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496
Published on the web September 12, 2013
Hydrogen-transfer Oxidation of Primary Alcohols Catalyzed
by Iridium Complexes Bearing a Functional Pyridonate Ligand
Using Isopropenyl Acetate as a Hydrogen Acceptor
Ken-ichi Fujita,* Takehisa Uejima, and Ryohei Yamaguchi*
Graduate School of Human and Environmental Studies, Kyoto University, Sakyo-ku, Kyoto 606-8501
(
Received August 17, 2013; CL-130760; E-mail: fujita.kenichi.6a@kyoto-u.ac.jp, yamaguchi.ryohei.75s@st.kyoto-u.ac.jp)
A new catalytic system for the hydrogen-transfer oxidation
aliphatic primary alcohols into aldehydes have not been
11
of alcohols catalyzed by iridium complexes bearing a functional
pyridonate ligand has been developed. By using isopropenyl
acetate as a hydrogen acceptor, a variety of primary alcohols
were efficiently converted to the corresponding aldehydes in
moderate to excellent yields.
achieved yet.
In the course of our studies on the oxidation of alcohols, we
1
0
examined the catalytic activity of 2a in dehydrogenation of
1-octanol (6a) (eq 1). Reaction using 2.0 mol % of 2a under
reflux in toluene for 20 h resulted in the formation of octanal
(
7a) in only 26% yield. On the other hand, addition of one
equivalent of isopropenyl acetate as a hydrogen acceptor
effectively improved the yield of 7a to 48%. Inspired by these
results, we started to investigate the hydrogen-transfer oxidation
of alcohols. Here, we wish to report a new system for the
hydrogen-transfer oxidation of aliphatic and benzylic primary
alcohols leading to aldehydes catalyzed by iridium complexes
bearing a functional pyridonate ligand using isopropenyl acetate
as a hydrogen acceptor.
The oxidation of alcohols to carbonyl compounds is one of
the most fundamental and important reactions in synthetic
organic chemistry, and it is conventionally accomplished by the
use of stoichiometric amounts of harmful reagents, such as
1
2
1
chromium reagents. Recently, much effort has been devoted to
2
catalytic oxidation using oxidants such as oxygen and hydro-
3
gen peroxide. However, those oxidants often cause safety
problems linked with the employment of flammable organic
solvents especially in large-scale processes. Hydrogen-transfer
oxidation using an organic hydrogen acceptor is a promising
protocol for the green-oxidation of alcohols, because it can be
performed under mild conditions and tolerates a broad range of
functional groups in many cases. Oppenauer-type oxidation
using acetone as a hydrogen acceptor is a well-known hydro-
Cp*
cat.
Ir
Cl
N
2a
F3C
OH
O
(
2.0 mol%)
CHO
ð1Þ
reflux in toluene, 20 h
6a
7a
hydrogen acceptor none: Conv. = 31%, Y = 26%
4
gen-transfer oxidation of alcohols. But it usually suffers from
equilibrium problem: large excess of acetone is required to
: Conv. = 51%, Y = 48%
AcO
5
obtain good yield of product. Whereas many catalytic systems
6
7
8
using alkene, alkyne, and other organic molecules accepting
hydrogen atoms in an irreversible manner have been also
reported, it is important to seek a safe and easily-available
hydrogen acceptor which enables environmentally benign
oxidation of alcohols.
First, we studied the oxidation of 6a to 7a using isopropenyl
acetate as a hydrogen acceptor in the presence of various
catalysts (Table 1). When the reactions were carried out in
the presence of 2.0 mol % of catalysts 2a and 2b bearing a
functional pyridonate ligand with and without a trifluoromethyl
group, 7a was obtained in 48 and 63% yields, respectively,
indicating a negative effect of electron-withdrawing substituent
on the catalytic activity (Entries 1 and 2). On the other hand,
introduction of electron-donating groups to the functional ligand
at 3- or 5-position enhanced the catalytic activity (Entries 3, 6,
and 8). Thus, the highest yield of 7a (up to 84%) was achieved
by the reaction in the presence of 3.0 mol % of catalyst 2e using
Meanwhile, we have previously reported the dehydrogen-
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10
ation of alcohols and cyclic amines which proceeds accom-
panying the evolution of hydrogen gas using Cp*Ir catalysts
(Cp*: pentamethylcyclopentadienyl) bearing functional ligands
with an α-hydroxypyridine or α-pyridonate skeleton (Figure 1).
Various secondary alcohols and benzylic primary alcohols could
be dehydrogenatively oxidized to ketones and benzaldehydes,
respectively. However, efficient dehydrogenative oxidation of
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3
three equivalents of isopropenyl acetate (Entry 7).
We next examined various hydrogen acceptors (Table 2).
The reactions using vinyl acetate and vinyl benzoate gave 7a
in moderate yields. However, selectivity of the reaction was
Cp*
Ir
Cp*
Ir Cl
Cp*
HO
Cl
Cl
Ir Cl
N
N
N
14
F C
3
relatively low in these cases (Entries 2 and 3). Methyl
OH
O
19a
Cp*
Ir
2+
2a10
Cp*
39b
methacrylate and butyl vinyl ether both showed lower efficiency
(Entries 4 and 5). The reactions using ethylene and styrene
resulted in moderate yields of 7a with relatively high selectivity
HO
O
OH2
OH
Ir OH2
N
N
O
N
N
(
Entries 6 and 7). α-Methylstyrene and diphenylacetylene did
49c
59d
not act as an efficient hydrogen acceptor (Entries 8 and 9). Based
on these results, we concluded that isopropenyl acetate was the
optimal hydrogen acceptor.
Figure 1. Cp*Ir catalysts active for dehydrogenative reactions.
Chem. Lett. 2013, 42, 14961498
© 2013 The Chemical Society of Japan