Vicinal-diaryl interactions in stilbenoid hydrocarbons as observed in the through-space charge delocalization of their cation radicals

Article abstract of DOI:10.1139/v99-081

The conformational preference of vicinal or 1,2-phenyl groups is probed in two classes of ring-substituted 1,2-diphenylbicyclooctene (stilbenoid) hydrocarbons 1a-1d and 2a-2c. UV-vis spectroscopy reveals, and X-ray crystallography verifies, the intramolecular (edge-to-face) orientation for the phenyl-phenyl interaction in stilbenoids 1a-1d. Most importantly, when two pairs of ortho-methyl substituents are present, the cofacial phenyl groups in the stilbenoid donors are established by X-ray crystallography and spectrally observed in the cation radicals (2a^+?-2c^+?) by the appearance of new bands with strong absorptions in the near IR with λ_max = 1100-1315 nm, analogous to those previously observed in intermolecular (aromatic) interactions of aromatic cation radicals.

Full text of DOI:10.1139/v99-081

913
Vicinal–diaryl interactions in stilbenoid
hydrocarbons as observed in the through-space
charge delocalization of their cation radicals
Rajendra Rathore and Jay K. Kochi
Abstract: The conformational preference of vicinal or 1,2-phenyl groups is probed in two classes of ring-substituted
1,2-diphenylbicyclooctene (stilbenoid) hydrocarbons 1a–1d and 2a–2c. UV–vis spectroscopy reveals, and X-ray
crystallography verifies, the intramolecular (edge-to-face) orientation for the phenyl–phenyl interaction in stilbenoids
1a–1d. Most importantly, when two pairs of ortho-methyl substituents are present, the cofacial phenyl groups in the
stilbenoid donors are established by X-ray crystallography and spectrally observed in the cation radicals (2a+•–2c+•) by
the appearance of new bands with strong absorptions in the near IR with λmax = 1100–1315 nm, analogous to those
previously observed in intermolecular (aromatic) interactions of aromatic cation radicals.
Key words: stilbenoid hydrocarbon, cation radical, aryl–aryl interaction.
Résumé : On a étudié la préférence conformationnelle de groupes phényles vicinaux (ou en positions 1,2) de deux
classes d’hydrocarbures, des 1,2-diphénylbicycloocènes (stilbénoïdes) substitués sur le cycle, 1a–1d et 2a–2c. La spectro-
scopie UV-visible révèle, et la diffraction des rayons X permet de confirmer, l’orientation intramoléculaire (arête vers la
face) de l’interaction phényle-phényle dans les stilbénoïdes, 1a-1d. La diffraction des rayons X a permis d’établir un fait
encore plus important, à savoir que, lorsque deux paires de substituants ortho-méthyles sont présents, les groupes phényles
des stilbénoïdes donneurs sont coaxiaux et on peut les observer spectralement dans les cations radicaux (2a+•–2c+•) par
l’apparition de nouvelles bandes de forte intensité dans le proche infrarouge, avec un λmax de 1100–1315 nm, analogues à
celles observées antérieurement dans les interactions intermoléculaires (aromatiques) des cations radicaux aromatiques.
Mots clés : hydrocarbure stilbénoïde, cation radical, interaction aryle–aryle.
[Traduit par la Rédaction] Rathore and Kochi 921
amongst the various chromophores. For example, inter-
molecular interactions of aromatic hydrocarbons (ArH) have
The through-space interaction between a pair of vicinal
aromatic π-systems is known to decrease the ionization po-
tential in a variety of aromatic hydrocarbons such as 1,1,2,2-
tetraphenylethane, 1,2-diarylethanes, triptycenes, cyclophanes,
benzpinacols, etc. (1–3). In the latter, Penn et al. (4) noted
that such cofacial aryl–aryl interactions can explain the in-
creased electron-transfer reactivity in carbon–carbon bond
cleavage. Recently, Gano et al. (5) pointed out that similar
1,2-diaryl interactions are also responsible for the observed
decrease in the ionization potentials in (Z)-stilbenes as com-
pared to the corresponding (E) isomers. Since these findings
suggest that the cation-radical intermediates resulting from
1-electron oxidation of such systems may be unusually sta-
ble species, a detailed (UV–vis) spectroscopic analysis of
these cationic intermediates must be carried out to further
explore the structural requirements of aryl–aryl interactions.
