8
738
S. Kumar et al. / Tetrahedron Letters 46 (2005) 8737–8739
2
ÆxH O-mediated deoxygenation of aromatic N-oxides
Table 1. RuCl
3
X
X
a
1
2
b
RuCl .xH O
3
2
Product
R /X
R
Time (min) Yield (%)
CH CN
3
2
2
2
2
2
2
4
4
4
4
6
6
6
8
8
a
b
c
d
e
f
H
H
H
4-Cl
H
4-Cl
H
4-Cl
H
4-Cl
4-CH
3-Cl
—
—
—
25
25
20
20
30
30
40
35
40
40
45
50
55
50
55
94
97
91
88
89
87
82
80
78
80
76
72
70
68
70
N
N
4-Cl
4-Cl
4-CH
4-CH
H
4-Cl
4-CH
3-Cl
H
O
5
6
3
3
Scheme 3.
a
b
c
d
a
b
c
a
b
X
X
N
3
3
RuCl .xH O
3
2
CH CN
Cl
OCH
H
3
N
O
2
CH@CH
2
—
—
Cl
8
7
a
1
Melting points, IR, H NMR were in accordance with those of
authentic samples.
Isolated yields.
Scheme 4.
b
Acknowledgements
(
9
hexane/ethyl acetate) to afford benzylidine aniline 2a in
4% yield, mp 52 °C. Similarly, other nitrones gave the
The authors thank the Council of Scientific and Indus-
trial Research, New Delhi, for financial assistance.
1
9
corresponding imines (2b–f) in excellent yields (Table 1),
without the formation of any side products. Encouraged
by these results, we extended this to azoxybenzenes 3
and N-heteroarene N-oxides 5 and 7. Azoxybenzenes
gave the corresponding azobenzenes 4 (Scheme 2, Table
References and notes
1
2
3
. Tenaglia, A.; Terranova, E.; Waegell, B. Tetrahedron Lett.
991, 32, 1169–1170.
. Thivolle-Cazat, J.; Tkaqtchenko, I. J. Chem. Soc., Chem.
Commun. 1982, 52, 1128–1129.
. De, S. K.; Gibbs, R. A. Synthesis 2005, 1748–1750.
4. Jaouhari, R.; Gu e´ not, P.; Dixneuf, P. H. J. Chem. Soc.,
Chem. Commun. 1986, 1255–1256.
1
1
) and N-heteroarene N-oxides, that is, pyridine N-oxi-
des 5 and quinoline N-oxides 7 afforded the deoxygen-
ated products 6 and 8 (Schemes 3 and 4), respectively,
in quantitative yields (Table 1).
The selectivity of the present method was illustrated by
several functionalities, such as cyano, halogen, ethers
and esters, which were unaffected. Ethers and esters
are prone to cleavage when strong Lewis acids like alu-
5. Fazio, F.; Wong, C. H. Tetrahedron Lett. 2003, 44, 9083–
9
085.
. Iranpoor, N.; Kazemi, F. Tetrahedron 1998, 54, 9475–
480.
6
9
2
0
7. For review see: (a) Murahashi, S. I. Angew. Chem., Int. Ed.
Engl. 1995, 2452; (b) Ganesh, K. N.; Sharma, N. K.
Tetrahedron Lett. 2004, 45, 1403–1406; (c) Moshraqui, S.
H.; Karnik, M. A. Tetrahedron Lett. 1998, 39, 4895–4898;
minium halides are used for deoxygenation. There was
no evidence for the formation of any undesirable rear-
2
1
ranged or other side products from aldonitrones. At-
tempts to perform the deoxygenation with less reagent
afforded low yields but still with high selectivity. Fur-
thermore, no halogenation of heteroarene nucleus was
observed, a serious drawback of chlorine-containing re-
(
d) Yong, D.; Zhang, C. J. Org. Chem. 2001, 66, 4814–
4818; (e) Quittmann, W.; Roberge, D. M.; Bessard, Y.
Org. Process Res. Dev. 2004, 8, 1036–1041; (f) Yamaoka,
H.; Moriya, N.; Ikunka, M. Org. Process Res. Dev. 2004,
2
2
8, 931–938; (g) Niwa, H.; Mori, T.; Hsegawa, T.; Yamada,
agents like PCl , PO Cl, SO Cl , etc. Finally, the pres-
5
3
2
2
K. J. Org. Chem. 1986, 58, 1015–1018.
8. Barton, D. H. R.; Fekih, A.; Lusinchi, X. Tetrahedron
Lett. 1985, 26, 4603–4606.
ent method is a one-pot procedure and does not involve
2
0
any prior preparation of the reagent system.
9
. (a) Olah, G. A.; Gupta, B. G. B.; Narang, S. C. J. Org.
Chem. 1978, 43, 4503–4505; (b) Howard, E., Jr.; Olszew-
ski, W. F., J. Am. Chem. Soc. 1959, 81, 1483–1484; (c)
Lunn, G.; Sansone, E. B.; Keefer, L. K. Synthesis 1985,
1104–1107.
In conclusion, the present protocol is mild, efficient and
of fairly wide scope for the deoxygenation of N-oxides.
Excellent yields, short reaction periods, the survival of
ether and halogen substituents, avoidance of harsh re-
agents and mild reaction conditions are the main advan-
tages of this method.
10. (a) Bjorsvik, H.-R.; Gambarotti, C.; Jensen, V. R.;
Gonzalez, R. R. J. Org. Chem. 2005, 70, 3218–3224, and
O
1
1
R
R
RuCl .xH O
3
2
N
N
N
CH CN
N
R2
3
R2
3
4
Scheme 2.