Baeyer-villiger oxidation of cyclic ketones using aqueous hydrogen peroxide catalyzed by potassium salts of tungstophosphoric acid

Article abstract of DOI:10.1246/cl.140159

The salts of 12-tungstophosphoric acid catalysts are prepared with varying potassium content. The resulting catalysts are active for the Baeyer-Villiger oxidation of cyclic ketones with 30 wt% H₂O₂ and achieve good conversions and yields. KHPW is the preferred catalyst in the reaction because of its reusability and environmental benefits.

Full text of DOI:10.1246/cl.140159

Received: February 26, 2014 | Accepted: March 19, 2014 | Web Released: March 26, 2014
CL-140159
Baeyer­Villiger Oxidation of Cyclic Ketones Using Aqueous Hydrogen Peroxide
Catalyzed by Potassium Salts of Tungstophosphoric Acid
Qingguo Ma, Wanzhen Xing, Junhui Xu, and Xinhua Peng*
School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, P. R. China
(
E-mail: xhpeng@mail.njust.edu.cn)
¹
1
The salts of 12-tungstophosphoric acid catalysts are
prepared with varying potassium content. The resulting catalysts
are active for the Baeyer­Villiger oxidation of cyclic ketones
with 30 wt % H O and achieve good conversions and yields.
KHPW is the preferred catalyst in the reaction because of its
reusability and environmental benefits.
nitrogen flow of 5 mL min . Samples (3 mg) are heated from 50
¹
1
to 800 °C at a rate of 10 °C min . GC (Shimadzu GC-2014C) is
used to determine the product composition.
The salts of 12-tungstophosphoric acid catalysts are pre-
pared with varying potassium content. In a typical method, the
required amount of HPW is dissolved in distilled water and a
calculated amount of K2CO3 is added dropwise to this solution
2
2
¹
1
at a rate of 0.1 mL min
with vigorous stirring at room
The Baeyer­Villiger oxidation of ketones is a reaction of
major interest with regard to syntheses in organic chemistry with
a large range of possible applications spanning diverse areas
such as syntheses of antibiotics and steroids, pheromones for
temperature. HPW and K2CO3 are used after pretreatment at
423 K in vacuum. The resultant mixture is stirred for 3 h, and a
white solid is precipitated and separated by filtration from the
resultant mixture. Prior to use, these salts are pretreated at 423 K
in vacuum for 2 h. These salts are denoted as KPW (the quantity
of K in KPW is 3.9 wt %) and KHPW (the quantity of K in
KHPW is 1.3 wt %).
The typical procedure for the oxidation of 2-heptylcyclo-
pentanone is as follows. KHPW (25 mg) and 2-heptylcyclopen-
tanone (2.5 mmol, 450 mg) are mixed and the oxidizing agent,
aqueous hydrogen peroxide (30 wt %; 6.2 mmol), is slowly
added dropwise to this mixture with stirring at 40 °C. The
resultant mixture is stirred for 12 h. The catalyst is separated by
filtration. The aqueous phase is removed from the filtration.
Conversion of 2-heptylcyclopentanone and selectivity of δ-
dodecalactone are determined by quantitative GC analyses of the
organic phase. The organic phase is evaporated under reduced
pressure and further purified by column chromatography over
