Vanadium-catalyzed green oxidation of benzylic alcohols in water under air atmosphere

Article abstract of DOI:10.1016/j.tet.2014.02.032

The oxidation of alcohols using catalytic amounts of metal complexes is highly attractive from the viewpoint of green chemistry principles. However, examples of metal complex-catalyzed oxidations of alcohols with O₂ using water as the solvent are still rare, and precious metals, high-pressure O₂ or air, and a stoichiometric amount of base are often required. In this study, it was found that an oxovanadium-4,4′-t-Bubpy (4,4′-di-tert-butyl-2,2′-bipyridyl) complex exhibited high catalytic activity in the oxidation of benzhydrols under an atmosphere of O₂ in water as the sole solvent. Interestingly, this catalytic oxidation method could be applied to the gram-scale aerobic oxidation of alcohols in water under the atmosphere.

Full text of DOI:10.1016/j.tet.2014.02.032

Tetrahedron 70 (2014) 2431e2438
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Vanadium-catalyzed green oxidation of benzylic alcohols in water
under air atmosphere
Kuniaki Marui a, Yuuki Higashiura a, Shintaro Kodama a, Suguru Hashidate a,
,
Akihiro Nomoto a, Shigenobu Yano b, Michio Ueshima a, Akiya Ogawa a
*
a Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531,
Japan
b Graduate School of Materials Sience, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 October 2013
Received in revised form 10 February 2014
Accepted 13 February 2014
Available online 20 February 2014
The oxidation of alcohols using catalytic amounts of metal complexes is highly attractive from the
viewpoint of green chemistry principles. However, examples of metal complex-catalyzed oxidations of
alcohols with O2 using water as the solvent are still rare, and precious metals, high-pressure O2 or air, and
a stoichiometric amount of base are often required. In this study, it was found that an oxovanadium-4,40-
t-Bubpy (4,40-di-tert-butyl-2,20-bipyridyl) complex exhibited high catalytic activity in the oxidation of
benzhydrols under an atmosphere of O2 in water as the sole solvent. Interestingly, this catalytic oxidation
method could be applied to the gram-scale aerobic oxidation of alcohols in water under the atmosphere.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Oxidation
Vanadium catalyst
Water solvent
Gram-scale
Air condition
1. Introduction
the oxidation of alcohols with O2;11 nonetheless, no reports are
known to exist of an efficient vanadium catalytic system for aerobic
The oxidation of alcohols to aldehydes and ketones is one of the
most important synthetic transformations for organic chemists with
respect to the useful intermediates obtained for the preparation of
more complex organic compounds.1 Conventionally, stoichiometric
amounts of heavy metals (e.g., chromium(VI)) are used as oxidants.
Therefore, the oxidation of alcohols using catalytic amounts of metal
complexes is a highlyattractive prospect from the viewpointof green
chemistry principles,2e7 in which the following reaction character-
istics are desired: (1) the use of earth-abundant transition metals in
terms of sustainability and cost; (2) use of molecular oxygen (O2) as
a terminal oxidant, where water is the only by-product; (3) use of
water in place of organic solvents due to its low cost, inflammability,
and great abundance.8 However, examples of metal complex-
catalyzed oxidations of alcohols with O2 using water as the solvent
are still rare.9 Precious metals (e.g., Au, Ru, and Pd), high-pressure O2
or air, and a stoichiometric amount of base are often required.
Vanadium is a relatively abundant element, ranking among the
top 20 elements occurring in the earth’s crust. The oxidative
properties of pentavalent vanadium are well known.10 Recent
studies have focused on the development of vanadium catalysts for
oxidation of alcohols using water as the only solvent under atmo-
spheric conditions.
Recently, we have reported the aerobic oxidation of benzyl alco-
hols catalyzed by (VO)4(hpic)4 or VO(Hhpic)2, which are synthesized
from VOSO4 and 3-hydroxypyridine-2-carboxylic acid (H2hpic).12a,b
Recyclability was observed in (VO)4(hpic)4, while VO(Hhpic)2 was
noted to have relatively high catalytic activity (Scheme 1).
* Corresponding author. Tel./fax: þ81 72 254 9290; e-mail address: ogawa@
Scheme 1. Oxidation of benzhydrol using VO(Hhpic)2 or (VO)4(hpic)4.
chem.osakafu-u.ac.jp (A. Ogawa).
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.02.032

2432
K. Marui et al. / Tetrahedron 70 (2014) 2431e2438
Table 1
Opportunely, we also found that VO(Hhpic)2 could also oxidize
Examination of ligand effects on vanadium-catalyzed oxidation
benzylamines to the corresponding imines with high selectivity
using an ionic liquid solvent under an atmosphere of O2 (0.1 MPa),
also with good recyclability (Scheme 2).12c
VOSO4 (2 mol%), ligand (4 mol%)
O2 (1.0 MPa)
O
Ph
OH
MeCN, 120 oC, 3 h
Ph
H
Entry
1
Ligand
Yielda (%)
Entry
Ligand
Yielda (%)
OH
48
7
38
Scheme 2. Oxidation of benzylamine using recyclable vanadium catalyst VO(Hhpic)2
in ionic liquid.
