Lanthanum triflate-catalyzed rapid oxidation of secondary alcohols using hydrogen peroxide urea adduct (UHP) in ionic liquid

Article abstract of DOI:10.1080/00397911.2013.831904

A convenient and efficient protocol for the oxidation of secondary hydroxyl group to ketone using hydrogen peroxide-urea adduct and catalytic (CF ₃SO₃)₃La in ionic liquid has been developed. A number of 1,2-diols, -hydroxyketones, and other aromatic and aliphatic secondary alcohols have been successfully oxidized to the corresponding ketones using this protocol in good yields and short reaction times. [Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications for the following free supplemental resource(s): Full experimental and spectral details.]

Full text of DOI:10.1080/00397911.2013.831904

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Lanthanum Triflate–Catalyzed Rapid
Oxidation of Secondary Alcohols Using
Hydrogen Peroxide Urea Adduct (UHP) in
Ionic Liquid
Pooja Saluja a , Devanshi Magoo a & Jitender M. Khurana a
a Department of Chemistry , University of Delhi , Delhi , India
Published online: 20 Feb 2014.
To cite this article: Pooja Saluja , Devanshi Magoo & Jitender M. Khurana (2014) Lanthanum
Triflate–Catalyzed Rapid Oxidation of Secondary Alcohols Using Hydrogen Peroxide Urea Adduct (UHP)
in Ionic Liquid, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 44:6, 800-806, DOI: 10.1080/00397911.2013.831904
To link to this article: http://dx.doi.org/10.1080/00397911.2013.831904
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Synthetic Communications1, 44: 800–806, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2013.831904
LANTHANUM TRIFLATE–CATALYZED RAPID
OXIDATION OF SECONDARY ALCOHOLS USING
HYDROGEN PEROXIDE UREA ADDUCT (UHP) IN IONIC
LIQUID
Pooja Saluja, Devanshi Magoo, and Jitender M. Khurana
Department of Chemistry, University of Delhi, Delhi, India
GRAPHICAL ABSTRACT
Abstract A convenient and efficient protocol for the oxidation of secondary hydroxyl group
to ketone using hydrogen peroxide–urea adduct and catalytic (CF3SO3)3La in ionic liquid
has been developed. A number of 1,2-diols, a-hydroxyketones, and other aromatic and
aliphatic secondary alcohols have been successfully oxidized to the corresponding ketones
using this protocol in good yields and short reaction times.
[Supplementary materials are available for this article. Go to the publisher’s online
edition of Synthetic Communications1 for the following free supplemental resource(s):
Full experimental and spectral details.]
Keywords Ionic liquid; ketones; lanthanum triflate; oxidation; secondary alcohols; UHP
INTRODUCTION
Oxidation of alcohols to carbonyl compounds is a fundamental reaction in
organic chemistry and several methods have been reported in the literature covering
a wide variety of reagents.[1] Recently Fe(NO3)2 ꢀ 9H2O,[2] NBS=2,6-lutidine,[3] 1,4-
diazabicyclo[2.2.2]octane (DABCO) = Br2,[4] N-bromosuccinimide (NBS)-CCl4-Py,[5]
NBS-Al2O3,[6] and 4-MeO-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)[7] have
been used to accomplish this transformation. However, some of these catalytic systems
Received June 22, 2013.
Address correspondence to Jitender M. Khurana, Department of Chemistry, University of Delhi,
Delhi 110007, India. E-mail: jmkhurana1@yahoo.co.in
800

