Fast selective oxidation of alcohols under solvent-free conditions

Article abstract of DOI:10.1007/s11164-012-0846-9

A combination of acetic anhydride, H₂SO₄-nano silica, wet-SiO₂ (60 %), and K₂Cr₂O₇ as a new oxidizing system for the selective oxidation of different types of alcohols to the corresponding aldehydes and ketones at room temperature under solvent-free conditions is introduced. Mild reaction conditions, high yields of the products, short reaction time, no further oxidation to the corresponding carboxylic acid, and easy work-up make this new system a useful method for oxidizing alcohols.

Full text of DOI:10.1007/s11164-012-0846-9

Res Chem Intermed
DOI 10.1007/s11164-012-0846-9
Fast selective oxidation of alcohols under solvent-free
conditions
Ali Saffar-Teluri
Received: 22 August 2012 / Accepted: 1 October 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract A combination of acetic anhydride, H SO -nano silica, wet-SiO
2
2
4
(
60 %), and K Cr O as a new oxidizing system for the selective oxidation of
2 2 7
different types of alcohols to the corresponding aldehydes and ketones at room
temperature under solvent-free conditions is introduced. Mild reaction conditions,
high yields of the products, short reaction time, no further oxidation to the corre-
sponding carboxylic acid, and easy work-up make this new system a useful method
for oxidizing alcohols.
Keywords Selective oxidation Á Alcohols Á Acetic anhydride Á Solvent-free
Introduction
The oxidation of the hydroxyl group to the corresponding carbonyl compounds is
one of the most important reactions in synthetic organic chemistry, which have long
been the objective of many research papers [1–6]. Chromium(VI) reagents such as
Collins reagent [7], Jones reagent [8], chromic anhydride chlorotrimethylsilane [9],
caffeinilium chlorochromate [10], guanidinium chlorochromate [11], cetyltrimeth-
ylammonium dichromate [12], N-methyl morpholine chlorochromate [13], benzyl-
dimethyltelluronium dichromate [14], isoquinolinium bromochromate [15],
pyridinium dichromate [16], pyridinium chlorochromate [17], Collins reagent-
acetic anhydride [18], chromium(VI) oxide-pyridine-acetic anhydride [19], and
pyridinium dichromate-acetic anhydride [20] are widely used for this transforma-
tion. Unfortunately, most of these methods suffer from one or more disadvantages,
such as a long reaction time, formation of a hemiacetal, overoxidation, use of toxic
A. Saffar-Teluri (&)
Faculty of Material Science & Engineering, Najafabad Branch, Islamic Azad University,
P.O. Box 85141-43131, Najafabad, Iran
e-mail: a.saffar.t@iaun.ac.ir
123

A. Saffar-Teluri
solvent, strongly acidic conditions, high temperatures, stoichiometric amounts of
oxidant, tedious work-up, detrimental side reactions, and unsatisfactory yields.
In spite of a large number of methods reported for the oxidation reaction, there is
still a need to develop a more efficient, simple, milder, and high-yielding product
using laboratorial and industrial oxidants. Herein, we wish to report, for the first
time, a combination of potassium dichromate (K Cr O ), sulfuric acid immobilized
2
2 7
on nano silica, and acetic anhydride as an affective oxidizing system for the
oxidation of alcohols to their corresponding aldehydes or ketones in solvent-free
conditions at room temperature (Scheme 1).
Experimental
Commercial reagents from Fluka, Merck, and Aldrich chemical companies were
used without further purification. The composition of the synthesized powder was
obtained by Fourier transform infrared (FT-IR) spectroscopy (IR Prestige-21,
-
1
Shimadzu) in the wavelength range of 400–4,000 cm . Information about the
phase was obtained by an X-ray powder diffractometer (Philips, CuKa,
˚
k = 1.54178 A) in the Bragg angle ranging between 10° and 60°.
Preparation of H SO -nano silica
2
4
H SO -nano silica was synthesized according to the previously reported procedure
2
4
[
available concentrated H SO (3 ml) with shaking for 5 min. The solvent was
21]. To a slurry of silica gel (10 g) in CCl (50 ml) was added commercially
4
2
4
evaporated and the produced H SO -nano silica was then dried at 110 °C for 3 h.
2
4
?
The amount of H in the H SO -nano silica was determined by acid–base titration
2
4
(
by standard NaOH) and it was 0.11 mmol (0.1 g H SO -nano silica, equal to
2 4
?
0
.11 mmol of H ) [22].
The prepared catalyst was characterized with FT-IR and X-ray diffraction
XRD). The XRD spectrum of H SO -nano silica is shown in Fig. 1. This pattern
(
2
4
exhibits a broad peak (2h = 15–30°), attributed to amorphous nano silica.
In the FT-IR spectrum, the band at about 800 cm belongs to the Si–OH bond
-
1
-
1
stretching vibration (Fig. 2). The band in the region 1,040–1,230 cm is attributed
to the stretching vibrations of the Si–O–Si and S=O bonds. A peak appeared at
-
about 3,400 cm , due to the stretching of OH groups in the SO H.
1
3
OH
O
Acetic anhydride/K Cr O /H SO -Silica
2
2
7
2
4
R1
R
2
Wet-SiO (60%), Solvent-free, r. t.
R1
R2
2
1
R = aryl, alkyl
2
R = H, alkyl
Scheme 1 Selective oxidation of alcohols
1
23

