Oxygenation and chlorination of aromatic hydrocarbons with hydrochloric acid photosensitized by 9-mesityl-10-methylacridinium under visible light irradiation

Article abstract of DOI:10.1007/s11164-012-0643-5

Efficient photocatalytic oxygenation of toluene occurs under visible light irradiation of 9-mesityl-10-methylacridinium (Acr⁺-Mes) in oxygen-saturated acetonitrile containing toluene and aqueous hydrochloric acid with a xenon lamp for 15 h. The oxygenated products, benzoic acid (70 %) and benzaldehyde (30 %), were formed after the photoirradiation. The photocatalytic reaction is initiated by intramolecular photoinduced electron transfer from the mesitylene moiety to the singlet excited state of the Acr⁺ moiety of Acr⁺-Mes, which affords the electron-transfer state, Acr ^?-Mes^?+. The Mes^?+ moiety can oxidize chloride ion (Cl^-) by electron transfer to produce chlorine radical (Cl^?), whereas the Acr^? moiety can reduce O₂ to O ₂ ^?-. The Cl ^? radical produced abstracts a hydrogen from toluene to afford benzyl radical in competition with the bimolecular radical coupling of Cl ^?. The benzyl radical reacts with O₂ rapidly to afford the peroxyl radical, leading to the oxygenated product, benzaldehyde. Benzaldehyde is readily further photooxygenated to yield benzoic acid with Acr^?-Mes^?+. In the case of an aromatic compound with electron-donating substituents, 1,3,5-trimethoxybenzene, photocatalytic chlorination occurred efficiently under the same photoirradiation conditions to yield a monochloro-substituted compound, 2,4,6-trimethoxychlorobenzene.

Full text of DOI:10.1007/s11164-012-0643-5

Res Chem Intermed (2013) 39:205–220
DOI 10.1007/s11164-012-0643-5
Oxygenation and chlorination of aromatic
hydrocarbons with hydrochloric acid photosensitized
by 9-mesityl-10-methylacridinium under visible light
irradiation
Kei Ohkubo
Shunichi Fukuzumi
•
Kentaro Mizushima
•
Received: 3 October 2011 / Accepted: 21 October 2011 / Published online: 21 June 2012
Springer Science+Business Media B.V. 2012
ꢀ
Abstract Efficient photocatalytic oxygenation of toluene occurs under visible
?
light irradiation of 9-mesityl-10-methylacridinium (Acr –Mes) in oxygen-saturated
acetonitrile containing toluene and aqueous hydrochloric acid with a xenon lamp for
1
5 h. The oxygenated products, benzoic acid (70 %) and benzaldehyde (30 %),
were formed after the photoirradiation. The photocatalytic reaction is initiated by
intramolecular photoinduced electron transfer from the mesitylene moiety to the
?
singlet excited state of the Acr moiety of Acr –Mes, which affords the electron-
?
•
•?
•?
-
transfer state, Acr –Mes . The Mes moiety can oxidize chloride ion (Cl ) by
•
•
electron transfer to produce chlorine radical (Cl ), whereas the Acr moiety can
•
•
-
2
reduce O to O . The Cl radical produced abstracts a hydrogen from toluene to
2
•
afford benzyl radical in competition with the bimolecular radical coupling of Cl .
The benzyl radical reacts with O rapidly to afford the peroxyl radical, leading to the
oxygenated product, benzaldehyde. Benzaldehyde is readily further photooxygen-
2
•
•?
ated to yield benzoic acid with Acr –Mes . In the case of an aromatic compound
with electron-donating substituents, 1,3,5-trimethoxybenzene, photocatalytic chlo-
rination occurred efficiently under the same photoirradiation conditions to yield a
monochloro-substituted compound, 2,4,6-trimethoxychlorobenzene.
Keywords Electron transfer ꢀ Oxygenation ꢀ Chlorination ꢀ Radical cation ꢀ
Photocatalysis
K. Ohkubo (&) ꢀ K. Mizushima ꢀ S. Fukuzumi (&)
Department of Material and Life Science, Graduate School of Engineering, Osaka University and
ALCA (JST), Suita, Osaka 565-0871, Japan
e-mail: ookubo@chem.eng.osaka-u.ac.jp
S. Fukuzumi
e-mail: fukuzumi@chem.eng.osaka-u.ac.jp
S. Fukuzumi
Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
123

