Dimerization of 1-phenylcycloalkene cation radicals dependent on their structure

Article abstract of DOI:10.1021/ja950770o

Irradiation of 1-phenylcyclopentene (1a) with 1,4-dicyanobenzene (DCNB) in acetonitrile gave [2 + 4] cycloadducts, tetralin-type dimers (2-4), together with a 2:1 adduct of 1a with DCNB (5), through an electron transfer reaction. However, similar irradiation of 1-phenylcyclohexene (1b) gave 1-cyano-2-phenylcyclohexane (7) and 1:1 adducts of 1b with DCNB (8 and 9), instead of yielding dimeric products. Transient absorption spectra obtained by pulse radiolysis and laser flash photolysis of 1a showed simultaneous formation of a new absorption band with 1/4 (max) around 480 nm, attributable to dimer cation radicals, and decreasing absorption bands for monomer cation radicals (1a^.+, λ(max): 400 and 670 nm). In the case of 1b, by contrast, only absorption bands for monomer cation radicals 1b^.+ were observed in a region similar to those of 1a^.+. The optimized structure of the monomer olefin cation radicals, calculated by the PM3 method, suggested that, unlike the chairlike structure of 1b^.+, the fully planar structure of 1a^.+ would, on interacting with another neutral 1a, promote the formation of dimer cation radicals.

Full text of DOI:10.1021/ja950770o

2612
J. Am. Chem. Soc. 1996, 118, 2612-2617
Dimerization of 1-Phenylcycloalkene Cation Radicals
Dependent on Their Structure
Masanobu Kojima,*,† Akikazu Kakehi,‡ Akito Ishida,§ and Setsuo Takamuku§
Contribution from Faculty of Agriculture, Shinshu UniVersity, Asahi, Matsumoto 390, Japan,
Faculty of Engineering, Shinshu UniVersity, Wakasato, Nagano 380, Japan, and The Institute of
Scientific and Industrial Research, Osaka UniVersity, Ibaraki, Osaka 567, Japan
ReceiVed March 8, 1995X
Abstract: Irradiation of 1-phenylcyclopentene (1a) with 1,4-dicyanobenzene (DCNB) in acetonitrile gave [2 + 4]
cycloadducts, tetralin-type dimers (2-4), together with a 2:1 adduct of 1a with DCNB (5), through an electron
transfer reaction. However, similar irradiation of 1-phenylcyclohexene (1b) gave 1-cyano-2-phenylcyclohexane (7)
and 1:1 adducts of 1b with DCNB (8 and 9), instead of yielding dimeric products. Transient absorption spectra
obtained by pulse radiolysis and laser flash photolysis of 1a showed simultaneous formation of a new absorption
band with λmax around 480 nm, attributable to dimer cation radicals, and decreasing absorption bands for monomer
cation radicals (1a•+, λmax: 400 and 670 nm). In the case of 1b, by contrast, only absorption bands for monomer
cation radicals 1b•+ were observed in a region similar to those of 1a•+. The optimized structure of the monomer
olefin cation radicals, calculated by the PM3 method, suggested that, unlike the chairlike structure of 1b•+, the fully
planar structure of 1a•+ would, on interacting with another neutral 1a, promote the formation of dimer cation radicals.
and a JEOL JNX-DX303HF mass spectrometer. Gas chromatography
was performed on a Shimadzu GC-14A gas chromatograph equipped
with a flame ionization detector. Irradiations were carried out with a
400-W high-pressure mercury lamp (Riko UVL-400HA).
Photoinduced Electron Transfer Dimerization of 1a. When 1a
(0.1 M) was irradiated with DCNB (0.1 M) in acetonitrile with a 400-W
high-pressure mercury lamp through a Pyrex filter for 8 h, 1a was
almost consumed and three dimers (2-4) were produced with yields
of 11, 23, and 7%, respectively, together with 2:1 and 1:1 adducts of
1a with DCNB [5 (31%) and 6 (∼1%), respectively]. One of the dimers
was purified by column chromatography on silica gel using hexane as
an eluent, and X-ray crystallographic analysis revealed the structure
of the purified dimer to be that of a tetralin-type dimer 3. The same
method was used in the case of adduct 5. On the basis of their MS
and 13C NMR spectra data, dimer 2 was identified as a stereoisomer of
3, while dimer 4 was found to be the dehydro dimer of 2 and 3.
