360 Bull. Korean Chem. Soc. 2013, Vol. 34, No. 2
Communications to the Editor
found to be ideal for the complete conversion of amine-N-
Synth. Commun. 1989, 19, 897. (d) Yoo, B. W.; Choi, J. W.; Kim,
D. Y.; Hwang, S. K.; Choi, K. I.; Kim, J. H. Bull. Korean Chem.
Soc. 2002, 23, 797.
17
oxide 1 into amine 2 in methanol. Methanol has been found
to be suitable to favour the reaction at room temperature. In
order to explore the scope and limitations of this reagent
system, we have tested its applicability for selective deoxy-
genation of amine-N-oxides bearing other potentially sensi-
tive functional groups in the heteroaromatic ring. A variety
of functional groups (methoxy, cyano, chloro, hydroxy, ester,
and amide) were tolerated under the reaction conditions. It is
worth mentioning that the chloro substituent in 2-chloro-
pyridine-N-oxide (entry 7) remains unchanged, even though
3
. Balick, R.; Cybulski, M.; Maciejewski, G. Synth. Commun. 1989,
1
9, 897.
4. Ilias, M.; Barman, D. C.; Prajapati, D.; Sandhu, J. S. Tetrahedron
Lett. 2002, 43, 1877.
. (a) Kozuka, S.; Akasaka, K.; Furumai, T.; Dae, S. Chem. Ind.
London) 1974, 452. (b) Jousseaume, B.; Chanson, E. Synthesis
987, 55.
. Balicki, R. Synthesis 1989, 645.
5
6
(
1
7. Ilankumaran, P.; Chandrasekaran, S. Tetrahedron Lett. 1995, 36,
4881.
1
8
8. Handa, Y.; Tanaka, W.; Ohta, A. J. Chem. Soc. Chem. Commun.
it is prone to dehalogenation reaction. And the position of
the substituent in the heteroaromatic ring does not seem to
have any influence in the process. We have been able to
1
994, 10, 1225.
. Yadav, J. S.; Reddy, B. V. S.; Reddy, M. M. Tetrahedron Lett.
000, 41, 2663.
10. Yoo, B. W.; Choi, J. W.; Yoon, C. M. Tetrahedron Lett. 2006, 47,
125.
1. Singh, S. K.; Reddy, M. S.; Mangle, M.; Ganesh, K. R. Tetrahedron
9
2
3 2
demonstrate the utility of the easily accessible CeCl ·7H O/
Zn system as a convenient reagent for effecting chemoselec-
tive deoxygenation of amine-N-oxides. Although the role of
cerium(III) chloride is still not clearly understood at this
time, it is assumed that cerium(III) chloride is involved in
complexation with the substrates. The reduction probably
proceeds by a reductive cleavage of polarized N-O bonds
through a single electron transfer from zinc metal to the
cerium-substrate complex due to the high oxophilic nature
of the cerium species. The reducing property exhibited by
metal-metal salt combinations proceeds through transfer of
one electron from the metal surface to the substrate. We
1
2
007, 63, 126.
2. Sanjay, K.; Anil, S.; Sandhu, J. S. Tetrahedron Lett. 2005, 46,
737.
1
8
13. Saini, A.; Kumar, S.; Sandhu, J. S. Synlett 2006, 395.
14. (a) Yoo, B. W.; Choi, K. H.; Lee, S. J.; Yoon, C. M.; Kim, S. H.;
Kim, J. H. Synth. Commun. 2002, 32, 63. (b) Yoo, B.W.; Lee, S.
J.; Yoo, B. S.; Choi, K. I.; Kim, J. H. Synth. Commun. 2002, 32,
2489. (c) Yoo, B. W.; Min, S. K. Synth. Commun. 2011, 41, 2993.
(d) Yoo, B. W.; Kim, D. I.; Kim, H. M.; Kim, S. H. Bull. Korean
Chem. Soc. 2012, 33, 2851. (e) Yoo, B. W.; Kim, S. H.; Min, G. H.
Bull. Korean Chem. Soc. 2012, 33, 27.
1
5. (a) Sarmah, B. K.; Barua, N. C. Tetrahedron 1991, 47, 8587. (b)
Bezbarua, M. S.; Bez, G.; Barua, N. C. Chem. Lett. 1999, 325. (c)
Wang, W. B.; Shi, L. L.; Huang, Y. Z. Tetrahedron Lett. 1990, 31,
3 2
believe that the present procedure using CeCl ·7H O/Zn
system proceeds through a SET (single electron transfer)
process. The notable advantages of the present method are
the mild reaction conditions, the high yields, the experi-
mental simplicity, and the tolerance of other labile functional
groups.
1185. (d) Barua, M.; Boruah, A.; Prajapati, D.; Sandhu, J. S.
Tetrahedron Lett. 1996, 37, 4559. (e) B. Rama, R.; Gitali, D.;
Yoofisaca, S. N.; Anil, K. S. Synlett 2005, 358.
1
6. (a) Molander, G. A. Chem. Rev. 1992, 92, 29. (b) Imamoto, T.
Lanthanides in Organic Synthesis, academic Press: London, 1994.
3 2
In conclusion, we have discovered that CeCl ·7H O/Zn
(c) Giuseppe, B.; Enrico, M.; Letizia, S. Synlett 2003, 2101.
system mediates a selective and efficient deoxygenation of
amine-N-oxides to the corresponding amines. Although the
scope and limitations have not been fully established, the
present procedure offers an attractive alternative to the conv-
entional methods with its mildness and chemoselectivity as
well as high yields. Further investigations of more useful
applications with this system are currently in progress.
1
7. A general procedure for the deoxygenation of amine-N-oxides is
as follows: Zinc powder (131 mg, 2.0 mmol), and cerium(III)
chloride heptahydrate (373 mg, 1.0 mmol) were mixed in
methanol (4 mL). The resulting mixture was sonicated for 0.5 h.
2-Chloropyridine-N-oxide (65 mg, 0.5 mmol) was added to this
solution and the reaction mixture was stirred for 0.5 h at room
temperature. After completion of the reaction, as indicated by
TLC, the mixture was diluted with water, extracted with ethyl
acetate. The combined organic extract was washed with brine,
2 4
dried over anhydrous Na SO , and concentrated under reduced
Acknowledgments. This work was financially supported
pressure. The crude product was purified by silica gel column
chromatography (hexane:ethyl acetate = 2:1) to afford 2-chloro-
pyridine (54 mg, 95%). All of the products were characterized by
comparison of their spectroscopic and physical data with reported
by Korea University.
References
19
values.
1
8. Katritzky, A. R.; Monro, A. M. J. Chem. Soc. 1958, 1263.
9. Dictionary of Organic Compounds, 4th ed.; Pollock, J. R. A.;
Stevens, R., Eds.; Eyre & Spottiswoode: London, 1965.
1
. Ochiai, E. In Aromatic Amine Oxides; Elsevier: Amsterdam, 1967;
pp 184-209.
1
2
. (a) Malinoswaki, M. Synthesis 1987, 732. (b) Balicki, R. Chem.
Ber. 1990, 647. (c) Balicki, R.; Kaczmarek, L.; Malinnowski, M.