Addition versus oxygenation of alkylbenzenes with 10-methylacridinium ion via photoinduced electron transfer

Article abstract of DOI:10.1021/ja9538952

Addition of alkylbenzenes with 10-methylacridinium ion (AcrH⁺) occurs efficiently under visible light irradiation in deaerated acetonitrile containing H₂O to yield 9-alkyl-10-methyl-9,10-dihydroacridine selectively. On the other hand, the photochemical reaction of AcrH⁺ with alkylbenzenes in the presence of perchloric acid in deaerated acetonitrile yields 10-methyl-9,10-dihydroacridine, accompanied by the oxygenation of alkylbenzenes to the corresponding benzyl alcohols. The photooxygenation of alkylbenzenes occurs also in the presence of oxygen, when AcrH⁺ acts as an efficient photocatalyst. The studies on the quantum yields and fluorescence quenching of AcrH⁺ by alkylbenzenes as well as the laser flash photolysis have revealed that the photochemical reactions of AcrH⁺ with alkylbenzenes in both the absence and presence of oxygen proceed via photoinduced electron transfer from alkylbenzenes to the singlet excited state of AcrH⁺ to produce alkylbenzene radical cations and 10-methylacridinyl radical (AcrH·). The competition between the deprotonation of alkylbenzene radical cations and the back electron transfer from AcrH· to the radical cations determines the limiting quantum yields. In the absence of oxygen, the coupling of the deprotonated radicals with AcrH· yields the adducts. The photoinduced hydride reduction of AcrH⁺ in the presence of perchloric acid proceeds via the protonation of acridinyl radical produced by the photoinduced electron transfer from alkylbenzenes. In the presence of oxygen, however, the deprotonated radicals are trapped efficiently by oxygen to give the corresponding peroxyl radicals which are reduced by the back electron transfer from AcrH· to regenerate AcrH⁺, followed by the protonation to yield the corresponding hydroperoxide. The ratios of the deprotonation reactivity from different alkyl groups of alkylbenzene radical cations were determined from both the intra- and intermolecular competitions of the deprotonation from two alkyl groups of alkylbenzene radical cations. The reactivity of the deprotonation from alkylbenzene radical cations increases generally in the order methyl < ethyl < isopropyl. The strong stereoelectronic effects on the deprotonation from isopropyl group of alkylbenzene radical cations appear in the case of the o-methyl isomer.

Full text of DOI:10.1021/ja9538952

8566
J. Am. Chem. Soc. 1996, 118, 8566-8574
Addition versus Oxygenation of Alkylbenzenes with
10-Methylacridinium Ion via Photoinduced Electron Transfer
Morifumi Fujita,† Akito Ishida,‡ Setsuo Takamuku,‡ and Shunichi Fukuzumi*,†
Contribution from the Department of Applied Chemistry, Faculty of Engineering, Osaka
UniVersity, Suita, Osaka 565, Japan, and The Institute of Scientific and Industrial Research,
Osaka UniVersity, Ibaraki, Osaka 567, Japan
ReceiVed NoVember 20, 1995X
Abstract: Addition of alkylbenzenes with 10-methylacridinium ion (AcrH+) occurs efficiently under visible light
irradiation in deaerated acetonitrile containing H2O to yield 9-alkyl-10-methyl-9,10-dihydroacridine selectively. On
the other hand, the photochemical reaction of AcrH+ with alkylbenzenes in the presence of perchloric acid in deaerated
acetonitrile yields 10-methyl-9,10-dihydroacridine, accompanied by the oxygenation of alkylbenzenes to the
corresponding benzyl alcohols. The photooxygenation of alkylbenzenes occurs also in the presence of oxygen, when
AcrH+ acts as an efficient photocatalyst. The studies on the quantum yields and fluorescence quenching of AcrH+
by alkylbenzenes as well as the laser flash photolysis have revealed that the photochemical reactions of AcrH+ with
alkylbenzenes in both the absence and presence of oxygen proceed via photoinduced electron transfer from
alkylbenzenes to the singlet excited state of AcrH+ to produce alkylbenzene radical cations and 10-methylacridinyl
radical (AcrH•). The competition between the deprotonation of alkylbenzene radical cations and the back electron
transfer from AcrH• to the radical cations determines the limiting quantum yields. In the absence of oxygen, the
coupling of the deprotonated radicals with AcrH• yields the adducts. The photoinduced hydride reduction of AcrH+
in the presence of perchloric acid proceeds via the protonation of acridinyl radical produced by the photoinduced
electron transfer from alkylbenzenes. In the presence of oxygen, however, the deprotonated radicals are trapped
efficiently by oxygen to give the corresponding peroxyl radicals which are reduced by the back electron transfer
from AcrH• to regenerate AcrH+, followed by the protonation to yield the corresponding hydroperoxide. The ratios
of the deprotonation reactivity from different alkyl groups of alkylbenzene radical cations were determined from
both the intra- and intermolecular competitions of the deprotonation from two alkyl groups of alkylbenzene radical
cations. The reactivity of the deprotonation from alkylbenzene radical cations increases generally in the order methyl
< ethyl < isopropyl. The strong stereoelectronic effects on the deprotonation from isopropyl group of alkylbenzene
radical cations appear in the case of the o-methyl isomer.
Introduction
light irradiation via photoinduced electron transfer. Benzylation
of aromatic nitriles can also be effected by UV irradiation in
the presence of alkylbenzenes via photoinduced electron
transfer.8-10 In these cases alkybenzene radical cations formed
by the photoinduced electron transfer oxidation of alkylbenzenes
are believed to act as reactive and important intermediates. Since
alkylbenzene radical cations are known as extremely strong
carbon acids in solution,11 they undergo proton loss from an
R-carbon to yield benzyl radical and analogs.12-15 Such
Recent advancement in photoinduced electron transfer chem-
istry has uncovered a large number of useful chemical and
biological processes which involve electron transfer being a key
step accompanied by the cleavage and formation of chemical
bonds.1,2 We have recently demonstrated that visible light
irradiation of the absorption band of 10-methylacridinium ion
(AcrH+, green color) in the presence of organometallic com-
pounds as well as alkenes results in efficient C-C bond
formation between these electron donors and AcrH+ via
photoinduced electron transfer from the donors to the singlet
excited state of AcrH+ to yield the alkylated or allylated adducts
selectively.3-5 Mariano et al.6,7 have shown that photoaddition
reactions of alkylbenzenes to iminium salts occur under UV
(6) Mariano. P. S. In Photoinduced Electron Transfer; Fox, M. A.,
Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part C, p 372.
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Chem. Soc. 1984, 106, 6439.
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M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part C, p 88. (b)
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† Department of Applied Chemistry.
‡ The Institute of Scientific and Industrial Research.
X Abstract published in AdVance ACS Abstracts, August 15, 1996.
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S0002-7863(95)03895-9 CCC: $12.00 © 1996 American Chemical Society

Addition Versus Oxygenation of Alkylbenzenes
J. Am. Chem. Soc., Vol. 118, No. 36, 1996 8567
deprotonation processes from alkylbenzenes have attracted
considerable interest not only because of the mechanistic aspects
but also in view of the practical utility.
zenes and iminium ions, requiring irradiation with UV light.
Such high-energy photochemistry has usually afforded a mixture
of various products.
In particular, the role of stereoelectronic effects (the rate of
deprotonation could be influenced by the relative orientation
of the CR-H bond and the aromatic π systems)16-18 has been
one of the central issues of radical cation chemistry. Contro-
versial results have so far been obtained on the role of
stereoelectronic effects. Serious doubts have been raised on
the actual role of stereoelectronic effects in the competitive
deprotonation between methyl and isopropyl groups of the
radical cation of p-cymene,19 although convincing experimental
evidence has recently been reported to support the existence of
stereoelectronic effects in the deprotonation from 9-ethyl-
anthracene radical cation.20 Certainly more data on the depro-
tonation from alkylbenzene radical cations in definitive systems
are required to gain a more comprehensive and confirmative
understanding of the reality of stereoelectronic effects.
