Catalytic Oxidative Cracking of Benzene Rings in Water

Article abstract of DOI:10.1021/acscatal.8b04004

Efficient degradation of harmful benzene rings in water is indispensable for achieving a clean water environment. We report herein unprecedented catalytic oxidative benzene cracking (OBC) in water using a ruthenium(II)-aqua complex having an N-heterocyclic carbene ligand as a catalyst and a cerium(IV) salt as a sacrificial oxidant under mild conditions. The OBC reactions produced carboxylic acids such as formic acid, which can be converted to dihydrogen directly from the OBC solution using a rhodium(III) catalyst with adjustment of the solution pH to 3.3. The OBC reactions can be applied to monosubstituted benzene derivatives such as ethylbenzene, chlorobenzene, and benzoic acid. Initial rates of the OBC reactions showed a linear relationship in the Hammett plot with a negative slope, indicating the electrophilicity of a Ru(III)-oxyl complex as the reactive species in the catalytic OBC reaction. Also, we discuss a plausible mechanism of the catalytic OBC reactions based on the kinetic analysis and the product stoichiometry for the OBC reaction of nonvolatile sodium m-xylene sulfonate. The addition of an electrophilic radical to the aromatic ring to form arene oxide/oxepin is proposed as the initial step of the OBC reaction.

Full text of DOI:10.1021/acscatal.8b04004

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Article
Catalytic Oxidative Cracking of Benzene Rings in Water
Yoshihiro Shimoyama, Tomoya Ishizuka, Hiroaki Kotani, and Takahiko Kojima
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04004 • Publication Date (Web): 07 Dec 2018
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Catalytic Oxidative Cracking of Benzene Rings in Water
Yoshihiro Shimoyama, Tomoya Ishizuka, Hiroaki Kotani, and Takahiko Kojima*
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Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki
3
05-8571, Japan.
ABSTRACT: Efficient degradation of harmful benzene rings in water should be indispensable for gaining clean water environment. We report
herein unprecedented catalytic oxidative benzene cracking (OBC) in water using a ruthenium(II)-aqua complex having an N-heterocyclic
carbene (NHC) ligand as a catalyst and a cerium(IV) salt as a sacrificial oxidant under mild conditions. The OBC reactions produced carbox-
ylic acids such as formic acid, which can be converted to dihydrogen directly from the OBC solution using a rhodium(III) catalyst with ad-
justment of the solution pH value to be 3.3. The OBC reactions can be applied to mono-substituted benzene derivatives such as ethylbenzene,
chlorobenzene and benzoic acid. Initial rates of the OBC reactions showed a linear relationship in the Hammett plot with showing a negative
slope, indicating the electrophilicity of a Ru(III)-oxyl complex as the reactive species in the catalytic OBC reaction. Also, we discuss on a plau-
sible mechanism of the catalytic OBC reactions based on the kinetic analysis and the product stoichiometry for the OBC reaction of non-
volatile sodium m-xylene sulfonate. An electrophilic radical addition to the aromatic ring to form arene oxide/oxepin is proposed as the initial
step of the OBC reaction. KEYWORDS: N-heterocyclic carbene, ruthenium(III)-oxyl species, oxidative benzene cracking, formic acid, water
benzene ring was available to produce a useful compound. A strate-
gy to develop more useful OBC reactions can be attained on the
basis of the fact that metal-bound oxygen-centered radical (metal-
oxyl) species can react with benzene derivatives through addition
of the oxyl radical to a double bond of the benzene derivative to
INTRODUCTION
Benzene has been widely used as solvents for organic synthesis and
also as starting materials for industrial production of aromatic
compounds. Vapor concentration of benzene, however, is strictly
1, 2
restricted due to its high carcinogenicity and noxiousness. Once
1
6, 17
afford phenol derivatives in high selectivity.
discharged to the environment, benzene and the derivatives are
difficult to be decomposed due to their high stability derived from
its resonance stabilization and strong C-H and C-C bonds; there-
fore, cracking reactions of benzene rings require large thermal en-
Recently, we have reported the formation and characterization of
III
a ruthenium(III)-oxyl (Ru -O•) complex bearing a single bond
character between the Ru and O atoms and the application to cata-
3
–5
6, 7
18
III
ergy or strictly limited conditions. For example, catalytic crack-
ing reactions of benzene derivatives into C1 compounds using Pd
lytic oxidation of organic substrates in water. The Ru -O• com-
plex as the active species in the catalytic oxidation can be formed
through proton-coupled electron transfer (PCET) oxidation of a
ruthenium(II)-aqua complex, bearing a tridentate NHC ligand,
3
4
6, 7
catalysts, Fe oxide or microbes have been reported; however,
the products are CH or CO , which are too stable to be used as
4
2
II
2+
starting materials for syntheses of useful chemicals and even known
to exert strong greenhouse effects. Thus, the utility of the reactions
reported so far is quite limited. It has been also demonstrated that
the activation of benzene rings can be achieved at ambient temper-
[Ru (BPIm)(bpy)(OH
2
)]
( ) (BPIm = 1,3-bis(2-pyridyl-
1
8
methyl)imidazol-2-ylidene, bpy = 2,2’-bipyridyl; see Figure 1e),
IV
using (NH
4
)
2
[Ce (NO
3
)
6
] (CAN) as a sacrificial oxidant in acidic
1
8
water. Inspired by the oxidation reactions of benzene derivatives
9
, 10
16, 17
ature with reactive organic molecules, or transition-metal com-
with the metal-oxyl species mentioned above,
we applied the
1
1, 12
III
plexes.
These reactions, however, are limited to stoichiometric
Ru -O• species to OBC reactions in aqueous media. Herein, we
describe catalytic conversion of benzene and the derivatives into
carboxylic acids such as formic acid, which can serve as a hydrogen
reactions and not applicable to catalytic reactions. Under the situa-
tion as referred above, development of efficient catalysts, capable of
cracking benzene rings into useful C1 compounds at ambient tem-
perature, should be strongly required for solving environmental
issues derived from benzene derivatives and also for gaining indus-
trial benefits. Catalytic oxidative cracking reactions of mono-
19
III
reservoir, by using the Ru -O• complex as an active species and
dihydrogen evolution from the reaction solution with use of a rho-
2
0
dium(III) catalyst. The kinetic studies revealed that the OBC
reaction in this work proceeds through an electrophilic addition of
III
substituted benzene derivatives at ambient temperature have been
Ru -O• species on a benzene ring in the substrate followed by
VIII
reported with use of ruthenium(VIII) tetraoxide (Ru
O
4
) as a
benzene-ring cleavage reaction to afford formic acid and carbon
dioxide.
