100% selective oxygenation of p-xylene to p-tolualdehyde via photoinduced electron transfer

Article abstract of DOI:10.1021/ol0065571

(matrix presented) The 100% selective oxygenation of p-xylene to p-tolualdehyde is initiated by photoinduced electron transfer from p-xylene to the singlet excited state of 10-methyl-9-phenylacridinium ion under visible light irradiation, yielding p-tolualdehyde exclusively as the final oxygenated product. The reason for the high selectivity in the photocatalytic oxygenation of p-xylene is discussed on the basis of the photoinduced electron transfer mechanism.

Full text of DOI:10.1021/ol0065571

ORGANIC
LETTERS
2
000
Vol. 2, No. 23
647-3650
1
00% Selective Oxygenation of p-Xylene
3
to p-Tolualdehyde via Photoinduced
Electron Transfer
Kei Ohkubo and Shunichi Fukuzumi*
Department of Material and Life Science, Graduate School of Engineering,
Osaka UniVersity, CREST, Japan, and Science and Technology Corporation,
Suita, Osaka 565-0871, Japan
fukuzumi@ap.chem.eng.osaka-u.ac.jp
Received September 7, 2000
ABSTRACT
The 100% selective oxygenation of p-xylene to p-tolualdehyde is initiated by photoinduced electron transfer from p-xylene to the singlet
excited state of 10-methyl-9-phenylacridinium ion under visible light irradiation, yielding p-tolualdehyde exclusively as the final oxygenated
product. The reason for the high selectivity in the photocatalytic oxygenation of p-xylene is discussed on the basis of the photoinduced
electron transfer mechanism.
Selective oxygenation of ring-substituted toluenes to aromatic
aldehydes has been one of the most important organic
reactions in industrial chemistry because of useful applica-
tions of aromatic aldehydes as key chemical intermediates
for production of a variety of fine or specialty chemicals
such as pharmaceutical drugs, dyestuffs, pesticides, and
oxygenation of ring-substituted toluenes to aromatic alde-
hydes. However, their synthetic utility has been limited
because of low yield and poor selectivity. In addition, the
use of stoicihometric amounts of inorganic oxidants results
in the generation of copious amounts of inorganic waste,
which causes serious pollution of the environment. The
electrochemical recycle of these oxidants has been reported
1
perfume compositions. A number of methods using inor-
2
3
8,9
ganic oxidants such as chromium(IV), cobalt(III), manga-
to avoid the stoicihometric use of toxic inorganic oxidants.
nese(III),4 cerium(IV),5 benzeneseleninic anhydride, or
peroxydisulfate/copper ion have so far been reported for
6
However, the development of catalytic alternatives employ-
7
10
2
ing clean oxidants such as O is highly desired.
This study reports that 10-methyl-9-phenylacridinium
perchlorate (AcrPh ClO , green color) acts as an efficient
4
photocatalyst for highly selective oxygenation of p-xylene
to p-tolualdehyde under visible light irradiation via photo-
(
1) ) (a) Franz, G.; Sheldon, R. A. In Ullmann’s Encyclopedia of
+
-
Industrial Chemistry, 5th ed.; VCH: Weinheim, 1991. (b) Sheldon, R. A.;
Kochi, J. K. In Metal Catalyzed Oxidation of Organic Compounds;
Academic Press: New York, 1981; Chapter 10.
(2) (a) Clarke, R.; Kuhn, A. T.; Okoh, E. Chem. Br. 1975, 11, 59. (b)
Periasamy, M.; Bhatt, M. V. Tetrahedron Lett. 1978, 4561.
(
3) Ballard, R. E.; McKillop, A. U.S. Patent 4, 1984, 482, 438.
(8) (a) Steckhan, E. Angew. Chem., Int. Ed. Engl. 1986, 25, 683. (b) (b)
Kreh, R. P.; Spotniz, R. M.; Lundquist, J. T. J. Org. Chem. 1989, 54, 1526.
(9) For direct anodic oxidation, see: (a) Brinkhaus, K.-H. G.; Steckhan,
E. Tetrahedron 1986, 42, 553. (b) Nishiguchi, I.; Hirashima, T. J. Org.
Chem. 1985, 50, 539. (c) Torii, S.; Shiroi, T. J. Synth. Org. Chem. Jpn.
1979, 37, 914.
(4) Udupa, H. V. K. Trans. Soc. AdV. Electrochem. Sci. Technol. 1976,
1
1, 143.
(
5) (a) Baciocchi, E.; Giacco, T. D.; Roi, C.; Sebastiani, G. V.
Tetrahedron Lett. 1985, 28, 3353. (b) Ho, T.-L. Synthesis 1973, 347. (c)
Syper, L. Tetrahedron Lett. 1966, 4493.
(
6) Barton, D. H. R.; Hui, R. A. H. F.; Lester, D. J.; Ley, S. V.
Tetrahedron Lett. 1976, 3331.
7) Bhatt, M. V.; Perumal, P. T. Tetrahedron Lett. 1981, 22, 2605.
(10) (a) Sheldon, R. A. J. Chem. Technol. Biotechnol. 1997, 68, 381.
(b) Sheldon, R. A. Chem. Ind. (London) 1997, 12. (c) Oxygenases and Model
Systems; Funabiki, T., Ed.; Kluwer: Dordrecht, 1997.
(
1
0.1021/ol0065571 CCC: $19.00 © 2000 American Chemical Society
Published on Web 10/14/2000

