Table 1 Epoxidation of olefins by Mn1 and the reference catalysts MnTPP and MnTMPP
Mn1a
MnTPPa
MnTMPPa
Substrate
Axial ligand Yieldb
Ratec
c+td
Yieldb
Ratec
c+td
Yieldb
Ratec
c+td
f
f
f
a-pinene
Pye
81
57
72
82
57
72
12.0
19.9g
18.9
10.9
15.5g
24.1
—
10
39
9
80
70
63
1.2
3.7g
3.8
12.9
57.0g
21.2
—
—
—
—
cis-stilbene
trans-stilbene
a-pinene
Pye
96+4
65+35
33
8
—
2.9
63+37
Pye
< 1.0
h
h
h
f
f
f
Bupyi
Bupyi
Bupyi
—
—
—
—
cis-stilbene
trans-stilbene
90+10
90+10
52
65
39.3
20.0
92+8
h
h
h
a Standard reaction conditions.† b Yield (%) after 3 h. c Initial rate 3 105 mol dm23 s21
.
d Ratio cis–trans epoxide product after 3 h. e 1 equiv. per Mn(III)-
catalyst. f Not determined. g Rate of formation of the cis-epoxide. h No cis-epoxide was detected. i 500 equiv. per Mn-catalyst.
substrate. Remarkably, the rate of epoxidation of cis-stilbene
catalysed by the Mn1–Bupy system (epoxidation within the
cavity) is similar to that catalysed by the Mn1–Py system
(epoxidation outside the cavity) (Table 1). This implies that
additional factors play a role in the epoxidation reaction. Further
studies are currently under investigation.
In summary, we have shown that by means of a unique
supramolecular activation of Mn1 only one equiv. of the axial
ligands pyridine or imidazole are required to activate the
catalyst for the epoxidation of olefins. Coordination of a bulky
axial ligand on the outside of the cavity of Mn1 strongly
enhances the stability of the catalyst, which in this approach
is protected from further oxidative decomposition. Current
research is focused on the functionalisation of Mn(III) porphyr-
ins with molecular clip receptors on both faces, so that both
approaches of supramolecular activation and catalyst protection
are combined.
Fig. 2 Epoxidation of a-pinene using 1 equivalent of an axial ligand.
MnTPP–Py (-), Mn1–Py (5), Mn1–Im (:).
amount of unreacted olefin decreases. This phenomenon has
been attributed to the formation of m-oxo-bridged Mn(IV
)
Notes and references
porphyrin dimeric structures, which are unreactive in further
catalysis and rapidly decompose.9 This rapid decomposition
was also found to occur in the case of the Mn1–Py system used
in approach A. To prevent this, the bulky axial ligand 4-tert-
butylpyridine (Bupy) was used to coordinate to Mn1 on the
outside of the cavity (Fig. 1, approach B). It was expected that
coordination of this ligand, which does not fit within the
cavity,‡ would efficiently prevent m-oxo dimer formation since
the other face of the porphyrin is protected by the receptor
cavity. When the epoxidation reactions using Mn1 were carried
out in the presence of 500 equiv. of Bupy, catalyst destruction
was indeed prevented, as was concluded from the fact that the
organic layer retained its brown colour, and, more importantly,
that newly added amounts of substrate were epoxidised. The
epoxidation experiment was repeated sequentially several
times. The observed initial rate was found in all cases to be
almost identical, indicating that no decomposition of the
catalyst occurred. This gave turnover numbers of > 1000 per
catalyst.§ When MnTPP or MnTMPP were used as the
catalyst in combination with Bupy, this stabilization did not
occur and the catalysts decomposed.
Assuming complete shielding of the outside of Mn1 by Bupy
implies that the oxygen transfer to the substrate has to occur
within the cavity. From molecular modelling studies it became
clear that the substrates cannot react via the side of the cavity,
i.e. they need to enter the cavity completely to reach the
manganese–oxo species. The epoxidation results of cis- and
trans-stilbene using the Mn1–Bupy system are summarised in
Table 1 and compared to those when MnTPP or MnTMPP are
used as the catalyst. Whilst for the epoxidation of trans-stilbene
not much difference is observed between the catalysts with
regard to initial rate and epoxide yield, the epoxidation rate of
cis-stilbene clearly decreases going from MnTPP to MnTMPP
to Mn1. This decrease in rate coincides with a simultaneous
increase in steric hindrance by the substituents of the meso-
phenyl rings. Apparently, these groups have more steric
influence on a cis-stilbene substrate than on a trans-stilbene
† Reaction conditions: to a CH2Cl2 solution (0.65 ml) of the substrate (0.626
M), the manganese catalyst (2.5 mM), the phase transfer catalyst
tetrabutylammonium chloride (5 mM), the axial ligand pyridine (2.5 mM)
or 4-tert-butyl pyridine (1.25 M), and an internal standard (1,3,5-tri-tert-
butylbenzene (0.17 M) in a Schlenk tube was added an aqueous NaOCl
solution (2 ml, 0.6 M). The mixture was stirred at a constant rate under
nitrogen for 3 h, and during the course of the reaction samples were taken
from the organic layer which were analysed by GLC and 1H NMR.
‡ This was concluded from 1H NMR experiments on mixtures of Zn1 and
Bupy in CDCl3, which indicated no binding within the cavity of the host
even when 500 equiv. of the axial ligand were added.
§ More than 4 portions of substrate could be oxidised without any
decomposition of the catalyst. Due to phase separation between the solvent
and the epoxidation products it then became more difficult to measure a
reliable rate of conversion when the number of portions were increased
beyond 4.
1 J. T. Groves and Y. Z. Han, Cytochrome P450: Structure, Mechanism
and Biochemistry, ed. P. R. Oritz de Montellano, Plenum Press, New
York, 1995, 2nd edn., pp. 3–48.
2 For reviews see: B. Meunier, Chem Rev., 1992, 92, 1411; J. P. Collman
and L. Fu, Acc. Chem. Res., 1999, 32, 455.
3 For a review see: J. P. Collman, X. Zhang, V. J. Lee, E. S. Uffelman and
J. I. Brauman, Science, 1993, 261, 1404.
4 B. Meunier, M. E. de Carvalho, O. Bortolini and M. Momenteau, Inorg.
Chem., 1988, 27, 161.
5 R. Breslow, Y. Huang and X. Zhang, J. Am. Chem. Soc., 1997, 119,
4535.
6 D. R. Benson, R. Valentekovich, S.-W. Tam and F. Diederich, Helv.
Chim. Acta, 1993, 76, 2034.
7 For a review see: A. E. Rowan, J. A. A. W. Elemans and R. J. M. Nolte,
Acc. Chem. Res., 1999, 32, 995.
8 A. E. Rowan, P. P. M. Aarts and K. W. M. Koutstaal, Chem. Commun.,
1998, 611; J. A. A. W. Elemans, M. B. Claase, P. P. M. Aarts, A. E.
Rowan, A. P. H. J. Schenning and R. J. M. Nolte, J. Org. Chem., 1999,
64, 7009.
9 A. W. van der Made, R. J. M. Nolte and W. Drenth, Recl. Trav. Chim.
Pays-Bas., 1990, 109, 537.
10 B. Meunier, E. Guilmet, M.-E. de Carvalho and R. Poilblanc, J. Am.
Chem. Soc., 1984, 106, 6668.
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Chem. Commun., 2000, 2443–2444