Reactivities of Isomerization, Oxidation, and Dimerization of Radical Cations of Stilbene Derivatives

Article abstract of DOI:10.1021/jp9609904

Reactions of radical cations of eight stilbene derivatives (S(.+)) have been studied using pulse radiolysis and γ-ray radiolysis in 1,2-dichloroethane or butyl chloride.Unimolecular isomerization from cis-S(.+) to trans-S(.+) and bimolecular reactions with O2 (oxidation) and a neutral stilbene (dimerization) occur depending on the substituents.The unimolecular c-t isomerization and the oxidation proceed preferably in S(.+) substituted with a p-methoxyl group (as an electron-donating substituent) with rate constants of k_i = 4.5E6 to 1,4E7 s^-1 and k₀₂ = (1.2-4.5)E7 M^-1 s^-1, respectively.On the basis of transient absorption measurements, it is concluded that separation and localization of a positive charge and an unpaired electron play the most important role as the controlling factors in the reactivities of the unimolecular isomerization and the oxidation.The dimerization involves initial formation of a ?-complex with overlapping of two benzene rings and is inhibited by steric hindrance of substituents on the benzene rings and olefinic carbons.

Full text of DOI:10.1021/jp9609904

J. Phys. Chem. 1996, 100, 13615-13623
13615
Reactivities of Isomerization, Oxidation, and Dimerization of Radical Cations of Stilbene
Derivatives
Tetsuro Majima,*,† Sachiko Tojo, Akito Ishida, and Setsuo Takamuku*
The Institute of Scientific and Industrial Research, Osaka UniVersity, Mihogaoka 8-1,
Ibaraki, Osaka 567, Japan
ReceiVed: April 2, 1996X
Reactions of radical cations of eight stilbene derivatives (S•+) have been studied using pulse radiolysis and
γ-ray radiolysis in 1,2-dichloroethane or butyl chloride. Unimolecular isomerization from cis-S•+ to trans-
S•+ and bimolecular reactions with O2 (oxidation) and a neutral stilbene (dimerization) occur depending on
the substituents. The unimolecular c-t isomerization and the oxidation proceed preferably in S•+ substituted
with a p-methoxyl group (as an electron-donating substituent) with rate constants of ki ) 4.5 × 106 to 1.4 ×
107 s-1 and kO2 ) (1.2-4.5) × 107 M-1 s-1, respectively. On the basis of transient absorption measurements,
it is concluded that separation and localization of a positive charge and an unpaired electron play the most
important role as the controlling factors in the reactivities of the unimolecular isomerization and the oxidation.
The dimerization involves initial formation of a π-complex with overlapping of two benzene rings and is
inhibited by steric hindrance of substituents on the benzene rings and olefinic carbons.
Introduction
SCHEME 1
It is well-known that cis(c)-trans(t) isomerization, dimer-
ization, and addition of a nucleophile occur in radical cations
of aromatic olefins as typical reactions.1-5 The reactivities
depend on the structure or substituents of the radical cations.
However, a rather small number of the rate constants for the
reactions have been reported despite numerous studies from
synthetic and mechanistic points of view.1-5 Therefore, factors
that control the reactivities of the radical cations are still unclear.
Even those of the stilbene radical cation (St•+) are not
completely established.2-4 No unimolecular c-t isomerization
occurs in St•+,2 although it does in the radical cations of c-4,4′-
dibromostilbene and c-4,4′-dimethylstilbene.2c Dimerization3
of St•+ with St occurs with a rate constant of kd ) (3.5-3.9) ×
108 M-1 s-1 3c,d to yield a π-dimer radical cation (π-St2•+) with
overlapping π-electrons between the two benzene rings, which
converts to a σ-dimer radical cation (σ-St2•+) with an acyclic
linear structure having both a radical and a cation on the 1- and
4-positions of the C4 linkage, decomposing to t-St•+ and St as
final products.3d It is reported that t-St•+ reacts with a
superoxide molecule4a-e at a rate constant of kSO ) 1.9 × 109
M-1 s-1 near the diffusion rate constant,4e while t-St•+ has little
reactivity toward O2.3a,4,5 On the other hand, a high reactivity
of kO2 ) 2.6 × 108 M-1 s-1 has been reported for the radical
cation of (E)-2,3-diphenyl-2-butene.4g
On the basis of the rate constants of the three types of reactions,
we have found that the unimolecular c-t isomerization and
oxidation with O2 of radical cations of stilbene derivatives,
particularly with substitution of a p-methoxyl group as an
electron-donating substituent in the S•+ derivatives, enhance
remarkably the isomerization and oxidation at room temperature.
It is concluded that separation and localization of a positive
charge and an unpaired electron play the most important role
in increasing the reactivities.6
In this article we wish to report three reactions of radical
cations of eight stilbene derivatives (S•+ ) RCHdCR′R′′ )
1•+-8•+) (Scheme 1) using pulse radiolysis and γ-ray radiolysis
in 1,2-dichloroethane or butyl chloride,6 unimolecular isomer-
ization from cis to trans isomer radical cations (eq 1), and
bimolecular reactions with O2 (oxidation, eq 2) and a neutral
stilbene (dimerization, eq 3).
Experimental Section
General. Pulse radiolyses were performed, as described
previously,3d using an electron pulse (28 MeV, 8 ns, 0.7 kGy
per pulse) from a linear accelerator at Osaka University. The
temperature was controlled by circulating thermostated aqueous
ethanol around the quartz sample cell. γ-Radiolysis of 1,2-
dichloroethane (DCE) solutions or butyl chloride solutions at a
concentration of 1.0 × 10-2 M was carried out in a Pyrex tube
with an inner diameter of 1.0 cm or in 1.5-mm-thick Suprasil
cells at 77 K for UV-vis absorption measurements, respectively,
using a 60Co γ source (dose, 2.6 × 102 Gy).3d Optical
absorption spectra were taken by a spectrophotometer and a
multichannel photodetector.
ki
c-S•+ 98 t-S•+
(1)
(2)
kO2
S
•+ + O2 8 RC+HCR′CR′′
|
OO•
kd
98 S2
•+
S
•+ + S
(3)
† Tel: Japan+6-879-8496; FAX: Japan+6-875-4156; e-mail: majima@
sanken.osaka-u.ac.jp.
The S-containing solutions for spectral measurements at 77
K were degassed by freeze-pump-thaw cycles, while those
X Abstract published in AdVance ACS Abstracts, July 15, 1996.
S0022-3654(96)00990-2 CCC: $12.00 © 1996 American Chemical Society

13616 J. Phys. Chem., Vol. 100, No. 32, 1996
Majima et al.