206
K. Ohkubo et al.
Introduction
Oxygenation and halogenations of hydrocarbons are important and fundamental
chemical reactions, providing key precursors for various transformations. Although
there have been many reports on oxygenation and halogenation of hydrocarbon, it is
highly desired to develop more effective and environmentally benign methods,
because the current methodology requires drastic conditions or excessive use of
heavy metals as oxidants [1–4]. With this background in mind, it has recently been
reported that catalytic oxidation processes with hydrogen peroxide or molecular
oxygen can diminish waste [5–11]. From a ‘‘green chemistry’’ perspective, the best
candidate for an oxidant would be molecular oxygen because of its abundant
availability and non-toxicity [12–16].
Radical species which can react with molecular oxygen can be readily generated
by photoinduced electron-transfer reactions [17–25]. The radical cation of ring-
substituted toluenes readily deprotonates to give the corresponding benzyl radicals,
which can react with molecular oxygen directly to produce the benzyl peroxyl
radicals, leading to the final oxygenated products [26–30]. The key step in such
reactions is electron-transfer oxidation of substrates. A main problem in such a case
is that the one-electron reduction potentials of the excited states of photosensitizers
should be higher than the one-electron oxidation potentials of substrates, because
the photocatalytic reactions are initiated by the electron-transfer oxidation of
substrates [26].
We have reported that the photoexcitation of 9-mesityl-10-methylacridinium
?
Acr –Mes) results in ultrafast electron transfer from the Mes moiety to the singlet
(
?
•
•?
excited state of the Acr moiety to form the electron-transfer state (Acr –Mes ),
which has an extremely long lifetime (e.g., 2 h at 203 K) and a high energy
•
•?
(
with the Mes moiety as well as a strong reducing agent with the Acr moiety [31,
2.37 eV). The electron-transfer state (Acr –Mes ) has a strong oxidizing agent
•
•?
?
3
2]. In such a case, Acr –Mes acts as an efficient electron-transfer photocatalyst for
highly selective oxygenation of substrates. However, substrates that can be oxidized
•
•?
by Acr –Mes via electron transfer have been limited to those with the lower one-
•
electron oxidation potentials than the one-electron reduction potential of Acr –
•?
?
Mes . Thus, the use of Acr –Mes as an efficient electron-transfer photocatalyst
has yet to be expanded to photooxidation of toluene with O to yield benzoic acid
2
•
•?
because one-electron reduction potential of Acr –Mes (Ered = ? 2.06 V vs. SCE)
[
33] is lower than the oxidation potential of toluene (Eox = ? 2.20 V) [34].
?
Herein, we expand the scope of the use of Acr –Mes as an efficient electron-
transfer photocatalyst for oxidation of organic compounds with O other than
2
oxygenation reactions by examining photocatalytic oxygenation of toluene with O2
and hydrochloric acid. We have also found the photocatalytic chlorination of
?
methoxy-subsituted benzenes with HCl and O by Acr –Mes, where the radical
cation of substrate acts as a strong electrophile to efficiently react with a nuclephile,
2
-
Cl to yield the chlorinated product. The photocatalytic mechanism is clarified by
detecting radical intermediates involved in the photocatalytic reactions with use of
nanosecond laser flash photolysis.
1
23

Oxygenation and chlorination of aromatic hydrocarbons
207
Experimental
Materials
?
-
9
-Mesityl-10-methylacridinium perchlorate (Acr –Mes ClO ) and substrates
4
shown in Table 1 except for benzaldehyde and toluene were obtained from Tokyo
Chemical Industry. Hydrochloric acid (30 %) and toluene, benzaldehyde, sodium
iodide and hydrogen peroxide were obtained from Wako Pure Chemicals and used
without further purification except for benzaldehyde. Benzaldehyde was washed
with aqueous Na CO solution in three times and distilled to remove the
2
3
contaminants. Acetonitrile (MeCN) was of spectral grade, obtained commercially,
and used without further purification.
Table 1 Products and quantum yields for photocatalytic oxygenation of aromatic hydrocarbons by O
?
with HCl and Acr –Mes in O
2
?
-saturated MeCN (conditions: [substrate] = 10 mM, [Acr -Mes
2
-
ClO
4
] = 1.0 mM, HCl = 1.0 mM)
a
Substrate
Carboxylic acid
Yield Aldehyde
%)
Yield Conv. Time
(%)
U
(%)
(
(%)
(h)
8
9
9
7
5
5
13
100
13
0.44
1.13
0.63
b
95
5
5
4
100
12
9
6
100
20
12
10
0.56
–
2
0
1
4
7
3
44
10
13
0.01
2
3
1
8
6
13
100
–
Photoirradiation by use of a xenon lamp attached with a color glass filter (k \ 420 nm)
a
U was determined from the amount of the formation of benzaldehyde
b
U was determined from the amount of benzoic acid
123

