Cis-trans isomerization and oxidation of radical cations of stilbene derivatives

Article abstract of DOI:10.1021/jo960598m

Isomerization from cis stilbene derivatives (c-S (S = RCH=CHC₆H₅: 1, R = C₆H₅; 2, R = 4-CH₃C₆H₄; 3, R = 4-CH₃OC₆H₄ (= An); 4, R = 2,4-(CH₃O)₂C₆H₃; 5, R = 3,4-(CH₃O)₂C₆H₃; 6, R = 3,5-(CH₃O)₂C₆H₃; 7, AnCH=C(CH₃)C₆H₅; 8, AnCH=CHAn)) to trans isomers (t-S) and oxidation of S with O₂ were studied in γ-ray radiolyses of c-S in Ar-saturated 1,2-dichloroethane (DCE) and of S in O₂-saturated DCE, respectively. On the basis of product analyses, it is suggested that a smaller barrier to c-t unimolecular isomerization for c-3^.+-5^.+ and 8^.+ than for c-1^.+, 2^.+, and 6^.+ due to the single bond character of the central C=C double bond for c-3^.+-5^.+ and 8^.+ with a p-methoxyl group but not for c-l^.+, 2^.+, and 6^.+ without a p-methoxyl group because of the contribution of a quinoid-type structure induced by charge-spin separation. The isomerization proceeds via chain reaction mechanisms involving c-t unimolecular isomerization and endergonic hole transfer or dimerization and decomposition. The isomerization of c-3^.+ to t-3^.+ is catalyzed by addition of 1,4-dimethoxybenzene but terminated by triethylamine. The regioselective formation of 3d in oxidation of 3^.+ with O₂ is explained by spin localization on the β-olefinic carbon in 3^.+. The results of product analyses are compared with the rate constants of the unimolecular isomerization and the oxidation for S^.+ measured with pulse radiolyses.

Full text of DOI:10.1021/jo960598m

J . Org. Chem. 1996, 61, 7793-7800
7793
Cis-Tr a n s Isom er iza tion a n d Oxid a tion of Ra d ica l Ca tion s of
Stilben e Der iva tives
Tetsuro Majima,*,† Sachiko Tojo, Akito Ishida, and Setsuo Takamuku*
The Institute of Scientific and Industrial Research, Osaka University,
Mihogaoka 8-1, Ibaraki, Osaka 567, J apan
Received April 1, 1996X
Isomerization from cis stilbene derivatives (c-S (S ) RCHdCHC6H5: 1, R ) C6H5; 2, R ) 4-CH3C6H4;
3, R ) 4-CH3OC6H4 () An); 4, R ) 2,4-(CH3O)2C6H3; 5, R ) 3,4-(CH3O)2C6H3; 6, R ) 3,5-
(CH3O)2C6H3; 7, AnCHdC(CH3)C6H5; 8, AnCHdCHAn)) to trans isomers (t-S) and oxidation of S
with O2 were studied in γ-ray radiolyses of c-S in Ar-saturated 1,2-dichloroethane (DCE) and of S
in O2-saturated DCE, respectively. On the basis of product analyses, it is suggested that a smaller
barrier to c-t unimolecular isomerization for c-3•+-5•+ and 8•+ than for c-1•+, 2•+, and 6•+ due to
the single bond character of the central CdC double bond for c-3•+-5•+ and 8•+ with a p-methoxyl
group but not for c-1•+, 2•+, and 6•+ without a p-methoxyl group because of the contribution of a
quinoid-type structure induced by charge-spin separation. The isomerization proceeds via chain
reaction mechanisms involving c-t unimolecular isomerization and endergonic hole transfer or
dimerization and decomposition. The isomerization of c-3•+ to t-3•+ is catalyzed by addition of 1,4-
dimethoxybenzene but terminated by triethylamine. The regioselective formation of 3d in oxidation
of 3•+ with O2 is explained by spin localization on the â-olefinic carbon in 3•+. The results of product
analyses are compared with the rate constants of the unimolecular isomerization and the oxidation
for S•+ measured with pulse radiolyses.
In tr od u ction
stilbene and c-4,4′-dimethylstilbene.2c t-St•+ has little
reactivity toward O2,3a,4,5 while the radical cation of (E)-
2,3-diphenyl-2-butene has a high reactivity toward O2.4g
Factors which control the reactivities of the radical
cations are still unclear.
Numerous studies on reactions of radical cations of
aromatic olefins have been reported from the synthetic
and mechanistic points of view.1-5 It is well-known that
cis(c)-trans(t) isomerization and addition of a nucleophile
occur in radical cations of aromatic olefins as typical
reactions. The reactivities depend on the structure or
substituents of the radical cations. No unimolecular c-t
isomerization occurs in the stilbene radical cation (St•+),2
although it does in the radical cations of c-4,4′-dibromo-
We have recently reported reactions of radical cations
of stilbene derivatives using pulse radiolysis in 1,2-
dichloroethane,6 unimolecular isomerization from cis to
trans isomeric radical cations as well as bimolecular
reactions with O2 (oxidation). On the basis of the rate
constants for the reactions, we have found that the
unimolecular c-t isomerization and oxidation with O2 of
such stilbene radical cations, particularly with substitu-
tion of a p-methoxyl group as an electron-donating
substituent in radical cations of stilbene derivatives,
remarkably enhances 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 reactivi-
ties.6 In order to confirm this conclusion, product analy-
ses of c-t isomerization and oxidation by O2 of radical
cations of eight stilbene derivatives (S•+ ) 1•+-8•+) as
shown in Scheme 1 were carried out using γ-ray radi-
olyses and chloranil photosensitization. We have also
found that isomerization from c-S•+ to t-S•+ proceeds via
†
Tel: J apan+6-879-8496, FAX: J apan+6-875-4156, e-mail:
majima@sanken.osaka-u.ac.jp.
X Abstract published in Advance ACS Abstracts, October 1, 1996.
(1) Reviews: (a) Fox, M. A., Chanon, M., Eds. Photoinduced Electron
Transfer; Elsevier: Amsterdam, The Netherlands, 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 Netherlands, 1988; Part C,
Chapter 4. (d) Lewis, F. D. In Photoinduced Electron Transfer; Fox,
M. A., Chanon, M., Eds.; Elsevier: Amsterdam, The Netherlands, 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,
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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:
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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 .
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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. J pn. 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.; Koyama, K.; Matsuoka, T.;
Tsutsumi, S. Bull. Chem. Soc. J pn. 1967, 40, 162. (i) Futamura, S.;
Ohta, H.; Kamiya, Y. Chem. Lett. 1982, 381.
(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.; Toku-
maru, K. Chem. Phys. Lett. 1990, 174, 80. (c) Kuriyama, Y.; Sakuragi,
H.; Tokumaru, K.; Yoshida, Y.; Tagawa, S. Bull. Chem. Soc. J pn. 1993,
66, 1852. (d) Tojo, S.; Morishima, K.; Ishida, A.; Majima, T.; Taka-
muku, S. Bull. Chem. Soc. J pn. 1995, 68, 958.
(5) Tokumaru and his co-workers reported that t-1•+ reacts toward
O2 at a rate constant of kO ) 1.3 × 106 M-1 s-1 in the laser flash
2
photolysis of 9-cyanoanthracene-t-1 in DMSO.4f,g
(6) Tojo, S.; Morishima, K.; Ishida, A.; Majima, T.; Takamuku, S.