Fast selective oxidation of alcohols
1
0
20
30
40
50
60
2
Theta [deg]
2 4
Fig. 1 X-ray diffraction (XRD) pattern of H SO -nano silica
4000 3600 3200 2800 2400 2000 1600 1200
800
400
-1
Wavenumber (cm )
Fig. 2 Fourier transform infrared (FT-IR) spectrum of H SO -nano silica
2
4
General procedure for the selective oxidation of alcohols
A typical reaction was carried out as follows: 1 mmol of alcohol was added to a
suspension of 1.5 mmol of acetic anhydride, 0.2 mmol of K Cr O , 0.3 g of H SO -
nano silica, and an appropriate amount of wet-SiO (60 %). The mixture was stirred
2
2
7
2
4
2
under solvent-free conditions at room temperature for various lengths of time
(
Table 1). The progress of the reaction was monitored by thin layer chromatography
(
reaction, 20 ml CH Cl was added to the heterogeneous mixture, and it was then
TLC) (eluent; diethyl ether: CCl , various ratios). After the completion of the
4
2
2
filtered. The organic layer was washed with water, dried over MgSO , and the
4
solvent was removed. The residue was chromatographed on a silica gel column. All
of the products are known compounds and were identified by comparison of their
physical data with those of authentic samples.
Results and discussion
The oxidation of 3-nitrobenzyl alcohol (m-NO –C H –CH OH) was first investi-
2
6
4
2
gated as a model reaction. The oxidation provided 3-nitrobenzaldehyde in excellent
123