1
3-15
reactive oxidant,
formed in situ from a reaction between ruthe-
), in wa-
In the reported OBC reac-
nium(III) trichloride and sodium periodate (NaIO
ter/organic solvents biphasic media.
4
1
3, 14
RESULTS AND DISCUSSION
Oxidative Benzene Cracking in Water. An OBC reaction was
tions of mono-substituted benzene derivatives, the products have
been the corresponding carboxylic acids having the substituents on
the benzene rings of the substrates and the other five carbons were
performed at 283 K under Ar in D
(75 mM) as a sacrificial oxidant and ·2ClO (0.14 mM) as
2
O (pD 1.2, 1 mL) using CAN
1
2
1
1
3
4
oxidized to CO . Therefore, only one carbon among six of the
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Figure 1. (a) H NMR spectra of the aqueous phase of the reaction mixture after stirring for 24 h in the presence (top; 0.14 mM) and absence
2
2
(below) of the catalyst
1
. The reactions were performed at 283 K in D
2
O, whose pH was set at pD 1.2 and which included CAN (75 mM). (b) A
after stirring for 24 h. (c) An HSQC spectrum of the
after stirring for 24 h. H, C NMR, and HSQC spectra were measured at 298 K. (d) A
GC chart of the gaseous phase in the experimental setup. (e) A schematic representation of the OBC reaction of benzene as a substrate.
1
3
C NMR spectrum of the aqueous phase of the above reaction mixture in the presence of
1
1
13
aqueous phase of the reaction mixture in the presence of
1
~
ꢀ
aqueous phase of the reaction mixture was analyzed spectroscopi-
cally to identify the products (Figure 1).
C D
6!
(a)ꢀ
6
HDO !
1
In the H NMR spectrum of the reaction mixture, a signal assign-
in D Oꢀ
2
1
able to HCOOH was observed at d 8.29 ppm (Figure 1a) and a
13
13
C NMR signal for H COOH was observed at d 168 ppm in the
1
13
H-decoupled C NMR spectrum (Figure 1b). The 2D hetero-
nuclear single quantum coherence (HSQC) spectrum of the reac-
δ, ppmꢀ
~ꢀ
1
tion mixture exhibited a correlation cross peak between the H
1
3
(b)ꢀ
NMR signal at d 8.29 ppm and the C NMR one at d 168 ppm
(Figure 1c). Thus, we assigned the main product, observable in the
NMR spectra, of the OBC reaction of benzene to formic acid. The
concentration of formic acid in the aqueous phase of the reaction
HDO !
in D Oꢀ
2
1
mixture was determined by H NMR spectroscopy with use of DSS
(= sodium 4,4-dimethyl-4-silapentane-sulfonate) as an internal
standard, and the turnover number (TON) of the catalysis was
determined to be 55 ± 5. This catalytic OBC reaction proceeded
without any induction period as seen in the time profile of TON for
the formic acid production measured at 283 K (Figure S1b). This
suggests that the active species in the OBC reaction is not decom-
δ, ppmꢀ
(
c)ꢀ
D
D
D
D
D
D
1, CAN
O
2
3
+
CO2
posed products of the catalyst. In the gas chromatogram (GC) of
the gaseous phase of the reaction mixture, a peak derived from CO
was observed at 7.3 min of the retention time (Figure 1d). The
amount of CO formed was determined by GC and the TON based
D
OH
H O, 283 K
2
2
2
(d)ꢀ
D
on the amount of the catalyst was calculated to be 17 ± 6. Carbon
D
D
CD3
1, CAN
H O, 283 K
O
O
monoxide and formaldehyde were not observed both in the gase-
+
+ CO2
D
OH
D C OH
24
D
2
3
ous and liquid phases. To produce formic acid from benzene, the
D
theoretical equivalent number of the oxidant is calculated to be 3
molecules of the oxidant against one molecule of HCOOH based
Figure 2. 2H NMR spectra of the reaction mixtures for the OBC reac-
tions of benzene-d (a) and toluene-d (b) in the presence of CAN (75
(0.14 mM) at pH 0.8.22 Schematic representations of the
mM) and
catalytic OBC reactions of (c) benzene-d and (d) toluene-d in H O.
on the following chemical equation: C
6
H
6
+ 12H
2
O → 6HCOOH +
from benzene,
6
8
+
–
18H + 18e . On the other hand, to produce CO
2
1
–
5e removal from a benzene carbon per one molecule of CO
required based on the following chemical equation: C + 12H
2
is
O
6
8
2
6
H
6
2
a catalyst, which was prepared according to the literature proce-
dure. 1 mL of a substrate was added to the D
followed by addition of CAN and the biphasic reaction mixture was
stirred vigorously. The color of the mixture turned from yellow to
pale red-brown in the course of the reaction time of 24 h. The
+
–
® 6CO + 30H + 30e . Therefore, the efficiencies of these oxida-
2
1
8
2
O solution of ,
1
tion processes can be calculated with the following equations: 100
IV
´ {([HCOOH] ´ 3)/[Ce ]} for formic acid, 100 ´ {([CO
2
] ´
2
2
IV
5)/[Ce ]} for CO
2
. Thus, the efficiencies were determined to be
(32 ± 2)% for formic acid and (16 ± 5)% for CO
2
. Since CO can
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Table 1. Effects of Reaction Atmosphere on the OBC Reaction of
be also produced by overoxidation of formic acid obtained, the
stoichiometry of the OBC reaction was discussed in the part of
a
1
2
3
4
5
6
7
8
9
Benzene
2
5
mechanistic investigation (See below). In addition, to improve
the TON value, we performed the OBC reaction of benzene with
1
(0.1 µM) at higher tempera-
ture (303 K) for longer reaction time (48 h); as the result, the
atmosphere TON for
efficiency
TON for efficiency
b c
HCOOH for HCOOH CO
2
for CO
16
2
lower concentration of the catalyst
under Ar
under Air
53
55
57
30
31
33
17
12
20
TON value for formic acid reached to ca. 15300.
13
To confirm the origin of formic acid, deuterium-labeled ben-
zene-d was used as a substrate for the catalytic OBC reaction in
O (pH 0.8). The H NMR spectroscopic analysis allowed us to
observe a H NMR signal assigned to DCOOH at d 8.2 ppm (Fig-
ure 2a and c). Thus, we concluded that formic acid observed was
definitely derived from benzene as the substrate in the OBC reac-
tion. The OBC reaction presented here is the first example afford-
ing formic acid as a product from benzene, in sharp contrast to
under O
2
19
6
2
a
H
2
Reactions were performed under the same conditions (see ref. 22)
except the atmosphere.