208
K. Ohkubo et al.
Reaction procedures
The photocatalytic oxygenation of toluene with O in the presence of HCl was
2
carried out by the following procedure. Typically, an MeCN solution (0.60 mL)
?
containing Acr –Mes ClO (0.25 mg, 6.0 9 10 mol) and toluene (0.64 mg,
-
-7
4
-
.0 9 10 mol) in an NMR tube with a rubber septum was saturated with oxygen
6
6
by bubbling oxygen through a Teflon tube for 5 min. The solution was then
irradiated with a 500-W xenon lamp (Ushio Optical ModelX SX-UID 500XAMQ)
through a color filter glass (Asahi Techno Glass Y43) transmitting k [ 430 nm at
2
78 K. After photoirradiation for 13 h, the corresponding benzaldehyde and benzoic
acid were identified by comparison with those of an authentic samples in the GC
1
retention time and the GC/MS spectra (Shimadzu QP-5000), and also by H NMR
1
spectra. Benzaldehyde: H NMR (300 MHz, CD CN, ppm): d 10.0 (s, 1H), 7.89
3
(
d, J = 6.9 Hz, 2H), 7.68 (t, J = 7.3 Hz, 1H), 7.58 (t, J = 7.9 Hz, 2H). Benzoic
1
acid: H NMR (300 MHz, CD CN, ppm) d 8.00 (d, J = 7.8 Hz, 2H), 7.62 (m, 1H),
3
7
.48 (m, 2H).
The photocatalytic chlorination of 1,3,5-trimethoxybenzene (TMB) by O
2
?
-
with Acr –Mes ClO4 in the presence of HCl was carried out as follows.
?
Typically, an acetonitrile-d solution (0.60 mL) containing Acr –Mes (0.050 mg,
3
-
.2 9 10 mol) and TMB (0.40 mg, 2.4 9 10 mol) in an NMR tube with a
7
-6
1
rubber septum was saturated with oxygen by bubbling oxygen gas through a
stainless steel needle for 5 min. The solution was then irradiated with a 500-W
xenon lamp through a color filter glass transmitting (k \ 430 nm) at 278 K. After
photoirradiation for 6 h, 2,4,6-trimethoxychlorobenzene was identified by compar-
1
ing the H NMR spectrum in comparison with that of an authentic sample. 2,4,6-
1
Trimethoxychlorobenzene: H NMR (300 MHz, CD CN, ppm) d 6.27 (s, 2H), 3.82
3
(
s, 3H), 3.78 (s, 6H).
The amount of H O was determined by titration by iodide ion as follows
2
2
?
35–37]. An O -saturated MeCN solution (0.60 mL) of Acr –Mes (1.0 9 10 M)
-3
[
and toluene (1.0 9 10 M) was photoirradiated and the photoirradiated product
2
-
2
mixture was diluted with an MeCN solution containing excess amount of NaI.
-
The amount of I3 formed was then determined from the UV–Vis spectrum
4
-1
-1
(
kmax = 361 nm, k = 2.5 9 10 M cm ) [36, 37].
Quantum yield determinations
Potassium trisoxalatoferrate(III) as a standard actinometer [38] was used to
determine the quantum yield of the photocatalytic oxygenation of toluene with O in
2
?
the presence of Acr –Mes. A square quartz cuvette (10 mm i.d.), which contained a
?
-
-3
degassed MeCN solution (3.0 mL) of Acr –Mes ClO
(1.0 9 10 M) and
toluene (0–0.25 M), was irradiated with steady-state monochromatized light
k = 430 nm) from a Shimadzu RF-5300PC fluorescence spectrophotometer.
4
(
?
Under the conditions of actinometry experiments, the actinometer and Acr –Mes
absorbed essentially all the incident light of k = 430 nm. The light intensity of
monochromatized light of k = 430 nm was determined to be 9.9 9 10 ein-
stein s . In addition, a square quartz cuvette (10 mm i.d.), which contained a
-
7
-
1
1
23

Oxygenation and chlorination of aromatic hydrocarbons
209
?
-4
degassed MeCN solution (3.0 mL) of Acr –Mes (2.0 9 10 M) and toluene
0–0.2 M), was irradiated with monochromatized laser flash (k = 430 nm) from
Nd:YAG laser (Continuum, SLII-10, 4-6 ns fwhm). The light intensity of
(
-
8
monochromatized light of k = 430 nm was determined to be 8.6 9 10 ein-
stein/pulse. The photochemical reaction was monitored using a Hewlett Packard
8
453 diode-array spectrophotometer.
Laser flash photolysis measurements
Measurements of transient absorption spectra in the photochemical reactions of
?
Acr –Mes with toluene were performed according to the following procedure.
?
A degassed MeCN solution containing Acr –Mes ClO (2.0 9 10 M), toluene
-
-4
4
-
4
1.5 9 10 M) and HCl (2.0 9 10 M) was excited by Nd:YAG laser (Contin-
-4
(
uum, SLII-10, 4–6 ns fwhm) at 430 nm. Time courses of the transient absorption
spectra were measured by using a continuous xenon lamp (150 W) and an InGaAs-
PIN photodiode (Hamamatsu 2949) as a probe light and a detector, respectively.
The output from the photodiodes and a photomultiplier tube was recorded with a
digitizing oscilloscope (Tektronix, TDS3032, 300 MHz). The transient spectra were
recorded using fresh solutions in each laser excitation. All experiments were
performed at 298 K.
Results and discussion
Photocatalytic oxygenation of toluene with O2
Visible light irradiation (k [ 430 nm) of the absorption band of 9-mesityl-10-
?
methylacridinium perchlorate (Acr –Mes ClO4
-
-3
5.0 9 10 M) in an
;
O -saturated acetonitrile (MeCN) solution containing toluene (1.5 9 10 M) for
-
2
2
5 h by using a 500-W xenon lamp resulted in the formation of an oxygenated
product in very low yield, i.e., benzaldehyde in 2 % yield (Eq. 1).
ð1Þ
In the presence of HCl under the same irradiation conditions, toluene was
consumed with a concomitant formation of benzaldehyde (60 %), benzoic acid
123

210
K. Ohkubo et al.
1
?
CN solutions containing Acr –Mes ClO
-
Fig. 1
H
NMR spectra of oxygen-saturated CD
3
4
-
3
-2
-3
1.0 9 10 M), toluene (1.0 9 10 M) and HCl (1.0 9 10 M) a before and after photoirradiation
(
for b 6 h and c 15 h. Benzene was used as an internal standard
(
The yields of oxygenated products and H O were determined by H NMR and
20 %), and hydrogen peroxide (H O , 60 %) after 7 h photoirradiation (Fig. 1).
2 2
1
2
2
iodemetry measurements, respectively. The time course in photocatalytic oxygen-
ation is shown in Fig. 2. Prolonged photoirradiation afforded benzoic acid with the
consumption of benzaldehyde. The yields of final products after 15 h photoirra-
diation were 30 % for benzaldehyde, 70 % for benzoic acid and 20 % for H O . The
2
2
total stoichiometry for oxygenation of toluene to benzoic acid is given by Eq. 2. The
photocatalytic turnover
Fig. 2 Irradiation time profiles
of product formation and
reactant consumption in
?
Acr –Mes catalyzed
photooxygenation of toluene
-
1.0 9 10 M) in O
2
(
MeCN (0.6 mL) at 298 K;
2
-saturated
?
-
4
Acr –Mes ClO :
1
1
-3
.0 9 10 M, HCl:
M
-3
.0 9 10
1
23

