Table 1 Asymmetric epoxidation of alkenes catalysed by polymer-bound
chiral (salen)MnIII complex 2 or its homogeneous analogue 1aa
conditions (to minimise catalyst decomposition) are currently in
progress.
This research was supported by a grant from Ministry of
Science and Technology in Korea.
Notes and references
† Asymmetric epoxidation using polymer-bound catalyst 2.
Using NaOCl as the oxidant: resin beads of 2 (202 mg) were stirred in
CH2Cl2 (2 mL) for 1 h. Alkene (0.54 mmol) and 4-phenylpyridine N-oxide
(18.5 mg, 0.108 mmol) were then added and the mixture was cooled to 0 °C.
To this mixture buffered NaOCl (0.81 mmol, pH = 11.3) was added and the
reaction was stirred at 0 °C. At the end of the reaction, the suspension was
filtered off and resin beads were washed with CH2Cl2. The filtrate was
washed with brine and dried (MgSO4). After removal of the solvent under
reduced pressure, the residue was purified by column chromatography.
Using MCPBA as the oxidant: resin beads of 2 (202 mg) were poured into
a solution containing 2.7 mmol (316 mg) of N-methylmorpholine N-oxide
dissolved in CH2Cl2 (4 mL). Alkene (0.54 mmol) was then added and the
mixture was cooled to 278 °C. Solid MCPBA (1.08 mmol, 186 mg) was
then added as a solid in four roughly equal portions over a 2 min period. The
reaction mixture was stirred at 278 °C and the progress of the reaction was
monitored by TLC. At the end of the reaction, the suspension was filtered
off and resin beads were washed with CH2Cl2. The filtrate was washed with
1 M NaOH, brine and dried. After removal of the solvent under reduced
pressure, the residue was purified by column chromatography.
The ee values of products were determined by chiral HPLC or chiral GC:
For 3,4-epoxy-2,2-dimethylchroman: Daicel Chiralpak AD, propan-2-ol–
hexane (5+95), 0.8 mL min21; 9.30 min (3R,4R), 10.63 min (3S,4S); for
3,4-epoxy-6-cyano-2,2-dimethylchroman: Daicel Chiralcel OJ, propan-
2-ol–hexane (30+70), 1 mL min21; 13.95 min (3R,4R), 26.88 min (3S,4S);
for 1-phenylcyclohexene oxide: see ref. 7(b).
1 E. N. Jacobsen and M. H. Wu, Epoxidation of Alkenes Other than Allylic
Alcohols, in Comprehensive Asymmetric Catalysis II, ed. E. N. Jacobsen,
A. Pfaltz and H. Yamamoto, Springer Verlag, Berlin–Heidelberg–New
York, 1999, p. 649.
2 (a) B. B. De, B. B. Lohray and P. K. Dhal, Tetrahedron Lett., 1993, 34,
2371; (b) B. B. De, B. B. Lohray, S. Sivaram and P. K. Dhal,
Macromolecules, 1994, 27, 2191; (c) B. B. De, B. B. Lohray, S. Sivaram
and P. K. Dhal, Tetrahedron: Asymmetry, 1995, 6, 2105; (d) B. B. De,
B. B. Lohray, S. Sivaram and P. K. Dhal, J. Polym. Sci., Polym. Chem.
Ed., 1997, 35, 1809; (e) F. Minutolo, D. Pini and P. Savadori,
Tetrahedron: Asymmetry, 1996, 7, 2293; (f) F. Minutolo, D. Pini, A. Petri
and P. Savadori, Tetrahedron Lett., 1996, 37, 3375; (g) L. Canali, E.
Cowan, H. Deleuze, C. L. Gibson and D. C. Sherrington, Chem.
Commun., 1998, 2561; (h) M. D. Angelino and P. E. Laibinis,
Macromolecules, 1998, 31, 7581; (i) M. D. Angelino and P. E. Laibinis,
J. Polym. Sci. Part A. Polym. Chem., 1999, 37, 3888; (j) G.-J. Kim and
J.-H. Shin, Tetrahedron Lett., 1999, 40, 6827; (k) D. Pini, A. Mandoli, S.