Dimer 2: 13C NMR (CDCl3) δ 151.36, 146.81, 140.12, 129.88,
127.77, 127.45, 126.09, 125.39, 125.24, 125.13, 57.81, 55.81, 47.58,
47.09, 39.17, 30.03, 29.39, 29.13, 23.13, 22.60; HRMS calcd for C22H24
m/z 288.1878, found 288.1883, major fragment (relative intensity) 288
(15%), 245 (46%), 210 (100%), 144 (41%), 129 (31%), 91 (37%), 44
(29%).
Dimer 3: mp 83-84 °C from hexane; 13C NMR (CDCl3) δ 150.39,
143.95, 138.35, 130.21, 128.79, 127.85, 127.77, 125.66, 125.40, 125.16,
58.83, 48.92, 44.36, 39.41, 38.22, 35.08, 30.41, 27.79, 23.00, 21.76;
HRMS calcd for C22H24 m/z 288.1878, found 288.1871, major fragment
(relative intensity) 288 (46%), 246 (31%), 245 (100%), 220 (41%),
217 (38%), 91 (32%).
Dimer 4: HRMS calcd for C22H22 m/z 286.1722, found 286.1708,
major fragment (relative intensity) 286 (100%), 243 (87%), 209 (45%),
208 (43%), 167 (66%), 165 (48%), 142 (31%), 115 (36%), 91 (44%),
44 (44%).
It is well-known that on direct UV irradiation aromatic acyclic
olefins like styrenes undergo [2 + 2] cycloaddition to give cis-
cyclobutane-type dimers through the formation of their
excimers,1-3 while photoinduced electron-transfer dimerization
of the olefins with cyanoaromatics as electron acceptors yields
trans-cyclobutane-type and/or tetralin-type dimers through the
cation radicals of the olefins.2-7 Similarly, it has been reported
that direct irradiation of 1-phenylcycloalkenes (1) also yields
[2 + 2] cyclodimers.8,9 However, there has so far been no
investigation of photoinduced electron transfer (ET) dimerization
of 1. We now report that irradiation of 1-phenylcyclopentene
(1a) in the presence of 1,4-dicyanobenzene (DCNB) gave four
kinds of tetralin-type dimers, while similar irradiation of
1-phenylcyclohexene (1b) produced two kinds of 1:1 adducts
of 1b combined with DCNB. Secondly, it was found that the
transient absorption spectra obtained by pulse radiolysis and
laser flash photolysis of 1a showed absorption bands attributable
to monomer and dimer cation radicals; however, in the case of
1b, only the monomer cation radical was observed. Thirdly,
the experiment demonstrated that dimerization of the olefins
depends on the structure of their monomer olefin cation radicals.
Experimental Section
13C NMR spectra were recorded on a Bruker AC-250 spectrometer
and a Bruker DRX-500 spectrometer. Mass spectra were determined
with a Shimadzu GCMS-QP1000 gas chromatograph mass spectrometer
* To whom correspondence should be addressed.
† Faculty of Agriculture, Shinshu University.
‡ Faculty of Engineering, Shinshu University.
§ Osaka University.
Adduct 5: mp 205-207 °C from dichloromethane; 13C NMR
(CDCl3) δ 156.32, 149.83, 143.54, 141.92, 131.77, 130.24, 129.67,
128.74, 127.96, 127.81, 125.84, 125.74, 125.56, 119.07, 109.52, 56.27,
55.52, 49.57, 49.40, 38.38, 38.18, 27.94, 27.87, 21.58, 21.53; HRMS
calcd for C29H27N m/z 389.2144, found 389.2130, major fragment
(relative intensity) 389 (100%), 346 (88%), 217 (36%), 91 (89%).
Adduct 6: HRMS calcd for C18H17N m/z 247.1362, found 247.1369,
major fragment (relative intensity) 247 (28%), 218 (42%), 144 (100%),
129 (41%), 115 (44%).
