Useful application of acidic ionic liquids as solvents in Baeyer-Villiger oxidation with hydrogen peroxide for the synthesis of lactones

Article abstract of DOI:10.1080/00397910802138926

Cyclic ketones have been efficiently oxidized with hydrogen peroxide using acidic ionic liquids (ILs) as solvents. This is a new method for the synthesis of lactones with high yields that does not utilize any additional catalysts and enables ILs to be recycled. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910802138926

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Useful Application of Acidic
Ionic Liquids as Solvents in
Baeyer–Villiger Oxidation with
Hydrogen Peroxide for the
Synthesis of Lactones
Stefan Baj a & Anna Chrobok a
a Silesian University of Technology, Department of
Chemical Organic Technology and Petrochemistry ,
Krzywoustego, Gliwice, Poland
Published online: 11 Jul 2008.
To cite this article: Stefan Baj & Anna Chrobok (2008) Useful Application of Acidic
Ionic Liquids as Solvents in Baeyer–Villiger Oxidation with Hydrogen Peroxide for
the Synthesis of Lactones, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 38:14, 2385-2391, DOI:
10.1080/00397910802138926
To link to this article: http://dx.doi.org/10.1080/00397910802138926
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Synthetic Communications1, 38: 2385–2391, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910802138926
Useful Application of Acidic Ionic Liquids as
Solvents in Baeyer–Villiger Oxidation with
Hydrogen Peroxide for the Synthesis of Lactones
Stefan Baj and Anna Chrobok
Silesian University of Technology, Department of Chemical Organic
Technology and Petrochemistry, Krzywoustego, Gliwice, Poland
Abstract: Cyclic ketones have been efficiently oxidized with hydrogen peroxide
using acidic ionic liquids (ILs) as solvents. This is a new method for the synthesis
of lactones with high yields that does not utilize any additional catalysts and
enables ILs to be recycled.
Keywords: Baeyer–Villiger oxidation; Hydrogen peroxide; Ionic liquids;
Lactones; Urea–hydrogen peroxide
INTRODUCTION
The Baeyer–Villiger (BV) oxidation of ketones is a reaction of great
interest in organic chemistry with a large range of possible applications,
including synthesis of antibiotics, steroids, pheromones, and monomers
for polymerization.[1]
The most often used oxidants in this process are peracids and hydro-
gen peroxides. The use of organic peracids involves several disadvantages
(e.g., it results in the formation of 1 equivalent of the corresponding
carboxylic acid salt as waste). Moreover, peracids are expensive and
hazardous, which limits their commercial application.
Because of these complications, organic peracids have generally been
replaced by hydrogen peroxide[2] as an oxidant. These types of experi-
ments are generally carried out in the presence of a catalyst to increase
Received in Poland 25 October 2007
Address correspondence to Anna Chrobok, Silesian University of Technology,
Department of Chemical Organic Technology and Petrochemistry, Krzywoustego
4, 44-100 Gliwice, Poland. E-mail: Anna.Chrobok@polsl.pl
2385

