Efficient Baeyer-Villiger electro-oxidation of ketones with molecular oxygen using an activated carbon fiber electrode in ionic liquid [bmim][OTf]

Article abstract of DOI:10.4314/bcse.v30i2.14

A new and efficient method for the synthesis of lactones and esters involving the application of an molecular oxygen-based electro-catalytic oxidation system and ionic liquid [bmim][OTf] as electrolyte has been developed. The reaction between various ketones with molecular oxygen proceeds in a three-electrode cell under constant current conditions in [bmim][OTf] at room temperature to give the corresponding esters and lactones in good to excellent isolated yield. Additionally, the possible mechanism of Baeyer-Villiger oxidation of ketones in the electro-catalytic system is proposed.

Full text of DOI:10.4314/bcse.v30i2.14

Bull. Chem. Soc. Ethiop. 2016, 30(2), 297-306.
Printed in Ethiopia
DOI: http://dx.doi.org/10.4314/bcse.v30i2.14
ISSN 1011-3924
 2016 Chemical Society of Ethiopia
EFFICIENT BAEYER-VILLIGER ELECTRO-OXIDATION OF KETONES WITH
MOLECULAR OXYGEN USING AN ACTIVATED CARBON FIBER ELECTRODE IN
IONIC LIQUID [bmim][OTf]
Yu Lin Hu1,2,*, Yi Bi Xie2, and De Jiang Li2
1College of Chemistry and Environmental Engineering, Yancheng Teachers University,
Yancheng 224002, P. R. China
2College of Materials and Chemical Engineering, China Three Gorges University, Yichang
443002, People’s Republic of China
(Received April 5, 2015; revised July 3, 2016)
ABSTRACT. A new and efficient method for the synthesis of lactones and esters involving the application of an
molecular oxygen-based electro-catalytic oxidation system and ionic liquid [bmim][OTf] as electrolyte has been
developed. The reaction between various ketones with molecular oxygen proceeds in a three-electrode cell under
constant current conditions in [bmim][OTf] at room temperature to give the corresponding esters and lactones in
good to excellent isolated yield. Additionally, the possible mechanism of Baeyer-Villiger oxidation of ketones in
the electro-catalytic system is proposed.
KEY WORDS: Baeyer-Villiger oxidation, Ketone, Molecular oxygen, Electrocatalysis, Ionic liquid
INTRODUCTION
Lactones and esters are important classes of chemicals that have been used extensively as
synthetic intermediates in the preparation of a variety of fine or special chemicals such as drugs,
steroids, pheromones, etc [1, 2]. Up to now, many publications in the open literature have been
found in synthesis of these types of compounds, and a well known method for such a synthesis
constitutes the Baeyer-Villiger oxidation of ketones [3, 4]. Traditional methods for performing
such a transformation generally involve the use of stoichiometric amount of the strongest
oxidizing reagents (e.g., trifluoroperacetic acid, peroxyacetic acid, and perbenzoic acid) [5-8],
and suffer from considerable drawbacks such as low yield, harsh or delicate reaction condition,
and a large amount of waste by-products. Various reagents employed as the oxygen donors have
been developed for this conversion include sodium perborate [9], potassium
peroxydisulfate (K2S2O8) [10, 11], sodium perborate (NaBO3) [11], m-CPBA [12, 13], NADPH
[14, 15], phenylacetone monooxygenase (PAMO) [16], H2O2 [17-22], oxone [23],
h-SiO2·KHSO5 [24], and other complexes [25-30]. However, some of these procedures are
invariably associated with one or more disadvantages such as the use of expensive reagents,
long reaction time, difficulties in work up, environmental hazards, and difficulties in recycling
of the catalyst. Molecular oxygen is a safe, economical and environmentally-benign reagent,
molecular oxygen has recently been utilized as an attractive oxidant due to cheap price and
environmental friendly nature. Therefore, molecular oxygen-based protocols are powerful
methods for the oxidation of ketones and attract a great attention, which make the reactions of
choice in large and industrial scale applications. A number of catalytic oxidation processes
based on the combination of transition metals or organocatalysts and O2 have been developed
for such a conversion [31-34]. However, these methods still suffered from environmental
hazards, poor recovery of expensive metal catalysts and laborious workup procedures.
Consequently, search for new and environmentally benign synthetic methodologies for Baeyer-
Villiger oxidation of ketones that address these drawbacks remains to be of value and interest.