As such, the observation of new electronic transitions in
multichromophoric assemblies would provide valuable
information regarding the spatial (electronic) interactions
been previously observed by the presence of new broad ab-
sorption bands in the near-IR region in the electronic ab-
sorption spectra of a variety of dimeric aromatic cation
radicals (6, 7), and the broad NIR absorption bands have
been assigned to charge-resonance transitions within the
dimeric cation radicals (ArH)2+• arising from the favorable π,
π-orbital overlap between the two coplanar arene moieties
arranged in a sandwich-like structure (also compare ref. 8).
In this study, we report the preparation of cation-radical
salts of various stilbenoid donors with cofacially juxtaposed
aryl groups, and show how analysis of the electronic absorp-
tion spectra provides spectroscopic evidence for direct 1,2-
diaryl interactions. The two series of stilbenoid donors in
Chart 1 are readily synthesized by an efficient palladium-
catalyzed synthetic method (9). The spatial arrangements of
the neighboring aryl groups in these stilbenoid donors are
evaluated by monitoring the UV–vis spectra (and chemical
reactivity) and further confirmed by X-ray crystallography.
Moreover, the spectral comparison of the cation radical de-
Received November 24, 1998.
This paper is dedicated to Professor A.J. Kresge in recognition of his many seminal contributions to physical organic chemistry.
R. Rathore and J.K. Kochi.1 Department of Chemistry, University of Houston, Houston, TX 77204-5641, U.S.A.
1Author to whom correspondence may be addressed. Telephone: (713) 743-3290. e-mail: jkochi@pop.uh.edu
Can. J. Chem. 77: 913–921 (1999)
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Can. J. Chem. Vol. 77, 1999
Chart 1. Stilbenoid donors.
OMe
Class A.
Class B.
OMe
OMe
1a
2a
1b
2b
1c
2c
1d
2d
OMe
rived from other cofacial π-systems oriented at different di-
hedral angles (such as m-cyclophane and a cofacially
oriented phenylene donor) sheds additional light on the an-
gular requirements for such cofacial aryl–aryl interactions.
conformations, i.e., face-to-face (cofacial) (11) or edge-to-
face (perpendicular) (12), as diagrammatically presented in
Chart 2. Optimum π–π interactions between aryl groups in
Chart 2. Aromatic–aromatic orientations.
Results and discussion
1. Synthesis of stilbenoid donors
The various stilbenoid donors in Chart 1 were readily pre-
pared by the palladium-catalyzed coupling of aryl Grignard
reagents with a dibromoalkene, e.g.
Br
Ar
(PPh3)PdCl2
[1]
+ 2 ArMgBr
+ 2 MgBr2
Br
Ar
In a typical example, a freshly prepared solution of phenyl-
magnesium bromide (2.1 equiv.) was mixed with a solution
of 2,3-dibromobicyclo[2.2.2]oct-2-ene in anhydrous tetrahydro-
furan containing a catalytic amount of bis(triphenyl-
phosphine)palladium(II) chloride (10) under an argon
atmosphere at 25°C. The resulting pale-yellow solution was
refluxed for 8 h, and a standard aqueous work-up, followed
by recrystallization, afforded 2,3-diphenylbicyclo[2.2.2]oct-
2-ene (1a) in essentially quantitative yield. The generality of
the procedure was demonstrated by coupling various aryl-
magnesium bromides (with increasing substitution) with
dibromobicyclooctene to afford the diarylbicyclooctenes
listed in Table 1 in excellent yields.
various stilbenoid donors are expected to arise from the
cofacial arrangement of the aryl groups, and we will deter-
mine how such a preferred conformation can be achieved by
varying the substituents on the aryl groups as follows.
X-ray crystallography
Single crystal structure analysis was performed on 1a in
class A and 2a/2b in class B in order to ascertain the molec-
ular structures and conformations of the stilbenoid donors.
For the sake of simplicity, we identify the structural parame-
ters used for the comparison of all three structures as d, the
olefinic C=C bond length; l, the C–C bond length to the aryl
group; θ, the twist of the olefinic C=C bond; and φ1/φ2, the
coplanarity of aryl groups with the C=C bond (as designated
in the generic structure in Table 2). (Note that equal values
of φ1and φ2 indicate that the aryl groups are cofacial.)
