Aerobic alcohol oxidation catalyzed by supported ruthenium hydroxides

Article abstract of DOI:10.1016/j.jcat.2009.10.004

The supported ruthenium hydroxide (Ru(OH)_x) catalysts prepared with three different TiO₂ supports and an Al₂O₃ support showed the high catalytic activity for the oxidation of alcohols with molecular oxygen. In the presence of the most active catalyst, various kinds of alcohols could be converted into the corresponding carbonyl compounds in high yields. In addition, the catalyst could be applied to the aerobic amine oxidation. The observed catalysis was truly heterogeneous and the catalyst retrieved after the reaction could be reused with keeping its high catalytic performance. A reaction mechanism involving the ruthenium alcoholate formation/hydride abstraction (β-elimination) has been proposed. The alcoholate formation and hydride abstraction are reversible reactions. The kinetic isotope effects (κ_H/κ_D = 4.9- 5.3) show that the C-H bond breaking is included in the rate-determining step. The present Ru(OH)_x-catalyzed aerobic alcohol oxidation was dependent on the coordination number (CN) of nearest-neighbor Ru atoms in Ru(OH)_x and the suitable CN existed.

Full text of DOI:10.1016/j.jcat.2009.10.004

Journal of Catalysis 268 (2009) 343–349
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Aerobic alcohol oxidation catalyzed by supported ruthenium hydroxides
a,b
a
a
a,b,*
Kazuya Yamaguchi , Jung Won Kim , Jinling He , Noritaka Mizuno
a
Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan
Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The supported ruthenium hydroxide (Ru(OH) ) catalysts prepared with three different TiO supports and
x
2
Received 2 September 2009
Revised 1 October 2009
Accepted 1 October 2009
Available online 31 October 2009
2 3
an Al O support showed the high catalytic activity for the oxidation of alcohols with molecular oxygen.
In the presence of the most active catalyst, various kinds of alcohols could be converted into the corre-
sponding carbonyl compounds in high yields. In addition, the catalyst could be applied to the aerobic
amine oxidation. The observed catalysis was truly heterogeneous and the catalyst retrieved after the
reaction could be reused with keeping its high catalytic performance. A reaction mechanism involving
the ruthenium alcoholate formation/hydride abstraction (b-elimination) has been proposed. The alcohol-
Keywords:
Alcohol oxidation
Heterogeneous catalysis
Molecular oxygen
Ruthenium hydroxide
ate formation and hydride abstraction are reversible reactions. The kinetic isotope effects (k
.3) show that the C–H bond breaking is included in the rate-determining step. The present Ru(OH)
alyzed aerobic alcohol oxidation was dependent on the coordination number (CN) of nearest-neighbor Ru
atoms in Ru(OH) and the suitable CN existed.
/k
H D
= 4.9–
5
x
-cat-
x
Ó 2009 Elsevier Inc. All rights reserved.
1
. Introduction
The oxidation of alcohols is of paramount importance in organic
functional group transformations such as the oxidative
dehydrogenation and oxygenation of amines [14,15], hydration of
nitriles [16], and hydrogen-transfer reactions [17,18] also effi-
syntheses in laboratories as well as chemical industries because of
the versatile use of products such as aldehydes and ketones as
important intermediates for pharmaceuticals, agricultural chemi-
cals, and fine chemicals [1–5]. Until now, many efficient heteroge-
neous catalysts based on noble metal clusters, metal oxides, and
metal hydroxides have been developed for the oxidation of alco-
hols with molecular oxygen as a sole oxidant [6,7]. The scope
and limitation of heterogeneously catalyzed aerobic alcohol oxida-
tion systems have been summarized in the recent excellent review
articles [6,7]. Among heterogeneous catalysts, we [8,9] and other
x 2 3
ciently proceeded with Ru(OH) /Al O . The outstanding catalytic
performance is likely attributed to the presence of coordinatively
unsaturated ruthenium centers (Lewis acid sites) [19–21] and ba-
sic hydroxide groups (Brønsted base sites) [21]. After our first re-
x x
port for the supported Ru(OH) catalyst [8], Ru(OH) supported
on zeolites [11], titanium oxide nanotubes [12], and carbon nano-
tubes [13] have been reported to be active for the aerobic alcohol
oxidation.
It has been reported that Ru(OH)
like core structure [11,21,22]. It is very difficult to control the
structure and size of (unsupported) Ru(OH) because the dehydra-
x
has one-dimensional chain-
research groups [10–13] focused on ruthenium hydroxide Ru(OH)
x
x
(
hydrate oxide RuO
Matsumoto and Watanabe first reported that Ru(OH)
2
ꢀxH
2
O) for the aerobic alcohol oxidation.