Since benzyl radicals and analogs produced by the deproto-
nation of alkylbenzenes are readily trapped by oxygen,21 there
have been extensive studies concerning the oxygenation of
alkylbenzenes initiated by photoinduced electron transfer.8,22-24
However, most of the photoinduced electron transfer reactions
of alkylbenzenes have so far been limited to those with
sensitizers having high-energy excited sates such as dicyanoben-
This study reports that 10-methylacridinium perchlorate which
has an absorption maximum at 358 and 417 nm is highly
effective for the selective photoaddition of various alkylbenzenes
and that the oxygenation of alkylbenzenes occurs in the presence
of oxygen when AcrH+ acts as an efficient photocatalyst.25 The
oxygenation of alkylbenzenes has also been made possible
without oxygen by the photoinduced hydride transfer from
alkylbenzenes to AcrH+ in the presence of perchloric acid.26
The novel oxygenation mechanism without oxygen will be
revealed in relation to the photoaddition mechanism. The clean
and simple products formed by the photoreduction of AcrH+
by alkylbenzenes together with the excellent stability of AcrH+
as a photocatalyst for the photooxygenation reactions provide
a nice opportunity to gain a more comprehensive and confir-
mative understanding of the photoinduced electron transfer
reactions of alkylbenzenes as well as the reality of the
stereoelectronic effects on the deprotonation from alkylbenzene
radical cations.
Experimental Section
Materials. 10-Methylacridinium iodide was prepared by the reaction
of acridine with methyl iodide in acetone, and it was converted to the
perchlorate salt (AcrH+ClO4-) by addition of magnesium perchlorate
to the iodide salt, and purified by recrystallization from methanol.27
Alkylbenzenes and perchloric acid (HClO4; 70%) were obtained
commercially. Acetonitrile and methanol used as solvents were purified
and dried by the standard procedure.28
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Reaction Procedure. Typically, a deaerated [2H3]acetonitrile (CD3-
CN)/deuterium oxide (D2O) (7:1 v/v) solution (0.8 mL) containing
AcrH+ (8.0 × 10-3 M) was added to an NMR tube sealed with a rubber
septum under an atmospheric pressure of argon. After PhMe (4.0 ×
10-2 M) was added to the solution by means of a microsyringe and
mixed, the solution was irradiated with a high-pressure mercury lamp
through an acetophenone-methanol filter transmitting λ > 300 nm at
room temperature. After the reaction was complete, when the solution
became colorless, the products were analyzed by 1H NMR spectroscopy.
The 1H NMR measurements were performed using Japan Electron
Optics JNM-PS-100 (100 MHz) and JNM-GSX-400 (400 MHz) NMR
spectrometers. 1H NMR (CD3CN/D2O (7:1 v/v): (AcrHCH2Ph) δ 2.76
(d, 2H, J ) 7.3 Hz), 3.27 (s, 3H), 4.16 (t, 1H, J ) 7.3 Hz), 6.7-7.3
(m, 13H); (AcrHCH(Me)Ph) δ 1.10 (d, 3H, J ) 7.3 Hz), 2.84 (quintet,
1H, J ) 7.3 Hz), 3.14 (s, 3H), 3.99 (d, 1H, J ) 7.3 Hz), 6.7-7.3 (m,
13H); (AcrHCMe2Ph) δ 1.17 (s, 6H), 3.01 (s, 3H), 4.05 (s, 1H), 6.8-
7.3 (m, 13H); (AcrHCH(OMe)Ph) δ 3.02 (d, 3H, J ) 4.9 Hz), 3.46 (s,
3H), 4.11 (d, 1H, J ) 6.8 Hz), 4.29 (d, 1H, J ) 6.8 Hz), 6.7-7.4 (m,
13H); (AcrHCHPh2) δ 3.45 (s, 3H), 4.06 (d, 1H, J ) 10.7 Hz), 4.80
(d, 1H, J ) 10.7 Hz), 6.6-7.4 (m, 18H); (AcrHCH(c-C3H5)Ph) δ 0.20-
0.25 (m, 2H), 0.42-0.46 (m, 2H), 1.1-1.2 (m, 1H), 1.85 (dd, 1H, J )
6.4, 10.3 Hz), 3.00 (s, 3H), 4.26 (d, 1H, J ) 6.4 Hz), 6.6-7.3 (m,
13H); (AcrHCH2CHdCHPh) δ 2.41 (t, 2H, J ) 6.6 Hz), 3.35 (s, 3H),
4.09 (t, 1H, J ) 6.6 Hz), 5.9-6.1 (m, 2H), 6.9-7.4 (m, 13H);
(AcrHCH2C6H4-p-Me) δ 2.24 (s, 3H), 2.71 (d, 2H, J ) 7.3 Hz), 3.27
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8568 J. Am. Chem. Soc., Vol. 118, No. 36, 1996
Fujita et al.
(s, 3H), 4.13 (t, 1H, J ) 7.3 Hz), 6.6-7.2 (m, 12H); (AcrHCH2C6H3-
3,5-Me2) δ 2.13 (s, 6H), 2.66 (d, 2H, J ) 6.8 Hz), 3.27 (s, 3H), 4.11
(t, 1H, J ) 6.8 Hz), 6.3-7.2 (m, 11H); (AcrHCH2C6H4-p-Et) δ 1.16
(t, 3H, J ) 7.3 Hz), 2.54 (q, 2H, J ) 7.3 Hz), 2.72 (d, 2H, J ) 6.8
Hz), 3.24 (s, 3H), 4.14 (t, 1H, J ) 6.8 Hz), 6.6-7.3 (m, 12H);
(AcrHCH2C6H4-m-Et) δ 1.07 (t, 3H, J ) 7.3 Hz), 2.45 (q, 2H, J ) 7.3
Hz), 2.73 (d, 2H, J ) 6.8 Hz), 3.23 (s, 3H), 4.14 (t, 1H, J ) 6.8 Hz),
6.5-7.3 (m, 12H); (AcrHCH2C6H4-o-Et) δ 0.97 (t, 3H, J ) 7.8 Hz),
2.26 (q, 2H, J ) 7.8 Hz), 2.79 (d, 2H, J ) 7.3 Hz), 3.37 (s, 3H), 4.11
(t, 1H, J ) 7.3 Hz), 6.5-7.3 (m, 12H); (AcrHCH(Me)C6H4-p-Me) δ
1.07 (d, 3H, J ) 6.8 Hz), 2.22 (s, 3H), 2.81 (quint, 1H, J ) 7.3 Hz),
3.14 (s, 3H), 3.97 (d, 1H, J ) 7.3 Hz), 6.6-7.3 (m, 12H); (AcrHCH-
(Me)C6H4-m-Me) δ 1.08 (d, 3H, J ) 7.3 Hz), 2.15 (s, 3H), 2.80 (quint,
1H, J ) 6.8 Hz), 3.12 (s, 3H), 3.98 (d, 1H, J ) 6.8 Hz), 6.5-7.3 (m,
12H); (AcrHCH(Me)C6H4-o-Me) δ 1.02 (d, 3H, J ) 7.3 Hz), 1.66 (s,
3H), 3.21 (quint, 1H, J ) 7.3 Hz), 3.28 (s, 3H), 3.96 (d, 1H, J ) 7.8
Hz), 6.5-7.3 (m, 12H); (AcrHCH2C6H4-p-Pri) δ 1.17 (d, 6H, J ) 6.8
Hz), 2.73 (d, 2H, J ) 6.8 Hz), 2.81 (sept, 1H, J ) 6.8 Hz), 3.20 (s,
3H), 4.16 (t, 1H, J ) 6.8 Hz), 6.7-7.2 (m, 12H); (AcrHCH2C6H4-m-
Pri) δ 1.08 (d, 6H, J ) 6.8 Hz), 2.69 (sept, 1H, J ) 6.8 Hz), 2.73 (d,