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
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0
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2
3
4
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0
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2
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0
18
18
from the OBC reaction under air, O-labelled water (H
2
O) was
used as a solvent, and a peak was observed at m/z = 50 for
18 18
16 16
HC O OH, not at m/z = 46 for HC O OH, in the GC-MS spec-
tra (Figure S4 in the SI). These results indicate that the origin of
oxygen atoms should be water as a solvent and the OBC reaction
does not involve any free radical species that can react with dioxy-
gen as intermediates. Other ruthenium(II)-aqua complexes and
metal salts were also employed as catalysts for the OBC reactions in
1
3, 14
other benzene oxidation reactions,
acid but CO as a product as mentioned above. Furthermore, for-
mic acid formed in the OBC reaction was efficiently converted to
which do not afford formic
2
III
dihydrogen as a mixture of H
2
2+
, HD, and D
2
by adding a Rh cata-
III
lyst, [Rh (Cp*)(bpy)(H
2
O)] (Cp* = pentamethylcyclopentadi-
place of ; however, they could not catalyze the OBC reactions so
1
2
0
enyl), into the reaction solution of OBC by adjusting the pH val-
ue to be 3.3 with addition of an aqueous NaOH solution (Figure 3).
Thus, it should be emphasized that catalytic conversion of harmful
efficiently under the same conditions in comparison with
S2 and S3).
1
(Tables
Oxidative Cracking of Benzene Derivatives. The OBC reaction
catalyzed by was also applied to other aromatic compounds as
1
9, 20, 26
benzene into useful formic acid
has been accomplished effi-
1
ciently at ambient temperature in this study.
listed in Table 2. The OBC reaction can be applied not only to
benzene derivatives having electron-donating groups such as
ethylbenzene and xylene, but also to those having electron-
withdrawing groups such as chloro- and nitro-benzenes. In addition,
polyaromatic substrates such as naphthalene were also oxidized
under the same conditions to afford formic acid as a product. When
toluene was employed as a substrate under the same conditions as
To assess the influence of dioxygen on the reaction, catalytic
OBC reactions were performed under air, Ar, and O . The results
indicate that the atmosphere does not affect the TON values of
formic acid production (Table 1), demonstrating that the OBC
2
reaction catalyzed by
1
proceeds without influence of dioxygen,
which can participate in radical chain reactions. In addition, to
confirm the origin of the oxygen atoms of formic acid obtained
1
mentioned above, the H NMR spectrum of the aqueous phase of
the reaction mixture showed a signal assigned to acetic acid as a
product at d 2.09 ppm as well as that of formic acid (Figure S6a). In
(a)ꢀ
1, CAN
O
the reaction of toluene-d
8
as a substrate in distilled water, signals
H
OH
2+
D O (pD 1.2)
assigned to DCOOH and CD
3
COOH were both observed at d 8.2
2
2
and 1.9 ppm, respectively, in the H NMR spectrum (Figure 2b).
Thus, the origin of the formic and acetic acids was explicitly con-
firmed to be toluene as the substrate. In contrast, oxidation reac-
tions of toluene and the derivatives by ruthenium(IV)-oxo com-
plexes generally afford benzyl alcohol, benzaldehyde, and benzoic
Rh
N
D O
Rh cat. =
N
2
OH2
Rh cat.
(
pD 3 ~ 4)
H2 +
CO2
acid as products of the C-H abstraction reactions at the methyl
(b)ꢀ
27, 28
group.
This exemplifies the uniqueness of the OBC reaction by
III
the Ru -O• complex, in which oxidation of the aromatic ring pri-
marily proceeds rather than the C-H oxidation at the benzylic posi-
III
tion of toluene. The reactivity of the Ru -O• complex was also
found in the case of ethylbenzene as a substrate (Figure 4a); formic
1
and propionic acids were observed as main products in the H
—
: pD 3.3!
—: pD 3.7!
—
—
NMR spectrum (Figure S6b) and high-performance liquid chro-
matography (HPLC) analysis (Figure S5). In the case of benzyl
chloride as a substrate, formic and chloroacetic acids were obtained
as products (Figure S6c). Biphenyl employed as a substrate was
oxidized to give formic and benzoic acids in this OBC system (Fig-
ure S6d).
: pD 7.8!
: pD 1.3!
Time, hꢀ
Figure 3. (a) A schematic representation of the catalytic decomposition
of formic acid to evolve dihydrogen after the catalytic OBC reaction.
Mechanistic Insights into the OBC Reaction. In our previous
work, an acetophenone derivative was obtained in 33% yield
(
b) Time-profiles of dihydrogen evolution from the reaction mixture of
(
TON: 75) from the oxidation reaction of sodium p-ethylbenzene
sulfonate as a substrate catalyzed by complex (Figure 4a). The
1
sharp contrast to the result using ethylbenzene as the substrate
the OBC reaction, whose pD was adjusted by adding of NaOH aq as
described in the figure after the OBC reaction, at room temperature in
the presence of the rhodium catalyst (3.1 mM).
18
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Table 2. Catalytic OBC Reactions of Various Benzene Derivatives
aromatic ring rather than the cracking of the benzene ring. The
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with 1 as the Catalyst and CAN as the Sacrificial Oxidant in D
283 K
2
O at
OBC reactions of other substrates having electron-withdrawing
groups also resulted in low yields of formic acid as the product of
the OBC reactions (Table 2; entries 5, 7, 9, 11~15).
a
b
b
entry substrate
1
product
TON efficiency
Therefore, the OBC reaction is expected to proceed through an
electrophilic attack on an aromatic ring, and thus, the electron-
withdrawing substituents lower the reaction efficiencies in the aro-
matic ring oxidation. Further scrutiny was given to the electronic
formic acid
53
30
formic acid
acetic acid
39
20
28
13
5
22
11
16
7
2
–
effects of the substituents on the aromatic ring oxidation. The 1e -
2
9
oxidation potentials of benzene (+2.48 V vs SCE) and chloroben-
formic acid
propionic acid
2
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
zene (+2.46) are comparable. The TON values, however, based
on the amount of formic acid obtained were largely different be-
tween these substrates; the TON value for the oxidation of chloro-
benzene having an electron-withdrawing substituent was apparent-
ly lower than that of benzene. The result indicates that the OBC
reaction does not proceed through electron transfer from a sub-
c
3
acetic acid
7
phenylethanol
acetophenone
formic acid
1
<1
2
3
14
2
8
III
strate to the Ru -O• complex, whereas Fujii and co-workers pro-
chloroacetic acid
benzaldehyde
benzoic acid
1
posed an electron-transfer mechanism for an oxidation reaction of
Cl
4
benzene with an iron(IV)-oxo complex having a porphyrin p-
1
1
3
0
radical cation ligand. Furthermore, the total efficiencies of the
6
7
OBC reactions are dependent on the Hammett parameters (s) of
Cl
5
formic acid
8
4
the substituents of the substrates (Figure 4). The s
electron-donating substituents were employed as the Hammett
p
values for the
formic acid
38
14
21
8
6
7
d
benzoic acid
(
a)ꢀ
b)ꢀ
O
O
+
NO
2
OH
H
OH
formic acid
4
2
R = H
formic acid
9
5
R
O
naphthoquinone
naphthoquionediol
phthalic acid
14
10
17
11
12
38
8
R = SO Na
3
NaO S
3
(
Cl
X = Me, Cl, Br, C H
X = COOH, CF3!
