Simultaneous production of p-tolualdehyde and hydrogen peroxide in photocatalytic oxygenation of p-xylene and reduction of oxygen with 9-mesityl-10-methylacridinium ion derivatives

Article abstract of DOI:10.1039/b920606j

Photooxygenation of p-xylene by oxygen occurs efficiently under photoirradiation of 9-mesityl-2,7,10-trimethylacridinium ion (Me ₂Acr⁺-Mes) to yield p-tolualdehyde and hydrogen peroxide, which is initiated via photoinduced electron transfer of Me₂Acr ⁺-Mes to produce the electron-transfer state.

Full text of DOI:10.1039/b920606j

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Simultaneous production of p-tolualdehyde and hydrogen peroxide
in photocatalytic oxygenation of p-xylene and reduction of oxygen
with 9-mesityl-10-methylacridinium ion derivativesw
Kei Ohkubo,a Kentaro Mizushima,a Ryosuke Iwata,a Kazunori Souma,b Nobuo Suzukib and
Shunichi Fukuzumi*ac
Received (in Cambridge, UK) 5th October 2009, Accepted 4th November 2009
First published as an Advance Article on the web 18th November 2009
DOI: 10.1039/b920606j
Photooxygenation of p-xylene by oxygen occurs efficiently
under photoirradiation of 9-mesityl-2,7,10-trimethylacridinium
ion (Me2Acr+–Mes) to yield p-tolualdehyde and hydrogen
peroxide, which is initiated via photoinduced electron transfer
of Me2Acr+–Mes to produce the electron-transfer state.
photoexcitation of Acr+–Mes, has both strong oxidizing and
reducing ability, and this enables not only to oxidize alkyl
aromatic compounds but also to reduce O2.12,14
ꢁ
Visible light irradiation of [Acr+–Mes]ClO4 (lmax
=
430 nm, 0.20 mM) in oxygen-saturated acetonitrile (MeCN)
containing p-xylene (4.0 mM) with a xenon lamp attached with
a colour glass filter (l = 380–500 nm) for 80 min resulted in
formation of an oxygenated product, p-tolualdehyde (34%),
p-methylbenzyl alcohol (10%) and H2O2 (30%). The products
and quantum yields in the photocatalytic oxygenation were
determined by 1H NMR, GC-MS and iodometry.15 The
photooxygenation of mesitylene and durene also occur to
afford the corresponding benzaldehyde and H2O2 with a slight
amount of benzyl alcohol, as listed in Table 1. It is important
to note that 9-phenyl-10-methylacridnium ion (Acr+–Ph),
which contains no electron donor moiety, is not effective
Oxygenation of aromatic compounds to produce aromatic
aldehydes is key chemical reaction for production of precursors
of a variety of fine or specialty chemicals such as pharma-
ceutical drugs, dyestuffs, pesticides and perfume compositions.
A number of methods using inorganic heavy metal oxidants
have so far been reported for oxygenation of aromatic
compounds to produce the corresponding aromatic aldehyde.1–4
However, their synthetic utility has been limited because of
low yield and poor selectivity. Moreover, the use of stoichio-
metric amounts of inorganic oxidants should be avoided
because of environmental problems.5 For this reason, catalytic
oxygenation processes with molecular oxygen or hydrogen
peroxide (H2O2), have merited increasing attention.6 H2O2 is
desirable as a clean and mild oxidant because the by-product
in oxygenation of substrates is only H2O.6,7
as
a photocatalyst for the simultaneous formation of
p-tolualdehyde and H2O2 (Table 1).
The photocatalytic reactivity was enhanced by the presence
of H2O (0.9 M) and sulfuric acid (1.0 mM) to yield
p-tolualdehyde (75%), p-methylbenzyl alcohol (15%) and
H2O2 (74%) with a high quantum yield (0.25). The 100%
yields of tolualdehyde and H2O2 with a higher quantum yield
(0.37) were achieved by using 9-mesityl-2,7,10-trimethyl-
acridinium ion (Me2Acr+–Mes), where the hydrogens at
2 and 7 positions of the acridinium ring are replaced by methyl
groups (Table 1). The synthetic procedures for Me2Acr+–Mes
are described in ESI S2.w
There have been extensive studies on photocatalytic
formation of H2O2,8 or photocatalytic oxygenation of
aromatic substrates.9–11 If valuable aromatic aldehydes can
be produced together with H2O2 in the photocatalytic
oxygenation of alkyl aromatic compounds by O2, such a
process would be superior as compared to the conventional
methods to produce either or both H2O2 and aromatic aldehydes.