2H, J ) 6.8 Hz), 3.22 (s, 3H), 4.14 (t, 1H, J ) 6.8 Hz), 6.4-7.3 (m,
12H); (AcrHCH2C6H4-o-Pri) δ 0.96 (d, 6H, J ) 6.8 Hz), 2.82 (d, 2H,
J ) 7.3 Hz), 2.84 (sept, 1H, J ) 6.8 Hz), 3.38 (s, 3H), 4.10 (t, 1H, J
) 7.7 Hz), 6.7-7.3 (m, 12H); (AcrHCMe2C6H4-p-Me) δ 1.15 (s, 6H),
2.25 (s, 3H), 2.99 (s, 3H), 4.01 (s, 1H), 6.7-7.2 (m, 12H);
(AcrHCMe2C6H4-m-Me) δ 1.16 (s, 6H), 2.18 (s, 3H), 2.98 (s, 3H),
4.01 (s, 1H), 6.4-7.3 (m, 12H); (Ph2CHCHPh2) δ 5.08 (s, 2H). 1H
NMR (CD3OD): (AcrHCH2C6H4-p-Et) δ 1.19 (t, 3H, J ) 7.8 Hz),
2.57 (q, 2H, J ) 7.8 Hz), 2.71 (d, 2H, J ) 6.8 Hz), 3.20 (s, 3H), 4.06
(t, 1H, J ) 6.8 Hz), 6.5-7.2 (m, 12H); (AcrHCH2C6H4-m-Et) δ 1.07
(t, 3H, J ) 7.8 Hz), 2.58 (q, 2H, J ) 7.8 Hz), 2.72 (d, 2H, J ) 6.4
Hz), 3.19 (s, 3H), 4.07 (t, 1H, J ) 6.4 Hz), 6.3-7.3 (m, 12H);
(AcrHCH2C6H4-o-Et) δ 0.97 (t, 3H, J ) 7.8 Hz), 2.21 (q, 2H, J ) 7.8
Hz), 2.79 (d, 2H, J ) 7.3 Hz), 3.35 (s, 3H), 4.04 (t, 1H, J ) 7.3 Hz),
6.5-7.3 (m, 12H); (AcrHCH(Me)C6H4-p-Me) δ 1.10 (d, 3H, J ) 7.3
Hz), 2.22 (s, 3H), 2.79 (quint, 1H, J ) 7.1 Hz), 3.09 (s, 3H), 3.90 (d,
1H, J ) 6.8 Hz), 6.5-7.2 (m, 12H); (AcrHCH(Me)C6H4-m-Me) δ 1.12
(d, 3H, J ) 6.8 Hz), 2.13 (s, 3H), 2.79 (quint, 1H, J ) 6.8 Hz), 3.08
(s, 3H), 3.90 (d, 1H, J ) 6.8 Hz), 6.3-7.3 (m, 12H); (AcrHCH(Me)-
C6H4-o-Me) δ 1.05 (d, 3H, J ) 7.3 Hz), 1.61 (s, 3H), 3.2 (m, 1H),
3.25 (s, 3H), 3.89 (d, 1H, J ) 8.3 Hz), 6.5-7.3 (m, 12H);
(AcrHCH2C6H4-p-Pri) δ 1.21 (d, 6H, J ) 6.8 Hz), 2.72 (d, 2H, J ) 6.8
Hz), 2.83 (sept, 1H, J ) 6.8 Hz), 3.16 (s, 3H), 4.07 (t, 1H, J ) 6.8
Hz), 6.5-7.2 (m, 12H); (AcrHCH2C6H4-m-Pri) δ 1.07 (d, 6H, J ) 6.8
Hz), 2.65 (sept, 1H, J ) 6.8 Hz), 2.73 (d, 2H, J ) 6.8 Hz), 3.17 (s,
3H), 4.08 (t, 1H, J ) 6.8 Hz), 6.5-7.2 (m, 12H); (AcrHCH2C6H4-o-
Pri) δ 0.96 (d, 6H, J ) 6.8 Hz), 2.78 (sept, 1H, J ) 6.8 Hz), 2.82 (d,
2H, J ) 7.3 Hz), 3.36 (s, 3H), 4.04 (t, 1H, J ) 7.3 Hz), 6.7-7.3 (m,
12H); (AcrHCMe2C6H4-p-Me) δ 1.17 (s, 6H), 2.25 (s, 3H), 2.96 (s,
3H), 3.93 (s, 1H), 6.5-7.2 (m, 12H); (AcrHCMe2C6H4-m-Me) δ 1.19
(s, 6H), 2.16 (s, 3H), 2.94 (s, 3H), 3.93 (s, 1H), 6.5-7.3 (m, 12H).
The isolation of products was carried out with about 100 times as
large a scale as compared to that of the procedure described above.
Typically, a deaerated MeCN (50 mL)/H2O (15 mL) solution containing
AcrH+ClO4- (100 mg) and PhMe (2.0 mL) was irradiated with a high-
pressure mercury lamp through an acetophenone-methanol filter
transmitting λ > 300 nm at room temperature. After the reaction was
complete, water was added to the resulting solution. The vacuum
concentration resulted in precipitation of the products. The isolated
products were analyzed by 1H NMR spectroscopy in CDCl3. The
product was identified as 9-benzyl-10-methyl-9,10-dihydroacridine
(AcrHCH2Ph). The isolation yield was 93%. The elemental analysis
of the isolated products gave satisfactory results. Anal. Calcd for
C21H19N (AcrHCH2Ph): C, 88.38; H, 6.71; N, 4.91. Found: C, 88.38;
H, 6.62; N, 5.02. Anal. Calcd for C28H23N(AcrHC14H11): C, 90.04;
H, 6.21; N, 3.75. Found: C, 90.27; H, 6.10; N, 3.77.
The AcrH+-catalyzed photooxidation of alkylbenzenes with oxygen
was carried out in an oxygen-saturated CD3CN solution (0.80 mL)
containing AcrH+ (8.0 × 10-3 M). After PhMe (3.0 × 10-2 M) was
added to the solution by means of a microsyringe and mixed, the
solution was irradiated with a high-pressure mercury lamp through an
acetophenone-methanol filter transmitting λ > 300 nm at room
temperature. The oxidized products were identified by comparing the
1H NMR spectra of the products with those of the authentic samples.
The photochemical reactions of AcrH+ with alkylbenzenes were also
carried out in the presence of HClO4 (70%) in acetonitrile. The products
were identified by comparison with authentic sample by using HPLC
and 1H NMR. The increase of photoreduction products of AcrH+ and
the decrease of AcrH+ were monitored by using HPLC and UV-vis,
respectively.
Quantum Yield Determinations. A standard actinometer (potas-
sium ferrioxalate)29 was used for the quantum yield determination of
the photoaddition of alkylbenzenes with AcrH+ and the AcrH+-
catalyzed photooxygenation of alkylbenzenes. Typically a square quartz
cuvette (10 mm i.d.) containing an MeCN-H2O or MeOH solution
(3.0 cm3) of AcrH+ (3.0 × 10-4 M) and alkylbenzenes (3.0 × 10-3 to
8.0 × 10-2 M) was irradiated with monochromatized light of λ ) 358
nm from a Shimadzu RF-5000 fluorescence spectrophotometer. Under
the conditions of actinometry experiments, both the actinometer and
AcrH+ absorbed essentially all the incident light of λ ) 358 nm. The
light intensities of monochromatized light of λ ) 358 nm were
determined as 1.83 × 10-8 and 5.35 × 10-8 einstein s-1 with slit widths
of 10 and 20 nm, respectively. The photochemical reaction was
monitored using a Shimadzu UV-160A spectrophotometer. The
quantum yields of the photoreduction of AcrH+ by alkylbenzenes were
determined from the decrease in absorbance due to AcrH+ (λ ) 396
nm, ꢀ ) 3.5 × 103 M-1 cm-1) and those of the photooxygenation with
oxygen were determined from the rate of formation of the oxygenated
products.
