Ionic liquid promoted selective oxidation of organic compounds with NaBrO3

Article abstract of DOI:10.1007/s00706-007-0840-x

1-Butyl-3-methylimidazolium bromide ([bmim]Br) as an ionic liquid promoted selectively the oxidation of alkyl arenes and alcohols to the corresponding carbonyl compounds with NaBrO₃ in excellent yields under neutral conditions at the 70°C. Among the various ionic liquids examined, the [bmim]Br exhibited the best performances with NaBrO₃. The ionic liquid can be recycled and reused for several runs without any significant loss of activity.

Full text of DOI:10.1007/s00706-007-0840-x

Monatsh Chem 139, 905–908 (2008)
DOI 10.1007/s00706-007-0840-x
Printed in The Netherlands
Ionic liquid promoted selective oxidation of organic compounds with NaBrO3
Ahmad Shaabani, Elham Farhangi, Abbas Rahmati
Department of Chemistry, Shahid Beheshti University, Tehran, Iran
Received 2 October 2007; Accepted 28 October 2007; Published online 19 June 2008
# Springer-Verlag 2008
Abstract 1-Butyl-3-methylimidazolium bromide
([bmim]Br) as an ionic liquid promoted selectively
the oxidation of alkyl arenes and alcohols to the
corresponding carbonyl compounds with NaBrO3
in excellent yields under neutral conditions at the
70ꢀC. Among the various ionic liquids examined,
the [bmim]Br exhibited the best performances with
NaBrO3. The ionic liquid can be recycled and reused
for several runs without any significant loss of activity.
Sodium bromate is commercially available as an
inexpensive stable solid, which potentially is an in-
teresting candidate for the oxidation of organic com-
pounds because of its capability in multi-electron
transfer. Although sodium bromate is thermodynam-
ically a strong oxidant, according to literature bro-
mate itself is not able to oxidize organic compounds.
The standard redox potential of the bromate ion,
BrO3ꢂ, is 0.61 V in neutral and alkaline aqueous
solution while in aqueous acidic media it has a po-
tential of 1.52 V. Due to the low standard redox po-
tential in neutral or weak acidic media and very low
solubility in most of organic solvents, it is usually
used in aqueous media in the presence of strong
acidic co-reactants, such as NaHSO3 [9–12], HBr
[13], cerium(IV) ammonium nitrate (CAN) [14, 15],
H2SO4 [16, 17], KHSO4 [18], Bu4NHSO4 [19],
HClO4 [20], Mg(HSO4)2 [21], silica sulfuric acid
[22], and cation exchange resin [23].
In spite of their potential utility and solubility
in these media, most of these homogeneous acids
present limitations due to use of corrosive reagents,
tedious work-up procedures, the necessity of neutral-
ization of strong acidic media, and producing unde-
sired washes. In addition, the high acidity of solution
not only increased the oxidizing power of BrO3ꢂ and
restricted to apply this reagent for the selective oxi-
dation of most of organic compounds, but due to
bromine generation under acidic conditions in the
presence of reducing agents [24–27], the main pro-
ducts becomes contaminated by the formation of
ꢀ-bromo substituted of the corresponding carbonyl
Keywords 1-Butyl-3-methylimidazolium bromide; Oxida-
tion; Alkylarene; Alcohol.
Introduction
Oxidation reactions are among the most important
transformations in synthesis chemistry and offer an
important methodology for the introduction and mod-
ification of functional groups. During the last two
decades, there has been a spectacular development
in the field and a large number of novel and useful
oxidation reactions have been discovered. The oxida-
tion of alkyl arenes and alcohols to their correspond-
ing carbonyl compounds is of significant importance
in organic chemistry, both for fundamental research
and industrial manufacturing. The world-wide annual
production of carbonyl compounds is over 1ꢁ107
tons and most of these compounds are produced from
the oxidation of alkyl arenes and alcohols [1–8].
Correspondence: Ahmad Shaabani, Department of Chemistry,
Shahid Beheshti University, P.O. Box 19839-4716, Tehran,
Iran. E-mail: a-shaabani@cc.sbu.ac.ir