(RuO
tive condensation easily proceeds upon the heat treatment (>ca.
100 °C) [22]. Very recently, we have successfully prepared highly
x
2
ꢀx-
H
2
O) could catalyze the aerobic oxidation of allylic alcohols [10].
dispersed Ru(OH)
Al [21]. The coordination numbers (CNs) of one-dimensional
chain-like Ru(OH) could be controlled by choosing appropriate
supports and the supported Ru(OH) catalysts were highly ther-
x 2
on metal oxide supports such as TiO and
The turnover number (TON) and turnover frequency (TOF) were
low and the applicability was limited to only allylic alcohols [10].
In 2002, we reported that Ru(OH)
2 3
O
x
x
supported on Al
2
O
3
(Ru(OH)
x
/
x
Al ) could act as an efficient reusable heterogeneous catalyst
2
O
3
mally stable under reaction conditions (up to ca. 150 °C) [21]. This
for the aerobic oxidation of various kinds of structurally diverse
alcohols including benzylic, allylic, aliphatic, and heteroatom-
containing ones [8]. Besides the aerobic alcohol oxidation, many
offered an opportunity to investigate the CN-dependent catalytic
activity of the Ru(OH) species. For example, the reaction rates
x
(TOFs) for the hydrogen-transfer reactions such as the racemiza-
tion of chiral secondary alcohols and Meerwein–Ponndorf–Ver-
ley-type (MPV-type) reduction of carbonyl compounds were
much dependent on the CNs of Ru(OH) and monotonically in-
x
creased with the decrease in the CNs (Fig. S1) [21]. As for the aer-
obic alcohol oxidation, no studies have been devoted to address the
*
Corresponding author. Address: Department of Applied Chemistry, School of
Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656,
Japan. Fax: +81 3 5841 7220.
E-mail address: tmizuno@mail.ecc.u-tokyo.ac.jp (N. Mizuno).
0
021-9517/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2009.10.004

3
44
K. Yamaguchi et al. / Journal of Catalysis 268 (2009) 343–349
relationship between the CNs of Ru(OH)
as far as we know. In this study, the supported Ru(OH)
with the different CNs were utilized for the aerobic alcohol oxida-
tion and the CN-dependent catalytic activity was examined. In
x
addition, the scope of the present Ru(OH) -catalyzed aerobic alco-
x
and the catalytic activity,
catalysts
mined by GC analysis. All products have been identified by
1
13
x
comparison of their H and C NMR, and mass spectra with the lit-
erature data. The retrieved catalyst was washed with an aqueous
solution of NaOH (pH 13) and water, and then dried in vacuo be-
fore being recycled. The TOF values were calculated based on the
total Ru in the catalysts.
hol oxidation and possible reaction mechanism were also investi-
gated in detail.
3
. Results and discussion
2
. Experimental section
3.1. The effect of catalysts on the aerobic alcohol oxidation
2.1. General
We prepared four kinds of supported Ru(OH)
x
catalysts
supports (anatase
(A), BET surface area: 316 m g ), anatase TiO (TiO (B),
(C), 3.2 m g )) and an Al sup-
port (160 m g ) (Table 1, see Supporting information for prepara-
tion). The ruthenium contents in Ru(OH) /TiO (A), Ru(OH)
TiO (B), Ru(OH) /TiO (C), and Ru(OH) /Al were 2.1, 2.2, 2.2,
The NMR spectra were recorded on JEOL JNM-EX-270. The 1H
(
Ru(OH)
x
/support) with three different TiO
2
and 1 C NMR spectra were measured at 270 and 67.8 MHz, respec-
3
2
ꢁ1
TiO
2
(TiO
2
2
2
1
tively, in [D ]chloroform or [D
8
]toluene with TMS as an internal
2
ꢁ1
2
ꢁ1
7
3 m g ), and rutile TiO
2
(TiO
2
2 3
O
standard. The GC analyses were performed on Shimadzu GC-17A
using a flame ionization detector (FID) equipped with a DB-WAX
capillary column (internal diameter = 0.25 mm, length = 30 m) or
a Rt b-CDEXM capillary column (internal diameter = 0.25 mm,
length = 30 m). The mass spectra were recorded on Shimadzu
GCMS-QP2010 equipped with a TC-5HT capillary column (internal
diameter = 0.25 mm, length = 30 m). The ICP-AES analyses were
performed with Shimadzu ICPS-8100. The X-ray absorption spectra
were recorded at the NW10A beamline of PF at KEK, Japan (pro-
posal No. 2007G096) [21]. The data were analyzed using
REX2000 software (version 2.5, Rigaku) [21]. The heterolytic ben-
zylic C–H bond dissociation energies of p-substituted benzyl alco-
hols were calculated at the B3LYP/6-311++G(d, p) level theory with
Gaussian 03 program package [23].
