Table 2 Epoxidation of internal alkenes catalysed by MTO in tri-
fluoroethanol
At a substrate to catalyst ratio of 3000 the catalyst decomposed
and only 90% conversion could be reached. These high
turnovers require very careful addition of the alkene. A slightly
faster addition of the alkene than the optimum addition rate
causes the reaction mixture to warm up, resulting in complete
loss of catalytic activity.
One disadvantage of the present catalytic system is the high
polarity of the reaction medium. Very apolar alkenes (C12 or
higher alkenes and stilbenes) do not dissolve in the reaction
medium, and are therefore not epoxidised. In other cases (the
experiments of Table 1 and 2) the alkene does not dissolve
completely at the beginning of the reaction. During the course of
the reaction the medium becomes homogeneous due to the
formation of polar, soluble epoxide. The time required for a
homogeneous reaction mixture depends on the polarity of the
alkene and ranges from 1 h for hex-1-ene to 6 h for dec-1-ene.
The more reactive internal alkenes give a homogeneous reaction
mixture within minutes after the addition.
a
Alkene
t/h
Yield (%)
Cyclohexene
Cycloheptene
Cyclooctene
0.5
1
1
0.5
1
1
> 99
> 99
> 99
> 99
99
1
-Methylcyclohexene
Methylenecyclohexane
Indene
b
65
2
2
-Methylhept-1-ene
-Methylhept-2-ene
1.5
1
98
99
a
Conditions:1 mmol Bu
2
O (internal standard), 0.010 mmol MeReO
3
(0.1
2 2
mol%), 1 mmol pyrazole and 20 mmol 60% H O
were dissolved in 5 ml
trifluoroethanol. The stirred mixture was cooled in an ice bath to ca. 5 °C.
The alkene (10 mmol) was then added dropwise in 20–30 min. The mixture
b
was warmed to room temperature and analysed by GC. Several hydrolysis
products were also detected.
In summary, the use of MTO–pyrazole in trifluoroethanol
allows for the highly selective epoxidation of a variety of olefins
with 0.1 mol% of catalyst. The epoxide was formed with high
selectivity and no noticeable by-products were observed. The
only notable exception was styrene. Its very sensitive epoxide
was partly decomposed to phenylethane-1,2-diol and benzalde-
hyde. The yield, however, is comparable to the best results
2 2
with 30–60% aqueous H O at low (0.1 mol%) catalyst loading.
The methodology should have wide applicability in organic
synthesis.
Notes and references
6
claimed by Sharpless and co-workers. These high turnovers
obtained for terminal alkenes are unprecedented and can only be
1 G. Strukul and R.A. Michelin, J. Chem. Soc., Chem. Commun., 1984,
1538; C. Venturello and R. D’Aloisio, J. Org. Chem., 1988, 53, 1553;
Y. Ishii, K. Yamawaki, T. Ura, H. Yamada, T. Yoshida and M. Ogawa,
J. Org. Chem., 1988, 53, 3587; K. Sato, M. Aoki, M. Ogawa, T.
Hashimoto and R. Noyori, J. Org. Chem., 1996, 61, 8310; T.
Kamiyama, M. Inoue, H. Kashiwagi and S. Enomoto, Bull. Chem. Soc.
Jpn., 1990, 63, 1559; P. L. Anelli, S. Banfi, F. Montanari and S. Quici,
J. Chem. Soc., Chem. Commun., 1989, 779; D. de Vos and T. Bein,
Chem. Commun., 1996, 917; M. G. Clerici and P. Ingallina, J. Catal.,
1993, 140, 71.
approached by the recently improved manganese triazonane
catalyst.9
The high turnovers could also be obtained in the oxidation of
the more reactive internal alkenes. However, the reaction was in
several cases too fast, resulting in the decomposition of the
catalyst and incomplete conversion of the alkene. When
appropriate cooling was applied, no catalyst decomposition was
observed. A better procedure was to add the substrate slowly
(
ca. 20 to 30 min) to the ice-cooled reaction mixture. In most
2 M. Rüsch, gen. Klaas, Thesis RWTH Aachen, 1993.
3 W. A. Herrmann, R. W. Fischer and D. W. Marz, Angew. Chem., 1991,
cases the reaction was complete within minutes after the
addition of the last portion of alkene. The presence of the active
catalyst, even after 1000 turnovers, was indicated by the intense
yellow colour of the peroxo complex. The results for some
internal alkenes are shown in Table 2. Most epoxides give
1
03, 1706; W. A. Herrmann, D. W. Marz, W. Wagner, J. G. Kuchler, G.
Weichselbaumer and R. W. Fischer, Ger. Pat., 3902357, 1989 to
Hoechst AG.
4
5
W. A. Herrmann, R. W. Fischer, M. U. Rauch and W. Scherer, J. Mol.
Catal., 1994, 86, 221.
J. Rudolph, K. L. Reddy, J. P. Chiang and K. B. Sharpless, J. Am. Chem.
Soc., 1997, 119, 6189.
>
99% yield of epoxide, except indene. Indene oxide is known
to be very sensitive towards hydrolysis. The homogeneous
reaction mixture with water present in the same phase as the
epoxide results in decomposition of the product. The two-phase
6 C. Copéret, H. Adolfsson and K. B. Sharpless, Chem. Commun., 1997,
1565.
7 W. A. Herrmann, R. M. Kratzer, H. Ding, W. R. Thiel and H. Glas,
J. Organomet. Chem., 1998, 555, 293.
8 M. C. A. van Vliet, I. W. C. E. Arends and R.A. Sheldon, manuscript in
preparation.
system with CH
2
Cl
2
generally gives higher yields, because the
water and epoxide are present in separate phases. This
behaviour is known for other catalytic systems as well.10 Most
other epoxides, however, are stable enough to resist hydrolysis
in a monophasic system.
The catalyst is still active after 1000 turnovers, and 2500
turnovers could be obtained by slow addition of alkene to the
reaction mixture. At a substrate to catalyst ratio of 2000
complete conversion was reached within 1 h with cyclohexene.
9
D. E. de Vos, B. F. Sels, M. Reynaers, Y. V. Subba Rao and P. Jacobs,
Tetrahedron Lett., 1998, 39, 3221.
0 K. Sato, M. Aoki, M. Ogawa, T. Hashimoto, D. Panyella and R. Noyori,
Bull. Chem. Soc. Jpn., 1997, 70, 905.
1
Communication 9/02133G
822
Chem. Commun., 1999, 821–822