9
formic acid
6
3
Slope = –1.41ꢀ
6
5!
aꢀ
Cl
R = 0.976ꢀ
X
X
formic acid
acetic acid
20
27
11
14
bꢀ
10
cꢀ
eꢀ
SO
3
Na
dꢀ
ortho, para!
positionꢀ
meta!
positionꢀ
1
1
1
2
formic acid
3
2
e
fꢀ
n = 3 (σp < σ ), n = 2 (σ > σm)ꢀ
m p
SO
3
Na
formic acid
acetic acid
formic acid
acetic acid
formic acid
acetic acid
formic acid
acetic acid
8
5
e
e
σ
or σ ꢀ
p m
6
3
SO
3
Na
16
38
7
9
Figure 4. Reactions of ethylbenzene derivatives under the OBC condi-
tions (a) and a Hammett plot of the efficiencies for the OBC reactions
1
1
1
3
4
5
21
4
of benzene derivatives, C
used are as follows: X = CH
normalized by division of total efficiency by n = 6), X = Cl (c, s
6
H
5
X (b). In the Hammett plot, substrates
NO2
3
(a, s = –0.17), X = C (b, s = –0.01,
p
6
H
5
p
5
3
p
3
=
(f,
NO
2
27
54
15
30
+
0.23), X = Br (d, s
p
= +0.23), X = COOH (e, s
m
= +0.37), X = CF
s
m
= +0.43). [1]: 0.14 mM, [CAN]: 75 mM, [Substrate]: ca. 1 mM
(
The concentration of substrates is not accurate due to the insolubility
The results of the control experiments in the absence of catalyst
are shown in the SI. As for the detailed reaction conditions, see ref. 22.
The values were based on the NMR yield determined with DSS as the
internal standard. Acetic acid was probably formed by C-C bond
cleavage of pyruvic acid derived from cracking of acetophenone ob-
tained by the C-H oxidation of ethylbenzene. 10 mg of biphenyl was
added. [Substrate] = 13 mM.
1
in water and thus cannot be controlled.), Temp.: 283 K, reaction time:
a
2
4 h. Hammett parameters were obtained from ref. 31.
b
c
31
parameters, whereas the s
m
values for the electron-withdrawing
substituents were applied, since the OBC reactions were consid-
ered to proceed through the electrophilic mechanism as mentioned
above. The total efficiencies were calculated by summations of
d
e
yields of products, HCOOH, XCOOH (X = CH
3
, C
; the values were divided by three in the case
of substituents having an electron-donating substituent, and those
6
H
5
, CF
3
)
(see above) indicates that the C-H oxidation at the benzylic posi-
tion proceeds primarily for the aromatic substrate having an elec-
tron-withdrawing group such as the sulfonate (-SO
3
2
(
Figure 4) and CO
2
–
3
) group on the
ACS Paragon Plus Environment

Page 5 of 9
ACS Catalysis
were divided by two in the case of substituents having an electron- plex under the reaction conditions, as confirmed by a separate ex-
3
7
1
2
3
4
5
6
7
8
9
withdrawing substituent for normalization (see right in Figure 4b).
Logarithm values of the normalized total efficiencies were plotted
against the Hammett parameters (Figure 4b). A linear correlation
was observed with a slope of –1.41, confirming that the OBC reac-
periment. To consider the product ratio mentioned above, one
mXS molecule is assumed to afford one formic acid, two acetic
acids and three CO molecules (Figure 5a). Based on this assump-
2
tion, the conversion and the product yields well match within the
experimental errors. This is a very important clue to discuss the
reaction mechanism (see below).
III
tions proceed through electrophilic attack of the Ru -O• complex
1
8
as the active species to the aromatic rings. Negative r values have
been also observed for benzene hydroxylation by high-valent iron-
oxo complexes.
Kinetic Analysis on the OBC Reaction. We also performed kinetic
analysis on the OBC reaction using mXS as a substrate (Figure 5a)
by H NMR spectroscopy, and particularly investigated the de-
pendence of the rate constants on the concentrations of the sub-
strate and catalyst. Time-courses of the concentrations of formic
and acetic acids in the initial stage of the reaction showed a linear
3
3-35
1
In the case of benzene or other benzene derivatives, having rela-
tively low boiling points, used as substrates, the stoichiometry of
the OBC reaction was difficult to be determined due to the volatili-
ty and adsorption into rubber septums. The kinetics of the OBC
reactions was also difficult to be investigated due to the low solubil-
ity of the substrates in water. Thus, substituents such as the sul-
fonate group was introduced to the benzene ring to raise the boiling
points and improve the solubility of the substrate in water; however,
the reactivity of those substrates in the OBC reaction was lowered
due to the electron-withdrawing nature of the substituent. There-
fore, we employed sodium m-xylene sulfonate (mXS) as a non-
volatile substrate to calculate the conversion yields (Figure 5a); the
two electron-donating methyl groups of mXS is expected to sup-
press the negative effect of the electron-withdrawing sulfonate
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
correlation to determine the reaction rate (v
0
) (Figure. S8a and
S8b). The dependence of the v values for acetic acid formation on
0
the mXS concentration exhibited saturation behavior at higher
concentrations, which was analyzed on the basis of the Michaelis-
3
8
Menten equation (Figure 5b). The Michaelis constant, K
m
, and
–
4
–1
the Vmax value were determined to be 1.4 mM and 7.3 ´ 10 mM s ,
respectively. This small K
strong interaction between the substrate and the reactive species
prior to the oxidation reaction. The dependence of the v value on
m
value indicates that there are relatively
0
the catalyst concentration was investigated to determine the cata-
lytic rate constants (kcat) under the conditions where the substrate
concentration was large enough to show the saturation of the v
0
(
a)
H
H
value. As a result, kcatF for the formic acid formation and kcatA for
the acetic acid formation in the OBC reaction of mXS were deter-
H
SO Na
3
1, CAN
1HCOOH + 2CH COOH + 3CO
–3
–3 –1
3
2
mined to be 1.5 ´ 10 and 4.2 ´ 10 s , respectively (Figure
D O, 283 K
2
39
40
H
S8c). In the case of deuterated mXS (mXS-d
) as a substrate,
3
mXS
whose three protons on the aromatic ring were selectively labeled
D
(b)
(c)
by deuterium, kcatA values for the deuterated acetic acid obtained
H
H
–3 –1
were determined to be 4.1 ´ 10 s . Therefore, kinetic isotope
H
SO Na
3
H
D
effect (KIE) was calculated to be 1.0 (KIE = kcatA /kcatA ) (Figure
5c and Figure S10 in the SI). This indicates that hydrogen-atom
III
Hꢀ
Dꢀ
transfer (HAT) from C-H bonds of the aromatic rings to the Ru -
D
O• complex is not involved in the rate-limiting step of the OBC
reactions. The similar KIE value (1.04) has been also reported by
Kodera and co-workers in benzene oxidation to afford phenol as
D
SO Na
3
1
7
D
the product. The KIE value obtained also suggests that the active
species of the OBC reaction in this study is not hydroxyl radical,
since the KIE values of a Fenton-type benzene oxidation reaction
reported so far, using hydroxyl radical as the active species and
0ꢀ
0ꢀ
0
ꢀ
[
mXS], mM
[1], mM
Figure 5. (a) Schematic representation of the OBC reaction of mXS.
