Oxygenation of α-methylstyrene with molecular oxygen, catalyzed by 10-methylacridinium ion via photoinduced electron transfer

Article abstract of DOI:10.1021/jp034213x

Photooxygenation of α-methylstyrene with oxygen occurs efficiently in the presence of 10-methylacridinium perchlorate (AcrH⁺ClO₄^-) under visible light irradiation in oxygen-saturated acetonitrile (MeCN) to yield acetophenone as the main oxygenated product. No photoinduced oxygenation of α-methylstyrene occurs in the absence of AcrH⁺ under otherwise the same experimental conditions. Little photodegradation of AcrH^- occurs in the present photocatalytic system, which provides a clean method of photoinduced oxygenation reaction with molecular oxygen, alternate to the ene reaction of singlet oxygen. The photocatalytic oxygenation of α-methylstyrene with oxygen is shown to proceed via photoinduced electron transfer from α-methylstyrene to the singlet excited state of AcrH^- (¹AcrH⁺*) on the basis of the fluorescence quenching of ¹AcrH⁺* by α-methylstyrene, the quantum yield determination, and the detection of radical intermediates by laser flash photolysis and ESR measurements.

Full text of DOI:10.1021/jp034213x

J. Phys. Chem. A 2003, 107, 4339-4346
4339
Oxygenation of r-Methylstyrene with Molecular Oxygen, Catalyzed by 10-Methylacridinium
Ion via Photoinduced Electron Transfer
Kyou Suga, Kei Ohkubo, and Shunichi Fukuzumi*
Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity,
CREST, Japan Science and Technology Corporation (JST), Suita, Osaka 565-0871, Japan
ReceiVed: January 27, 2003; In Final Form: March 14, 2003
Photooxygenation of R-methylstyrene with oxygen occurs efficiently in the presence of 10-methylacridinium
-
perchlorate (AcrH+ClO4 ) under visible light irradiation in oxygen-saturated acetonitrile (MeCN) to yield
acetophenone as the main oxygenated product. No photoinduced oxygenation of R-methylstyrene occurs in
the absence of AcrH+ under otherwise the same experimental conditions. Little photodegradation of AcrH+
occurs in the present photocatalytic system, which provides a clean method of photoinduced oxygenation
reaction with molecular oxygen, alternate to the ene reaction of singlet oxygen. The photocatalytic oxygenation
of R-methylstyrene with oxygen is shown to proceed via photoinduced electron transfer from R-methylstyrene
1
to the singlet excited state of AcrH+ (1AcrH+*) on the basis of the fluorescence quenching of AcrH+* by
R-methylstyrene, the quantum yield determination, and the detection of radical intermediates by laser flash
photolysis and ESR measurements.
Introduction
acridinium perchlorate (AcrH+ClO4-) under visible light ir-
radiation in oxygen-saturated acetonitrile to yield acetophenone.
The oxidative cleavage of alkenes to carbonyl compounds has
been used as a useful synthetic tool in natural product
synthesis.18 The photocatalyst (AcrH+) is found to be relatively
stable under the photocatalytic conditions, as compared with
other reported photosensitizers.19,20 Thus, the present study
provides a useful method of photoinduced double bond cleavage
of alkenes with molecular oxygen under visible light irradiation.
In addition, the detection of reactive intermediates in the
photocatalytic oxygenation reaction by laser flash photolysis
and ESR measurements as well as the kinetic study based on
the quantum yield determination provides valuable insight into
the mechanism of AcrH+-photosensitized oxygenation of R-me-
thylstyrene with oxygen.
Photosensitized oxygenation of organic compounds with
molecular oxygen has attracted considerable attention, since the
development of catalytic oxygenation systems employing mo-
lecular oxygen, which is the cleanest and cheapest oxidant, is
highly desired from the environmental and economical points
of view.1,2 The mechanisms of photosensitized oxygenation
reactions have been the subject of intense research, since many
reactive intermediates such as singlet oxygen (1O2), superoxide
anion (O2•-), and substrate radical cations are involved in the
photosensitized oxygenation reactions and the role of reactive
intermediates is different depending on the photocatalytic
systems.3-7 Singlet oxygen is normally produced by dye-
sensitized photooxygenation,3 and a variety of substrates are
known to be oxygenated by the reactions with singlet oxygen.4-7
However, a sensitizer for the formation of 1O2 is often converted
Experimental Section
•-
to O2 by the photoinduced electron-transfer reactions in the
presence of an electron donor and acceptor.8-10 Both O2•- and
1O2 can be formed under the same reaction conditions, resulting
in a competition between two different types of oxidation
Materials. R-Methylstyrene, styrene, and acetophenone were
obtained from Tokyo Kasei Kogyo Co., Ltd. trans-Stylbene and
1,1-diphenylethylene were obtained from Nacalai Tesque, Inc.