2
ꢁ1
x
2
x
/
2
x
2
x
2 3
O
and 2.1 wt%, respectively (Table 1). The detailed structural charac-
terization of these catalysts has been reported elsewhere [21]. The
CNs of the nearest-neighbor Ru atoms (EXAFS analysis, Table 1, Ta-
ble S1 and Fig. S2) decreased in the order of Ru(OH)
C) > Ru(OH) /Al > Ru(OH) /TiO (B) > Ru(OH) /TiO (A) [21].
First, the catalytic activities for the oxidation of 1-phenyletha-
x 2
/TiO
(
x
2
O
3
x
2
x
2
nol (1a) to acetophenone (2a) with 1 atm of molecular oxygen
were compared among various ruthenium catalysts (Table 2).
The supported Ru(OH)
mance for the oxidation (Table 2, entries 1–4). Among the sup-
ported Ru(OH) catalysts examined, the observed TOFs (TOFsobs
x
catalysts showed high catalytic perfor-
x
)
ꢁ1
increased in the order of Ru(OH)
x
/TiO
2
(C) (TOFobs = 75 h ) <
2
.2. Reagents and catalysts
ꢁ1
ꢁ1
Ru(OH)
TiO
a were >99% in all cases. The TOFobs value of the most active
Ru(OH) /TiO (B) catalyst was about twice higher than that of the
previously reported Ru(OH) /Al [8,9].
No reaction proceeded in the absence of the catalysts or in the
presence of supports such as TiO and Al (Table 2, entries 14–
8). The oxidation hardly proceeded in the presence of Ru(OH)
x
/Al
2
O
3
(90 h ) < Ru(OH)
x
/TiO
2
(A) (100 h ) < Ru(OH)
x
/
ꢁ1
2
(B) (160 h ) (see the later section), while the selectivities to
Substrates and solvents were commercially obtained from To-
2
kyo Kasei, Aldrich, and Fluka (reagent grade) and purified before
the use [24]. Anatase TiO (TiO (A), BET surface area: ST-01,
(TiO (B), JRC-TIO-1, 73 m g ), rutile
(C), SUPER-TITANIA G-2, 3.2 m g ), and Al
x
2
2
2
2 3
O
x
2
ꢁ1
2
ꢁ1
3
16 m g ), anatase TiO
TiO (TiO
4, 160 m g ) were obtained from Ishihara Sangyo Kaisya Ltd.,
2
2
2
ꢁ1
2
2
2
2 3
O (KHS-
2 3
O
2
ꢁ1
2
1
x
the Catalysis Society of Japan, Showa Denko K.K., and Sumitomo
Chemical, respectively. RuHAP (9.1 wt%) was purchased from
Wako. The supported Ru(OH) catalysts and Ru(OH) were pre-
x x
pared according to the literature procedures (see Supporting infor-
mation) [21].
2
and anhydrous RuO (Table 2, entries 5 and 6). The catalytic activ-
ity of RuHAP (Ru–Cl species supported on hydroxyapatite) [25] was
much lower than those of the supported Ru(OH) catalysts (Table 2,
entry 7). In the case of the catalyst precursor of RuCl O, the
x
3
ꢀnH
2
selectivity to 2a was very low because of the formation of 1-phe-
0
nyl-1-tolylethane and 1,1 -(oxydiethylidene)-bis-benzene as
2.3. Catalytic aerobic oxidation
2 3 3
byproducts (Table 2, entry 8). Although RuCl (PPh ) was reported
to be active for the aerobic alcohol oxidation in the presence of
0
0
A suspension of the supported Ru(OH)
x
catalyst in a solvent was
2,2 ,6,6 -tetramehylpiperidine N-oxyl (TEMPO) [26] and hydroqui-
none [27], it gave only a stoichiometric amount of 2a in the ab-
sence of these additives (Table 2, entry 9). Other ruthenium
stirred for 5 min. Then, a substrate was added and molecular oxy-
gen was passed through the suspension. The mixture was stirred
(
800 rpm) at reaction temperature under 1 atm of molecular oxy-
complexes such as RuCl
2 2 2 2 3
(bpy) , [RuCl (benzene)] , Ru(acac) , and
gen. The yield and product selectivity were periodically deter-
3
Ru (CO)12 were completely inactive (Table 2, entries 10–13).
Table 1
x
Various supported Ru(OH) catalysts.