J . Org. Chem. 1995, 60, 4684. Majima, T.; Tojo, S.; Ishida, A.;
Takamuku, S. J . Phys. Chem. 1996, 100, 13615.
S0022-3263(96)00598-1 CCC: $12.00 © 1996 American Chemical Society

7794 J . Org. Chem., Vol. 61, No. 22, 1996
Majima et al.
Sch em e 1
for c-7 with a p-methoxyl group which were significantly
smaller than those for other S. On the other hand, c-S
was not formed in γ-radiolyses of Ar-saturated DCE
solutions containing t-S.
Effects of additives on the G value were studied in
γ-radiolysis of c-3 in DCE (Table 2). Both triethylamine
(TEA) at 2.0 × 10-2 M and O2 at 1.3 × 10-2 M completely
terminated the isomerization, while addition of 1,4-
dimethoxybenzene (DMB) at 2.0 × 10-2 M accelerated
the isomerization. The oxidation potentials measured for
c-3, TEA, and DMB are 0.94, 0.80, and 0.90 eV which
are in the same order of the ionization potentials of 8.07,
7.50, and 7.96 eV for c-3, TEA, and DMB, respectively.
P r od u ct An a lyses of Oxid a tion of S•+
. Product
analyses of the γ-radiolyses of S (1.0 × 10-2 M) in O2-
saturated DCE ([O2] ) 1.3 × 10-2 M) were carried out at
room temperature. Table 3 summarizes the G values for
the yields of oxidation products (a -d ). The total yields
of a -d based on consumption of S were more than 70%;
therefore, a -d were formed as the main products.8 On
the other hand, a -d were not formed in the γ-radiolyses
of Ar-saturated DCE solutions containing S. Formation
of a , b, and c was observed in the oxidation of all S•+
with O2. 1d , 2d , and benzyl 4-methylphenyl ketone, and
8d were produced as the minor products in reactions with
small G values (0.05-0.3) in γ-radiolyses of 1-O2, 2-O2,
and 8-O2, respectively, while 3d was the main product
with the largest G values (2.6-3.0) in the γ-radiolysis of
3-O2. It should be emphasized that 2d and benzyl
4-methylphenyl ketone were produced as the minor
products in the oxidation of 2, while 3d was formed
without benzyl 4-methoxyphenyl ketone in the oxidation
of 3.
chain reaction mechanisms at a higher concentration of
S, that the isomerization of c-3•+ to t-3•+ is catalyzed by
addition of 1,4-dimethoxybenzene but terminated by
triethylamine, and that a quinoid-type structure of 3•+
with a positive charge and an unpaired electron sepa-
rately is responsible for regiospecific formation of an
oxygenated product of a 3•+ with a p-methoxyl group.
Product analyses of the oxidation of t-1•+ or t-3•+
generated by photoinduced electron transfer from t-1 or
t-3 to a chloranil triplet were also carried out on mixtures
Resu lts
P r od u ct An a lyses of Isom er iza tion of c-S•+. Prod-
uct analyses of the γ-radiolyses of c-S (1.0 × 10-2 M) in
Ar-saturated 1,2-dichloroethane (DCE) were carried out
at room temperature. The main product was t-S the
yields of which were almost equivalent to those of c-S
consumed. Table 1 summarizes the conversions of c-S,
the yield of t-S based on the conversion of c-S respec-
tively, the G values for the yield of t-S with a definition
of the number of t-S molecules produced per 100 eV of
radiation energy absorbed by a sample solution, and Glim
for the G value when the concentration of c-S is extrapo-
lated to infinity in the plots of inverse G values vs the
inverse concentration of c-S. Since the G value of the
formation of c-S•+ is approximately 2.5 in butyl chloride
at 77 K,7 the G and Glim values of c-S consumed and final
products in unimolecular reactions of c-S•+ are less than
2.5 at room temperature.
The G values for the yield of t-S were 10-20 for c-1, 2,
and 6 without a p-methoxyl group and approximately 100
for c-3-5 and 8 with a p-methoxyl group. The G values
increased significantly with increasing concentration of
c-3-5 and 8, while the concentration of c-1, 2, and 6 had
less significant influence on the G values (Figure 1). Plots
of the reciprocal of the G values vs the reciprocal of the
concentration of c-S in the range of 5.0 × 10-3 to 5.2 ×
10-2 M provided Glim values over the range between 30
and more than 104 as shown in Table 1. The G value
and Glim for the yield of t-7 were 5.4 and 20, respectively,
of t-1 or t-3 at 5.0 × 10-3 M and chloranil at 5 × 10-3
M
in O2-saturated acetonitrile using a high pressure Hg
lamp. The products were found to be equivalent to those
obtained in γ-radiolyses in O2-saturated DCE (Table 3).
La ser F la sh P h otolysis of Ch lor a n il-S. The chlo-
ranil triplet with an absorption maximum of λmax ) 510
nm9 was generated in the flash photolysis of mixtures of
t-1 or t-3 and chloranil in acetonitrile at 355 nm and
quenched by t-1 or t-3 with a second-order rate constant
of kq ) 7.8 × 109 M-1 s-1 equivalent to the diffusion-
controlled rate constant. Triplet quenching was ac-
companied by the formation of t-1•+ or t-3•+ with λmax
)
480 or 500 nm, respectively, and the chloranil radical
anion with λmax ) 450 nm.10 The decay rate of t-1•+ with
λmax ) 480 nm did not depend on the concentration of
O2, while that of t-3•+ with λmax ) 500 nm did. The rate
constant of the reaction of t-3•+ and O2 (kO ) was calcu-
2
lated to be kO ) (0.9 - 1.4) × 107 M-1 s-1 from the
2
dependence of the decay rate on the concentration of O2.
Discu ssion
Isom er iza tion of c-S•+. It is well established that S•+
is formed initially by hole transfer from the radical cation
(8) Isomerization products were also observed with small G values
at higher conversions in prolonged γ-radiolyses of several St. For
example G ) 0.29 and 0.14 were obtained for the formation of c-7 and
t-7 at 52-56% conversion in the γ-radiolyses of t-7 and c-7, respectively.
It is suggested that the isomerization occurs via complicated processes
including decomposition of the main products formed initially.
(9) Gshwind, R.; Haselbach, E. Helv. Chim. Acta 1979, 62, 941.
(10) Andre, J . J .; Weill, G. Mol. Phys. 1968, 15, 97.
(7) Hamill, W. Radical Ions, Kaiser, E. T., Kevan, L., Eds.; New York
Intersciences: New York, 1968; p 321. Klinshpont, E. R. Organic
Radiation Chemistry Handbook; Milinchuk, V. K.; Tupikov, V. I. Eds.;
Ellis Hariwood: New York, 1989; p 26.