10-Methylacridinium iodide (AcrH+I-) was prepared by the
reaction of acridine with methyl iodide in acetone, was converted
to the perchlorate salt (AcrH+ClO4-) by the addition of
magnesium perchlorate to the iodide salt (AcrH+I-), and was
purified by recrystallization from methanol.21 Potassium ferri-
oxalate used as an actinometer was prepared according to the
literature and purified by recrystallization from hot water.22
Acetonitrile (MeCN) and propionitrile (EtCN) used as solvents
were purified and dried by the standard procedure.23 Deuterated
[2H3]acetonitrile (CD3CN, 99.8%) was obtained from EURI SO-
TOP, CEA, France, and used as received.
reactions.11,12 In some cases, O2 can be reduced to O2 by
thermal and photoinduced electron transfer from electron
donors.12 Such a complex situation has precluded the exact
delineation of the photooxygenation mechanism. In contrast,
1
•-
sensitizers such as 10-methylacridinium ion and 2,4,6-triphen-
•- 13-17
ylpyrylium ion are known to generate neither 1O2 nor O2
.
This situation simplifies the mechanistic study on the photo-
sensitized oxygenation reactions using these sensitizers.13-16
Nonetheless, the detailed photocatalytic oxygenation mecha-
nisms have yet to be fully understood because of the lack of
information about the reactive intermediates.
We report herein that photooxygenation of R-methylstyrene
with oxygen involving the cleavage of the carbon-carbon
double bond occurs efficiently in the presence of 10-methyl-
Reaction Procedure. Typically, an [2H3]acetonitrile (CH3-
CN) solution (0.6 cm3) containing AcrH+ (1.0 × 10-2 M) and
R-methylstyrene (3.0 × 10-2 M) in an NMR tube sealed with
a rubber septum was saturated with oxygen by bubbling with
oxygen through a stainless steel needle for 5 min. The solution
was then irradiated with a mercury lamp through an acetophen-
* To whom correspondence should be addressed. E-mail: fukuzumi@ap.
chem.eng.osaka-u.ac.jp.
10.1021/jp034213x CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/08/2003

4340 J. Phys. Chem. A, Vol. 107, No. 22, 2003
Suga et al.
one-methanol filter transmitting λ > 300 nm light at room
porting electrolyte at 298 K. The platinum working electrode
(BAS) was polished with BAS polishing alumina suspension
and rinsed with acetone before use. The counter electrode was
a platinum wire (BAS). The measured potentials were recorded
with respect to the Ag/AgNO3 (0.01 M) reference electrode.
The E°ox values (vs Ag/Ag+) are converted to those versus SCE
by adding 0.29 V.27
temperature. The irradiated solution was analyzed periodically
1
1
by H NMR spectroscopy. The H NMR measurements were
performed using a JEOL JMN-AL300 (300 MHz) NMR
1
spectrometer. Chemical shifts of H NMR were expressed in
parts per million downfield from tetramethylsilane as an internal
standard (δ ) 0). The products of the photoreaction of
R-methylstyrene (3.0 × 10-2 M) with AcrH+ (1.0 × 10-2 M)
ESR Measurements. An O2-saturated propionitrile solution
of AcrH+ (1.0 × 10-3 M) and R-methylstyrene (1.0 × 10-1
M) was irradiated at 193 K with a high-pressure mercury lamp
(USH-1005D) through a water filter focusing at the sample cell
in the ESR cavity. The ESR spectra were taken on a JEOL JES-
RE1XE spectrometer and were recorded under nonsaturating
microwave power conditions. The magnitude of the modulation
was chosen to optimize the resolution and the signal-to-noise
ratio (S/N) of the observed spectra. The g values and hyperfine
splitting constants (hfs’s) were calibrated using an Mn2+ marker.
