Dimeric chiral Mn(III) Schiff base complex-catalysed enantioselective epoxidation of non-functionalised alkenes

Article abstract of DOI:10.1016/S0040-4039(01)00314-8

Dimeric chiral Mn(III) Schiff base complex 1 has been investigated as a catalyst for enantioselective epoxidation of chromenes, indene and styrene with an objective to explore its efficiency and recycling capability. Excellent conversions were obtained wi

Full text of DOI:10.1016/S0040-4039(01)00314-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2915–2918
Dimeric chiral Mn(III) Schiff base complex-catalysed
enantioselective epoxidation of non-functionalised alkenes
Rukhsana I. Kureshy,* Noor-ul H. Khan, Sayed H. R. Abdi, Sunil T. Patel and Raksh V. Jasra
Silicates and Catalysis Discipline, Central Salt and Marine Chemicals Research Institute, Bhavnagar 364 002, India
Received 27 September 2000; revised 5 December 2000; accepted 21 February 2001
Abstract—Dimeric chiral Mn(III) Schiff base complex 1 has been investigated as a catalyst for enantioselective epoxidation of
chromenes, indene and styrene with an objective to explore its efficiency and recycling capability. Excellent conversions were
1
obtained with all alkenes. More than 99% chiral induction, as determined by H NMR using the chiral shift reagent Eu(hfc)3, was
obtained in the case of electron deficient chromenes, viz. nitro and cyano chromene. © 2001 Elsevier Science Ltd. All rights
reserved.
Simple synthesis, ease of handling, high activity and
selectivity are some of the advantages associated with
Jacobsen’s Mn SALEN complexes reported1 for the
epoxidation of a wide range of non-functionalised cis
olefins. While adequate enantioselective epoxidation is
achieved with these catalysts under homogeneous
biphasic conditions, separation and recycling of the
catalyst is still a problem as it limits its application
for a batch process. Therefore, various approaches,
viz. anchoring the catalyst on a solid support, or a
polymer chain,3–5 encapsulation in a zeolite,6 ‘van der
Waals wrapping’ in the elastomer network of a poly-
dimethylsiloxane membrane,7–9 use of a two-phase
system,2b and using ionic liquids10 have all been
reported to overcome this problem. All of these
approaches are interesting but demand additional
modifications to the structure of the catalyst or
require the reaction to be carried out in the presence
of some reagent which often results in loss of activity
and selectivity.
Scheme 1.
Keywords: dimeric; chiral; non-functionalized alkenes; manganese.
* Corresponding author. Fax: +91-0278-566970; e-mail: salt@csir.res.in
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00314-8