(b) Dependence of the initial rates (black-filled circles) of the acetic
acid formation in OBC on the concentration of mXS and the fitting
4
1,42
giving CO
2
as the product, are in the range of 1.7 – 1.8.
In the
Fenton-type reaction, the rate-determining step is assumed to be
the hydrogen-atom abstraction from aromatic C-H bonds by hy-
droxyl radical or deprotonation from C-H bonds of oxidized inter-
curve based on the Michaelis-Menten equation (red solid line): [
.14 mM, [CAN] = 75 mM. Temperature: 283 K. (c) Dependence of
the initial rates (red-filled circles for mXS and blue-filled circles for
mXS-d ) of the acetic acid formation in OBC on a lower range of the
concentration of mXS and mXS-d and the fitting curves based on the
second-order dependence (red and blue solid lines). [Substrate] = 8.5
1
] =
0
42
mediates. The assumption that HAT from the C-H bond of ben-
zene does not occur in the present OBC reaction is consistent with
the large bond-dissociation energy of the C-H bond of benzene (ca.
3
3
–1 43
113 kcal mol ).
mM (mXS) and 8.4 mM (mXS-d
283 K. Concentrations of the products were determined by H NMR
spectroscopy with DSS as an internal standard.
3
). [CAN] = 75 mM. Temperature:
In addition, as one of possible candidates for intermediates de-
rived from benzene in the course of the OBC reaction, phenol was
subjected to the OBC reaction as a substrate, since plenty of reports
1
group on the reactivity. The conversion of mXS was determined to
have been delivered for benzene oxidation to afford phenol as a
16, 17, 44-46
3
6
product.
In the phenol oxidation under the same conditions
be 84%, and yields of formic and acetic acids, and CO were esti-
2
mated to be 77, 151, and 211%, respectively, based on mXS. This
result indicates that all of the aromatic C-H bonds on the benzene
ring are not involved in formic acid, but one of the three aromatic
as those of the OBC reactions, a complicated mixture of aromatic
compounds was obtained as brown precipitates. Moreover, a rapid
reaction of phenol with CAN proceeded even in the absence of
1
(Figure S11 in the SI), indicating that phenol can be excluded as an
intermediate of the OBC reaction. To obtain further information
C-H bonds of mXS is involved in formic acid. In addition, formic
III
acid obtained was partially converted to CO
2
by the Ru -O• com-
ACS Paragon Plus Environment

ACS Catalysis
Page 6 of 9
O
X
nucleophilic attack of water molecules to decompose and afford
CO (step viii).
2
CO + HCOOH + XCOOH
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
O
2
vii
H
X
H
H
H
H
H
O
CONCLUSIONS
H
X
H
H
X
H
RuIII-O•
H
– RuII
H
X
H
H
H
O
H
H
H
We have established an efficient OBC reaction of benzene and
the derivatives for the first time, which allows us to obtain formic
acid and carboxylic acids having substituents originally on the aro-
matic rings by suppressing over-oxidation of the products. Formic
acid obtained can be used as a dihydrogen source in aqueous media
i
RuIII
ii
O
H
H
Electrophilic Addition
Arene oxide/oxepin
Equilibrium
H
X
O
H
X
O
H
HO
H
OH
3HCOOH + XCOOH + 2CO
2
iii
iv
O
H
H
O
H
H
v
vi
viii
at low temperature; actually, we succeeded in the formation of H
2
CO
2
CO
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
X = Cl, Br,
COOH)
as a clean energy resource from the OBC reaction mixture using a
Rh(III) catalyst with simple pH adjustment of the reaction solu-
20
III
tion. In addition, we have revealed that substituent effects on the
Figure 6. A proposed mechanism of the OBC reaction by the Ru -O•
complex (X = CH , C , Cl, Br, COOH, or CF ).
aromatic rings can switch the oxidation sites; i.e., the benzylic C-H
moiety attached to an electron-deficient aromatic ring was oxidized
3
6
H
5
3
on organic intermediates in the catalytic OBC reaction, we have
performed time-dependent product analysis by H NMR meas-
urements for the catalytic OBC reaction of benzene (Figure S12).
A H NMR signal derived from 1,4-benzoquinone was observed at
.9 ppm in the time range of 1 to 3 hour. To confirm whether 1,4-
benzoquinone can be involved as an intermediate in the catalytic
OBC reaction, catalytic oxidation of 1,4-benzoquinone was con-
as a catalyst; both of formic acid and CO were ob-
tained and the TON values were determined to be 39 and 52, re-
spectively. The H NMR spectroscopic analysis on the OBC reac-
tion mixture was also performed with lower concentration of CAN
19 mM) to suppress overoxidation of the organic intermediates.
