4714 J . Org. Chem., Vol. 66, No. 13, 2001
Yudin et al.
MTO) by cheaper and more readily available inorganic
rhenium oxides (e.g., Re2O7, ReO3(OH), and ReO3) was
accomplished with bis(trimethylsilyl) peroxide (BTSP) as
oxidant in place of aqueous H2O2 (eq 2).7 Systematic
investigation of the oxorhenium catalyst precursors and
various additives in olefin epoxidation with BTSP are
reported in the present paper.
despite extensive efforts in the Herrmann laboratory.2,10
Elimination of water from the reaction could become an
alternate path toward increased turnovers because cata-
lyst decomposition should be largely suppressed. At the
same time, the presence of pyridine should prevent the
epoxide ring opening and allow one to observe ligand
effects on the selectivity features of the oxygen atom
transfer event. A water-free environment should also
eliminate possible complications from phase transfer
effects. Obviously, any process that involves H2O2 as the
oxygen atom source produces at least 1 equiv of water
as byproduct, which will defeat an anhydrous system
unless an efficient water removal can be incorporated in
the process design. A possible solution to the problem
could be an oxidant that acts as an “anhydrous” analogue
of H2O2. Readily accessible bis(trimethylsilyl) peroxide
(BTSP)11,12 has been previously used in this capacity.13
To our surprise, however, MTO showed little to no
reactivity toward BTSP in CDCl3 (eq 3) under stoichio-
metric conditions,14a in marked contrast to the reaction
between MTO and aqueous H2O2 that instantaneously
generates a mixture of mono- and bisperoxorhenium
Resu lts a n d Discu ssion
Discovery of the beneficial effect of pyridine in the
MTO-catalyzed epoxidation prompted detailed study of
this phenomenon with the goal of further improving the
system.8 From the very beginning, salient features of the
pyridine-modified protocol seemed counterintuitive. For
example, base-mediated decomposition pathways of MTO
in aqueous H2O2 have been established.9 The hydroper-
oxide (HOO-) species is known to induce decomposition
of MTO into methanol and catalytically inactive perrhe-
nate (ReO4-). Pyridine would be expected to facilitate this
detrimental process by increasing the pH of the medium.
Indeed, pyridinium perrhenate is formed during MTO-
catalyzed epoxidations mediated by pyridine, but this
does not adversely affect the epoxidations of most olefins,
since full conversion is reached well before significant
levels of catalyst decomposition are reached. Another
important role attributed to pyridine in these systems is
that of a buffer for the Lewis acidic Re(VII) species,
thereby enabling even sensitive epoxides to survive.
Despite overall efficiency of the original pyridine-
modified system, lower conversions were observed for less
reactive substrates such as terminal olefins, due to
premature destruction of the catalyst. Although 3-cyan-
opyridine provided a remedy for this class of olefins, a
more general way of extending catalyst lifetime became
a challenge. Among the known organometallic oxorhe-
nium(VII) species (R-ReO3) capable of catalyzing olefin
epoxidation, MTO is most stable with respect to oxidative
and/or hydrolytic removal of the alkyl group (vide infra).
Hence, catalyst modification by variation of the R-
substituent on the rhenium center was not rewarding
(10) (a) Herrmann, W. A.; Ku¨hn, F. E.; Fischer, R. W.; Thiel, W. R.;
Romao, C. C. Inorg. Chem. 1992, 31, 4431. (b) For the most recent,
and best, procedure, see: Herrmann, W. A.; Kratzer, R. M.; Fischer,
R. W. Angew. Chem., Int. Ed. Engl. 1997, 36, 2652.
(11) (a) Cookson, P. G.; Davies, A.; Fazal, N. J . Organomet. Chem.
1975, 99, C31. (b) Taddei, M.; Ricci, A. Synthesis 1986, 633. (c) For a
convenient, large-scale (0.5 mol) preparation of BTSP from bis-
(trimethylsilyl)urea and urea/H2O2 complex in dichloromethane, see:
J ackson, W. P. Synlett 1990, 536. The product obtained according to
this method is virtually free of hexamethyldisiloxane, a common, albeit
harmless, byproduct in cognate BTSP preparations. (d) Babin, P.;
Bennetau, B.; Dunogue`s, J . Synth. Commun. 1992, 22, 2849. (e) BTSP
is now commercially available from Gelest.
(12) WARNING: Thermal stabilities of silylated organic peroxides
have been studied: Vesnovskii, B. P.; Thomadze, A. V.; Suchevskaya,
N. P.; Aleksandrov, Yu. A. Zh. Prikl. Khim. 1982, 55, 1005. Pure BTSP
has an active oxygen content of only 9% (cf. tert-butyl hydroperoxide
17.8%; di-tert-butyl peroxide 10.9%; hydrogen peroxide 47%). We would
like to stress, however, that despite its great thermal stability, BTSP
is subject to facile hydrolysis in the presence of water and acids which
results in formation of hazardous 100% H2O2. Additionally, Professors
Henri Kagan and Dieter Seebach recently brought to our attention
two reports that document explosions upon contact between BTSP and
metal needles: (a) Riant, O.; Samuel, O.; Flessner, T.; Taudien, S.;
Kagan, H. B. J . Org. Chem. 1997, 62, 6733. (b) Neumann, H.; Seebach,
D Chem. Ber. 1978, 111, 2785. Thus, only plastic or glass pipets should
be used in handling BTSP.
