Photo-Catalytic oxidation of benzyl alcohols using oxone in the presence of n-Bu4NBr in water

Article abstract of DOI:10.2174/157017809788681392

A simple, mild and efficient method has been developed for the oxidation of alcohols to aldehydes and ketones using Oxone as oxidant catalyzed by n-Bu₄NBr at room temperature in water under UV lamp for 1 h. Oxone smoothly oxidizes benzyl alcohol and its derivatives, benzhydrol and its derivatives.

Full text of DOI:10.2174/157017809788681392

424
Letters in Organic Chemistry, 2009, 6, 424-427
Photo-Catalytic Oxidation of Benzyl Alcohols Using Oxone® in the
Presence of n-Bu4NBr in Water
Shang Wu, Hengchang Ma, Penghua Yan, Jianqiang Wang, Juanjuan Ding and Ziqiang Lei*
Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest
Normal University, Lanzhou 730070, China
Received April 25, 2009: Revised May 13, 2009: Accepted May 13, 2009
Abstract: A simple, mild and efficient method has been developed for the oxidation of alcohols to aldehydes and ketones
using Oxone® as oxidant catalyzed by n-Bu4NBr at room temperature in water under UV lamp for 1 h. Oxone® smoothly
oxidizes benzyl alcohol and its derivatives, benzhydrol and its derivatives.
Keywords: Photo-catalytic oxidation, alcohols, Oxone®, water, aldehydes, ketones.
INTRODUCTION
However, organic solvents often creating a great deal of
safety, health and environmental issues due to their
flammability, toxicity and volatility [6]. Water is cheap,
readily available, nontoxic, non-flammable and safe to
environment, and would provide a more environmentally
benign medium for oxidation. Therefore, a series of catalytic
oxidation procedures using water as the only solvent have
been investigated [3-5]. In this letter, we introduce our study
on the generality of this photo-oxidation of alcohols in water
under UV lamp with n-Bu4NBr using Oxone® as oxidant.
Benzyl alcohol used as test substrate, was oxidized to the
corresponding aldehydes in quantitative GC yield.
The oxidation of alcohols to carbonyl compounds is a
fundamental reaction in organic chemistry, the
corresponding carbonyl compounds can be used as important
and versatile intermediates for the synthesis of fine
chemicals, and several methods covering a wide variety of
reagents have been developed for this important synthetic
transformation. However, these reactions essentially
involved the use of large quantities of heavy metals and
complex organic compounds, which generate large amounts
of waste, and they were not environmentally benign at all [1,
2]. In recent years, the notion of green chemistry was well
established in organic chemistry, and the development of
environmentally friendly processes is the goal of various
research projects [3-6]. Hence, improving the alternative
methods by using cheaper and less toxic reagents coupled
with simpler reaction conditions and easier work-up
procedures are still in demand urgently.
Recently, Oxone® (2KHSO5·KHSO4·K2SO4) has received
much attention in the field of oxidation which is
commercially available [7-9]. Bolm reported that
TEMPO/Oxone®/n-Bu4NBr was effective for the oxidation
of alcohols [10]. In a previous report, we described a
procedure for the oxidation of alcohols in that we used tetra-
n-butyl ammonium Oxone® (n-Bu4NHSO5) as oxidant [7].
RESULTS AND DISCUSSION
Typical procedure was carried out as follows: alcohols (1
equiv.), Oxone® (0.8 equiv.) and bromide (0.2 equiv.) was
added in an aqueous solution. The mixture was stirred for 1h
under irradiation of UV by a high-pressure mercury lamp
(500W). After completion of the reaction, the mixture was
analyzed by GC/MS or NMR.
In fact, the use of n-Bu4NBr and Oxone® or Oxone®
alone did not proceed at all at r.t. without irradiation. Further
studies showed that the activity of n-Bu4NBr and N-
bromosuccinimide was higher than other organic bromide
sources (Table 1), and the oxidation proceeds more smoothly
when the bromide is released more readily. In the case of
NBS, the selectivity of formation of aldehyde decreased
greatly. A plausible mechanism for oxidation was shown in
Scheme 1 based on the literature [11-12]. Bromo radical was
formed by irradiating bromo anion, and then a series of
reactions were occurred. This reaction includes bromine
formation because the color of the reaction mixture
immediately changed to yellow under irradiation of the
lamp. After the completion of the reaction, the anticipated
products were detected by GC/MS or NMR analysis.
In recent years, photo-catalytic oxidation has became
interest in keeping with the requirement of green chemistry.
Bromo radical, formed by continuous photo-oxidation of the
bromo anion, which promoted oxidize alcohols reactions
[11-14]. Zolfigol and co-workers recently presented a
catalytic and transition metal-free procedure for oxidation of
alcohols using periodic acid or iodic acid in the presence of a
catalytic amount of KBr [15]. Itoh and co-workers
investigated photo-oxidation of alcohols and a methyl group
at the aromatic nucleus in the presence of organic or
inorganic bromo sources in organic solvent [12-14].
The oxidation of a series of alcohols was explored under
optmized conditions, and the results were listed in Table 2.
Regarding of oxidation of benzyl alcohol derivatives,
electronwithdrawing substituents were propitious to the
reaction, and afforded the corresponding aldehydes in high
*Address correspondence to this author at the Key Laboratory of Polymer
Materials of Gansu Province, College of Chemistry and Chemical
Engineering, Northwest Normal University, Lanzhou 730070, China;
Fax: +86 9317970359; E-mail: leizq@nwnu.edu.cn
1570-1786/09 $55.00+.00
© 2009 Bentham Science Publishers Ltd.