TABLE 1: Transient Absorption Peaks (λmax) and the Molar Absorption Coefficients at the Peak (Eλ) of D1 and D2 of c-S•+ and
t-S•+ (S•+ ) 1•+-11•+) Obtained by Pulse Radiolyses of c- and t-S (S ) 1-11) (5.0 × 10-3 M) in DCE at Room Temperaturea
RCHdCR′R′′
λ
max/nm (ꢀλ/M-1 cm-1
)
S•+
1•+
R
R′
R′′
D2 of c-S•+
515
(18 000)
520
(30 900)
530
(13 200)
530
(nd)
540
(nd)
520
D2 of t-S•+
D1 of c-S•+
D1 of t-S•+
750
(11 400)
790
(19 600)
720, 800
(14 700, 41 700)
680
(14 700)
700
(14 700)
>900
(>11 000)
740, 800
(9810, 184 000)
780, 880
(24 500, 61 300)
Ph
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
480
(65 000)
490
(80 900)
470, 500
(39 200, 74 800)
480
(105 000)
480
(84 600)
460, 510
(24 500, 20 800)
440, 490
(18 400, 29 400)
480, 540
(33 100, 76 000)
780
(6180)
780
(11 500)
nd
2•+
3•+
4•+
5•+
6•+
7•+
8•+
9•+
11•+
4-CH3C6H4
H
4-CH3OC6H4
2,4-(CH3O)2C6H3
3,4-(CH3O)2C6H3
3,5-(CH3O)2C6H3
4-CH3OC6H4
4-CH3OC6H4
4-CH3OC6H4
4-CH3C6H4
H
H
nd
nd
H
H
700
(3270)
450, 500
(8820, 9710)
nd
(1240)
760, 780
(7940, 7940)
nd
CH3
H
4-CH3OC6H4
4-CH3OC6H4
4-CH3C6H4
(CH2)3
H
490, 560
(40 000, 16 000)
500, 570
790, >850
(13 000, 20 000)
790, 900
nd
nd
(20 000, 27 500)
(8130, >17 500)
a The molar absorption coefficients of ꢀλ are estimated from the optical density at λ nm and the optical length of 1 cm with the assumption of
a concentration of 8 × 10-6 M for S•+, which is calculated from the optical density at 480 nm, the optical length of 1 cm, and ꢀ480 ) 6.5 × 104
M-1 cm-1 for 1•+ in DCE.13
for pulse radiolyses were saturated by bubbling with Ar or O2
for 20 min. Oxygen concentrations were varied by bubbling
with a mixture gas of N2 and O2 premixed in a high-pressure
cylinder at several ratios.
of S•+ are shown in Figures 1 and 2. Their characteristics are
summarized in Table 1 and discussed later. The transient
absorptions of t-S•+ decayed monotonously with first-order
kinetics and did not show any shifts or spectral changes. The
apparent rate constants (kobs) were calculated from the decay
of the transient absorptions at a wavelength λ and are shown in
Figures 1 and 2.
Half-wave oxidation potentials (Ep/2ox) of 1.0 × 10-2
M
samples were measured by cyclic voltametry using with a
potentiostat and a function generator having a scan rate of 100
mV s-1, a reference electrode of Ag/AgNO3, a working electrode
of a platinum disk, a pair electrode of platinum wire, and a
supporting electrolyte of 10-1 M tetraethylammonium tetrafluo-
roborate in acetonitrile.
The transient absorptions of c-1•+, c-2•+, and c-6•+ without
a p-methoxyl group at λ decayed with kobs, similarly to those
of t-1•+-8•+ (Figures 1 and 2). On the other hand, formation
of the band of t-3•+-5•+ and t-8•+ after the electron pulse was
observed with decay of the band of c-3•+-5•+ and c-8•+ with
a p-methoxyl group, respectively. For example, the transient
absorption spectrum at 20 ns after the electron pulse was
composed of bands of c-3•+ and t-3•+ at 530 and 500 nm,
respectively (Figure 1). The band of t-3•+ at 500 nm increased
in the range of 1 µs, while the band of c-3•+ at 530 nm decayed
in the range of a few hundred nanoseconds and became constant
in the range of 1 µs because of formation of the band of t-3•+.
The decay profile of the band of c-3•+ at 530 nm was consistent
with the formation profile of the band of t-3•+ at 500 nm.
Similar time profiles were observed in the transient absorptions
of c-4•+, c-5•+, and c-8•+. The rate constants (ki) of the
unimolecular isomerization of c-S•+ to t-S•+ were calculated
from the rise in the absorption peaks of t•+ (Figures 1 and 2
and Table 2), which did not depend on the concentration of
c-S (ki ) 4.5 × 106 to 1.4 × 107 s-1). The ki value for c-8•+
was measured at various temperatures. From the Arrhenius plot
of ln ki and the reciprocal of temperature (T-1), the activation
energy and the preexponential factor were determined to be 1.3
kcal mol-1 and 107.5 s-1, respectively (Figure 3).
Materials. c-1 (Aldrich, 97%) and t-1 (>99.5%) were
purchased from Aldrich and Tokyo Kasei and purified by means
of distillation and recrystallization from ethanol, respectively,
before use. The other stilbenes (2-8 and 11) were synthesized
by the Wittig reaction of the corresponding substituted benzal-
dehyde and benzyltriphenylphosphonium chloride with sodium
ethoxide in absolute ethanol at room temperature, according to
literature procedures,7 and purified by means of distillation and
column chromatography on silica gel and/or recrystallization
from ethanol, respectively, before use. Analyses of the purified
S by GC showed purities higher than 99.5%. 1,2-Bis(4-
methoxyphenyl)cyclo-1-pentene (9) was prepared from the
Grignard reaction of 2-chlorocyclopentanone and (4-methox-
yphenyl)magnesium bromide, according to the literature,8 and
was purified by recrystallization from methanol. Other chemi-
cals were purchased from Tokyo Kasei and purified by
distillation or recrystallization prior to use.
1,2-Dichloroethane (DCE) used as a solvent was distilled over
calcium hydride. Butyl chloride was shaken with concentrated
sulfuric acid, washed with water, dried over calcium chloride,
and fractionally distilled.
Dimerization was observed in 1•+-3•+ and t-6•+, because the
decay of the absorption of 1•+-3•+ and t-6•+ increased with
increasing concentration of 1•+-3•+ and t-6•+, respectively. The
transient absorption of c-1•+ shifted to that of t-1•+ during the
decay because of the bimolecular isomerization of c-1•+ to t-1•+
via σ-St2•+, as reported previously.1c,d Similarly, c-2•+ shifted
to that of t-2•+ through bimolecular isomerization. The rate
Results
Formation and Reactions of S•+ (S•+ ) 1•+-8•+). Forma-
tion and reactions of S•+ (S•+ ) 1•+-8•+) were investigated
using a pulse radiolysis technique in 1,2-dichloroethane (DCE)
at room temperature.3d,6 The transient absorption spectra
immediately after the electron pulse were assigned to c-1•+-
constant of the dimerization (kd ) (2.0-4.3) × 108 M-1 s-1
absorption of 1•+-3•+ and t-6•+ on the concentration of 1-3
)
7
•+ and t-1•+-8•+ 9 formed from capture of the hole generated
was calculated from the dependence of decay of the transient
in the initiation step. The spectral changes and the time profiles

Radical Cations of Stilbene Derivatives
J. Phys. Chem., Vol. 100, No. 32, 1996 13617
Figure 1. Transient absorption spectra of c-S•+ and t-S•+ (S•+ ) 1•+-4•+) recorded at time t after an electron pulse during the pulse radiolysis of
an Ar-saturated DCE solution of c-S and t-S (S ) 1-4) with a concentration of 5.0 × 10-3 M at room temperature. Insets: kinetic traces illustrating
the time profiles of the D2 absorption band at a wavelength λ as a function of t. Both t and the observed rate constants (kobs and ki) are mentioned
in the figure.
