7
786
F. A. Khan et al. / Tetrahedron Letters 44 (2003) 7783–7787
Table 4. Recycling ionic liquids: crossed experiments
Chem., Int. Ed. 2000, 39, 3772–3789; (c) Sheldon, R. A.
Chem. Commun. 2001, 2399–2407; (d) Dupont, J.; de-
Souza, R. F.; Suarez, P. A. Z. Chem. Rev. 2002, 102,
3667–3692.
2
3
. (a) Steines, S.; Drieben-Holscher, B.; Wasserscheid, P. J.
Prakt. Chem. 2000, 342, 348–354; (b) Dyson, P. J.; Ellis,
D. J.; Parker, D. G.; Welton, T. Chem. Commun. 1999,
Substrate (Ar)
Ph
Cycle
Time (h)
Yield (%)a
25–26.
. (a) Ansari, I. A.; Gr e´ e, R. Org. Lett. 2002, 4, 1507–1509;
1
2
3
7
8
7
91
95
89
(
(
b) Farmer, V.; Welton, T. Green Chem. 2002, 4, 97–102;
c) Ley, S. V.; Ramarao, C.; Smith, M. D. Chem. Com-
(
(
p-MeC H )
6 4
p-ClC H )
6
4
mun. 2001, 2278–2279.
a
4. (a) Earle, M. J.; McCormac, P. B.; Seddon, K. R. Green
Chem. 1999, 23–25; (b) Fischer, T.; Sethi, A.; Welton, T.;
Woolf, J. Tetahedron Lett. 1999, 40, 793–796.
Isolated yields of pure amines. No contamination of products was
observed for the experiments conducted.
a
Table 5. Reduction of azobenzene to hydrazobenzene
5. Boulaire, V. L.; Gr e´ e, R. Chem. Commun. 2000, 2195–
2
196.
. (a) Gordon, C. M.; Ritchie, C. Green Chem. 2002, 4,
24–128; (b) Law, M. C.; Wong, K. Y.; Chan, T. H.
6
1
Green Chem. 2002, 4, 161–164.
7
. For a singe report on Sm promoted reduction of aro-
matic nitro compounds in ionic liquid, see: Zheng, X. L.;
Zhang, Y. M. Chinese J. Chem. 2002, 20, 925–928.
. Tafesh, A. M.; Weiguny, J. Chem. Rev. 1996, 96, 2035–
8
9
2
052.
. (a) Martin, E. L. Org. Synth. 1943, Coll. Vol. II, 501–503;
b) Yu, C.; Liu, B.; Hu, L. J. Org. Chem. 2001, 66,
19–924; (c) Brady, E. D.; Clark, D. L.; Keogh, D. W.;
Scott, B. L.; Watkin, J. G. J. Am. Chem. Soc. 2002, 124,
(
9
7007–7015; (d) Moody, C. J.; Pitts, M. R. Synlett 1998,
1028.
1
1
0. Tsukinoki, T.; Tsuzuki, H. Green Chem. 2001, 3, 37–38.
1. 1-Butyl-3-methylimidazolium hexafluorophosphate,
bmim][PF6], and 1-butyl-3-methylimidazolium tetra-
[
fluoroborate, [bmim][BF ], were prepared following: (a)
4
Guernik, S.; Wolfson, A.; Herskowitz, M.; Greenspoon,
N.; Geresh, S. Chem. Commun. 2001, 2314–2315, (b)
Suarez, P. A. Z.; Dullius, J. E. L.; Einloft, S.; de-Souza,
R. F., Dupont, J. Polyhedron 1996, 15, 1217–1219.
12. Gowda, S.; Abiraj, K.; Gowda, D. C. Tetrahedron Lett.
2
002, 43, 1329–1331.
3. General procedure for reduction of nitroarenes in
bmim][PF ] (Method A): Nitroarene (0.5 mmol), zinc
1
[
6
metal (2 mmol), NH Cl (1.5 mmol) and water (0.1 ml)
4
were added to [bmim]PF (1 ml) and vigorously stirred at
6
room temperature for the specified time under an argon
atmosphere (Table 2). After consumption of the starting
Table 6. Recycling of [bmim][BF ] for the reduction of
4
azobenzene
material, as monitored by TLC, Et O (3×5 ml) was added
2
to the reaction mixture (a white suspension) with vigor-
ous stirring for 5 min. The mixture was allowed to stand
for 4 min and the clear supernatant liquid was decanted
Cycle
1
2
3
4
Yield (%)
98
94
100
96
(
through a filter paper). The combined ether extracts were
concentrated to dryness and the crude product was found
1
13
to be sufficiently pure (400 MHz H NMR, 100 MHz
C
Research (CSIR), New Delhi for financial assistance.
J.D., R.K.G. and C.S. thank CSIR, U.G.C. for
fellowships.
NMR) in most of the cases. Where necessary, the crude
product was purified by SiO column chromatography. In
2
cases (Table 2, entries 11–14), where the products were
not fully extracted with diethyl ether, the reaction mix-
ture was dissolved in CH Cl (EtOAc can also be used in
2
2
References
place of CH Cl ), filtered, concentrated and then passed
2 2
through a small silica gel column to afford the pure
1
. For recent reviews, see: (a) Welton, T. Chem. Rev. 1999,
amines first, in the specified yields (Table 2), followed by
the recovery of the ionic liquid (400 MHz H NMR).
1
9
9, 2071–2083; (b) Wasserscheid, P.; Keim, W. Angew.