Direct oxygenation of benzene to phenol using quinolinium ions as homogeneous photocatalysts

Article abstract of DOI:10.1002/anie.201102931

Photocatalytic oxygenation of benzene with oxygen and water to phenol has been achieved under ambient conditions by using the 3-cyano-1-methylquinolinium ion as a photocatalyst (see picture). The mechanism was clarified by detecting the π-dimer benzene radical cation produced by photoinduced electron transfer from benzene to the photocatalyst, and by monitoring the reaction of the radical cations with water molecules. Copyright

Full text of DOI:10.1002/anie.201102931

Communications
DOI: 10.1002/anie.201102931
Photocatalysis
Direct Oxygenation of Benzene to Phenol Using Quinolinium Ions as
Homogeneous Photocatalysts**
Kei Ohkubo, Takaki Kobayashi, and Shunichi Fukuzumi*
Phenol, which is currently produced from benzene by a three-
step cumene process, is one of the most important chemicals
in industry. Because the cumene process affords very low
yields (around 5%) with byproducts such as acetone and a-
[
1–4]
methylstyrene under severe conditions,
extensive efforts
have been made to develop a one-step oxygenation process of
benzene to phenol using heterogeneous inorganic cata-
[
5–15]
lysts.
However, only low yields of phenol have so far
been obtained under high-temperature conditions. Thus,
direct oxygenation of benzene to phenol with oxygen in
homogeneous media remains a formidable challenge.
We report herein that the 3-cyano-1-methylquinolinium
+
ion (QuCN ) acts as an efficient photocatalyst for selective
oxygenation of benzene to phenol using oxygen and water
+
under homogeneous and ambient conditions. The QuCN ion
has a strong oxidizing ability at the singlet excited state (E
red
[
16,17]
vs. SCE = 2.72 V),
which is capable of oxidizing benzene
[18,19]
(Eox vs. SCE = 2.32 V)
through photoinduced electron
transfer. Such a photocatalytic transformation through photo-
induced electron transfer has recently gained increased
attention, because it provides a valuable means for metal-
[
20]
Figure 1. Irradiation time profile of benzene (circles), phenol (trian-
free and environmentally benign synthesis.
Photocatalytic oxygenation of benzene with oxygen
gles), and H O (squares) for the photocatalytic oxygenation of
2
2
+
benzene by oxygen and water with QuCN in oxygen-saturated MeCN
1.0 mL) at 298 K: [benzene]=30 mm, [QuCN ]=2.0 mm, and
+
À
occurs under photoirradiation of QuCN ClO4 (lmax
=
+
(
3
30 nm, 5.0 mm) in an oxygen-saturated acetonitrile
[
H O]=2.0m.
2
(
(
MeCN) solution containing benzene (30 mm) and H O
3.0m) by a xenon lamp (500 W, l = 290–600 nm), to which
2
a color-cut glass filter was attached. Phenol and hydrogen
peroxide were selectively produced [Eq. (1)] after photo-
irradiation, which were identified by H NMR spectroscopy
and iodometry (see Figure S1 in the Supporting Information).
The selectivity of formation of phenol was 98% with a
quantum yield of 16% after 1 h of irradiation and 51% after
5 h of irradiation (Figure 1). The photocatalytic turnover
number (TON) was 7.5. This is the first example of photo-
catalytic oxygenation of benzene to phenol in a homogeneous
system. A preparative gram-scale photocatalytic reaction
with benzene (2.3 g, 29 mmol) and QuCN (210 mg,
0.8 mmol) in MeCN (200 mL) for 48 h was also examined to
afford phenol (1.1 g, 12 mmol) in 41% yield.
1
+
Phenol was also detected by GC–MS. A mass peak was
observed at m/z = 94 in a crude solution after photoirradia-
[
*] Dr. K. Ohkubo, T. Kobayashi, Prof. Dr. S. Fukuzumi
Department of Material and Life Science
Graduate School of Engineering
Osaka University and ALCA (JST)
Suita, Osaka 565-0871 (Japan)
+
tion of a MeCN solution containing benzene, QuCN , and
1
6
H2 O (see Figure S2 in the Supporting Information). When
1
6
18
H2 O was replaced by H2 O to clarify the oxygen source, the
mass number increased to m/z = 96. Thus, the origin of the
phenol oxygen was confirmed to be water.