N
COOH
N
COOH
OH
2
3
4
5
12
8
9
49
Furthermore, the oxidation of alcohols with H2O2 using heter-
otetranuclear complexes bearing bipyridyl ligands in water as sol-
vent was already reported (Scheme 3).12d
N
OMe
COOH
OH
7
7
Herein, we report the efficient oxidation of benzyl alcohol de-
MeO
OMe
N
rivatives using an oxovanadium-2,20-bipyridyl complex using water
5
10
11
17 (29)
55
N
N
N
N
COOH
tBu
O
M
40 (9)
O
V
OMe
N
tBu
tBu
O
O
N
MeO
N
N
O
O
N
OH
tBu
V
O
M
O
6
55
O
O
N
V2W2 ; M = W
V2Mo2 ; M = Mo
Determined by 1H NMR. Yield of benzoic acid is shown in parentheses.
a
OH
Me
O
catalyst, H2O2 (1.6 mmol)
H2O (3.0 mL), 90 oC, 24 h
products were identified by NMR. Unfortunately, VO(Hhpic)2 and
VOSO4 were not effective under the reaction conditions (Table 2,
entries 2 and 3). Also, the addition of H2hpic was found to be in-
effective for the VOSO4-catalyzed reaction (entry 4). In sharp con-
trast, when 4,40-t-Bubpy was used as a ligand, the oxidation of 1a
proceeded successfully in moderate yield (entry 5). The addition of
4,40-t-Bubpy was also effective for the VOSO4-catalyzed reaction
(entry 6). A cursory examination of several bipyridyl derivatives in
the oxidation reaction demonstrated that 4,40-t-Bubpy was the
Ph
Ph
Me
V2W2 (0.1 mol%)
V2Mo2 (0.1 mol%)
63%
28%
Scheme 3. Oxidation of benzhydrol using heterotetranuclear complexes.
as the sole solvent under an atmosphere of O2; the ligand 4,40-di-
tert-butyl-2,20-bipyridyl (4,40-t-Bubpy) provides the greatest yields
of oxidation products.12e Furthermore, this reaction system could
also be used for gram-scale oxidation reactions under atmospheric
pressure in air. To the best of our knowledge, this is the first report
of an efficient catalytic system employing vanadium for the aerobic
oxidation of alcohols using water as the only solvent under atmo-
spheric conditions. We have also focused on using H2O as safer and
abundant.
Table 2
Vanadium catalyst activities for oxidation of benzhydrola
catalyst (5 mol%), ligand (10 mol%)
O2 (0.1 MPa)
H2O (0.5 mL), 90 oC
OH
Ph
O
Ph
Ph
Ph
1a
2a
2. Results and discussion
Entry
Catalyst
Ligand
Time (h)
Yield of 2ab (%)
Firstly, to determine the most effective ligand for catalysis, as
preliminary experimental, a series of H2hpic analogues were
screened (Table 1, entries 1e5). It was found that the pyridine unit
was essential for the useful catalytic activity of vanadium (entry 5).
An examination of several pyridine derivatives revealed that the
functional group identity on the pyridine unit affects reaction ef-
ficiency to some extent (entries 6e9). Next, the bidentate 2,20-
bipyridyl ligand consisting of two pyridine units was applied to this
oxidation. In entry 11, the yield of the corresponding ketone was
found to have increased compared with entry 5. More interestingly,
an over-oxidation product (benzoic acid) was observed when 2,20-
bipyridyl was used (entry 10). These data suggest that a bidentate
ligand is more efficient than monodentate ligand in the oxidation,
and that the bipyridyl unit in particular appears to be the most
effective for the oxidative activity of vanadium.
1
2
3
4
5
6
7
8
None
VOSO4
VO(Hhpic)2
VOSO4
[VO(t-Bubpy)2]SO4
VOSO4
VOSO4
VOSO4
VOSO4
VOSO4
None
None
None
H2hpic
None
4,40-t-Bubpy
4,40-DCbpy
6,60-Mebpy
4,40-t-Bubpy
5,50-Mebpy
4,40-Mebpy
4,40-MeObpy
4,40-Phbpy
Phen
3
3
3
3
3
3
3
3
6
6
6
6
6
6
6
N.D.
Trace
N.D.
Trace
43
47
2
N.D.