Comparison of the various parameters d, l, θ, φ1, and φ2
for 1a, 2a, and 2b in Table 2 shows that the stilbenoid do-
nors (2a and 2b), that contain ortho-methyl groups have es-
sentially the same molecular structure that is characterized
by the presence of cofacial aryl groups. It is clear that steric
hindrance of the ortho-methyl groups forces both aryl
groups to lie parallel to each other and (almost) orthogonal
to the ethylenic bond with φ1 = φ2 = 65°, and the rotations of
the aryl groups (around the C1–C3 and the C2–C5 bonds) are
strongly coupled (i.e., mutually dependent), as indicated by
the identical values of φ1 and φ2 in 2a and 2b. By contrast,
the absence of ortho-methyl groups in the parent analog 1a
Table 1. Synthesis of various stilbenoid donors by the Pd-catalyzed
coupling of arylmagnesium bromides with 1,2-dibromobicyclo-
octene in tetrahydrofuran.a
Stilbenoid hydrocarbon
Timeb (h)
% Yieldc
1a
1b
1c
1d
2a
2b
2c
2d
8
6
96
94
92
91
94
94
91
82
12
12
18
18
24
24
aSee the experimental section for the general procedure.
bThe course of the reaction monitored by GC.
cIsolated yields.
2. Conformational analysis of stilbenoid donors
Owing to the rigid backbone structure, the various
stilbenoid donors can attain one of the two stable (idealized)
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Rathore and Kochi
915
Table 2. Comparison of selected structural parameters for
various stilbenoid donors.
Fig. 1. The superimposition of the molecular structures of
diphenyl- (1a, light lines) and dimesityl- (2b, dark lines)
bicyclooctenes to show the coplanarity of a single phenyl group
with the olefinic bond in 1a.
Parameter
1a
1.351(1)
1.484(1)
6.6°
45.6°
60.1°
2a
1.328(5)
1.51(2)
4.9°
64.8°
65.5
2b
1.352(1)
1.496(1)
4.5°
59.9°
62.1°
d
la
θb
φ1
c
d
φ2
aAverage of two bonds (C1–C3 and C2–C5).
bTorsional angle C3-C1-C2-C5.
cTorsional angle C2-C1-C3-C4.
dTorsional angle C1-C2-C5-C6.
allows a greater degree of rotational freedom around the C1–
C3 or the C2–C5 bond, as indicated by the different torsion
angles φ1 (46°) and φ2 (60°).2 Furthermore, an arbitrary su-
perposition of the molecular structures of 1a and 2b (such as
that illustrated in Fig. 13) shows that at least one of the
phenyl groups in 1a can readily acquire a coplanar confor-
mation with the olefinic bond.
Fig. 2. Space-filling representation of 2a to show the complete
entombment of the olefinic carbons (darkened spheres) by four
ortho-methyl groups and the bicyclooctene framework.
Electronic (UV–vis) spectra
Structural analysis (vide supra) reveals that the presence
of four ortho-methyl groups in various stilbenoid donors in
class B forces both aryl groups to lie orthogonal to the
olefinic bond, and thus effectively prevents π-conjugation
with the olefinic bond. As such, the UV–vis spectral data show
that all donors in class B absorb at λmax < 260 nm, whereas
the class A derivatives show well-defined absorption bands
that are relatively red-shifted to λmax ≥ 260–310 nm (see Ex-
perimental). Importantly, the UV–vis absorption spectra of
various derivatives in class A are strongly reminiscent of
those of the parent stilbenes (14). In contrast, the cofacial stil-
benoid donors in class B show absorption spectra that closely
resemble those of the isolated (non-conjugated) aryl groups.