Orlandi and P. Salvadori, Tetrahedron: Asymmetry, 1999, 10, 3883.
3 Several other efforts to immobilize (salen)Mn-based epoxidation cata-
lysts have been described: M. J. Sabater, A. Corma, A. Domenech, V.
Fornés and H. García, Chem. Commun., 1997, 1285; S. B. Ogunwumi and
T. Bein, Chem. Commun., 1997, 901; L. Frunza, H. Kosslick, H.
Ladmesser, E. Hoft and R. Fricke, J. Mol. Catal. A: Chem., 1997, 123,
179; I. F. J. Vankelecom, D. Tas, R. F. Parton, V. Van de Vyver and
P. A. Jacobs, Angew. Chem., Int. Ed. Engl., 1996, 35, 1346; K. B. M.
Janssen, I. Laquiere, W. Dehaen, R. F. Parton, I. F. J. Vankelecom and
P. A. Jacobs, Tetrahedron: Asymmetry, 1997, 8, 3481; G. Pozzi, F.
Cinato, F. Montanari and S. Quici, Chem. Commun., 1998, 877.
4 K. Srinvasan, P. Michaud and J. K. Kochi, J. Am. Chem. Soc., 1986, 108,
2309.
which was carried out under two-phase conditions, required 24
h for a comparable degree of conversion and gave the epoxide
with 87% ee (entry 3). The polymeric catalyst 2 could be simply
recovered by filtration so enabling catalyst recycling. However,
the polymer-bound catalyst 2 underwent partial decomposition
under epoxidation condition. The initially dark brown resin was
more or less decolourised after the reaction. When MCPBA was
used as oxidant, much more severe decolourisation was
observed than when NaOCl was used. This result is in accord
with results of Skarzewski et al., who also observed a fairly high
resistance to degradation when NaOCl is used as an oxidant for
salen complexes with tert-butyl groups at the 5- and 5A-
positions.8 This tendency of salen manganese catalysts to
undergo decomposition under epoxidation conditions could
limit successful recycling of immobilised catalyst.9
In summary, we have prepared a promising class of polymer-
bound catalyst for heterogeneous asymmetric epoxidation of
alkenes. Our results showed that polymeric catalyst 2 showed a
comparable degree of enantioselectivity (92% ee for 2,2-dime-
thylchromene) in asymmetric epoxidation of alkenes to that
obtained with its homogeneous analogue 1a. However, the
activity and enantioselectivity of the polymeric catalyst 2 are
highly dependent on epoxidation conditions, and, moreover, the
degree of decomposition of catalyst also seems to be highly
influenced by oxidation conditions. Therefore, to carry out
heterogeneous asymmetric epoxidation successfully, not only
the design of supported catalysts, but also the selection of
appropriate oxidation conditions are very important. More
detailed studies concerning the optimisation of both catalyst
structure (to increase activity and enantioselectivity by modify-
ing the spacer length and resin morphology, etc.) and reaction
5 NovaSyn® TG amino resin LL (0.29 mmol g21) was purchased from
Calbiochem-Nova Biochem Japan Ltd.
6 R. G. Konsler, J. Karl and E. N. Jacobsen, J. Am. Chem. Soc., 1998, 120,
10780.
7 (a) For NaOCl: B. D. Brandes and E. N. Jacobsen, J. Org. Chem., 1994,
59, 4378; (b) for MCPBA: M. Palucki, G. J. McCormick and E. N.
Jacobsen, Tetrahedron Lett., 1995, 36, 5457.
8 J. Skarzewski, G. Gupta and A. Vogt, J. Mol. Catal. A: Chem., 1995, 103,
L63.
9 Very recently, the degradation of Mn-loaded polymeric salen ligands
under epoxidation conditions was also noted by Angelino et al. [see ref.
2(i)].
Communication b000876l
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Chem. Commun., 2000, 615–616