83
76
33
19
Trace
22
17
9
10
11
12
13
14
15
VOSO4
VOSO4
VOSO4
VOSO4
VOSO4
Bpy
a
Reaction conditions: catalyst (0.005 mmol), ligand (0.01 mmol), 1a (0.1 mmol),
H2O (0.5 mL), O2 (0.1 MPa), 90 ꢀC.
b
1
Determined by H NMR. N.D.: not detected. 4,40-DCbpy: 2,20-bipyridyl-4,40-di-
A number of oxovanadium compounds including VO(Hhpic)2,
previously identified as an oxidant with good catalytic activity (cf.
Scheme 1), were examined in the oxidation of benzhydrol (1a) in
water under an O2 atmosphere (0.1 MPa) at 90 ꢀC, and the resulting
carboxylic acid. 6,60-Mebpy: 6,60-dimethyl-2,20-bipyridyl. 5,50-Mebpy: 5,50-di-
methyl-2,20-bipyridyl. 4,40-Mebpy: 4,40-dimethyl-2,20-bipyridyl. 4,40-MeObpy: 4,40-
dimethoxy-2,20-bipyridyl. 4,40-Phbpy: 4,40-diphenyl-2,20-bipyridyl. Phen: phenan-
throline. Bpy: 2,20-bipyridyl.

K. Marui et al. / Tetrahedron 70 (2014) 2431e2438
2433
most effective ligand (entries 7e15). Steric bulk around the ligand
co-ordination site had varying effects on reaction efficiency; 6,60-
dimethyl-2,20-bipyridyl (6,60-Mebpy) was ineffective (entry 8),
most likely because approach of the substrate to the central metal
was inhibited by the functional groups blocking the metal site.
Oxidation of other substrates using 4,40-t-Bubpy as a ligand was
also investigated under an atmosphere of O2 in water (Table 3).
Benzhydrol derivatives (1) with an electron-donating or electron-
withdrawing group could be oxidized to the corresponding car-
bonyl compounds in high yields (entries 1e5).
proceeding in moderate to high yields. It was apparent that p-
nitrobenzhydrol (1d) was most effective as an additive in the
present oxidation system, with the corresponding ketone obtained
in 92% yield (entry 7).
Next, the time-course of the vanadium-catalyzed oxidation of 3a
using catalytic 1a as an additive was investigated in further detail
by 1H NMR analysis. Over the reaction course, the yield of 4a
continued to increase with the complete consumption of 1a, which
was oxidized to 2a (Table 5). The oxidation of 3a using 2a as an
additive was also investigated, but the reaction proceeded in-
efficiently. Furthermore, oxidation of 3c suggested that sub-
stituents on the aromatic ring of benzhydrol additives may affect
the activity of the vanadium catalyst (see Table 4, entries 5e8).13
Table 3
Vanadium-catalyzed oxidation of benzhydrol derivatives under an O2 atmosphere in
watera
Table 5
VOSO4 (5 mol%), ligand (10 mol%)
O2 (0.1 MPa)
H2O (0.5 mL), 90 oC
Time-course change of oxidation of 1aa
OH
R2
O
VOSO4 (5 mol%), 4,4'-t-Bubpy (10 mol%)
R1
R1
R2
OH
Me
1a (10 mol%), O2 (0.1 MPa)
H2O (0.5 mL), 90 oC
1
2
Ph
3a
Entry
R1
R2
Time (h)
Yield of 2b (%)
O
O
OH
Ph
1a
1
2
3
4
5
6
Ph
Ph
Ph
Ph
Ph
4-MeC6H4
Me
1a
1b
1c
1d
1e
3a
6
8
6
8
6
3
2a
2b
2c
2d
2e
4a
(87)
83
(84)
(83)
86
+
+
4-MeOC6H4
4-BrC6H4
4-NO2C6H4
4-MeC6H4
Ph
Ph
Me
Ph
Ph
Ph
4a
2a
Entry
Time (h)
Yieldb (%)
Trace
a
Reaction conditions: VOSO4 (0.005 mmol), 4,40-t-Bubpy (0.01 mmol), substrate
3a
4a
1a
2a
(0.1 mmol), H2O (0.5 mL), O2 (0.1 MPa), 90 ꢀC.
1
2
3
4
5
0.5
2
4
13
23
88
83
81
58
44
1
3
6
22
31
67
6
N.D.
N.D.
N.D.
13
42
63
63
70
b
Determined by 1H NMR. Isolated yields are shown in parentheses.