X Abstract published in AdVance ACS Abstracts, March 1, 1996.
(1) Brown, W. G. J. Am. Chem. Soc. 1968, 90, 1916-1917.
(2) Asanuma, T.; Yamamoto, M.; Nishijima, Y. J. Chem. Soc., Chem.
Commun. 1975, 608-609.
(3) Kojima, M.; Sakuragi, H.; Tokumaru, K. Bull. Chem. Soc. Jpn. 1989,
62, 3863-3868.
(4) Neunteufel, R. A.; Arnold, D. R. J. Am. Chem. Soc. 1973, 95, 4080-
4081.
(5) Mattes, S. L.; Farid, S. J. Am. Chem. Soc. 1983, 105, 1386-1387.
(6) Mattes, S. L.; Farid, S. J. Am. Chem. Soc. 1986, 108, 7356-7361.
(7) Yamamoto, M.; Asanuma, T.; Nishijima, Y. J. Chem. Soc., Chem.
Commun. 1975, 53-54.
(9) Dauben, W. G.; van Riel, H. C. H. A.; Robbins, J. D.; Wagner, G.
J. Am. Chem. Soc. 1979, 101, 6383-6389.
(8) Tada, M.; Shinozaki, H. Bull. Chem. Soc. Jpn. 1970, 43, 1270.
0002-7863/96/1518-2612$12.00/0 © 1996 American Chemical Society

Dimerization of 1-Phenylcycloalkene Cation Radicals
J. Am. Chem. Soc., Vol. 118, No. 11, 1996 2613
with a spectrophotometer (Hitachi 323) and a multichannel photode-
tector (Otsuka Electronics, MCPD-100). After the irradiated sample
at 77 K was placed in a precooled quartz Dewar vessel, the absorption
spectra were determined as follows. Immediately after the liquid
nitrogen was removed from the vessel, the sample was again placed in
the vessel and the absorption spectra were measured with the MCPD
every 3 s during an increase in temperature up to room temperature.
Laser Flash Photolysis. Laser flash photolysis was carried out with
a XeCl (308 nm) excimer laser (Lambda Physik EMG-101) with a
pulsed xenon arc (Wacom KXL-151, 150 W) as the monitoring light
source. The monitoring light coming from a monochromator (JASCO
CT-25C) was amplified by a photomultiplier (Hamamatsu Photonics
R-928) and stored in a storage scope (Iwatsu TS-8123), and the signals
were transferred to a personal computer (NEC PC-9801VX21) and
accumulated 3-5 times in order to be averaged. The system was
controlled by a computer, which was also used to analyze the decay
curves. Sample solutions containing 1 (20 mM) and DCNB (150 mM)
in acetonitrile were used under air.
Results and Discussion
Figure 1. ORTEP drawing of dimer 3.
Photoinduced Electron Transfer Dimerization of 1a with
DCNB. When 1a (0.1 M) was irradiated with DCNB (0.1 M)
in acetonitrile through a Pyrex filter for 8 h with a 400-W high-
pressure mercury lamp, 1a was almost consumed and three
dimers (2-4) were produced with yields of 11, 23, and 7%,
respectively (eq 1), together with 2:1 and 1:1 adducts of 1a
Ph
H
Ph
H
hν
H
H
H
H
+
+
DCNB
MeCN
1a
3
2
Figure 2. ORTEP drawing of adduct 5.
Ph
Ph
H
(1)
Photoinduced Electron Transfer Reaction of 1b with DCNB.
Olefin 1b (0.1 M) with DCNB (0.1 M) was irradiated in acetonitrile
under a nitrogen stream at room temperature using a 400-W high-
pressure mercury lamp. The concentrated reaction mixtures were
separated by column chromatography, on silica gel with hexane as an
eluent, to give cis- and trans-1-cyano-2-phenylcyclohexane (7, 13%)
and two kinds of 1:1 adducts of 1b combined with DCNB (8 and 9;
8/9 ) 1.6; 34%), without yielding dimeric products. The structures of
the products were identified on the basis of the following spectra data.
