Table 1 Asymmetric epoxidation of olefins catalysed by silica-bound chiral (salen)Mn(III) complexesa
Mn loading/
mmol g21
Abs.
confign.e
Run
Catalyst
Silica
Olefinb
Yield (%)c
Ee (%)d
1
2
3
4
5
6
7
8
9
10
11
12
13
(S,S)-7A
(S,S)-7A
(R,R)-7B
(R,R)-7B
(R,R)-7B
(R,R)-7B
(R,R)-5B
(R,R)-7B
(R,R)-7B
(R,R)-7B
(R,R)-7B
(R,R)-7B
(R,R)-7B
MCM-41
MCM-41
MCM-41
MCM-41
MCM-41
KG60
Homog.
MCM-41
MCM-41
MCM-41
KG60
0.3
0.1
0.1
W
W
W
W
W
W
W
X
Y
Z
Y
Z
40
31
49
38
36
42
71
70
48
68
55
73
43
26
34
82
84
84
82
89
30
45
70
43
65
81
1R, 2R
1R, 2R
1S, 2S
1S, 2S
1S, 2S
1S, 2S
1S, 2S
R
1R, 2S
1R, 2S
1R, 2S
1R, 2S
1S, 2S
0.08
0.06
0.06
—
0.06
0.06
0.06
0.1
KG60
KG60
0.1
0.1
W
a According to literature procedure (0 °C, 4 h). b Olefin employed: W = 1-phenylcyclohexene, X = styrene, Y = 1,2-dihydronaphthalene, Z = indene.
c Determined by GC after 4 h using PhBr as internal standard or by 1H NMR using (CHCl2)2 as internal standard. d Determined by GC using Chiraldex B
chiral column or by 1H NMR with chiral Eu(hfc)3. e Confirmed by polarimetry.
agreement with the expected chemical structure of the organic
moieties. In particular the formation of the supported complex
e.g. MCM-7A was confirmed by the shift from 1628 cm21 to a
lower wavenumber of the imine stretching vibration, that cannot
be resolved from the triazine band at 1600 cm21, and by a band
interesting results were obtained for catalyst 7B supported on
silica-gel. Indeed in the second cycle there was a modest
reduction of yield and ee (from 82 to 78%).
In summary, we have prepared a promising class of silica
supported catalysts for heterogeneous epoxidation of alkenes,
characterised by a new type of single linker bounding salen
ligands to a cheap solid support. The asymmetric induction is
good, being higher or comparable with results obtained using
MCPBA with similar catalysts supported on silica or an organic
polymer.2a,b,10b
This research was supported by MURST (National Project
“Stereoselezione in Chimica Organica. Metodologie e Applica-
zioni). We thank Professor R. Capelletti for help with the UV–
Vis measurements. CIM is acknowledged for the use of NMR
and MS instruments.
typical of metallosalen complexes at 1537 cm21 7
.
The electronic spectra of supported salens 6 presented the
same absorption bands as the corresponding free ligand in
methanolic solution (e.g. MCM-6B l = 228, 260, 336, 410 nm
and 5B l = 225, 258, 327 nm). As expected, after metal
complexation there is a d–d transition band at about 500 nm,
characteristic for Mn(III) in these complexes.8
Then we tested the activity of the prepared catalysts in the
model epoxidation reaction of 1-phenylcyclohex-1-ene with
MCPBA–NMO in CH2Cl2 at 0 °C and in the presence of 4
mol% of the catalyst. The data reported in Table 1 show that the
catalyst 7B is an efficient promoter of the enantioselective
reaction, giving the 1,2-epoxy-1-phenylcyclohexane in ee
higher than 82% (entries 3–5). In our opinion the lower
asymmetric induction of the catalyst 7A is mainly due to the
lack of a tert-butyl group at the 3-position, that is known to
strongly affect the enantioselectivity,1a although the variation in
the electronic properties could be important too.9
As a control experiment we investigated the catalytic
efficiency of the soluble analogue 5B of the supported catalyst
7B (entry 7). We could observe that this catalyst was only
slightly less effective than Jacobsen’s catalyst (yield 84%, ee
95%), probably due to the electron-withdrawing effect of the
linker.
Comparing entries 5 and 7 we can see that the heterogenisa-
tion of the catalyst did not cause a significant decrease of
asymmetric induction, a problem commonly reported in the
literature, while the yield was more affected.
