effect can not be explained easily, it is apparent that the different
hydrophobic character of 2 depending on the anion plays a
decisive role in this reaction. Thus, we examined the ARO of
other epoxides in the hydrophobic ionic liquid 2a. All the yields
and enantiomeric excesses were quite comparable to those2a
obtained under homogeneous conditions (Table 1, entries 5 and
nature of the anion [X] in the ionic liquids [bmim][X] 2.
Detailed studies for the optimisation of this process and the
extension of this methodology to other ARO’s are currently in
progress.
This research was supported by a grant from the Korea
Institute of Science and Technology.
6). Although, as mentioned above, excellent results were
achieved using the ionic liquid 2a, the catalyst existed in a
suspended form in the ionic liquid 2a when hexane was added
to the reaction mixture after reaction. On the other hand,
although the reaction hardly occurred in the ionic liquids 2c and
Notes and references
† The ee values of products were determined by chiral GC: for 1-azido-
2
-(trimethylsiloxy)cyclopentane: Chrompak Chiralsil-dex CB, 95 °C iso-
thermal, 25.1 min (1R,2R), 27.3 min (1S,2S). For 1-azido-2-(trimethyl-
siloxy)-cyclohexane; Chrompak Chiralsil-dex CB, 110 °C isothermal, 24.3
min (1R,2R), 27.1 min (1S,2S). For 3-azido-4-(trimethylsiloxy)tetrahy-
2d, the catalyst was immobilised more efficiently in these
solvents after reaction than in 2a, and thus formed a clear red–
brown solution phase, which can make its separation from the
hexane phase more easy. We expected that a system combining
the hydrophobic and hydrophilic ionic liquids might provide
beneficial effects on the catalyst immobilisation. As we
expected, in the mixture of 2a and 2d with a volume ratio of
21
drofuran; Chrompak Chiralsil-dex CB, 75 °C for 10 min, then 2 °C min
,
3
4.4 min (3R,4S), 35.2 min (3S,4R).
1
E. N. Jacobsen and M. H. Wu, Ring Opening of Epoxides and Related
Reactions, in Comprehensive Asymmetric Catalysis III, ed. E. N.
Jacobsen, A. Pfaltz and H. Yamamoto, Springer-Verlag, Berlin-
Heidelberg-New York, 1999, p. 1309.
5
+1, the reaction proceeded with comparable yield and
2a
enantiomeric excess to those obtained under homogeneous
conditions (Table 2, entry 1). Moreover, the catalyst could be
much better immobilised in this mixture after reaction than in
the ionic liquid 2a alone, and thus the ionic liquid phase
containing the catalyst was almost quantitatively recovered
from the hexane phase.8 The recovered ionic liquid phase
containing the catalyst was reused several times without any
loss of activity and enantioselectivity even after the fifth use
2
(a) L. E. Martínez, J. L. Leighton, D. H. Carsten and E. N. Jacobsen,
J. Am. Chem. Soc., 1995, 117, 5897; (b) J. F. Larrow, S. E. Schaus and
E. N. Jacobsen, J. Am. Chem. Soc., 1996, 118, 7420; (c) H. Lebel and E.
N. Jacobsen, J. Org. Chem., 1998, 63, 9624; (d) S. E. Schaus, J. F. Larrow
and E. N. Jacobsen, J. Org. Chem., 1997, 62, 4197; (e) M. H. Wu and E.
N. Jacobsen, Tetrahedron Lett., 1997, 38, 1693; (f) J. L. Leighton and E.
N. Jacobsen, J. Org. Chem., 1996, 61, 389; (g) L. E. Martínez, W. A.
Nugent and E. N. Jacobsen, J. Org. Chem., 1996, 61, 7963.
(
Table 2).
3 (a) B. Pugin and H.-U. Blaser, Catalyst Immobilization: Solid Supports,
in Comprehensive Asymmetric Catalysis III, ed. E. N. Jacobsen, A. Pfaltz
and H. Yamamoto, Springer-Verlag, Berlin-Heidelberg-New York,
Table 2 Enantioselective ring opening of cyclopentene oxide with catalyst
recycling using a mixture of ionic liquids 2a and 2d
a
1
999, p. 1367; (b) G. Oehme, Catalyst Immobilization: Two-Phase
System, in Comprehensive Asymmetric Catalysis III, ed. E. N. Jacobsen,
A. Pfaltz and H. Yamamoto, Springer-Verlag, Berlin-Heidelberg-New
York, 1999, p. 1377.
4
5
(a) T. Welton, Chem. Rev., 1999, 99, 2071; (b) K. R. Seddon, J. Chem.
Tech. Biotechnol., 1997, 68, 351; (c) Y. Chauvin and H. Olivier,
CHEMTECH, 1995, 26.
(a) A. Stark, B. L. MacLean and R. D. Singer, J. Chem. Soc., Dalton
Trans., 1999, 63; (b) T. Fischer, A. Sethi, T. Welton and J. Woolf,
Tetrahedron Lett., 1999, 40, 793; (c) B. Ellis, W. Keim and P.
Wasserscheid, Chem. Commun., 1999, 337; (d) W. A. Herrmann and
V. P. W. Bohm, J. Organomet. Chem., 1999, 572, 141; (e) W. Keim, D.
Vogt, H. Waffenschmidt and P. Wasserscheid, J. Catal., 1999, 186, 481;
(f) W. Chen, L. Xu, C. Chatterton and J. Xiao, Chem. Commun., 1999,
1
247; (g) C. E. Song and E. J. Roh, Chem. Commun., 2000, 843.
6
Preparation of ionic liquids: (a) For 2a and 2c: P. A. Z. Suarez, J. E. L.
Dullius, S. Einloft, R. F. de Souza and J. Dupont, Polyhedron, 1996, 15,
1
217; (b) The synthesis of 2b was similar to that of 2a and 2c with the
exception that NaSbF was used in place of NaPF or NaBF ; (c) For 2d:
6
6
4
In summary, we have developed a new and highly practical
recycling procedure of Cr(salen) catalyst by using the air and
moisture stable ionic liquids 2. This procedure does not include
hazardous work-up stages such as distillation of the azide
product, and moreover, provides not only simple recycling of
catalyst but also the additional advantage that the catalyst can be
used without any modification of the structure. The catalytic
activity and enantioselectivity were strongly dependent on the
P. Bonhôte, A.-P. Dias, N. Papageorgiou, K. Kalyanasundaram and M.
Grätzel, Inorg. Chem., 1996, 35, 1168. All ionic liquids used in this paper
were pre-dried under reduced pressure (0.5 mmHg) at 50 °C for 24 h.
The recovered catalyst displayed a strong IR absorbance at 2058 cm21;
see ref. 2(a).
The Cr(salen) catalyst 1 is slightly soluble in hexane. However, in this
case, the amount of catalyst dissolved in the hexane phase is
negligible.
7
8
1744
Chem. Commun., 2000, 1743–1744