Supporta
BET surface area (m2
Support
g
ꢁ1
)
CNb
d (Å)c
Catalyst (wt%)
Catalyst
Ru(OH)
Ru(OH)
Ru(OH)
Ru(OH)
Ru(OH)
x
x
x
x
x
/TiO
/TiO
/TiO
2
2
2
(A) (2.1)
(B) (2.2)
(C) (2.2)
Anatase TiO
Anatase TiO
2
2
316
73
3.2
160
–
298
74
7.0
163
15
0.37 (±0.17)
0.76 (±0.21)
0.94 (±0.22)
0.91 (±0.20)
1.4 (±0.2)
3.07 (±0.02)
3.09 (±0.01)
3.10 (±0.01)
3.08 (±0.01)
3.10 (±0.01)
Rutile TiO
2
/Al
2
O
3
(2.1)
Al O
2 3
–
a
b
c
See Section 2.
CN = average coordination number of nearest-neighbor Ru atoms.
d = average interatomic Ruꢀ ꢀ ꢀRu distance.

K. Yamaguchi et al. / Journal of Catalysis 268 (2009) 343–349
345
Table 2
a
The aerobic oxidation of 1a by various catalysts.
OH
O
catalyst
1a
2a
TOFobs (h 1)b
ꢁ
Entry
Catalyst
Conversion of 1a (%)
Selectivity to 2a (%)
1
2
3
4
5
6
7
8
9
Ru(OH)
Ru(OH)
Ru(OH)
Ru(OH)
Ru(OH)
x
x
x
x
x
/TiO
/TiO
/TiO
2
2
2
(A)
(B)
(C)
49
62
27
45
>99
>99
>99
>99
–
100
160
75
90
–
–
1.0
–
0.5
–
–
–
–
2
/Al O
3
No reaction
No reaction
2
>99
1
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
RuO
2
(anhydrous)
–
RuHAP
>99
c
RuCl
RuCl
RuCl
3
ꢀnH
2
(PPh
2
(bpy)
2
O
<1
)
3 3
>99
–
–
–
–
–
–
–
–
10
11
12
13
14
15
16
17
18
2
[RuCl
Ru(acac)
Ru (CO)12
2
(benzene)]
2
3
3
d
d
d
d
TiO
TiO
TiO
2
2
2
(A)
(B)
(C)
–
–
–
–
2
Al O
3
None
–
–
a
Reaction conditions: 1a (1 mmol), catalyst (Ru: 1 mol%), toluene (3 mL), 80 °C, 0.5 h, under 1 atm of molecular oxygen. Conversion and selectivity were determined by GC
using an internal standard.
b
Based on the observed reaction rate.
c
0
1
4
-Phenyl-1-tolylethane and 1,1 -(oxydiethylidene)-bis-benzene were formed as byproducts.
0 mg.
d
3.2. The scope of the present aerobic oxidation
x 2
the Ru(OH) /TiO (B) catalyst was removed from the reaction mix-
ture by hot filtration at ca. 50% conversion of 1a. After removal of
the catalyst, the reaction was again carried out with the filtrate un-
der the same conditions. In this case, the reaction was completely
stopped. It was confirmed by the ICP-AES analysis that no ruthe-
nium was detected in the filtrate (below detection limit of
7 ppb). All these facts can rule out any contribution to the observed
catalysis from ruthenium species that leached into the reaction
solution and the observed catalysis is intrinsically heterogeneous
[29]. In addition, it was confirmed by XANES and EXAFS spectra
that the local structure of the ruthenium species in the used
x 2
In the presence of the most active Ru(OH) /TiO (B) catalyst, the
aerobic oxidation of various kinds of structurally diverse alcohols
was examined (Table 3). Secondary (1a–1d) and primary (1e–1h)
benzylic alcohols were oxidized to afford the corresponding car-
bonyl compounds in quantitative yields (Table 3, entries 1–8).
Notably, the oxidation efficiently proceeded even at room temper-
ature (ca. 25 °C) (Scheme 1). The oxidation of 1c with the unstable
cyclopropane ring exclusively proceeded to give the corresponding
ketone 2c without formation of the ring-opened products (Table 3,
entry 3). It has been reported that 1d gave the corresponding ke-
tone 2d with two-electron transfer oxidants and that 2e and tert-
butyl radical were obtained as primary products with one-electron
transfer oxidants (radical mechanism) [28]. In the present
Ru(OH)
(Fig. S3). The Ru(OH)
x
/TiO
2
(B) catalyst was the same as that in the fresh catalyst
/TiO (B) catalyst could be reused for the oxi-
x
2
dation of 1a at least three times with retention of its high catalytic
performance (>99% yield of 2a for the third reuse experiment).
