2
66 J. CHEM. RESEARCH (S), 1998
J. Chem. Research (S),
An Efficient Deoxygenation of Nitrones and
Heteroarene N-Oxides$
Dilip K. Dutta* and Dilip Konwar
1998, 266±267$
Organic Chemistry Division, Regional Research Laboratory, Jorhat-785006, Assam, India
Selective deoxygenation of nitrones and heteroarene N-oxides with the Zn±AlCl
sponding imines and heteroarenes in high yield.
3
Á6H
2
O-THF system produces the corre-
The selective deoxygenation of organic N-oxides is
subject of current interest in organic synthesis. Consequently
a
of the organic solvent gave the corresponding imine 2a.
Yield 0.34 g (92%).
1
a number of reagents, e.g. low-valent titanium, phos-
Interestingly, the carbon±carbon double bonds (entries
1g and 1h in Table 1) furan (entry 1d) and thiophene
(entry 1f ), which are susceptible to saturation, remain
intact and only selective deoxygenation of the nitrone
takes place. Moreover, aromatic methoxy (entry 1d) or
chloro (entries 1b and 1c) substituted nitrones were
selectively reduced. At the same time, when the reaction was
extended to heteroarene N-oxides (entries 3a and 3b), the
corresponding deoxygenated products were produced in
good yields.
2
phorus, sulfur, sodium hydrogen telluride, tributyltin
3
4
4
hydride, Pd/C, aluminium/nickel alloy, tetrathiomolyb-
6
7
8
date, etc. may be used to reduce the N±O bonds of
4
nitrones and heteroarene N-oxides.
1,5
However, there
are various limitations to their general use: (i) expensive or
not readily available reagents, (ii) low temperature and dry
1
(a,b)
reaction conditions,
with the isolation of the product,
conditions which eect the substituents, (v) uncontrolled
(iii) side reactions and diculties
2(b)
(iv) drastic reaction
9
4
8
reaction of nitrones and (vi) long reaction time. Therefore,
the development of ecient, cheap and readily available
reagents requires great attention. We report here that
nitrones and heteroarene N-oxides can be deoxygenated
in excellent yields under mild conditions and with short
Although the mechanism of the reaction can not be
assigned at this stage the single-electron transfer (SET)
10
process cannot be ignored. Accordingly, we believe that
0
Al generated from the reaction of zinc and aluminium
chloride (formation of black slurry) is responsible for deoxy-
genation. Further work is in progress to explore the possible
use of this reagent in other synthetic transformations.
In conclusion, we hope that the present method ®nds
wide application in organic synthesis because of its short
reaction time, high yields and use of a readily available
reagent under mild conditions.
reaction times by addition of Zn±AlCl
THF.
3
2
Á 6H O to nitrones in
Experimental
Mps were determined on a Buchi apparatus and are uncorrected.
Mass spectra were obtained using a Finnigan-MAT (1NCOS-50)
spectrometer. IR spectra were recorded on a Perkin-Elmer 237B
1
spectrophotometer and H NMR spectra on a Varian T-60 spec-
trometer. Commercially available (BDH) AlCl
were used directly. Tetrahydrofuran was used after distillation over
3
2
Á6H O and zinc dust
KOH. Nitrones (1a±h) and heteroarene N-oxides (3a±b) were
prepared as reported.
12
Deoxygenation of Nitrones and Heteroarene N-oxides.ÐNitrone
a (0.4 g, 2 mmol) was added to a vigorously stirred mixture of Zn
The readily available nitrone 1a (0.4 g, 2 mmol) was
added to a well-stirred mixture of Zn (0.26 g, 4 mmol)
1
(
0.26 g, 4 mmol) and AlCl
3
Á6H
2
O (1 g, 4 mmol) in THF (25 ml)
and AlCl
temperature for 10 min. Aqueous work-up and evaporation
3
2
Á 6H O (1.0 g, 4 mmol) in THF (25 ml) at room
at room temperature. An exothermic reaction occurred at the
beginning of the reaction, which subsided after 10 min. The reaction
Table 1 Deoxygenation of nitrones and heteroarene N-oxides
Mp/Bp (8C)
a
Reaction
time (t min.)
Yield
(%)
1
2
R
Entry
R
Found
Reported
Ref.
1
1
1
1
1
1
1
1
3
3
a
b
c
d
h
f
g
h
a
b
Ph
p-ClC
Ph
p-CH
Ph
Thiophenyl
Furyl
Ph-CH1CH
H
CH
Ph
Ph
p-ClC
Ph
p-MeOC
Ph
Ph
Ph
H
10
10
15
15
15
15
15
10
20
25
92
90
88
81
82
80
82
84
76
80
52
64±65
62
63±64
70
122±124
57±58
108±109
164±163
162±163
52
11
12
12
12
12
13
14
15
16a
16b
6
H
4
64±65
63
64
6 4
H
3
OC
6
H
4
6
H
4
70±71
b
b
122±125
58
109
165±166
163
c
c
c
c
3
CH
3
a
1
b
IR, H NMR and mass spectra were in accord with those of authentic samples. At 2 mmHg. Mp of the
c
picrate derivative.
*To receive any correspondence.
mixture was then poured into a saturated NH
4
Cl solution (50 ml)
$
This is a Short Paper as de®ned in the Instructions for Authors,
and extracted with diethyl ether (3 Â 50 ml). The organic layer was
washed with brine (2Â50 ml) and dried over anhydrous sodium sul-
fate. Removal of solvent under reduced pressure gave a solid mass
Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).
Dutta, Dilip K.