Adduct 8: 13C NMR (CDCl3) δ 152.23, 141.75, 139.02, 136.74,
132.22, 128.80, 128.66, 127.23, 126.71, 119.09, 109.90, 31.91, 27.25,
21.48; HRMS calcd for C19H17N m/z 259.1362, found 259.1376, major
fragment (relative intensity) 259 (27%), 130 (100%), 129 (35%), 115
(39%), 91 (23%).
Adduct 9: 13C NMR (CDCl3) δ 151.00, 141.09, 136.74, 131.99,
129.24, 128.34, 128.22, 125.69, 125.15, 119.09, 109.60, 42.70, 32.41,
25.90, 17.50; HRMS calcd for C19H17N m/z 259.1362, found 259.1349,
major fragment (relative intensity) 259 (87%), 231 (57%), 230 (46%),
143 (44%), 129 (37%), 128 (49%), 116 (35%), 115 (92%), 91 (100%),
38 (78%).
Pulse Radiolysis. The L-band linear accelerator at Osaka University
was used as the source of electron pulse, with an energy value of 28
MeV, pulse width of 8 ns, and dose of 0.7 kGy per pulse. The diameter
of the electron beam spot on the surface of the cell was ca. 3 mm. A
450-W xenon lamp (Osram, XBO-450) was used as the analyzing light
source. The light passing through the sample solution was monitored
first by using a monochromator (CVI-Laser, DIGIKROM-240) and then
by a photomultiplier (Hamamatsu Photonics, R-1477). The light signal
was amplified on a transient digitizer (Tektronix, 7912AD).
γ-Radiolysis. The γ-radiolysis of glassy rigid matrix of degassed
chlorobutane solutions was carried out in 1.5-mm-thick Suprasil cells
cooled in liquid nitrogen at 77 K by a 60Co γ source (dose, 4.5 × 106
Gy). The optical absorption spectra of the 77 K matrix were measured
+
+
H
Ar
Ar
6
(Ar: 4-CNC6H4)
4
5
with DCNB [5 (31%) and 6 (∼1%), respectively]. The
structures of dimer 3 and adduct 5 were determined by X-ray
crystallographic analysis (Figures 1 and 2; Table 1), and other
dimers were identified by their spectra data.
Although it has been reported, mainly on the basis of MS
spectra, that direct UV irradiation of 1a forms cyclobutane-
type dimers, their structure has not been determined yet.8 In
the cases of acyclic olefins such as styrene, 4-methylstyrene,
and 4-methoxystyrene, both [2 + 2] and [2 + 4] cyclodimers
were formed through ET reaction, depending on the initial
concentration and the substituents on the benzene rings.2,3,10
Therefore, it is significant that 1a produces [2 + 4] cyclodimers
without yielding any [2 + 2] dimers through similar ET
reactions.
Photoinduced Electron Transfer Reaction of 1b with
DCNB. In the case of 1-phenylcyclohexene (1b), direct
irradiation gives [2 + 2] cyclodimers at room temperature
through an excimer and a [2 + 4] cyclodimer at -70 °C through
its trans isomer.9 By contrast, irradiation of 1b with DCNB
under conditions similar to those created in the case of 1a gave
no dimers but produced 1-cyano-2-phenylcyclohexane (7, 13%
at 20% conversion of 1b) and 1:1 adducts of 1b combined with
(10) Schepp, N. P.; Johnston, L. J. J. Am. Chem. Soc. 1994, 116, 6895-
6903.

2614 J. Am. Chem. Soc., Vol. 118, No. 11, 1996
Kojima et al.