Photo-Catalytic Oxidation of Benzyl Alcohols Using Oxone®
Letters in Organic Chemistry, 2009, Vol. 6, No. 5
425
Table 1. Comparison of Different Organic Bromides in the Oxidation of Benzyl Alcohol Using Oxone®
O
H
hv
OH
Oxone
H2O
Bromides
Entry
Bromide
Conv. (%)
Sele. (%) (Aldehyde)
1
2
3
4
5
C2H4Br2
C4H8Br2
<5
<5
100
100
47
C7H7Br
46
(CH2CO)2NBr
n-Bu4NBr
99
47
100
86
Reaction condition: benzyl alcohol 0.10 mmol, Oxone® (0.8 equiv. to alcohol), bromide (0.2 equiv. to alcohol), water 3 mL, irradiation 1 h at room temperature. Percentage
conversion and reaction selectivity were determined by GC analysis.
-
HSO5
O
O
O
OH
OH
HBr
H2O
Br
R1
R1
H
OH
R1
Br
R2
R1
R2
R2
R1
-
-
HSO5
O
C
HSO5
O
hv
Br2
Br-
Br
HBr
HBr
+
R1
R1
Br
HBr
Br2
Br
Scheme 1.
conversion and selectivity (Entry 2, 3). However,
electrondonating groups retarded the reaction (Entry 4, 5).
The reaction also provided good-to-excellent yields and
purities in the oxidation of benzhydrol, 4-methylbenzhydrol
and 4-chlorobenzhydrol in the identical reaction conditions
(Entry 6-8). On the other hand, for the cyclic and acyclic
aliphatic alcohols, the catalyst system demonstrated poor
activity. (Entry 9-11).
(Table 2, Entry 7): 1H NMR (CDCl3, 400MHz): ꢀ=7.80-7.78
(2H, Ar-H), 7.78-7.76 (2H, Ar-H), 7.59-7.56 (1H, Ar-H),
7.49-7.46 (2H, Ar-H), 7.29-7.26 (2H, Ar-H), 2.45 (3H, CH3);
13C NMR (100 MHz): ꢀ=196.51, 143.23, 137.90, 134.85,
132.15, 130.28, 129.91, 128.95, 128.16, 21.65. 4-
1
Chlorobenzophenone (Table 2, Entry 8): H NMR (CDCl3,
400MHz): ꢀ=7.80-7.78 (2H, Ar-H), 7.78 -7.76 (2H, Ar-H),
7.63 -7.59 (1H, Ar-H), 7.51-7.49 (2H, Ar-H), 7.47-7.45 (2H,
Ar-H); 13C NMR (100 MHz): ꢀ=195.52, 138.88, 137.22,
135.83, 132.63, 131.46, 129.92, 128.62, 128.39.
To obtain the pure isolated products, the aqueous phase
was extracted with ethyl acetate (5 mL ꢀ 3), organic layer
was dried over anhydrous MgSO4, and removed under
reduced pressure to give the desired crude product.
Analytically pure products were obtained after column
chromatography using petroleum ether and ethyl acetate as
eluent (Entries 2-3, 6-8). The NMR data: 4-
CONCLUSION
In summary, we have presented a green, clean and simple
methodology for the photo-catalytic oxidation of benzyl
alcohols in aqueous media under UV lamp. Benzyl alcohols,
benzhydrols have been oxidized to the corresponding
aldehydes, ketones in moderate to high conversion and
selectivity in 1 h at room temperature. This new oxidation
reaction presents several advantages as metal-free, water as
solvent, less waste, cheap and nontoxic materials.
1
Chlorobenzaldehyde (Table 2, Entry 2): H NMR (CDCl3,
400MHz): ꢀ=10.01 (1H, CHO), 7.83-7.85 (2H, Ar-H), 7.54-
7.52 (2H, Ar-H); 13C NMR (100 MHz): ꢀ=190.85, 140.94,
134.68, 130.89, 129.41. 4-Nitrobenzaldehyde (Table 2,
1
Entry 3): H NMR (CDCl3, 400MHz): ꢀ=10.16 (1H, CHO),
8.42-8.40 (2H, Ar-H), 8.10-8.08 (2H, Ar-H); 13C NMR (100
MHz): ꢀ=190.29, 151.11, 140.02, 130.48, 124.31.
Benzophenone (Table 2, Entry 6): 1H NMR (CDCl3,
400MHz): ꢀ=7.82-7.79 (4H, Ar-H), 7.60-7.59 (2H, Ar-H),
7.56-7.46 (4H, Ar-H); 13C NMR (100 MHz): ꢀ=196.69,
137.48, 132.36, 129.98, 128.21. 4-Methylbenzophenone
EXPERIMENTAL
Oxone® was obtained from commercial sources.
Bromides were all chemical pure reagent. All of the alcohols

