Sodium selenite-dimethylsulfoxide: a highly efficient reagent for dehydrogenation

Article abstract of DOI:10.3184/030823408X313591

Sodium selenite in dimethylsulfoxide is a highly efficient reagent for dehydrogenation reactions and can be used under both thermal as well as microwave irradiation conditions. Using this reagent benzoins have been dehydrogenated to benzils, flavanones to flavones and 2'-hydroxy chalcones have been converted into flavones by cyclodehydrogenation.

Full text of DOI:10.3184/030823408X313591

JOURNAL OF CHEMICAL RESEARCH 2008
APRIL, 225–226
RESEARCH PAPER 225
Sodium selenite-dimethylsulfoxide: a highly efficient reagent for
dehydrogenation
Mandeep Lambaa and Jagdish Kumar Makrandib
aOrdnance Factory Chanda, Ministry of Defence 442501 India
bDepartment of Chemistry, M.D. University, Rohtak, India
Sodium selenite in dimethylsulfoxide is a highly efficient reagent for dehydrogenation reactions and can be
used under both thermal as well as microwave irradiation conditions. Using this reagent benzoins have been
dehydrogenated to benzils, flavanones to flavones and 2'-hydroxy chalcones have been converted into flavones by
cyclodehydrogenation.
Keywords: sodium selenite, dimethylsulfoxide, benzoins, benzils, flavanones, flavones, 2¢-hydroxy chalcones
Dehydrogenation is one of the common reactions involved in
synthetic organic chemistry. Some of the important reagents
used for dehydrogenation include ceric ammonium nitrate,1
selenium dioxide,2 seleniumdioxide-DMSO,3 triphenylmethyl
perchlorate,4 DDQ,5 Pd on carbon,6 N-chlorosuccinimide,7
H2O2- NaOH8 and InBr3.9 Selenium dioxide, the reagent
which has been used for dehydrogenation in number of
compounds, is quite hazardous due to its volatility. Sodium
selenite, in which selenium is present in the + 4 oxidation
state, has not been exploited as an oxidising agent perhaps due
to its insolubility in the common organic solvents being used
for the reactions. We have observed that sodium selenite acts
as an excellent dehydrogenating agent in dimethylsulfoxide
medium. SeO2 is volatile but selenium and sodium selenite
are not that volatile,10,11 and thus sodium selenite not as
harmful as selenium dioxide and we wish to report the utility
of this efficient dehydrogenating reagent in the present work.
It is possible, however, that its reduction product is volatile.
Dehydrogenation of benzoins was chosen as the first example
and these were converted into benzils 1a–c by heating with
sodiumseleniteindimethylsulfoxideat140oCunderanhydrous
conditions. It is well known that a number of organic reactions
when carried out using microwave irradiations (M.W.), are
markedly improved in terms of temperature needed and time
taken.12,13 Continuous microwave radiation could not be
applied to these systems due to rapid boiling of the solutions;
hence, exposure was given intermittently (Table 1). Thus
the above reaction was carried out by using microwave
irradiation and the reaction was found to be completed (TLC)
in 2.5 min. at 100oC. Flavanones were the next class of
naturally occurring compounds chosen for the study and eight
compounds of this class with substituents at various positions
could be successfully dehydrogenated to flavones 2a–h with
sodium selenite in dimethylsulfoxide under thermal as well as
under microwave conditions (Table 1).
Encouraged by the above results, the cyclodehydrogenation
of 2'-hydroxychalcones was studied. This reaction is considered
to be one of the important reactions in flavanoid chemistry for
the direct synthesis of flavones from 2¢-hydroxychalcones.
Flavones constitute a large class of natural products. Synthesis
of flavones and their derivatives has attracted considerable
attention due to their significant biocidal,14 pharmaceutical,15
antioxidant,16 anti-anxiolytic,17 anticancer18 and anti-
inflammatory effects.19 This conversion is usually carried
out by heating 2¢-hydroxychalcones with selenium dioxide20
in biphenyl ether. Other reagents that have been used include
sodium periodate,21 iodine,22 iodine-DMSO,23 Pd-black24
or Pd(II),25 DDQ26 and organic disulfides.27 These reactions
usually require long reaction periods (20–40 h) or the reaction
is accompanied by formation of other products such as aurones
Table 1 Dehydrogenation of benzoins to benzils 1a–c, flavanones to flavones 2a–h and cyclodehydrogenation of 2¢-hydroxy
chalcones to flavones 3a–j
Compd R1
R2
R3
R4
Method A
Yield/%
Method B
Lit. M.p. /°C
Time/h
M.p./°C
Time/min Yield/%
M.p. /°C
1a
1b
1c
2a
2b
2c
2d
2e
2f
2g
2h
3a
3b
3c
3d
3e
3f
H
H
H
H
1.5
1.5
2.0
3.0
4.0
2.5
3.0
3.0
4.0
2.5
3.0
3.0
2.5
1.5
1.5
4.0
4.0
2.5
2.5
2.0
3.0
80
80
85
80
65
70
60
76
70
70
80
80
75
70
75
80
70
75
80
70
75
94
1.5
1.5
2.0
2.0
3.0
2.0
2.5
1.5
2.5
2.5
2.0
2.5
2.5
1.5
2.0
2.5
3.0
1.0
2.0
2.5
1.5
90
85
90
85
75
76
75
85
80
75
85
85
80
75
80
80
75
75
80
80
80
95
9528
OCH3
CH3
H
OCH3
CH3
H
H
H
129
130
132–13328
10228
H
H
OCH3
H
OCH3
H
OCH3
H
CH3
H
OCH3
H
OCH3
H
H
100
100
H
97–98
97–98
97–9829
157–15830
110–11132
143–14431
12229
H
H
H
155
156
OCH3
OCH3
H
H
H
109
108–109
143
H
H
142
CH3
CH3
H
H
120–121
168
119–120
168–169
127–128
110–111
96–97
156–158
120–121
169–170
108–109
142–144
109–110
128–129
151–153
192–193
H
H
17031
H
OCH3
H
128–129
109–110
97–98
129–13027
111–11227
97–9829
157–15830
12229
H
H
H
H
H
H
H
H
156–157
120–121
168–169
108–110
142–143
110–112
126–128
150–152
193–194
H
CH3
CH3
H
H
H
H
17031
OCH3
OCH3
H
H
110–11132
143–14431
111–11233
129–13027
15434
H
OCH3
CH3
H
H
OCH3
H
3g
3h
3i
H
H
H
H
OCH3
H
H
OCH3
OCH3
3j
H
H
194–19535
* Correspondent. E-mail: jkmakrandi_chem@rediffmail.com
PAPER: 08/5004