Table 1. Crystal and Structure Analysis Data of Compounds 3
and 5
formula
formula weight
crystal system
space group
lattice parameter
a/Å
b/Å
c/C
R/deg
â/deg
3: C22H24
288.43
triclinic
5: C29H27N
389.54
monoclinic
P21/n; Z ) 4
P1; Z ) 2
10.810(8)
12.587(5)
6.212(2)
93.77(3)
98.97(4)
10.936(2)
10.842(4)
18.059(2)
90
96.43(1)
γ/deg
95.47(5)
828.4(8)
1.156
90
2127.8(8)
1.216
V/Å3
D
calcd/g cm-3
crystal size/mm3
diffractometer
radiation
0.140 × 0.560 × 0.940
0.240 × 0.160 × 0.480
Rigaku AFC5S
Rigaku AFC5S
Mo KR (λ ) 0.710 69 Å) Mo KR (λ ) 0.710 69 Å)
monochrometer
scan type
graphic
ω-2θ
55.1°
graphic
ω-2θ
55.0°
2θ max
computer program TEXSAN systema
TEXSAN systema
MITHRILc
calcd, not refined
structure solution
hydrogen atom
treatment
SIR88b
calcd, not refined
refinement
full-matrix,
anisotropic
full-matrix,
anisotropic
least-squares weight 4Fo /σ2(Fo )
no. of measurement total 3991,
reflns
4Fo /σ2(Fo )
total 5409,
unique 5145
1718
2
2
2
2
unique 3792
988
199
0.084; 0.096
0.40
no. of obsnsd
no. of variables
residuals R; Rw
max shift/error
∆Fmax/e- Å3
121
0.083; 0.090
0.01
Figure 3. Transient absorption spectra obtained at various times after
an 8-ns pulse irradiation of 1 (10 mM) in chlorobutane at room
temperature: (a) 1a and (b) 1b.
0.25
0.49
a See ref 26. b Direct method, see ref 27. c Direct method, see ref
difference in the products resulting from the photoinduced ET
reactions of 1a,b suggests that the reaction routes depend on
the reactivity of the monomer cation radicals of the olefins
generated (1a•+ and 1b•+) but not on the initial ET process.
Transient Absorption Spectra of 1•+. Upon pulse radiolysis
of 1a (10 mM) in chlorobutane under argon at room temperature,
simultaneous formation of a new transient absorption band with
λmax around 480 nm was observed after the decay around 400
and 670 nm of the initial absorption bands, due to the presence
of monomer cation radical 1a•+. An increase in the concentra-
tion of 1a intensifies the 480-nm band: therefore, the 480-nm
band is attributed to the resulting dimer cation radicals (Figure
3a). The transient absorption spectra obtained by γ-ray irradia-
tion of 1a (8 mM) in degassed chlorobutane at 77 K was
confirmed to be consistent with those observed above (Figure
4a); in particular, we observed that the 480-nm transient band
occurred on warming, but not at 77 K. However, in the case
of 1b, no transient absorption band attributable to the dimer
cation radical of 1b was observed by pulse radiolysis and γ-ray
irradiation (Figures 3b and 4b). Laser (308 nm) flash photolysis
of 1a (20 mM) in acetonitrile with DCNB (150 mM) gave
transient bands for monomer and dimer cation radicals of 1a,
similar to those observed through the techniques described
above, while only transient bands attributable to monomer cation
radical were observed in the case of 1b (Figure 5). From these
results, there is no doubt that 1b•+ cannot react with another
neutral 1b to form the dimer cation radical.
28. d I > 3.00σ(I).
Table 2. Oxidation Potentials (Eox), Free Energy Change (∆G),
and Ionization Potentials (I), and Heat of Formation
Eox/Va
H(n)d
H(•+)e
∆Hf
1
(vs Ag/AgCl) I/eVb ∆G/eVc (kcal/mol) (kcal/mol) (kcal/mol)
1a
1b
1.60
1.64
8.90 -1.08
9.11 -1.03
26.91
9.80
218.83
211.46
191.92
191.66
a Measured by cyclic voltammetry. b Calculated by the PM3 method.
d
c See text. Heat of formation for neutral 1. e Heat of formation for
1•+ f ∆H ) H(•+) - H(n).
.