silica gel using 1:5 ethyl acetate­hexane as the eluent to furnish
pure δ-dodecalactone.
1
agrochemistry, and monomers for polymerization. However, the
traditional Baeyer­Villiger reaction is carried out with various
peroxyacids; the performance of this process on an industrial
scale is associated with some technological problems caused by
2
the necessity of synthesis and isolation of peroxyacids.
3
Two alternative oxidants, molecular oxygen and aqueous
hydrogen peroxide, have been used in the Baeyer­Villiger
4
reaction to solve some environmental problems. When molecu-
lar oxygen and aqueous hydrogen peroxide are used as oxidizing
agents, catalysts are required for the active transfer of oxygen
from the oxidizer to the substrate. Low-concentration aqueous
hydrogen peroxide is a promising oxidant for the Baeyer­
Villiger reaction, because hydrogen peroxide is converted into
environmentally benign water.5 An efficient catalytic system
comprising Sn­zeolites was used by Corma et al. for the
Baeyer­Villiger reaction of cycloketones with aqueous hydro-
6
gen peroxide. Recently, much effort has been devoted to finding
The FT-IR spectra of HPW and KHPW are displayed in
Figure 1. The spectrum of HPW contains vibration bands
characteristic of the Keggin anion: 1080, 990, 890, and
other effective catalysts to activate the reagent. Reported
examples include Sn­clay, Sn-MWW,8 β-zeolite,9 pillared
7
clays, MCM-41, hydrotalcite,12 Sn(salen)­NaY zeolite,
1
0
11
13
810 cm , corresponding to vibrations P­Oa, W­Od, W­Ob­W,
¹1
1
4
and TS-1. When these catalysts are used, high temperature is
required to activate the reagent. While many studies on finding
effective catalysts to activate the reagent have been reported
and W­Oc­W, respectively (Oa: oxygen in the central PO4
tetrahedron, O : terminal oxygen bonding to W atom, O :
d
b
8
,9,15
until now,
the study on the oxidation of cycloketones using
low concentration of aqueous hydrogen peroxide and solid acids
is lacking. 12-Tungstophosphoric acid (HPW) is an inorganic
solid acid. Its acid strength is higher than that of conventional
solid acids; however, it is not efficient because of its high
solubility in polar media. Generally, the incorporation of
alkaline metals into 12-tungstophosphoric acid can enhance its
water tolerance and simultaneously adjust acidity.17 In view of
this, we attempt to synthesize lactones by using the salts of 12-
tungstophosphoric acid.
KHPW
HPW
1
6
All chemicals are analytical grade without further purifica-
tion. The structures of tungstophosphoric acid and its potassium
salts are characterized by IR spectroscopy (Nicolet FTIR IS10)
and X-ray diffractometer (Rigaku MiniFlex 600). Thermogravi-
metric analysis of tungstophosphoric acid and its potassium
salts are performed using a TGA/SDTA851e instrument under a
0
500
1000
1500
2000
2500
3000
3500
4000
4500
-1
Wavenumber/cm
Figure 1. FT-IR spectra of HPW and KHPW.
Chem. Lett. 2014, 43, 941–943 | doi:10.1246/cl.140159
© 2014 The Chemical Society of Japan | 941