x 2
Ru(OH) /TiO (B)-catalyzed system, the corresponding ketone 2d
was exclusively obtained as a sole product without the formation
of 2e (Table 3, entry 4). In the case of 1i, the corresponding unsat-
urated aldehyde 2i was obtained without the intramolecular
hydrogen-transfer reaction and geometrical isomerization of the
carbon–carbon double bond (Table 3, entry 9). The present system
could oxidize linear (1j) and cyclic aliphatic (1k) alcohols in high
3
.3. Mechanistic studies
The reaction mechanism for the present aerobic oxidation of
x 2
alcohols was examined with the most active Ru(OH) /TiO (B) cat-
alyst. The addition of a radical scavenger of 2,6-di-tert-butyl-4-
methylphenol (1 mol% with respect to 1a) did not affect the
reaction rate and product selectivity for the oxidation of 1a.
Furthermore, the oxidations of 1c and 1d exclusively proceeded
to give the corresponding ketones 2c and 2d without the formation
of ring-opened and cleaved products, respectively (Table 3, entries
yields (Table 3, entries 10 and 11). Also, Ru(OH)
catalyzed the oxidation of heteroatom-containing alcohols such as
l and 1m (Table 3, entries 12 and 13). In addition, the present sys-
x 2
/TiO (B) efficiently
1
tem could be applied to the aerobic oxidation of secondary (3a and
3
b) and primary (3c)1 amines (Table 3, entries 14–16).
3
and 4). These results show that free-radical intermediates are not
In order to verify whether the observed catalysis is derived from
solid Ru(OH) /TiO (B) or leached ruthenium species, the oxidation
x 2
of 1a was carried out under the conditions described in Table 3 and
involved in the present alcohol oxidation. In the competitive oxida-
tion of 1a and 1e, the oxidation of a primary alcohol 1e proceeded
much faster than that of a secondary alcohol 1a (Fig. 1). The faster
oxidation of a primary alcohol in the presence of a secondary one
suggests the formation of an alcoholate species via the ligand
exchange between the ruthenium hydroxide species and an alco-
hol [25–27,30–33].
1
For the oxidation of primary amines with Ru(OH)
x
/TiO
2
(B), the selectivities to the
/Al . This is likely
due to the decomposition of the intermediate imines by the acidic TiO support.
desired nitriles were lower (by ca. 10%) than those with Ru(OH)
x
2 3
O
2

3
46
K. Yamaguchi et al. / Journal of Catalysis 268 (2009) 343–349
Table 3
The aerobic oxidation of various alcohols and amines catalyzed by Ru(OH)
/TiO
2
(B).a
x
Entry
1
Substrate
OH
Time (min)
Conversion (%)
>99
Product
Selectivity (%)
>99
1a
1b
1c
1d
120
O
2a
2b
2c
2d
2
3
4
OH
72
>99
>99
>99
O
>99
>99
>99
OH
OH
60
O
O
60
5
6
7
1e
1f
60
40
40
>99
>99
>99
2e
2f
>99
>99
>99
OH
OH
O
O
1g
2g
OH
O
MeO
Cl
MeO
Cl
8
9
1h
60
>99
2h
>99
OH
O
1i
1j
420
180
>99
84
2i
2j
>99
>99
OH
O
1
0b
O
1
1
1
1k
1l
180
360
76
2k
2l
>99
>99
OH
OH
O
2c
>99
S
N
S
N
O
1
1
3c
4
1m
3a
270
150
>99
99
2m
4a
>99
>99
OH
O
N
H
N
H
1
1
5
6
3b
3c
210
74
4b
>99
N
H
N
240
>99
CN
4c
76
NH2
OMe
OMe
x 2
/TiO (B) (Ru: 1 mol% for alcohols, 3 mol% for amines), toluene (3 mL), 80 °C for alcohols, 100 °C for amines, under
a
Typical reaction conditions: substrate (1 mmol), Ru(OH)
atm of molecular oxygen. Yields were determined by GC analyses using an internal standard.
1
b
2
5
mol%.
mol%.
c
OH
Ru(OH) /TiO (B) (5 mol%)
x
2
O
+
1/2O2
1 atm)
+ H O
2
toluene (3 mL), rt, 12 h
(
1
e (1 mmol)
2
e (84% yield)
x 2
Scheme 1. The Ru(OH) /TiO (B)-catalyzed oxidation of 1e at room temperature.