DCNB (8 and 9, 34%; eq 2).11,12 Olefins 1a,b possess almost
hν
+
DCNB
MeCN
1b
CN
+
7
(2)
Ar
Ar
8
9
(Ar: 4-CNC6H4)
the same oxidation potentials (Eox ) 1.60 and 1.64 V vs Ag/
AgCl, respectively) as well as similar ionization potentials
(Table 2). The free energy change (∆G) associated with the
ET process from ground state 1 to excited DCNB (Ered ) -1.57
V vs Ag/AgCl) was calculated using Weller’s equation to be
exothermic for both olefins.13 Therefore, the remarkable
Pulse Radiolysis of dimer 3. Figure 6 shows transient
absorption spectra measured by pulse radiolysis of dimer 3 in
1,2-dichloroethane. The absorption bands were observed around
320, 440, 480-500, and 600 nm. It is interesting that the
transient spectra obtained resemble those of 1a at 60 and 270
ns after pulse (see Figure 3a).
(11) (a) Arnold, D. R.; Wong, P. C.; Maroulis, A. J.; Cameron, T. S.
Pure Appl. Chem. 1980, 52, 2609-2619. (b) Borg, R. M.; Arnold, D. R.;
Cameron, T. S. Can. J. Chem. 1984, 62, 1785-1802.
(12) Maroulis, A. J.; Shigemitsu, Y.; Arnold, D. R. J. Am. Chem. Soc.
1978, 100, 535-541.
(14) The calculations for the optimized structure were carried out using
MOPAC Ver. 6.0: Stewart, J. J. P. QCPE Bull. 1989, 9, 10.
(13) Rehm, D; Weller, A. Isr. J. Chem. 1970, 8, 259-271.

Dimerization of 1-Phenylcycloalkene Cation Radicals
J. Am. Chem. Soc., Vol. 118, No. 11, 1996 2615
Figure 6. Transient absorption spectra obtained by pulse radiolysis
of 3 (10 mM) in 1,2-dichloroethane at room temperature.
Figure 4. Transient absorption spectra obtained by γ-ray irradiation
of 1 (8 mM) in chlorobutane (1 at 77 K; 2-4 on warming): (a) 1a and
(b) 1b.
Figure 7. Optimized structure of monomer cation radicals of 1a,b
obtained by semiempirical molecular orbital calculation with the PM3
method.
the optimized structure for neutral 1a has a dihedral angle of
33.6° between the benzene and cyclopentene rings. On the other
hand, the optimized 1b•+ does not have a planar but chairlike
structure with a dihedral angle of 6.7° between the benzene ring
and ethylene bond, while that of neutral 1b was calculated to
be 16.6°, smaller by 17° than that of 1a (See Figure 7).
Calculation of Heat of Formation. Using the PM3 method,
the heat of formation of 3 was calculated to be 27.09 kcal/mol,
which is lower by ca. 4.2 kcal/mol than that of 2. This is one
of the reasons why the formation of 3 is preferred to that of 2.
Mechanism of Dimerization. Dimerization of 1a through
ET would be explained similarly by the mechanism proposed
previously for the dimerization of acyclic olefins as shown in
Scheme 1. When the monomer cation radical 1a•+ reacts with
another neutral olefin 1a, two probable π-complexes (10 and
11) are expected to be formed by their syn-head-to-head and
anti-head-to-head interaction, finally to collapse into dimers 2
and 3, respectively. On interacting with another neutral 1a, the
planer structure of 1a•+ would promote the formation of cis-
and trans-cyclobutane dimer cation radicals (12 and 13) or
acyclic 1,4-dimer cation radicals (14 and 15) as primary
intermediates.15-18 A 1,3-sigmatropic shift from 12 and 13 or
1,6-cyclization from 14 and 15 would give substituted hexatriene
cation radicals (16 and 17).6,16,19 ET followed by a proton shift
gives dimers 2 and 3, or deprotonation followed by loss of a
hydrogen atom yields dehydrodimer 4. In addition, formation
Figure 5. Transient absorption spectra obtained at various times after
laser (308 nm) flash photolysis of 1 (20 mM) in acetonitrile under air
in the presence of DCNB (150 mM): (a) 1a and (b) 1b.
(15) Bauld, N. L.; Pabon, R. J. Am. Chem. Soc. 1983, 105, 633-634.
(16) Bauld, N. L. Tetrahedron 1989, 45, 5307-5363.