OXIDATION OF SECONDARY ALCOHOLS
801
are performed in aromatic or halogenated solvents or have drawbacks in terms of
cost of reagents, harsh reaction conditions, poor yields, environmentally unfriendly
oxidants, or wastes.[8] Recently much attention has been directed toward the
reduction or replacement of volatile organic compounds (VOCs) from the reaction
media because of environmental concerns.[9] A variety of environmentally benign
media, such as water, ionic liquids, and supercritical fluids, have been promoted
as replacements to VOCs. Ionic liquids have been recognized as promising green
solvents because of their unique properties, including low volatility, high polarity,
good stability over a wide temperature range, and capacity to dissolve a variety
of organic, inorganic, and organometallic compounds. Furthermore, the proper
choice of cations and anions for an ionic liquid can allow for fine-tuning of their
miscibility with water and organic solvents to simplify separation processes by
liquid–liquid extraction.[10] The advantages of ionic liquids as reaction media for
transition-metal-catalyzed oxidations has yet to be widely explored. Also, the
development of new catalytic oxidation system using urea–hydrogen peroxide
adduct (UHP) as terminal oxidant is particularly attractive as replacing H2O2 by
its urea adduct requires a simple reaction setup.[11] Thus, we explored the possibility
of using urea-H2O2 adduct in ionic liquids for the oxidation of 1,2-diols, a-hydroxy
ketones, and alcohols.
RESULTS AND DISCUSSION
In this article, we report a rapid and environmentally benign protocol for the
oxidation of a variety of 1,2-diols, a-hydroxyketones, and other secondary alcohols
using hydrogen peroxide–urea adduct (UHP) as the oxidant and lanthanum triflate
as the catalyst in 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4). The
reaction conditions were optimized by attempting reactions of hydrobenzoin (1a)
(1.0 mmol) with UHP (2.0 mmol) in different ionic liquids and different catalysts.
The reactions attempted in different ionic liquids using InCl3 (10 mmol%) as catalyst
gave poor yields, though among these greatest yield was obtained in [bmim]BF4
(Table 1, entries 1–4). Subsequently, other catalysts namely, L-proline, DBU
Table 1. Optimization of the reaction conditions for oxidation of hydrobenzoin to benzila
Entry
Ionic liquid
Catalyst
Time (h)
Yieldb (%)
1.
2.
3
[bmim]Cl
InCl3
InCl3
6
6
30
55
40
35
20
25
20
90
[bmim]BF4
[bmim]PF6
[bmim]Br
InCl3
InCl3
6
4
6
5.
6.
7
[bmim]BF4
[bmim]BF4
[bmim]BF4
[bmim]BF4
L-Proline
DBU
CTAB
6
6
6
1.5
8.
(CF3SO3)3La
aEmploying 1.0 mmol hydrobenzion, 10 mol% catalyst, 2.0 mmol UHP, and 5 mmol ionic liquid at
70 ꢁC.
bIsolated yields.

802
P. SALUJA, D. MAGOO, AND J. M. KHURANA
(1,8-diazabicyclo-[5.4.0]undec-7-ene), cetyltrimethylammonium bromide (CTAB),
and lanthanum triflate were screened in [bmim]BF4 to identify optimal conditons.
While reactions in the presence of DBU, CTAB, and L-proline gave poor product
yields (Table 1, entries 5–7), the reaction in the presence of lanthanum triflate
proved to be the best with 90% yield of benzil (2a) using 1:2 molar ratio of sub-
strate–UHP (Table 1, entry 8). It was further observed that though the reaction
of hydrobenzoin was complete using 1:1 molar ratio of substrate–UHP, it yielded
a mixture of benzoin and benzil. Increasing the molar ratio of hydrobenzoin–
UHP to 1:3 had no marked effect on the product yield or time of the reaction.
Reactions at higher temperatures affected the oxidation adversely in terms of
yield of the benzil, probably due to rapid decomposition of H2O2 at higher
temperature.
Subsequently, oxidation of different hydrobenzoins was carried out under
optimized reaction conditions (Table 2, entries 1–9). All reactions were clean and
gave the corresponding substituted benzils in good yields.
Table 2. Oxidation of secondary alcohols using 10 mol% lanthanum triflate as catalyst and UHP as oxi-
dant in bmim[BF4]a
Entry Substrate UHP (equiv.) Time (min.) Product Yieldb (%) MP (found) MP (literature)
1
2
1a
1b
1c
1d
1e
1f
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
3
3
3
2
2
2
2
2
2
2
3
2
60
30
2a
2b
2c
2d
2e
2f
90
94
87
89
90
91
90
75
78
93
89
90
87
85
90
92
91
80
84
80
82
82
84
78
74
95
80
92–94
104–105
196–198
120–124
90–92
92–94[12]
103–104[12]
198–199[12]
128–130[12]
91–93[13]
121–122[12]
116–117[14]
34–38[15]
82–83[16]
92–94[12]
3
40
45
4
5
40
35
6
118–120
114–116
32–34
7
1g
1h
1i
40
2g
2h
2i
8
180
150
45
9
78–80
92–94
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
3a
3b
3c
3d
3e
3f
2a
2b
2c
2d
2e
2f
30
40
103–104
199–200
124–126
90–92
103–104[12]
198–199[12]
128–130[12]
50
35
[13]
91–93
30
45
118–120
48–50
121–122[12]
47–49[17]
144–145[18]
79–80[19]
—
4a
4b
4c
4d
4e
4f
5a
5b
5c
5d
5e
5f
30
40
144–146
78–80
50
55
Liquid
50–52
48–52[20]
109–110[21]
—
90
50
106–108
Liquid
Liquid
Liquid
Liquid
80–82
4g
4h
4i
5g
5h
5i
60
—
—
120
120
50
4j
5j
—
82–83[22]
—
4k
4l
5k
5l
120
Liquid
aReaction using 1.0 mmol alcohol, UHP, 10 mol% (CF3SO3)3La, and 5 mmol [bmim]BF4 at 70 ꢁC for the
appropriate time.
bIsolated yield.