and t-6, respectively (Table 2).10 For example, plots of kobs for
c-1•+, c-2•+, or c-3•+ vs the concentration of c-1, c-2, or c-3,
respectively, are shown in Figure 4. When the linear plots of
kobs vs concentration of c-1 and c-2 were extrapolated to 0 M,
the intercepts were essentially equal to ki. However, the plots
gave significantly small values of the intercepts, which indicates
that ki < 106 s-1. No dimerization of c-6•+ was observed,
because the decay of the absorption did not depend on the
concentration of c-6.
The transient absorption of c-7•+ with a p-methoxyl group
and a methyl group on the olefinic carbon decayed with kobs
according to first-order kinetics without any shifts or influence

13618 J. Phys. Chem., Vol. 100, No. 32, 1996
Majima et al.
Figure 2. Transient absorption spectra of c-S•+ and t-S•+ (S•+ ) 5•+-8•+) recorded at time t after an electron pulse during the pulse radiolysis of
an Ar-saturated DCE solution of c-S and t-S (S ) 5-8) with a concentration of 5.0 × 10-3 M at room temperature. Insets: kinetic traces illustrating
the time profiles of the D2 absorption band at a wavelength λ as a function of t. Both t and the observed rate constants (kobs and ki) are mentioned
in the figure.
of the concentration. In other words, neither unimolecular
isomerization nor dimerization was observed in c-7•+ (Figure
2).
Pulse radiolyses of t-S and c-S were carried out in O2-
saturated DCE at room temperature. The decay of the transient
absorption of t-S•+ was accelerated in the presence of O2 in the
cases of t-3•+-5•+ and t-7•+ with a p-methoxyl group. The
decay rates were measured as a function of the concentration
of O2, and the rate constant of the oxidation (kO2) was calculated
to be kO2 ) (1.2-4.5) × 107 M-1 s-1 (Table 2). A typical
example of the plot of kobs vs concentration of O2 for t-3•+ is
shown in Figure 5. On the other hand, no oxidation was

Radical Cations of Stilbene Derivatives
J. Phys. Chem., Vol. 100, No. 32, 1996 13619
TABLE 2: Rate Constants for the Reactions of S•+ (S•+
)
1•+-8•+) in Pulse Radiolyses in DCE at Room Temperaturea
c-S•+
t-S•+
S•+
ki/s-1
kd/M-1 s-1 kO2/M-1 s-1 kd/M-1 s-1 kO2/M-1 s-1
1•+ <106
3.9 × 108
3.4 × 108
<106
<106
nd
3.9 × 108
4.3 × 108
3.0 × 108
<106
<106
2•+ <106
<106
3•+ 4.5 × 106 2.0 × 108
4.5 × 107
4.0 × 107
1.2 × 107
<106
4•+ 1.3 × 107 nd
nd
5•+ 1.4 × 107 nd
nd
<106
6•+ <106
7•+ <106
<106
<106
<106
3.4 × 108
<106
2.5 × 107 <106
2.8 × 107
8•+ 5.5 × 106 nd
<106
<106
a ki, calculated from the formation of t-S•+ at 5.0 × 10-3 M of c-S;
kd, measured from the dependence of the decay of S•+ on the
concentration of S; kO2, obtained from the dependence of the decay of
c-S•+ or t-S•+ on the concentration of O2. ki < 106 s-1 denotes that the
formation of t-S•+ was not observed. kd < 106 M-1 s-1 and kO2 < 106
M-1 s-1 denote that there was no dependence of the decay of c-S•+ or
t-S•+ on the concentration of c-S, t-S, or O2.
Figure 4. Plots of the observed decay rate constants (kobs) of the
transient absorptions of c-1•+ (O), c-2•+ (b), c-3•+ (0) (a) and t-1•+
(O), t-2•+ (b), t-3•+ (0) (b) vs concentration of c-1, c-2, c-3, t-1, t-2,
or t-3 ([c-1], [c-2], [c-3], [t-1], [t-2], or [t-3]), respectively, during the
pulse radiolysis of an Ar-saturated DCE solution.
Figure 3. Arrhenius plot of ln ki and the reciprocal of temperature
(T-1) for the unimolecular isomerization of c-8•+ to t-8•+ during the
pulse radiolysis of an Ar-saturated DCE solution of c-8 (5.0 × 10-3
M).
observed on a similar time scale in t-1•+, t-2•+, and t-6•+ without
a p-methoxyl group. For example, no effect of the concentration
of O2 on kobs in t-1•+ is shown in Figure 5. No oxidation was
also observed in t-8•+ with two p-methoxyl groups. Oxidation
of c-7•+ was also observed with a kO2 value similar to that for
t-7•+, while no oxidation was observed in c-1•+, c-2•+, and c-6•+
without a p-methoxyl group and in c-8•+ with two p-methoxyl
groups. In the case of c-3•+-5•+ with a p-methoxyl group, kO2
could not be estimated because of the fast unimolecular
isomerization of c-3•+-5•+ to t-3•+-5•+ at ki ) 4.5 × 106 to
Figure 5. Plots of the observed decay rate constants (kobs) of the
transient absorptions of t-1•+ (O) or t-3•+ (b) vs concentration of oxygen
([O2]) during the pulse radiolysis of a DCE solution of t-1 or t-3 (5.0
× 10-3 M), respectively.
1.3 × 107 M-1 s-1
.
Absorption Spectra of S•+. c- and t-2•+-8•+ show two
typical absorption bands in the range 400-600 and 700-1000
nm assigned to D2 r D0 and D1 r D0 transitions, respectively,
similarly to 1•+ (Figure 1 and Table 1).9 Although the shape
and wavelength of the absorption bands depend on the com-
pounds, c-S•+ shows an absorption peak at λmax at a longer
wavelength than t-S•+, similarly to the case of c-1•+ and t-1•+.
The characteristics of the absorption spectra are summarized
with respect to substitution as follows.
was measured, while the absorption bands of c-3•+-5•+ and
c-8•+ were not detected because c-3•+-5•+ and c-8•+ isomerized
rapidly to t-3•+-5•+ and t-8•+.