The reaction mixture was treated with an excess amount of sodium
18
through the C-H abstraction, whereas the C-C double bonds of
1
the electron-rich aromatic rings were oxidatively cleaved. The clear
selectivity of the aromatic-ring oxidations in the OBC reaction
derives from the electrophilic and strong radical character of the
1
6
III
Ru -O• complex as the active species and the OBC reactions pro-
III
ceed through the electrophilic addition of the Ru -O• complex to
the aromatic ring.
ducted using
1
2
1
EXPERIMENTAL SECTION
General Procedure of Oxidative Cracking of Benzene Deriva-
tives. In a sample vial (10 mL) shielded by a rubber septum was
(
filled with biphasic reaction mixture composing of D
2
O solution (1
oxalate at the reaction time of 40 min to quench the remaining
IV
mL) of
1
(0.14 mM) and (NH
4
)
2
[Ce (NO
)
3 6
] (CAN; 75 mM) as
4
7 1
oxidants; H NMR signals assigned to muconic acid were ob-
served (Figure S13a). The products of the OBC reaction, which
was also treated with an excess amount of sodium oxalate at the
reaction time of 40 min, were separated by HPLC (Figure S13b).
The ESI-TO-MS measurement of the HPLC fraction at the reten-
tion time of 36.4 min exhibited a MS peak derived from muconic
acid at m/z = 140.99 (Simulation for [M – H ] : m/z = 141.02)
Figure S13c). Muconic acid also can be further oxidized into for-
mic acid and carbon dioxide. Since it is probably impossible to
form muconic acid from 1,4-benzoquinone, 1,4-benzoquinone and
muconic acid should be formed through different pathways.
an aqueous layer, whose pD was set to be 1.2 by addition of acidic
CAN, and 1 mL of a substrate as an organic layer. In most cases, the
reactions were performed at 283 K in a water bath equipped with a
temperature controller, COOL STIRRER SAC-900. A schematic
representation of the reaction setup is shown in Figure 7.
+
–
A rubber septumꢀ
(
Sample vial (10 mL)ꢀ
Substrate (1 mL)ꢀ
4
8
A Proposed Mechanism of the OBC Reaction. Based on the
product analysis and the kinetic studies mentioned above, we pro-
pose a plausible mechanism of the OBC reaction catalyzed by
shown in Figure 6. As the first step, the electrophilic addition of the
Ru -O• complex, which is formed from catalyst
ring occurs to generate a radical-coordinated intermediate (step i in
Figure 6). This step is reminiscent of the previously reported ben-
zene oxidation with a Cu(II)-oxyl complex.
intermediate releases a Ru(II) species that reproduces
D
2
O (pD 1.2)!
[1]: 0.14 mM!
CAN] 75 mMꢀ
Water bath (283 K)ꢀ
1
as
A stirrer chipꢀ
[
III
1
, to the benzene
Figure 7. A simple representation of the experimental setup in this
work.
Kinetic Analysis on the OBC Reactions of Sodium m-Xylene-
16,17
The radical-bound
in water
Sulfonate and its Deuterated Derivative (mXS and mXS-d
3
). The
H NMR signals of formic and acetic acids derived from oxidation
of mXS were observed to monitor the reaction progress in D
1
1
and an arene oxide that can be in an equilibrium with oxepin (step
ii). Further oxidation of the arene oxide/oxepin mixture can af-
ford E/Z-isomers of muconaldehyde as reported (step iii). The
2
O
49
(
pD 1.2). The amounts of formic and acetic acids were determined
50
using DSS as an internal standard. As for mXS-d , only the for-
3
aldehydes can be easily oxidized into the E/Z-isomers of muconic
acid (step iv), which are further oxidized into formic acid and car-
bon dioxide (steps v and vi). In addition, oxidation of arene oxide
can also form 1,4-benzoquinone through a different pathway from
that giving muconic acid (step vii). Such direct conversion of arene
oxide into 1,4-benzoquinone has been proposed. In the case of
chloro- and bromobenzene, plausible products include chloro- and
bromoformic acids (X = Cl, Br); however, the acids easily undergo
mation of acetic acid was monitored, since formic acid derived from
2
mXS-d was deuterated and insensitive deuterium ( H) NMR spec-
3
troscopy did not allow us to quantify DCOOD in an appropriate
period of the reaction time for kinetic analysis. A linear least-
squares fitting was conducted for the time profile of the product
amounts observed in the range of 0 to 50 min and the slopes of the
5
1
fitting lines obtained were used as the initial rates (v ) of the mXS
0
ACS Paragon Plus Environment

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ACS Catalysis
and mXS-d
3
oxidation. The v
0
values on formic and acetic acid for-
(5) Weiss, J. M.; Downs, C. R. Catalytic Oxidation of Benzene to Ma-
1
2
3
4
5
6
7
8
9
mation were determined at various catalyst concentrations and the
catalytic rate constants, kcat and kcat , were determined with eq 1.
(1)
leic Acid. J. Chem. Educ. 1925, 2, 1178-1180.
H
D
(6) Farhadian, M.; Vachelard, C.; Duchez, D.; Larroche, C. In Situ Bi-
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0
v = kcat[ ]
1
(
7) Masumoto, H.; Kurisu, F.; Kasuga, I.; Tourlousse, D. M.; Furumai,
H. Complete Mineralization of Benzene by a Methanogenic En-
richment Culture and Effect of Putative Metabolites on the Deg-
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8) Lashof, D. A.; Ahuja, D. R. Relative Contributions of Greenhouse
Gas Emissions to Global Warming. Nature 1990, 344, 529-531.
9) Inagaki, Y.; Nakamoto, M.; Sekiguchi, A. A Diels-Alder Super
Determination of K
mXS. The initial rates, v
concentrations (1.1 – 18 mM) were obtained according to the
procedure described above. The v values obtained were plotted
against the substrate concentration. The v values exhibited satura-
m
and Vmax Values for the OBC Reaction of
0
, for the OBC reactions of mXS at various
(
(
0
0
tion behavior to the concentration (See Figure 5b). Based on the
Michaelis-Menten equation as described in eq 2, the Michaelis
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Diene Breaking Benzene into C
2
H
2
and C
4
4
H . Nat. Commun.
2014, 5, 3018.
constant, K
m
, and the maximum rate in the system, Vmax, were de-
(10) Ellis, D.; McKay, D.; Macgregor, S. A.; Rosair, G. M.; Welch, A. J.
Room-Temperature C-C Bond Cleavage of an Arene by a Metal-
termined by the fitting analysis.
lacarborane. Angew. Chem., Int. Ed.
2010, 49, 4943-4945.
v
0
= (Vmax[mXS]) / (K
m
+ [mXS])
(2)
(
11) Hu, S.; Shima, T.; Hou, Z. Carbon-Carbon Bond Cleavage and
Rearrangement of Benzene by a Trinuclear Titanium Hydride. Na-
ture 2014, 512, 413-415.
ASSOCIATED CONTENT
AUTHOR INFORMATION
(12) Sattler, A.; Parkin, G. Cleaving Carbon-Carbon Bonds by Inserting
Tungsten into Unstrained Aromatic Rings. Nature 2010, 463, 523-
526.