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R. I. Kureshy et al. / Tetrahedron Letters 42 (2001) 2915–2918
An interesting dimeric form of Jacobsen’s catalyst was
recently reported11 for its improved retention in a poly-
dimethylsiloxane membrane. In the present report, a
similar dimeric dissymmetric Mn(III) complex 1 was
synthesized (Scheme 1) by modifying a reported proce-
dure.11 1R,2R (−) Diamino cyclohexane, obtained from
its mono tartrate salt, was condensed with 3,5-di-tert-
butylsalicylaldehyde in a 2:1 molar ratio in CHCl3 at
0°C to give a 1:1 condensed product which was reacted
with 5,5%-methylene-di-3-tert-butylsalicylaldehyde to
obtain a dimeric ligand. This dimeric ligand was com-
plexed with manganese(II) acetate which after work-up
gave the desired complex 5,5-methylene di-[(R,R)-{N-
(3 - tert - butylsalicylidine) - N% - (3%,5% - di - tert - butylsali-
cylidine)}-1,2-cyclohexanediaminato (2-) manganese(III)
chloride]12 1 in quantitative yield. The complex 1 was
used as a catalyst in the enantioselective epoxidation of
chromenes, indene and styrene using pyridine N-oxide
as a proximal ligand and sodium hypochlorite as an
oxidant. The complex 1 has a tendency to precipitate
from hexane and thus could be easily separated from
the products and reused up to five times.
2.75 mmol of NaOCl as the oxidant. Various substrates
were studied and include: 2,2-dimethylchromene (entry
1), 6-cyano-2,2-dimethylchromene (entry 2), 6-nitro-2,2-
dimethylchromene (entry 3), 6-methoxy-2,2-dimethyl-
chromene (entry 4), spiro[cyclohexane-1,2%-[2H][1]-
chromene (entry 5), styrene (entry 6) and indene (entry
7). Reaction conversions and enantiomeric excess val-
ues are given in Table 1.
Excellent conversions were obtained with all the alkenes
but the highest chiral induction (>99%) (as determined
1
by H NMR using the chiral shift reagent (+)Eu(hfc)3)
was observed for nitro and cyano chromene (entry 2
and 3). However, the ee’s were not encouraging in the
case of styrene (entry 6).
The monomeric Jacobsen complex1 with 2 mol% cata-
lyst loading (with 4-phenyl pyridine N-oxide) gave 96%
conversion with 97% ee’s for cyano chromene in 9 h.
However, the dimeric complex 1 takes only 2.5 h with
100% conversion and >99% ee’s under similar reaction
conditions even with simple pyridine N-oxide. More-
over, a catalyst loading of 0.6 mol% is sufficient to
achieve similar conversion and selectivity within 6 h.
Further reduction in catalyst loading (0.2 mol%) caused
a reduction in reaction rate with marginal decrease in
ee’s value in 24 h. Below 0.2 mol% of catalyst loading,
the reaction rate and selectivity deteriorate rapidly. The
Asymmetric epoxidation reactions were performed fol-
lowing an established procedure.1 Typically, the reac-
tion was conducted at 0°C using 2 mol% of complex 1
and 1.29 mmol of substrate in 1 ml of dichloromethane
in the presence of 0.13 mmol of pyridine N-oxide and
Table 1. Data for enantioselective epoxidation of non-functionalised alkenes catalysed by dimeric Mn(III) chiral Schiff base
complex 1 in presence of pyridine N-oxide

R. I. Kureshy et al. / Tetrahedron Letters 42 (2001) 2915–2918
2917
Table 2. Data for the enantioselective epoxidation of styrene with catalyst 1 using pyridine N-oxide and substituted pyridine
N-oxides as an axial base
Axial base
Conversion (%)
Time (h)
ee’s (%)
PyNO
4-PhPyNO
4-PPPyNO
100
100
100
3
1.5
0.5
35
35
35
Table 3. Data for enantioselective epoxidation of styrene
with recycled catalyst in presence of pyridine N-oxide
enhanced activity of dimeric catalyst 1 indicates that
the two units are not working in isolation but have a
cooperative interaction. It is reported13 that the
monomeric catalyst with appropriate N-oxide deriva-
tives as additive serves to activate and stabilize the
catalyst system. Analogously, for dimeric catalyst 1 the
epoxidation of styrene was observed to be faster in the
presence of the 4-phenyl pyridine N-oxide (4-PhPyNO)
and 4-(3-phenyl propyl) pyridine N-oxide (4-PPPyNO)
as compared to pyridine N-oxide. However, no im-
provement in chiral induction was observed (Table 2).
Run
1
2
3
4
5
Conversion (%)
ee’s (%)
Time (h)
100
35
3
90
35
3
80
35
5
60
35
6
55
35
9
Acknowledgements
Furthermore, after one epoxidation cycle the catalyst
was separated from the reaction mixture by precipita-
tion using hexane. The recovered catalyst was reused
for catalytic reactions without further purification. The
results with recycled catalyst are shown in Table 3.
R.I.K. and N.H.K. are thankful to DST and TWAS for
financial assistance and also thankful to Dr. P. K.
Ghosh, Director, of the Institute for providing instru-
mentation facility.
The activity of the recycled catalyst gradually decreased
upon successive use, indicating some degradation of the
catalyst 1 under the epoxidation conditions. However,
no loss was observed in the product ee’s.
References
1. (a) Jacobson, E. N.; Zhang, W.; Guler, M. L. J. Am.
Chem. Soc. 1991, 113, 6703–6704; (b) Jacobson, E. N.;
Zhang, W.; Muci, A. R.; Ecker, J. R.; Deng, L. J. Am.
Chem. Soc. 1991, 113, 7063–7064; (c) Zhang, W.; Jacob-
sen, E. N. J. Org. Chem. 1991, 56, 2296–2298.
2. (a) Pugin, B.; Blaser, H.-U. In Catalyst Immobilization
Solid Support in Comprehensive Asymmetric Catalysis III;
Jacobsen, E. N.; Pfaltz, A.; Yamamoto, H., Eds.;
Springer: New York, 1999; p. 1367; (b) Ochme, G. In
Catalyst Immobilization; Two Phase System, in Compre-
hensive Asymmetric Catalysis III; Jacobsen, E. N.; Pfaltz,
A.; Yamamoto, H., Eds.; Springer-Vertag: New York,
1999; p. 1378.
3. (a) Brandes, B. D.; Jacobsen, E. N. J. Org. Chem.
1994, 59, 4378–4380; (b) Jacobsen, E. N.; Deng, L.;
Furukawa, Y.; Martinez, L. E. Tetrahedron 1994, 50,
4323–4334.
4. (a) De, B. B.; Lohray, B. B.; Sivaram, S.; Dhal, P. K.
Tetrahedron: Asymmetry 1995, 6, 2105–2108; (b) De, B.
In conclusion, the dimeric dissymmetric chiral Mn(III)
Schiff base complex worked very well with all non-
functionalised alkenes and the best chiral induction was
obtained with nitro and cyano chromene. Higher rates
of reaction were observed in the presence of substituted
pyridine N-oxides. However, the activity of the recycled
catalyst gradually decreased upon successive use,
showing the degradation of the catalyst 1 under epoxi-
dation conditions. As the catalyst loading could be
reduced to more than half without adversely effecting
activity and selectivity, the two units in the dimer seem
to be working with cooperativity. Further, the symme-
try of individual SALEN units is affected due to the
absence of a tertiary butyl group at the point of connec-
tivity. However, in spite of this, the catalyst still works
well.