The absorption spectra of c-3•+ and t-3•+ generated by
γ-radiolyses of c-3 and t-3 in butyl chloride at 77 K appear
similar to those observed during pulse radiolyses of c-3•+ and
t-3•+ in DCE (Figure 6). Because c-3•+ has a larger ꢀλ only in
the range 510-670 nm than t-3•+, it is expected that the optical
densities decrease in the range 510-670 nm but increase in
other ranges because of the unimolecular isomerization of c-3•+
to t-3•+.
(1) p-Methyl Substitution. 2•+ shows a spectrum similar to
that of 1•+, although the λmax shifts at a longer wavelength of
approximately 5-10 nm.
(2) p-Methoxyl Substitution. 3•+, 7•+, and 8•+ show a λmax
and a shoulder peak of D1 at 700-900 nm with relatively larger
molar absorption coefficients (ꢀλ) than those of 1•+.11 c-3•+,
t-3•+, and t-8•+ show absorption peaks of D2 at a longer
wavelength of approximately 15-60 nm than 1•+. t-3•+ and
t-8•+ show a shoulder peak of D2 at 480 nm. t-8•+ also shows
the two λmax of D1 at 760-900 nm. The spectrum of c-7•+
Measurement of the absorption spectra of c-8•+ and t-8•+
using a multichannel photodetector was also carried out in the
γ-radiolyses of c-8 and t-8 in butyl chloride at 77 K. The same
absorption spectrum was observed with a peak at 540 nm, which
is assigned to t-8•+. This indicates that c-8•+ isomerizes to t-8•+

13620 J. Phys. Chem., Vol. 100, No. 32, 1996
Majima et al.
SCHEME 2
Discussion
Formation of S•+. It is well established that the radical cation
of 1 (1•+) is formed during pulse radiolyses and γ-radiolyses
in alkyl halide solutions such as DCE and butyl chloride.9,11,14
Absorption spectra of c-3•+ and t-3•+ generated by γ-radiolyses
of c-3 and t-3 in butyl chloride at 77 K show absorption bands
similar to those of c-1•+ and t-1•+: c-3•+, 420-580 with λmax
) 500 nm; t-3•+, 400-520 and >700 with λmax ) 495, 730,
and >800 nm (Figure 1). Similar absorption spectra of c-2•+,
c-4•+-8•+, t-2•+, and t-4•+-8•+ were observed in the γ-radi-
olyses. Therefore, c-S•+ and t-S•+ (S•+ ) 1•+-8•+) are formed
initially by hole transfer from the radical cation of DCE (DCE•+)
as shown in the following initiation processes (eqs 4-7),
Figure 6. Absorption spectra of c-3•+ and t-3•+ measured by γ-radi-
olyses of butyl chloride solutions containing 5.0 × 10-3 M c-3 and t-3
at 77 K.
e-
DCE Df DCE•+ + es-
DCE•+ + S f DCE + S•+
es- + DCE f DCE•-
(4)
(5)
(6)
(7)
Figure 7. Transient absorption spectra of 9•+ recorded at t after an
electron pulse in the pulse radiolysis of an Ar-saturated DCE solution
of 9 (5.0 × 10-3 M) at room temperature. Inset: kinetic trace
illustrating the time profile of the D2 absorption band at 560 nm as a
function of t. Both t and the observed rate constant (kobs) are mentioned
in the figure.
•
DCE•- f ClCH2CH2 + Cl-
where es- denotes a solvated electron and is stabilized by
dissociative attachment to DCE.
Assignments of Absorption Bands of S•+. To study the
electronic properties of c-S•+ and t-S•+, we have measured the
transient absorption spectra of c-S•+ and t-S•+ during pulse
radiolyses of S at room temperature (Figure 1). The spectral
characteristics are shown with respect to the λmax and ꢀλ of D1
and D2 (Table 1).
during the spectral measurement by a multichannel photodetector
even at 77 K.12 To confirm the absorption spectrum of c-8•+,
that of the 1,2-bis(4-methoxyphenyl)cyclo-1-pentene (9) radical
cation was measured during the pulse radiolysis in DCE (Figure
7). 9 has a rigid planar structure, with the c-8 chromophore
constrained structurally by the cyclopentene ring, and is stable
for geometrical isomerizations as the structurally constrained
derivative of c-8. From the λmax ) 490 and 560 nm and ꢀλ of
9•+, c-8•+ is expected to show a λmax at longer wavelengths,
with a smaller ꢀλ than those of t-8•+ and 9•+, similarly to the
difference between the values for other c-S•+ and t-S•+. The
absorption spectrum of 9•+ collapsed on a microsecond time
scale.
It is established that t-1•+ shows two absorption bands in the
range 400-510 and 650-900 nm with a relatively sharp λmax
) 480 nm and a weak λmax ) 750 nm assigned to D2 r D0 and
D1 r D0 transitions, respectively, while c-1•+ also shows two
absorption bands in the range 400-580 and 650-900 nm with
a relatively strong λmax ) 515 nm and a weak λmax ) 780 nm,
respectively.9 The absorption bands of c-1•+ are broader and
weaker than those of t-1•+, while the absorption bands of c-1•+
shift to longer wavelengths than those of t-1•+. The differences
depend on the electronic configurations of D0, D1, and D2 of
c-S•+ and t-S•+. It is reported that t-1•+ has a planar structure
with two benzene rings and a CdC double bond as the most
stable conformation, while c-1•+ has a twist angle of 26°
between the two benzene rings on the basis of MO calculations
(Scheme 2).15 Therefore, delocalization of π-electrons is less
in c-1•+ than in t-1•+. The energy level of D0 is suggested to
increase in c-1•+ compared that with in t-1•+.
(3) o-Methoxyl and m-Methoxyl Substitution. Both t-4•+ and
t-5•+ show a λmax of D2 at 480 nm, which is same as that of
t-1•+, while D1 of t-4•+ and t-5•+ is observed at 680-700 nm,
with an extremely smaller ꢀλ than that of t-1•+.
(4) m,m-Dimethoxyl Substitution. t-6•+ shows two λmax of
D2 at 460 and 510 nm and a λmax of D1 at a wavelength longer
than 900 nm, with a considerably small ꢀλ. On the other hand,
c-6•+ shows very weak and broad absorption bands in the range
360-900 nm.
(5) Methyl on the Central CdC Double Bond. c-7•+ and t-7•+
show broader absorption bands, with a smaller ꢀλ than those of
t-3•+.
Effects of Substituents on Electronic Properties of 2•+-
8•+. On the basis of absorption spectra (Figures 1 and 2 and
Table 1), the effects of substituents on the electronic properties
of 2•+-8•+ are summarized as follows.
The most remarkable characteristics of the absorption spectra
of c- and t-2•+-8•+ are that the λmax values of 3•+-5•+ and 8•+
shift to longer wavelengths with larger ꢀλ values than those of
1•+ (Table 1) and that c-6•+ show a λmax with an extremely
small ꢀλ, similar to that of the 1,3-dimethoxybenzene radical
cation.13
(1) p-Methyl (2•+) and p-methoxyl substitutions (3•+-5•+,
7•+, and 8•+) enhance the delocalization of π-electrons of c-1•+
and t-1•+.