Fluorescence Quenching. Fluorescence measurements were carried
out on a Shimadzu RF-5000 spectrofluorophotometer. The excitation
wavelength of AcrH+ was 360 nm in MeCN or MeOH. The monitoring
wavelength was that corresponding to the maxima of the emission band
at 488 nm. The solution was deoxygenated by argon purging for 10
min or saturated with oxygen prior to the measurements. Relative
emission intensities were measured for an MeCN or MeOH solution
containing AcrH+ (3.0 × 10-5 M) and alkylbenzene at various
concentrations (3.0 × 10-3 to 8.0 × 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 alkylbenzene. The Stern-Volmer
relationship (eq 1) was obtained for the ratio of the emission intensities
I0/I ) 1 + Kq[RH]
(1)
in the absence and presence of alkylbenzene (I0/I) and the concentrations
of alkylbenzene ([RH]). The fluorescence lifetime τ of AcrH+ was
determined as 37 ns in MeCN by single photon counting using a Horiba
NAES-1100 time-resolved spectrofluorophotometer. The quenching
rate constants kq ()Kqτ-1) in MeCN and MeOH were obtained from
the quenching constants Kq and the fluorescence lifetime; τ ) 37 and
31 ns in MeCN and MeOH, respectively.30
Laser-Flash Photolysis. A sample is contained in a 10 × 10 mm2
quartz cell. Deaerated MeCN containing AcrH+ (5.0 × 10-5 M) and
alkylbenzene was excited by third harmonic light (355 nm) of a Nd:
YAG laser (4-ns pulses; 180 mJ/pulse). The excitation light was parallel
to the analyzing light from a 450-W Xe lamp (Osram, XBO-450). The
analyzing light passing through a sample cell was focused on a
computer-controlled monochromator (CVI Digikrom-240) by two lenses
and four mirrors. The output light of the monochromator was monitored
by a photomultiplier tube (PMT; Hamamatsu Photonix, R1417). The
signal from a PMT was recorded on a transient digitizer (Tektronix,
7912AD with plug-ins, 7A19 and 7B92A). Total system control and
date processing were carried out with a microcomputer (Sharp, X-6800)
which was connected to the measurement components with a GP-IB
interface.
Theoretical Calculations. The theoretical studies were performed
using the PM3 molecular orbital method.31,32 The MOPAC program
(QCPE No. 455), which was revised as OS/2 Version 5.01 to adapt
(29) (a) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A
1956, 235, 518. (b) Calvert, J. C.; Pitts, J. N. Photochemistry; Wiley: New
York, 1966; p 783.
(30) Poulos, A. T.; Hammond, G. S.; Burton, M. E. Photochem.
Photobiol. 1981, 34, 169.
(31) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209, 221.

Addition Versus Oxygenation of Alkylbenzenes
J. Am. Chem. Soc., Vol. 118, No. 36, 1996 8569
Table 1. Photoaddition of Alkylbenzenes (4 × 10-2 to 2 × 10-2 M) with AcrH+ (8.5 × 10-3 M) in Deaerated CD3CN Containing D2O at
298 K
time
(h)
no.
alkylbenzene
PhMeb
product (yield, %)a
AcrHCH2Ph (100) [93]
AcrHCH(Me)Ph (100)
AcrHCMe2Ph (100) [92]
AcrHCH2C6H4-p-Me (100) [88]
AcrHCH2C6H3-3,5-Me2 (100)
AcrHCH(OMe)Ph (100)
1
2
3
4
5
6
7
8
9
3
3
3
6
17
3
3
3
3
3
PhEtb
PhPri b
p-Me2C6H4
c
c
1,3,5-Me3C6H3
PhCH2OMeb
PhCH2(c-C3H5)b
PhCH2CHdCH2
AcrHCH(c-C3H5)Ph (100)
b
AcrHCH2CHdCHPh (100)
AcrHCHPh2 (77), (PhCH2)2 (12), (AcrH)2 (12)
AcrHC14H11 (84), (AcrH)2 (8)
PhCH2Phb
10
9,10-dihydroanthraceneb
a The values in brackets refer to the isolated product yields (%). b CD3CN/D2O (7:1 v/v). c CD3CN/D2O (3:1 v/v).
for the use on a NEC PC computer, was obtained through the Japan
Chemistry Program Exchange (JCPE).33 The calculations were also
performed by using the MOL-GRAPH program, Version 2.8, by Daikin
Industries, Ltd. The structural output was recorded by using the MOPC
program (JCPE No. P038). The final geometry and energetics were
obtained by optimizing the total molecular energy with respect to all
structural variables. The geometries of alkylbenzene benzyl radicals
were optimized using the unrestricted Hartree-Fock (UHF) formalism.
Results and Discussion
Photoaddition of Alkylbenzenes with 10-Methylacridinium
Ion. Visible light irradiation of the absorption band of
10-methylacridinium perchlorate (AcrH+ClO4-) in deaerated
acetonitrile (MeCN)/H2O (7:1 vol %) solution containing toluene
for 3 h gave 9-benzyl-10-methyl-9,10-dihydroacridine (AcrHCH2-
Ph), as shown in eq 2. The products are well identified by the
Figure 1. Photoaddition of p-xylene (1.4 × 10-2 M) with AcrH+ (3.0
× 10-4 M) in deaerated MeCN containing H2O: 0 M (O), 9.3 × 10-2
M (b), 1.9 × 10-1 M (4), 3.7 × 10-1 M (2), 5.6 × 10-1 M (0), 5.6
M (9) at 298 K; irradiation with monochromatized light of λ ) 358
nm, slit width 20 nm from a xenon lamp of a Shimadzu RF-5000
fluorescence spectrophotometer.
1H NMR spectra and elemental analysis (see the Experimental
Section). The yield of AcrHCH2Ph determined by H NMR
addition of H2O to the MeCN solution results in a remarkable
acceleration of the rate to approach a constant rate with an
increase in [H2O]. As shown in eq 2, the photoaddition of
alkylbenzenes with AcrH+ is accompanied by the liberation of
H+ from the benzyl position of alkylbenzenes. The liberation
of H+ is observed as the low-field shift of H2O protons in the
1H NMR spectra as the reaction proceeds. Thus, the acceleration
of the rate with an increase in [H2O] may be ascribed to that of
the deprotonation step, which may be involved as a rate-
determining step for the photoaddition reaction, because of the
strong solvation of H+ with H2O as discussed later.
The presence of a rate-determining deprotonation step is
confirmed by the deuterium isotope effects on the photoaddition
reactions (eq 2). The deuterium isotope effects (YH/YD) on the
deprotonation process are determined from intermolecular
competition between toluene-h8 and toluene-d8 as shown in eq
3, where YH and YD are the product yields of the toluene-h8
1
was 100%, and AcrHCH2Ph was isolated as the sole product
(the isolated yield was 93%; see the Experimental Section). The
AcrH+ is also photoreduced by other alkylbenzenes to yield
the corresponding 9-substituted 10-methyl-9,10-dihydroacridine
(AcrHR) as shown in Table 1. The presence of a sufficient
amount of H2O is essential to obtain high yields of AcrHR. In
most cases the alkyl group which is deprotonated from the
benzyl position is introduced selectively at the C-9 position of
the acridine moiety (nos. 1-8 in Table 1). R-Cyclopropyl-
toluene was introduced at the C-9 position without opening the
cyclopropyl ring (no. 7). The photoaddition of allylbenzene
yields the cinnamyl adduct (no. 8). In the case of photoreduction
of AcrH+ by diphenylmethane (no. 9) and 9,10-dihydro-
anthracene (no. 10), however, 10,10′-dimethyl-9,9′,10,10′-
tetrahydro-9,9′-biacridine [(AcrH)2] is also obtained as a minor
product in the photoreduction of AcrH+. In this case, the same
amounts of the other homocoupling products, i.e., 1,1,2,2-
tetraphenylethane and 9,9′-bis(9,10-dihydroanthracene), as that
of (AcrH)2 are also obtained.
Irradiation of the absorption band of AcrH+ (λmax ) 358 nm)
in deaerated MeCN containing alkylbenzene results in the
decrease in the absorption band of AcrH+ and the concomitant
increase in the absorption band (λmax ) 285 nm) due to the
adduct. The rate of formation of the adduct decreases gradually
with a progress of the photoaddition as shown in Figure 1. The
adduct (H) and toluene-d8 (D), respectively. The YH/YD values
of toluene and ethylbenzene are determined as 3.7 and 2.5,
respectively.
Photoinduced Electron Transfer. Irradiation of the absorp-
tion bands of AcrH+ causes fluorescence at 488 nm in MeCN.
The fluorescence of 1AcrH+* is quenched efficiently by
alkylbenzenes. The quenching rate constants kq are determined
(32) (a) Dewar, M. J. S.; Thiel, W. J. Am. Chem. Soc. 1977, 99, 4899.
(b) Dewar, M. J. S.; Thiel, W. J. Am. Chem. Soc. 1977, 99, 4907.
(33) Toyoda, J. JCPE News Lett. 1990, 2, 37.