__________
*Corresponding author. E-mail: huyulin1982@163.com

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Ionic liquids (ILs) have attracted considerable attention during the past few years due, in
some cases, to their favourable properties such as very low vapor pressure, wide liquid
temperature range, good ionic conductivity, excellent electrochemical properties, extraction and
catalytic ability [35-38]. Therefore, ILs have recently gained recognition of scholars from
various fields such as separation science [39], chemical synthesis [40], electrochemistry [41],
and catalysis [42, 43]. The good chemical stability, high electronic conductivity and wide
electrochemical windows made ILs more suitable for electrochemical research [44-46].
Electrons are green reactants and can be used in oxidation and reduction reactions. Also,
electrodes can act as catalysts and are easily separated from the reaction mixture [47]. Between
different electrochemical reactions, the electrochemical oxidation method is a powerful strategy,
and different types of electrochemical oxidation have been reported [48-50]. Herein, we would
like to report a green and efficient protocol for the anodic electro-mediated aerobic oxidation of
ketones to lactones and esters via constant current electro-catalysis in a three-electrode cell, in
the presence of ionic liquid [bmim][OTf] at room temperature (Scheme 1), aiming to supply a
new way for the synthesis of lactones and esters from ketones.
Scheme 1. Baeyer-Villiger electro-oxidation of ketones.
EXPERIMENTAL
Apparatus and reagents
All the chemicals were from commercial sources without any pretreatment. All reagents were of
analytical grade. The ionic liquids were synthesized according to the literature procedure [38].
1H NMR spectra and 13C NMR spectra were recorded on a Bruker 500-MHz spectrometer using
CDCl3 as the solvent with tetramethylsilane (TMS) as an internal standard. GC analyses were
performed on a Shimadzu-14B gas chromatography equipped with HP-1 capillary column (30
m, 0.25 mm, 0.25 mm). Melting points were recorded on a Buchi R-535 apparatus and are
uncorrected. Elemental analysis was performed on a Vario EL III instrument (Elmentar
Anlalysensy Teme GmbH, Germany). Constant-current coulometry was performed using a
potentiostat/galvanostat model 283 (PAR EG&G, USA). The working electrode used in the
oxidation experiments was an activated carbon fiber and platinum wire was used as the counter
electrode and a platinum wire as a reference electrode with a diameter of 0.5 mm.
General procedure for oxidation reactions
A mixture of [bmim][OTf] (20 mL) and ketone (0.1 mol) in a three-electrode cell fitted with
activated carbon fiber as the anode and Pt cathode was subjected to electro-catalytic oxidation at
a constant current at room temperature for an appropriate time. O2 gas was charged into the cell
through a O2 bomb to a desired amount at the flow rate of 15 mL min-1. A magnetic stirrer was
employed during the electro-oxidation. The progress of the reaction was monitored by GC.
After completion of the reaction, the organic phase was extracted with dichloromethane (3×20
mL). The solvent was removed and the residue was purified by preparative thin-layer
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Efficient Baeyer-Villiger electro-oxidation of ketones with molecular oxygen
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chromatography on silica gel (ethyl acetate: hexane, 1:10) to afford the desired pure product.
The rest of the ionic liquid was recovered. Fresh substrates were then recharged to the recovered
electro-catalytic system and then recycled under identical reaction conditions. The target
substrates were characterized by Elemental analysis, NMR spectra or compared with their
authentic samples. Spectroscopic data for selected products is as follows.
Methyl benzoate (Table 3, entry 1). Colourless oil, bp: 196–199 °C/760 mm (Ref. [54] 198–199
°C/760 mm).
Methyl 4-methylbenzoate (Table 3, entry 2). Colourless oil, bp: 221–223 °C/15 mm (Ref. [12]
222–223 °C/760 mm).
Methyl 2-methylbenzoate (Table 3, entry 3). Colourless oil, bp: 205–208°C/760 mm (Ref. [54]
207–208°C/760 mm).
Methyl 4-methoxybenzoate (Table 3, entry 4). White solid, mp: 46-48 °C (Ref. [10] 47–49 °C).
1H NMR: δ 3.78 (s, CH3, 3H), 3.86 (s, CH3, 3H), 6.82-6.87 (m, Ar-H, 2H), 7.07-7.15 (m, Ar-H,
2H). 13C NMR: δ 50.7, 55.4, 113.8, 123.1, 131.2, 163.5, 167.8. Anal. calcd. for C9H10O3: C,
65.02; H, 6.04; O, 28.86. Found: C, 65.05; H, 6.07; O, 28.88.