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
Prof. Dr. S. Fukuzumi
Department of Bioinspired Science, Ewha Womans University
Seoul 120-750 (Korea)
+
When QuCN was replaced by 1-methylquinolinium
+
+
(
QuH ) and 1,2-dimethylquinolinium (QuMe ), the oxygen-
[
**] This work was supported by the Ministry of Education, Culture,
Sports, Science and Technology (Japan) with a Grant-in-Aid (grant
number 20108010 to S.F. and 23750014 to K.O.) and a Global COE
program, “The Global Education and Research Center for Bio-
Environmental Chemistry” and by the KOSEF/MEST through the
WCU project (R31-2008-000-10010-0), Korea.
ation of benzene to phenol by oxygen and water yielded 5.7
and 18% of phenol, respectively (Table 1). In the case of
chlorobenzene and QuCN , selective formation of phenol
was also observed to afford p- and o-chlorophenol in 27 and
3% yield at 31% conversion, respectively.
+
The efficient and selective photocatalytic oxygenation of
benzene to phenol is made possible by the difference in the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201102931.
8652
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 8652 –8655

Table 1: Rate constants of photoinduced electron transfer (k ), conversions of benzene, selectivities, and quantum yields of phenol after
et
[
a]
photocatalytic oxygenation of benzene by oxygen and water in the presence of quinolinium ions.
À1 À1
[b]
Catalyst
Substrate
ket [m
s
]
Irradiation time [h]
Conversion [%]
Yield [%]
Selectivity [%]
Quantum yield [%]
+
10
QuCN
C6H6
C6H6
C6H6
C6H6
C H Cl
1.1ꢀ10
1.0
5.0
5.0
5.0
0.5
1.0
31
73
88
21
31
7
30
51
5.7
18
98
70
6.5
87
16
+
QuCN
+
9
QuH
5.8ꢀ10
6.7ꢀ10
1.3ꢀ10
1.1ꢀ10
26
3.0
3.4
+
8
QuMe
+
10
10
QuCN
27 (p-), 3 (o-)
–
88 (p-), 11 (o-)
6
5
+
[c]
QuCN
C H OH
3.7
6
5
+
[
a] Conditions for photocatalytic reactions: [QuCN ]=2.0 mm, [substrate]=30 mm, [H O]=3.0m. [b] Determined from the initial rate for the yield of
2
the corresponding phenol derivative(s). [c] Based on the consumption of phenol.
reactivity of benzene and phenol. When benzene was
replaced by phenol as a starting material, the reactivity
significantly decreased relative to that obtained by benzene.
+
Prolonged photoirradiation of phenol with QuCN afforded
only small amounts of further oxygenated products such as
[
21]
quinones and diphenol derivatives.
The mechanism of oxygenation of benzene was examined
by fluorescence quenching and transient absorption experi-
+
ments. The fluorescence lifetime (t) of QuCN (l = 430 nm)
em
in the absence and presence of benzene or phenol were
determined by femtosecond laser flash photolysis. A transient
+
absorption spectrum of QuCN was observed in deaerated
MeCN after femtosecond laser excitation at 355 nm. The
spectrum is assigned to the singlet–singlet absorption because
1
+
of the singlet excited state of QuCN * detectable at 450 nm
Figure 2a). Photoinduced electron transfer from benzene or
(
1
+
phenol to QuCN * is energetically feasible, because the one-
electron reduction potential of QuCN * (E vs. SCE =
2
of benzene (E vs. SCE = 2.32 V).
and QuMe photocatalysts the rates of photoinduced electron
transfer were slower than that of QuCN , because the E
values of QuH * (2.46 V) and QuMe * (2.54 V) are lower
than that of QuCN * (2.72 eV).
phenol with QuH and QuMe photocatalysts (Table 1) are
ascribed to the lower oxidizing ability of QuH * and
QuMe * as compared with that of QuCN *.