However, no photocatalytic reaction to produce both aromatic
aldehydes and H2O2 together has ever been reported.
No further oxygenated product, p-toluic acid or
p-phthaladehyde, was produced as shown in 1H NMR spectra
(ESI S3w).17 The photocatalytic turnover number (TON) was
20 under the experimental conditions. When the concentration
of Me2Acr+–Mes (1.0 ꢂ 10ꢁ4 M) was decreased, TON was
We report herein that 9-mesityl-10-methylacridinium ion
(Acr+–Mes)12–14 and its derivative act as efficient photo-
catalysts for simultaneous production of aromatic aldehydes
and H2O2 in the photocatalytic reduction of O2 by
alkyl aromatic compounds under visible light irradiation.
The electron-transfer state (Acrꢀ–Mesꢀ+), produced upon
increased to 200.
A
preparative scale photocatalytic
reaction with p-xylene (0.5 g, 4.7 mmol) and Me2Acr+–Mes
(60 mg, 0.14 mmol) in MeCN (150 mL) under irradiation with
a xenon lamp for 48 h was also examined to afford
p-toluadehyde (59%), p-benzyl alcohol (28%) and H2O2
(51%) with 87% conversion of substrate determined by 1H
NMR.18 The catalyst can be recycled because no decomposition
of photocatalyst occurred under the present experimental
conditions.19 Fig. 1 shows the time profiles of the reactant
and products in the photocatalytic reactions. p-Methylbenzyl
alcohol was formed as a reaction intermediate and this was
readily oxygenated to form p-benzaldehyde under the present
reaction conditions.
a Department of Material and Life Science, Graduate School of
Engineering, Osaka University and SORST (JST),
2-1 Yamada-oka, Suita, Osaka 565-0871, Japan.
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp; Fax: +81-6-6879-7370;
Tel: +81-6-6879-7368
b Tokyo Chemical Industry Co., Ltd., 4-10-2 Honcho, Nihombashi,
Chuo-ku, Tokyo, 103-0023, Japan
c Department of Bioinspired Science, Ewha Womans University, Seoul,
120-750, Korea
w Electronic supplementary information (ESI) available: Synthetic
procedures and spectral data. See DOI: 10.1039/b920606j
ꢃc
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Table 1 Products and quantum yields (F) for photocatalytic oxygenation of methyl-substituted benzenes with 10-methylacridinium derivatives
Yield, (%)
RCHO : RCH2OH : H2O2
Photocatalyst
Acr+–Mes
Substrate
Eoxa/V (vs. SCE)
Fc,d
p-Xylene
Mesitylene
Durene
p-Xylene
Mesitylene
Durene
1.93
1.71
1.63
1.93
1.71
1.63
1.93
75 : 15 : 74 (34 : 3 : 30)
77 : 19 : 75 (47 : 3 : 44)
75 : 15 : 70 (35 : 1 : 33)
100 : 0 : 100 (54 : 5 : 52)
66 : 2 : 65 (40 : 1 : 39)
64 : 10 : 62 (37 : 2 : 36)
12 : 18 : 10 (5 : 2 : 4)
0.25 (0.13)
0.30 (0.078)
0.30 (0.065)
0.37 (0.13)
0.26 (0.14)
0.25 (0.15)
0.042 (0.019)
Me2Acr+–Mes
Acr+–Ph
p-Xylene
a
b
Taken from ref. 16. After photoirradiation for 80 min. Conditions: [photocatalyst] = 0.2 mM, [substrate] = 4.0 mM, [H2O] = 0.9 M,
c
d
[H2SO4] = 1.0 mM. Values in parentheses are determined. Values in parenthesis are determined without H2O and H2SO4. Based on formation
of the corresponding aldehyde.