2386
S. Baj and A. Chrobok
the nucleophilicity of hydrogen peroxide. A number of transition-metal
systems based on Pt, Re, and Ti have been widely used.[3] BV oxidations
typically make use of chlorinated hydrocarbons as solvents. However,
there is a strong environmental need for a method that utilizes aqueous
hydrogen peroxide and avoids the use of such solvents. Recently, ionic
liquids have been described as some of the most promising new reaction
media. These liquids dissolve many organic and inorganic substances and
are readily recycled and tunable to specific chemical tasks.[4] There is little
information in the literature about successful synthetic approaches employ-
ing ionic liquids as solvents in BV oxidations (e.g. using of peracids[5] as
oxidants). However, interesting results have been obtained using ionic
liquids as solvents in the presence of H2O2 as an oxidant using methyltriox-
orhenium,[6] Sn-b molecular sieves,[7] and Pt (II)[8] as catalysts.
Herein, we present a new method for the synthesis of lactones involv-
ing acidic ionic liquids as solvents and hydrogen peroxide as an oxidant.
Acidic ionic liquids have recently become widely used as task-specific
substances–both as solvents and catalysts. The first examples of such a
use of ILs were chloroaluminate ionic liquids.[4c,d] Nevertheless, in the
presence of water, the composition of the salt can change with consequ-
ences for the acidity of the solution. This is the reason why water- and
air-stable acidic ionic liquids were developed. First, SO3H-functionalized
ionic liquids were used for porphyrin synthesis.[9] Later, other ionic
liquids (e.g., based on [HSO4] À anion) have been used in esterification[10]
or alkylation[11] processes. Also, protic ionic liquids used for esterifi-
cation,[12] Mannich reaction,[13] and others are interesting.
RESULTS AND DISCUSSION
The method of synthesis of lactones presented here involves acidic ionic
liquids as solvents and hydrogen peroxide as an oxidant. All ionic liquids
used in this study are based on dialkylimidazolium cations. This moiety
of IL is acidic. Additionally, bmim[HSO4] possesses an acidic anion.
The use of acid or base catalysis with H2O2 in the BV oxidation has been
known for a long time.[14] However, this method avoids hazardous
organic solvents, such as methylene chloride, acetonitryle, or chloroform,
and toxic=expensive catalysts, which are typically necessary when using
H2O2 as an oxidant.
As model reactants, we chose strained ketones such as cyclobutanone
and cyclopentanone, which have long been known to afford lactones in
high yield with alkaline hydrogen peroxide[15] and nonstrained ketone
cyclohexanone, which is much more difficult to oxidize with H2O2.[1]
As oxidants, we chose 30%, 50%, and 68% hydrogen peroxide.

Useful Application of Acidic Ionic Liquids
2387
Additionally, for these studies we selected the strongly H-bonded,
anhydrous, solid urea-hydrogen peroxide adduct (UHP) as the oxidant.
The UHP is stable, nontoxic, commercially available, inexpensive, and
easily handled. This adduct, which contains 35% of H2O2, sets the hydro-
gen peroxide free during its application.
It is known that hydrogen peroxide can relatively easily undergo
decomposition in the presence of acids. For this reason, it was important
to test the stability of H2O2 and UHP under the chosen reaction condi-
tions by stirring them with studied ionic liquids at 50ꢀC. After 5 h, the
concentration of H2O2 in the tested samples did not change. This was
tested by an iodometric titration.
Syntheses of lactones were carried out at 50 ꢀC for 5 h. The concentration
of cyclic ketone in ionic liquid was 1.9 mol=dm3. Three and a half times the
stoichiometric amount of hydrogen peroxide was used. Products were
extracted with diethyl ether. Although the solvent extraction process may
be controversial (using an ionic liquid to avoid atmospheric emissions and
substituting a volatile organic solvent to extract the product), it could have
an environmental benefit.[4c] The organic extractant does not necessarily
have to be a good solvent for the synthesis, so ether or hexane can be used
instead of chlorinated hydrocarbons. As a result, we obtained lactones with
high yields (Table 1) in the presence of 68% H2O2 as well as UHP.
It is known from the literature[2] that the first step of BV oxidation is
the formation of the Criegee adduct. Probably introduced by us, acidic
ionic liquids can influence the Criegee rearrangement step in BV oxidation.
Table 1. Baeyer–Villiger oxidation with acidic ionic liquids as solvents
Ketone
Lactone
Solvent=oxidant
Yield (%)a
[bmim]CF3COO=H2O2
[bmim]HSO4=H2O2
[bmim]HSO4=urea H2O2
[bmim]CF3COO=H2O2
[bmim]HSO4=H2O2
89 (84)
92 (88)
95 (92)
86 (82)
85 (81)
91 (88)
[bmim]HSO4=urea H2O2
[pmim]PF3(C4F9)3=H2O2
[hmim]PF3(C2F5)3=H2O2
[bmim]CF3COO=H2O2
[bmim]HSO4=H2O2
96
96
90 (85)
90 (87)
93 (88)
[bmim]HSO4=urea H2O2
aYields determined by gas chromatography; in parentheses are yields after
chromagraphic purification of the reaction mixture.