Reactions of Stilbene Radical Cations
J . Org. Chem., Vol. 61, No. 22, 1996 7795
Ta ble 1. P r od u cts fr om γ-Ra d iolyses of c-S (S ) 1-8),a Ra te Con sta n t of Un im olecu la r Isom er iza tion of c-S•+ (ki),b a n d
Oxid a tion P oten tia ls of Sc
γ-radiolyses
oxidation potentials
ox
ox
S
conv of c-S, %
yield of t-S, %
G-value of t-S
Glim
30
ki/s-1
<106
<106
E1/2 of c-S, V
E1/2 of t-S, V
∆E1/2ox(c-t), V
1
2
3
4
5
6
7
8
6.0
5.4
32.5
26.0
22.7
2.9
94
100
76
100
97
100
85
100
18.9
18.2
81.5
105.6
94.7
10.2
5.4
1.25
1.16
0.94
0.80
0.87
1.23
0.90
0.70
1.14
1.05
0.82
0.64
0.72
1.03
0.77
0.55
0.11
0.11
0.12
0.16
0.15
0.20
0.13
0.15
200
>103
>104
>4 × 103
140
4.5 × 106
1.3 × 107
1.4 × 107
<106
2.0
39.1
20
<106
120.6
>104
5.5 × 106
Dose, 2.6 × 102 Gy; concentration of substrate, 1.0 × 10-2 M in Ar-saturated DCE; room temperature; conversion of c-S, the conversion
a
of c-S; yield(t-S), the yield of t-S based on conversion of c-S; G value of t-S, the yield of t-S with definition of the number of molecules per
100 eV energy; Glim, the G value of t-S when the concentration of c-S is extrapolated to infinity in the plots of inverse G values of t-S vs
the inverse concentration of c-S. ki, calculated from the formation of t-S•+ during pulse radiolyses of c-S at 5.0 × 10-3 M in Ar-saturated
b
DCE.6 c Halfwave 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 acetonitrile; ∆E1/2ox(c-t) ) E1/2ox(c-S) - E1/2ox(t-S).
p-methoxyl group. The large difference is consistent with
the rate constant of unimolecular c-t isomerization (ki
listed in Table 1) and suggests a difference in the reaction
mechanism. The difference may be explained by a
smaller barrier to isomerization for c-3•+-5•+ and 8•+ than
for c-1•+, 2•+, and 6•+ due to the single bond character of
the central CdC double bond for c-3•+-5•+ and 8•+ but
not for c-1•+, 2•+, and 6•+ because of the contribution of a
quinoid-type structure, B, with separation and localiza-
tion of the positive charge on the oxygen of the p-
methoxyl group and an unpaired electron on the â-olefinic
carbon (Scheme 3). 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•+
.
F igu r e 1. Plots of the G values of the yield of t-1-6 vs
concentration of c-1-6 in γ-radiolyses of c-1 (O), c-2 (2), c-3
(b), c-4 (0), c-5 (9), c-6 (4), respectively (1.0 × 10-2 M), in an
Ar-saturated DCE at room temperature.
The G value for the isomerization increased with
increasing concentration of c-S (S ) 1-8) in the range
of 10-2-10-1 M (Figure 1). Glim was calculated from the
extrapolation of linear plots of the reciprocal of the G
values vs the reciprocal of the concentration of c-S (Table
1). Because the G and Glim values are expected to be less
than 2.5 for the yield of the DCE radical cation initially
produced by γ-radiolysis at room temperature, the large
Glim values indicate that the isomerization of c-S to t-S
proceeds via a chain reaction mechanism.
Ta ble 2. Effects of Ad d itives on Isom er iza tion of c-3 to
t-3 in γ-Ra d iolysis of c-3a
additives
concentration, M E1/2ox, V G value
none
oxygen
triethylamine
1,4-dimethoxybenzene
-
0.94b
-
0.80
0.90
67
0
0
1.3 × 10-2
2 × 10-2
2 × 10-2
154
a
Dose, 2.4 × 102 Gy; Ar-saturated DCE solution containing c-3
Ch a in Mech a n ism In volvin g Un im olecu la r
Isom er iza tion of c-S•+ (S ) 3-5 a n d 8). The unimo-
lecular isomerization of c-3•+-5•+ and c-8•+ to t-3•+-5•+
and t-8•+, respectively, occurs predominantly with a rate
at
a
concentration of 1 × 10-2 M and the additive at the
concentration mentioned at room temperature; E1/2ox, halfwave
b
oxidation potential of the additive; G value of t-3 formed. Value
of c-3.
constant of ki ) 4.5 × 106 to 1.4 × 107 s-1
. Since
dimerization of c-3•+ with c-3 preferably occurs at the rate
constant of kd ) 2.0 × 108 M-1 s-1 particularly at a high
concentration of 5.0 × 10-2 M, the bimolecular isomer-
ization of c-3•+ to t-3•+ might be considered. However,
in contrast to the dimer of c-1•+ (c-12•+), c-32•+ decomposes
slowly into t-3•+ and t-3 because there is no effect of the
concentration of c-3 on the formation of the absorption
of t-3•+. Therefore, the bimolecular isomerization of c-3•+
of DCE (DCE•+) as shown in the following initiation
-
processes where es denotes a solvated electron and is
stabilized by dissociative attachment to DCE during
γ-radiolyses of S in DCE (Scheme 2).7,11-13 Formation of
c-S•+ must be responsible for the isomerization of c-S to
t-S in the γ-radiolyses of c-S in Ar-saturated DCE. The
yields of t-S indicate that c-S isomerizes stoichiometric-
ally to t-S (Table 1). The G values for c-3-5 and 8 with
a p-methoxyl group are much larger than those for c-1,
2, and 6 without a p-methoxyl group and for c-7 with a
to t-3•+ through c-32 is not important. The significant
•+
increase in the G values of the isomerization of c-3•+
-
5•+ and c-8•+ must correspond to the unimolecular
isomerization of c-3•+-5•+ and c-8•+ involving another
chain reaction mechanism in the γ-radiolysis. The chain
reaction mechanism involving the unimolecular c-t
isomerization and the hole transfer from t-S•+ formed into
c-S is considered to reproduce c-S•+ and t-S, where c-S•+
denotes c-3•+-5•+ and c-8•+. For example, Scheme 4 is
(11) Shida, T.; Hamill, W. J . Chem. Phys. 1966, 44, 2375. Shida,
T. Electronic Absorption Spectra of Radical Ions, Elsevier: Amsterdam,
The Netherlands, 1988; p 113.
(12) Yamamoto, Y.; Aoyama, T.; Hayashi, K. J . Chem. Soc., Faraday
Trans. 1 1988, 84, 2209.
(13) Arai, S.; Grev, D. A.; Dorfman, L. M. J . Chem. Phys. 1967, 46,
2572.