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 Acrꢀ–Mesꢀ+, as shown in Scheme 1.12–14 The
+
Mesꢀ moiety can oxidize p-xylene to produce thꢀe radical
cation, whereas the Acrꢀ moiety can reduce O2 to O2 ꢁ 14 The
.
produced p-xylene radical cation gives the corresponding
benzyl type radical via deprotonation. This is followed by
rapid O2 addition to afford the peroxyl radical, leading to the
final oxygenated product, p-tolualdehyde.11 The radical inter-
mediates were detected by ESR (gJ = 2.101, g> = 2.009 for
ꢁ
ꢀ
O2
and gJ = 2.033, g> = 2.006 for p-methylbenzyl
peroxyl radical) in frozen MeCN: see ESI S4w).11b,14,21,22 The
disproportionation of the peroxyl radical affords p-tolualdehyde,
Fig.
1
Irradiation time profiles of the Acr+–Mes (---) and
Me2Acr+–Mes (—) catalyzed photooxygenation of p-xylene
(4.0 ꢂ 10ꢁ3 M) in oxygen-saturated MeCN (0.6 mL) at 298 K;
[photocatalyst] = 2.0 ꢂ 10ꢁ4 M; [H2O] = 0.9 M; [H2SO4] = 1.0 ꢂ
10ꢁ3 M.
p-methylbenzyl alcohol and O2. p-Methylbenzyl alcohol is
+ 23
readily oxygenated to yield p-tolualdehyde with Acrꢀ–Mesꢀ
.
ꢁ
ꢀ
On the other hand, O2 disproportionates in the presence of
protons to yield H2O2 and O2 (Scheme 1).24 H2O and H2SO4
enhance the deprotonation of p-xylene radical cation and the
The photocatalytic oxygenation also occurred for durene
and mesitylene. The one-electron oxidation potentials (Eox) of
the toluene derivatives in MeCN are listed in Table 1. The
disproportionation process of O2 ꢁ, respectively,25 leading to
ꢀ
a remarkable enhancement of photocatalytic reactivity by
addition of aqueous sulfuric acid. The reaction pathway of
photooxygenation is not the radical chain process because
there was no dependence of the product quantum yield on
concentration of p-xylene (ESI S5w) and no induction period,
typical for an autoxidation process was observed (Fig. 1).11
The overall stoichiometry of the photocatalytic reaction is
given by eqn (1).
E
ox values are lower than the one-electron reduction potential
(Ered
)
of the electron-transfer state of Acr+–Mes
(Acrꢀ–Mesꢀ+: 2.06 V vs. SCE in MeCN). Thus, electron
transfer from toluene derivatives such as p-xylene to
Acrꢀ–Mesꢀ+ is energetically feasible, whereas electron transfer
+
from toluene itself (Eox = 2.20 V)16 to the Mesꢀ moiety is
energetically unfavourable and so no photocatalytic oxidation
of toluene occurred under the same experimental conditions.
The Eox values of 3,5-dimethylbenzaldehyde and 1,3,4-
trimethylbenzaldehyde were determined by cyclic voltammetry
(CV) and second harmonic alternative current voltammetry
(SHACV)20 to be 2.30 and 2.07 V, respectively. No CV and
SHACV wave for the oxidation of p-tolualdehyde was
observed before the solvent oxidation. The Eox value of
p-tolualdehyde is higher than 2.4 V due to an electron
withdrawing formyl group of p-tolualdehyde. The Eox values
of the alkylbenzenes, the oxygenated products of the corres-
ð1Þ
In contrast to the case of Acr+–Mes, the photocatalytic
oxygenation of p-xylene with O2 in the presence of Acr+–Ph
proceeds via electron transfer from p-xylene to Acr+–Ph.
However, the lifetime of the singlet excited state of Acr+–Ph
(t = 1.5 ns in MeCN)11a is much shorter than that of the
electron-transfer state of Acr+–Mes. A high concentration of
substrate is needed to quench the short-lived singlet excited
state of Acr+–Ph.
ponding benzaldehyde, are also higher than the Ered value
+
(2.06 V). This is the reason why the
of Acrꢀ–Mesꢀ
oxygenated product of p-xylene was only p-tolualdehyde and
no further oxygenated product such as p-phthalaldehyde
was formed. On the other hand, the one-electron reduction
The reducing ability of the electron-transfer state of
Acr+–Mes is improved by the electron-donating methyl substi-
tution of the acridinium ring of Acr+–Mes. Cyclic voltammo-
grams of Acr+–Mes and Me2Acr+–Mes in deaerated MeCN
are shown in Fig. 2 to compare the reducing abilities of the
+
of O2 by Acrꢀ–Mesꢀ is energetically feasible to produce
ꢁ 12,14
ꢀ
O2
.