Corresponding Author
(
13) Nuñez, M. T.; Martín, V. S. Efficient Oxidation of Phenyl Groups
to Carboxylic Acids with Ruthenium Tetraoxide. A Simple Synthe-
sis of (R)-g-Caprolactone, the Pheromone of Trogoderma Gran-
arium. J. Org. Chem. 1990, 55, 1928-1932.
*kojima@chem.tsukuba.ac.jp
Notes
The authors declare no competing financial interest.
(14) Caputo, J. A.; Fuchs, R. The Oxidation of Cyclobutanols and Ar-
omatic Rings with Ruthenium Tetroxide. Tetrahedron Lett. 1967
, 4729-4731.
,
8
(
15) Djerassi, C.; Engle, R. R. Oxidations with Ruthenium Tetroxide. J.
SUPPORTING INFORMATION
The Supporting Information is available free of charge on the ACS
Publications website.
Am. Chem. Soc. 1953, 75, 3838-3840.
(16) Viella. L.; Conde, A.; Balcells, D.; Diaz-Requejo, M. M.; Lledós, A.;
Pérez, P. J. A Competing, Dual Mechanism for Catalytic Direct
Benzene Hydroxylation from Combined Experimental-DFT Stud-
ies. Chem. Sci. 2017, 8, 8373-8383.
(17) Tsuji, T.; Zaoputra, A. A.; Hitomi, Y.; Mieda, K.; Ogura, T.; Shiota,
Y.; Yoshizawa, K.; Sato, H.; Kodera, M. Specific Enhancement of
Catalytic Activity by a Dicopper Core: Selective Hydroxylation of
Benzene to Phenol with Hydrogen Peroxide. Angew. Chem., Int.
Ed. 2017, 56, 7779-7782.
(18) Shimoyama, Y.; Ishizuka, T.; Kotani, H.; Shoita, Y.; Yoshizawa, K.;
Mieda, K.; Ogura, T.; Okajima, T.; Nozawa, S.; Kojima, T. A Ru-
thenium(III)-Oxyl Complex Bearing Strong Radical Character.
Angew. Chem., Int. Ed. 2016, 55, 14041-14045.
1
Synthetic procedures of mXS-d
3
, kinetic data, control experiments, H
NMR spectra of the reaction mixtures, including Figures S1-S13 and
Table S1-S5 (PDF)
ACKNOWLEDGMENT
This work was supported by JST CREST (JPMJCR16P1) and
Grants-in-Aid (Nos. 15H00915, 17H03027 and 18K19089) from
the Japan Society of Promotion of Science (JSPS, MEXT) of Japan.
T. K. also appreciates financial supports from The Mitsubishi
Foundation and Yazaki Memorial Foundation for Science and
Technology. Y. S. appreciates a support from Research Fellowship
for Young Scientists provided by JSPS (18J12050). The authors
appreciate Mr. M. Miura and Mr. M. Yamatake of JASCO for their
help to HPLC analysis.
(
19) Johnson, T. C.; Morris, D. J.; Wills, M. Hydrogen Generation from
Formic Acid and Alcohols Using Homogeneous Catalysts. Chem.
Soc. Rev. 2010, 39, 81-88.
(20) Fukuzumi, S.; Kobayashi, T.; Suenobu, T. Efficient Catalytic De-
composition of Formic Acid for the Selective Generation of H
2
and H/D Exchange with a Water-Soluble Rhodium Complex in
Aqueous Solution. ChemSusChem 2008, 1, 827-834.
(
21) Other sacrificial oxidants were also used for these catalytic OBC
REFERENCES
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(
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Data in Exposure and Human Health Risk Assessment: Benzene
Case Study. Crit. Rev. Toxicol. 2013, 43, 119-153.
(22) The details of general reaction conditions are shown in Methods in
the Experimental Section. [ ] = 0.14 mM, [CAN] = 75 mM, reac-
1
tion temperature: 283 K, reaction time: 24 h. 1 mL of liquid sub-
strate was added to the reaction vessel.
(
(
2) Lamm, S. H.; Grünwald, W. Benzene Exposure and Hematotoxici-
ty. Science 2006, 312, 998-999.
3) He, C.; Li, J.; Cheng, J.; Li, L.; Li, P.; Hao, Z; Xu, Z. P. Compara-
tive Studies on Porous Material-Supported Pd Catalysts for Cata-
lytic Oxidation of Benzene, Toluene, and Ethyl Acetate. Ind. Eng.
Chem. Res. 2009, 48, 6930-6936.
(
(
(
23) Nanoparticles with a diameter of ca. 400 nm were observed in dy-
namic light scattering (DLS) measurements of the reaction mix-
ture after the OBC reaction; however, the nanoparticles did not
show reactivity for the OBC reaction. See Figure S1c and in the SI.
24) The absence of carbon monoxide and formaldehyde as products
was confirmed by the GC analyses of the reaction mixture and the
Nash method with UV-Vis spectroscopy, respectively (Figure S2
in the SI).
(
4) Tamhankar, S. S.; Tsuchiya, K.; Riggs, J. B. Catalytic Cracking of
Benzene on Iron Oxide-Silica: Catalyst Activity and Reaction
Mechanism. Appl. Catal. 1985, 16, 103-121.
25) We have also conducted the OBC reaction of benzene in a ben-
zene-saturated aqueous solution (~20 mM), not under the bipha-
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sic conditions. As a result, formic acid and carbon dioxide were ob-
(38) Karich, A.; Kluge, M.; Ullrich, R.; Hofrichter, M. Benzene
Oxygenation and Oxidation by the Peroxygenase of Agrocybe
Aegerita. AMB Express 2013, 3, 5-13.
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tained with the TON values of 50 and 9, respectively; however, we
could not determine the accurate conversion yield of the benzene
oxidation due to the high volatility of benzene. The details of the
reaction setup were shown in Figure S3 in the SI.
(39) Since two acetic acid molecules are released from one mXS mole-
cule based on stoichiometry on the catalytic oxidation of mXS, the
H
–3 –1
(
26) Fukuzumi, S.; Suenobu, T. Hydrogen Storage and Evolution Cata-
lyzed by Metal Hydride Complexes. Dalton Trans. 2013, 42, 18-28.
value of kcatA should be normalized as 2.1 ´ 10
s
by dividing by
2. Therefore, the substantial catalytic rate constant of the OBC re-
(27) Kojima, T.; Nakayama, K.; Ikemura, K.; Ogura, T.; Fukuzumi, S.
Formation of Ruthenium(IV)-Oxo Complex by Electron-
–3 –1
actions of mXS should be 2.1 ´ 10
s
. The discrepancy of the
a
rates is derived from further oxidation of formic acid to CO2, which
reduces the amount of formic acid formed to lower the kcatF value,
as described in Figure S7.