8570 J. Am. Chem. Soc., Vol. 118, No. 36, 1996
Fujita et al.
1
Table 2. Gibbs Energy Change (∆Get) of Photoinduced Electron Transfer from Alkylbenzenes to AcrH+*, Fluorescence Quenching Rate
1
Constants (kq) of AcrH+* by Alkylbenzenes, and Rate Constants (kobs) and Limiting Quantum Yields Φ∞ in the Photoaddition of
Alkylbenzenes with AcrH+ in Deaerated MeCN Containing H2O or in Deaerated MeOHa at 298 K
alkylbenzene
∆Getb (kcal mol-1
)
kq (M-1 s-1
)
kobs (M-1 s-1
)
Φ∞
PhMec
+0.7
-6.0
3.4 × 108 (6.0 × 108)
1.4 × 1010 (1.2 × 1010)
(1.3 × 1010)
3.8 × 108 (5.2 × 108)
1.1 × 1010 (1.1 × 1010)
(1.2 × 1010)
0.13 (0.13)
0.04 (0.050)
(0.057)
(0.051)
(0.078)
(0.068)
(0.079)
∼10-3 (0.033)
e
0.16 (0.17)
0.20 (0.18)
0.22
0.04
(0.43)
p-MeC6H4Med
p-Me(Et)C6H4
p-Me(Pri)C6H4
p-Et2C6H4
(1.1 × 1010)
(7.5 × 109)
(1.1 × 1010)
(1.1 × 1010)
m-Me2C6H4
(1.2 × 1010)
(1.0 × 1010)
m-Me(Et)C6H4
1,3,5-Me2C6H4Med
p-MeOC6H4Med
PhCH2Mef
(1.0 × 1010)
(9.7 × 109)
-4.8
1.3 × 1010 (1.3 × 1010)
1.8 × 1010
e (1.1 × 1010)
e
-1.2
-0.7
3.5 × 108 (5.7 × 108)
1.6 × 108 (3.2 × 108)
8.5 × 108
4.3 × 108 (5.2 × 108)
3.2 × 108 (4.2 × 108)
9.2 × 108
f
PhCHMe2
PhCH2Phf
f
PhCH2CHdCH2
PhCH2OH
p-MeC6H4CH2OH
2.5 × 108
2.7 × 108
(6.5 × 108)
(7.1 × 108)
(9.0 × 109)
(1.1 × 1010)
(0.30)
a The values in parentheses are those obtained in MeOH. b The E°ox values of alkylbenzenes are taken from ref 34. c [H2O] ) 0.55 M. d [H2O]
) 5.5 M. e Too slow to be determined accurately or no reaction. f [H2O] ) 0 M.
from the slopes of the Stern-Volmer plots and lifetime of the
singlet excited state 1AcrH+* (τ ) 37 ns in MeCN and τ ) 31
ns in MeOH). The kq values thus determined are listed in Table
2. The Gibbs energy change of photoinduced electron transfer
from alkylbenzenes to 1AcrH+* (∆G°et) is given by eq 4, where
∆G°et ) F(E°ox - E°red)
(4)
E°ox and E°red are the one-electron oxidation potentials of
alkylbenzenes and the one-electron reduction potential of
1AcrH+* (2.32 V).4 Since the one-electron oxidation potentials
of various alkylbenzenes have previously been reported,34 the
∆G°et values are determined by using eq 4 as listed in Table 2
for comparison. The kq value increases with a decrease in the
∆G°et value to reach a diffusion limit value in MeCN (2.0 ×
1010 M-1 s-1) as the photoinduced electron transfer becomes
energetically more favorable (i.e., more exergonic). Thus, the
fluorescence quenching of 1AcrH+* by alkylbenzenes may occur
via photoinduced electron transfer from alkylbenzenes (RH)
Figure 2. Transient absorption spectra observed in laser flash
photolysis of AcrH+ (5 × 10-5 M) in deaerated MeCN containing
diphenylmethane (5 x 10-2 M). Spectra were recorded 1 µs (O), 7 µs
(b), and 17 µs (4) after the laser pulse.
Photooxygenation of Alkylbenzenes with AcrH+ in the
Presence of HClO4. In contrast with the photoaddition of
alkylbenzenes with AcrH+ (eq 2), the photooxygenation of
ethylbenzene to 1-phenyl-1-ethanol occurs in the photochemical
reaction of AcrH+ with ethylbenzene in the presence of
perchloric acid [HClO4 (70%); 1.2 M, containing 2.9 M H2O]
in deaerated MeCN as shown in eq 6, where AcrH+ is reduced
1
acting as electron donors to the singlet excited state AcrH+*
(eq 5).
RH + 1AcrH+* f RH•+ + AcrH•
(5)
Laser flash irradiation (355 nm from a Nd:YAG laser) of
10-methylacridinium ion (5.0 × 10-5 M) in deaerated MeCN
solution containing diphenylmethane (1.3 × 10-2 M) gave
transient absorption of 10-methylacridinyl radical (AcrH•; a
broad absorption band between 450 and 540 nm)35 as shown in
Figure 2. When ethylbenzene is used instead of diphenyl-
methane, the transient absorption of AcrH• is observed as well.
The concomitant formation of alkylbenzene radical cations could
not be confirmed because of the overlap of the absorption bands
(e.g., 445 nm for ethylbenzene radical cation)36 with the
depletion of AcrH+. However, the direct observation of AcrH•
in Figure 2 confirms the occurrence of photoinduced electron
transfer from alkylbenzenes to the singlet excited state 1AcrH+*
to yield AcrH• and alkylbenzene radical cation.
to 10-methyl-9,10-dihydroacridine (AcrH2) instead of the adduct.
In the case of the other alkylbenzenes such as cumene and
diphenylmethane, the photoinduced hydride reduction of AcrH+
to AcrH2 occurs in the presence of HClO4, accompanied by the
oxygenation of alkylbenzenes to the corresponding benzyl
alcohol as shown in Table 3. When the HClO4 concentration
is reduced from 1.2 to 0.10 M in the photochemical reaction of
AcrH+ with diphenylmethane, both AcrH2 and AcrHCHPh2 are
obtained in the ratio 59:41. In the case of the photoinduced
hydride reduction of AcrH+ by diphenylmethane, the corre-
sponding dimer, i.e., 1,1,2,2-tetraphenylethane was also obtained
in addition to benzhydrol. The yield of 1,1,2,2-tetraphenyl-
ethane in the presence of HClO4 corresponds to that of 1,1,2,2-
tetraphenylethane and 10,10′-dimethyl-9,9′-biacridine [(AcrH)2]
in the absence of HClO4. Figure 3 shows that the formation of
(34) (a) Fukuzumi, S.; Kochi, J. K. Bull. Chem. Soc. Jpn. 1983, 56, 969.
(b) Howell, J. O.; Goncalves, J. M.; Amatore, C.; Klasinc, L.; Wightman,
R. M.; Kochi, J. K. J. Am. Chem. Soc. 1984, 106, 3968. (c) Miller, L. L.;
Nordblom, G. D.; Mayeda, E. A. J. Org. Chem. 1972, 37, 916.
(35) (a) Peters, K. S.; Pang, E.; Rudzki, J. J. Am. Chem. Soc. 1982, 104,
5535. (b) Poulos, A. T.; Hammond, G. S.; Burton, M. E. Photochem.
Photobiol. 1981, 34, 169.
(36) Bockman, T. M.; Karpinski, Z. J.; Sankararaman, S.; Kochi, J. K.
J. Am. Chem. Soc. 1992, 114, 1970.

Addition Versus Oxygenation of Alkylbenzenes
J. Am. Chem. Soc., Vol. 118, No. 36, 1996 8571
Table 3. Photochemical Reaction of AcrH+ (1.5 × 10-3 M) with
Alkylbenzenes (4.0 × 10-2 M) in the Presence of HClO4 (1.2 M) in
Deaerated CD3CN
yield (%)
alkylbenzene
AcrH2
alcohol
dimer
PhCH2Me
PhCHMe2
PhCH2Ph
88
92
78
53 (PhCH(Me)OH)
46 (PhCMe2OH)
44 (Ph2CHOH)
13 ((PhCH2)2)
Figure 4. Dependence of the quantum yields (Φ) on [RH] for the
photoaddition of RH [PhMe (O), PhEt (b), PhPri (4), PhCH2Ph (2),
p-Me2C6H4 (0)] with AcrH+ (3.0 × 10-4 M) in deaerated MeCN at
298 K.
Figure 3. Photochemical reaction of AcrH+ (O, 2.4 × 10-2 M) with
diphenylmethane (6.0 × 10-2 M) in the presence of HClO4 (1.2 M) in
deaerated CD3CN under irradiation with λ > 366 nm light from a high-
pressure mercury lamp, monitored by 1H NMR: AcrH2 (b), Ph2CHOH
(4), and (Ph2CH)2 (2).