ꢃc
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602 | Chem. Commun., 2010, 46, 601–603

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This work was supported by KOSEF/MEST through WCU
project (R31-2008-000-10010-0), a Seeds Innovation program
‘‘Practicability Verification Stage’’ from Japan Science and
Technology Agency (JST), a Grant-in-Aid (Nos. 21750146
and 19205019) and a Global COE program from MEXT.
Notes and references
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2 R. A. Sheldon and J. K. Kochi, Metal Catalyzed Oxidation of
Organic Compounds, Academic Press, New York, 1981, ch. 10.
3 R. E. Ballard and A. McKillop, US. Pat., 1984, 4,482,438.
4 (a) R. Clarke, A. Kuhn and E. Okoh, Chem. Brit., 1975, 11, 59;
(b) L. Syper, Tetrahedron Lett., 1966, 7, 4493; (c) T.-L. Ho,
Synthesis, 1973, 347.
5 M. Oelgemoller, C. Jung, J. Ortner, J. Mattay and
¨
E. Zimmermann, Green Chem., 2005, 7, 35.
6 F. Cavani and J. H. Teles, ChemSusChem, 2009, 2, 508.
7 W. T. Hess, in Kirk-Othmer Encyclopedia of Chemical Technology,
Wiley, New York, 4th edn, 1995, vol. 13, p. 961.
Scheme 1
8 (a) A. Das, V. Joshi, D. Kotkar, V. S. Pathak,
V. Swayambunathan, P. V. Kamat and P. K. Ghosh, J. Phys.
Chem. A, 2001, 105, 6945; (b) W. Song, J. Ma, C. Chen and
J. Zhao, J. Photochem. Photobiol., A, 2006, 183, 31.
9 (a) M. A. Fox, Photoinduced Electron Transfer, ed. M. A. Fox and
M. Chanon, Elsevier, Amsterdam, 1988, pp. 1–27; (b) M. Julliard,
C. Legris and M. Chanon, J. Photochem. Photobiol., A, 1991, 61, 137;
(c) K. Ohkubo and S. Fukuzumi, Bull. Chem. Soc. Jpn., 2009, 82, 303.
10 F. D. Lewis, A. M. Bedell, R. E. Dykstra, J. E. Elbert, I. R. Gould
and S. Farid, J. Am. Chem. Soc., 1990, 112, 8055.
11 (a) K. Ohkubo, K. Suga, K. Morikawa and S. Fukuzumi, J. Am.
Chem. Soc., 2003, 125, 12850; (b) K. Suga, K. Ohkubo and
S. Fukuzumi, J. Phys. Chem. A, 2005, 109, 10168.
12 S. Fukuzumi, H. Kotani, K. Ohkubo, S. Ogo, N. V. Tkachenko
and H. Lemmetyinen, J. Am. Chem. Soc., 2004, 126, 1600.
13 The electron-transfer state of Acr+–Mes forms a bimolecular
p-dimer with Acr+–Mes; see: S. Fukuzumi, H. Kotani and
K. Ohkubo, Phys. Chem. Chem. Phys., 2008, 10, 5159.
Fig.
2
Cyclic voltammograms of (a) Acr+–Mes and (b)
Me2Acr+–Mes (10 mM) in deaerated MeCN containing TBAPF6
(0.1 M) at 298 K.
14 H. Kotani, K. Ohkubo and S. Fukuzumi, J. Am. Chem. Soc., 2004,
126, 15999.
electron-transfer states of Acr+–Mes and Me2Acr+–Mes. The
Ered value of Me2Acr+–Mes (ꢁ0.67 V vs. SCE) is 0.1 eV more
negative than that of Acr+–Mes (ꢁ0.57 V).