Oxygenation and chlorination of aromatic hydrocarbons
211
ð2Þ
number was determined to be 50. No oxygenation of the mesitylene moiety of
?
Acr –Mes occurred under photoirradiation in the presence of substrate and HCl
?
because the electron-transfer state of Acr –Mes is efficiently quenched by the large
?
excess of HCl to give the ground state of Acr –Mes [39]. It was also confirmed that
no chlorinated substrate was observed under the present experimental conditions.
?
When Acr –Mes was replaced by 9-phenyl-10-methylacridinium (Acr –Ph), the
?
photooxygenated products were also obtained as 8 % yield of benzoic acid after
?
5 h photoirradiation. The photocatalytic reactivity of Acr –Ph is significantly
1
lower than the case of Acr –Mes. In contrast to the case of Acr –Mes, the
?
?
?
photocatalytic oxygenation of toluene with O in the presence of Acr –Ph proceeds
2
-
via electron transfer from Cl to Acr –Ph. However, the lifetime of the singlet
?
?
excited state of Acr –Ph (k = 1.5 ns in MeCN) [26] is much shorter than that of the
electron-transfer state of Acr –Mes [31]. A high concentration of substrate is
?
?
needed to quench the short-lived singlet excited state of Acr –Ph.
?
When Acr –Mes was replaced by 9-mesityl-2,7,10-trimethylacridinium
?
Me Acr –Mes) which has a stronger reducing ability of the electron-transfer state
(
than Acr –Mes [33], the yield of benzoic acid was 13 % in the Me Acr –Mes
2
?
?
2
?
catalyzed photooxygenatyion. The low catalytic reactivity of Me Acr –Mes is
2
?
attributed to the lower quantum yield of the electron-transfer state of Me Acr –Mes
2
?
60 %) as compared to Acr –Mes (98 %) [31, 33]. When benzaldehyde was used as
(
a starting material, the photocatalytic oxygenation in MeCN also occurred to yield
benzoic acid. The yield of benzoic acid was 70 % after 8 h photoirradiation as
shown in Fig. 3. The catalytic reactivity was only slightly increased by addition of
Fig. 3 Irradiation time profiles
of the Acr –Mes—
photosensitized oxygenation of
?
-2
benzaldehyde (1.0 9 10 M)
in oxygen-saturated MeCN
?
0.6 mL) containing Acr –Mes
(
ClO
-
4
-3
(1.0 9 10 M), HCl
-
3
(1.0 9 10 M) and H
and 1.0 mM) at 298 K
2 2
O (0 M
123

212
K. Ohkubo et al.
H O (1.0 mM) as shown in Fig. 3. Thus, H O does not play a major role in the
2
2
2 2
photocatalytic oxygenation of benzaldehyde to benzoic acid in this catalytic system.
The quantum yields (U) of formation of the photooxygenated product under
1
photoirradiation (k [ 430 nm) were determined by following appearance of H
NMR peaks due to toluene. The dependence of U value on concentration of toluene
?
in the Acr –Mes catalyzed photooxygenation of toluene in O -saturated MeCN is
2
shown in Fig. 4a. The U value increases with increasing concentration of toluene
(
[Toluene]) to reach a constant value. Such a saturated dependence of U on the
-1
• -1
[
Toluene] is expressed by Eq. 3, where Kobs is defined as k k [Cl ] . k is the
2
1
1
U ¼ U K ½Tolueneꢁ=ð1 þ K ½TolueneꢁÞ
ð3Þ
rate constant of back electron transfer from Cl to Acr –Mes, and k is the rate
1
obs
obs
-
•
2
•
constant of hydrogen abstraction of toluene by Cl (Scheme 1). Equation 3 is
rewritten as Eq. 4. The saturated dependence of U on [Toluene] was converted
h
i
ꢂ1
¼ U 1 þ ðKobs½TolueneꢁÞꢂ1
ꢂ1
U
ð4Þ
1
-
1
-1
to a linear plot of U versus [Toluene] as shown in Fig. 4b. From the slope and
-
2
2
intercept, the U and Kobs values were determined to be 2.6 9 10 and 2.7 9 10
?
-
1
M
, respectively.
Table 1 shows the yields and U values of the photocatalytic oxygenation of a
series of substrates including those with electron-withdrawing groups. Each reaction
proceeds in high conversion to yield the corresponding carboxylic acid and
aldehyde. In the case of p-xylene as a substrate, telephthalic acid was obtained in
1
6 % yield.
In order to elucidate the photocatalytic oxygenation mechanism, laser flash
photolysis was performed to detect the intermediates. Nanosecond laser excitation
?
at 430 nm of a degassed MeCN solution of Acr –Mes resulted in formation of the
2
Fig. 4 a Dependence of quantum yield of benzaldehyde on [Toluene] for the photocatalytic reaction O -
?
saturated MeCN containing Acr –Mes ClO
the photoirradiation (80 min, k [ 430 nm). b Plot of U versus [Toluene]
-
4
(1.0 mM), toluene (0.5–240 mM) and HCl (1.0 mM) after
-1
-1
1
23