Reactivity and steric crowding
The olefinic carbons in class B donors are completely en-
tombed due to the presence of four ortho-methyl groups (as
well as the bicyclooctene framework), as shown by the space-
filling representation of 2a in Fig. 2. Such steric crowding
around the olefinic bonds makes these donors completely in-
ert to hydrogenation and epoxidation reactions. (Note that the
2 Such a restricted rotation around the C–Ar single bond can lead to atropisomers as observed in a variety of biphenyls (ref. 13). For example, spec-
tral (NMR) analysis of naphthyl derivative 1d showed the presence of two isomers (see Experimental) that could not be separated or
interconverted by heating up to 180°C. A (molecular mechanics) minimization of the molecular structure of 2d with the aid of the QUANTA
graphics (Molecular Simulation Inc., 16 New England Executive Park, Burlington, MA 081803) confirmed the presence of two (syn and
anti) atropisomers. An X-ray structural study (in progress) should establish this point.
3 All eight of the carbon atoms of the bicyclooctene moiety in 1a were superimposed on those of 2a for Fig. 1.
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Can. J. Chem. Vol. 77, 1999
1-naphthyl derivative 2d is similarly unreactive.4) In contrast,
the class A derivatives (which lack ortho-methyl groups)
readily undergo hydrogenation and epoxidation reactions
owing to a greater rotational freedom of the aryl groups.4, 5
[2]
1c + MA+• → 1c+• + MA
The same exchange procedure was used for the prepara-
tion of the UV–vis absorption spectra of the other stilbenoid
cation radicals in Fig. 3a. It is important to emphasize, how-
ever, that the spectra were not dependent on the method of
oxidation of the neutral donors, i.e., either electrochemical
or chemical oxidation (vide supra). Thus the absorption
spectra for the various stilbenoid cation radicals presented in
Figs. 3 and 4 were obtained by at least three independent
methods of oxidation and found to be the same, as follows.
3. Cation radicals of the stilbenoid donor
Methods of oxidation
All the stilbenoid donors in Chart 1 undergo reversible
electrochemical oxidation in the range E0 = 0.98–1.45 V
ox
vs. SCE6 (see Table 3). The electrochemical reversibility in
such an easily accessible potential range allows the ready
preparation of the corresponding cation radicals by either
chemical or electrochemical oxidation in dichloromethane
solution. For example, the chemical oxidation can be readily
carried out by electron exchange with stable aromatic cat-
ion-radical salts MA+• and EA+•, as selective organic oxi-
dants with reversible reduction potentials that differ by only
190 mV, as listed in Chart 3 (16). Similarly, a mixture of
Class A stilbenoid cation radicals
The class A stilbenoid cation radicals showed the charac-
teristic (UV–vis) absorption spectra (in Fig. 3a) with twin
absorption bands I and II (consisting of similar band shapes
and fine structure in the high-energy band). These spectra
are strongly reminiscent of the absorption spectrum previ-
ously observed for the transient cation radical of the parent
(Z)-stilbene in frozen matrices (18) (see Fig. 3b). Interest-
ingly, a change of the aryl group from phenyl to tolyl to
anisyl in class A stilbenoid cation radicals leads to a
monotonic red shift in the absorption maxima of both bands
I and II as well as a hyperchromic increase in the (relative)
intensity of band II. Such an observation is in accord with
the increasing stabilization of the cationic charge by a
coplanar aryl group containing a para substituent (19) in the
order: H < CH3 < OCH3.
Chart 3. Cation-radical oxidants.
OMe
OMe
OMe
EA+.
1.30
OMe
MA+.
1.11
Eo
(V vs SCE)
red
Class B stilbenoid cation radicals
In marked contrast, the various class B stilbenoid cation
radicals in Fig. 4a showed an intense new (broad) absorption
band (band III) in near-IR region (1100–1315 nm) in addi-
tion to the twin absorption bands I and II in the UV–vis re-
gion. It is important to reemphasize that the NIR absorption
band III was completely absent in all the class A stilbenoid
cation radicals, as shown in Fig. 4b.