1
+
red
[
17]
.72 V) is higher than the one-electron oxidation potential
[
18,19]
+
In the case of QuH
ox
+
+
red
+
1
+
1
+
Figure 2. a) Transient absorption spectra of QuCN with benzene
1
+
[17,22]
(1.5m) and without benzene in deaerated MeCN taken at 200 ps after
The lower yields of
femtosecond laser excitation (l =355 nm). Inset: The rise time
+
+
ex
profiles at 760 nm and at various concentrations of benzene (0–1.5m).
b) Plot of the observed rate constant (kobs) versus [C H ]. c) Plot of k
1
+
6
6
obs
+
1
+
versus [H O] determined from the decay of absorbance at 760 nm
2
+
The addition of benzene to QuCN in MeCN solution
because of the reaction of the p-dimer benzene radical cation with
H O (0, 25, 50 mm) in MeCN. d) Plot of k versus [O ] determined
from the decay of absorbance because of QuCNC at various concen-
followed by laser photoexcitation results in formation of
electron-transfer products, that is, the quinolinyl radical
2
obs
2
trations of O by nanosecond laser flash photolysis at 355 nm of an
2
(
QuCNC) detectable at 520 nm and the p-dimer benzene
+
MeCN solution containing QuCN (0.40 mm) and benzene (100 mm).
radical cation detectable in the near-IR region (Figure 2a).
The p-dimer benzene radical cation is generated by p–p
association of the radical cation of benzene with a large excess
[
23]
(1.60 V) is smaller than that of benzene, electron transfer
[
18]
1
+
of benzene (1.5m). The rate constant of formation of the p-
dimer benzene radical cation increased with increasing
concentration of benzene (inset of Figure 2a). The second-
order rate constant of formation of the p-dimer benzene
radical cation is determined from the linear plot shown in
from phenol to QuCN * is also highly exergonic. As a result
1
0
À1 À1
the electron-transfer rate constant (k = 1.1 ꢀ 10 m s ) is
et
close to the diffusion-limited value in MeCN. The ket values
obtained are summarized in Table 1.
The near-IR absorption band of the p-dimer benzene
radical cation was also observed by nanosecond laser flash
photolysis (see Figure S4 in the Supporting Information). The
association constant (Kdimer) of benzene to the radical cation
1
0
À1 À1
Figure 2b to be 2.1 ꢀ 10 m s , which is close to the
diffusion-limited value in MeCN as expected from the
exergonic electron transfer. The rate constant (k ) of electron
et
1
+
À1
transfer from benzene to QuCN * was determined by
fluorescence quenching of QuCN * with benzene and by a
of benzene was determined to be 11m , which was obtained
1
+
from the transient absorption intensities at 800 nm and at
1
0
À1 À1
Stern–Volmer plot to be 1.1 ꢀ 10 m
s
(see Figure S3 in the
various concentrations of benzene. The K
well with the reported value of 12 m . The transient species
value agrees
dimer
À1 [18]
Supporting Information). Because the Eox value of phenol
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www.angewandte.org
8653

Communications
of QuCNC and the p-dimer benzene radical cation is the
solvent-separated radical ion pair generated by photoinduced
electron transfer.
The rates of decay of QuCNC and the p-dimer benzene
radical cation obeyed second-order kinetics because of the
bimolecular back electron transfer (see Figure S4b in the
Supporting Information). The rate constant (kbet) was deter-
the radical ion pair was not observed by nano- and femto-
second laser flash photolysis (see Figures S8 and S9 in the
Supporting Information), although the fluorescence of
1
+
QuCN * was efficiently quenched by phenol. This indicates
that the back electron transfer is much faster than the electron
transfer from phenol to QuCNC. The back electron transfer in
the case of the benzene radical cation is highly exergonic and
thereby the process is located deep in the Marcus inverted
mined using the molar absorption coefficient of QuCNC (e
=
20
5
À1
À1 [24]
10 À1 À1
2
000m cm )
of 1.5 ꢀ 10 m s , which is close to the
region, where the rate of the back electron transfer becomes
1
0
À1 À1
[19,25]
diffusion-limited rate constant in MeCN (2.0 ꢀ 10 m s ).
