NaBrO3/bmim[HSO4]: a versatile system for the selective oxidation of 1,2-diols, α-hydroxyketones, and alcohols

Article abstract of DOI:10.1007/s00706-016-1749-z

Abstract: Sodium bromate with bmim[HSO₄] has been found to be an excellent oxidizing agent in aqueous medium. NaBrO₃:bmim[HSO₄] oxidized 1,2-diols, α-hydroxyketones, and alcohols to the corresponding carbonyl compounds in excellent yields. This method offers advantages such as low cost reagents, aqueous reaction conditions, moderate temperatures and short reaction times and hence environmentally benign reaction. Moreover, the ionic liquid bmim[HSO₄] could be recycled for at least three times without loss of significant activity. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s00706-016-1749-z

Monatsh Chem
DOI 10.1007/s00706-016-1749-z
ORIGINAL PAPER
NaBrO /bmim[HSO ]: a versatile system for the selective
3
4
oxidation of 1,2-diols, a-hydroxyketones, and alcohols
1
1
1
Jitender M. Khurana
•
Anshika Lumb
•
Ankita Chaudhary
Received: 8 May 2015 / Accepted: 29 March 2016
Ó Springer-Verlag Wien 2016
Abstract Sodium bromate with bmim[HSO ] has been
4
Introduction
found to be an excellent oxidizing agent in aqueous med-
ium. NaBrO :bmim[HSO ] oxidized 1,2-diols, a-
Oxidation reactions are an important component of organic
synthesis [1]. Selective oxidation of alcohols to corre-
sponding carbonyl compounds has been reported by variety
of reagents invariably in organic solvents [2–5]. In contin-
uation of our ongoing efforts for developing inexpensive
and efficient reagents for functional group transformations
[6–9], we decided to investigate the potential and selectivity
of sodium bromate, as an oxidizing agent. Sodium bromate
is capable of multi-electron transfer and is normally used in
3
4
hydroxyketones, and alcohols to the corresponding car-
bonyl compounds in excellent yields. This method offers
advantages such as low cost reagents, aqueous reaction
conditions, moderate temperatures and short reaction times
and hence environmentally benign reaction. Moreover, the
ionic liquid bmim[HSO ] could be recycled for at least
4
three times without loss of significant activity.
Graphical abstract
conjunction with acidic co-reactants such as NaHSO [10],
3
HBr [11], ceric(IV) ammonium nitrate [12], H SO [13],
2
4
KHSO [14], Bu NHSO [15], Mg(HSO ) [16], silica sul-
4
4
4
4
furic acid [17], and cation exchange resin [18].
Ionic liquids offer a variety of advantages over con-
ventional solvents and are green alternatives [19, 20].
Recently, there has been lot of interest in acidic ionic liq-
uids which serve the dual purpose of catalysis and solvent
[
21, 22]. Therefore, we believed that sodium bromate could
be used as an oxidant in acidic ionic liquid which could act
as a co-reactant and a green solvent. Furthermore, the
oxidations [23, 24] using ionic liquids are highly efficient
as ionic liquids are easily separable, and recyclable which
makes the process more economic. We have explored the
versatility of inexpensive sodium bromate in presence of
Keywords Oxidation Á Alcohols Á Green chemistry Á
Sodium bromate Á Ionic liquid
acidic ionic liquid, i.e. bmim[HSO ] as an efficient oxi-
4
dation system.
Results and discussion
&
Jitender M. Khurana
jmkhurana1@yahoo.co.in
In this paper, we report a new, convenient, and simple
procedure for the selective oxidation of 1,2-diols to a-hy-
droxyketones or 1,2-diketones, a-hydroxyketones to 1,2-
1
Department of Chemistry, University of Delhi, Delhi 110007,
India
123