(17) Lewis, F. D.; Kojima, M. J. Am. Chem. Soc. 1988, 110, 8664-
8670.
(18) Ledwith, A. Acc. Chem. Res. 1972, 5, 133-139.
(19) Bauld, N. L. AdV. Electron Transfer Chem. 1992, 2, 1-66.
Optimized Structure of 1•+. Using semiempirical molecular
orbital calculation with the PM3 method,14 1a•+ was optimized
to give a fully planar structure, although it was calculated that

2616 J. Am. Chem. Soc., Vol. 118, No. 11, 1996
Kojima et al.
Scheme 1
Scheme 2
On the other hand, as in the pulse radiolysis of dimer 3, the
transient spectra obtained closely resemble those of the dimer
cation radicals generated by pulse radiolysis, γ-radiolysis, and
laser photolysis of 1a. If π-complex 11 is generated upon the
pulse radiolysis of 3, decomposition from 3•+ to 11 must proceed
rapidly through several steps. For the reasons described above,
and given that hexatriene-type dimer cation radicals 16 and 17
are the nearest intermediates that produce final neutral dimers
2-4, 16 and 17 seem to be the most plausible candidates at the
present stage. However, other possibilities for the 480-nm
transient cannot be ruled out completely for lack of further
evidence, for example distonic 1,4-dimer cation radicals, where
π-electrons of two phenyl groups interact with each other as in
charge transfer and/or charge resonance interactions.
Addition of 1b•+ to DCNB•-. The photoreaction of 1b with
DCNB is considered to be analogous to that of 2,3-dimethyl-
2-butene (TME) with DCNB and 1,4-dicyanonaphthalene
(DCN) reported by Arnold et al.,11 who initially suggested that
the photosubstitution reaction of DCNB and DCN with TME
occurred via an electron transfer, proton transfer, radical
coupling mechanism.11a However, according to a more recent
study, addition of an allyl radical generated by deprotonation
from the cation radical of TME (TME•+) to DCNB•- or
formation of a zwitterion by the combination of TME•+ and
DCNB•- can better account for the observed results.11b Depro-
tonation from 1b•+ is expected to generate two kinds of neutral
radicals 20 and 21 as shown in Scheme 2. Calculation of the
heat of formation for 20 and 21 using PM3 suggests that radical
21 is more stable by ca. 4.3 kcal/mol than 20, which is probably
due to the resonance effect between the allyl radical and the
phenyl group. However, the adducts between 21 and DCNB•-
were not observed although addition of 20 to DCNB•- followed
by loss of CN- would give adduct 9. Furthermore, in the case
of TME, the homocoupling of the allyl radical generated from
deprotonation of TME•+ occurred to give several dimers of
TME;11a by contrast, no dimer of 1b was produced in the
photoreaction of 1b with DCNB. Formation of adducts 8 and
9, therefore, is explained using Scheme 2, where combination
of 1b•+ and DCNB•- gives a zwitterion intermediate (19) as a
primary adduct, followed by loss of H+ and CN- to yield the
adducts. UV irradiation of 1b and KCN in acetonitrile-2,2,2-
trifluoroethanol solution with 1-cyanonaphthalene has been
reported to give cis- and trans-2-cyano-1-phenylcyclohexane
(7);12 therefore, adduct 7, produced by irradiation of 1b with
DCNB in acetonitrile, should result from the reaction between
1b•+ and HCN produced under the reaction conditions.
of adduct 5 suggests the trapping of the hexatriene cation
radicals by neutral DCNB or its anion radical (DCNB•-).3
In dimerization of styrene, R-methylstyrene, and 4-methoxy-
styrene through their cation radicals, transient dimer species with
absorption in the region of 450-500 nm have been observed,20-23
and a mixture of [2 + 2] and [2 + 4] cyclodimers was
produced.2,3,7,10 Therefore, it should be noted that photoinduced
ET dimerization of 1a yields [2 + 4] dimers. The fact that the
reaction produced only [2 + 4] dimers may be due to the large
steric repulsion between the benzene and cyclopentene rings
of the dimer cation radicals 12-15, which makes the cyclobu-
tane-type dimer cation radicals (12 and 13) unstable and
accelerates the collapse to hexatriene-type dimer cation radicals
(16 and 17).
