Published on Web 10/23/2004
FeCl
3
-Activated Oxidation of Alkanes by [Os(N)O
3
]-
Shek-Man Yiu, Zhi-Biao Wu, Chi-Keung Mak, and Tai-Chu Lau*
Contribution from the Department of Biology and Chemistry, City UniVersity of Hong Kong,
Tat Chee AVenue, Kowloon Tong, Hong Kong, China
Abstract: Although the ion [OsVIII(N)(O)
organic substrates, it is readily activated by FeCl
]- is a stable species and is not known to act as an oxidant for
in CH Cl /CH CO H to oxidize alkanes efficiently at
3
3
2
2
3
2
room temperature. The oxidation can be made catalytic by using 2,6-dichloropyridine N-oxide as the
terminal oxidant. The active intermediates in stoichiometric and catalytic oxidation are proposed to be
VIII
III
VIII
III
[
3 4
(O) Os tNsFe ] and [Cl (O)Os tNsFe ], respectively.
Introduction
to activate metal-oxo species. The activating effects of Brønsted
acids on metal-oxo species are well known; however, the use
of Lewis acids to enhance the reactivity of metal-oxo species
is much less studied. We recently reported that the oxidation
Nature has evolved reactive oxoiron species in enzymes such
as cytochrome P-450 and methane monooxygenases that can
oxidize alkanes under mild conditions.1-3 The search for highly
reactive metal-oxo species in chemical systems that can match
the reactivities of these enzymes continues to be a challenge to
17
18
of alkanes by oxo species of ruthenium, iron, manganese,
19
and chromium is greatly enhanced by Lewis acids. The Lewis-
acid-assisted oxidation of various other organic substrates by
permanganate was also subsequently reported by Lee and co-
4-8
chemists. Metal-oxo species such as chromate and perman-
ganate have long been used as oxidants for a variety of organic
20,21
workers.
9
functional groups. However, their use in the oxidation of
We report here a study of the remarkable activating effects
of FeCl3 on the oxidation of alkanes by the nitridoosmate(VIII)
alkanes is limited; refluxing conditions are often required to
oxidize unactivated C-H bonds. Much more reactive metal-
oxo species have been generated by using synthetic porphyrin
ligands,2 and a number of oxometalloporphyrins of iron,
-
ion, [Os(N)(O)3] . Although OsO4 is a well-established reagent
2
2
for the dihydroxylation of alkenes, it does not oxidize alkanes.
10
-
The [Os(N)(O)3] ion is an even weaker oxidant than OsO4
due to the stronger electron-donating properties of the N ion
than the O ion, and it is not known to act as an oxidant for
any organic substrates. However, the addition of just a few
equivalents of FeCl3 to [Os(N)(O)3] results in a system that
11-13
14,15
16
chromium,
ruthenium
and manganese, usually gener-
3-
ated in situ, are capable of oxidizing a variety of organic
substrates, including alkanes. Our approach is to use Lewis acids
2
-
-
(
1) Ortiz de Montellano, P. R., Ed. Cytochrome P450. Structure, Mechanism
and Biochemistry; Plenum Press: New York, 1995.
can perform stoichiometric and catalytic oxidation of alkanes
efficiently at room temperature.
(
2) McLain, J. L.; Lee, J.; Groves, J. T. In Biomimetic Oxidations Catalyzed
by Transition Metal Complexes; Meunier, B., Ed.; Imperial College Press:
London, 2000; pp 91-170.
Experimental Section
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2
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(
4) Hu, Z.; Gorun, S. M. In Biomimetic Oxidations Catalyzed by Transition
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pp 269-308.
Materials. All chemicals were of reagent grade unless otherwise
n
23
n
24
4 3 4 4
noted. [ Bu N][Os(N)(O) ] and [ Bu N][Os(N)Cl ] were synthesized
according to published procedures. The purity of these complexes was
determined by CHN analysis and UV-vis spectrophotometry. Anhy-
drous ferric chloride (98%), cis- and trans-1,2-dimethylcyclohexane,
propane, isobutane, and n-butane were purchased from Aldrich and
were used as received. Other alkanes and solvents were purified
(
5) Robert, A.; Meunier, B. In Biomimetic Oxidations Catalyzed by Transition
Metal Complexes; Meunier, B., Ed.; Imperial College Press: London, 2000;
pp 543-562.
(
(
(
(
6) Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879-2932.
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Y. J. Am. Chem. Soc. 1988, 110, 8443-8452.
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(18) Ho, C. M.; Lau, T. C. New J. Chem. 2000, 24, 587-590.
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T. J.; Thiruvazhi, M. J. Org. Chem. 2000, 65, 1008-1015.
(21) Lai, S.; Lee, D. G. Tetrahedron 2002, 58, 9879-9887.
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Mijs, W. J., de Jonge, C. R. H. I., Eds.; Plenum Press: New York, 1986;
pp 633-693.
1
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1
(
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10.1021/ja0487832 CCC: $27.50 © 2004 American Chemical Society
J. AM. CHEM. SOC. 2004, 126, 14921-14929
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