J. M. Khurana et al.
diketones and alcohols to corresponding carbonyl com-
pounds with sodium bromate in the presence of
Scheme 1
bmim[HSO ] which acts as a green solvent as well as the
4
co-catalyst.
Hydrobenzoin (1a) was selected as a model substrate to
optimize the reaction conditions. The reactions were car-
ried out under different conditions by changing the molar
ratio of substrate to NaBrO and also the v/v amount of
3
[
bmim]HSO to water as an additive. The reactions were
4
carried out both at room temperature and at higher tem-
peratures. The products expected from oxidation of
hydrobenzoin are benzoin and benzil. The aim was to
identify reaction conditions so that selective oxidation of
hydrobenzoin to benzoin or benzyl could be achieved. The
details of reactions carried out for optimization are listed in
Table 1, entries 1–10. The reactions were attempted using
Subsequently, oxidation of a-hydroxyketones was also
attempted using 1:3 molar ratio of substrate:NaBrO and
3
bmim[HSO ]:H O 3:1 (v/v) at 60 °C. All the a-hydrox-
4
2
yketones underwent oxidation under these conditions to
give the corresponding 1,2-diketones in high yields
1
.0 mmol of hydrobenzoin. It can be inferred from the
(Scheme 3; Table 4).
listed reactions that water plays an important role in the
selective oxidation of 1,2-diols to a-hydroxyketones or 1,2-
diketones with sodium bromate in bmim[HSO4].
The recyclability of the ionic liquid was also investi-
gated. The recovered ionic liquid gave comparable yields
of the product 3a in second and third cycles. However, the
yields decreased gradually in fourth and fifth cycles as
shown in Fig. 1.
Therefore, it is clear from above results of Table 1 that
the selective oxidation of hydrobenzoin to benzoin could
be achieved using 1:3 molar ratio of substrate:NaBrO and
3
The scope of the reagent, NaBrO3 in bmim[HSO4],
was also investigated for oxidation of alcohols. We
observed that primary alcohols could be selectively oxi-
dized to the corresponding aldehydes and secondary
alcohols, both aromatic and aliphatic, underwent oxida-
tion to the corresponding carbonyl compounds 5a–5p
bmim[HSO ]:H O (1:1, v/v) at room temperature and
4
2
selective oxidation to benzil could be achieved by carrying
out the reaction using 1:3 molar ratio of substrate:NaBrO3
and bmim[HSO ]:H O (3:1, v/v) at 60 °C. These optimized
4
2
reaction conditions were then extended to a variety of aryl
and alkyl 1,2-diols. These could be oxidized to the corre-
sponding a-hydroxyketones (Scheme 1; Table 2) or 1,2-
diketones (Scheme 2; Table 3) in high yields by changing
the reaction conditions.
(Table 5) in high yields using 1:3 molar ratio of substrate
to NaBrO3 and bmim[HSO ]:H O 3:1 (v/v) at 60 °C
4
2
(Scheme 4).
3 4
Table 1 Optimization of reaction conditions for the oxidation of hydrobenzoin with NaBrO /bmim[HSO ]
a
O (v/v)
b
Run
1a:NaBrO
3
[bmim]HSO
:H
4 2
Temp./°C
Time
Product (yield/%)
c
1
2
3
4
5
6
7
8
9
1:1
1:3
1:2
1:2
1:3
1:3
1:3
1:3
1:3
1:4
1:3
4:0
0:4
4:0
1:1
1:1
1:3
3:1
1:1
3:1
3:1
3:1
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
60
6 h
6 h
6 h
6 h
–
No reaction
c
–
d
2a ? 3a
30 min
6 h
2a (88)
d
2a ? 3a
e
30 min
4 h
2a ? 3a
e
2a ? 3a
60
30 min
35 min
30 min
3a (87)
3a (86)
3a (85)
1
1
a
0
60
1
100
3
For 1.0 mmol of substrate, 4 cm of solvent was used
b
c
2
a = Benzoin; 3a = Benzil
Incomplete reaction; number of products
Incomplete reaction
d
e
Reaction complete
1
23