The reaction was extended to different conjugated olefins
(entries 8–10) and moderate to high enantioselectivities were
obtained. The degree of chirality transfer depends on the olefin
structure, in agreement with the literature,2b,10 and the stereo-
chemistry of the epoxides is in accordance with the findings in
homogeneous catalysis, suggesting that the transition states are
quite similar. The lower yields could be attributed to diffusion
limitation usually present when a heterogeneous catalyst is
employed.11 The alkenes different reactivity could probably be
due to steric effects that play a role in heterogeneous
epoxidation.
The good results obtained with catalyst 7B supported on
amorphous silica KG60 indicate that the structure of the solid
support does not influence the activity of the catalyst with low
Mn-loading. Lastly we examined the problem of catalyst
recycling. Whereas the MCM-41-supported catalyst used three
times gave progressive reduction of yield and ee,12 as often
observed by other authors for similar supported catalysts,13,2a,c
Notes and references
1 (a) W. Zhang, J. L. Loebach, S. R. Wilson and E. N. Jacobsen, J. Am.
Chem. Soc., 1990, 112, 2801; (b) For a review: E. N. Jacobsen and M.
H. Wu in Comprehensive Asymmetric Catalysis, ed. E. N. Jacobsen, A.
Pfaltz and H. Yamamoto, Springer, Berlin, 1999, vol. II, 649.
2 Recent papers: (a) C. E. Song, E. J. Roh, B. M. Yu, D. Y. Chi, S. C. Kim
and K.-J. Lee, Chem. Commun., 2000, 615; (b) L. Canali, E. Cowan, H.
D. Deleuze, C. L. Gibson and D. C. Sherrington, J. Chem. Soc., Perkin
Trans. 1, 2000, 2055; (c) T. S. Reger and K. D. Janda, J. Am. Chem.
Soc., 2000, 122, 6929; (d) H. Sellner, J. K. Karjalainen and D. Seebach,
Chem. Eur. J., 2001, 7, 2873 ; and references therein.
3 P. Salvadori, D. Pini, A. Petri and A. Mandoli, in Chiral Catalyst
Immobilization and Recycling, ed. D. E. De Vos, I. F. J. Vankelecom
and P. A. Jacobs, Wiley-VCH, Weinheim, 2000.
4 F. Bigi, L. Chesini, R. Maggi and G. Sartori, J. Org. Chem., 1999, 64,
1033; G. Sartori, F. Bigi, R. Maggi, A. Mazzacani and G. Oppici, Eur.
J. Org. Chem., 2001, 2513.
5 (a) R. Menicagli, C. Malanga and P. Peluso, Synth. Commun., 1994, 24,
2153; (b) J. P. Mathias, C. T. Seto, E. E. Simanek and G. M. Whitesides,
J. Am. Chem. Soc., 1994, 116, 1725.
6 D. A. Annis and E. N. Jacobsen, J. Am. Chem. Soc., 1999, 121, 4147.
7 A. Domenech, P. Formentin, H. Garcia and M. J. Sabater, Eur. J. Inorg.
Chem., 2000, 1339.
8 S. Ogunwumi and T. Bein, Chem. Commun., 1997, 901.
9 E. N. Jacobsen, W. Zhang and M. L. Güler, J. Am. Chem. Soc., 1991,
113, 6703.
10 (a) M. J. Sabater, A. Corma, A. Domenech, V. Fornés and H. Garcia,
Chem. Commun., 1997, 1285; (b) D. Pini, A. Mandoli, S. Orlandi and P.
Salvadori, Tetrahedron: Asymmetry, 1999, 10, 3883.
11 The acid catalysed ring-opening of the epoxides was not observed in the
1H NMR spectra of the crude reactions. The by-products observed in the
reactions involving 1-phenylcyclohexene and 1,2-dihydronaphtalene
were biphenyl and naphthalene respectively.
12 The characterisation of the recovered MCM-41-supported catalyst 7B
evidenced a Mn loss of ca. 40%, the FTIR spectra showed a partial
degradation of the salen ligand, and elemental analyses suggested the
entrapment of reactants within the silica matrix.
13 M. D. Angelino and P. E. Laibinis, Macromolecules, 1998, 31, 7581.
CHEM. COMMUN., 2002, 716–717
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