(2) p-Methoxyl substitution (3•+-5•+ and 8•+) leads to two
types of D2 r D0 and D1 r D0 transitions. The λmax values at

Radical Cations of Stilbene Derivatives
J. Phys. Chem., Vol. 100, No. 32, 1996 13621
SCHEME 3
although no direct evidence for them was observed. Similar
values of kd for c-1•+-c-3•+ suggest that p-methyl and
c
p-methoxyl substitutions have little effect on the dimerization
•+
•+
of c-S•+. Structures of π-(c-2)2 and π-(c-3)2 are assumed
to overlap between the same benzene rings for a greater
perturbation of the π-orbitals, as shown in Scheme 3. It is
•+
suggested that σ-22 decomposes rapidly into the thermody-
namically stabler t-2•+ and t-2, while σ-32•+ decomposes slowly
into t-3•+ and t-3 (eq 11) because there is no effect of the
concentration of c-3 on the formation of the absorption of t-3•+.
This is probably attributed to p-methoxyl substitution, which
•+
makes σ-32 stable toward â-fission because of electron
donating to the positive charge and radical centers.
The decay of the absorption of other c-S•+ at higher
concentrations could not be performed, because the decay was
too fast to be detected within the experimental limits, or the
concentration of c-4-8 could not be increased because of low
solubilities in DCE. The kdc value for c-8•+ with a p-methoxyl
group is expected to be 108 M-1 s-1 from analogy with c-3•+,
while the steric hindrance due to the twisted structure with the
3,5-dimethoxyphenyl ring rotating relative to the CdC double
bond and to the methyl group on the olefinic carbon probably
inhibits the dimerization of c-6•+ and 7•+, respectively.
Dimerization of t-S•+. Similarly to c-1•+, t-1•+ decays via
neutralization with Cl- (eq 8) and dimerization with t-1 at a
the longer wavelength of the D2 r D0 shift to a longer
wavelength in the order t-1•+, t-2•+, t-3•+, c-1•+, c-2•+, c-3•+,
t-8•+, and c-8•+. This order indicates that the λmax shifts to
longer wavelength with increasing deviation from the planarity
of the two benzene rings and the CdC double bond induced
by increasing the single-bond character of the CdC double bond
or the charge-spin separation character because of the p-
methoxyl group, as discussed below.
(3) o,m-Dimethoxyl substitution (4•+, 5•+), m,m-dimethoxyl
substitution (6•+), and methyl substitution on the central CdC
double bond (7•+) interfere with the delocalization of π-electrons
of t-1•+ because of the steric repulsion of the planar structure.
The significant weak broad absorptions of 6•+ suggest that 6•+,
particularly c-6•+, has a twisted structure with the 3,5-dimeth-
oxyphenyl ring rotating relative to the CdC double bond.
Dimerization of c-S•+. The decay of c-1•+ has been analyzed
by neutralization with Cl- formed from dissociative electron
attachment to DCE (eq 7) at a rate constant of kn ) 1.6 × 1011
M-1 s-1 (eq 8) when the concentration of c-1 is low (10-4
M).3d,11
rate constant of kd ) 3.9 × 108 M-1 s-1 to form a π-type dimer
t
radical cation of t-1 (π-(t-1)2•+, eq 12) with overlapping of
π-electrons between the two benzene rings of t-1•+ and t-1
(Scheme 3).3d,11 Neither the unimolecular nor the bimolecular
isomerization of t-1•+ to c-1•+ occurs. It is found that π-(t-
•+
•+
1)2 generated at 77 K converts to σ-12 upon warming (eq
13) and finally decomposes into the thermodynamically stabler
t-1•+ and t-1 (eq 11) on the basis of spectroscopic measurements
as discussed above for c-1•+.3d
kd
9
t-S•+ + c-S
8 π-(t-S)2•+ (S ) 1-3)
(12)
(13)
kn
S
•+ + Cl- 98 neutral products
(8)
•+
π-(t-S)2•+ f σ-S2
It is reported that the dimerization of c-1•+ with c-1 occurs at
a rate constant of kdc ) 3.5 × 108 M-1 s-1 at room temperature
to form a π-type dimer radical cation of c-1 (π-(c-1)2•+, eq 9),
which changes to a σ-type dimer radical cation of 1 (σ-12•+, eq
10) (Scheme 3) on the basis of spectroscopic measurements
t-2•+, t-3•+, and t-6•+ show behavior similar to that of t-1•+
(Table 2). Similar kdt values for t-1•+-t-3•+ and t-6•+ suggest
that p-methyl and m-methoxyl substitutions have little effect
on the dimerization of t-S•+ and that a similar dimerization
mechanism has been suggested (eqs 11-13).16 On the other
hand, the decay rates of the absorption bands of t-4•+, t-5•+,
t-7•+, and t-8•+ were much slower than those of t-1•+-3•+ and
t-6•+, because the decay rate did not change with varying
concentration (5 × 10-3-10-1 M) of t-4, t-5, t-7, and t-8,
respectively. Therefore, dimerizations of t-4•+, t-5•+, t-7•+, and
t-8•+ do not occur. The dimerization occurs even in t-3•+ with
kd
9
c-S•+ + c-S
8 π-(c-S)2•+ (S ) 1-3)
(9)
•+
π-(c-S)2•+ f σ-S2
(10)
during the pulse radiolyses and γ-radiolyses of c-1 and 1,2,3,4-
tetraphenylcyclobutanes as precursors of the σ-dimer model
compounds in DCE at room temperature and in butyl chloride
at 77 K.3d The unimolecular isomerization of c-1•+ to t-1•+
does not occur, although the bimolecular isomerization of c-1•+
a large kd value but not in t-4•+, t-5•+, and t-7•+; therefore, it is
t
suggested that the dimerization involves the initial formation
of a π-complex with overlapping of the two benzene rings and
that the π-complex formation is inhibited by steric hindrance
of the substituents on the benzene rings and olefinic carbons.
Enhancements of Unimolecular Isomerization and Oxida-
tion by Charge-Spin Separation (Effect of p-Methoxyl
Substituent). In contrast to c-1•+ and 2•+, the unimolecular
isomerization of c-3•+-5•+ and c-8•+ having a p-methoxyl group
occurs to yield t-3•+-5•+ and t-8•+, respectively. The ki value
for c-3•+ is almost equivalent to that for c-8•+, while those for
c-4•+ and c-5•+ are larger than those for c-3•+ and c-8•+. Neither
the unimolecular nor the bimolecular isomerization occurs in
c-6•+ and c-7•+. The oxidation of t-3•+-5•+ and 7•+ having a
p-methoxyl group with O2 occurs at kO2 ) (1.2-4.5) × 107
M-1 s-1, while the oxidation does not occur in c-1•+, c-2•+, or
to t-1•+ is proposed to occur through σ-12 on the time scale
•+
of 100-300 ns with the concentration of (5-10) × 10-3 M at
room temperature.3c,d Thermally unstable σ-12 decomposes
•+
rapidly into the thermodynamically stabler t-1•+ and t-1 (eq 11),
as proposed.3c,d Therefore, the bimolecular isomerization is
enhanced with increasing concentration of c-1.
