Published on Web 07/19/2008
Efficient Catalytic Oxidation of Alkanes by Lewis Acid/
VI
-
[
Os (N)Cl
4
] Using Peroxides as Terminal Oxidants. Evidence
for a Metal-Based Active Intermediate
Shek-Man Yiu, Wai-Lun Man, and Tai-Chu Lau*
Department of Biology and Chemistry, City UniVersity of Hong Kong, Tat Chee AVenue,
Kowloon Tong, Hong Kong, China
Received April 10, 2008; E-mail: bhtclau@cityu.edu.hk
t
Abstract: The oxidation of alkanes by various peroxides ( BuOOH, H
2
O
2
, PhCH
CO H to give alcohols and ketones.
Oxidations occur rapidly at ambient conditions, and excellent yields and turnover numbers of over 7500
2 3 2
C(CH ) OOH) is efficiently
VI
-
catalyzed by [Os (N)Cl
4
] /Lewis acid (FeCl
3
or Sc(OTf)
3
) in CH
2 2
Cl /CH
3
2
t
and 1000 can be achieved in the oxidation of cyclohexane with BuOOH and H
2 2
O , respectively. In particular,
this catalytic system can utilize PhCH C(CH OOH (MPPH) efficiently as the terminal oxidant; good yields
2
3 2
)
of cyclohexanol and cyclohexanone (>70%) and MPPOH (>90%) are obtained in the oxidation of
cyclohexane. This suggests that the mechanism does not involve alkoxy radicals derived from homolytic
cleavage of MPPH but is consistent with heterolytic cleavage of MPPH to produce a metal-based active
intermediate. The following evidence also shows that no free alkyl radicals are produced in the catalytic
oxidation of alkanes: (1) The product yields and distributions are only slightly affected by the presence of
O
2
. (2) Addition of BrCCl
3
does not affect the yields of cyclohexanol and cyclohexanone in the oxidation of
cyclohexane. (3) A complete retention of stereochemistry occurs in the hydroxylation of cis- and trans-
1
,2-dimethylcyclohexane. The proposed mechanism involves initial O-atom transfer from ROOH to
VI
-
VIII
-
[Os (N)Cl
4 4
] /Lewis acid to generate [Os (N)(O)Cl ] /Lewis acid, which then oxidizes alkanes via H-atom
abstraction.
Introduction
The selective oxidation of alkanes under mild conditions
active toward alkanes. We have reported that the oxidation of
1
3
14
alkanes by anionic oxo species of ruthenium, iron, chro-
1
5
15,16
1
,2
mium, and manganese
is greatly enhanced by just a few
continues to be a challenge for chemists. A number of
enzymes, such as cytochrome P-450 and methane monooxy-
genase, are able to make use of highly reactive iron-oxo species
equivalents of Lewis acids. In particular, we have recently
-
17
studied the activation of [Os(N)(O)3] by FeCl3 in detail.
-
3
–5
Although [Os(N)(O) ] is a very stable osmium-oxo species,
3
to oxidize alkanes, and this has inspired the development of
synthetic models that can mimic the reactivity of these
4
,6–12
enzymes.
Our approach to design catalytic systems for
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10.1021/ja802625e CCC: $40.75
2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 10821–10827 9 10821