Shimadzu UPLC DAD detector and an Acquity
UPLC HSS C18 column (Waters, 50 mm  2.1 mm  1.8 mm).
Notes and references
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Methods
General method of the epoxidation reaction with [bmim][HSO5].
Olefin (0.44 mmol) was suspended in 1 ml of water followed by the
addition of solid NaHCO3 (6.60 mmol) and TFA (1.32 mmol). Next
[bmim][HSO5] (1.10 mmol) was added dropwise and the reaction
was stirred at RT. Periodically, 20 ml of the samples diluted with
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