Laser Flash Photolysis. The measurements of transient
absorption spectra in the AcrH+-photosensitized oxygenation
of R-methylstyrene with oxygen were performed according to
the following procedures. The Ar- or O2-saturated MeCN
solution containing R-methylstyrene (1.0 × 10-1 M) and AcrH+
(5.0 × 10-5 M) was excited by an Nd:YAG laser (Continuum,
SLII-10, 4-6 ns fwhm) at λ ) 355 nm with the power of 30
mJ per pulse. Photoinduced events were estimated by using a
continuous Xe-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 per-
formed at 298 K.
1
in oxygen-saturated CD3CN (0.6 cm3) were determined by H
NMR spectra. 1H NMR (300 MHz, CD3CN): acetophenone δ
2.56 (s, 3H), 7.4-8.0 (m, 5H); 2-(1-hydroxy-2-phenyl)propyl
hydroperoxide δ 1.49 (s, 3H), 3.52 and 3.54 (asym, 2H).24
Quantum Yield Determination. A standard actinometer
(potassium ferrioxalate)22 was used for the quantum yield
determination of the AcrH+-photosensitized oxygenation of
R-methylstyrene with oxygen. A square quartz cuvette (10 mm
i.d.) which contained an acetonitrile solution (3.0 cm3) of AcrH+
(5.0 × 10-5 M) and R-methylstyrene (1.0 × 10-3 to 8.0 × 10-3
M) was irradiated with monochromatized light of λ ) 358 nm
from a Shimadzu RF-5300PC fluorescence spectrophotometer.
Under the conditions of actinometry experiments, both the
actinometer and AcrH+ absorbed essentially all the incident
light. The light intensity of monochromatized light of λ ) 358
nm was determined as 2.53 × 10-9 einstein s-1 with the slit
width of 20 nm, respectively. The photochemical reaction was
monitored by using a Shimadzu UV-3100PC spectrophotometer.
The quantum yields were determined from the disappearance
of R-methylstyrene determined by GC as well as an increase in
absorbance due to acetophenone (λ ) 278 nm, ꢀ ) 1.1 × 103
M-1 cm-1). The photochemical reaction was monitored using
a Shimadzu GCMS-QP5000 mass spectrometer. The quantum
yields were determined from the decrease in yield of R-meth-
ylstyrene. A 10-mm square quartz cuvette which contained an
O2-saturated MeCN solution (3.0 cm3) of AcrH+ (0-1.5 × 10-2
M) and R-methylstyrene (1.0 × 10-3 to 1.0 × 10-2 M) was
irradiated with monochromatized light of λ ) 358 nm using a
Shimadzu spectrofluorophotometer (RF-5300PC). To avoid the
contribution of light absorption of the products, only the initial
rates were determined for determination of the quantum yields.
Fluorescence Quenching. Quenching experiments of the
fluorescence of AcrH+ by electron donors were performed using
a Shimadzu RF-5300PC fluorescence spectrophotometer with
the excitation wavelength 358 nm. The monitoring wavelength
was that corresponding to the maximum of the emission band
at 488 nm.15 The solutions were deoxygenated by argon purging
for 10 min prior to the measurements. Relative emission
intensities were measured for MeCN solution containing AcrH+
(5.0 × 10-5 M) with electron donors at various concentrations
(0-1.5 × 10-2 M). There was no change in the shape, but there
was a change in the intensity of the fluorescence spectrum by
the addition of an electron donor. The Stern-Volmer relation-
ship (eq 1) was obtained for the ratio of the emission intensities
Theoretical Calculations. Density-functional theory (DFT)
and ZINDO calculations were performed on a COMPAQ
DS20E computer. Geometry optimizations were carried out
using the Becke3LYP functional and 6-311++G** or 3-21G*
basis set28,29 with the unrestricted Hartree-Fock (UHF) formal-
ism and as implemented in the Gaussian 98 program.30 The hfs
values are evaluated from the spin density of each hydrogen
multiplied by 506.82 G.
Results and Discussions
AcrH+-Photosensitized Oxygenation of r-Methylstyrene
with Oxygen. Irradiation of the absorption band of 10-
methylacridinium perchlorate (AcrH+ClO4-) in an O2-saturated
MeCN solution containing R-methylstyrene for 1.0 h gave a
C-C double bond cleaved oxygenated product (acetophenone)
as the major product in addition to 2-(1-hydroxy-2-phenyl)propyl
hydroperoxide, as shown in eq 2. The products are identified
I0/I ) 1 + KSV[D]
(1)
in the absence and presence of electron donor (I0/I) and the
concentrations of quenchers [D]. The fluorescence lifetime τ
of AcrH+ is 37 ns in MeCN.15 The observed quenching rate
constants kq ()KSVτ-1) were obtained from the Stern-Volmer
constants KSV and the emission lifetimes τ (see Supporting
Information Figure S1).