-
2
induced electron transfer from p-xylene to the singlet excited
solution of xylenes and toluene (3.0 × 10 M) containing
+
1
+
+ -2
state of AcrPh ( AcrPh *). In contrast to the oxidation by
inorganic oxidants, the rate of photoinduced electron transfer
is highly sensitive to the oxidation potentials of electron
donors, and thus, no further oxidation of p-tolualdehyde has
occurred via photoinduced electron transfer from p-tolual-
AcrPh (1.0 × 10 M) with a high-pressure Hg lamp (λ >
300 nm) decreases in the following order: p- f o- f
m-xylene > toluene. The selectivity for tolualdehyde de-
creases in the following order: p- (100%) > m- (99%) >
o-xylene (94%). The further oxygenation of m- and o-xylene
occurs to yield small amounts of the corresponding phthala-
ldehyde. In the case of p-xylene, it was confirmed that no
photooxygenation of p-tolualdehyde occurs when p-tolual-
dehyde was used as a starting material.
We have previously reported that oxygenation of ring-
substituted toluenes to aromatic aldehydes proceeds via
photoinduced electron transfer from toluenes to the singlet
excited state of 10-methylacridinium ion ( AcrH *) as shown
in Scheme 1 for the case of p-xylene. The photoinduced
1
+
dehyde to AcrPh *, leading to formation of p-tolualdehyde
as the sole oxygenated product of p-xylene.
Visible light irradiation of the absorption band (λmax
)
+
3
58 and 417 nm) of 10-methylacridinium perchlorate (AcrH -
-
ClO
4
: 10 mM) in oxygen-saturated acetonitrile containing
p-xylene (30 mM) with a xenon lamp through a UV cutoff
filter (λ > 310 nm) results in formation of p-tolualdehyde
accompanied by the disappearance of p-xylene. After 24 h
1
+
1
1
12
irradiation, the yield of p-tolualdehyde was 37%. The
product yield is improved to 66%, when the solvent,
acetonitrile, is replaced by a less polar solvent, chloroform-
d, under otherwise identical experimental conditions. The
Scheme 1
+
photocatalyst AcrH remained largely unchanged, but a small
amount of the adduct, 9-p-xylenyl-10-methyl-9,10-dihy-
droacridine [AcrH(CH C H CH -p)], was produced after the
2 6 4 3
photooxygenation reaction. The photooxygenated product
+
yield is further improved to 100% when AcrH is replaced
+
by AcrPh in chloroform as shown in Table 1. There was
Table 1. Photooxygenation of Xylenes and Toluene (3.0 ×
-
2
+
-2
1
O
0
M), Catalyzed by AcrPh (1.0 × 10 M) with O
2
in
-Saturated Chloroform at 298 Ka
2
a
Irradiation time is 10 h. b Benzaldehyde.
1
+
electron transfer from p-xylene to AcrH * (ket) is followed
by the deprotonation of the p-xylene radical cation in
b
competition with the back electron transfer (k ) to the reactant
no dioxygenated product following prolonged photoirradia-
tion. It was confirmed that there was no adduct formation
between the photocatalyst, AcrPh , and p-xylene. Thus, the
pair to produce the p-xylenyl radical which couples with
•
AcrH in the absence of oxygen to yield the adduct [AcrH-
+
12
(
CH
2
C
6
H
4
CH
3
-p)]. In the presence of oxygen, the p-xylenyl
1
00% selective photooxygenation of p-xylene to p-tolual-
radical is readily trapped by oxygen to give a p-xylenylp-
+
dehyde has been accomplished by using AcrPh as a
photocatalyst in chloroform. The photoirradiation time to
obtain a 100% yield of p-tolualdehyde (3.0 × 10 M) was
reduced from 24 to 10 h when the xenon lamp was replaced
by a high-pressure Hg lamp and an acetophenone-methanol
filter (λ > 300 nm).
eroxyl radical that is reduced by back electron transfer from
•
AcrH to yield p-xylenyl hydroperoxide, accompanied by
-
2
+
regeneration of AcrH (Scheme 1). The hydroperoxide
12
decomposes to yield p-tolualdehyde selectively.
The 100% selective photocatalytic oxygenation of p-xylene
is made possible by the difference in the reactivity of
p-xylene and the oxygenated product, p-tolualdehyde, as
indicated by the following fluorescence quenching experi-
Other isomers, o- and m-xylene, are also converted to o-
and m-tolualdehyde, respectively (Table 1). The product
yields of o-, m-, and p-tolualdehyde and benzaldehyde after
+
ments. The fluorescence lifetimes (τ) of AcrH (λem ) 488
10 h of photoirradiation of an oxygen-saturated chloroform
(
12) Fujita, M.; Ishida, A.; Takamuku, S.; Fukuzumi, S.J. Am. Chem.
1
(
11) The products were analyzed by H NMR spectroscopy.
Soc. 1996, 118, 8566.
3648
Org. Lett., Vol. 2, No. 23, 2000