Oxygenation and chlorination of aromatic hydrocarbons
213
Scheme 1 Proposed reaction mechanism for photooxygenation of toluene derivatives catalyzed by
Acr –Mes and HCl
?
•
•?
electron-transfer state (Acr –Mes ) via photoinduced electron transfer from the
?
Mes moiety to the singlet excited state of the Acr moiety as shown in Fig. 5. The
•
•?
quantum yield for the formation of Acr –Mes
duced electron transfer has been previously determined to be nearly quantitative
98 %) [31].
The absorption band at 500 nm is due to the Acr moiety overlapped with the
by intramolecular photoin-
(
•
•?
Mes moiety [31, 32]. The absorption band at 500 nm was previously assigned to
?
the triplet excited state of the Acr moiety [40–44]. However, the absorption band
at 500 nm is accompanied by a near-IR absorption, which was clearly assigned to
•
•?
the p-dimer radical cation formed between the electron-transfer state (Acr –Mes
and the ground state of Acr –Mes [45, 46]. A similar near-IR absorption was
)
?
observed for the p-dimer radical cation formed between the electron-transfer state of
Fig. 5 Transient absorption
spectrum of electron-transfer
?
state of Acr –Mes in deaerated
MeCN without HCl taken at
6
ls after nanosecond laser
excitation (k = 430 nm)
123

214
K. Ohkubo et al.
2
-phenyl-4-(1-naphthyl)quinolinium and the ground state [47, 48]. Thus, the
absorption band at 500 nm does not originate from the triplet excited state but the
electron-transfer state. It should be pointed out that the energy of acridinium triplet
was mistakenly reported to be 1.96 eV, which is lower than the electron-transfer
•
•?
state (Acr –Mes ) [41, 42]. However, the reported low triplet energy value
corresponds to the strong phosphorescence of the corresponding acridine, which
remains as impurity according to the reported synthetic method [41, 42]. It is well
known that acridine derivatives exhibit phosphorescence with the same energy of
1
.96 eV as the mistakenly reported value for the acridinium ion [49]. It is also well
known that the triplet energy of acridinium ion is significantly higher than that of
the corresponding acridine [50, 51]. It should be emphasized that the electron-
•
•?
transfer state (Acr –Mes ) has not only strong oxidizing ability but also strong
reducing ability, both of which have been utilized a variety of photoinduced redox
processes [52–69].
•
?
The decay rate constant of the transient absorption band due to the Mes moiety
•
•?
of Acr –Mes increases with increasing concentration of HCl as shown in Fig. 6.
The formation rate obeys pseudo-first-order kinetics, and the pseudo-first-order rate
constant increases linearly with increasing concentration of HCl (Fig. 6). The
-
•
•?
second-order rate constant of electron transfer from Cl to Acr –Mes
8
determined from the slope of the linear plot in Fig. 6 to be 9.5 9 10 M
was
-1 -1
s .
•
•?
Because the one-electron reduction potential of Acr –Mes (Ered = ?2.06 V vs.
-
SCE) [33] is more positive than the one-electron oxidation potential of Cl
-
Eox = ?1.15 V vs. SCE) [39], electron transfer from Cl to the Mes moiety in
•?
(
Acr –Mes is energetically quite feasible. Thus, in the presence of HCl, electron
•
•?
-
transfer from Cl to the Mes moiety occurs to produce Cl and Acr –Mes [39].
•?
•
•
•
-
This indicates clearly that Cl is formed by electron transfer from Cl to Acr –
•
•?
-
Mes rather than by direct photoinduced electron transfer from Cl to the singlet
?
excited state of the Acr moiety in Acr –Mes.
?
•
On the other hand, the rate constant of electron transfer from toluene to Acr –
-1 -1
•
?
6
Mes was determined to be 2.0 9 10 M
s
(Fig. 6), which is significantly
-
smaller than the rate of electron transfer from Cl to Acr –Mes . Electron transfer
•
•?
Fig. 6 Plots of kdecay versus
[Toluene or HCl] for the
electron-transfer oxidation of
toluene or HCl with Acr –Mes
in deaerated MeCN at 298 K
•
•?
1
23

Oxygenation and chlorination of aromatic hydrocarbons
215
•
•?
oxidation of toluene with Acr –Mes is energetically unfavorable, because the
one-electron oxidation potential of toluene (Eox = ?2.20 V vs. SCE) [34] is
•?
more positive than the reduction potential of the Mes moiety (?2.06 V) of
•?
•
Acr –Mes
.
The photocatalytic mechanism involving the radical intermediates detected in
?
this study is shown in Scheme 1. Photoexcitation of Acr –Mes gives the long-lived
•
•?
electron-transfer state (Acr –Mes ) by intramolecular electron transfer from the
mesitylene moiety to the acridinium moiety that has strong reducing and oxidizing
abilities (Ered = ?2.06 V, Eox = –0.57 V vs. SCE) [33]. The electron-transfer state
•
•?
is stabilized as the p-dimer radical cation composed of Acr –Mes and the ground
state of Acr –Mes (Scheme 1) [45, 46]. The Acr and Mes moieties can reduce
?
•
•?
-
and oxidize O and Cl to give superoxide anion (O2 ) and Cl , respectively [33,
•-
•
2
•
3
radical. This is followed by rapid O addition to afford the peroxyl radical, leading
9]. A hydrogen of the methyl group of toluene is abstracted by Cl to afford benzyl
2
to benzaldehyde and benzyl alcohol via the disproportionation. Benzyl alcohol is
•
•?
readily further oxygenated to yield benzaldehyde with Acr –Mes . On the other
•
-
2
hand, O disproportionates with proton to yield H O and O (Scheme 1) [33]. The
2
2
2
?
further oxygenation of benzaldehyde photosensitized by Acr –Mes with HCl occurs
to yield benzoic acid via abstraction of a hydrogen from the formyl group of
•
benzaldehyde by Cl .
Photocatalytic chlorination of trimethoxybenzene
No photooxygenation of TMB occurred, because the hydrogen abstraction from
•
TMB by Cl is difficult due to the strong C–H bond of the methoxy group. However,
?
aerobic photocatalytic aromatic chlorination of TMB occurs with Acr –Mes and
?
HCl. Visible light irradiation of the absorption band (k [ 430 nm) of Acr –Mes
-
5.0 9 10 M) in an O -saturated MeCN solution of TMB containing HCl
3
(
2
-
2
6.0 9 10 M) resulted in the formation of a chlorinated product, 2,4,5-
(
trimethoxychlorobenzene. The overall stoichiometry of the photocatalytic reaction
is given by Eq. 5
ð5Þ
The yield of 2,4,6-trimethoxychlorobenzene and H O was determined to be
2
2
1
8 % and 46 % by the H NMR and iodometry measurements, respectively (see the
‘Experimental’’ section). The photocatalytic chlorination of other methoxybenzene
5
‘
derivatives was also examined and the results are summarized in Table 2.
Laser flash photolysis was employed to elucidate the mechanistic details for
photocatalytic chlorination. Transient absorption spectra taken after the nanosecond
?
laser excitation at 430 nm of a degassed MeCN solution of Acr –Mes in the absence
•
?
and presence of TMB are shown in Fig. 7a. The absorption band due to the Mes
123