7796 J . Org. Chem., Vol. 61, No. 22, 1996
Majima et al.
a
Ta ble 3. P r od u cts fr om γ-Ra d iolyses of S (S ) 1-8) in th e P r esen ce of O2 or Ch lor a n il-P h otosen sitized Oxid a tion s of
c
t-1 a n d t-3,b a n d Ra te Con sta n t of Oxid a tion of S•+ (kO
)
2
G value or yield
RCH[O]CHPhf RCH2C(dO)Phg
RCHdCR′Ph
PhCHO
RCHO
b
S
R
R′
methodd
conversion,e %
a
c
d
kO , M-1 s-1
2
c-1
t-1
t-2
c-3
t-3
t-4
t-5
t-6
t-7
c-7
t-8
t-1
t-3
Ph
Ph
H
H
H
H
H
H
H
H
CH3
CH3
-
H
H
γ
nd
nd
nd
nd
nd
nd
nd
nd
52
56
nd
64
68
1.7
1.6
1.1
1.0
0.9
1.3
1.3
1.0
3.8i
3.5i
-
-
0.9
1.1
0.7
1.2
1.4
nd
nd
nd
nd
0.1
<106
γ
-
0.3
0.05h
3.0
2.6
nd
nd
nd
0
<106
4-CH3C6H4
4-CH3OC6H4
4-CH3OC6H4
2,4-(CH3O)2C6H3
3,4-(CH3O)2C6H3
3,5-(CH3O)2C6H3
4-CH3OC6H4
4-CH3OC6H4
(4-CH3OC6H4)2
γ
1.3
1.6
1.4
1.5
2.3
0.9
1.7j
1.7j
1.0j
-
<106
nd
γ
γ
4.5 × 107
4.0 × 107
1.2 × 107
<106
γ
γ
γ
γ
2.8 × 107
2.5 × 107
<106
γ
nd
0
0.8l
19
0.2m
0
k
γ
Ph
hν
hν
50
<106
4-CH3OC6H4
17
29
12
15
(0.9-1.4) × 107
Dose, 4.9 × 104 Gy; concentration of substrate, 1.0 × 10-2 M in O2-saturated DCE; G value, the yield of oxidation products. Isomerization
a
products were also observed with small G values at higher conversions in prolonged γ-radiolyses.8 nd denotes that the values were not
measured. Photoirradiation of O2-saturated acetonitrile solution containing t-S at 5.0 × 10-3 M and chloranil at 5.0 × 10-3 M using a
b
high pressure Hg lamp at 405 nm. Product yields in % based on the consumption of t-1 and t-3. c kO , obtained from the dependence of
2
the decay of S•+ on the concentration of O2 during pulse radiolyses of S (5.0 × 10-3 M) in the presence of O2 in DCE,6 or flash photolyses
of S-chloranil (5.0 × 10-3 M each) at 355 nm in the presence of O2 in acetonitrile. Method for oxidation: γ, γ-radiolysis in the presence
d
of O2; hν, chloranil-photosensitized oxidation. e Conversion of t-S; yield, the yield of oxidation produced based on consumption of t-S.
f 1-Aryl-2-phenyl-1,2-epoxyethane (aryl ) R). Arylmethyl phenyl ketone (aryl ) R). Total G value of 2d and benzyl 4-methylphenyl
g
h
ketone. Acetophenone. j 4-Methoxybenzaldehyde () 3b). k 8 ) (4-CH3OC6H4)CHdCH(4-CH3OC6H4). l 4,4′-Dimethoxystilbene oxide. Des-
i
m
oxyanisoin.
Sch em e 2
Sch em e 4
Sch em e 3
3.9 × 106 to 3.9 × 107 s-1 at concentration of 5.0 × 10-3
to 5.0 × 10-2 M and could compete with the unimolecular
isomerization. It is concluded that the hole transfer
mechanism is reasonably suggested for the isomerization
with the large Glim values.
It is reasonably explained that TEA terminates the
isomerization via hole transfer quenching of c-3•+ because
of the lower oxidation potential (0.80 V) than that of c-3
(0.94 V) (Table 2). The isomerization of c-3•+ was also
terminated by O2 because of the reaction of c-3•+ with
O2. DMB•+ catalyzed the isomerization of c-3 to t-3 (Table
2); therefore, the catalytic c-t isomerization is similarly
proposed for catalytic reactions such as the c-t isomer-
ization of olefins, the cycloreversion of cyclobutane com-
pounds, and the anti-Markovnikov addition of methanol
to olefins by aromatic hydrocarbon radical cations.14
proposed for the isomerization of c-3 to t-3. If the hole
transfer would proceed at a diffusion controlled rate (kdiff
) 7.8 × 109 M-1 s-1), the apparent rate constants of the
hole transfer are calculated to be 3.9 × 107 to 3.9 × 108
s-1 at concentrations of 5.0 × 10-3 to 5.0 × 10-2 M. These
rates show that the unimolecular isomerization process
of c-3•+-5•+ and c-8•+ with ki ) 4.6 × 106 to 1.4 × 107 s-1
is the rate-determining process. Because the hole trans-
fer process is slightly endergonic with 0.11-0.2 V (Table
1), it probably proceeds slower than the diffusion-
controlled rate. If the rate constant would be one order
smaller than kdiff ) 7.8 × 109 M-1 s-1, the apparent rate
constants of the hole transfer would be calculated to be
(14) Majima, T.; Pac, C.; Takamuku, S.; Sakurai, H. Chem. Lett.
1979, 1149. Majima, T.; Pac, C.; Sakurai, H. Chem. Lett. 1979, 1136.
Majima, T.; Pac, C.; Sakurai, H. J . Am. Chem. Soc. 1980, 102, 5265.
Majima, T.; Pac, C.; Kubo, J .; Sakurai, H. Tetrahedron Lett. 1980, 21,
377. Majima, T.; Pac, C.; Sakurai, H. J . Chem. Soc., Perkin Trans. 1
1980, 2705. Majima, T.; Pac, C.; Nakasone, A.; Sakurai, H. J . Am.
Chem. Soc. 1981, 103, 4499.
(15) Kikuchi, O.; Oshiyama, T.; Takahashi, O.; Tokumaru, K. Bull.
Chem. Soc. J pn. 1992, 65, 2267.

Reactions of Stilbene Radical Cations
J . Org. Chem., Vol. 61, No. 22, 1996 7797
dimerization of c-2•+ with c-2 occurs at kd ) 3.4 × 108
M-1 s-1, the bimolecular isomerization of c-2•+ to t-2•+ is
proposed by analogy to σ-22•+. A chain mechanism is
possibly considered to involve the bimolecular isomer-
ization of c-1•+ and c-2•+ into t-1•+ and t-1 and t-2•+ and
Sch em e 5
•+
t-2, respectively, via π- and σ-12 and 22•+, and the
reproduction of π- and σ-12•+ and 22•+ by reactions of t-1•+
and c-1 and t-2•+ and c-2, respectively (Scheme 5).
Neither unimolecular nor bimolecular isomerization of
c-6•+ and c-7•+ to t-6•+ and t-7•+ was observed in the time
scale of microseconds (Table 1). However, the isomer-
ization of c-6 and c-7 to t-6 and t-7, respectively, were
observed with the large G and Glim values. This indicates
that the isomerization proceeds via a chain reaction
mechanism in the time scale of minutes. The chain
reaction mechanism involving slow unimolecular isomer-
ization at ki < 106 s-1 and the exergonic hole transfer
could be operative (Scheme 4), while another mechanism
involving slow dimerization to form π- and σ-S2•+ at kd <
106 M-1 s-1 could be alternatively proposed (Scheme 5).
Oxid a tion of S•+ w ith O2. The reactivity of S•+
toward O2 is enhanced by charge-spin separation in-
duced by the p-methoxyl group as shown from the
bimolecular rate constant of S•+ with O2 (kO listed in
2
Table 3).6 However, a -c were produced in similar range
of G values (0.7-3.8) from oxidation of all S•+ with O2 in
the γ-radiolyses of S in DCE (Table 3). Products a -c
would be produced not directly from S•+ but from
complicated reactions of peroxy radicals from DCE with
S. Because radicals such as 2-chloroethyl radical are
formed from dissociative electron attachment to DCE in
the initiation processes of the γ-radiolysis, the radicals
would react with O2 to yield peroxy radicals which would
react toward S to yield oxidation products such as a -c.