Such a remarkable difference in the electronic absorption
spectra of class A and B stilbenoid cation radicals can be as-
cribed to the steric effects of the ortho-methyl groups. Thus,
in class A derivatives, the absence of ortho-methyl groups
allows ready attainment of a conformation (edge-to-face) in
which one of the aryl groups is coplanar with the ethylenic
bond. However, such a coplanar conformation is strongly
discouraged in class B derivatives due to the presence of
ortho-methyl groups, and leads to only a face-to-face
(cofacial) conformation (for example, see Chart 4). The ob-
servation of new electronic absorption bands in the NIR
region (band III in Fig. 4a) is thus ascribed to charge-
resonance transitions between the rigidly held cofacial aryl
groups in 2a–2d cation radicals. Such a spectral assignment
is in accord with the observations of Badger and Brockle-
hurst (6) and Rodgers (7), who showed the presence of in-
chloranil and methanesulfonic acid (15) can also serve as an
effective oxidant for the preparation of dichloromethane so-
lutions of the cation radicals of the various stilbenoid donors
in Chart 1. Moreover, the isolation of crystalline cation-
radical salts from a number of stilbenoid donors can be ef-
fected with triethyloxonium hexachloroantimonate (17) as
the 1-electron oxidant (see Experimental). Alternatively, the
stable cation-radical solutions from stilbenoid donors in
Chart 1 can be obtained by anodic oxidation in anhydrous
dichloromethane containing tetrabutylammonium hexafluoro-
phosphate as the supporting electrolyte (see Experimental ).
Electronic absorption spectra
A solution of stilbenoid derivative 1c when mixed with 1
–
equiv. of MA+•SbCl6 in dichloromethane (under an argon
atmosphere at 25°C) immediately turned red, and the UV–
vis absorption spectrum showed the characteristic twin ab-
sorption bands, at λmax = 546 nm (band I) and 880 nm (band
II), of dianisylbicycloctene cation radical 1c+•, as shown in
Fig. 3a. Quantitative spectral analysis of the magenta solu-
tion (log ε546 = 4.16) indicated that the electron exchange in
eq. [2] was displaced completely to the right, i.e.,
4 A palladium-catalyzed hydrogenation of 2d under high pressure led to a partial reduction of the naphthyl moiety. For the hydrogenation pro-
cedures, see R. Rathore et al. (9)
5 A standard epoxidation procedure using m-chloroperbenzoic acid was employed, see ref. 15.
6 (a) Reversible electrochemical oxidation potentials (E0 in V vs. SCE) were measured for the various stilbenoid donors (5 mM) by cyclic
ox
voltammetry in dichloromethane (containing 0.2 M tetra-n-butylammonium hexafluorophosphate as supporting electrolyte). Note that the
naphthyl derivative 1d was irreversibly oxidized even at a scan rate of v = 2 V s–1. (b) It is noteworthy that E0 values of the class B donors
ox
are not significantly less positive than those of the class A analogues, despite the presence of additional electron-releasing methyl groups
(24). No doubt part of the discrepancy lies in the orthogonal aromatic rings, which completely remove them from ethylene conjugation. We
hope to develop other qualitative methods for determining the stabilization energy of the through-space delocalized cation radicals 2a+•–2c+.
© 1999 NRC Canada

Rathore and Kochi
917
Table 3. Cation radicals of various stilbenoid donors in solution.a
Cation radical
Donor
(D)
UV–vis band
UV–vis band
IR band
b
E0
ox
λmax (I)c
λmax(II)c
λmax(III)c
log ε
d
d
d
f
d
d
d
f
1a
1b
1c
1d
2a
2b
2c
2d
3
1.47
1.36
0.98
1.28e
1.28
1.40
1.21
1.33
0.80
1.41
493, 472(sh)
518, 478(sh)
722
781
547, 506(sh)
880
f
f
426
410
428
534
440
475
740
681
700
782
460
—
1162
1100
1315
1294
1200
1180
3.87
3.73
3.98
g
3.98
g
4
aFor the oxidation procedures, see Experimental.
bV vs. SCE at 25°C. In dichloromethane containing 0.2 M n-tetrabutylammonium
hexafluorophosphate at 100 mV s–1.
cλmax in nm.
dNo absorption band observed in NIR region.
ePeak potential, irreversible cyclic voltammogram at v = 1000 mV s–1.
fTransient cation radical.
gNot determined.
Fig. 3. (a) The UV–vis absorption spectra of various cation radicals from class A stilbenoid donors (as indicated) obtained by
oxidation with EA+• SbCl6 in dichloromethane at 22°C. (b) Comparison of the UV–vis absorption spectra of the cation radicals of
–
parent (Z)-stilbene (18) and 2,3-diphenylbicyclooctene 1a (as indicated).