OXIDATION OF SECONDARY ALCOHOLS
803
The application of UHP as an oxidant for the oxidation of a-hydroxyketones
in [bmim]BF4 was also investigated. a-Hydroxyketones also underwent oxidation
using 1:2 molar ratio of substrate–UHP in [bmim]BF4 at 70 ꢁC to give the corre-
sponding 1,2-diketones in good yields (Table 2, entries 10–15). Reaction using
1:1 molar ratio of benzoin to UHP was incomplete even after a prolonged reaction
time.
The efficiency of this method was further evaluated for oxidation of alcohols. It
was observed that benzhydrols could be oxidized to corresponding benzophenones
in good yields (Table 2, entries 16–18). Similarly other aromatic alcohols and ali-
phatic alcohols could be oxidized to corresponding ketones (Table 2, entries
19–27). The reactions of diaryl alcohols required 3 molar equivalents of UHP for
completion of reaction.

804
P. SALUJA, D. MAGOO, AND J. M. KHURANA
EXPERIMENTAL
All the chemicals purchased from Sigma-Aldrich were used as received. All the
products are known and were identified by co-TLC with authentic samples and by
mp (wherever applicable). Silica gel 60 F254 (precoated aluminium plates) from
Merck were used to monitor the reaction progress. Melting points were determined
on a Tropical Labequip apparatus and are uncorrected. Infrared (IR) (KBr) spectra
1
were recorded on a Perkin-Elmer FTIR spectrophotometer. The H NMR spectra
were recorded on a Hitachi FT-NMR model R-600 (60 MHz) using tetramethylsi-
lane (TMS) as an internal standard. The ionic liquids were synthesized by reported
procedures.[23,24] The products were identified by co-TLC with authentic samples,
mp, mixed mp (wherever applicable), and spectral data.
Typical Procedure for the Oxidation of 4,4’-Dimethylhydrobenzoin
(1b) as a Representative Example
In a typical experiment, a mixture of 4,40-dimethylhydrobenzoin (1b)
(1.0 mmol), UHP (2.0 mmol), 10 mol% (CF3SO3)3La, and [bmim]BF4 (5 mmol) was
placed in a 50-mL, round-bottom flask fitted with a calcium chloride guard tube.
The reaction mixture was stirred magnetically in an oil bath maintained at 70 ꢁC
for 30 min. (Table 2). The progress of the reaction was monitored by TLC using pet-
roleum ether–ethyl acetate (70:30). After completion of the reaction, the reaction
mixture was cooled to room temperature and washed with diethyl ether (3 ꢂ 10 mL).
The combined ethereal layer was washed with water (2 ꢂ 10 mL) and dried over
anhydrous. Na2SO4. The ethereal extract was concentrated on a rotary evaporator
to give a white solid, which was identified as 4,40-dimethylbenzil by mp, IR, and
1H NMR spectra.
Yield (94%); mp 104–105 ꢁC (lit. 103–104[12]); IR (KBr, cmꢃ1): nmax ¼ 2919,
1
1661, 1605, 1172; H NMR (60 MHz, CDCl3) d 7.86 (d, J ¼ 8.0 Hz, 4H, Ar), 7.30
(d, J ¼ 8.0 Hz, 4H, Ar), 2.43 (s, 6H, CH3).
Supporting Information
General procedures for the oxidation of 1,2-diols, a-hydroxyketones, and other
aromatic or aliphatic secondary alcohols are available in the Supporting Information.
CONCLUSION
We have reported a mild, efficient, and green catalytic oxidation procedure for
the oxidation of hydrobenzoins, a-hydroxyketones, and alcohols to the correspond-
ing benzils and ketones.
ACKNOWLEDGMENTS
P. S. and D. M. thank the University Grants Commission and the Council of
Scientific and Industrial Research, respectively, New Delhi, India, for the grants of
junior and senior research fellowships.

OXIDATION OF SECONDARY ALCOHOLS
805
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