σ-S2•+ f t-S•+ + t-S
(11)
Similarly, concentration effects suggest that the bimolecular
isomerizations of c-2•+ and c-3•+ to t-2•+ and t-3•+ involve the
dimerization of c-2•+ and c-3•+ to form π-(c-2)2 and π-(c-
•+
•+
•+
•+
3)2 and then σ-22 and σ-32 (eqs 9 and 10, respectively),

13622 J. Phys. Chem., Vol. 100, No. 32, 1996
Majima et al.
TABLE 3: Oxidation Potentials (E1/2ox) of c-S and t-S (S )
1-6) and Difference between E1/2ox(c-S) and E1/2ox(t-S)a
SCHEME 4
S
R of RCHdCHPh E1/2ox(c-S)/V E1/2ox(t-S)/V ∆E1/2ox(c-t)/V
1
2
3
4
5
6
Ph
1.25
1.16
0.94
0.80
0.87
1.23
1.14
1.05
0.82
0.64
0.72
1.03
0.11
0.11
0.12
0.16
0.15
0.20
4-CH3C6H4
4-CH3OC6H4
2,4-(CH3O)2C6H3
3,4-(CH3O)2C6H3
3,5-(CH3O)2C6H3
a Half-wave oxidation potentials (E1/2ox(c-S) and E1/2ox(t-S)) of c-S
and t-S at 1.0 × 10-2 M, reference electrode of Ag/AgNO3, supporting
electrolyte of 10-1 M tetraethylammonium tetrafluoroborate in aceto-
nitrile; ∆E1/2ox(c-t) ) E1/2ox(c-S) - E1/2ox(t-S).
appears on the olefinic carbon on the side of the p-methoxy-
phenyl group in 3•+ because of contribution of B, while it
appears on both olefinic carbons in 8•+ with two symmetrical
p-methoxyl groups. Therefore, the density of an unpaired
electron on the olefinic carbon in t-8•+ is lower than that in
t-3•+.
Neither the unimolecular c-t isomerization nor the oxidation
of c-6•+ occurred. It should be noted that c-6•+ shows a λmax
with an extremely small ꢀλ compared with c-1•+-5•+, c-7•+,
and c-8•+, but similar to the ꢀλ value of the 1,3-dimethoxyben-
zene radical cation.13 These results may be interpreted by a
twisted structure with the 3,5-dimethoxyphenyl ring rotating
relative to the CdC double bond, because a quinoid-type
structure, B, is not possible in c-6•+ having an m-methoxyl
group.
Tokumaru and his co-workers reported that ki ) 3 × 105 s-1
for radical cations of cis-4,4′-dibromostilbene (c-10•+) and cis-
4,4′-dimethylstilbene (c-11•+) and ki > 3 × 105 s-1 for c-8•+.2c
They proposed that the unimolecular c-t isomerization might
be accelerated by reduction of the electron density on the
unsaturated linkage induced by the substituents on the basis of
lower coupling constants of the R-H of t-10 and t-11 having
p-substituents rather than that of t-1 itself in ESR studies.21 They
also proposed that the oxidation depends on the structures of
the olefins, because the kO2 values varied over a wide range.4e,f
The ki values for c-3•+-5•+ and c-8•+ are 1 or 2 orders larger
than those for c-10•+ and c-11•+,2c as shown in Table 2.
Remarkable enhancements of the unimolecular c-t isomeriza-
tion by p-methoxyl substitution are explained by charge-spin
separation in c-3•+-5•+ and c-8•+.
c-6•+ without a p-methoxyl group and in t-8•+ with two
p-methoxyl groups. The unimolecular isomerization and the
oxidation with O2 are remarkably enhanced in 3•+-5•+ and 7•+
with a p-methoxyl group.
On the basis of these experimental results, it is clearly shown
that a p-methoxyl group as an electron-donating substituent on
the benzene ring remarkably changes the reactivities of 1•+-
8•+. It is reported that t-1•+ has a planar structure with the two
benzene rings and the CdC double bond as the most stable
conformation, while c-1•+ has a twist angle of 26° between the
two benzene rings on the basis of MO calculations.15 Although
the positive charge and unpaired electron are delocalized on
the singly occupied molecular orbital (SOMO) involving
π-orbitals of sp2 carbon atoms and the n-orbital of the oxygen
atom of the p-methoxyl group in 1•+-8•+, as shown in structure
A, a quinoid-type structure, B, is considered to contribute in
3•+-5•+, 7•+, and 8•+ with a p-methoxyl group but not in 1•+,
2•+, and 6•+ without a p-methoxyl group (Scheme 4).17
Alternatively, the n-orbital of the oxygen atom of the p-methoxyl
group and the π-orbital of the olefinic â-carbon participate
considerably in the SOMO of 3•+-5•+, 7•+, and 8•+. Therefore,
it is suggested that p-methoxyl substitution in 3•+-5•+, 7•+, and
8
•+ causes separation and localization of the positive charge on
the oxygen atom of the p-methoxyl group and an unpaired
electron on the olefinic â-carbon (charge-spin separation),
which induces a twisted conformation with a large twist angle
and C-C single-bond character for the central CdC double
bond in B. On the other hand, such charge-spin separation is
less important in 1•+, 2•+, and 6•+ than in 3•+-5•+, 7•+, and
8•+.
The occurrence of unimolecular isomerization and oxidation
in 3•+-5•+, 7•+, and 8•+, the large G values of the formation of
t-3-5 in the γ-radiolyses of c-3-5 in Ar-saturated DCE,18 and
the regioselective formation of 4-methoxybenzyl phenyl ketone
in the γ-radiolysis of t-3 in O2-saturated DCE18 are consistent
with the order of ki and kO2 and charge-spin separation induced
by the p-methoxyl group in 3•+-5•+, 7•+, and 8•+. It is
suggested that distonic radical cations known in the gas phase19
can be used to explain the reactivities of radical cations even
in solution.20
Neither the unimolecular isomerization of c-7•+ nor the
oxidation of t-8•+ occurred, although 7•+ and 8•+ have a
p-methoxyl group. These are possibly explained in terms of a
barrier to the twisting of the CdC double bond and the spin
density on the olefinic carbon, respectively. The contribution
of B is decreased by the electron-donating methyl group on the
olefinic carbon; therefore, it is suggested that the single-bond
character of the CdC double bond is lower in c-7•+ than in
c-3•+ and that the barrier to the twisting of the CdC double
bond is higher in c-7•+ than in c-3•+. An unpaired electron
The ki value for c-8•+ was measured at various temperatures.