15 The yield of H2O2 was determined by 1H NMR spectroscopy (see
ESI S1bw) as well as by titration by I3ꢁ using UV-vis; see (a) R. D. Mair
and A. J. Graupner, Anal. Chem., 1964, 36, 194; (b) S. Fukuzumi,
S. Kuroda and T. Tanaka, J. Am. Chem. Soc., 1985, 107, 3020.
16 S. Fukuzumi, K. Ohkubo, T. Suenobu, K. Kato, M. Fujitsuka and
O. Ito, J. Am. Chem. Soc., 2001, 123, 8459.
Dynamics of the electron-transfer reduction of O2 by the
electron-transfer states of Acr+–Mes and Me2Acr+–Mes were
examined by nanosecond laser flash photolysis. The rates of
the electron-transfer reduction were determined from
the quenching of the transient absorption due to the
electron-transfer state by O2 to be 6.8 ꢂ 108 Mꢁ1 sꢁ1 for
17 No oxygenation of p-xylene by singlet oxygen was observed in
photoreaction of p-xylene with zinc tetraphenylporphyrin as a
singlet oxygen photosensitizer.
18 The yield of H2O2 is smaller than that of aldehydes since H2O2 may
have partially decomposed under the present experimental
conditions. The isolated yield of p-tolualdehyde was 40%
(see ESI S2w for the experimental procedure for the isolation).
19 Photooxygenation of the mesitylene moiety in Acr+–Mes with O2
may occur in the absence of substrate under extended photo-
irradiation. See: A. C. Benniston, K. J. Elliott, R. W. Harrington
and W. Clegg, Eur. J. Org. Chem., 2009, 253.
+ 14
+
Acrꢀ–Mesꢀ
,
and 2.0 ꢂ 1010 Mꢁ1 sꢁ1 for Me2Acrꢀ–Mesꢀ
(see ESI S6w). Thus, the reducing ability of Me2Acrꢀ–Mesꢀ+ is
significantly enhanced by the methyl substitution. This may be
the reason why a 100% yield of tolualdehyde and H2O2 with
a
higher quantum yield (0.37) was achieved by using
Me2Acr+–Mes.
20 A. J. Bard and L. R. Faulkner, Electrochemical Methods:
Fundamentals and Applications, John Wiley & Sons, New York,
2001, ch. 10, pp. 368–416.
In conclusion, the electron-transfer states of Acr+–Mes
and Me2Acr+–Mes, which have both high oxidizing and
reducing ability, make it possible to produce both aromatic
aldehydes and H2O2 selectively in the photocatalytic oxygena-
tion of alkyl aromatic compounds with oxygen for the first
time. After aromatic aldehydes were formed, no further
21 For ESR spectrum of O2 ꢁ, see: R. N. Bagchi, A. M. Bond,
ꢀ
¨
F. Sholz and R. Stosser, J. Am. Chem. Soc., 1989, 111, 8270.
22 (a) M. Bersohn and J. R. Thomas, J. Am. Chem. Soc., 1964, 86,
959; (b) J. A. Howard, in Peroxyl Radicals, ed. Z. B. Alfassi, Wiley,
Chichester, 1997, pp. 283–334.
23 K. Ohkubo, K. Suga and S. Fukuzumi, Chem. Commun., 2006, 2018.
24 D. T. Sawyer, T. S. Calderwood, K. Yamaguchi and C. T. Angelis,
Inorg. Chem., 1983, 22, 2577.
oxidation takes place because electron transfer from aromatic
+
moiety is thermodynamically
aldehydes to the Mesꢀ
25 The deprotonation of toluene radical cation occurs efficiently in the
presence of aqueous sulfuric acid since the acidity of the toluene
radical cation (pKa = ꢁ13 in MeCN) is much stronger than that
of sulfuric acid, see: A. M. de P. Nicholas and D. R. Arnold,
Can. J. Chem., 1982, 60, 2165.
unfavorable. Thus, the use of charge-separation dyads as
photocatalysts paves a new way for the selective oxygenation
of alkyl aromatic compounds with simultaneous formation
of H2O2.
ꢃc
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Chem. Commun., 2010, 46, 601–603 | 603