In the case of 1-phenylethanol (3a), however, the reaction pro-
ceeded ineffectively (Table 3, entry 6). After further optimization of
the reaction conditions, it was noticed that addition of a catalytic
amount of 1a promoted the oxidation of 3a; acetophenone (4a) was
obtained in this manner in 22% yield (Table 4, entry 1). Further-
more, using satd MgSO4(aq) and scaling up substrate loading from
0.1 mmol to 0.5 mmol improved reaction efficiency and yield of the
desired product (67%, entry 2). Further substrate scale-up increased
the product yield only minimally (entry 3). For oxidation of other
benzyl alcohols, the benzhydrol additive was less effective (entries
4 and 5). Several benzhydrol derivatives were examined for the
a
Reaction conditions: VOSO4 (0.005 mmol), 4,40-t-Bubpy (0.01 mmol), 1a
(0.01 mmol), H2O (0.5 mL), substrate (0.1 mmol), O2 (0.1 MPa), 90 ꢀC.
b
Determined by 1H NMR. N.D.: Not detected.
Oxidation of benzyl alcohol derivatives (3) was studied next
using the optimized conditions (Table 6). Oxidation of secondary
benzyl alcohols proceeded smoothly, with the corresponding ke-
tones obtained in good to high yields (entries 1e6). Benzyl alcohol
oxidation of p-bromo-a-methylbenzyl alcohol (3c), with oxidation
Table 6
Oxidation of benzyl alcohol derivatives under an O2 atmospherea
VOSO4 (5 mol%), 4,4'-t-Bubpy (10 mol%)
MgSO4 (7.5 mmol), 1d (10 mol%)
Table 4
Oxidation in the presence of benzhydrol additivesa
OH
R2
3
O
O2 (0.1 MPa)
VOSO4 (5 mol%), 4,4'-t-Bubpy (10 mol%)
H2O (2.5 mL), 90 oC
R1
R1
R2
MgSO4 (7.5 mmol), benzhydrols (10 mol%)
O2 (0.1 MPa)
H2O (2.5 mL), 90 oC
OH
Me
O
4
R1
R2
Time (h)
Yield of 4b (%)
R
R
Me
Entry
3
4
1
2
3
4
5
6
7
8
4-NO2C6H4
4-MeOC6H4
Ph
Ph
Ph
Me
Me
nBu
cPr
cHex
Me
H
H
H
H
H
3b
3d
3e
3f
3g
3h
3i
3j
3k
3l
48
12
23
22
23
40
22
22
22
22
22
22
4b
4d
4e
4f
4g
4h
4i
4j
4k
4l
75
54
86
55
77
48
24 (61)
35 (26)
77 (23)
65 (12)
56 (22)
43 (31)
Entry
R
Benzhydrols
Yield of 4b (%)
1c
2
Ph
Ph
Ph
3a
3a
3a
3b
3c
3c
3c
3c
1a
1a
1a
1a
1a
1c
1d
1e
4a
4a
4a
4b
4c
4c
4c
4c
22
67
72
27
51
82
92
44
3d
4d
5d
6
2-Thienyl
Ph
4-NO2C6H4
4-BrC6H4
4-BrC6H4
4-BrC6H4
4-BrC6H4
4-MeC6H4
4-MeOC6H4
4-BrC6H4
4-ClC6H4
4-NO2C6H4
9
7
8
10
11
12
3m
3n
4m
4n
H
a
Reaction conditions: VOSO4 (0.025 mmol), 4,40-t-Bubpy (0.05 mmol), substrate
a
(0.5 mmol), MgSO4 (7.5 mmol), H2O (2.5 mL), O2 (0.1 MPa), 90 ꢀC, 22 h.
Reaction conditions: VOSO4 (0.025 mmol), 4,40-t-Bubpy (0.05 mmol), 1d
(0.05 mmol), H2O (2.5 mL), MgSO4 (7.5 mmol), substrate (0.5 mmol), O2 (0.1 MPa),
90 ꢀC.
Determined by 1H NMR.
b
c
Substrate (0.1 mmol) in the absence of MgSO4 reaction time: 6 h.
Substrate (1.0 mmol), MgSO4 (15 mmol).
d
b
Determined by 1H NMR. Yield of carboxylic acids are shown in parentheses.

2434
K. Marui et al. / Tetrahedron 70 (2014) 2431e2438
Table 7
with an electron-withdrawing group required prolonged reaction
times, giving the corresponding ketone in moderate yield (entry 1).
This system could be applied to the oxidation of 1-(2-thienyl)eth-
anol (3h), a heterocyclic example; 2-acetylthiophene (4h) was
obtained in 48% yield (entry 6).
Oxidation of benzhydrol derivatives under open-air atmospherea
We next applied our catalytic system to the oxidation of benz-
hydrol under an open-air atmosphere, the mildest possible condi-
tion in the oxidation reaction (Scheme 4). Interestingly, the
oxidation proceeded successfully by prolonging the reaction time
to 48 h; the corresponding ketone was successfully obtained in 75%
yield.
Entry
1
Substrate 1
Yield of 2(%)b
OH
1b
1c
1d
2b
2c
2d
90
MeO
Br
OH
OH
2
3
88
Scheme 4. Oxidation of 1a under open-air conditions in water.