nm) in the absence and presence of xylenes, toluene, or the
corresponding aldehydes were determined by using a time-
resolved fluorescence spectrofluorophotometer. The rate
ylsemiquinone radical anion. The ESR spectrum of AcrPh•
was persistent for several hours in deaerated acetonitrile. The
hyperfine splitting constants and the maximum slope line
widths (∆Hmsl) were determined from a computer simulation
of the ESR spectra. The ∆Hmsl value thus determined
increases linearly with an increase in the concentration of
-
1
constants of fluorescence quenching k
q
() K τ ) by pho-
q
toinduced electron transfer are determined from the slopes
of the linear Stern-Volmer plots of τ
MeCN ) vs the quencher concentration. The k
0
/τ (τ
0
) 37 ns in
values thus
13
+
q
AcrPh in MeCN. Such line width variations of the ESR
determined are listed in Table 2.
spectra can be used to investigate the rate processes involving
the radical species. The rate constants (kex) of the electron-
+
•
transfer exchange reactions between AcrPh and AcrPh were
0
determined using eq 1, where ∆H msl is the maximum slope
line width of the ESR spectra in the absence of AcrPh ,
Table 2. Fluorescence Quenching Rate Constants of AcrH+
+
with Xylenes, Toluene, and Aldehydes in Deaerated MeCN at
i
respectively, and P is a
2
98 K
7
0
1
.57 × 10 (∆H - ∆H msl)
msl
k )
(1)
ex
+
(
1 - P )[AcrPh ]
i
statistical factor which can be taken as nearly zero (see
1
4
Supporting Information). The reorganization energies (λ)
of the electron-transfer reactions are obtained from the kex
1
1
-1 -1
values using eq 2 (Z ) 10
M
s ), where the effect of
-
1
-1
-1
[
(k ) - (kdiff) ] ) Z exp(λ/4k T)
(2)
ex
B
diffusion (kdiff ) 2.0 × 10 M-1 s-1 in MeCN and 1.2 ×
10
1
010
M
-1
s
-1
in chloroform, respectively) is taken into
1
+
The AcrH * fluorescence was quenched efficiently by
account and k is the Boltzmann constant. The λ value in
chloroform is determined as 0.27 eV which is smaller than
the value in MeCN (0.34 eV).
B
1
+
electron transfer from xylenes to AcrH *, whereas no
7
quenching was observed by p-tolualdehyde (k
q
, 1 × 10
value decreases in the following order:
p-xylene > o-xylene > m-xylene > o-tolualdehyde > toluene
m-tolualdehyde . p-tolualdehyde (not observed). This
-
1
-1
M
s ). The k
q
Since the λ values (0.27, 0.34 eV) are much smaller than
0
the driving force of the back electron transfer (-∆G
et
)
•
0
15
>
2.36 eV) from AcrH (E vs SCE ) -0.43 V) to the
p-xylene radical cation (E ) 1.93 V), the back electron
ox
0
16
order is consistent with the order of the monooxygenated
and dioxygenated product yields in Table 1. Thus, the faster
the photoinduced electron transfer, the larger is the product
red
transfer is deeply in the Marcus inverted region, where the
back electron transfer rate is expected to slow as the λ value
1
7
yield. However, the k
q
value for p-xylene determined in
decreases. The slower back electron transfer rate with
decreasing the solvent polarity leads to an increase in the
product yield as observed experimentally.
9
-1 -1
chloroform (4.2 × 10 M s ) is smaller than the value in
acetonitrile in Table 2, in contrast to the improved product
yield in chloroform as compared to that in the more polar
solvent acetonitrile (vide supra). The improved product yield
in chloroform may result from a decrease in the reorganiza-
tion energy of the electron transfer with a decrease in the
solvent polarity, which results in a slower back electron
The further improvement of the product yield by employ-
+
+
ing AcrPh instead of AcrH can also be ascribed to the
slower back electron transfer rate for the former than the
latter. In the Marcus inverted region, the back electron
transfer becomes slower as the driving force increases. Since
•
0
•
0
18
transfer from AcrH to the p-xylene radical cation in Scheme
(vide infra). Since the deprotonation of the p-xylene radical
the E ox value of AcrPh (E ox vs SCE ) -0.55 V) is more
•
0
15
1
negative than the value of AcrH (E ox vs SCE ) -0.43 V),
the driving force of the back electron transfer from AcrPh•
cation, which leads to the oxygenated product, competes with
the back electron transfer, the slower back electron transfer
results in the larger product yield.
The reorganization energies are evaluated by determining
the rate constants of electron-transfer self-exchange reactions
between 9-phenyl-10-methylacridinium ion (AcrPh ) and the
corresponding one-electron reduced radical (AcrPh ) in
acetonitrile and chloroform. The AcrPh radical was produced
by the electron-transfer reduction of AcrPh by tetrameth-
(14) Cheng, K. S.; Hirota, N. InVestigation of Rates and Mechanisms of
Reactions; Hammes, G. G., Ed.; Wiley-Interscience: New York, 1974; Vol.
VI, p 565.
(15) Fukuzumi, S.; Koumitsu, S.; Hironaka, K.; Tanaka, T. J. Am. Chem.
Soc. 1987, 109, 305.
+
0
0
(
16) The E ox value of p-xylene, which corresponds to the E red value of
•
the radical cation, was determined by the second harmonic ac voltammetry
(SHACV) measurements. The SHACV method is known to provide a
superior approach to the direct evaluation of the one-electron redox potentials
in the presence of a follow-up chemical reaction; Arnett, E. M.; Amarnath,
K.; Harvey, N. G.; Cheng, J.-P. J. Am. Chem. Soc. 1990, 112, 344.
(17) (a) Marcus, R. A. Annu. ReV. Phys. Chem. 1964, 15, 155. (b) Marcus,
R. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 1111.
•
+
(
13) Poulos, A. T.; Hammond, G. S.; Burton, M. E. Photochem.
Photobiol. 1981, 34, 169.
Org. Lett., Vol. 2, No. 23, 2000
3649