480-nm Transient. The 480-nm transient in Figures 3a, 4a,
and 5a is probably attributable to dimer cation radicals; however,
there are several candidates for the transient, from 10 to 17, as
shown in Scheme 1. Under the conditions described above, no
neutral [2 + 2] cyclodimers were produced at all; therefore,
dimer cation radicals 12 and 13 can be excluded from the list
of possible candidates. In the case of acyclic dimer cation
radicals 14 and 15, transient bands from both the benzyl radical
site (λmax, ∼300 nm)24 and the benzyl cation site (λmax, ∼320
nm)25 are likely to be observed; however, 14 and 15 do not
display an absorption band around 480 nm. Furthermore, it is
not possible that the 480-nm transient is attributable to
π-complexes 10 and 11 formed on initial interaction. Because
the π-complexes undergo rapid σ-bond formation to give dimer
cation radicals 12-15, they cannot come into existence until
ca. 300 ns after the pulse irradiation.
(20) Tojo, S.; Toki, S.; Takamuku, S. J. Org. Chem. 1991, 56, 6240-
6243.
(21) Egusa, S.; Arai, S.; Kira, A.; Imamura, M.; Tabata, Y. Radiat. Phys.
Chem. 1977, 9, 419-431.
(22) Egusa, S.; Tabata, Y.; Kira, A.; Imamura, M. J. Polym. Sci. 1978,
16, 729-741.
(23) Irie, M.; Masuhara, H.; Hayashi, K.; Mataga, N. J. Phys. Chem.
1974, 78, 341-347.
(24) Tokumaru, K.; Ozaki, T.; Nosaka, H.; Saigusa, Y.; Itoh. M. J. Am.
Chem. Soc. 1991, 113, 4974-4980.
(25) (a) McClelland, R. A.; Chan, C.; Cozens, F.; Modro, A.; Steenken,
S. Angew. Chem., Int. Ed. Engl. 1991, 30, 1337-1339. (b) McClelland, R.
A.; Cozens, F. L.; Steenken, S.; Amyes, T. L.; Richard, J. P. J. Chem.
Soc., Perkin Trans. 2 1993, 1717-1722.
(26) TEXSAN TEXRAY, Structure Analysis Package, Molecular Struc-
ture Corp., 1985.
(27) Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo, C.; Polidori,
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(28) Gilmore, C. J. J. Appl. Crystallogr. 1984, 17, 42-46.

Dimerization of 1-Phenylcycloalkene Cation Radicals
J. Am. Chem. Soc., Vol. 118, No. 11, 1996 2617
Conclusion
its dimer cation appeared. Consistent with the direct observation
of dimer cation radical species, irradiation of 1a with DCNB
in acetonitrile gives tetralin-type dimers 2-5 as major products,
together with a minor 1:1 adduct of 1a with DCNB (6).
However, irradiation of 1b under similar conditions yields 1:1
adducts (8 and 9) of 1b combined with DCNB, without forming
dimeric products. Furthermore, it is noteworthy that 1-phenyl-
cyclopentene gives only [2 + 4] cycloadducts as dimers, while
aromatic acyclic olefins like styrenes produce [2 + 2] and/or
[2 + 4] dimers depending on the concentration of the olefins
and the electron donating groups on the benzene rings.
The reactivity of monomer cation radicals of 1-phenylcyclo-
pentene (1a) and 1-phenylcyclohexene (1b) is determined by
their structures. Calculation of their optimized structures shows
that 1a•+ is completely planar but 1b•+ is distorted and
nonplanar. Pulse radiolysis, γ-ray radiolysis, and laser flash
photolysis of 1a give transient absorption spectra which
resemble each other, in that the λmax occurs around 480 nm, a
band attributable to dimer cation radicals of 1a. The 480-nm
transient can probably be ascribed to hexatriene dimer cation
radicals 16 and 17, although other possibilities cannot be entirely
excluded. In the case of 1b, however, no band attributable to
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