Transfer Oxidation of a Coordinatively Saturated Ruthenium(II)
Complex and Detection of Oxygen-Rebound Intermediates in C-
H Bond Oxygenation. J. Am. Chem. Soc. 2011, 133, 11692-11700.
28) Thompson, M. S.; Meyer, T. J. Kinetics and Mechanism of Oxida-
(
(
40) Synthesis and characterization of mXS-d
3
is described in Figure S9
0
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0
(
(
in the SI.
2
+
tion of Aromatic Hydrocarbons by Ru(trpy)(bpy)O . J. Am.
Chem. Soc. 1982, 104, 5070-5076.
41) Wang, D.; Wang, M.; Li, Z. Fe-Based Metal-Organic Frameworks
for Highly Selective Photocatalytic Benzene Hydroxylation to
Phenol. ACS Catal. 2015, 5, 6852-6857.
29) Merkel, P. B.; Luo, P.; Dinnocenzo, J. P.; Farid, S. Accurate Oxida-
tion Potentials of Benzene and Biphenyl Derivatives via Electron-
Transfer Equilibria and Transient Kinetics. J. Org. Chem. 2009, 74,
(
42) Augusti, R.; Dias, A. O.; Rocha, L. L.; Lago, R. M. Kinetics and
Mechanism of Benzene Derivative Degradation with Fenton's Re-
agent in Aqueous Medium Studied by MIMS. J. Phys. Chem. A
5
163-5173.
(
30) Asaka, M.; Fujii, H. Participation of Electron Transfer Process in
Rate-Limiting Step of Aromatic Hydroxylation Reactions by
1
998, 102, 10723-10727.
(
(
43) Luo, Y.-R. Handbook of Bond Dissociation Energies in Organic
Compounds, 1st ed.; CRC Press LLC: Boca Raton, London, New
York, Washington, D. C. 2003; p 39.
44) Morimoto, Y.; Bunno, S.; Fujieda, N.; Sugimoto, H.; Itoh, S. Di-
rect Hydroxylation of Benzene to Phenol Using Hydrogen Perox-
ide Catalyzed by Nickel Complexes Supported by Pyridylalkyla-
mine Ligands. J. Am. Chem. Soc. 2015, 137, 5867-5870.
Compound I Models of Heme Enzymes. J. Am. Chem. Soc. 2016
38, 8048-8051.
31) Hansch, C.; Leo, A.; Taft, R. W. A Survey of Hammett Substituent
Constants and Resonance and Field Parameters. Chem. Rev. 1991
1, 165-195.
,
1
(
(
,
9
32) Benzoic acid is probably oxidized to afford oxalic acid as one of the
products (X = COOH in Figure 4). Oxalic acid formed should be
(
45) Xu, J.; Chen, Y.; Hong, Y.; Zheng, H.; Ma, D.; Xue, B.; Li, Y.-X. Di-
rect Catalytic Hydroxylation of Benzene to Phenol Catalyzed by
Vanadia Supported on Exfoliated Graphitic Carbon Nitride. Appl.
Catal. A: General 2018, 549, 31-39.
oxidized under the reaction conditions to afford CO
uct, which was confirmed by oxidation of oxalic acid as a substrate
to afford CO in 162% yield. Reaction conditions are as follows:
] = 0.14 mM or 0 mM, [CAN] = 75 mM, [sodium oxalate] = 10
mM, temp.: 283 K, reaction time: 2 h. In addition, this reaction
2
as the prod-
2
[
1
(
(
46) Ohkubo, K.; Kobayashi, T.; Fukuzumi, S. Direct Oxygenation of
Benzene to Phenol Using Quinolinium Ions as Homogeneous
Photocatalysts. Angew. Chem., Int. Ed. 2011, 50, 8652-8655.
proceeded even in the absence of
yield.
1
as a catalyst in the comparable
1
47) The reason why the H NMR signals observed were not complete-
(
(
(
33) Kang, M.-J.; Song, W. J.; Han, A.-R.; Choi, Y. S.; Jang, H. G.; Nam,
W. Mechanistic Insight into the Aromatic Hydroxylation by High-
Valent Iron(IV)-oxo Porphyrin π-Cation Radical Complexes. J.
Org. Chem. 2007, 72, 6301-6304.
34) Visser, S. P. D.; Oh, K.; Han, A.-H.; Nam. W. Combined Experi-
mental and Theoretical Study on Aromatic Hydroxylation by
Mononuclear Nonheme Iron(IV)−Oxo Complexes. Inorg. Chem.
ly matched with the signals of authentic trans,trans-muconic acid is
probably that stereoisomers of muconic acid (e.g. cis,cis- or
cis,trans-muconic acid) were also formed.
(
48) Catalytic oxidation of trans,trans-muconic acid also gave formic
acid and CO
2
as products, and formic acid and CO were obtained
2
very fast through the oxidation with CAN even in the absence of
Details are shown in Table S5 in the SI.
1
.
2
007, 46, 4632-4641.
(49) Vogel, E.; Günther, H. Benzene Oxide-Oxepin Valence Tautomer-
ism, Angew. Chem., Int. Ed. Engl. 1967, 6, 385-401.
35) Korzekwa, K. R.; Swinney, D. C.; Trager, W. F. Isotopically La-
beled Chlorobenzenes as Probes for the Mechanism of Cyto-
chrome P-450 Catalyzed Aromatic Hydroxylation. Biochemistry
(
50) Golding, B. T.; Barnes, M. L.; Bleasdale, C.; Henderson, A. P.;
Jiang, D.; Li, X.; Mutlu, E.; Petty, H. J.; Sadeghi, M. M. Modeling
the Formation and Reactions of Benzene Metabolites. Chemico-
Biological Interactions 2010, 184, 196-200.
1
989, 28, 9019-9027.
(36) Reaction conditions were as follows: [
mM, [mXS] = 5 mM, reaction time: 24 h, temp.: 283 K, solvent:
O (pD 1.2).
37) Formic acid as a substrate was catalytically converted into CO
the presence of as a catalyst. See Figure S7 in the SI.
1
] = 0.14 mM, [CAN] = 225
(51) Ramu, R.; Wanna, W. H.; Janmanchi, D.; Tsai, Y.-F.; Liu, C.-C.;
Mou, C.-Y.; Yu, S. S.-F. Mechanistic Study for the Selective Oxida-
D
2
(
2
in
tion of Benzene and Toluene Catalyzed by Fe(ClO
4
)
2
in an H
2
2
O -
1
H
2
O-CH
3
CN System. Mol. Catal. 2017, 441, 114-121.
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