AcrH2 is concomitant with the decrease of AcrH+, indicating
that AcrH2 is formed directly from AcrH+ rather than a
subsequent photochemical reaction of AcrHR which could be
initially formed.
Quantum Yields. The quantum yields (Φ) of the photoad-
dition of alkylbenzenes with AcrH+ in deaerated MeCN
containing H2O (0-5.5 M) or in MeOH and those of the
photoinduced hydride reduction of AcrH+ by alkylbenzenes in
the presence of HClO4 (0.40 M) in MeCN containing H2O (1.0
M) were determined from the decrease in the absorption band
due to AcrH+ (λmax ) 358 nm) and the increase in the
concentration of AcrH2 determined by HPLC, respectively. The
Φ values in the absence and presence of HClO4 increase with
an increase in the concentration of alkylbenzene ([RH]), to
approach a limiting value (Φ∞) according to eq 7 as shown in
Figure 4. Equation 7 is rewritten by eq 8, and the linear plots
Figure 5. Φ-1 vs [RH]-1 for the photoaddition of RH [PhMe (O),
PhEt (b), PhPri (4), PhCH2Ph (2), p-Me2C6H4 (0)] with AcrH+ (3.0
× 10-4 M) in deaerated MeCN at 298 K.
Table 4. Limiting Quantum Yields Φ∞, Observed Quenching
Constants Kobs, and Rate Constants kobs in the Photochemical
Reaction of AcrH+ with Alkylbenzenes and Fluorescence
1
Quenching Rate Constants kq of AcrH+* by Alkylbenzenes in the
Presence of HClO4 (0.40 M) in MeCN at 298 K
Kobs (M-1
)
kobs (M-1 s-1
)
kq (M-1 s-1
)
Φ ) Φ∞Kobs[RH]/(1 + Kobs[RH])
Φ-1 ) Φ∞-1[1 + (Kobs[RH])-1]
(7)
(8)
alkylbenzene
Φ∞
PhCH2Me
PhCHMe2
PhCH2Ph
p-MeOC6H4Me
0.10
0.12
0.11
a
1.2 × 10
1.0 × 10
3.2 × 10
a
3.3 × 108
2.8 × 108
8.7 × 108
a
3.5 × 108
1.6 × 108
8.4 × 108
1.8 × 1010
of Φ-1 vs [RH]-1 are shown in Figure 5. From the slopes and
intercepts are obtained the Φ∞ and Kobs values. The Kobs values
a No reaction.
can be converted to the corresponding rate constants (kobs
)
provided that the excited state of AcrH+ involved in the
photochemical reaction is singlet (1AcrH+*; kobs ) Kobsτ-1, τ
) 37 ns in MeCN, τ ) 31 ns in MeOH). The Φ∞ and kobs
values of photoaddition of alkylbenzene to AcrH+ in MeCN
containing H2O and in MeOH are listed in Table 2. The Φ∞
and kobs values of the photoinduced hydride reduction of AcrH+
by alkylbenzene in the presence of HClO4 are given in Table
4. As shown in both Tables 2 and 4, the kobs values agree well
with the kq values determined independently by the fluorescence
Addition vs Oxygenation of Alkylbenzenes via Photoin-
duced Electron Transfer. On the basis of the above results,
the reaction mechanism for both the photoaddition of alkyl-
benzenes to AcrH+ and the photooxygenation of alkylbenzenes
with AcrH+ in the presence of HClO4 may be summarized as
shown representatively for the reaction with ethylbenzene in
Scheme 1. Both reactions are initiated by photoinduced electron
transfer from the alkylbenzene to the singlet excited state
(1AcrH+*) to give the alkylbenzene radical cation-acridinyl
radical pair. The deprotonation from the alkylbenzene radical
cation gives the corresponding benzyl radical (R•). The
acceleration of the photoaddition rate by the presence of H2O
(Figure 1) may be ascribed to that of the deprotonation rate of
1
quenching of AcrH+*, indicating that both the photoaddition
of alkylbenzenes to AcrH+ and the photoinduced hydride
reduction of AcrH+ proceed via photoinduced electron transfer
1
from alkylbenzenes to the singlet excited state, AcrH+*.

8572 J. Am. Chem. Soc., Vol. 118, No. 36, 1996
Fujita et al.
Scheme 1
R• to AcrH2•+ is highly exergonic, and thereby it may proceed
efficiently to yield AcrH2 and R+. The benzyl cation and
analogs (R+) may undergo the nucleophilic addition of H2O to
yield the oxygenated product, i.e., the corresponding benzyl
alcohol derivatives (ROH). In the case of diphenylmethane,
the oxidation of diphenylmethyl radical by AcrH2•+ in the cage
leads to formation of benzhydrol, while some diphenylmethyl
radicals escape from the cage to undergo the radical coupling
to yield the corresponding dimer, i.e., 1,1,2,2-tetraphenylethane
(Table 3), the yield of which agrees with that obtained in the
absence of HClO4 (Table 1).
According to Scheme 1, the quantum yield is expressed by
eq 9, which agrees well with the experimental result (eq 7).
The limiting quantum yield Φ∞ is then expressed by eq 10,
Φ ) [kdketτ/(kd + kb)][RH]/(1 + ketτ[RH])
Φ∞ ) kd/(kd + kb)
(9)
(10)
where the competition between the rate of deprotonation from
the alkylbenzene radical cation (kd) and the back electron transfer
(kb) from AcrH• to the alkylbenzene radical cation determines
the limiting quantum yield. The deprotonation reactivity from
the radical cation decreases with increasing electron donor
property of alkylbenzenes, because of the stabilization of the
cationic charge. Thus, the strongly electron donating alkyl-
benzenes have the low Φ∞ values despite the diffusion-limited
values of kobs and kq as shown in Table 2. In addition, the Φ∞
values in MeOH are generally larger than those in MeCN, since
the deprotonation from alkylbenzene radical cation may be
facilitated in MeOH because of the high solvating ability of
MeOH to H+ as compared to that of MeCN. Thus, the
deprotonation from alkylbenzene radical cations is an important
step in determining the limiting quantum yields. The decrease
in the Φ∞ values with an increase in the electron-donor ability
of alkylbenzenes (Table 2) may be ascribed to the decrease in
the deprotonation rate constant (kd) as the acidity decreases.3,4a,10
Thus, no photoaddition of p-MeOC6H4Me, the strongest electron
donor employed in this study, with AcrH+ occurs because of
the slow deprotonation rate as compared with the fast back
electron transfer despite the efficient fluorescence quenching
of AcrH+ by p-MeOC6H4Me (Table 2).
the alkylbenzene radical cation because of the strong solvation
of H+ with H2O. The observed deuterium isotope effects (Vide
supra) may correspond to those on the deprotonation from the
alkylbenzene radical cation. In the absence of HClO4, the two
radical species (AcrH• and R•) may couple efficiently to yield
the adduct (AcrHR) selectively. In the case of diphenylmethane,
however, the radical coupling process in the cage may in part
compete with escape of the radicals from the cage because of
the steric effects of two phenyl groups. The escaped radicals
may couple with each other to yield 1,1,2,2-tetraphenylethane
Stereoelectronic Effects on Intra- and Intermolecular
Competition in Deprotonation from Alkylbenzene Radical
Cations. The photoaddition of p-ethyltoluene yields two
different adducts, 9-(4-ethylbenzyl)-10-methyl-9,10-dihydroacri-
dine (Me-add) and 9-(4-methyl-R-methylbenzyl)-10-methyl-9,-
10-dihydroacridine (Et-add) as shown in eq 11. In such a case
and
10,10′-dimethyl-9,9′,10,10′-tetrahydro-9,9′-biacridine
[(AcrH)2] as shown in Table 1.