48 (67)
Gram-scale aerobic oxidation of benzhydrol was also examined
(Scheme 5). The oxidation proceeded to yield benzophenone in
moderate yield (57%). The addition of satd MgSO4 (aq) to the re-
action mixture allowed for benzophenone to be obtained in
a vastly improved yield (94%). Moreover, the oxidation also pro-
ceeded efficiently, when the catalyst amount was reduced to
1 mol %.
O2N
OH
OH
OH
OH
4
5
6
7
1e
1f
2e
2f
61
90
90
69
Me
Me
Me
Cl
1g
1h
2g
2h
Scheme 5. Gram-scale oxidation of 1a under open-air conditions.
MeO
OMe
Cl
From these results, gram-scale oxidations of benzhydrol de-
rivatives under open-air conditions in satd MgSO4 (aq) were per-
formed. Stirring under open-air condition in water, the
corresponding ketones were obtained in moderate to high yields
(Table 7).
OH
8
1i
2i
57
Cl
a
Reaction conditions: VOSO4 (0.5 mmol), 4,40-t-Bubpy (1 mmol), H2O (50 mL),
Gram-scale oxidation of benzyl alcohol derivatives was also
investigated. Secondary benzyl alcohols were oxidized successfully
in water under open-air conditions and the corresponding ketones
were obtained in high yields (Table 8). The procedure could also be
successfully applied to alcohols having long-chained alkyl groups.
The effects of additives were studied. VOSO4 was soluble in
water in our oxidation system; in contrast, the ligands and sub-
strates were insoluble, and would float on the aqueous solution of
vanadium. The complexation of vanadium with ligands began by
heating the mixture, which melted the substrate; the vanadium
complex was then able to solubilize in this organic phase. From
these results, we presume that this oxidation proceeds in the or-
ganic layer composed of the substrate. To improve the efficiency of
substrate contact with the vanadium complex, several salts were
added to the reaction system; MgSO4 was found to be the most
effective additive for solubilization in the established reaction
system (Table 9).13
MgSO4 (150 mmol), substrate (10 mmol), 90 ꢀC, open-air atmosphere.
b
Isolated yield. 1H NMR yield is shown in parenthesis.
oxidized by molecular oxygen and generates V(V) species B, and C
is formed through intramolecular hydrogen abstraction. To cor-
roborate this reaction pathway, we examined the catalyst com-
plex via 51V NMR spectroscopy. A weak and broad peak was
detected at ca.
d
ꢁ510 ppm in the 51V NMR spectrum (CDCl3) of
the mixture after reaction (see Supplementary data).14 From this,
we assume the formation of active catalytic V(V) species in this
reaction. Elimination of H2O2 (D) and ligand exchange of product
by alcohol (E) completes the catalytic cycle. We suppose that
benzhydrols may generate H2O2, as a more active oxidant than
molecular O2, act as a ligand for the vanadium catalyst, or elimi-
nate some active species to improve catalytic activity (see
Supplementary data).15
A plausible reaction pathway is proposed in Fig. 1.11e Firstly,
complex A is provided by the reaction of alcohol and catalyst. A is

K. Marui et al. / Tetrahedron 70 (2014) 2431e2438
2435
Table 8
Oxidation of secondary benzyl alcohol derivatives under open-air atmospherea
Table 9
Effect of salts on the oxidation of 1aa
Entry
Additive
Time (h)
Yieldb (%)
Entry
1
Substrate 3
Yield of 4 (%)b
1
2
3
4
5
6
7
MgSO4 (7.5 mmol)
MgSO4 (150 mmol)
NaCl (330 mmol)
MgSO4 (150 mmol)
Na2SO4 (152 mmol)
(NH4)2SO4 (150 mmol)
K2SO4 (150 mmol)
48
48
48
96
96
96
96
24
42
(11)
94
(80)
43
OH
3e
3g
3o
4e
4g
4o
83 (88)
73
OH
OH
a
Reaction conditions: VOSO4 (0.5 mmol), 4,40-t-Bubpy (1 mmol), H2O (50 mL),
additive, benzhydrol (10 mmol), 90 ꢀC, open-air atmosphere. The amounts of ad-
ditives were adjusted to be saturated in 90 ꢀC.
2
3
81
b
Isolated yield. 1H NMR yield is shown in parentheses.
85 (98)
VOSO4
+
O
L2V
4,4'-tBubpy (L)
OH
OH
4
5
3p
3q
4p
4q
92 (97)
80 (84)
R
R
MeO
Cl
OH
L2VIV
O
O2
OH
H
R
R
A
O
V OH
L2V IV
O
O
L2V
O
OH
OH
E
B
H
R
H
6
7
3r
3s
3t
4r
4s
4t
70 (89)
(92)
R
R
R
O
OH
R
R
O
L2V IV
O
IVOH
L2V
OH
O
OH
R
R
O
OH
H2O2
R
R
R
R
8
83 (98)
D
C
Fig. 1. A plausible catalytic cycle.
a
Reaction conditions: VOSO4 (0.5 mmol), 4,40-t-Bubpy (1 mmol), 1d (1 mmol),
H2O (50 mL), MgSO4 (150 mmol), substrate (10 mmol), 90 ꢀC, open-air atmosphere.
b
Isolated yield. 1H NMR yield is shown in parentheses.