The decay of absorbance at 800 nm because of the p-dimer
faster with decreasing driving force.
The back electron
transfer of the phenol radical cation may be much faster
because of the much smaller driving force of the back electron
benzene radical cation was accelerated by addition of H O.
2
The decay time profile obeys first-order kinetics in the
transfer (ÀDG = 3.29 eV) relative to the driving force of the
bet
presence of H O (see Figure S6 in the Supporting Informa-
back electron transfer of the benzene radial cation (ÀDG
=
bet
2
tion). The decay rate constant (kobs) increased linearly with
3.08 eV). This difference may be the reason why benzene was
oxidized but phenol was not oxidized in the photocatalytic
increasing concentration of H O as shown in Figure 2c. The
2
+
rate constant for the reaction of the benzene radical cation or
oxygenation catalyzed by QuCN . Thus, benzene was selec-
p-dimer benzene radical cation with H O was determined
tively and photocatalytically oxidized to phenol by oxygen
and water and no further oxygenation of phenol was
observed.
2
from the slope of k
versus [H O] to be k = 1.8 ꢀ
2 H2O
obs
7
À1 À1
1
0 m s . On the other hand, QuCNC was efficiently
quenched by O (see Figure S7 in the Supporting Informa-
tion). The decay rate constant of the transient absorption
In summary, the efficient and selective photooxygenation
of benzene to phenol has been accomplished in the presence
2
band at 500 nm for QuCNC increased linearly with increasing
of oxygen and H O through photoinduced electron-transfer
2
concentration of O as shown in Figure 2d. The rate constant
oxidation of benzene under homogeneous conditions using
2
+
for the electron-transfer reduction of O with QuCNC was
QuCN as photocatalyst. The quantum yield for the forma-
2
8
À1 À1
+
determined to be kO2 = 5.1 ꢀ 10 m s .
tion of phenol (26% for QuH ) is the highest value ever
The photocatalytic reaction is initiated by photoinduced
reported for the direct photocatalytic oxygenation of benzene
to phenol.
1
+
[26]
electron transfer from benzene to QuCN * as shown in
Scheme 1. The benzene radical cation, which is in equilibrium
with the p-dimer benzene radical cation, formed by photo-
induced electron transfer reacts with H O to yield the OH- Experimental Section
2
adduct radical. On the other hand, O can be reduced by
Reaction procedures: A quinolinium ion derivative (1.0–5.0 mm) and
3
2
À
À
D O (1.0–3.0m) were added to a CD CN solution (1.0 cm ). The
2 3
QuCNC to O C followed by protonation of O C to afford HO C.
2
2
2
solution was sealed in a sample tube and saturated with oxygen. Then,
benzene (30–50 mmol) was added to the solution. The mixture was
irradiated with a 500 W xenon lamp through a color glass filter of
transmittance at l > 290 nm. After photoirradiation, the oxygenated
products were identified and quantified by comparison of the
The hydrogen abstraction of HO C from the OH-adduct
2
radical affords phenol and H O (Scheme 1). When benzene
2
2
was replaced by phenol, the transient absorption spectrum of
1
H NMR spectra with those of identical samples using cyclohexane
as internal standard. The spectra confirmed that the reaction of
cyclohexane did not occurred in this photocataytic system. The yield
of H O was determined by titration with excess NaI (100 mm). The
2
2
À
amount of I3 formed was determined from the UV/Vis spectrum
e361 nm = 25000m cm ).
Preparative synthesis of phenol: QuCN ClO (210 mg,
.80 mmol) and benzene (2.3 g, 29 mmol) were dissolved in an O2-
À1
À1 [27]
(
+
À
4
0
saturated MeCN solution (200 mL) containing H O (3.6 mL,
2
0.2 mol). The solution was stirred and saturated with oxygen under
photoirradiation using a 300 W mercury lamp for 48 h. The temper-
ature of the solution was held constant at 208C by cooling with water.
The isolated yield of phenol was 41% (1.1 g, 12 mmol).
Received: April 28, 2011
Revised: July 1, 2011
Published online: August 1, 2011
Keywords: electron transfer · homogeneous catalysis · oxygen ·
.
photooxidation · radical ions
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
8655