Fig. 4. (a) The pronounced NIR absorption band III in the electronic spectra of the cation radicals from class B stilbenoid donors (as
indicated) in the region between λmax = 1100–1315 nm in dichloromethane at 22°C. (b) For comparison, the UV–vis absorption spectra
of the cation radicals 1b+• and 1c+• showing the absence of the NIR band.
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Can. J. Chem. Vol. 77, 1999
Chart 4. Interplanar dihedral angles.
tense NIR absorption bands in the electronic spectra of a
variety of (intermolecular) dimeric aromatic cation radicals.
They further suggested that the efficient orbital overlap be-
tween the parallel aromatic π-systems (in a sandwich-like
structure) is the structural basis for the observed charge-
resonance transitions (i.e., NIR absorption bands). Note that
a sandwich-like arrangement of the dimeric aromatic cation
radicals (of naphthalene and its derivatives) has been veri-
fied by isolation of single crystals and their X-ray crystallo-
graphic characterization (for a review, see ref. 20).
Fig. 5. The NIR bands in the UV–vis absorption spectra of the
cation radicals from cofacial donors 3 and 4 (as indicated) showing
the presence of intense charge-resonance bands in the 1200-nm region.
Angular requirements for the charge-resonance transitions
Intense charge-resonance absorption bands of the stil-
benoid cation radicals 2a–2d are observed despite the fact
that the interplanar (dihedral) angle of Ψ > 70° between the
two aryl π-systems deviates significantly from that of a par-
allel arrangement (see Chart 4). To address the question of
the angular requirements for observation of charge-
resonance transitions in the cation radicals, we extend the
donors to a cyclophane (2,4,2′,4′-tetramethoxy-m-cyclophane,
3 (21)) and a phenylene donor (octamethyl-9,10-dihydro-
9,10-ethanoanthracene 4 (22)) in which the cofacial π-systems
are oriented at different interplanar Ψ angles as established
by X-ray crystallography (see Chart 4). (Also note that the
closest interplanar contacts between two aromatic π-systems
in 2a, 3, and 4 are 3.08, 2.94, and 2.42 Å, which are much
shorter than the sum of van der Waals thickness 2r = 3.4 Å.)
charge-resonance transitions in the UV–vis–NIR absorption
spectra similar to those observed for the dimeric cation radi-
cals. Theoretical analysis and quantitative comparison of the
(NIR) charge-resonance absorption bands with changes in
various structural parameters in a variety of mixed-valence
cation-radical salts are currently under investigation.
Cyclophane 3 and the phenylene donor 4 undergo revers-
ible electrochemical oxidation in dichloromethane at poten-
tials of E0 = 0.80 V and 1.41 V vs. SCE, respectively,
ox
which are lower by 380 and 230 mV than those of the parent
The efficient synthesis of the stilbenoid donors in Chart 1
allows the steric requirement for the cofacial interaction of
aromatic groups to be established. Thus the presence of ortho-
methyl substituents in the class B donors 2a–2c forces the
aromatic rings to lie parallel and orthogonal to the ethylenic
bond (Fig. 1). As a result, the electronic (UV–vis) transitions
in the cation radicals 2a+•–2c+• show strong new absorption
bands in the near-IR region (1100 –1315 nm) arising from
the charge-resonance interaction between cofacial aromatic
rings. However, the energy of such a through-space charge
delocalization is not sufficient to overcome the through-
bond conjugation inherent to stilbene cation radicals, and the
UV–vis spectra of the cation radicals of the class A donors
1a–1d (with no ortho substituents) are singularly bereft of
the near-IR absorption. We believe, however, that 1,2-diaryl
interactions of the type described for stilbenoids 2a–2c in
this study will also pertain to the saturated analogues such as
4,6-dimethyl-1,3-dimethoxybenzene (E0 = 1.18 V vs. SCE)
ox
(23) and hexamethylbenzene (E0 = 1.64 V vs. SCE) (24).