4. Experimental section
4.1. General information
3. Conclusion
1H NMR spectra were recorded on
a JEOL JNM-ECX400
(400 MHz) FT NMR system using CDCl3 as the solvent with tetra-
methylsilane (TMS) as the internal standard. 13C NMR spectra were
recorded on a JEOL JNM-ECX400 (100 MHz) FT NMR system using
CDCl3 as the solvent. Alcohols were prepared by reduction of the
corresponding ketones with NaBH4 (except 1a, 1f, 1g, 1h, 1i, 3o, and
3s). Unless otherwise noted, other reactants and reagents were
purchased from commercial source and used without further
purification.
In conclusion, we have developed a novel vanadium-catalyzed
system for the oxidation of benzyl alcohol derivatives, using wa-
ter as the sole solvent under atmospheric pressure of O2. Further-
more, oxidation proceeds under open-air atmosphere, and the
corresponding carbonyl compounds are obtained in good yields.
The advantages of our oxidation method are three-fold: (1) special
devices such as autoclaves are unnecessary; (2) there is no cost for
the oxidant, as air functions as the terminal oxidant; (3) the gram-
scale oxidation of several alcohols proceeds efficiently using water
as a benign reaction medium. Finally, because our oxidation
method is safe, operationally simple, and amenable to gram-scale
synthesis under an air atmosphere, we expect this novel vana-
diumebipyridyl ligand complex to be useful for green oxidation in
organic synthesis and low-costly industrial chemical applications.
4.2. General procedure of synthesis of substrates16
To a solution of ketone or aldehyde (20.0 mmol) in MeOH
(50 mL) (or a mixture of MeOH and MeCN), NaBH4 (40 mmol) was
added and stirred at room temperature. After confirming by TLC

2436
K. Marui et al. / Tetrahedron 70 (2014) 2431e2438
that the starting material was consumed entirely, the reaction
mixture was poured into a separatory funnel, and organic layer was
extracted with EtOAc. Extracted organic layers were washed with
brine, and dried with anhydrous MgSO4, and concentrated to give
the pure product.
4.7. Analytical data
4.7.1. Benzophenone18aec (2a, Scheme 5). White solid; mp
47e48 ꢀC; yield 1.7149 g (94%); 1H NMR (400 MHz, CDCl3)
d
7.82e7.80 (m, 4H), 7.62e7.58 (m, 2H), 7.51e7.47 (m, 4H); 13C NMR
(100 MHz, CDCl3)
d 196.7, 137.6, 132.4, 130.0, 128.2.
17
4.3. Synthesis of [VO(t-Bubpy)2]SO4
4.7.2. 4-Methoxyphenyl(phenyl)methanone18b,c,j (2b, Table
7). White solid; mp 56e57 ꢀC; yield 1.9185 g (90%); 1H NMR
To a solution of 4,40-di-tert-butyl-2,20-bipyridyl (930.4 mg,
3.5 mmol) in EtOH (40 mL) was added a solution of VOSO4$5H2O
(437.6 mg, 1.7 mmol) in EtOH (20 mL). After stirring for 2.5 h at
room temperature, the solution was concentrated under reduced
pressure, and the precipitate was filtered using diethyl ether and
dried in vacuo to afford [VO(t-Bubpy)2]SO4 as a green powder
(700.6 mg). HR-ESI MS (MeOH) calcd for C36H49N4O5SV [MþH]þ
700.2863, found 700.2881.
(400 MHz, CDCl3)
d
7.82 (d, J¼7.3 Hz, 2H), 7.74 (d, J¼7.8 Hz, 2H),
7.54e7.43 (m, 3H), 6.95 (d, J¼7.3 Hz, 2H), 3.85 (s, 3H); 13C NMR
(100 MHz, CDCl3)
128.1, 113.5, 55.4.
d 195.4, 163.1, 138.2, 132.4, 131.8, 129.6, 129.3,
4.7.3. 4-Bromophenyl(phenyl)methanone18a,b,j (2c, Table 7). White
solid; mp 74e75 ꢀC; yield 2.2998 g (88%); 1H NMR (400 MHz,
CDCl3)
d 7.77e7.74 (m, 2H), 7.67e7.56 (m, 5H), 7.49e7.45 (m, 2H);
4.4. Oxidation of benzhydrols with molecular oxygen
13C NMR (100 MHz, CDCl3)
129.8, 128.3, 127.4.