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R. I. Kureshy et al. / Tetrahedron Letters 42 (2001) 2915–2918
B.; Lohray, B. B.; Sivaram, S.; Dhal, P. K. J. Poly. Sci.
9. Katsuki, T. J. Mol. Cat. A: Chem. 1996, 113, 87–107.
10. Song, C. E.; Roh, E. J. Chem. Commun. 2000, 837–838.
11. Janssen, K. B. M.; Laquiere, I.; Dehaen, W.; Parton, R.
F.; Vankelecom, I. F. J.; Jacobs, P. A. Tetrahedron:
Asymmetry 1997, 8, 3481–3487.
12. Spectroscopic data for the complex 1. IR (KBr): 3431
(br), 2947 (s), 2866 (s), 1612 (s), 1538 (s), 1475 (sh), 1435
(s), 1388 (m), 1342 (s), 1309 (s), 1285 (sh), 1238 (sh), 1201
(m), 1170 (m), 1100 (w), 1030 (m), 940 (w), 833 (m), 780
(w), 731 (w), 690 (w), 568 (s), 475 (w). UV–vis: (CH2Cl2)
umax (m) 250 (50000), 266 (49960), 345 (47540), 420
(41460), 451 (41040), 510 (36060)
A: Polym. Chem. 1997, 35, 1809–1818; (c) De, B. B.;
Lohray, B. B.; Dhal, P. K. Tetrahedron Lett. 1993, 34,
2371–2374.
5. (a) Minutolo, F.; Pini, D.; Salvadori, P. Tetrahedron Lett.
1996, 37, 3375–3378; (b) Minutolo, F.; Pini, D.; Petri, A.;
Salvadori, P. Tetrahedron: Asymmetry 1996, 7, 2293–
2302.
6. Ogunwumi, S. B.; Bein, T. Chem. Commun. 1997, 901–
902.
7. Vankelecom, I. F. J.; Tas, D.; Parton, R. F.; Vyver, V.
Van de; Jacobs, P. A. Angew. Chem., Int. Ed. Engl. 1996,
35, 1346–1348.
8. Parton, R. F.; Vankelecom, I. F. J.; Tas, D.; Janssen, K.
B. M.; Knops-Gerrits, P.-P.; Jacobs, P. A. J. Mol. Catal.
A: Chem. 1996, 113, 283–292.
13. Senanayke, C. H.; Smith, G. B.; Fredenburgh, L. E.; Liu,
J.; Ryan, K. M.; Roberts, F. E.; Larsen, R. D.; Verho-
even, T. R.; Reider, P. J. Tetrahedron Lett. 1996, 37,
3271–3274.
.