A. Saffar-Teluri
2 2 7
Table 1 Selective oxidation of different types of alcohols in the presence of acetic anhydride/K Cr O /
H
2
SO
4
-nano silica/wet-SiO
2
(60 %) under solvent-free conditions at room temperature
a
b
Entry
Substrate
Product
Time (min)
Yield (%)
1
2
3
4
5
6
7
m-NO
m-NO
m-NO
m-NO
m-NO
m-NO
m-NO
2
2
2
2
2
2
2
–C
–C
–C
–C
–C
–C
–C
6
6
6
6
6
6
6
H
H
H
H
H
H
H
4
4
4
4
4
4
4
–CH
2
OH
OH
OH
OH
OH
OH
OH
m-NO
2
2
–C
–C
6
H
H
4
4
–CHO
–CH OCOCH
3
120
3
96
88
–
–CH
–CH
–CH
–CH
–CH
–CH
2
2
2
2
2
2
m-NO
6
2
3
No detection of aldehyde
m-NO
m-NO
m-NO
2
2
2
–C
–C
–C
6
H
6
H
6
H
4
4
4
–CHO
–CHO
–CHO
60
30
30
30
92
91
90
50[
Mixture of m-NO
and m-NO –C
o-NO –C –CHO
2
–C
6
H
4
–CHO
OCOCH
2
6
H
4
–CH
2
3
8
9
o-NO
2
–C
6
H
4
–CH OH
2
2
6
H
4
3
91
95
94
89
96
95
88
94
95
93
91
93
92
94
o-Br–C
6
H
H
4
4
–CH
2
2
OH
OH
o-Br–C
6
6
H
4
4
–CHO
–CHO
2.5
3
10
11
12
13
14
15
16
17
18
19
20
21
a
p-Br–C
6
–CH
p-Br–C
H
o-H
p,o-Cl
o-F–C
p-H C–C
PhCH CH
PhCH(OH)CH
PhCH CH CH
n-CH (CH CH
(CH CH(OH)CH
CH CH OH
CH(OH)CH
3
CO–C
–C
–CH
–CH
OH
6
H
4
–CH
–CH OH
OH
OH
2
OH
o-H
p,o-Cl
o-F–C
p-H C–C
PhCH CHO
PhCOCH CH
PhCH CH
n-CH (CH
(CH
3
CO–C
–C
–CHO
–CHO
6
H
4
–CHO
5
2
6
H
3
2
2
6
H
3
–CHO
2
6
H
4
2
6
H
4
2
3
6
H
4
2
3
6
H
4
5
2
2
2
2.5
2
2
CH
OH
OH
3
2
3
2
2
2
2
2
CHO
3
3
2
)
6
2
3
2
)
6
CHO
COCH
CH CHO
COCH
5
CH
CH
CH
3
3
3
2
)
5
3
CH
CH
CH
3
3
3
2
)
5
3
3
CH
2
2
2
2
CH
2
2
2
4
CH
3
CH
3
3
Reaction carried out in a molar ratio of substrate:acetic anhydride:K
free conditions at room temperature
2
2 7
Cr O of 1:1.5:0.2 under solvent-
b
All of the products are known compounds and were identified by comparison of their physical data with
those of authentic samples
yield when the reaction was allowed to stir in the presence of acetic anhydride,
K Cr O , H SO -nano silica, and wet-SiO (60 %) under solvent-free conditions at
2
2
7
2
4
2
room temperature. No overoxidation to 3-nitrobenzoic acid was observed.
In order to improve the yields, we performed reactions using different quantities
of reagents. The best result was obtained using a 1:1.5:0.2 mol ratio of alcohol,
acetic anhydride, and K Cr O at 3 min (Table 1).
2
2 7
To illustrate the need for a catalyst, oxidant, and acetic anhydride for this
reaction, a series of experiments was carried out. As the first try, in the absence of
K Cr O , the treatment of 3-nitrobenzyl alcohol with acetic anhydride and H SO -
2
2
7
2
4
nano silica yielded the corresponding 3-nitrobenzyl acetate, which the obtained
product did not produce in the main oxidation reaction (Table 1, entry 2). Next,
3
3
-nitrobenzyl acetate was reacted with K Cr O to give the expected product,
2 2 7
-nitrobenzaldehyde, but the reaction did not occur. This result showed that
K Cr O as an oxidant is necessary for the oxidation of alcohols and, in this process,
2 2 7
1
23