(13) For applications of BTSP in organic synthesis, see: (a) Brandes,
D.; Blaschette, A. J . Organomet. Chem. 1973, 49, C6. (b) Brandes, D.;
Blaschette, A. Ibid. 1974, 73, 217. (c) Tamao, K.; Kumada, M.;
Takahashi, T. ibid. 1975, 94, 367. (d) Salomon, M. F.; Salomon, R. G.
J . Am. Chem. Soc. 1979, 101, 4290. (e) Adam, W.; Rodriguez, A. J .
Org. Chem. 1979, 44, 4969. (f) Suzuki, M.; Takada, H.; Noyori, R. Ibid.
1982, 47, 902. (g) Weber, W. P. Silicon Reagents in Organic Synthesis;
Springer-Verlag: New York, 1983. (h) Kanemoto, S.; Oshima, K.;
Matsubara, S.; Takai, K.; Nozaki, H. Tetrahedron Lett. 1983, 24, 2185.
(i) Matsubara, S.; Takai, K.; Nozaki, H. ibid. 1983, 24, 3741. (j)
Matsubara, S.; Takai, K.; Nozaki, H. Bull. Chem. Soc. J pn. 1983, 56,
2029. (k) See ref 6b. (l) Hayakawa, Y.; Uchiyama, M.; Noyori, R.
Tetrahedron Lett. 1986, 27, 4195. (m) Curci, R.; Mello, R.; Troisi, L.
Tetrahedron 1986, 42, 877. (n) Kanemoto, S.; Matsubara, S.; Takai,
K.; Oshima, K.; Utimoto, K.; Nozaki, H. Bull. Chem. Soc. J pn. 1988,
61, 3607. (o) Davis, F. A.; Lal, S. G.; Wei, J . Tetrahedron Lett. 1988,
29, 4269. (p) Olah, G. A.; Ernst, T. D. J . Org. Chem. 1989, 54, 1204.
(q) Camporeale, M.; Fiorani, T.; Troisi, L.; Adam, W.; Curci, R.;
Edwards, J . O. Ibid. 1990, 55, 93. (r) Shibata, K.; Itoh, Y.; Tokitoh,
N.; Okazaki, R.; Inamoto, N. Bull. Chem. Soc. J pn. 1991, 64, 3749. (s)
Chemla, F.; J ulia, M.; Uguen, D. Bull. Soc. Chim. Fr. 1993, 130, 547.
(t) Irie, R.; Hosoya, N.; Katsuki, T. Synlett 1994, 255. (u) Prouilhac-
Cros, S.; Babin, P.; Bennetau, B.; Dunogue`s, J . Bull. Soc. Chim. Fr.
1995, 132, 513. (v) Adam, W.; Korb, M. N. Tetrahedron 1996, 52, 5487.
(w) Adam, W.; Golsch, D.; Sundermeyer, J .; Wahl, G. Chem. Ber. 1996,
129, 1177. (x) Barton, D. H. R.; Chabot, B. M. Tetrahedron 1997, 53,
487. (y) Barton, D. H. R.; Chabot, B. M. Ibid. 1997, 53, 511. (z) see ref
12. (aa) Sakurada, I.; Yamasaki, S.; Gottlich, R.; Iida, T.; Kanai, M.;
Shibasaki, M. J . Am. Chem. Soc. 2000, 122, 1245. (bb) Cox, P. B.; Loh,
V. M.; Monteils, C.; Baxter, A. D.; Boyd, E. A. Tetrahedron Lett. 2001,
42, 125.
(6) (a) Rudolph, J .; Reddy, K. L.; Chiang, J . P.; Sharpless, K. B. J .
Am. Chem. Soc. 1997, 119, 6189. (b) Cope´ret, C.; Adolfsson, H.;
Sharpless, K. B. Chem. Commun. 1997, 16, 1565. (c) Adolfsson, H.;
Cope´ret, C.; Chiang, J . P.; Yudin, A. K. J . Org. Chem. 2000, 65, 8651.
(7) Yudin, A. K.; Sharpless, K. B. J . Am. Chem. Soc. 1997, 119,
11536.
(8) For a recent mechanistic investigation of the MTO-catalyzed
olefin epoxidation, see ref 2p.
(9) (a) Herrmann et al. used N-bases in order to suppress epoxide
ring opening (see ref 5) albeit at the expense of detrimental effect on
catalytic activity. For the most recent study of the MTO/Lewis base
catalysts in olefin epoxidation, see: Herrmann, W. A.; Ding, H.;
Kratzer, R. M.; Ku¨hn, F. E.; Haider, J . J .; Fischer, R. W. J . Organomet.
Chem. 1997, 549, 319. (b) For a comprehensive study on the base-
induced decomposition of MTO, see: Abu-Omar, M. M.; Hansen, P.
J .; Espenson, J . H. J . Am. Chem. Soc. 1996, 118, 4966. (c) In the
presence of pyridine and H2O2, MTO is slowly oxidized, producing
pyridinium perrhenate and CH3OH: Yudin, A. K.; Sharpless, K. B.,
unpublished results. (d) Herrmann, W. A.; Kratzer, R. M.; Ding, H.;
Thiel, W. R.; Glas, H. J . Organomet. Chem. 1998, 555, 293. {e)
Adolfsson, H.; Converso, A.; Sharpless, K. B. Tetrahedron Lett. 1999,
40, 3991.