The Ru(OH)
p-substituted benzyl alcohols gave the following reactivity order:
p-CH O (R /R = 2.4) > p-CH (1.6) > p-H (1.0) > p-Cl (0.95) (the
values in the parentheses were the relative rates and the rate of 1e
p-H) was taken as a unity). Therelativerates (log(R /R )) are plotted
x
/TiO
2
(B)-catalyzed competitive oxidations of
served, suggesting the formation of the carbocation-type transition
state via the hydride abstraction. The formation of the ruthenium
hydride species was evidenced by the fact that Ru(OH) /TiO (B)
x 2
showed high catalytic activity for the hydrogen-transfer reactions
such as the racemization of chiral secondary alcohols and MPV-type
reduction of carbonyl compounds using 2-propanol [21].
3
X
H
3
(
X
H
against the heterolytic benzylic C–H bond dissociation energies of
p-substituted benzyl alcohols calculated at the B3LYP/6-
On the basis of the above results, we here propose a possible
reaction mechanism as shown in Scheme 2. The mechanism is
3
11++G(d, p) level theory (Fig. 2). The good liner correlation was ob-

K. Yamaguchi et al. / Journal of Catalysis 268 (2009) 343–349
347
1
00
HO
R
H
R
R'
step 1
OH
H O
2
R'
H
O
HO
1
e
8
6
4
2
0
0
0
0
0
Run+
Run+
step 2
O
R
R'
H
R
R'
O
H
OH
β-elimination
Run+
Run+
hydride re-addition (racemization)
1
a
step 3
O2
1/2O2
H
Run+
HOO
Run+
HO
Run+
0
5
10
15
t/ min
20
25
30
Scheme 2. A proposed reaction mechanism for the present Ru(OH)
aerobic oxidation of alcohols.
x
-catalyzed
Fig. 1. The reaction profiles of the competitive oxidation of 1a and 1e. Reaction
conditions: 1a (0.5 mmol), 1e (0.5 mmol), Ru(OH)
3 mL), 80 °C, under 1 atm of molecular oxygen.
x
2
/TiO (B) (Ru: 1 mol%), toluene
(
Table 4
a
The kinetic isotope effects for the oxidation of D-1f .
0
0
0
0
0
.5
.4
.3
.2
.1
0
H D
OH
D
H
catalyst
O
O
+
D-1f
D-2f
2f
1g
Entry
Catalyst
Yield (%)
H D
k /k
D-2f
2f
1f
1
2
3
4
5
Ru(OH)
Ru(OH)
Ru(OH)
Ru(OH)
Ru(OH)
x
/TiO
/TiO
2
(A)
(B)
83
84
75
60
17
16
15
12
4.9
5.3
5.0
5.1
x
x
x
x
2
/Al
2
O
3
/TiO
2
(C)
No reaction
1e
a
Reaction conditions: D-1f (0.5 mmol), catalyst (Ru: 1 mol%), [D
1.5 mL), 80 °C, 1.5 h, under 1 atm of molecular oxygen. Yields were determined by
8
]toluene
1
h
(
1
H NMR.
-
0.1
8.4
8.6
8.8
9
9.2
9.4
Heterolytic bond dissociation energy/ eV
Fig. S4). The k
of -deuterio-p-methylbenzyl alcohol (D-1f) with the supported
Ru(OH) catalysts were in the range of 4.9–5.3 (Table 4). These re-
sults show that the C–H bond breaking (step 2) is included in the
H D
/k values (kinetic isotope effects) for the oxidation
X H
Fig. 2. The relationship between the relative rates (log(R /R )) and heterolytic
benzylic C–H bond dissociation energies of p-substituted benzyl alcohols. Reaction
conditions: 1e (0.5 mmol), p-substituted benzyl alcohol (1f, 1g, or 1h, 0.5 mmol),
a
x
Ru(OH)
x 2
/TiO (B) (Ru: 1 mol%), toluene (3 mL), 80 °C, under 1 atm of molecular
oxygen.
rate-determining step in all cases with supported Ru(OH)
x
cata-
lysts [34].