Furthermore, the cofacial donorsox3 and 4 were both readily
oxidized to the corresponding cation radicals by electron ex-
change with MA+• and chloranil – methanesulfonic acid
mixture (vide supra), and the UV–vis spectral analysis of the
yellow-green solutions in both cases showed intense charge-
resonance absorption bands at 1200 and 1180 nm, respec-
tively (see Fig. 5). Such an observation of the NIR absorp-
tion bands in 2a, 3, and 4, with drastically varied interplanar
angles from Ψ = 30°–117°, suggests that minimal orbital
overlap between interacting π-systems is sufficient for
charge-resonance transitions to occur. Thus, a close, but not
necessarily complete, cofacial approach of π-systems in vari-
ous cation radicals (see Chart 4) leads to strong electronic
interactions between the two aryl moieties, and results in
© 1999 NRC Canada

Rathore and Kochi
919
vicinal diarylalkanes, etc. (1–3), in which restricted (single)
bond rotations and through-bond conjugation will be less re-
strictive.
7.14 (d, J = 8.4 Hz, 4H); 13C NMR (CDCl3) δ: 21.10, 26.23,
37.52, 128.28, 128.56, 135.26, 138.23, 139.52; GC–MS m/z,
calcd. for C22H24: 288; found (M+): 288. Anal. calcd. for
C22H24: C 91.61, H 8.39; found: C 91.45, H 8.35.
2,3-Bis(4-methoxyphenyl)bicyclo[2.2.2]oct-2-ene (1c): Yield
94%, mp 72–73°C (ethanol); UV–vis (CH2Cl2) λmax: 237,
291 nm; H NMR (CDCl3) δ: 1.62 (br d, 4H), 1.74 (br d,
4H), 2.94 (s, 2H), 3.79 (s, 6H), 6.77 (d, J = 8.7 Hz, 4H),
7.09 (d, J = 8.7 Hz, 4H); 13C NMR (CDCl3) δ: 26.23, 37.44,
54.97, 113.23, 129.44, 133.63, 138.60, 157.59; GC–MS m/z,
calcd. for C22H24O2: 320; found (M+): 320. Anal. calcd. for
C22H24O2: C 82.46, H 7.55; found: C 82.38, H 7.53.
Materials
1
The synthesis of 2,3-dibromobicyclo[2.2.2]oct-2-ene (25),
2, 4, 2′, 4′-tetramethoxy-[3.3]-m-cyclophane (3) (21), and
1,2,3,4,5,6,7,8-octamethyl-9,10-dihydro-9,10-ethanoanthracene
(4) (22) have been described previously. Bis(triphenylphos-
phine) palladium(II) chloride and triethyloxonium hexachloro-
antimonate (Aldrich) were stored in a Vacuum Atmosphere
HE-493 dry box kept free of oxygen. The cation radicals
MA+• and EA+• were readily obtained as the stable hexa-
chloroantimonate salts in quantitative yields from the reaction
of corresponding neutral hydroquinone ethers MA (9,10-
dimethoxy-1,4:5,8-dimethano-1,2,3,4,5,6,7,8-octahydroanthra-
cene) and EA (9,10-dimethoxy-1,4:5,8-dimethano-1,2,3,4,5,6,7,8-
octahydroanthracene), respectively, with antimony pentachloride
(16) or triethyloxonium hexachloroantimonate (17).
2,3-Bis(2-naphthyl)bicyclo[2.2.2]oct-2-ene (1d): Yield 91%,
mp 195–196°C (ethanol); UV–vis (CH2Cl2) λmax: 270, 315
nm; 1H NMR (CDCl3) δ: 1.73 (d, J = 8.7 Hz, 4H), 1.83 (d, J
= 8.7 Hz, 4H), 3.16 (s, 2H), 7.18–7.78 (m, 14 H); 13C NMR
(CDCl3) δ: 26.48, 38.01, 125.44, 125.81, 126.66, 127.20,
127.59, 127.82, 132.07, 133.60, 138.73, 140.85; GC–MS
m/z, calcd. for C28H24: 360; found (M+): 360. Anal. calcd.
for C28H24: C 93.29, H 6.71; found: C 93.12, H 6.54.