226 JOURNAL OF CHEMICAL RESEARCH 2008
OH
CH C
O
C
O
C
O
R1
R2
R1
R2
Na2SeO3 -DMSO
Na2SeO3 -DMSO
1a-c
R3
R4
R3
R4
R1
O
O
R1
O
R2
R2
O
2a-h
R3
R4
R3
R4
R1
R2
O
R1
R2
OH
O
Na2SeO3 -DMSO
O
3a-j
Method A:Thermal
Method B: M.W.
Scheme 1
and flavanones. The 2¢-hydroxychalcone was thus heated
with sodium selenite in dimethylsulfoxide at 140oC and the
flavone was obtained as a sole product after 3 h. The same
reaction was when repeated under microwave irradiations.
The reaction was found to be completed in 2.5 min. (TLC)
and flavone was obtained in 85% yield (Table 1). Using this
method 10 2'-hydroxychalcones were converted into flavones
3a–j. The identities of the products were confirmed from their
1H NMR spectroscopic data and by direct comparison with
authentic samples (CO-IR).
Received 17 December 2007; accepted 28 March 2008
Paper 07/5004
doi: 10.3184/030823408X313591
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Experimental
The reactions were carried out in a domestic microwave oven
(Samsung, output energy 900W, frequency 2450 MHz, with
temperature control arrangement model No.CE118KF using 50%
power for all experiments). The identities of the products were
confirmed from their 1H NMR spectroscopic data and direct
comparison with authentic samples (CO-IR).
CAUTION: Care should be taken when heating dimethylsulfoxide
by microwave irradiation due to its high polarity and heat capacity.
General procedure for dehydrogenation
Method A (Thermal): The substrate (0.05 mol) in dimethylsulfoxide
(4 ml) and sodium selenite (0.05 mole) under anhydrous conditions
using a calcium chloride guard tube were heated on an oil bath
maintained at 140oC for the specified period and the completion
of the reaction was monitored by TLC. After disappearance of the
starting material, the reaction mixture was cooled, and diluted with
ice cold water (50 ml) and extracted with diethyl ether (3 ¥ 30 ml).
The organic layer was washed with water and dried over Na2SO4.
The solvent was removed by distillation and the residue was
crystallised from aqueous methanol to afford the product.
Method B (M.W.): A solution of substrate (0.02 mol) in
dimethylsulfoxide (2 ml) and sodium selenite (0.02 mole) in a loosely
stoppered 20 ml round bottom flask was irradiated with microwave
irradiation (450 W) setting the microwave oven temperature at
100oC for the specified period. The completion of the reaction was
monitored by TLC. The reaction mixture was worked up as described
in Method A to give the product.
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PAPER: 08/5004