Fast selective oxidation of alcohols
an ester as an intermediate was not formed. Furthermore, it is noted that, when the
reaction of 3-nitrobenzyl alcohol with K Cr O and H SO -nano silica in the
2
2
7
2
4
absence of acetic anhydride was conducted under the same conditions and at 3 min,
the expected product was not detected. With continuing the reaction for 60 min,
3
interesting that, with adding wet-SiO (60 %) to the above reaction mixture, the
-nitrobenzaldehyde was produced in excellent yield (Table 1, entries 3 and 4). It is
2
reaction time decreased to 30 min (Table 1, entry 5). Finally, we examined the
effect of different solvents in this reaction. When the reaction was carried out in
CH Cl , good yield was observed at 30 min (Table 1, entry 6). The reaction in
2
2
acetic acid gave different products, such as 3-nitrobenzyl acetate and 3-nitrobenz-
aldehyde in moderate yields at 30 min (Table 1, entry 7). However, the reaction
under solvent-free conditions gave excellent yields at 3 min.
Due to the excellent results obtained, we applied the optimal protocol to various
aromatic benzyl alcohols carrying either electron-donating or electron-withdrawing
substituents in different positions (ortho, meta, and para). It was observed that the
presence of –OCH and –CH on the phenyl ring increase the time of oxidation
3
3
(
Table 1, entries 11 and 14), whereas –NO , –Cl, –Br, and –F groups decrease the
2
time of oxidation under the same conditions (Table 1, entries 1, 8–10, 12, and 13).
The lower times of electron-withdrawing groups substituted benzyl alcohols in
comparison to electron-donating derivatives in this study can be related to the
electronic effects of substituents. Another advantage of this procedure is its
efficiency for the high yield synthesis of aliphatic aldehydes and ketones from
aliphatic alcohols at short times, which normally show very poor yields in
traditional oxidation (Table 1, entries 18–21).
Based on previous works [14–16] and our experimental results, a plausible
mechanism for the selective oxidation of alcohols is proposed in Scheme 2.
O
Cr
O
O
Cr
O
2
-
O
+
Cr
2
O
7
O
O
O
+ OH
O
+
H+
4
O
O
O
O
O
-
2 CH COOH
3
5
3
2
+
OH
OH
O
Cr
O
O
O
O
Cr
.
2
.
Cr
2
H O
O
O
+
O
O
O
O
+
O
7
6
.
.
OH
..
OH
OH
O
RCH
2
H
Cr
O
1
Cr
HCrO3
RCHO
O
O
R
O
O
O
7
8
Scheme 2 A plausible mechanism for the selective oxidation of alcohols
123

A. Saffar-Teluri
Table 2 Comparison of the results obtained in this work with other groups
Entry Oxidant
Substrate to oxidant (molar Time
ratio)
Yield
(%)
Reference
1
2
3
4
5
Quinaldinium fluorochromate 1:1.5
2–4 h
74–94
62–77
70–95
[11]
[11]
[22]
[24]
[13]
Quinaldinium dichromate
SSA/sodium dichromate
1:1.5
1:0.35
1:3
3–4 h
30 min
Na
Cr
2 2
O
7
20–30 min 83–97
45–55 min 90–96
N-methyl morpholine
chlorochromate
1:1.5
6
Cetyltrimethylammonium
dichromate
1:17
0.5–6 h
80–100 [12]
7
9
Quinoxalinium dichromate
1:1.5
1:0.2
2 h
71–92
88–96
[23]
K
2
Cr
2
O
7
/acetic anhydride
2–5 min
This work
In the presence of H SO -nano silica as a catalyst, first, the acetic anhydride (2) is
2
4
activated to form a protonated intermediate (3). Then, the O-nucleophilic
dichromate (4) attacks the carbonyl of an intermediate (3) to produce acetic acid
and an intermediate (5), which, in turn, affords an intermediate (7) through
protonation by H SO -nano silica and then Cr–O bond cleavage with the
2
4
nucleophilic attack of H O, respectively. Subsequently, O-nucleophilic attack of
2
alcohol on the Cr–OCOCH bond of an intermediate (7) produces an intermediate
3
(8), which follows by proton transfer and Cr–O bond cleavage to yield the
corresponding aldehyde and HCrO3.
In order to show the excellent activity of the introduced oxidizing system, we
compared our results (time, yield, and molar ratio of substrate to oxidant) with the
results obtained by other reagents [11–13, 23–25] (Table 2). The comparison of
results indicated that our oxidizing system oxidized the alcohols in a lower molar
ratio of substrate to oxidant, shorter reaction time, and solvent-free conditions than
the other reagents, especially in the case of aliphatic alcohols and benzyl alcohols
with the electron-withdrawing substituents.
Conclusion
In this study, we have demonstrated, for the first time, the efficiency of the
combination of acetic anhydride, H SO -nano silica, wet-SiO (60 %), and K Cr O
2 7
2
4
2
2
as an oxidizing system for the selective oxidation of different types of alcohols to
the corresponding aldehydes and ketones. Mild reaction conditions, high yields of
the products, short reaction time, no further oxidation to the corresponding
carboxylic acid, and easy work-up make this new system a useful method for
oxidizing alcohols.
Acknowledgments We gratefully acknowledge the financial support from the Research Council of
Islamic Azad University, Najafabad Branch.
1
23

Fast selective oxidation of alcohols
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