1042
Chemistry Letters Vol.37, No.10 (2008)
Iron Catalyst for Oxidation in Water: Surfactant-type Iron Complex-catalyzed Mild
and Efficient Oxidation of Aryl Alkanes Using Aqueous TBHP as Oxidant in Water
Takashi Nagano and Shu¯ KobayashiÃ
Department of Chemistry, School of Science and Graduate School of Pharmaceutical Sciences,
The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 110-0033
The HFRE Division, ERATO, Japan Science and Technology Agency (JST)
(Received July 4, 2008; CL-080662; E-mail: shu kobayashi@chem.s.u-tokyo.ac.jp)
Surfactant-type iron(III) complex, Fe2O(DS)4, was found
to be effective for benzylic oxidation of simple aryl alkanes
using aqueous t-butyl hydroperoxide (TBHP) as an oxidant.
phenyl ketone (3a) in 40% yield (Entry 1). Use of 5 equiv of
TBHP resulted in a much higher yield of the product (Entry 2).
Simple iron(II) salts were not effective under the same condi-
tions (Entries 3 and 4). Although Fe(acac)3 did not catalyze
the reaction, water-soluble FeIII salts such as FeCl3 and
K3[Fe(CN)6] showed some catalytic activity (Entries 5–7), even
if somewhat lower than that of complex 1. These results indicate
the importance of both valency of iron and surfactant-type
ligand. Cummen hydroperoxide or hydrogen peroxide as an
oxidant did not give the product at all (Entries 8 and 9). In these
cases evolution of gas was observed, and decomposition of the
oxidants might occur in the presence of the iron catalyst under
the conditions. Finally we obtained 72% yield of the desired
ketone after 50 h (Entry 10). Prolonged reaction time or higher
catalyst loading did not improve the yield (Entries 11 and 12).
In the latter case, an excess of the solid catalyst retarded effec-
tive formation of the micellar solution.
Recently increasing attention has been paid to the develop-
ment of environmentally benign, sustainable, chemical process-
es. Use of ubiquitous metals, especially iron, as catalysts for or-
ganic synthesis is important in the development of such process-
es.1 Utilizing water as an alternative to toxic and harmful organic
solvents is also important and has been extensively studied.2
Thus, the combination of iron catalyst and water as a solvent
seems to be very attractive from a viewpoint of green sustainable
chemistry.3 In the course of our studies on Lewis acid surfactant-
combined catalysts (LASC) for organic reactions in water,4 re-
lated ꢀ-oxo-dinuclear iron(III) complex Fe2O(DS)4 (DS = n-
C12H25OSO3–) (1)5 was found to be effective for benzylic oxida-
tion of simple aryl alkanes in water. Although the combination
of iron salts and peroxides for oxidation of alkanes has been well
recognized as the Gif system established by Barton and Doller,6
the system requires the use of pyridine or acetonitrile and acid
additives, and benzylic oxidation of simple aryl alkanes has
not been well studied.7 Recently Nakanishi and Bolm focused
on the Gif-type system for benzylic oxidation, where pyridine
solvent and high temperature were still required and the yields
of oxidation products from non-activated hydrocarbons were
moderate.8 Here, we report mild and efficient benzylic oxida-
tion9 of simple aryl alkanes without additives in water.
With the optimized conditions in hand, we then investigated
substrate generality of the present reaction system (Table 2). A
variety of aryl alkanes bearing methoxy, fluoro, t-butyl and
phenyl functional groups at the para position gave the corre-
sponding aryl alkyl ketones in moderate to good yields (Entries
1–5). It is noteworthy that the present catalytic system can be ap-
Table 2. Iron-catalyzed oxidation in watera
Entry
Substrate 2
Yield/%b
Product 3
a: R = H, n = 6
72
84
52
77
71
90
86
1
2
3
4
5
Initially we examined several reaction conditions using 1-
phenyloctane (2a) as a model substrate (Table 1). The reaction
of 2a with 3 equiv of aqueous t-butyl hydroperoxide (TBHP)
in the presence of 2.5 mol % of the iron catalyst 1 in water, which
made a micellar solution, produced the corresponding heptyl
b: R = OMe, n = 6
c: R = F, n = 6
O
n
d: R = t-Bu, n = 6
e: R = Ph, n = 3
f: R = NO2, n = 0
g: R = OTf. n = 0
n
R
R
6
7
Table 1. Optimization of reaction conditionsa
n
h: n = 1
i: n = 2
8
9
91
87
n
Entry
Fe cat./mol %
Oxidant (equiv) Time/h Yield/%b
O
1
2
3
4
5
6
7
8
9
Fe2O(DS)4 (1) (2.5)
1 (2.5)
TBHP (3)
TBHP (5)
TBHP (5)
TBHP (5)
TBHP (5)
TBHP (5)
TBHP (5)
PhCMe2O2H (5)
H2O2 (5)
24
24
24
24
24
24
24
24
24
50
85
50
40
53
O
X = O
R = CH3
87
84
10
11
j:
Fe(OAc)2 (5)
FeSO4 (5)
Fe(acac)3 (5)
FeCl3 (5)
K3[Fe(CN)6] (5)
1 (2.5)
1 (2.5)
1 (2.5)
1 (2.5)
1 (5)
0c
R
X = NSO2Ph
R = H
12
0c
34
26
0c
0c
72
71
56
R
k:
X
X
R
R
O
O
12c
13c
94
99
Ph
Ph
l
Ph
Ph
10
11
12
TBHP (5)
TBHP (5)
TBHP (5)
m
aConditions: 1 (2.5 mol %), TBHP (5 equiv), H2O, 30 ꢀC, 50 h. bIsolated
yield. cRoom temperature for 24 h.
aThe reaction was carried out with 2a (0.5 mmol) in H2O at rt.
bDetermined by 1H NMR analysis. c2a was recovered quantitatively.
Copyright Ó 2008 The Chemical Society of Japan