On the other hand, the addition of HClO4 in MeCN solution
may result in the protonation of AcrH• to produce AcrH2•+. The
semiempirical molecular orbital calculation indicates that the
protonation of acridinyl radical occurs at the C-9 position to
give 10-methyl-9,10-dihydroacridine radical cation (AcrH2•+).37
The transient absorption of AcrH• in Figure 2 became signifi-
cantly weaker in the presence of HClO4 (0.40 M) as compared
to that in its absence because of the protonation of AcrH•.
•+
However, the concomitant formation of AcrH2 (λmax ) 640
nm)39 was not observed probably because of the rapid electron
•+
transfer from benzyl radical and analogs (R•) to AcrH2
.
the product ratio (Me-add/Et-add) directly gives the ratio of the
deprotonation rate constant from p-ethyltoluene radical cation,
since the other terms such as the forward and back electron
transfers are common. The results of such intramolecular
competition of p-, m-, and o-ethyltoluenes and p-, m-, and
Judging from the one-electron oxidation potential of benzyl
radical and analogs (R•), being more negative (E°ox ) 0.73,
•
•
0.37, 0.16, and 0.35 V vs SCE for PhCH2 , PhCHMe•, PhMe2 ,
and Ph2CH•, respectively)38 than the reduction potential of
•+
AcrH2 (E°red ) 0.81 V vs SCE),39 the electron transfer from
(38) (a) Wayner, D. D. M.; McPhee, D. J.; Griller, D. J. Am. Chem.
Soc. 1988, 110, 132. (b) Sim, B. A.; Milne, P. H.; Griller, D.; Wayner, D.
D. M. J. Am. Chem. Soc. 1990, 112, 6635.
(39) (a) Fukuzumi, S.; Tokuda, Y.; Kitano, T.; Okamoto, T.; Otera, J. J.
Am. Chem. Soc. 1993, 115, 8960. (b) Fukuzumi, S.; Koumitsu, S.; Hironaka,
K.; Tanaka, T. J. Am. Chem. Soc. 1987, 109, 305.
(37) The heat of formation values calculated by the PM3 method indicate
that the radical cation protonated at the C-9 position of AcrH• is more stable
by 9 kcal mol-1 than that at the N-10 position. The values calculated by
the MNDO method also indicate that the former is more stable by 8 kcal
mol-1 than the latter.

Addition Versus Oxygenation of Alkylbenzenes
J. Am. Chem. Soc., Vol. 118, No. 36, 1996 8573
Table 5. Intramolecular Competition of Deprotonation from
Ethyltoluenes (2.7 × 10-2 M) and Cymenes (2.4 × 10-2 M) in the
Photoaddition Reactions with AcrH+ (8.5 × 10-3 M) in Deaerated
CD3CN/D2O (3:1 v/v)
Table 6. Photooxygenation of Alkylbenzenes with O2, Catalyzed
by AcrH+ (1.0 × 10-2 M) in O2-Saturated CD3CN at 298 Ka
alkylbenzene
time
(h)
(concentration, M)
product (yield, %)
PhCHO (69)
PhCHO (74)
PhCOMe (52), PhCH(OH)Me (46)
PhCOMe (34), PhCH(OH)Me (29)
PhCMe2OH (49), PhCOMe (25)
PhCMe2OOH (6), Me2CO (6)
PhCMe2OOCMe2Ph (4)
Me2CO (76)
product (yield, %)a
PhMe (3.1 × 10-2
)
)
6
18
6
18
6
Pri-add
b
PhMe (3.1 × 10-2
alkylbenzene
Me-add
Et-add
PhEt (3.3 × 10-2
PhEt (3.3 × 10-2
)
)
)
p-ethyltoluene
m-ethyltoluene
o-ethyltoluene
p-cymene
16 (20) [21]
17 (20)
23 (24)
45 (48) [45]
50 (50)
84 (80) [79]
83 (80)
77 (76)
b
PhPri (3.1 × 10-2
55 (52) [55]
50 (50)
m-cymene
b
PhPri (3.1 × 10-2
)
18
6
o-cymene
100 (100)
0 (0)
p-Me2C6H4 (3.3 × 10-2
)
p-MeC6H4CHO (66)
a The values in parentheses refer to product yields obtained in the
photoaddition of ethyltoluenes (2.7 × 10-2 M) or cymenes (2.4 × 10-2
M) with AcrH+ (6.4 × 10-3 M) in deaerated CD3OD. The values in
brackets refer to product yields obtained in the intermolecular competi-
tion of ethylbenzene (1.02 × 10-1 M) and cumene (9.0 × 10-2 M) vs
toluene (1.18 × 10-1 M) in the photoaddition with AcrH+ (8.0 × 10-3
M) in deaerated CD3CN/D2O (3:1 v/v).
a Irradiated with a high-pressure mercury lamp through an acetophe-
none-methanol filter transmitting λ > 300 nm. b [HClO4] ) 0.19 M.
at 77 K. Thus, there may be no preferred conformation of the
isopropyl group with respect to the benzene ring at 298 K, in
agreement with the absence of the stereoelectronic effects.
It should be noted, however, that no adduct via the depro-
tonation from the isopropyl group is obtained in the case of
o-cymene (see Table 5). The heat of formation values calculated
by the PM3 method31 indicate that the adduct derived from the
deprotonation from the isopropyl group of o-cymene radical
cation is less stable by 14 kcal mol-1 than that from the methyl
group. Thus, the diminished deprotonation reactivity of the
isopropyl group of o-cymene radical cation may be ascribed to
the pronounced steric effects operating in the C-C bond
formation step between the sterically hindered tertiary radical
and the acridinyl radical.
However, we cannot discard the possibility that such de-
creased reactivity of the isopropyl group is due, at least in part,
to the stereoelectronic effects in the case of o-cymene. In order
to evaluate the stereoelectronic effects in such a case, we
examined the AcrH+-catalyzed photooxygenation of alkylben-
zenes with oxygen, since there is no C-C bond formation step
in the photooxygenation reaction (Vide infra).
o-cymenes in MeCN containing H2O and in MeOH are
summarized in Table 5. The yields of the adducts are about
the same for the photoaddition reactions in MeCN and MeOH.
The intermolecular competition was also performed between
toluene (1.18 × 10-1 M) and ethylbenzene (1.02 × 10-1 M) as
well as between toluene (1.18 × 10-1 M) and cumene (9.00 ×
10-2 M) for the photoaddition with AcrH+ (8.0 × 10-3 M).
The difference in the quenching efficiency between two alkyl-
benzenes, 1 and 2, was corrected on the basis of the dependence
on the quencher concentration using the fluorescence quenching
constants of the two alkylbenzenes (K1 and K2, respectively).