906
A. Shaabani et al.
Scheme 1
Table 1 Oxidation of fluorene and benzoin with NaBrO3 in
various ionic liquids at 70ꢀCa
compounds. So, due to bromine generation, the re-
action mixture must be neutralized with sodium thio-
sulfate before isolating the product. Furthermore, in
most of these oxidation systems the removal of ex-
cess sodium bromate from the reaction mixture and
separation of bromate or bromide from co-reactant
salts is difficult.
Therefore, the development of new approaches for
the selective oxidation of organic compounds with
NaBrO3 in the absence of acidic co-reactants in non-
aqueous media is of some importance in synthesis
organic compounds.
One can wonder whether inert ionic liquids are use-
ful solvents for NaBrO3-mediated organic transforma-
tions taking into account the popularity of ionic liquids
as alternative solvents for organic reactions [28–35].
To the best of our knowledge, ionic liquids have not
yet been explored for reactions with NaBrO3.
In continuation of our studies on the oxidation of
organic compounds [24–26, 36–40], in this paper
we evaluate various ionic liquids for BrO3ꢂ-mediated
oxidation of alkyl arenes and alcohols under neutral
conditions in non-aqueous media at 70ꢀC (Scheme 1).
Entry
Ionic liquid
Yieldb=%
Yieldb=%
1
2
3
[bmim]Br
[bmim]Cl
[bmim]PF6
92
50
30
90
70
20
a
NaBrO3 (0.5 g, 3.30 mmol); ionic liquid (0.30 g); fluorene
or benzoin (1 mmol)
b
The yields of oxidation fluorene and benzoin after 8 h and
100 min
Table 2 Oxidation of fluorene and benzoin with NaBrO3 in
[bmim]Br at various temperaturesa
Entry
Temperature=ꢀC
Yieldb=%
Yieldb=%
1
2
3
4
25
50
70
90
0
50
92
95
0
40
90
90
a
NaBrO3 (0.5 g, 3.30 mmol); [bmim]Br (0.30 g, 1.37 mmol);
fluorene or benzoin (1 mmol)
b
The yields of oxidizing fluorene and benzoin after 8 h and
100 min, respectively
liquids (0.30 g) followed by addition of fluorene
(0.17g, 1 mmol) or benzoin (0.21g, 1 mmol). As in-
dicated in Tables 1 and 2, 1-butyl-3-methylimida-
zolium bromide is the best suited ionic liquid for
these reactions at 70ꢀC in terms of yields and easy
work-up.
As can be seen from Table 3, alkyl arenes are con-
verted into the corresponding carbonyl compounds
in good yields under very mild reaction conditions.
The products are identical to those obtained under
heterogeneous conditions where the reductant is dis-
Results and discussion
In order to obtain the best ionic liquid, we
examined various ionic liquids, such as 1-butyl-
3-methylimidazolium bromide ([bmim]Br), 1-butyl-
3-methylimidazolium chloride ([bmim]Cl), and
1-butyl-3-methylimidazolium hexafluorophosphat
([bmim]PF6). Oxidation of fluorene to 9-fluorenone
and benzoin to benzil were selected as model re-
actions. Test reactions were carried out by mixing
of NaBrO3 (0.50 g, 3.30 mmol) with various ionic