1
by the H NMR spectra (see Experimental Section). Formal-
dehyde (10% yield after 40 min photoirradiation) was also
detected as the cleaved product, which was further oxidized
under the present photocatalytic conditions. Figure 1 shows the
time course of the photocatalytic reaction.31 The hydroperoxide
is formed initially but decomposes at prolonged reaction time,
accompanied by an increase in the yield of acetophenone. Thus,
Electrochemical Measurements. The second harmonic ac
voltammetry (SHACV)25,26 measurements of olefins and sty-
renes were performed on a BAS 100B electrochemical analyzer
in deaerated MeCN containing 0.10 M NBun ClO4 as a sup-
4

Oxygenation of R-Methylstyrene with Molecular Oxygen
J. Phys. Chem. A, Vol. 107, No. 22, 2003 4341
Figure 1. Time dependence of conversion of R-methylstyrene (O),
and yields of acetophenone (2) and 2-(1-hydroxy-2-phenyl)propyl
hydroperoxide (9) in the photoinduced oxygenation of R-methylstyrene
(3.0 × 10-2 M) in the presence of AcrH+ (1.0 × 10-2 M) in oxygen-
saturated acetonitrile at 298 K.
Figure 2. (a) Transient absorption spectrum observed by photoexci-
tation of a deaerated MeCN solution of AcrH+ (5.0 × 10-5 M) and
R-methylstyrene (1.0 × 10-1 M) at 40 µs after laser excitation at 298
K.
electron transfer (∆G°et , 0), the photoinduced electron transfer
is expected to occur at the diffusion-limited rate,33,34 as observed
experimentally (Table 1).
Detection of Reactive Intermediates. The occurrence of
photoinduced electron transfer from R-methylstyrene to 1AcrH+*
is confirmed by the laser flash photolysis experiments. Laser
flash excitation (355 nm from an Nd:YAG laser) of AcrH+ (5.0
× 10-5 M) in deaerated MeCN solution containing R-methyl-
styrene affords transient absorption bands at λmax ) 340 and
TABLE 1: Oxidation Potentials of Olefins (E°ox), Free
Energy Change of Photoinduced Electron Transfer (∆G°et),
1
and Fluorescence Quenching Rate Constants (kq) of AcrH+*
by Olefins in MeCN
olefin
E°ox vs SCE, V ∆G°et, eV 10-10kq, M-1 s-1
styrene
1.88
1.82
1.82
1.41
-0.44
-0.50
-0.50
-0.91
2.0
1.4
2.0
2.0
1,1-diphenylethylene
R-methylstyrene
trans-stilbene
360 nm due to R-methylstyrene radical cation35 and at λmax
)
520 nm due to AcrH•,15,17,36 as shown in Figure 2. Both
absorption bands appear immediately after nanosecond laser
exposure and decay at the same rate, obeying second-order
kinetics, as shown in Figure 3. From the slope of the linear
plot of ∆Abs-1 versus time in Figure 3b and the ꢀmax value
the hydroperoxide is an intermediate product for the formation
of the C-C double bond cleaved product, acetophenone.
After the photochemical reaction for 1.0 h, all R-methylsty-
rene molecules are consumed when the concentration of AcrH+
remains largely the same as that before the photochemical
reaction. Thus, AcrH+ acts as an efficient and stable photo-
catalyst for the photoinduced oxygenation of R-methylstyrene
with oxygen, in contrast with other photosensitizers employed
for the photoinduced oxygenation of alkenes, which readily
decompose under photoirradiation.19,20
Photoinduced Electron Transfer from Electron Donors
to AcrH+. Irradiation of the absorption band of AcrH+ results
in fluorescence at 488 nm in MeCN.15 The fluorescence of the
singlet excited state of AcrH+ (1AcrH+*) is known to be
quenched efficiently by a variety of aromatic electron donors
via electron transfer from aromatic electron donors to 1AcrH+*
in MeCN.17 The fluorescence of 1AcrH+* is also quenched by
a variety of olefins including R-methylstyrene. The quenching
rate constants kq are determined from the slopes of the Stern-
Volmer plots and the lifetime of the singlet excited state
1AcrH+* (see Supporting Information, S1). The kq values thus
obtained are summarized in Table 1.