2388
S. Baj and A. Chrobok
Table 2. Influence of H2O2 concentration (3.25 mmol) on cyclopentanone
(0.93 mmol) oxidation at 50 ꢀC (reaction time 5 h)
Concentration of
H2O2 [%]
Entry
Solvent
Yield (%)a
1
2
3
4
5
6
30
30
50
50
68
68
[bmim]CF3COO
[bmim]HSO4
[bmim]CF3COO
[bmim]HSO4
[bmim]CF3COO
[bmim]HSO4
58
52
70
73
86
85
aDetermined by gas chromatography.
The reagents [bmim]HSO4 and [bmim]CF3COO were very successful in the
oxidation process, making it feasible within a reasonable time period (5 h).
These ionic liquids are easily obtained via ion exchange between [bmim]Cl
and H2SO4. High yields of d-valerolactone were obtained when using per-
fluorinated ionic liquids (e.g., [pmim]PF3(C4F9)3 and [hmim]PF3(C2F5)3).
These ionic liquids are relatively expensive, so only the reactivity of
cyclopentanone was tested using them as a reaction medium.
Additionally, we have checked the influence of concentration of H2O2 on
the reaction course by applying commercially available hydrogen peroxide
(30% and 50% solution in water) (Table 2). As a result, we observed that with
increasing H2O2 concentration, the yield of d-valerolactone increases. This is
caused by the presence of water in the reaction medium. In cases with higher
amounts of water, the hydrolysis of lactones can be observed (with the
concomitant formation of hydroxycarboxylic acids). Anhydrous UHP seems
to be the best and the most convenient reagent in this situation.
The ionic liquids were recovered and reused for another reaction with
identical results. Table 3 shows [bmim]HSO4 having been recycled three
times after the oxidation of cyclopentanone.
Table 3. Recycling of [bmim]HSO4(2 mL) in the oxi-
dation process of cyclobutanone (3.72 mmol) with 68%
H2O2 (13.00 mmol) at 50 ꢀC (reaction time 5 h)
Recyclability of IL
Yield [%]a
Fresh, nonrecycled IL
First
second
Third
92
90
90
89
aDetermined by gas chromatography.

Useful Application of Acidic Ionic Liquids
CONCLUSIONS
2389
In summary, we report an efficient and mild method for the synthesis of
lactones that utilizes hydrogen peroxide as an oxidant in environmentally
attractive novel solvents such as ionic liquids, which can be used as both
solvent and catalyst. This method avoids toxic and expensive catalysts.
The simplicity of the methodology, ease of product isolation, high yields,
mild conditions, as well as the possibility of recycling the ionic liquids can
result in big steps toward greener processes for the manufacture of
lactones.
EXPERIMENTAL
All ionic liquids used in this study were purchased from Merck: 1-butyl-3-
methylimidazolium trifluoroacetate ([bmim]CF3COO); 1-butyl-3-methy-
limidazolium hydrogensulfate ([bmim]HSO4); 1-methyl-3-pentylimidazo-
lium tris(nonafluoro butyl)trifluorophosphate ([pmim]PF3(C4F9)3); and
1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate
[hmim]PF3(C2F5)3). Cyclobutanone, cyclopentanone, cyclohexanone,
and UHP were purchased from Aldrich. H2O2 (68% aqueous) was
obtained by distillation of 30% H2O2 in the presence of stabilizers:
Na4P2O7, H3PO4, and 8-hydroxychinoline.
Typical Procedure for BV Oxidation
H2O2 (68% aqueous, 3.25 mmol) was added to a solution of cyclobuta-
none (0.93 mmol) in [bmim]HSO4 (0.5 mL) and stirred at 50ꢀC for 5 h.
The progress of the reaction was monitored by gas chromatographic
(GC). After the completion of the reaction, water (10 mL) was added,
and the postreaction mixture was extracted with diethyl ether
(3 Â 10 mL). The organic layer was washed with 5 mL of 10% of a water
solution of NaHCO3, dried over anhydrous MgSO4, filtered, and concen-
trated in a vacuum. The yield of c–butyrolactone after the purification by
column chromatography with hexane–ethyl acetate (4:1) as eluent was
88% yield. All products were characterized by comparison of their
NMR spectra with authentic samples. The aqueous layer was concen-
trated under reduced pressure to recover the ionic liquid and reused.
For the experiments where IL was recycled, four times the amount of
reactants were used. GC analysis was performed using a Perkin-Elmer
chromatograph; 1H NMR and 13C NMR spectra were recorded at
300 MHz in CDCl3 (Varian Unity Inova Plus, internal TMS).

2390
S. Baj and A. Chrobok
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