216
K. Ohkubo et al.
Table 2 One-electron oxidation potentials of substrates, products and quantum yields for photocatalytic
chlorination of aromatic compounds by O
?
with HCl and Acr –Mes in O
2
-
2
saturated MeCN (conditions:
?
[
4
substrate] = 4.0 mM, [Acr –Mes ClO ] = 0.20 mM, [HCl] = 20 mM)
a
Substrate
E
ox versus SCE
Product
Conv.
(%)
Yield
(%)
Time
(h)
U
(%)
(
V)
1
.76
.49
Trace
9
–
–
1
1
0
Trace
9
.43
.96
74
23
14
9
0.30
1.1
100
1.5
Photoirradiation by use of a xenon lamp attached with a color glass filter (k [ 420 nm)
a
U was determined from the amount of the formation of chlorinated product
•
•?
moiety (kmax = 500 nm) of Acr –Mes was quenched by the addition of TMB. In
the presence of TMB, a new absorption band at 450 nm appeared at 12 ls as shown in
Fig. 7a, which is assigned to the TMB radical cation (TMB ) [64, 70]. The rate
•
?
•
?
constant of electron transfer from TMB to the Mes of the electron-transfer state of
?
Acr –Mes was determined from the slope of the linear plot of k versus [TMB] to be
rise
1
0
-1 -1
kTMB = 2.0 9 10 [64]. The decay rate was accelerated with increasing
concentration of HCl (Fig. 8a). The observed decay rate constant (kdecay) increases
M
s
•
?
linearly with increasing concentration of HCl as shown in Fig. 8b. Thus, TMB
-
efficiently reacts with Cl to form the Cl adduct radical [TMB(Cl) ]. The rate constant
•
-
for the addition of Cl to TMB was determined from the slope of kdecay versus [HCl]
•?
7
= 8.2 9 10 M
-1 -1
to be k
-
s
Cl in the photocatalytic reaction was also determined by quenching of the transient
(Fig. 8b). The rate of electron-transfer oxidation of
HCl
•
•?
8
absorption of Acr –Mes by HCl to be 2.9 9 10 M
-1 -1
s
TMB is significantly longer than that of Cl in MeCN under the photocatalytic
[39]. The lifetime of
•
?
•
•
conditions. Thus, the contribution of Cl that is formed by the electron-transfer
-
oxidation of Cl is negligible under the present reaction conditions.
The photocatalytic reaction is initiated by formation of intramolecular photoin-
?
duced electron transfer from the Mes moiety to the singlet excited state of the Acr
?
moiety of Acr –Mes to generate Acr –Mes as shown in Scheme 2. The electron-
•
•?
transfer state is stabilized as the p-dimer as mentioned in the previous section. The
1
23

Oxygenation and chlorination of aromatic hydrocarbons
217
•
•?
Fig. 7 a Transient absorption spectra for electron transfer from TMB (0–0.30 mM) to Acr –Mes in
MeCN after nanosecond laser flash photolysis (kex = 430 nm). b Transient absorbance time profiles at
•?
4
50 nm due to TMB observed in the presence of various concentration of TMB
Fig. 8 a Transient absorbance time profiles at 450 nm. b Plot of kdecay versus [HCl] for the reaction of
•?
TMB with HCl (kex = 430 nm)
•
?
•?
•
Mes moiety can oxidize TMB to produce TMB , whereas the Acr moiety can
•
•?
-
reduce O with proton to produce HO . The TMB thus produced reacts with Cl to
2
2
•
form the Cl adduct radical, which undergoes the dehydrogenation with HO to afford
2
the corresponding chlorinated product and hydrogen peroxide. Hydrogen peroxide
further reacts with HCl and substrate to produce another chlorinated product.
Conclusions
?
A donor–acceptor linked dyad, Acr –Mes, which has strong oxidizing and reducing
abilities in the photoinduced electron-transfer state, has been shown to act as an
efficient photocatalyst for the oxygenation of toluenes and chlorination of
123