d 195.4, 137.0, 136.2, 132.6, 131.5, 131.4,
Catalyst (0.005 mmol), ligand (0.01 mmol), and substrate
(0.1 mmol) were placed in a 10 mL two necked flask, and then water
(0.5 mL) was added. Next, the mixture was stirred at 90 ꢀC for the
appropriate time under O2 (0.1 MPa, O2 balloon). The yield of the
product was confirmed by 1H NMR spectroscopy using 1,2-
diphenylethane as the internal standard, after extraction of the
reaction mixture with diethyl ether. Purification of the products
was carried out by a short pad of silica gel using ethyl acetate and
hexane as eluent to afford the analytically pure ketones. The
products were identified by comparison with the commercially
available samples.
4.7.4. 4-Nitrophenyl(phenyl)methanone18b,j (2d, Table 7). Yellow
solid; mp 136e137 ꢀC; yield 1.1009 g (48%); 1H NMR (400 MHz,
CDCl3)
d
8.35 (d, J¼8.2 Hz, 2H), 7.94 (d, J¼8.2 Hz, 2H), 7.81 (d,
J¼7.3 Hz, 2H), 7.66 (t, J¼7.3 Hz, 1H), 7.53 (t, J¼7.3 Hz, 2H); 13C NMR
(100 MHz, CDCl3)
128.7, 123.5.
d 194.8, 149.8, 142.9, 136.3, 133.5, 130.7, 130.1,
4.7.5. Bis(4-methylphenyl)methanone18d,k (2e, Table 7). White solid;
mp 87e88 ꢀC; yield 1.3044 g (61%); 1H NMR (400 MHz, CDCl3)
d
7.70 (d, J¼8.2 Hz, 4H), 7.27 (d, J¼8.2 Hz, 4H), 2.44 (s, 6H); 13C NMR
(100 MHz, CDCl3)
d 196.2, 142.9, 135.2, 130.2, 128.9, 21.6.
4.5. Oxidation of benzyl alcohols with molecular oxygen
4.7.6. Phenyl(p-tolyl)methanone18a,b,k (2f, Table 7). Yellow solid; mp
52e53 ꢀC; yield 1.7642 g (90%); 1H NMR (400 MHz, CDCl3)
VOSO4$5H2O (6.3 mg, 0.025 mmol), 4,40-di-tert-butyl-2,20-
bipyridyl (13.4 mg, 0.05 mmol), and p-nitrobenzhydrol (0.05 mmol)
were placed in a 25 mL two necked round-bottomed flask, and then
water (2.5 mL) was added. Next, the mixture was stirred, and then
anhydrous MgSO4 (902.8 mg, 7.5 mmol) was slowly added into the
mixture (cooling by ice water). Then, substrate (0.5 mmol) was
added into the mixture at room temperature, and the mixture was
stirred at 90 ꢀC for the appropriate time under O2 (0.1 MPa, O2
balloon). The yield of the product was confirmed by 1H NMR
spectroscopy using 1,2-diphenylethane as the internal standard,
after extraction of the reaction mixture with diethyl ether. The
product was identified by comparison with the commercially
available sample.
d
7.79e7,76 (m, 2H), 7.73e7.71 (d, J¼8.2 Hz, 2H), 7.58e7.54 (tt,
J¼7.3 Hz, 1.37 Hz, 1H), 7.48e7.44 (m, 2H), 7.27 (d, J¼7.3 Hz, 2H), 2.43
(s, 3H); 13C NMR (100 MHz, CDCl3)
130.2, 129.9, 128.9, 128.1, 21.6.
d 196.4, 143.2, 137.9, 134.8, 132.1,
4.7.7. 4-Chlorophenyl(phenyl)methanone18b,c,k (2g, Table 7). White
solid; mp 70e71 ꢀC; yield 1.9627 g (90%); 1H NMR (400 MHz, CDCl3)
d
7.79e7.74 (m, 4H), 7.62e7.57 (m, 1H), 7.51e7.44 (m, 4H); 13C NMR
(100 MHz, CDCl3)
128.6, 128.3.
d 195.4, 138.8, 137.2, 135.8, 132.6, 131.4, 129.9,
4.7.8. Bis(4-methoxyphenyl)methanone18c,l (2h, Table 7). White
solid; mp 143e144 ꢀC; yield 1.6673 (69%); 1H NMR
g
4.6. Oxidation of alcohols under open-air atmosphere
(400 MHz, CDCl3)
d
7.78 (d, J¼7.3 Hz, 4H), 6.95 (d, J¼7.3 Hz, 4H), 3.87
(s, 6H); 13C NMR (100 MHz, CDCl3)
55.3.