From the Arrhenius plot of ln ki and the reciprocal of the
temperature (T-1) the activation energy (Ea) and the preexpo-
nential factor (A) were determined to be 1.3 kcal mol-1 and
107.5 s-1, respectively, which are smaller than Ea ) 7.7 kcal
mol-1 and A ) 1011(2 s-1 for c-10•+ and Ea ) 3.3 kcal mol-1
and A ) 109(2 s-1 for c-11•+.2c This result is consistent with
the larger value of ki for c-8•+ than for c-10•+ and c-11•+ and
suggests that Ea predominantly controls the unimolecular
isomerization. On the basis of the differences in oxidation
potentials between c-S and t-S (Table 3) and Ea values, the
energy barrier is too high to be overcome in the isomerization
of t-S•+ to c-S•+ (11-14 kcal mol-1). This makes the
isomerization one-way from c-S•+ to t-S•+ in 1-6.
The rate constant of free radical-O2 reactions has been
reported for several free radicals and found to be nearly equal
to the diffusion rate constant (kdiff). For example, kO2 ) 2.4 ×
109 M-1 s-1 for the benzyl radical-O2 reaction is similar to
kdiff ) 6.7 × 109 M-1 s-1 in cyclohexane.22 On the other hand,
values of kO2 ) (1.2-4.5) × 107 M-1 s-1 for the oxidation of
t-3•+-5•+ and t-7•+ are found to be 2 orders smaller than kO2
for free radicals and kdiff ) 7.8 × 109 M-1 s-1 in DCE. This
suggests that an unpaired electron is not completely localized

Radical Cations of Stilbene Derivatives
J. Phys. Chem., Vol. 100, No. 32, 1996 13623
on the olefinic carbon in t-3•+-5•+ and t-7•+ and that the positive
charge interferes with the reactivity of t-3•+-5•+ and t-7•+ as a
radical toward O2 because of the electrophilic character of O2.
Koyama, K.; Matsuoka, T.; Tsutsumi, S. Bull. Chem. Soc. Jpn. 1967, 40,
162. (i) Futamura, S.; Ohta, H.; Kamiya, Y. Chem. Lett. 1982, 381.
(5) Tokumaru and his co-workers reported that t-1•+ reacts toward O2
at a rate constant of kO2 ) 1.3 × 106 M-1 s-1 on the basis of the dependence
of the decay of the absorption of t-1•+ on the initial concentration of t-1•+
,
which changed with varying laser intensity in the laser flash photolysis of
9-cyanoanthracene-t-1 in DMSO.4f,g They proposed that the oxidation
depends on the structures of the olefins, because the kO2 values varied over
Conclusions
It is concluded that c-3•+-5•+ and c-8•+ with a p-methoxyl
group generated by pulse radiolyses or γ-radiolyses isomerize
unimolecularly to the corresponding t-S•+ at a unimolecular rate
constant of ki ) 4.5 × 106 to 1.4 × 107 s-1, while t-3•+-5•+
and 7•+ with a p-methoxyl group are oxidized with O2 at a
a wide range, kO2 ) 1.3 × 106, 7.2 × 105, and 2.6 × 108 M-1 s-1 for t-1•+
,
t-8•+, and the radical cation of (E)-2,3-diphenyl-2-butene, respectively.
(6) Tojo, S.; Morishima, K.; Ishida, A.; Majima, T.; Takamuku, S. J.
Org. Chem. 1995, 60, 4684.
(7) Maccarone, E.; Mamo, A.; Perrini, G.; Torre, M. J. Chem. Soc.,
Perkin Trans. 2 1981, 324.
(8) Mueller, G. P.; Fleckenstein, J. G.; Tallent, W. H. J. Am. Chem.
Soc. 1951, 73, 2651.
bimolecular rate constant of kO2 ) (1.2-4.5) × 107 M-1 s-1
.
On the other hand, neither the unimolecular isomerization from
t-S•+ to c-S•+ nor the oxidation with O2 occurs in t-S•+ without
a p-methoxyl group.
(9) Hamill, W. Radical Ions; Kaiser, E. T., Kevan, L., Eds.; New York
Intersciences: New York, 1968; p 405. Shida, T.; Hamill, W. J. Chem.
Phys. 1966, 44, 2375. Shida, T. Electronic Absorption Spectra of Radical
Ions, Elsevier: Amsterdam, The Netherland, 1988; p 113.
(10) (a) It might be suggested that the rate constant from the slopes of
the plots in Figure 4 should be analyzed not only with dimerization but
also with regeneration of S•+ from the dimer radical cations formed by the
dimerization. However, the contribution of the formation of S•+ from the
dimer radical cations is negligibly small at the initial period of the decay
of the transient absorption of S•+ (1•+-3•+ and t-6•+), where the concentra-
tion of the dimer radical cations is much lower than that of S•+. Therefore,
the decay can be analyzed with the dimerization at the initial period.
Because kobs values were obtained in the initial period, the plots of kobs vs
concentration of S gave linear lines from which the rate constants of
dimerization were calculated (Figure 4). (b) The dimerization was also
observed in t-8•+ on the basis of electrochemical measurements, although
the rate constant of kd ) (3.4-5) × 103 M-1 s-1 was too small to be
observed under the present conditions in this study. Steckhan, E. J. Am.
Chem. Soc. 1978, 100, 3526. Burgbacher, G.; Schaefer, H. J. J. Am. Chem.
Soc. 1979, 101, 7590.
It should be noted that an unpaired electron is not completely
localized on the olefinic carbon in t-3•+-5•+ and 7•+ and that
a positive charge interferes with the reactivity of t-3•+-5•+ and
7
•+ as a radical toward O2 because of the electrophilic character
of O2. The present work is the first example to clarify that the
reactivities of stilbene radical cations are controlled predomi-
nantly by charge-spin separation induced by p-methoxyl
substitution.
Acknowledgment. We wish to thank the members of the
Radiation Laboratory of ISIR, Osaka University, for running
the linear accelerator. This work was partly supported by a
Grant-in-Aid (No. 07455341) from the Ministry of Education,
Science, Sport and Culture of Japan.
(11) Yamamoto, Y.; Aoyama, T.; Hayashi, K. J. Chem. Soc., Faraday
Trans. 1 1988, 84, 2209.
(12) Such c-t isomerization has also been observed in c-1•+ during the
absorption measurement by a multichannel photodetector at 77 K. It is
considered that photochemical isomerization of c-1•+ to t-1•+ occurs
quantitatively upon irradiation of monitor light from the multichannel
photodetector. Similar to c-1•+, c-8•+ could not be detected even at 77 K
by a multichannel photodetector.3d
References and Notes
(1) Reviews: (a) Fox, M. A., Chanon, M., Eds. Photoinduced Electron
Transfer; Elsevier: Amsterdam, The Netherland, 1988. (b) Mattes, S. L.;
Farid, S. In Organic Photochemistry; Padwa, A., Ed.; Marcel Deckker: New
York, 1983; Vol. 6, pp 233-326. (c) Chanon, M.; Eberson, L. In
Photoinduced Electron Transfer; Fox, M. A., Chanon, M., Eds.; Elsevier:
Amsterdam, The Netherland, 1988; Part C, Chapter 4. (d) Lewis, F. D. In
Photoinduced Electron Transfer; Fox, M. A., Chanon, M., Eds.; Elsevier:
Amsterdam, The Netherland, 1988; Part C, Chapter 4. (e) Yoon, U. C.;
Mariano, P. S. Acc. Chem. Res. 1992, 25, 233. (f) Parker, V. D. Acc. Chem.