Products 1d , 2d and benzyl 4-methylphenyl ketone, and
8d were produced with a small number of the G value in
Formation of a π-complex between DMB•+ and c-3 as the
intermediate may be involved. When c-3 encounters
DMB•+ to form a π-complex (c-3/DMB)•+, a positive charge
is partly developed on c-3 and the C-C single bond
character appears on the CdC double bond because of
the p-methoxyl group. The unimolecular isomerization
via the twisting around the CdC double bond then
proceeds to yield t-3.
Lewis et al. reported that the quantum yield of t-isomer
formation for dicyanoanthracene-sensitized c-t isomer-
ization of the c-4-cyanostilbene radical cation (φ ) 13.0)
is much larger than those of 1, 2, and 3 (φ ) 0.32, 0.14,
and 0.12, respectively).2b Tokumaru and co-workers
reported that the rotational barriers assumed to be the
energy differences between the C1 90°-twisted structure
and c•+ were calculated to be 7-11 kcal mol-1 by the open-
shell RHF method as a function of the twist angle of the
central CdC double bond.15 Because the barriers of 2-4
are lower than that of 1 by 1-4 kcal mol-1, they
suggested that the isomerization of the c-4-cyanostilbene
radical cation may not proceed by the rotation around
the C-C bond. In a manner similar to the isomerization
via 3•+-5•+ described above, the chain reaction mecha-
nism involving hole transfer (Scheme 4) or catalytic
isomerization involving a π-complex would be suggested
in the isomerization of the c-4 cyanostilbene radical
cation.
the γ-radiolyses of 1, 2, and 8. The reactivities of 1•+
•+, and 8•+ toward O2 are low as shown in kO < 106 M-1
,
2
2
s-1 (Table 3). Therefore, 1d , 2d and benzyl 4-methylphe-
nyl ketone, and 8d may be formed from the secondary
reaction of 1c, 2c, and 8c, respectively, during a pro-
longed γ-irradiation. However, the G value of 3d in the
γ-radiolysis of 3d is significantly large, compared with
that for 1d and 2d and benzyl 4-methylphenyl ketone.
Selective formation of 3d without benzyl 4-methoxyphe-
nyl ketone suggests the regioselective oxidation of 3•+
with O2. The regioselective oxidation may be explained
by the addition of O2 to the â-olefinic carbon of 3•+
,
because of the contribution of B with separation and
localization of the positive charge on the oxygen of the
p-methoxyl group and an unpaired electron on the
â-olefinic carbon (Scheme 3). It has been suggested that
the density of an unpaired electron on the olefinic carbon
in t-8•+ is lower than that in t-3•+, because an unpaired
electron appears on both olefinic carbons in 8•+ with two
symmetrical p-methoxyl groups.6 The regioselective
formation of 3d can be also explained through a consid-
eration of the relative stabilities of the two possible
carbocations that could be formed from the addition of
Ch a in Mech a n ism In volvin g Bim olecu la r Isom er -
iza tion of c-S•+ (S ) 1, 2, 6, a n d 7). The unimolecular
isomerization was not observed for c-S•+ (S ) 1, 2, 6, and
7) (ki < 106 s-1, Table 1), while the bimolecular isomer-
izations of c-1•+ and c-2•+ to t-1•+ and t-2•+ were observed
in the time scale of 100-300 ns with the concentration
of (5-10) × 10-3 M during pulse radiolyses of c-1 and
c-2, respectively, in DCE at room temperature.3c,d The
bimolecular isomerization of c-1•+ to t-1•+ is proposed to
O2 to 3•+
.
Product 7d was formed in the oxidation of 7•+ in spite
of the contribution of B induced by charge-spin separa-
tion. It can be explained that migration of the methyl
group does not occur in the benzylic peroxy radical
generated by the addition of O2 to the benzylic position
•+
occur through π- and σ-12 formed by dimerization of
c-1•+ with c-1 at kd ) 3.9 × 108 M-1 s-1) (Scheme 5).3c,d
•+
Thermally unstable σ-12 decomposes rapidly into the
thermodynamically more stable t-1•+ and t-1.3c,d Since
of 7•+
.

7798 J . Org. Chem., Vol. 61, No. 22, 1996
Majima et al.
M-1 s-1 was obtained on the basis of the same O2
concentration in O2-saturated acetonitrile as in O2-
saturated DCE by the flash photolysis of t-3-chloranil
Sch em e 6
mixtures in the presence of O2 in acetonitrile. The kO
2
value parallels that (kO ) 4.5 × 107 M-1 s-1) determined
2
during the pulse radiolysis of t-3 in the presence of O2 in
DCE. Alternatively the difference between two kO values
2
is attributable to the different reactivities of t-3•+ and/or
the different O2 concentrations in two solvents. These
results show the regioselective oxidation of 3•+ with O2
because of the charge-spin separation with p-methoxyl
substitution.
In the chloranil-photosensitized oxidation, triplet en-
ergy transfer from 3Chl* to O2 occurs to give chloranil
and singlet oxygen (1O2*). However, 1O2* is not reactive
toward stilbene derivatives.4a Formation of a -c from t-1
or t-3 suggests that the electron transfer from Chl•- to
•-
O2 occurs slowly to produce a superoxide molecule (O2
)
which reacts with t-1•+ and t-3•+ to give the corresponding
a , b, and c as final products, since it has been reported
that t-1•+ reacts with O2 to give two 1a products via a
•-
dioxetane intermediate (Scheme 6).4a However, the
electron transfer from Chl•- to O2 must be slow on the
ns time scale because of the endergonic character. It is
suggested that products a -c are produced not directly
from t-1•+ and t-3•+ but from complicated reactions of
peroxy radicals generated from DCE with t-1 and t-3
(Scheme 6). Consequently, the formation of 3d is at-
tributed to regioselective oxidation of 3•+ with O2 because
of the structure, B, with charge-spin separation and
localization of an unpaired electron on the â-olefinic
carbon (Scheme 3).
It is well-known that O2 hardly reacts with radical
cations of aromatic olefins4 but reacts with triplets16,17
as well as radicals.18 No triplet formation was observed
during pulse radiolyses and γ-radiolyses of aromatic
hydrocarbons as well as S in alkyl halide solutions.
Formation of a superoxide anion (O2•-) during the pulse
radiolyses and γ-radiolyses in O2-saturated DCE cannot
be considered because of the smaller electron affinity of
O2 (11 kcal mol-1) than that of DCE (40 kcal mol-1).
Therefore, intermediacy of O2•- as a reactive species can
be safely ruled out of the reaction mechanism. It is
shown that charge-spin separation in 3•+ with a p-
methoxyl group is responsible for the regioselective
oxidation of 3•+ with O2 to give 3d .