426 Letters in Organic Chemistry, 2009, Vol. 6, No. 5
Wu et al.
Table 2. Oxidation of a Series of Alcohols by the Catalysis of n-Bu4NBr
Substrate
Product
Entry
Conv. (%)
Sele. (%)
O
OH
1
100
86
92
H
O
OH
2
H
100
Cl
Cl
O
OH
H
3
4
100
64
100
91
O2N
O2N
O
OH
H
O
OH
H
5
11
90
OH
OH
OH
O
O
O
6
7
100
100
100
100
8
100
100
Cl
Cl
O
9
25
16
<5
100
100
100
OH
OH
O
10
11
H
OH
4
4
O
Reaction condition: alcohol 0.10 mmol, Oxone® (0.8 equiv. to alcohol), n-Bu4NBr (0.2 equiv. to alcohol), water 3 mL, irradiation 1 h at room temperature. Percentage conversion and
reaction selectivity were determined by GC analysis.
used in the reaction were obtained from ABCR GmbH & Co.
KG. Other materials were A.R. and were used as received
without further treatment.
Gas chromatography (GC) analysis was performed on a
Shimadzu GC-2010 equipped with a 15mꢀ0.53mmꢀ1.5μm
RTX-1 capillary column and a oxyhydrogen flame detector.

Photo-Catalytic Oxidation of Benzyl Alcohols Using Oxone®
Letters in Organic Chemistry, 2009, Vol. 6, No. 5
427
GC/MS analyses were carried out on
a
trace HP
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