Res. 1984, 17, 243. (g) Eberson, L. AdV. Phys. Org. Chem. 1982, 18, 79.
(h) Mizuno, K.; Otsuji, Y. In Topics in Current Chemistry; Springer: Berlin,
1994; Vol. 169, pp 301-346. (i) Lopez, L. In Topics in Current Chemistry;
Mattay, J., Ed.; Springer: Berlin, 1990; Vol. 156. (j) Roth, H. In Topics
in Current Chemistry; Mattay, J., Ed.; Springer: Berlin, 1990; Vol. 156, p
1. (k) Mattay, J. In Topics in Current Chemistry; Mattay, J., Ed.;
Springer: Berlin, 1990; Vol. 156, p 219. (l) Mariano, P. S.; Stavinoha, J.
L. In Synthetic Organic Chemistry; Horspool, W. M., Ed.; New York, 1984;
p 145. (m) Mattay, J. Angew. Chem., Int. Ed. Engl. 1987, 26, 825. (n)
Kavarnos, G. J.; Turro, N. J. Chem. ReV. 1986, 86, 401. (o) Mattay, J.
Synthesis 1989, 233. (p) Ledwith, A. Acc. Chem. Res. 1972, 5, 133. (q)
Pac, C. Pure Appl. Chem. 1986, 58, 1249.
(13) Takamuku, S.; Komitsu, S.; Toki, S. Radiat. Phys. Chem. 1989,
34, 553.
(14) Arai, S.; Grev, D. A.; Dorfman, L. M. J. Chem. Phys. 1967, 46,
2572.
(15) Kikuchi, O.; Oshiyama, T.; Takahashi, O.; Tokumaru, K. Bull.
Chem. Soc. Jpn. 1992, 65, 2267.
(16) The dimerization of 3•+ has been reported to yield a dimer in the
anodic oxidation of 3 in the presence of acetic acid and sodium acetate in
acetonitrile. Eberson, L.; Parker, V. D. Acta Chem. Scand. 1970, 24, 3553.
(17) It might be suggested that Scheme 4 should be replaced by a series
of canonical valence bond representations including a quinoid-type structure,
B, which is favorable because of the stabilization of the positive charge by
the p-methoxyl group. Because the contribution of B can critically explain
both the unimolecular isomerization and the oxidation in 3•+-5•+, 7•+, and
8•+ with a p-methoxyl group, B with charge-spin separation is shown in
Scheme 4.
(18) Product analyses were performed in γ-radiolyses of S (S ) 1-8)
by a 60Co γ source in Ar- or O2-saturated DCE (1.0 × 10-2 M) at room
temperature. t-3-5 were formed in high yields in γ-radiolyses of c-3-5
in Ar-saturated DCE, and 4-methoxybenzyl phenyl ketone was regioselec-
tively formed in γ-radiolysis of 3 in O2-saturated DCE, although the details
will be published elsewhere.
(2) (a) Lewis, F. D.; Dykstra, R. E.; Gould, I. R.; Farid, S. J. Phys.
Chem. 1988, 92, 7042. (b) Lewis, F. D.; Bedell, A. M.; Dykstra, R. E.;
Elbert, J. E.; Gould, I. R.; Farid, S. J. Am. Chem. Soc. 1990, 112, 8055. (c)
Kuriyama, Y.; Arai, T.; Sakuragi, H.; Tokumaru, K. Chem. Phys. Lett. 1990,
173, 253.
(3) (a) Lewis, F. D.; Petisce, J. R.; Oxman, J. D.; Nepras, M. J. J. Am.
Chem. Soc. 1985, 107, 203. (b) Akaba, R.; Sakuragi, H.; Tokumaru, K.
Chem. Phys. Lett. 1990, 174, 80. (c) Kuriyama, Y.; Sakuragi, H.; Tokumaru,
K.; Yoshida, Y.; Tagawa, S. Bull. Chem. Soc. Jpn. 1993, 66, 1852. (d)
Tojo, S.; Morishima, K.; Ishida, A.; Majima, T.; Takamuku, S. Bull. Chem.
Soc. Jpn. 1995, 68, 958.
(19) (a) Yates, B. F.; Bouma, W. J.; Radom, L. J. Am. Chem. Soc. 1984,
106, 5805. (b) Yates, B. F.; Bouma, W. J.; Radom, L. Tetrahedron 1986,
22, 6225. (c) Hammerum, S. Mass Spectrom. ReV. 1988, 7, 123. Stirk, K.
M.; Kiminkinen, L. K. M.; Kentta¨maa, H. I. Chem. ReV. 1992, 92, 1649.
(e) Smith, R. L.; Chyall, L. J.; Chou, P. K.; Kentta¨maa, H. I. J. Am. Chem.
Soc. 1994, 116, 781.
(4) (a) Ericksen, J.; Foote, C. S. J. Am. Chem. Soc. 1980, 102, 6083.
(b) Manring, L. E.; Ericksen, J.; Foote, C. S. J. Am. Chem. Soc. 1980, 102,
4275. (c) Spada, L. T.; Foote, C. S. J. Am. Chem. Soc. 1980, 102, 391. (d)
Lewis, F. D.; Petisce, J. R.; Oxman, J. D.; Nepras, M. J. J. Am. Chem. Soc.
1985, 107, 203. (e) Yamashita, T.; Tsurusako, T.; Nakamura, N.; Yasuda,
M.; Shima, K. Bull. Chem. Soc. Jpn. 1993, 66, 857. (f) Tsuchiya, M.;
Ebbesen, T. W.; Nishimura, Y.; Sakuragi, H.; Tokumaru, K. Chem. Lett.
1987, 2121. (g) Konuma, S.; Aihara, S.; Kuriyama, Y.; Misawa, H.; Akaba,
R.; Sakuragi, H.; Tokumaru, K. Chem. Lett. 1991, 1897. (h) Inoue, T.;
(20) (a) Tojo, S.; Toki, S.; Takamuku, S. J. Org. Chem. 1991, 56, 6240.
(b) Brede, O.; David, F.; Steenken, S. J. Chem. Soc., Perkin Trans. 2 1995,
23.
(21) Bonazzola, L.; Michaut, J.-P.; Roncin, J.; Misawa, H.; Sakuragi,
H.; Tokumaru, K. Bull. Chem. Soc. Jpn. 1990, 63, 347.
(22) Maillard, B.; Ingold, K. U.; Scaiano, J. C. J. Am. Chem. Soc. 1983,
105, 5095.
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