Con clu sion s
Isomerization from c-S (S ) 1-8) to t-S was found to
occur stoichiometrically with large G values (5-121) and
Glim (20->10000) in γ-ray radiolyses of c-S in Ar-
saturated DCE at room temperature. The larger G
values for the yield of t-3-5 and t-8 in γ-ray radiolyses
of c-3-5 and c-8, respectively, with a p-methoxyl group
than those in those of c-1, 2, and 6 without a p-methoxyl
group suggest a smaller barrier to c-t isomerization for
c-3•+-5•+ and c-8•+ than for c-1•+, 2•+, and 6•+ due to the
greater single bond character of the former central CdC
double bond for c-3•+-5•+ and 8•+ relative to c-1•+, 2•+
,
and 6•+ because of the contribution of a quinoid-type
structure, B, with separation and localization of the
positive charge on the oxygen of the p-methoxyl group
and an unpaired electron on the â-olefinic carbon. The
isomerization of c-3-5 and c-8 proceeds via a chain
reaction mechanism involving c-t unimolecular isomer-
ization and endergonic hole transfer from t-S•+ to c-S.
On the other hand, the isomerization of c-1 and 2
proceeds via a chain reaction mechanism involving
dimerization and decomposition into t-S•+ and t-S. For-
mation of a -c in a range of G values (0.7-3.8) in the
γ-radiolyses of S in O2-saturated DCE suggests that a -c
would be produced from complicated reactions of peroxy
radicals from DCE with S. On the other hand, the
regioselective formation of 3d with large G values (2.6-
3.0) in oxidation of 3•+ with O2 is explained by spin
localization on the â-olefinic carbon because of the
contribution of B in 3•+. The results of products analyses
are essentially identical with predictions based on ki and
Oxidation of t-1•+ or t-3•+ generated by chloranil
photosensitization was also studied in the photoirradia-
tion of a mixture of t-1 or t-3, and chloranil in O2-
saturated acetonitrile using a Hg lamp. The products
were equivalent to those obtained in the γ-radiolysis of
t-1 or t-3 in O2-saturated DCE. The chloranil triplet
(3Chl*)19 generated through a fast intersystem crossing
of the singlet excited state (1Chl*) by photoexcitation is
quenched by t-1 or t-3 at a diffusion controlled rate
constant of kdiff ) 7.8 × 109 M-1 s-1 to give the chloranil
radical anion (Chl•-) and t-1•+ or t-3•+ via electron transfer
(Scheme 6). The rate constant of kO ) (0.9-1.4) × 107
2
(16) Murov, S. T.; Carmichael, I.; Hug, G. L. Handbook of Photo-
chemistry, 2nd ed.; Marcel Decker: New York, 1993; pp 223-259.
(17) Lessi, E. A.; Encinas, M. V. In CRC Handbook of Organic
Photochemistry; Scaiano, J . C., Ed.; CRC Press: Boca Raton, 1989; Vol.
2, pp 117-176.
(18) Lusztyk, J .; Kanabus-Kaminska, J . M. In CRC Handbook of
Organic Photochemistry; Scaiano, J . C., Ed.; CRC Press: Boca Raton,
1989; Vol. 2, pp 177-210.
(19) Hilinski, E. F.; Milton, S. V.; Rentzepis, P. M. J . Am. Chem.
Soc. 1983, 105, 5193.
kO for S•+ measured with pulses radiolyses. It should
be emphasized that the reactivities of c-t unimolecular
2

Reactions of Stilbene Radical Cations
J . Org. Chem., Vol. 61, No. 22, 1996 7799
isomerization and reaction of S•+ with O2 can be under-
stood in terms of charge-spin separation induced by
p-methoxyl substitution.
(6). Anal. Calcd for C15H14O: C, 86.39; H, 6.71. Found: C,
86.36; H, 6.77. t-3: 7.1 g (25%); mp 137-138 °C (lit.24 136-
137 °C); 1H NMR δ 7.35-7.21 (m, 7H), 6.83-6.80 (m, 2H), 6.98
(d, 1H, J ) 4.1 Hz), 6.97 (d, 1H, J ) 4.1 Hz), 3.73 (s, 3H); MS
m/z 210 (M+, 100), 195 (28), 179 (12), 165 (33), 152 (18), 105
(9), 89 (5). c-(1-(2,4-Dimethoxyphenyl)-2-phenylethylene (c-
4): 2.1 g (7.5%); bp 110 °C/0.5 mmHg; MS m/z 240 (M+, 100),
197 (12), 165 (20), 153 (8). Anal. Calcd for C16H16O2: C, 79.97;
H, 6.71. Found: C, 79.95; H, 6.78. t-4: 3.6 g (13%); mp 66-
67 °C (lit.25 62-63 °C); MS m/z 240 (M+, 100), 197 (12), 165
(16), 120 (6). Anal. Calcd for C16H16O2: C, 79.97; H, 6.71.
Found: C, 79.94; H, 6.74. c-(1-(3,4-Dimethoxyphenyl)-2-
phenylethylene (c-5): 2.6 g (8.9%); bp 107 °C/0.6 mmHg; MS
m/z 240 (M+, 100), 225 (34), 178 (9), 165 (18), 152 (11), 120
(6). Anal. Calcd for C16H16O2: C, 79.97; H, 6.71. Found: C,
79.98; H, 6.75. t-5: 4.8 g (16%); mp 108-109 °C (lit.26 111
°C); MS m/z 240 (M+, 100), 225 (32), 178 (8), 165 (18), 152 (10),
120 (7). c-(1-(3,5-Dimethoxyphenyl)-2-phenylethylene (c-6):
3.3 g (11%); bp 153 °C/0.2 mmHg (lit.27 150 °C/0.2 mmHg);
MS m/z 240 (M+, 100), 225 (8), 209 (15), 194 (7), 178 (6), 165
(5), 152 (6). t-6: 2.9 g (9.4%); mp 56 °C (lit.28 56.5 °C); MS
m/z 240 (M+, 100), 225 (7), 209 (15), 194 (6), 178 (9), 165 (5),
152 (8). c-1-(4-Methoxyphenyl)-2-phenylpropene (c-7): 0.95 g
(4.2%); mp 44-45 °C; MS m/z 224 (M+, 100), 209 (178), 194
(11), 178 (8), 165, (6) 121 (7), 115 (5). Anal. Calcd for
C16H16O: C, 85.68; H, 7.19. Found: C, 85.65; H, 7.23. t-7:
2.5 g (11%); mp 83-84 °C; MS m/z 224 (M+, 100), 209 (20),
194 (13), 178 (10), 165 (8), 121 (6), 115 (5). Anal. Calcd for
C16H16O: C, 85.68; H, 7.19. Found: C, 85.66; H, 7.26. c-1,2-
Bis(4-methoxyphenyl)ethylene (c-8): 6.4 g (6.1%); mp 35-36
Exp er im en ta l Section
Gen er a l. γ-Radiolysis of DCE solutions containing S was
carried out in a Pyrex tube with an inner diameter of 1.0 cm
at room temperature using a 60Co γ source (dose, 2.6 × 102
Gy).3d Continuous photoirradiation experiments were done in
Pyrex cells using a 500-W high-pressure mercury lamp. The
S-containing solutions 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. After γ-radiation and photoirra-
diation, the reaction mixtures were directly analyzed by GC
and GC-MS and compared with authentic samples with respect
of the retention times and MS fragment patterns as described
below. The product yields (a , b, 1c-3c, 1d -3d , and 8d ) were
determined by calibrations of the peaks with those of authentic
samples. GC and GC-MS analyses were carried out using 2%
silicon OV-17 on Uniport HP (80-100 mesh) as a column
packing. The G values were calculated from the products
yields and the absorbed dose measured by the ferrous sulfate
dosimeter (Fricke dosimeter).20
Laser flash photolysis was carried out in Ar- or O2-saturated
solutions containing t-1 or t-3 (5.0 × 10-3 to 7.0 × 10-2 M)
and chloranil (5.0 × 10-3 M) in acetonitrile at room temper-
ature. A flash at 355 nm (5 ns duration, 20 mJ pulse-1
,
diameter of 0.4 cm) was obtained by the third-harmonic
oscillation from a Nd:YAG laser (Quantel Model Brilliant). The
probe beam was obtained from a 450 W Xe-lamp (Osram, XBO-
450) synchronized with the laser flash. The probe beam was
sent into the sample solution and focused to a computer-
controlled monochromator (CVI Laser, Digikrom-240). The
output of the monochromator was monitored by a PMT
(photomultiplier tube; Hamamatsu Photonics, R1417 or R2497).