(7400 M-1 cm-1 36
is obtained the decay rate constant as 7.0
)
× 109 M-1 s-1. The bimolecular decay of AcrH• may be
ascribed to the back electron transfer from AcrH• to R-meth-
ylstyrene radical cation.
When oxygen was introduced to the AcrH+-R-methylstyrene
system, the absorption band at 360 nm due to R-methylstyrene
radical cation disappeared and instead a new absorption band
appeared at the long wavelength region at λmax ) 680 nm, as
shown in Figure 4. The absorbance at 680 nm decreases with
decreasing oxygen concentration. Thus, the absorption band at
680 nm is attributed to the intermediate product derived from
the reaction of R-methylstyrene radical cation with oxygen.
Oxygen is expected to add to the â-position of R-methylstyrene
radical cation to produce the peroxyl radical cation, judging from
the largest spin density (0.54) at the â-position of the radical
cation, obtained by the density functional calculation at the
Becke3LYP/6-311++G** level, as shown in Figure 5a (see
Experimental Section).
Dioxetane intermediates are often isolated in the photooxy-
genation of sterically hindered olefins such as adamantylide-
neadamantane and its derivatives.37,38 Thus, it is reasonable to
assign the new absorption band at λmax ) 680 nm due to the
dioxetane radical cation (1) derived from the peroxyl radical
cation (Scheme 1).39
The free energy change of photoinduced electron transfer
from electron donors to 1AcrH+* (∆G°et in eV) is given by eq
3 ,32 where e is the elementary charge and E°ox and E°red* are
∆G°et ) e(E°ox - E°red*)
(3)
the one-electron oxidation potentials of electron donors and the
1
one-electron reduction potential of AcrH+* (E°red* ) 2.32 V
The absorption band at 680 nm decays obeying first-order
kinetics (Figure 6a), in contrast with the absorption band at 520
nm due to AcrH•, which decays obeying second-order kinetics
(Figure 6b). When water is added to the oxygen-saturated MeCN
solution of AcrH+ (5.0 × 10-5 M) and R-methylstyrene (1.0 ×
vs SCE),15 respectively. Since the E°ox values of various olefins
in MeCN have been determined in this study (see Experimental
Section), the ∆G°et values are given from eq 3, as listed in Table
1. Judging from the exergonic nature of the photoinduced

4342 J. Phys. Chem. A, Vol. 107, No. 22, 2003
Suga et al.
Figure 5. Spin distribution and spin density of R-methylstyrene radical
cation obtained by DFT calculation using the B3LYP/6-311++G**
basis set.
Figure 3. Decay time profiles of absorbance at (a) 360 nm and (b)
520 nm observed by photoexcitation of a deaerated MeCN solution of
AcrH+ (5.0 × 10-5 M) and R-methylstyrene at 298 K. Inset: Second-
order plot.
Figure 6. Decay time profiles of absorbance at (a) 680 nm and (b)
520 nm observed by photoexcitation of an oxygen saturated MeCN
solution of AcrH+ (5.0 × 10-5 M) and R-methylstyrene (1.0 × 10-1
M) at 298 K. Insets: First- and second-order plots.
SCHEME 1
Figure 4. Transient absorption spectrum observed by photoexcitation
of a oxygen-saturated MeCN solution ([O2] ) 1.3 × 10-2 M) of AcrH+
(5.0 × 10-5 M) and R-methylstyrene (1.0 × 10-1 M) at 40 µs after
laser excitation at 298 K.
remains virtually the same (Supporting Information Figure S3).
This indicates that the dioxetane radical cation reacts with H2O
to produce the species which has no absorption in the long
wavelength region.
10-1 M), the decay rate of the absorption band at 680 nm
increases linearly with increasing H2O concentration (Supporting
Information Figure S2), whereas the decay of AcrH• at 520 nm

Oxygenation of R-Methylstyrene with Molecular Oxygen
J. Phys. Chem. A, Vol. 107, No. 22, 2003 4343
Figure 8. Spin distribution and aH values (in gauss) of (a) the
hydroxylated peroxyl radical (2) derived from R-methylstylene and (b)
that from styrene, obtained by the DFT calculation using the B3LYP/
3-21G* basis set. The hfs values are evaluated from the spin density
of each hydrogen multiplied by 506.82 G.
