Published on the web April 20, 2011
495
Vanadium-catalyzed Atmospheric Oxidation of Benzyl Alcohols Using Water as Solvent
Shintaro Kodama,1 Suguru Hashidate,1 Akihiro Nomoto,1 Shigenobu Yano,2 Michio Ueshima,1 and Akiya Ogawa*1
1Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University,
1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531
2Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192
(Received February 9, 2011; CL-110112; E-mail: ogawa@chem.osakafu-u.ac.jp)
tBu
tBu
Me
A vanadium complex combined with 4,4¤-di-tert-butyl-2,2¤-
bipyridyl as a ligand exhibited excellent catalytic activity in
atmospheric oxidation of benzhydrols using water as solvent to
afford benzophenones in good yields.
Me
Me
Me
N
N
N
N
N
N
4,4'-tBubpy
5,5'-Mebpy
4,4'-Mebpy
MeO OMe
N
N
N
N
N
N
Oxidation of alcohols is one of the most important reactions
in organic synthesis. Traditionally, stoichiometric amounts of
heavy metal reagents (e.g., chromium(VI)) are used as oxidants.
From a green chemical viewpoint, oxidation of alcohols using
catalytic amounts of metal complexes in the presence of
molecular oxygen (O2) has attracted much attention in recent
years.1 However, most of the reactions are performed in organic
solvent, such as acetonitrile, dichloromethane, and toluene.
Water is the most abundant and eco-friendly solvent available.
It is also nonflammable, and therefore, oxidation using water as
solvent could be cheaper, more eco-friendly, and safer than
current oxidation processes in organic solvents.2 But examples
of metal-complex-catalyzed oxidation of alcohols with O2 using
water as solvent are still rare,3-6 and precious metals (e.g.,
palladium, ruthenium, and gold), high pressure of air or O2, and
a stoichiometric amount of base are often required in the
reactions.
Vanadium exists on the surface of the earth more abundantly
than copper, and pentavalent vanadium acts as an oxidant.
Recently, some vanadium catalyst systems have been developed
for the oxidation of alcohols with O2.7 However, there has been
no report of an efficient catalytic system with vanadium for
aerobic oxidation of alcohols using water as the only solvent
under atmospheric conditions.
Phen
4,4'-MeObpy
Bpy
Figure 1. Structures of ligands in Table 1.
Table 1. Catalytic activities for oxidation of benzhydrol with
different catalysts and ligandsa
O
OH
catalyst (5 mol%), ligand (10 mol%)
H2O, O2 (0.1 MPa), 90 oC, 3 h
Ph
Ph
Ph
Ph
1a
2a
Yield of Recovery of
2a/%b
Entry Catalyst
Ligand
1a/%b
1
2
3
4
5
None
VOSO4
VO(Hhpic)2 None
VOSO4
VOSO4
None
None
ND
Trace
ND
Trace
47
>99
94
>99
>99
40
H2hpic
4,4¤-tBubpy
4,4¤-tBubpy
5,5¤-Mebpy
4,4¤-Mebpy
Phen
6c VOSO4
7c VOSO4
8c VOSO4
9c VOSO4
10c VOSO4
11c VOSO4
83
76
33
22
19
17
ND
4
56
77
76
4,4¤-MeObpy
Bpy
76
aReaction conditions: catalyst (0.005 mmol), ligand (0.01
mmol), benzhydrol (1a) (0.1 mmol), H2O (0.5 mL), O2 (0.1
MPa), 90 °C, 3 h. bDetermined by 1H NMR. ND: Not detected.
cReaction time was 6 h.
Herein, we report that oxovanadium complexes bearing
bipyridyl derivatives have excellent catalytic activity in oxida-
tion of benzhydrol derivatives using water as the only solvent
under an atmospheric pressure of O2. Furthermore, we found that
the vanadium-catalyzed atmospheric oxidation of various benzyl
alcohols proceeded successfully by the use of catalytic amounts
of benzhydrols as additives.
Phen, 4,4¤-MeObpy, and Bpy),9 4,4¤-tBubpy was found to be the
most effective as a ligand (Entries 6-11).
Oxidation of benzhydrol (1a) using different vanadium
catalysts and ligands (Figure 1) was examined using water as
solvent under O2 atmosphere (0.1 MPa) at 90 °C, and the results
are shown in Table 1. VOSO4 and VO(Hhpic)2 were not
effective as catalysts for the oxidation (Entries 2 and 3).8 Also,
3-hydroxypicolinic acid (H2hpic) was found to be ineffective as
an additional ligand to VOSO4 (Entry 4). In sharp contrast,
when VOSO4 was combined with 4,4¤-di-tert-butyl-2,2¤-bipyr-
idyl (4,4¤-tBubpy), the oxidation of 1a with atmospheric oxygen
proceeded successfully, affording benzophenone (2a) in 47%
yield (Entry 5). When the reaction time was prolonged to 6 h,
the yield of 2a increased to 83% yield (Entry 6). Among bi-
pyridyl ligands screened (4,4¤-tBubpy, 5,5¤-Mebpy, 4,4¤-Mebpy,
Various benzhydrol derivatives 1b-1d bearing electron-
withdrawing and electron-donating groups could be oxidized
effectively in the presence of catalytic amounts of VOSO4
and 4,4¤-tBubpy using water as solvent under an atmosphere
of O2 (Table 2, Entries 1-5).10 In the case of the oxidation of
1-phenylethanol (1f), however, a trace amount of acetophenone
(2f) was detected as an oxidation product (85% of 1f was
recovered) (Entry 6). The yield of 2f slightly increased to 3% by
prolonging the reaction time to 22 h, but this condition was not
practical. When a catalytic amount of 1a was employed as an
additive, to our surprise, 2f was obtained in 22% yield in 13 h.
As a result of further optimization of the reaction conditions
by using sat. MgSO4(aq)11 and scaling up from 0.1 mmol of
Chem. Lett. 2011, 40, 495-497
© 2011 The Chemical Society of Japan