The signal from the PMT was recorded on a transient digitizer
(Tektronix, 7912AD with plug-ins, 7A19 and 7B92A). The
signals were converted to transient optical densities.
1
°C (lit.29 37 °C); H NMR δ 7.20-7.16 (m, 4H), 6.76-6.73 (m,
4H), 6.43 (s, 2H), 3.76 (s, 6H); MS m/z 240 (M+, 100), 225 (35),
209 (17), 195 (20), 182 (15), 178 (8), 165 (7), 152 (5), 135 (6).
t-8: 23.0 g (39%); mp 215-216 °C (lit.24 214.5-215 °C); 1H
NMR δ 7.30-7.20 (m, 4H), 6.85-6.82 (m, 4H), 6.95 (s, 2H),
3.78 (s, 6H); MS m/z 240 (M+, 100), 225 (30), 209 (18), 195
(19), 182 (13), 178 (10), 165 (9), 152 (7), 135 (5).
Benzaldehyde (1a -6a ), acetophenone (7a ), substituted ben-
zaldehyde (3d -6b), stilbene oxide (1c), deoxybenzoin (1d ), and
desoxyanisoin (8d ) were purchased from Aldrich or Tokyo
Kasei, while substituted stilbene oxides (2c, 3c) and substi-
tuted phenylmethyl phenyl ketones (2d , 3d ) were prepared
from oxidation of the corresponding stilbenes (2, 3) using
3-chloroperoxybenzoic acid and from Friedel-Crafts acylation
of benzene using (4-methylphenyl)acetyl chlorides and (4-
methoxyphenyl)acetyl chlorides, respectively. Benzyl 4-me-
thylphenyl ketone and benzyl 4-methoxyphenyl ketone were
also prepared as authentic samples in GC analyses from
Friedel-Crafts acylation of anisole and toluene using pheny-
lacetyl chloride, respectively. The prepared compounds were
purified by column chromatograph on silica gel and identified
All chemical shifts (δ) are reported in parts per million and
ox
J values are in hertz. Halfwave oxidation potentials (Ep/2
)
of 1.0 × 10-2 M samples were measured by cyclic voltametry
using 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 platinum wire auxiliary
electrode, and a supporting electrolyte of 10-1 M tetraethy-
lammonium tetrafluoroborate in acetonitrile.
Ma ter ia ls. c-1 (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) were synthesized by the Wittig
reaction of the corresponding substituted benzaldehyde and
benzyltriphenylphosphonium chloride with sodium ethoxide
in absolute ethanol at room temperature, according to litera-
ture procedures,21 and purified by means of distillation, column
chromatography on silica gel and/or recrystallization from
ethanol, respectively, before use. c-(1-(4-Methylphenyl)-2-
phenylethylene (c-2): 2.8 g (10% yield); bp 127 °C/1 mmHg;
MS m/z 194 (M+, 100), 179 (86), 165 (20), 115 (17), 96 (11).
Anal. Calcd for C15H14: C, 92.74; H, 7.26. Found: 92.72; H,
7.28. t-2: 4.6 g (18%); mp 120-121 °C (lit.22 119.5-120 °C);
MS m/z 194 (M+, 100), 179 (75), 165 (25), 115 (16), 96 (8). c-(1-
(4-Methoxyphenyl)-2-phenylethylene (c-3): 4.3 g (15%); bp 105
°C/1 mmHg (lit.23 141-143 °C/3 mmHg); 1H NMR (270 MHz)
(CDCl3) δ 7.27-7.12 (m, 7H), 6.74-6.70 (m, 2H), 6.49 (d, 1H,
J ) 1.4 Hz), 6.48 (d, 1H, J ) 1.4 Hz), 3.72 (s, 3H); MS m/z 210
(M+, 100), 195 (21), 179 (10), 165 (27), 152 (17), 105 (8), 89
1
by H NMR and GC-MS. c-4-Methylstilbene oxide (c-2c): 1H
NMR δ 7.37-7.28 (m, 9H), 4.35 (d, 1H, J ) 1.0 Hz), 4.34 (d,
1H, J ) 1.0 Hz), 2.28 (s, 3H); MS m/z 210 (M+, 10), 194 (22),
181 (100), 165 (30), 149 (8), 140 (23), 107 (85), 105 (60), 89
(20), 79 (14), 77 (31). t-2c: 1H NMR δ 7.35-7.26 (m, 9H), 3.86
(d, 1H, J ) 2.9 Hz), 3.84 (d, 1H, J ) 2.9 Hz), 2.28 (s, 3H); MS
m/z 210 (M+, 12), 194 (30), 181 (100), 165 (23), 149 (12), 140
(29), 107 (77), 105 (57), 89 (15), 79 (17), 77 (35). c-4-
Methoxystilbene oxide (c-3c): 1H NMR δ 7.81-7.28 (m, 9H),
4.40 (d, 1H, J ) 1.2 Hz), 4.39 (d, 1H, J ) 1.2 Hz), 3.72 (s, 3H);
MS m/z 226 (M+, 8), 2.11 (26), 197 (100), 165 (20), 153 (14),
135 (36), 105 (10), 77 (15), 65 (3), 51 (6). t-3c: 1H NMR δ 7.81-
7.28 (m, 9H), 3.90 (d, 1H, J ) 3.2 Hz), 3.88 (d, 1H, J ) 3.2
Hz), 3.74 (s, 3H); MS m/z 226 (M+, 9), 211 (29), 197 (100), 165
(18), 153 (15), 135 (42), 105 (14), 77 (16), 65 (7), 51 (9). (4-
Methylphenyl)methyl phenyl ketone (2d ): 1H NMR δ 7.82-
7.70 (m, 2H), 7.27-7.05 (m, 7H), 4.16 (s, 2H), 2.27 (s, 3H); MS
m/z 210 (M+, 14), 149 (15), 121 (5), 105 (M+ - 4-CH3C6H4CH2,
(20) Tabata, Y. In CRC Handbook of Radiation Chemistry; Tabata,
Y., Ed.; CRC Press: Boca Raton, 1991; pp 63-95.