•
(
2.48 eV) is larger than that from AcrH (2.36 eV). The larger
highly selective photooxygenation of other isomers to the
corresponding aromatic aldehydes.
driving force results in the slower back electron transfer,
leading to the improved product yield.
The enhanced stability of AcrPh as a photocatalyst as
compared to AcrH is ascribed to the steric effect of the
phenyl group of AcrPh which hampers the radical coupling
with the deprotonated radicals, that is, the deactivation
process of the photocatalyst in Scheme 1.
In conclusion, the use of AcrPh as a photocatalyst in
chloroform has enabled us to accomplish the 100% selective
photooxygenation of p-xylene to p-tolualdehyde as well as
+
Acknowledgment. This work was partially supported by
a Grant-in-Aid for Scientific Research Priority Area (No.
+
•
11228205) from the Ministry of Education, Science, Culture
and Sports, Japan.
+
Supporting Information Available: ESR spectra of
•
+
AcrPh in chloroform (S1) and plot of ∆Hmsl vs [AcrPh ]
S2). This material is available free of charge via the Internet
(
0
•
0
(
18) The E ox value of AcrPh , which corresponds to the E red value of
AcrPh , was determined by the CV measurements in acetonitrile: Fuku-
at http://pubs.acs.org.
+
zumi, S.; Ohkubo, K.; Tokuda, Y.; Suenobu, T. J. Am. Chem. Soc. 2000,
1
22, 4286.
OL0065571
3650
Org. Lett., Vol. 2, No. 23, 2000