3
2
.0
.9
KHPW
KHPW
2.8
2
2
.7
.6
HPW
70
HPW
0
10
20
30
40
50
60
80
90
2
θ
/degree
0
100
200
300
400
500
600
700
800
900
T / °C
Figure 2. XRD patterns of HPW and KHPW.
Figure 3. TGA curves for HPW and KHPW.
edge-sharing oxygen connecting W, and Oc: corner-sharing
oxygen connecting W O units).18 These bands also appear in
3
13
the spectrum of KHPW and their positions are not changed
relative to those in the spectrum of HPW. This indicates that the
structures of the Keggin anions of HPW are preserved when
protons are replaced by potassium ions.
The XRD patterns of HPW and KHPW are shown in
Figure 2. The XRD pattern of KHPW is similar to that of HPW
and only slightly shifted in comparison to that of HPW toward
higher angles. Thus, we conclude that KHPW is formed.
Thermal stability is measured mainly by TGA. Figure 3
shows the TGA curves for HPW and KHPW. For HPW, three
weight loss regions can be observed at 58­100, 100­250, and
Table 1. Catalytic performance of various catalysts for the
oxidation of 2-heptylcyclopentanone with H2O2
a
Catalyst
Time/h
Conversion/%b
Yield/%c
®
HPW
H-ZSM-5
HY zeolite
KPW
KHPW
KHPW
KHPW
KHPW
KHPWf
KHPW
KHPW
KHPW
KHPW
24
10
24
24
24
24
18
12
18
8
trace
97
11
trace
67
8
9
7
trace
74
98
98
98
98
98
95
92
11
trace
49
72
81
68
70
81
76
73
7
d
e
330­415 °C on the TGA curve. Similarly, for KHPW, a small
e
weight loss region is found from 65 to 120 °C, and a larger one
is found from 330 to 360 °C. The acid form of HPW is usually
obtained with small amounts of physically adsorbed water and
large amounts of crystal water, and most of these water
molecules are released below 250 °C. The weight loss at 330­
e,g
12
12
12
12
e,h
e,i
e,j
415 °C is probably due to the decomposition of HPW. KHPW is
only obtained with small amounts of physically adsorbed water;
the weight loss due to decomposition is decreased because of
the replacement of protons.
aReaction conditions: 2-heptylcyclopentanone (2.5 mmol),
catalysts (25 mg), experiments performed at 40 °C, aqueous
b
hydrogen peroxide (30 wt %) (6.25 mmol). Calculated by GC.
A series of catalysts are studied using H2O2 as an oxidant
to produce δ-dodecalactone under the same reaction conditions.
The results of this study are summarized in Table 1. In the
absence of a catalyst, 2-heptylcyclopentanone is difficult to react
with H O to produce δ-dodecalactone. In the homogeneous
c
d
e
f
g
h
Isolated yield. 50 mg. 100 mg. 150 mg. 1st recycled. 2nd
i
j
recycled. 3rd recycled. KHPW is filtered out after 1 h.
hydroxydodecanoic acid, which is formed by the hydrolysis of
δ-dodecalactone with water present in the reaction media. The
amount of KHPW affects the oxidation rate of 2-heptylcyclo-
pentanone and the selectivity of δ-dodecalactone; it seems that
very large amounts of KHPW can also accelerate the hydrolysis
and oxidation of δ-dodecalactone. The appropriate amount of
KHPW is 100 mg. The catalyst is repeatedly isolated by filtration,
washed with ethyl acetate for 1 h to remove organic compounds,
and used again in the reaction without further treatment. The
results suggest that the KHPW catalyst can be recycled.
Table 2 summarizes the reaction of simple cyclic ketones in
the presence of KHPW. The yields of lactones depend on the
sizes of the rings. In the case of 2-adamantanone and cyclo-
dodecanone, the corresponding lactones are obtained in good
yields. Despite high conversions achieved for cyclopentanone,
cyclohexanone, and cycloheptanone, the yields of corresponding
lactones are low; this is tentatively ascribed to the hydrolysis of
lactones.
2
2
case, the HPW catalyst results in quite high conversion of
-heptylcyclopentanone because of its strong acidity and high
2
solubility in the liquid phase. Leaching of HPW is a common
problem in reactions. The use of several other insoluble solid
acids (H-ZSM-5, HY zeolite, KPW, and KHPW) instead of
HPW slows the reaction considerably under heterogeneous
conditions; especially, when using KPW as a catalyst, the
conversion of 2-heptylcyclopentanone is rather poor. It proves
that the catalytic activity is close to that of protons. Increasing
the amount of KHPW under this condition causes a significant
increase of the conversion of 2-heptylcyclopentanone and yield
of δ-dodecalactone. A further increase of the conversion rate of
2-heptylcyclopentanone is observed by increasing the amount of
KHPW, but an excess amount has a strong negative impact on
selectivity. It can be seen that δ-dodecalactone is an unstable
product, where 5-oxododecanoic acid is present as a by-product.
Indeed, 5-oxododecanoic acid is formed by the oxidation of 5-
942 | Chem. Lett. 2014, 43, 941–943 | doi:10.1246/cl.140159
© 2014 The Chemical Society of Japan

Table 2. Oxidation of cyclic ketones to give corresponding
lactones
Substrate Temperature/°C Time/h Conversion/%b Yield/%b
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aReaction conditions: Substrate (2.5 mmol), catalyst/sub-
4
strate = 20 wt %, aqueous hydrogen peroxide (30 wt %)
b
(
6.25 mmol), dioxane (3.5 mL). Calculated by GC.
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Although the yields of lactones obtained by the reactions
catalyzed by KHPW are different, this is a useful method for the
Baeyer­Villiger oxidation of cyclic ketones.
This project was funded by the Priority Academic Program
Development of Jiangsu Higher Education Institutions (PAPD).
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Chem. Lett. 2014, 43, 941–943 | doi:10.1246/cl.140159
© 2014 The Chemical Society of Japan | 943