218
K. Ohkubo et al.
Scheme 2 Proposed catalytic reaction mechanism of photochlorination of aromatic hydrocarbon by
Acr –Mes and HCl
?
methoxybenzenes with molecular oxygen as an oxidant. The radical intermediates
in the photocatalytic oxidation and chlorination of substrates with O2 were
successfully detected by nanosecond transient absorption measurements to clarify
the photocatalytic mechanisms.
Acknowledgments This work was supported by a Grant-in-Aid (Nos. 20108010 and 23750014), a
Global COE program from the Ministry of Education, Culture, Sports, Science and Technology, Japan
and KOSEF/MEST through WCU project (R31-2008-000-10010-0), Korea.
References
1. R.C. Larock, Comprehensive organic transformations: a guide to functional group preparations
(Wiley, New York, 1999)
2
. I. Hermans, E.S. Spier, U. Neunschwanedr, N. Turr a´ , A. Baiker, Top. Catal. 52, 1162–1174 (2009)
. M.B. Smith (ed), in March’s Advanced Organic Chemistry: Reactions, Mechanisms, and Structure,
3
6th edn. (Wiley, New York, 2007), pp. 698–705
4
5
6
. R.C. Larock (ed), in Comprehensive Organic Transformations, 2nd edn. (Wiley, New York, 1999),
pp. 622–624
. A. Bottino, G. Capannelli, F. Cerutti, A. Comite, R. Di Felice, Chem. Eng. Res. Des. 82, 229–235
(2004)
. D. Dumitriu, R. Barjega, L. Frunza, D. Macovei, T. Hu, Y. Xie, V.I. Parvulescu, S. Kaliaguine,
J. Catal. 219, 337–351 (2003)
7. Y. Ishii, S. Sakaguchi, Catal. Surv. Jpn. 3, 27–35 (1999)
8. R. Bandyopadhyay, S. Biswas, S. Guha, A.K. Mukherjee, R. Bhattacharyya, Chem. Commun.
1627–1628 (1999)
9. V.R. Durvasula, Synlett 495–496 (1992)
1
1
1
1
1
1
0. G. Barak, Y. Sasson, J. Chem. Soc., Chem. Commun. 637–638 (1988)
1. W. Bartok, D.D. Rosenfeld, A. Schriesheim, J. Org. Chem. 28, 410–412 (1963)
2. R.A. Sheldon, J. Chem. Technol. Biotechnol. 68, 381–388 (1997)
3. R.A. Sheldon, Chem. Ind. (London) 12–15 (1997)
4. T. Funabiki (ed) in Oxygenases and Model Systems. (Kluwer, Dordrecht, 1997)
5. D.H.R. Barton, A.E. Martell, D.T. Sawyer, The activation of dioxygen and homogeneous catalytic
oxidation (Plenum, New York, 1993)
16. L.L. Simandi, Dioxygen activation and homogeneous catalytic oxidation (Elsevier, Amsterdam,
1991)
1
23