Figure 7. ESR spectra observed under photoirradiation of an oxygen-
saturated EtCN solution of AcrH+ (1.0 × 10-3 M) containing (a)
R-methylstyrene (1.0 × 10-1 M) and (b) styrene (1.0 × 10-1 M) at
193 K.
To obtain further insight into the reactive intermediates in
the AcrH+-photosensitized oxygenation of R-methylstyrene with
oxygen, the ESR spectrum of an oxygen-saturated propionitrile
(EtCN) solution containing AcrH+ (1.0 × 10-3 M) and
R-methylstyrene (1.0 × 10-1 M) was measured under photoir-
radiation of visible light at low temperatures. Figure 7a shows
the detected ESR spectrum under such conditions.
The ESR spectrum consists of a single isotropic signal with
the g value of 2.0152 showing no detectable hyperfine structure.
This ESR feature is in good agreement with that of tertiary
alkylperoxyl radicals.40,41 The absence of the proton hyperfine
structure suggests the observed peroxyl radical is attributed to
neither the peroxyl radical cation nor the dioxetane radical cation
in Scheme 1, since the peroxyl radical cation would exhibit the
hyperfine splitting due to the â-proton and since the dioxetane
radical cation (1) has a quite different g value (2.0099) than
that of the complexed hyperfine structure.42 When R-methyl-
styrene is replaced by styrene, the ESR spectrum obtained under
the same experimental conditions as those for the case of
R-methylstyrene exhibits a doublet signal which corresponds
to the hyperfine structure due to one proton (Figure 7b). The
observed hfs value (aH ) 6.1 G) is in the range 2-8 G reported
for secondary alkylperoxyl radicals.43 This indicates that oxygen
is added to the R-position of styrene in the peroxyl radical. Thus,
the peroxyl radical cation in which oxygen is added to the
â-position is cyclized to give the dioxetane radical cation
(Scheme 1), and the C-O bond is cleaved by the reaction with
water to generate the hydroxylated peroxyl radical (2) that is
observed in ESR spectroscopy, as shown in Scheme 2. The DFT
calculation of 2 using the Becke3LYP/3-21G* basis set shown
in Figure 8a indicates the estimated largest aH value (0.33 G)
is smaller than the observed line width, in agreement with the
observation of no hyperfine structure of 2 in Figure 7a. The
same DFT calculation of the hydroxylated peroxyl radical
Figure 9. Spin distribution of the dioxetane radical cation (1), obtained
by DFT calculations using the B3LYP/3-21G* basis set.
SCHEME 2
derived styrene (Figure 8b) affords the aH value of 6.32 G, which
agrees with the experimental value in Figure 7b (aH ) 6.1 G).
The spin distribution of the dioxetane radical cation (1) in
Figure 9 indicates that the spin is delocalized over the molecule,
in contrast with the case of 2 in Figure 8a, where the spin density
of 2 is localized mainly on oxygen atoms.
Quantum Yield Determination. The quantum yields (Φp)
of the AcrH+-photosensitized oxygenation reaction of R-me-
thylstyrene with O2 were determined from the product formation
rate under irradiation of monochromatized light of λmax ) 358

4344 J. Phys. Chem. A, Vol. 107, No. 22, 2003
Suga et al.
indicated by Figure 10, where the calculated results using the
kqτ value (4.1 × 102 M-1) in eq 4 (solid line) agree with the
experimental results. Such agreement indicates that the photo-
catalytic reaction proceeds via photoinduced electron transfer
1
from R-methylstyrene to AcrH+*.
It has recently been reported that the same type of oxygen-
ation of R-methylstyrene, involving the cleavage of the carbon-
carbon double bond to yield acetophenone, as that of the present
photocatalytic reaction proceeds at a high temperature (90 °C)
with neither catalyst nor photoactivation although no reaction
occurs at room temperature.45 The thermal oxygenation reaction
of R-methylstyrene with oxygen is proposed to proceed via
radical chain processes, as shown in Scheme 3.38b,46,47 The
R-methylstyrene radical cation is assumed to be produced by
direct electron transfer from R-methylstyrene to oxygen in the
presumed charge-transfer complex between them.45 However,
the saturated dependence of Φp on concentration of R-methyl-
styrene in Figure 10 indicates that such an electron-transfer
radical chain process (Scheme 3) is not operative as the major
pathway in the present photocatalytic reaction. If the chain
process in Scheme 3 is the major pathway, the Φp value would
increase linearly with increasing concentration of R-methylsty-
rene.