x
intrinsically the same as the widely accepted one with Ru(OH) -
based catalysts [8–13]. First, the ruthenium alcoholate species is
formed (step 1). No kinetic isotope effect was observed for the oxi-
dation of 2-propanol-OD. Thus, the ligand exchange between the
ruthenium hydroxide and an alcohol is very fast and is not the
rate-determining step. Then, the typical b-elimination proceeds
to give the corresponding carbonyl compound and the ruthenium
hydride species (step 2). This step is reversible because the oxida-
tion of a chiral secondary alcohol proceeds with a simultaneous de-
crease in the enantiomeric excess (ee) of the substrate (see the
later section). Finally, the hydride species is reoxidized by molecu-
lar oxygen (step 3). The amine oxidation proceeds in a similar way;
the formation of ruthenium amide species followed by the b-elim-
ination [14]. The reaction rates for the oxidation of 1a were almost
independent of the partial pressure of molecular oxygen (>0.5 atm,
3.4. The reason why the CN-dependent catalytic activity is observed for
the aerobic alcohol oxidation
As mentioned in the previous section (Table 2), the TOFsobs for
the oxidation of 1a increased in the order of Ru(OH) /TiO (C) (TO-
x
2
ꢁ1
ꢁ1
F
h
obs
= 75 h ) < Ru(OH) /Al O
x
2
3
x 2
(90 h ) < Ru(OH) /TiO (A) (100
) < Ru(OH) /TiO (B) (160 h ). The TOFsobs are plotted against
x 2
ꢁ
1
ꢁ1
the CNs of nearest-neighbor Ru atoms in the supported Ru(OH)x
catalysts (Fig. 3). The TOFsobs increased with the decrease in the
CNs, reached maximum with Ru(OH) /TiO (B), and then
x
2
2
decreased.
2
For the aerobic oxidation of 3a, the TOFsobs also increased with the decrease in the
CNs, reached maximum with Ru(OH) /TiO (B), and then decreased (Fig. S5).
x
2

3
48
K. Yamaguchi et al. / Journal of Catalysis 268 (2009) 343–349
Table 5
1
1
50
20
The TOFobs, TOFrac, and TOFb-elim values for the oxidation of (R)-1a.a
B
TOFobs (h 1)b
ꢁ
TOFrac (h )c
ꢁ1
TOFb-elim (h )d
ꢁ1
Entry
Catalyst
1
2
3
4
5
Ru(OH)
Ru(OH)
Ru(OH)
x
/TiO
/TiO
2
(A)
(B)
100
136
92
65
52
10
2.6
0.20
152
146
95
x
x
2
A
/Al
2
O
3
C
Ru(OH) /TiO (C)
65
x
2
9
6
3
0
0
0
0
Ru(OH)
x
No reaction
a
Reaction conditions: (R)-1a (1 mmol), catalyst (Ru: 1 mol%), toluene (3 mL),
D
8
0 °C, under 1 atm of molecular oxygen.
b
Based on the observed reaction rate.
c
ꢁ1
Based on the initial rate. TOFrac (h ) = {log(ee/100)}/{log(1 ꢁ (x/100))}/t, where
ee, x, and t are the enantiomeric excess (%), total amount of Ru in the catalyst
(
mol%), and reaction time (h), respectively.
TOFb-elim (h ) = TOFobs (h ) + TOFrac (h ).
d
ꢁ1
ꢁ1
ꢁ1
E
0
0.5
1
1.5
2
1
1
60
20
80
CN of nearest-neighbor Ru atoms
(a)
Fig. 3. The relationship between TOFsobs and CNs of nearest-neighbor Ru atoms for
the aerobic oxidation of 1a. (A) Ru(OH) /TiO (A), (B) Ru(OH) /TiO (B), (C) Ru(OH)
Al , (D) Ru(OH) /TiO (C), and (E) Ru(OH) . Reaction conditions: 1a (1 mmol),
catalyst (Ru: 1 mol%), toluene (3 mL), 80 °C, under 1 atm of O
x
2
x
2
x
/
2
O
3
x
2
x
2
.
1
00
(
d)
(
b)
(
(
c)
40
b)
9
9
8
5
0
5
0
0
0.5
1
1.5
2
CN of nearest-neighbor Ru atoms
Fig. 5. The relationship between TOFs ((a)TOFsb-elim and (b)TOFsrac) and CNs of
nearest-neighbor Ru atoms for the aerobic oxidation of (R)-1a. Reaction conditions:
R)-1a (1 mmol), catalyst (Ru: 1 mol%), toluene (3 mL), 80 °C, under 1 atm of O
(
2
.
(
a)
ꢁ1
ꢁ1
(
(
2.6 hꢁ1) < Ru(OH)
Table 5 and Fig. 5b). This order was well consistent with that for
the racemization under anaerobic conditions (Fig. S1) [21].
