460
X.-W. Wu, B.-D. Li / Chinese Chemical Letters 25 (2014) 459–462
2.1. Preparation of catalysts
R
H2O2
R
O
HCOOH
R
HCOOOH
Cat. I
The catalyst [(C18H37)2N(CH3)2]3[PW4O16] (Cat. I) was prepared
according to the literature [17]. A suspension of tungstic acid (5.0 g,
20 mmol) in 16.5 mL of 30% aqueous H2O2 was heated at 60 8C for
15 min. H3PO4 (40%, 1.24 mL, 5.0 mmol) was added to the obtained
colorless solution at room temperature and the whole solution was
diluted with another 40 mL of water and then stirred for 30 min. To
the resulted solution, 6.0 g of dioctadecyl dimethyl ammonium
chloride (10 mmol) in dichloromethane (80 mL) was added
dropwise with stirring. After the solution was stirred continuously
for 30 min, the organic phase was separated, dried over anhydrous
Na2SO4, filtered and then the solvent was removed at 60 8C under
atmospheric pressure. A yellow dry powder was obtained by
further evaporating under reduced pressure.
OH
OH-
O
O
OH
R
O
OH
O
H
OH
O
H
HCOOH
R
OH-
+
OH
R
O
OH
R
R
Scheme 1. The process of phase-transfer catalysis for preparation of 1,2-diol.
840
The catalysts with different quaternary ammonium cations
were similarly prepared using cetylpyridinium chloride
(p-
A
C5H5NC16H33Cl), hexadecyl trimethyl ammonium chloride
(C16H33N(CH3)3Cl) and tetrabutylammonium hydrogen sulfate
(Bu4NHSO4), denoted as Cat. II, Cat. III and Cat. IV, respectively.
B
C
D
2.2. Characterization techniques
The laser Raman experiments were performed using a Jobin
Yvon ARAMIS, Raman spectrometer equipped with a CCD (Te
RAMAN-1024 Â 256-OPEN-SYN) detector and the spectrum reso-
lution of 1 cmÀ1 using a laser at 532 nm as the excitation source.
The infrared spectra were recorded on a Nicolet IS10 FT-IR
spectrometer.
2.3. Catalytic oxidation of terminal alkenes to 1,2-diol
600
700
800
900
1000
1100
1200
Wavenumbers/cm-1
The catalytic reactions were performed in a 50 mL four-necked
round-bottomed flask equipped with a reflux condenser. The
assembly was kept in an isothermal water bath at a constant
known temperature and mechanically agitated with an electric
motor. The oxidation was carried out as follows: catalyst
(0.12 mmol), terminal alkenes (30 mmol), H2O2 (30% aq., 40 mmol)
and FA (60 mmol) were charged in the reaction flask. The catalyst/
olefin ratio was 1:250 for all the reactions. When the oxidation
reaction was over, the catalyst was precipitated from the reaction
medium for further use when acetone was subsequently added to
the solution, the precipitate was removed by centrifugation and
filtration, and the filtrate extracted with CH2Cl2 (50 mL Â 3); the
organic layer was collected and dried under anhydrous Na2SO4.
After the evaporation of the solvent under reduced pressure, a
solution of sodium hydroxide was added dropwise to the organic
product to give high purity 1,2-diols. The organic product was
analyzed by GC9600 using an internal standard method. Typical
GC-MS analysis was also conducted to identify the products.
Fig. 1. IR spectra of the catalysts with different cations. (A) [(C18H37)2N(CH3)2]+; (B)
[Bu4N]+; (C) [C16H33N(CH3)3]+; (D) [ -C5H5NC16H33]+.
p
For catalyst Cat. I, its IR spectrum (Fig. 1A) gives a strong peak at
840 cmÀ1, which indicates that the catalyst still contain active
oxygen. For catalyst Cat. II, its IR spectrum (Fig. 1D) indicates it is a
7À
mixture of [PW11O39
]
and [PW12O40
]
3À, as demonstrated by the
(W55O) 967 cm
À1 n(W–
following resonances:
n
(PO4) 1077 cmÀ1
;
n
;
Ob–W) 880 cmÀ1. The IR spectra of fresh catalysts showed almost no
active oxygen (Fig. 1B and C). So the nature of quaternary
ammonium cation affects the composition of the catalyst, and
bigger cation can more efficiently stabilize active oxygen.
The oxidation of 1-octene catalyzed by a series of catalysts was
conducted in the H2O2/formic acid system, and the results are
summarized in Table 1. Cat. I showed the highest activity (entry 1)
and a 91.2% yield of 1,2-octanediol was obtained. The activity of
Cat. II was similar to that of Cat. I and a 80.4% yield of 1,2-
octanediol was obtained. The activity of Cat. III and Cat. IV was not
as good as that of Cat. I and the yields of 1,2-octanediol were 73.1%
3. Results and discussion
3.1. Catalysts with different cations
Table 1
Oxidation of 1-octene catalyzed by various catalyst.a
The oxidation of terminal olefins is the more difficult but
important step in the alkene/diol transformation. In our catalytic
system, the oxidation of terminal olefins generates some by-
products that can be transformed to the corresponding 1,2-diol in
high yield (Scheme 1).
Entry
Catalyst
Conversionb (mol%)
Yieldc (mol%)
1
2
3
4
5
Cat. I
Cat. II
Cat. III
Cat. IV
/
98
88
81
67
58
91.2
80.4
73.1
60.6
51.9
Chen et al. [14] have reported that cations played a significant
role in phase-transfer catalytic oxidation. It was found that the
choice of proper cations is crucial for the precipitation of
heteropoly species from the reaction media. Three more catalysts
a
Reaction conditions: olefin (30 mmol); H2O2 (40 mmol); catalyst (0.12 mmol);
FA (60 mmol).
b
Conversion was determined by gas chromatography using an internal standard
with different cations, [
(Bu4N)+, were synthesized.
p
-C5H5NC16H33]+, [C16H33N(CH3)3]+ and
technique and was based on 1-octene.
c
Yield was also based on olefin, yield (%) = 1,2-diol (mol)/1-octene (mol) Â 100%.