NaBrO
3
/bmim[HSO
4
]: a versatile system for the selective oxidation of 1,2-diols, a-…
3 4 2
Table 2 Selective oxidation of 1,2-diols to a-hydroxyketones with NaBrO (1:3 molar ratio) in bmim[HSO ]:H O (1:1,v/v) at room temperature
Entry
Substrate
Product
Time/min
Yield/%
M.p./°C (obs.)
M.p./°C (lit.)
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1f
2a
2b
2c
2d
2e
2f
30
75
90
60
50
60
90
90
88
76
80
72
74
70
78
78
128–130
86–88
133–135 [25]
86–88 [25]
112–113 [26]
88–90 [26]
75–76 [27]
94–96 [27]
76–77 [26]
63–64 [28]
110–112
80–82
74–76
90–92
1g
1h
2g
2h
72–74
62–64
3
For 1 mmol of substrate, 4 cm of solvent was used
Conclusion
Scheme 2
In conclusion, we have developed a simple and convenient
procedure for the selective oxidation of 1,2-diols, a-hy-
droxyketones, and alcohols using BrO3 anion as an
-
oxidant and bmim[HSO ] as a solvent and co-catalyst. We
4
have also shown the selectivity and efficiency of bromate
anion as an oxidant. Moreover, ionic liquid used could be
recycled three times without any significant loss of activity.
Easy work-up, mild conditions and high yields of products
make this method a benign alternative to the present
methodologies.
No over-oxidation of aldehydes to carboxylic acid was
observed. However, aldehydes could be oxidised subse-
quently to the corresponding carboxylic acids on addition
of 1:3 molar ratio of substrate to NaBrO3 and
bmim[HSO ]:H O 3:1 (v/v).
Experimental
All the melting points were recorded on a Tropical Labe-
quip apparatus. IR spectra were recorded on Perkin-Elmer
FT-IR SPECTRUM-2000 and NMR spectra were recorded
4
2
Table 3 Oxidation of 1,2-diols to 1,2-diketones with NaBrO
3
in bmim[HSO
4
]: H
2
O (3:1,v/v) at 60 °C
Entry
Substrate
Product
Time/min
Yield/%
M.p./°C (obs.)
M.p./°C (lit.)
9
1a
1b
1c
1d
1e
1f
3a
3b
3c
3d
3e
3f
25
25
45
30
60
40
45
60
45
40
50
30
40
88
87
84
89
82
81
78
80
82
80
78
72
78
90–92
92–94 [8]
10
11
12
13
14
15
16
17
18
19
20
21
198–200
130–132
104–106
116–118
224–226
90–92
198–199 [8]
132–133 [8]
104–105 [8]
116–117 [29]
226–228 [8]
91–93 [30]
134–135 [8]
128–130 [8]
129–130 [31]
118 [32]
1g
1h
1i
3g
3h
3i
132–134
128–130
126–128
116–118
Liquid
1j
3j
1k
1l
3k
3l
– [33]
1m
3m
84–86
82–83 [8]
3
For 1 mmol of substrate, 4 cm of solvent was used
123