Figure 10. Dependence of the quantum yield (Φp) on the concentration
of R-methylstyrene for the AcrH+-catalyzed photooxygenation of
R-methylstyrene in oxygen- (O) or air- (b) saturated MeCN at 298 K.
SCHEME 3
Photocatalytic Mechanism. On the basis of the above results,
the reaction mechanism for the AcrH+-photosensitized oxygen-
ation of R-methylstyrene is summarized, as shown in Scheme
4. The reaction is started by photoinduced electron transfer (ket)
from R-methylstyrene to the singlet excited state (1AcrH+*) to
•+
give the radical cation-acridinyl radical pair (PhC(Me)dCH2
AcrH•). This process occurs at the diffusion-limited rate, as
indicated by the fluorescence quenching of 1AcrH+* with
R-methylstyrene (Table 1). The formation of R-methylstyrene
radical cation and acridinyl radical is detected as the transient
absorption spectra in the absence of oxygen (Figure 2). In the
presence of oxygen, oxygen is added to R-methylstyrene radical
cation to give the peroxyl radical cation, which may be
converted to the more stable form, that is the dioxetane radical
cation (1), as detected as the transient absorption band at λmax
) 680 nm (Figure 4). The decay of the absorption band due to
the dioxetane radical cation is accelerated by the presence of
nm (see Experimental Section).44 The Φp values increase with
an increase in concentration of R-methylstyrene [S] to reach a
limiting value (Φp ) 0.62) in accordance with eq 4 , as shown
Φp ) Φp∞Kobs[S]/(1 + Kobs[S])
(4)
in Figure 10. It should be noted that the Φp values are virtually
the same irrespective of the difference in O2 concentration
(Figure 10). The observed saturated dependence of Φp on [S]
is well reproduced assuming that Kobs ) kqτ, obtained from the
1
fluorescence quenching of AcrH+* by R-methylstyrene, as
SCHEME 4

Oxygenation of R-Methylstyrene with Molecular Oxygen
J. Phys. Chem. A, Vol. 107, No. 22, 2003 4345
K. T.; Chen, J. S.; Greer, A.; Foote, C. S.; Houk, K. N. J. Am. Chem. Soc.
2003, 125, 1319.
water (Supporting Information Figure S2). This indicates that
water is added to the dioxetane radical cation to give the
hydroxylated peroxyl radical (2), which is detected by ESR
(Figure 7a). The back electron transfer from acridinyl radical
to the peroxyl radical gives the corresponding hydroperoxide,
which is detected as an intermediate product (Figure 1),
accompanied by regeneration of AcrH+ (Scheme 4). The
hydroperoxide decomposes to yield the final product, acetophen-
one. Formaldehyde formed together with the hydroperoxide
decomposes under the present photocatalytic conditions (vide
supra).
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According to Scheme 4, the quantum yield is expressed by
eq 5, where ka is the rate constant of addition of O2 to
Φp ) [ka[O2]ketτ/(ka[O2] + kbet)][S]/(1 + ketτ[S]) (5)
R-methylstyrene radical cation and kbet is the back electron
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quantum yield Φ∞ is then expressed by eq 6, where the
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Φp∞ ) ka[O2]ketτ/(ka[O2] + kbet)
(6)
competition between the rate of oxygen addition (ka) and the
back electron transfer (kbet) determines the limiting quantum
yield. Since the observed quantum yield is indepent of O2
concentration (vide supra), the rate of addition of O2 to
R-methylstyrene radical cation may be much faster than the back
electron-transfer process (kET): ka[O2] . kbet. In such a case,
the limiting quantum yield would be unity. In fact, the observed
limiting quantum yield determined from the acetophenone
formation (60 ( 5%) corresponds to the quantum yield of the
reactant conversion (97 ( 8%).
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: Stern-Volmer plot for
the fluorescence quenching of AcrH+ by olefins (S1), plot of
the decay rate constant of absorbance at 680 nm versus [H2O]
(S2), and decay time profile of absorbance at 520 nm in oxygen-
saturated MeCN containing 0.50 M H2O (S3). This material is
available free of charge via the Internet at http://pubs.acs.org.
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