As mentioned in the previous section: (i) no kinetic isotope ef-
fect was observed for 2-propanol-OD, (ii) the kinetic isotope effects
x
2
/TiO (B) (10 h ) < Ru(OH)
x
/TiO
2
(A) (52 h
)
0
20
40
60
80
100
Yield of 2a/ %
Fig. 4. The ee of the substrate vs. yield of 2a plots for the oxidation of (R)-1a with
a) Ru(OH) /TiO (A) and (b) Ru(OH) /TiO (B), (c) Ru(OH) /Al , and (d) Ru(OH)
TiO (C). Reaction conditions: (R)-1a (1 mmol), catalyst (Ru: 1 mol%), toluene (3 mL),
0 °C, under 1 atm of O
(
x
2
x
2
x
2
O
3
x
/
2
x
for the oxidation of D-1f with the supported Ru(OH) catalysts
8
2
.
were in the range of 4.9–5.3 (Table 4), and (iii) the reaction rates
were almost independent of the partial pressure of molecular oxy-
gen (Fig. S4), showing that the step 2 is the rate-determining step
for the present oxidation. Therefore, the TOFobs corresponds to that
for the step 2, i.e., TOFb-elim ꢁ TOFrac, where TOFb-elim is the TOF for
the b-elimination (forward reaction in step 2). The TOFsb-elim with
Next, the aerobic oxidations of (R)-1-phenylethanol ((R)-1a,
99% ee) were carried out with a series of the supported Ru(OH)
catalysts, and the yields of 2a and ees of the substrate were mon-
itored during the oxidation. As shown in Fig. 4, the oxidation of
R)-1a proceeded with a simultaneous decrease in the ees in all
cases, showing that the progress of the racemization of (R)-1a by
the re-addition of the hydride species (backward reaction in step
) even under aerobic conditions. The TOFs for the racemization
based on the initial rates (TOFsrac
>
x
Ru(OH)
TiO (C) were calculated to be 152, 146, 95, and 65 h , respectively
Table 5 and Fig. 5a). The TOFsb-elim and TOFsrac (for forward and
x 2 x 2 x 2 3 x
/TiO (A), Ru(OH) /TiO (B), Ru(OH) /Al O , and Ru(OH) /
(
ꢁ
1
2
(
backward reactions in step 2) intrinsically increased with the de-
crease in the CNs of nearest-neighbor Ru atoms (Fig. 5), likely be-
cause of the increase in the numbers of coordinatively
unsaturated sites [19–21]. The TOFsrac more increased than
TOFsb-elim with the decrease in CNs from 0.76 to 0.37 (Fig. 5b),
while the TOFsb-elim more increased than TOFsrac with the decrease
in CNs from 0.94 to 0.76 (Fig. 5a). Therefore, the Ru(OH) /TiO (B)
x 2
catalyst with the suitable CNs showed the highest catalytic activity
for the present aerobic alcohol oxidation (Fig. 3).
2
3
) monotonically increased with
the decrease in the CNs of nearest-neighbor Ru atoms and the order
was as follows: Ru(OH) /TiO (C) (TOFrac = 0.20 h ) < Ru(OH) /Al O
x 2 x 2 3
ꢁ
1
3
The TOFsrac based on the initial rates can be calculated by the following recursion
ꢁ
1
formula [17]: TOFrac (h ) = {log(ee/100)}/{log(1 ꢁ (x/100))}/t, where ee, x, and t are
the enantiomeric excess (%), total amount of Ru in the catalyst (mol%), and reaction
time (h), respectively.

K. Yamaguchi et al. / Journal of Catalysis 268 (2009) 343–349
349
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. Conclusions
The Ru(OH)
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-catalyzed aerobic alcohol oxidation was structure-
for both the b-elim-
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[
x
nearest-neighbor Ru atoms (the size of Ru(OH) species). The TOFs
[
[
[
for the racemization more increased than those of b-elimination
with the decrease in CNs from 0.76 to 0.37, while the TOFs for
the b-elimination more increased than those of racemization with
the decrease in CNs from 0.94 to 0.76. As a result, the observed
TOFs increased with the decrease in the CNs, reached maximum
with Ru(OH)
most active Ru(OH)
alcohols including benzylic, allylic, aliphatic, and heteroatom-con-
taining ones could be converted into the corresponding carbonyl
compounds in high to excellent yields. Moreover, the catalyst
could be applied to the aerobic amine oxidation. The observed
[
[
[
[
[
[
1
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x
/
TiO (B) catalyst retrieved after the reaction could be reused with-
out an appreciable loss of its high catalytic activity for the aerobic
oxidation.
2
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Acknowledgments
We thank Mr. T. Koike (The University of Tokyo) for his help
with experiments. This work was supported in part by the Core Re-
search for Evolutional Science and Technology (CREST) program of
the Japan Science and Technology Agency (JST), the Global COE
Program (Chemistry Innovation through Cooperation of Science
and Engineering), and Grants-in-Aid for Scientific Researches from
Ministry of Education, Culture, Sports, Science and Technology.
[
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