Methyl 4-ethylbenzoate (Table 3, entry 5). Colourless oil, bp: 106–109 °C/15 mm (Ref. [54]
105–107 °C/15 mm).
Methyl 4-(tert-butyl)benzoate (Table 3, entry 6). Colourless oil, bp: 134–136 °C/15 mm (Ref.
[12] 135–137 °C/15 mm). 1H NMR: δ 1.31 (s, CH3, 9H), 3.87 (s, CH3, 3H), 7.21-7.32 (m, Ar-H,
2H), 7.78-7.86 (m, Ar-H, 2H). 13C NMR: δ 30.8, 34.5, 51.1, 122.7, 126.4, 130.3, 157.8, 168.2.
Anal. calcd. for C12H16O2: C, 74.95; H, 8.36; O, 16.62. Found: C, 74.97; H, 8.39; O, 16.64.
Methyl 4-(trifluoromethyl)benzoate (Table 3, entry 7). Colourless oil, bp: 85–87 °C/15 mm (Ref.
[54] 86–87 °C/15 mm).
Methyl 4-nitrobenzoate (Table 3, entry 8). Light yellow solid, mp: 96-98 °C (Ref. [54] 95–
97°C). 1H NMR: δ 3.91 (s, CH3, 3H), 8.27-8.38 (m, Ar-H, 4H). 13C NMR: δ 51.7, 124.2, 130.9,
135.9, 158.7, 169.3. Anal. calcd. for C8H7NO4: C, 53.01; H, 3.84; N, 7.72; O, 35.31. Found: C,
53.04; H, 3.89; N, 7.73; O, 35.33.
Dimethyl terephthalate (Table 3, entry 9). White solid, mp: 140-142 °C (Ref. [54] 141–142 °C).
Methyl 4-bromobenzoate (Table 3, entry 10). Light yellow solid, mp: 79-82 °C (Ref. [54] 78–81
°C).
propyl 4-methylbenzoate (Table 3, entry 11). Colourless oil, bp: 112–114 °C/15 mm (Ref. [54]
112–113 °C/15 mm).
Pentyl 4-methylbenzoate (Table 3, entry 12). Colourless oil, bp: 284–287 °C/760 mm (Ref. [54]
1
285–286 °C/760 mm). H NMR: δ 0.93 (t, J = 7.6 Hz, CH3, 3H), 1.31-1.37 (m, CH2CH2, 4H),
1.84 (m, CH2, 2H), 2.37 (s, CH3, 3H), 4.25 (t, J = 7.6 Hz, CH2, 2H), 7.24-7.35 (m, Ar-H, 2H),
7.76-7.85 (m, Ar-H, 2H). 13C NMR: δ 14.5, 20.9, 22.1, 27.5, 28.4, 62.7, 127.5, 128.2, 130.1,
147.6, 165.5. Anal. calcd. for C13H18O2: C, 75.66; H, 8.77; O, 15.48. Found: C, 75.69; H, 8.80;
O, 15.51.
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Tetrahydro-2H-pyran-2-one (Table 3, entry 13). Colourless oil, bp: 229–232 °C/760 mm (Ref.
[54] 230–231 °C/760 mm).
Oxepan-2-one (Table 3, entry 14). Colourless oil, bp: 233–235 °C/760 mm (Ref. [54] 234–235
°C/760 mm).
6-Methyltetrahydro-2H-pyran-2-one (Table 3, entry 15). Colourless oil, bp: 111–113 °C/15 mm
1
(Ref. [54] 110–112 °C/15 mm). H NMR: δ 1.39 (d, J = 8.0 Hz, CH3, 3H), 1.73-1.89 (m,
CH2CH2, 4H), 2.36 (m, CH2, 2H), 4.32 (m, CH, 1H). 13C NMR: δ 19.4, 21.5, 29.8, 71.1, 170.3.
Anal. calcd. for C6H10O2: C, 63.13; H, 8.79; O, 28.02. Found: C, 63.14; H, 8.83; O, 28.03.
7-Methyloxepan-2-one (Table 3, entry 16). Colourless oil, bp: 226–229 °C/760 mm (Ref. [54]
1
227–228 °C/760 mm). H NMR: δ 1.38 (d, J = 7.8 Hz, CH3, 3H), 1.45-1.71 (m, CH2CH2CH2,
6H), 2.32 (m, CH2, 2H), 4.19 (m, CH, 1H). 13C NMR: δ 19.8, 23.6, 29.1, 34.7, 38.2, 71.3, 173.5.