Ionic liquid promoted selective oxidation
907
Table 3 Oxidation of alkyl arenes to carbonyl compounds with NaBrO3=[bmim]Br at 70ꢀC
Entry
Reductant
Product
Time=h
Yielda=%
Mp or Bp (reported)b=ꢀC
1
2
3
4
5
6
7
8
9
10
ethylbenzene
propylbenzene
butylbenzene
diphenylmethane
fluorene
indane
tetralin
1,3-dihydro isobenzofuran
xanthene
p-xylene
acetophenone
propiophenone
1-phenyl-1-butanone
benzophenone
9-fluorenone
1-indanone
ꢀ-tetralone
phthalide
9-xanthenone
4-methylbenzoic acid
10
11
13
17
8
15
10
15
9
75
79
78
79
92
83
85
84
90
72
18–19 (19–20)b
17–18 (17–19)b
11–12 (11–13)b
48–49 (47–49)b
80–81 (80–83)b
39–40 (39–41)b
6–7 (2–7)b
71–72 (70–73)c
173–174 (172–174)b
178–179 (177–180)b
12
a
Isolated yields
Ref. [41]
Ref. [42]
b
c
Table 4 Oxidation of alcohols to carbonyl compounds with NaBrO3=[bmim]Br at 70ꢀC
Entry
Reductant
Product
Time=h
Yielda=%
Mp or Bp (reported)b=ꢀC
1
2
3
4
5
6
7
8
cyclohexanol
cyclooctanol
cyclodecanol
4-methylbenzyl alcohol
4-methoxybenzyl alcohol
4-chlorobenzyl alcohol
4-nitrobenzyl alcohol
1-phenylethanol
1-indanol
diphenylmethanol
benzoin
4,40-dimethoxy benzoin
9-fluorenyl methanol
cyclohexanone
cyclooctanone
cyclodecanone
4-methyl benzaldehyde
4-methoxy benzaldehyde
4-chloro benzaldehyde
4-nitro benzaldehyde
acetophenone
1-indenone
benzophenone
benzil
4,40-dimethoxy benzil
9-fluorenone
180
150
150
100
100
90
79
91
88
81
79
83
85
76
153–155 (154–156)b
75–77 (77–79)b
23–24 (21–24)b
202–203 (204)d
247–248 (247–249)d
47–48 (45–50)b
105–106 (104–106)b
18–19 (19–20)b
39–40 (39–41)b
48–49 (47–49)b
94–95 (94–96)b
132–133 (132–134)b
80–81 (80–83)b
90
180
140
100
100
80
9
87
83
10
11
12
13
90, 91, 87c
92
86
140
a
Isolated yields
Ref. [41]
The same IL was used for each of the three runs
Ref. [42]
b
c
d
solved in an inert solvent; however, the reaction times
are reduced from a few days to a few hours at 70ꢀC.
As indicated in Scheme 1, the reaction is equally effec-
tive when oxygen is present in the side chain. Excellent
yields of the corresponding lactones are realized
when the side chain is an exocyclic ether. Some alkyl
arenes, such as indane and tetralin are readily oxidized
to their corresponding carbonyl compounds (entries
1–9), whereas others such as xylene are converted to
their corresponding carboxylic acids (entry 10).
As indicated in Scheme 1, secondary alcohols are
converted into the corresponding ketones in good
yields (Table 4), and aldehydes are obtained in com-
parable yields from the oxidation of primary alcohols
(entries 4–7, Table 4). The preparation of aromatic
aldehydes from the corresponding primary alcohols
has not previously been easily achieved using per-
manganate or other strong oxidants because alde-
hydes are so readily converted to carboxylic acids
under oxidizing conditions. Among the various alco-
hols studied, ꢀ-hydroxyketones were found to be
the most reactive and needed shorter reaction times
for their oxidation (Table 4, entries 11 and 12).
One of the advantages of ionic liquids is their
ability to function as a recyclable reaction medium.
We were able to separate [bmim]Br from the reaction
medium easily by extracting the product with n-hex-
ane and washing the residue with dichloromethane,
evaporating it under vacuum, and reuse it for subse-
quent reactions (entry 11, Table 4).

908
A. Shaabani et al.
6. Ishii Y, Sakaguchi S, Iwahama T (2001) Adv Synth Cat
343:393
In conclusion, we developed an efficient and simple
procedure for selective oxidation of organic com-
pounds using BrO3 as oxidant and [bmim]Br as an
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ꢂ
ionic liquid. Most importantly, the [bmim]Br could also
by recycled and reused in further reactions without any
significant loss of activity. This method possesses in-
creased compatibility with different functional groups
and the oxidation reaction is carried out in neutral con-
ditions. Unlike previously reported methods, this one
requires neither acids as co-oxidant nor protic solvents,
such as water. Further applications of this reagent
combination are under investigation in our laboratory.
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Experimental
Melting points were measured on an Electrothermal 9200 ap-
paratus. IR spectra were recorded on FT-IR 102MB BOMEM
apparatus. Mass spectra were recorded on a FINNIGAN-MAT
8430 mass spectrometer operating at an ionization potential
of 70eV. 1H NMR spectra were recorded on a BRUKER
1
DRX-300 AVANCE spectrometer at 300.13 MHz. H NMR
spectra were obtained on solutions in CDCl3.
All of the products of these reactions are known compounds
and were characterized from their IR and 1H NMR spectra and
by comparison of their melting points (or those of the deriva-
tives) with known compounds [40]. Most importantly, no
over-oxidized product was detected by H NMR analysis of
the crude reaction mixture in all the cases.
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(1972) Chem Abstr 77:39201z
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Synth Commun 33:2935
22. Shaabani A, Soleimani K, Bazgir A (2004) Synth
Commun 34:3303
1
23. Shaabani A, Lee DG (2003) Sulfur Lett 26:43
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39:3773
Oxidation of alkyl arenes or alcohols: general procedure
In a typical reaction, a mixture of 1 mmol alkyl arene or al-
cohol and 0.50 g sodium bromate (3.30 mmol) was successive-
ly added to a screw-capped vial containing a magnetic stirring
bar to 0.30 g [bmim]Br (1.37 mmol) and was heated at 70ꢀC
in a preheated oil bath, while the progress of the reaction was
followed by TLC and GC. Upon completion, the reaction
mixture was washed with n-hexane. Evaporation of the solvent
gave a product of sufficient purity for most purposes.
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Acknowledgements
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We gratefully acknowledge financial support from the
Research Council of Shahid Beheshti University.
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