Thus, the ratio of the deprotonation reactivity from two
alkylbenzene radical cations, 1•+ and 2•+, was determined from
the product ratio of 1-add/2-add times K2[2](1 + K1[1])/[K1[1]-
(1 + K2[2])]. The deprotonation reactivity ratios derived from
such intermolecular competition are also listed in Table 5, where
the values of the intermolecular competition agree well with
those of the intramolecular competition. Such agreement
indicates that there is no significant difference in the rate of
the back electron transfer process in the intermolecular competi-
tion between two monoalkylbenzenes. This is consistent with
a recent report on the rate constants of back electron transfer
from 1,2,4,5-tetracyanobenzene radical anion to alkylbenzene
radical cations, which increase only slightly with a significant
decrease in the oxidation potentials of alkylbenzenes.40
In Table 5, the significant amount of product via the
deprotonation from the isopropyl group is observed, in sharp
contrast to the pervious results,16 indicating the absence of
stereoelectronic effects of the isopropyl group. The relative
reactivity of isopropyl to ethyl to methyl is 3.7:7.9:1 after
correction for the statistical factor. The heat of formation values
of toluene, ethylbenzene, cumene, and the corresponding radical
cations with various dihedral angles between the benzylic ring
and the plane of the benzene ring were calculated by the PM3
method.31 The calculation indicates that the rotation barriers
of the isopropyl group are only 1.7 and 1.6 kcal mol-1 for
cumene and the radical cation, respectively, which are not
significantly larger than those of the methyl group (0.1 kcal
mol-1 for toluene and the radical cation).41 The EPR work by
Symons42 has demonstrated that free rotation of the isopropyl
group is possible even at 130 K although the rotation is hindered
AcrH+-Catalyzed Photooxygenation of Alkylbenzenes with
Oxygen. The oxygenation of the alkyl group of alkylbenzenes
occurs when the photochemical reaction of AcrH+ with alkyl-
benzenes is carried out in the presence of oxygen. The yields
of the oxygenated products are shown in Table 6. In the case
of the photooxygenation of cumene, cumene hydroperoxide is
detected as the initial photooxygenated product which decom-
poses to yield the other oxygenated products (Table 6). In the
presence of HClO4, the acid-catalyzed decomposition of cumene
hydroperoxide gives acetone and phenol. The rate constants
of fluorescence quenching of AcrH+ by alkylbenzenes as well
as the quantum yields were also determined in oxygen-saturated
MeCN.43 Figure 6 shows the comparison of the quantum yields
for the photochemical reactions of toluene with AcrH+ in the
absence and presence of oxygen as a function of the toluene
concentration, demonstrating the similar dependence in the
absence and presence of oxygen. The quantum yields obeyed
the same kinetic formulation as employed for the photochemical
reactions of AcrH+ with alkylbenzenes in the absence of oxygen
(i.e., eq 7). The Φ∞ and kobs values of the photooxygenation
reactions were obtained as 0.19 and 4.8 × 108 M-1 s-1 for
toluene, and 0.06 and 1.2 × 1010 M-1 s-1 for p-xylene. These
values agree well with those obtained in the photoaddition
reaction in the absence of oxygen (0.13 and 3.8 × 108 M-1 s-1
for toluene and 0.04 and 1.1 × 1010 M-1 s-1 for p-xylene).
(40) Arnold, B. R.; Noukakis, D.; Farid, S.; Goodman, J. L.; Gould, I.
R. J. Am. Chem. Soc. 1995, 117, 4399.
(41) There are two energy minima for cumene radical cation: one is the
conformation with hydrogen being perpendicular to the plane of the benzene
ring and the other is that with the methyl group perpendicular to the plane
of the benzene ring. The energy of the former is 1 kcal lower than that of
the latter.
(42) Rao, D. N. R.; Chanda, H.; Symons, M. C. R. J. Chem. Soc., Perkin
Trans. 2 1984, 1201.
(43) The singlet excited state of AcrH+ has been reported to be quenched
slightly by triplet oxygen: Kikuchi, K.; Sato, C.; Watanabe, M.; Ikeda, H.;
Takahashi, Y.; Miyashi, T. J. Am. Chem. Soc. 1993, 115, 5180.

8574 J. Am. Chem. Soc., Vol. 118, No. 36, 1996
Fujita et al.
evaluated from intramolecular competition of the AcrH+-
catalyzed photooxygenation of ethyltoluene and cymene with
oxygen. The photooxygenation of p-ethyltoluene gave p-
ethylbenzaldehyde (18% yield on the basis of the conversion),
p-methylacetophenone (43%), and 1-(4-methylphenyl)-1-ethanol
(39%) at 38% conversion.
From the product distribution, the relative reactivity of ethyl
to methyl after correction for the statistical factor is determined
as 6.8:1, agreeing with that determined from intramolecular
competition of the photoaddition reaction (7.9:1, Vide supra).
This indicates that the C-C bond formation step in the
photoaddition of ethyltoluene is not affected by the steric effects
between the deprotonated radicals and AcrH•.
Figure 6. Dependence of the quantum yields (Φ) on [RH] for the
photochemical reaction of AcrH+ (3.0 × 10-4 M) with toluene in
deaerated MeCN containing H2O (0.55 M) (O) and in O2-saturated
MeCN containing HClO4 (5.8 × 10-2 M) (b) at 298 K.
The photooxygenation of p-cymene gave p-isopropylbenzal-
dehyde (12% yield on the basis of the conversion), p-
methylacetophenone (55%), and acetone (33%) at 41% con-
version. From the product distribution, the relative deprotonation
reactivity of isopropyl to methyl after correction for the statistical
factor is determined as 22:1, which is much larger than that
determined from intramolecular competition in the photoaddition
reaction of p-cymene in the absence of oxygen (3.7:1, Vide
supra). The heat of formation values calculated by the PM3
method31 indicate that the adduct derived from the deprotonation
from the isopropyl group of p- and m-cymene radical cation is
less stable by 8.6 kcal mol-1 than that from the methyl group.
This indicates that the C-C bond formation between AcrH• and
the hindered radicals deprotonated from the isopropyl group of
the radical cations is affected by the steric effects. In contrast
to the C-C bond formation, there may be fewer steric effects
in the C-O bond formation of the deprotonated radicals with
oxygen. Thus, the relative deprotonation reactivity of isopropyl
to methyl (22:1) may be regarded as the actual value without
the influence of the steric effects in the bond formation step.
The same relative reactivity of isopropyl to methyl (22:1) is
derived from the photooxygenation of m-cymene.
Scheme 2
Such agreement indicates that the rate-determining step is
common in both cases, i.e., photoinduced electron transfer from
1
alkylbenzenes to AcrH+* as shown representatively for the
AcrH+-catalyzed photooxygenation of cumene in Scheme 2. The
photoinduced electron transfer may be followed by the depro-
tonation of cumene radical cation to produce cumyl radical in
competition with the back electron transfer to the reactant pair.
In the absence of oxygen, the coupling of cumyl radical with
AcrH• gives the adduct, while in the presence of oxygen cumyl
radical may be trapped efficiently by oxygen to give cumyl-
peroxyl radical which is reduced to yield cumene hydroperoxide
by the back electron transfer from AcrH•, accompanied by
regeneration of AcrH+.44 The cumene hydroperoxide decom-
poses to the other oxygenated products as shown in Table 6.
Although the comparable Φ∞ and kobs values in the absence
and presence of oxygen indicate that there is no significant
contribution of autoxidation (radical chain reactions) of alkyl-
benzenes in the photooxygenation reactions, the formation of
small amounts of peroxides in the case of photooxygenation of
cumene suggests some minor participation of the autoxidation
in this particular case.
In the case of the photooxygenation of o-cymene, only
o-isopropylbenzaldehyde was obtained at 100% yield on the
basis of the conversion (20%). Despite the fewer steric effects
on the C-O bond formation of the deprotonated radicals with
oxygen, no oxygenated products were derived from the depro-
tonation of the isopropyl group. The heat of formation values
calculated by the PM3 method31 indicate that the deprotonated
radical from the isopropyl group of o-cymene radical cation is
more stable by 6 kcal mol-1 than that from the methyl group.
Thus, the absence of oxygenated products via the deprotonation
from the isopropyl group of o-cymene radical cation despite
the favorable thermodynamic stability of the deprotonated
tertiary radical may be ascribed to the stereoelectronic effects
in this particular case. The methyl group at the ortho position
may prevent the orientation where the C-HR bond of the
isopropyl group is collinear with the aromatic π-system,
resulting in the significant decrease in the deprotonation
reactivity of the isopropyl group.
According to Scheme 2, the limiting quantum yields Φ∞ are
determined by the competition between the deprotonation of
alkylbenzene radical cations and the back electron transfer from
AcrH•. Thus, the difference in the deprotonation reactivity from
alkyl groups of alkylbenzene radical cations can also be
In conclusion, the reactivity ratio of the deprotonation from
alkylbenzene radical cations increases in the order methyl <
ethyl < isopropyl for the p- and m-isomers, and the strong
stereoelectronic effects appear in the case of the deprotonation
from the isopropyl group of o-cymene radical cation.
(44) The transient absorption of AcrH• in Figure 2 was decreased
significantly in the presence of oxygen, suggesting that AcrH• is also trapped
efficiently by oxygen. In such a case AcrHOO• may be combined with
cumyl radical to give the peroxide which decomposes with H+ to yield the
final products. At present, however, it cannot be distinguished whether cumyl
radical and/or AcrH• is trapped by oxygen.
Acknowledgment. This work was partially supported by a
Grant-in-Aid from the Ministry of Education, Science, and
Culture, Japan.
JA9538952