Anal. calcd. for C7H12O2: C, 65.58; H, 9.41; O, 24.94. Found: C, 65.60; H, 9.44; O, 24.97.
Oxocan-2-one (Table 3, entry 17). Colourless oil, bp: 235–237 °C/760 mm (Ref. [54] 236–237
°C/760 mm).
1
2-Oxabicyclo[3.2.1]octan-3-one (Table 3, entry 18). Colourless oil. H NMR: δ 1.57-2.23 (m,
7H), 2.38-2.71 (m, CH2, 2H), 4.82 (m, CH, 1H). 13C NMR: δ 29.5, 31.9, 32.8, 35.3, 40.7, 79.8,
172.6. Anal. calcd. for C7H10O2: C, 66.62; H, 7.98; O, 25.34. Found: C, 66.65; H, 7.99; O,
25.37.
Propyl acetate (Table 3, entry 19). Colourless oil, bp: 100–102 °C/760 mm (Ref. [54] 101–102
°C/760 mm).
Isopropyl acetate (Table 3, entry 20). Colourless oil, bp: 88–90 °C/760 mm (Ref. [54] 88–89
°C/760 mm).
RESULTS AND DISCUSSION
Preliminary studies were performed using acetophenone as the model substrate on an activated
carbon fiber (ACF) electrode in the presence of ionic liquid [bmim][OTf] electrolyte to examine
the effects of various reaction parameters on the typical oxidation (Table 1). In the absence of
ionic liquid no product was detected (Table 1, entry 1). When the reaction was performed in the
presence of the [bmim][OTf] electrolyte (5 mL), the product could be obtained in a higher yield
46% (Table 1, entry 2) in a shorter time (60 min). The yield improved with increasing amount of
[bmim][OTf] (Table 1, entries 2-5), and reached maximum with 20 mL of [bmim][OTf] (Table
1, entry 5). However, further addition the amount of [bmim][OTf] (Table 1, entry 6), under the
same conditions, the yield was not enhanced significantly. These experiments revealed that ionic
liquid [bmim][OTf] electrolyte are crucial for the oxidation. On investigating the effect of
current amount on the reaction yield, constant currents of 20, 25, 30, 35, and 40 mA were
utilized. The best results were obtained in 30 mA current (Table 1, entry 5).
The effects of different electrolytes, such as [hmim][OTf], [dema][OTf], [bmim][OTf],
[bmim][PF6], and [bmim][BF4], were tested on an ACF electrode in the reaction (Table 2,
entries 1-5), and it was observed that [bmim][OTf] demonstrated the best performance (Table 2,
entry 3). The different catalytic effects of ILs may be attributed to their different abilities of
stabilizing and dissolving O2, the catalyst and the substrate. Under reaction conditions, O2 may
be more soluble in [bmim][OTf], leading to higher effective concentration of the oxidant, which
increases the encounter probabilities between the substrate and reactive species, and so the
higher rate and yield of the reaction were observed. Furthermore, the electro-catalytic system
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could be typically recovered and reused with no appreciable decrease in yields and reaction rates
(Table 2, entries 6 and 7). Besides ACF, other types of electrode materials were also tested in
this model reaction (Table 2, entries 8-10). The results showed that ACF demonstrated the best
performance.
Table 1. Screening of the reaction condition for the aerobic electro-catalytic oxidation of acetophenonea.
Entry
1
2
3
4
5
6
7
8
[bmim][OTf] (mL)
I (mA)
60
40
30
30
30
30
35
40
Time (min)
Yield (%)b
--
5
100
60
20
20
20
20
20
20
20
20
0
46
81
88
90
90
87
83
88
85
15
18
20
22
20
20
20
20
9
10
25
20
aThe reactions were carried out with acetophenone (0.1 mol), [bmim][OTf] (20 mL) equipped with ACF as anode
and Pt cathode with the bubbling of O2 gas (15 mL min-1) at room temperature. bIsolated yield.
Table 2. Optimization of the conditions for the aerobic electro-catalytic oxidation of acetophenonea.