J. M. Khurana et al.
Scheme 3
100
Recycling of Ionic Liquid
8
8
90
80
70
60
50
85
83
7
8
74
Run 1
Run 2
Run 3
Run 4
Run 5
Cycles
on a Hitachi FT-NMR model R-600 (60 MHz) with TMS
as internal standard. All the products are known and were
identified by co-TLC with authentic samples whenever
available, m.p., IR and NMR spectra.
Fig. 1 Recycling yields: reaction of hydrobenzoin (1.0 mmol) with
NaBrO (3.0 mmol) in [bmim]HSO
(3:1, v/v) at 60 °C
3
4
1
benzoin (2) as characterized by m.p., IR and H NMR
spectra.
General procedure for the oxidation of 1,2-diols to a-
hydroxyketones
The aqueous layer containing ionic liquid was washed
with diethyl ether and reactivated at 80 °C under vacuum.
The recovered ionic liquid could be reused for the reactions
for up to three runs without any appreciable loss of
efficiency.
In a typical reaction, a mixture of hydrobenzoin (1a,
1
5
.0 mmol) and sodium bromate (3.0 mmol) was placed in a
3
0 cm RB flask fitted with a condenser and a magnetic stir
bar. Ionic liquid and water, [bmim]HSO :H O (1:1, v/v)
4
2
3
4 cm ) was added to the flask. The reaction mixture was
(
General procedure for the oxidation of 1,2-diols, a-hy-
droxyketones and alcohols to 1,2-diketones and aldehydes/
ketones
stirred at room temperature. The progress of the reaction
was monitored by TLC, using petroleum ether: ethyl
acetate (80:20, v/v) as eluent. After completion of the
In a typical experiment, a mixture of 1,2-diols/a-hydrox-
yketones/alcohols (1 mmol), sodium bromate (3 mmol),
and bmim[HSO ]:H O 3:1 (v/v) along with a stir bar was
reaction, the product was extracted with diethyl ether
3
3 9 15 cm ). The combined ether extract was washed
(
with saturated solution of Na S O to decompose excess
2
4
2
2 3
3
bromine and water (2 9 10 cm ). The combined ether
placed in a RB flask fitted with a condenser. The reaction
mixture was stirred magnetically in an oil-bath maintained
at 60 °C for an appropriate time as mentioned in Tables 3,
layer was dried over anhyd. Na SO and concentrated on a
2
4
rotary evaporator to give the product. The crude product
was recrystallised from aqueous ethanol and yielded pure
4
, and 5, respectively. After completion of the reaction, as
Table 4 Oxidation of a–hydroxyketones to 1,2-diketones with NaBrO
3
(1:3 molar ratio) in bmim[HSO
4
]:H
2
O (3:1,v/v) at 60 °C
Entry
Substrate
Product
Time/min
Yield/%
M.p./°C (obs.)
M.p./°C (lit.)
1
2
3
4
5
6
7
8
9
2a
2b
2c
2d
2e
2f
3a
3b
3c
3d
3e
3f
25
25
30
35
40
45
40
47
60
86
80
82
84
83
81
85
78
74
90–92
92–94 [8]
196–198
132–134
104–106
116–118
224–226
90–92
198–199 [8]
132–133 [8]
104–105 [8]
116–117 [34]
226–228 [8]
91–93 [30]
134–135 [8]
– [35]
2g
2h
2i
3g
3h
3n
132–134
Liquid
3
For 1 mmol of substrate, 4 cm of solvent was used
1
23

NaBrO
3
/bmim[HSO
4
]: a versatile system for the selective oxidation of 1,2-diols, a-…
Table 5 Oxidation of alcohols with NaBrO
3
in bmim[HSO
4
]:H
2
O (3:1,v/v) at 60 °C
Entry
Substrate
Product
Time/min
Yield/ %
M.p./ °C (obs.)
M.p./ °C (lit.)
1
2
3
4
5
6
7
8
9
4a
4b
4c
4d
4e
4f
5a
5b
5c
5d
5e
5f
45
35
60
45
40
90
45
45
40
60
30
30
35
30
30
45
87
82
84
82
82
80
74
70
72
69
65
78
76
81
74
78
82–84
46–48
76–78
142–144
42–44
Liquid
Liquid
Liquid
Liquid
100–102
Liquid
Liquid
Liquid
Liquid
Liquid
Liquid
82–83 [36]
47–49 [37]
79–80 [38]
144–145 [39]
48–52 [37]
– [40]
4g
4h
4i
5g
5h
5i
– [41]
– [42]
– [43]
10
11
12
13
14
15
16
4j
5j
100–105 [44]
– [45]
4k
4l
5k
5l
– [46]
4m
4n
4o
4p
5m
5n
5o
5p
– [46]
– [46]
– [47]
– [48]
3
For 1 mmol of substrate 4 cm of solvent was used
2
3
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