(21) Maccarone, E.; Mamo, A.; Perrini, G.; Torre, M. J . Chem. Soc.,
Perkin Trans. 2 1981, 324.
(22) Zechmeister, L.; McNeely, W. H. J . Am. Chem. Soc. 1942, 64,
1919.
(24) Oki, M.; Urushibara, Y. Bull. Chem. Soc. J pn. 1952, 109.
(25) Molho, D.; Coillard, J . Bull. Soc. Chem. Fr. 1956, 78.
(26) Cadogan, J . I. G.; Duell, E. G.; Inward, P. W. J . Chem. Soc.
1962, 4164.
(27) Erdtman, H.; Leopold, B. Acta. Chem. Scand. 1948, 2, 34.
(28) Cox, R. F. B. J . Am. Chem. Soc. 1940, 62, 3512.
(29) Weygand, C.; Siebenmark, T. Chem. Ber. 1940, 73, 765.
(23) Kon, G. A. R.; Spickett, R. G. W. J . Chem. Soc. 1949, 2724.

7800 J . Org. Chem., Vol. 61, No. 22, 1996
Majima et al.
100), 77 (16). (4-Methoxyphenyl)methyl phenyl ketone (3d ):
1H NMR δ 7.88-7.76 (m, 2H), 7.57-6.63 (m, 7H), 4.14 (s, 2H),
3.76 (s, 3H); MS m/z 226 (M+, 20), 121 (M+ - 4-CH3OC6H4,
100), 105 (93), 91 (5), 77 (34), 65 (2), 51 (9). Benzyl 4-meth-
ylphenyl ketone: 1H NMR δ 7.74 (m, 2H), 7.25-7.16 (m, 5H),
7.07 (m, 2H), 4.18 (s, 2H), 2.34 (s, 3H); MS m/z 210 (M+, 3),
119 (M+ - C6H5CH2, 100), 105 (16), 91 (22). Benzyl 4-meth-
oxyphenyl ketone: 1H NMR δ 7.87 (m, 2H), 7.29-7.15 (m, 5H),
6.86 (m, 2H), 4.18 (s, 2H), 3.70 (s, 3H); MS m/z 226 (M+, 1),
135 (M+ - C6H5CH2, 100), 107 (10), 92 (10), 77 (15), 64 (4), 51
(8).
Other chemicals including triethylamine, 1,4-dimethoxy-
benzene, and chloranil 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. Acetonitrile (Tokyo Kasei) was used
without further purification.
77 (13); 3d : 226 (M+, 18), 121 (M+ - 4-CH3OC6H4, 100), 105
(87), 91 (5), 77 (30), 65 (2), 51 (8). The regioselective formation
of 3d was clearly indicated by the retention time and fragment
pattern which are different from those of benzyl 4-methox-
yphenyl ketone in GC and GC-MS analyses.
4. 1a ; 2,4-dimethoxybenzaldehyde (4b): 166 (M+, 100), 165
(91), 149 (13), 135 (22), 122 (18), 106 (14), 77, 63; 2,4-
dimethoxystilbene oxide (4c): 256 (M+, 7), 227 (100), 165, 152,
91 (51); 2,4-dimethoxybenzyl phenyl ketone (4d ): 256 (M+, 18),
240 (70), 197 (8), 151 (M+ - 2,4-(CH3O)2C6H3, 100), 121 (26),
105 (7), 77 (7).
5. 1a ; 3,4-dimethoxybenzaldehyde (5b): 166 (M+, 100), 165
(56), 151 (14), 95 (12), 77 (8); 3,4-dimethoxystilbene oxide
(5c): 256 (M+, 15), 227 (100), 196 (10), 181 (8), 165 (6); 3,4-
dimethoxybenzyl phenyl ketone (5d ): 256 (M+, 8), 240 (20),
195 (71), 167 (24), 151 (M+ - 3,4-(CH3O)2C6H3, 100), 139 (28),
105 (11).
Ch a r a cter iza tion s of Oxygen a ted P r od u cts a-d . Oxy-
genated products a , b, 1c-3c, 1d -3d , and 8d of S (S ) c-
and t-1-8) were analyzed by GC and GC-MS using authentic
samples, while assignment of 4c-8c and 4d -6d was carried
out on the basis of the MS fragment patterns observed after
the γ-irradiation of S in O2-saturated DCE and after photoir-
radiation of chloranil-S mixture in O2-saturated acetonitrile
(Table 3). The m/z number of peaks and the relative intensity
in parentheses are described below.
6. 1a ; 3,5-dimethoxybenzaldehyde (6b): 166 (M+, 100), 165
(35), 135 (15), 109 (8); 3,5-dimethoxystilbene oxide (6c): 256
(M+, 44), 227 (100), 212 (24), 196 (28), 181 (9), 152 (8), 107
(88), 105 (55), 89 (20), 79 (23), 77 (23); 3,5-dimethoxybenzyl
phenyl ketone (6d ): 256 (M+, 19), 165 (45), 151 (M+ - 3,5-
(CH3O)2C6H3, 100), 137 (16), 122 (8), 105 (86), 77 (17).
7. Acetophenone (7a ): 120 (M+, 55), 105 (100), 77 (54), 51
(11); 7b () 3b); 4-methoxy-R-methylstilbene oxide (7c): 240
(M+, 16), 225 (20), 211 (100), 195 (30), 179 (13), 170 (18), 149
(36), 105 (41), 77 (23), 51 (9).
1. Benzaldehyde (1a ): 106 (M+, 100), 105 (100), 94 (12),
77 (98), 51 (34); stilbene oxide (1c): 196 (M+, 20), 178 (5), 167
(100), 152 (17), 105 (32), 91 (20), 77 (13); deoxybenzoin (1d ):
196 (M+, 3), 105 (M+ - C6H5CH2, 100), 91 (5), 77 (32), 51 (6).
2. 1a ; 4-methylbenzaldehyde (2b): 120 (M+, 100), 119 (100),
108 (10), 105 (63), 91 (97); 2c: 210 (M+, 16), 194 (18), 181 (100),
165 (32), 149 (16), 140 (20), 107 (88), 105 (55), 89 (20), 79 (23),
8. 8b () 3b); 4,4′-dimethoxystilbene oxide (8c): 256 (M+,
4), 227 (100), 212 (8), 114 (6); desoxyanisoin (8d ): 256 (M+,
5), 135 (M+ - 4-CH3OC6H4, 100), 121 (9), 107 (6), 77.
Ack n ow led gm en t. 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 (Nos.06239106, 07455341,
08240229, 08750954) from the Ministry of Education,
Science, Sport and Culture of J apan.
77 (23); 2d : 210 (M+, 10), 149 (13), 121 (7), 105 (M+
-
4-CH3C6H4CH2, 100), 77 (22); benzyl 4-methylphenyl ketone:
210 (M+, 2), 119 (M+ - C6H5CH2, 100), 105 (11), 91 (24).
3. 1a ; 4-methoxybenzaldehyde (3b): 136 (M+, 72), 135
(100), 107 (17), 92 (13), 77 (28), 64, 51, 39; 3c: 226 (M+, 6),
211 (25), 197 (100), 182, 165 (12), 153 (12), 135 (40), 105 (9),
J O960598M