Entry
1
2
3
4
5
6
7
8
Electrolyte
[hmim][OTf]
[dema][OTf]
[bmim][OTf]
[bmim][PF6]
[bmim][BF4]
[bmim][OTf]
[bmim][OTf]
[bmim][OTf]
[bmim][OTf]
[bmim][OTf]
Electrode material
ACF
Time (min)
Yield (%)b
20
20
20
20
20
20
20
20
30
20
78
86
90
ACF
ACF
ACF
ACF
ACF
ACF
82
80
90 c
89 d
78
Graphite rod
Glass carbon
Ti/TiO2
9
10
84
81
aThe reactions were carried out with acetophenone (0.1 mol), IL (20 mL) on a Pt cathode with the bubbling of O2
gas (15 mL min-1) at constant current of 30 mA. bIsolated yield. cThe first run. dThe second run.
The electro-catalytic oxidation of a wide variety of ketones with molecular oxygen was
successful and gave the desired products in good to excellent yields as summarized in Table 3.
Various types of aryl, aliphatic, and naphthenic ketones can be successfully electro-oxidated to
the corresponding esters and lactones with molecular oxygen (Table 3, entries 1-20), whereas
the aryl ketones were less reactive, higher reaction constant current and longer reaction time
were needed to reach good to excellent yields (Table 3, entries 1-12). Various functionalities
such as alkyl, alkoxy, bromo, trifluoromethyl, acetyl and nitro groups can tolerate the reaction.
It was also observed that the electronic nature of the substituents on the aromatic ring has some
impact on the reaction rate and the yield. Aryl ketones, especially those with electron-donating
substituents (Table 3, entries 2-6), were more reactive than those with electron-withdrawing
substituents (Table 3, entries 7-10), provided excellent yields under milder reaction conditions.
In addition, in order to examine a greater range of ketones to better illustrate the scope and
limitations of the method, we investigated the reactions with long carbon chain ketones such as
1-(p-tolyl)butan-1-one, 1-(p-tolyl)hexan-1-one, and pentan-2-one (Table 2, entries 11, 12 and
19), excellent yields of the expected esters were obtained.
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Table 3. Aerobic electro-catalytic oxidation of ketonesa.
Time
(min)
Yield
(%)b
Entry
1
Substrate
Product
I (mA)
30
20
20
20
90
95
94
O
2
3
30
30
4
5
30
30
20
20
97
98
6
30
20
96
7
8
35
35
30
30
87
88
9
35
30
87
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Efficient Baeyer-Villiger electro-oxidation of ketones with molecular oxygen
303
10
11
35
30
30
20
89
97
O
O
12
30
20
98
13
14
20
20
15
15
96
98
15
16
20
20
15
15
95
97
17
18
20
20
15
15
97
95
19
20
20
20
15
15
94
95
aUnless otherwise noted, the reactions were carried out with ketone (0.1 mol), [bmim][OTf] (20 mL) equipped
with ACF as anode and Pt cathode with the bubbling of O2 gas (15 mL min-1) at room temperature. bIsolated yield.
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According to the literatures [51-53] and the observations in our reactions, taking the electro-
catalytic oxidation of acetophenone with O2 as an example, a possible mechanism is proposed
(Scheme 2). In the reaction, the electro-catalytic system equipped with ACF as anode and Pt
cathode in [bmim][OTf] electrolyte can generate singlet oxygen (1) by visible constant current
of 30 mA electro-catalysis of O2 (triplet state). Then, 1 reacts with the acetophenone to form
acyl radical (2) and methylperoxyl radical (3), and the two radicals 2 and 3 are reacts with each
other to form peroxy acid ester (4). 4 subsequently undergoes reaction with another
acetophenone to form transition state (5), followed by Bayer-Villiger type rearrangement to
generate two molecules of the desired product (6). It looks like that the formations of 2 and 5 are
the rate-determining steps.
Scheme 2. Possible mechanism for the electro-oxidation of acetophenone.
CONCLUSIONS
In conclusion, a new and highly efficient electro-catalytic oxidation system equipped with ACF
as anode and Pt cathode in [bmim][OTf] electrolyte has been developed, which is capable of
converting ketones with molecular oxygen into the corresponding esters and lactones in good to
excellent isolated yield. Mild reaction conditions, simplicity of operation, high yields, stability,
easy isolation of products, and excellent recyclability of the catalytic system are the attractive
features of this methodology. The scope, definition of the mechanism and synthetic application
of this reaction are currently under study in our laboratory.
ACKNOWLEDGEMENTS
We thank the Natural Science Foundation of Jiangsu Province (no. BK20140460) for support of
this research.
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