d 194.3, 162.7, 132.1, 130.6, 113.3,
VOSO4$5H2O (126.5 mg, 0.5 mmol), 4,40-di-tert-butyl-2,20-
bipyridyl (268.4 mg, 1 mmol), and p-nitrobenzhydrol (229.3 mg,
0.05 mmol) were placed in a 100 mL round-bottomed flask, and
then water (50 mL) was added. Next, the mixture was stirred, and
then anhydrous MgSO4 (18.06 g, 150 mmol) was slowly added into
the mixture (cooling by ice water). After that, substrate (10 mmol)
was added into the mixture at room temperature, and the mixture
was stirred at 90 ꢀC for the appropriate time under open-air at-
mosphere. After the reaction, the mixture was extracted with ethyl
acetate and dried over anhydrous MgSO4. The extracts were con-
centrated in vacuo. Purification of the products was carried out by
silica gel column chromatography using hexane and diethyl ether
as eluent to afford the analytically pure ketones. In Table 7 (entries
3, 7, and 8), crude products were purified by recrystallization with
ethyl acetate in refrigerator, afforded analytically pure ketones. The
product was identified by comparison with the commercially
available sample using 1H NMR spectroscopy.
4.7.9. Bis(4-chlorophenyl)methanone18c,k (2i, Table 7). White solid;
mp 142e143 ꢀC; yield 1.4536 g (57%); 1H NMR (400 MHz, CDCl3)
d
7.73 (d, J¼8.7 Hz, 4H), 7.47 (d, J¼8.2 Hz, 4H); 13C NMR (100 MHz,
CDCl3)
d 194.2, 139.1, 135.5, 131.3, 128.8.
4.7.10. 1-Phenylpentan-1-one18g (4e, Table 8). Colorless oil; yield
1.3454 g (83%); 1H NMR (400 MHz, CDCl3)
7.94e7.92 (m, 2H),
d
7.51e7.47 (m, 1H), 7.42e7.38 (m, 2H), 2.91 (t, J¼7.3 Hz, 2H) 1.69
(quint, J¼7.3 Hz, 2H), 1.38 (sextet, J¼7.3 Hz, 2H) 0.93 (t, J¼7.3 Hz,
3H); 13C NMR (100 MHz, CDCl3)
37.8, 26.0, 22.1, 13.6.
d 199.8, 136.7, 132.4, 128.1, 127.6,
4.7.11. Cyclohexyl(phenyl)methanone18a,m (4g, Table 8). White solid;
mp 54e55 ꢀC; yield 1.519 g (81%); 1H NMR (400 MHz, CDCl3)
d
7.81e7.78 (m, 2H), 7.38e7.25 (m, 3H), 3.15e3.08 (m, 1H),

K. Marui et al. / Tetrahedron 70 (2014) 2431e2438
2437
1.75e1.06 (m, 10H); 13C NMR (100 MHz, CDCl3)
d 203.2, 135.9, 132.3,
128.2, 127.9, 45.1, 29.0, 25.6, 25.5.
ꢀ
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7.97e7.95 (m, 2H),
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199.8, 136.8, 132.5, 128.2, 127.7, 40.1, 17.4, 13.5.
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7.93 (d,
d
J¼8.7 Hz, 2H), 6.91 (d, J¼8.7 Hz, 2H), 3.83 (s, 3H), 2.87 (t, J¼7.3 Hz,
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7.89 (d, J¼8.2 Hz, 2H), 7.42 (d, J¼8.2 Hz, 2H), 2.91 (t, J¼6.9 Hz, 2H),
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4.7.16. 1-Phenylheptan-1-one18i (4t, Table 8). Colorless oil; yield
1.069 g (83%); 1H NMR (400 MHz, CDCl3)
7.97e7.95 (m, 2H),
d
7.57e7.53 (m, 1H), 7.47e7.44 (m, 2H) 2.96 (t, J¼7.3 Hz, 2H), 1.74
(quint, J¼7.3 Hz, 2H), 1.40e1.30 (m, 6H) 0.89 (t, J¼6.9 Hz, 3H); 13C
NMR (100 MHz, CDCl3)
29.0, 24.3, 22.5, 14.0.
d 200.6, 137.1, 132.8, 128.5, 128.0, 38.6, 31.7,
Acknowledgements
This research was supported by JST Research Seed Quest Pro-
gram (Lower Carbon Society), from the Ministry of Education,
Culture, Sports, Science, and Technology Promotion (KRF). S.K. ac-
knowledges Research Fellowship of Japan Society for the Promotion
of Science (JSPS) for Young Scientists.
Supplementary data
13. We suppose that satd MgSO4 (aq) as solvent to improve the contact of catalyst
and substrates; however, there is possibility that a high concentration of SO42ꢁ
or Mg2þ is desirable in this reaction system.
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2014.02.032.
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~
Adao, P.; Kuznetsov, M. L.; Martins, A. M.; Avecilla, F.; Costa Pessoa, J. Inorg.
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