Oxygenation and chlorination of aromatic hydrocarbons
219
1
1
1
2
2
2
2
2
7. F. M u¨ ller, J. Mattay, Chem. Rev. 93, 99–117 (1993)
8. J. Mattay, V. Martin, Top. Curr. Chem. 159, 219–255 (1991)
9. M.A. Fox, M. Chanon (eds.), in Photoinduced Electron Transfer. (Elsevier, Amsterdam, 1988)
0. M. Julliard, M. Chanon, Chem. Rev. 83, 425–506 (1983)
1. M. Julliard, C. Legris, M. Chanon, J. Photochem, Photobiol. A Chem. 61, 137–152 (1991)
2. M. Julliard, A. Galadi, M. Chanon, J. Photochem. Photobiol. A Chem. 54, 79–90 (1990)
3. L. Lopez, Top. Curr. Chem. 156, 117–166 (1990)
4. A. Heumann, M. Chanon, in Applied Homogeneous Catalysis with Organometallic Compounds, ed.
by B. Cornils, W.A. Herrmann (VCH, Weinheim, 1996), pp 929–948
5. W.P. Todd, J.P. Dinnocenzo, S. Farid, J.L. Goodman, I.R. Gould, J. Am. Chem. Soc. 113, 3601–3602
2
(1991)
2
2
2
2
3
3
6. K. Ohkubo, K. Suga, K. Morikawa, S. Fukuzumi, J. Am. Chem. Soc. 125, 12850–12859 (2003)
7. K. Suga, K. Ohkubo, S. Fukuzumi, J. Phys. Chem. A 109, 10168–10175 (2005)
8. K. Ohkubo, K. Suga, S. Fukuzumi, Chem. Commun. 2018–2020 (2006)
9. K. Ohkubo, S. Fukuzumi, Org. Lett. 2, 3647–3650 (2000)
0. K. Ohkubo, S. Fukuzumi, Bull. Chem. Soc. Jpn. 82, 303–315 (2009)
1. S. Fukuzumi, H. Kotani, K. Ohkubo, S. Ogo, N.V. Tkachenko, H. Lemmetyinen, J. Am. Chem. Soc.
126, 1600–1601 (2004)
3
3
2. K. Ohkubo, H. Kotani, S. Fukuzumi, Chem. Commun. 4520–4522 (2005)
3. K. Ohkubo, K. Mizushima, R. Iwata, K. Souma, N. Suzuki, S. Fukuzumi, Chem. Commun. 46,
6
01–603 (2010)
4. S. Fukuzumi, K. Ohkubo, T. Suenobu, K. Kato, M. Fujitsuka, O. Ito, J. Am. Chem. Soc. 123,
459–8467 (2001)
3
8
3
3
3
3
3
4
4
5. R.D. Mair, A.J. Graupner, Anal. Chem. 36, 194–204 (1964)
6. S. Fukuzumi, S. Kuroda, T. Tanaka, J. Am. Chem. Soc. 107, 3020–3027 (1985)
7. S. Fukuzumi, M. Ishikawa, T. Tanaka, J. Chem. Soc. Perkin Trans. 2, 1037–1045 (1989)
8. C.G. Hatchard, C.A. Parker, Proc. R. Soc. London Ser. A 235, 518–536 (1956)
9. K. Ohkubo, A. Fujimoto, S. Fukuzumi, Chem. Commun. 47, 8515–8517 (2011)
0. A.C. Benniston, K.J. Elliott, R.W. Harrington, W. Clegg, Eur. J. Org. Chem. 2, 253–258 (2009)
1. A.C. Benniston, A. Harriman, P. Li, J.P. Rostron, J.W. Verhoeven, Chem. Commun. 2701–2703
(2005)
42. A.C. Benniston, A. Harriman, P. Li, J.P. Rostron, H.J. van Ramesdonk, M.M. Groeneveld, H. Zhang,
J.W. Verhoeven, J. Am. Chem. Soc. 127, 16054–16064 (2005)
43. A.C. Benniston, A. Harriman, J.W. Verhoeven, Phys. Chem. Chem. Phys. 10, 5156–5158 (2008)
44. H. van Willigen, G. Jones II, M.S. Farahat, J. Phys. Chem. 100, 3312–3316 (1996)
45. S. Fukuzumi, H. Kotani, K. Ohkubo, Phys. Chem. Chem. Phys. 10, 5159–5162 (2008)
46. S. Fukuzumi, R. Hanazaki, H. Kotani, K. Ohkubo, J. Am. Chem. Soc. 132, 11002–11003 (2010)
47. Y. Yamada, T. Miyahigashi, H. Kotani, K. Ohkubo, S. Fukuzumi, J. Am. Chem. Soc. 133,
16136–16145 (2011)
48. H. Kotani, K. Ohkubo, S. Fukuzumi, Faraday Disccuss. 155, 89–102 (2012)
49. O. Morawski, J. Prochorow, Chem. Phys. Lett. 242, 253–258 (1995)
50. K. Kikuchi, C. Sato, M. Watabe, H. Ikeda, Y. Takahashi, T. Miyashi, J. Am. Chem. Soc. 115,
5180–5184 (1993)
5
5
5
5
5
5
5
5
5
6
6
6
6
6
1. K. Kasama, K. Kikuchi, Y. Nishida, H. Kokubun, J. Phys. Chem. 85, 4148–4153 (1981)
2. H. Kotani, K. Ohkubo, S. Fukuzumi, J. Am. Chem. Soc. 126, 15999–16006 (2004)
3. K. Ohkubo, T. Nanjo, S. Fukuzumi, Org. Lett. 7, 4265–4268 (2005)
4. K. Ohkubo, T. Nanjo, S. Fukuzumi, Catal. Today 117, 356–361 (2006)
5. K. Ohkubo, K. Yukimoto, S. Fukuzumi, Chem. Commun. 2504–2506 (2006)
6. K. Ohkubo, T. Nanjo, S. Fukuzumi, Bull. Chem. Soc. Jpn. 79, 1489–1500 (2006)
7. S. Fukuzumi, Bull. Chem. Soc. Jpn. 79, 177–195 (2006)
8. H. Kotani, K. Ohkubo, Y. Takai, S. Fukuzumi, J. Phys. Chem. B 110, 24047–24053 (2006)
9. H. Kotani, T. Ono, K. Ohkubo, S. Fukuzumi, Phys. Chem. Chem. Phys. 9, 1487–1492 (2007)
0. K. Ohkubo, R. Iwata, T. Yanagimoto, S. Fukuzumi, Chem. Commun. 3139–3141 (2007)
1. H. Kotani, K. Ohkubo, S. Fukuzumi, Appl. Catal. B Environ. 77, 317–324 (2008)
2. S. Fukuzumi, Eur. J. Inorg. Chem. 1351–1362 (2008)
3. S. Fukuzumi, Phys. Chem. Chem. Phys. 10, 2283–2297 (2008)
4. K. Ohkubo, K. Mizushima, R. Iwata, S. Fukuzumi, Chem. Sci. 2, 715–722 (2011)
123

220
K. Ohkubo et al.
65. S. Fukuzumi, Y. Yamada, T. Suenobu, K. Ohkubo, H. Kotani, Energy Environ. Sci. 4, 2754–2766
(2011)
6
6
6. K. Ohkubo, R. Iwata, S. Miyazaki, T. Kojima, S. Fukuzumi, Org. Lett. 8, 6079–6082 (2006)
7. M. Tanaka, K. Ohkubo, C.P. Gros, R. Guilard, S. Fukuzumi, J. Am. Chem. Soc. 128, 14625–14633
(
2006)
8. M. Murakami, K. Ohkubo, T. Nanjo, K. Souma, N. Suzuki, S. Fukuzumi, Chem. Phys. Chem. 11,
594–2605 (2010)
6
2
6
9. M. Murakami, K. Ohkubo, S. Fukuzumi, Chem. Eur. J. 16, 7820–7832 (2010)
0. T. Shida, Electronic absorption spectra of radical ions (Elsevier, Amsterdam, 1988)
7
1
23