Synthesis of 2,3-diarylbicyclo[2.2.2]oct-2-enes
2,3-Bis(pentamethylphenyl)bicyclo[2.2.2]oct-2-ene (2a): Yield
94%, mp 239–240°C (ethanol); UV–vis (CH2Cl2) λmax: 227,
251 nm; H NMR (CDCl3) δ: 1.77 (br d, J = 7.4 Hz, 4H),
General procedure
1
An approximately 0.4 M solution of pentamethylphenyl-
magnesium bromide was prepared from bromopentamethyl-
benzene (5.0 g, 22 mmol) and excess magnesium turnings
(2.4 g, 100 mmol) in tetrahydrofuran, under an argon atmo-
sphere by refluxing for 3 h. The Grignard solution thus ob-
tained was transferred via a cannula to a Schlenk flask
containing a solution of 2,3-dibromobicyclo[2.2.2]oct-2-ene
(25) (2.66 g, 10 mmol) and a catalytic amount of bis(triphenyl-
phosphine)palladium(II) chloride (0.10 g, 0.15 mmol) in an-
hydrous tetrahydrofuran (20 mL). The resulting yellow
mixture was refluxed for 18 h, cooled to room tempera-
ture, and quenched with saturated aqueous ammonium chlo-
ride (100 mL). The organic layer was separated and the
aqueous phase was further extracted with diethyl ether (3 ×
50 mL). The combined organic extracts were washed with
water, followed by brine solution, and dried over anhydrous
magnesium sulfate. Evaporation of the solvent afforded a
pale yellow residue that was filtered through a short pad of
silica gel with ether:hexanes (1:1) as the eluent. The resul-
tant product was further purified by recrystallization from
ethanol to afford bis(pentamethylphenyl) bicyclooctene 2a
as colorless prisms (3.7 g).
1.91 (br d, J = 7.4 Hz, 4H), 2.14 (s, 12H), 2.18 (s, 6H), 2.19
(s, 12H), 2.63 (br s, 2H); 13C NMR (CDCl3) δ: 16.63, 16.81,
19.83, 27.28, 37.79, 131.96, 132.08, 132.05, 139.45, 142.71;
GC–MS m/z, calcd. for C30H40: 400; found (M+): 400. Anal.
calcd. for C30H40: C 89.94, H 10.06; found: C 89.77, H 10.01.
2,3-Bis(2,4,6-trimethylphenyl)bicyclo-[2.2.2]oct-2-ene (2b):
Yield 94%, mp 134–136°C (ethanol); UV–vis (CH2Cl2) λmax
:
1
225, 258 nm; H NMR (CDCl3) δ: 1.49 (sym m, 8H), 2.16
(s, 12H), 2.20 (s, 6H), 2.64 (s, 2H), 6.74 (s, 4H); 13C NMR
(CDCl3) δ: 20.71, 21.90, 26.84, 36.65, 128.88, 135.20, 136.23,
138.43, 141.67; GC–MS m/z, calcd. for C26H32: 344; found
(M+): 344. Anal. calcd. for C26H32: C 90.64, H 9.36; found:
C 90.49, H 9.19.
2,3-Bis(2,6-dimethyl-4-methoxyphenyl)bicyclo[2.2.2]oct-2-ene
(2c): Yield 91%, mp 129–130°C (ethanol); UV–vis
(CH2Cl2) λmax: 221, 260 nm; 1H NMR (CDCl3) δ: 1.79 (br s,
8H), 2.23 (s, 12H), 2.67 (s, 2H), 3.76 (s, 6H), 6.54 (s, 4H);
13C NMR (CDCl3) δ: 22.41, 27.03, 36.91, 54.98, 113.45,
134.20, 137.98, 141.64, 157.30; GC–MS m/z, calcd. for
C26H32O2: 376; found (M+): 376. Anal. calcd. for C26H32O2:
C 82.94, H 8.57; found: C 82.69, H 8.73.
The characteristic spectral data for the various diarylbi-
cyclooctenes obtained using the above general procedure are
given as follows.
2,3-Bis(1-naphthyl)bicyclo[2.2.2]oct-2-ene (2d), a mixture of
atropoisomers:2 Yield 82%, mp 186–188°C (ethanol); UV–
1
vis (CH2Cl2) λmax: 231, 296 nm; H NMR (CDCl3) δ: 1.75–
2,3-Diphenylbicyclo[2.2.2]oct-2-ene (1a): Yield 96%, mp
126–127°C (ethanol); UV–vis (CH2Cl2) λmax: 228, 280 nm;
1H NMR (CDCl3) δ: 1.68 (sym m, 4H), 1.82 (sym m, 4H),
3.03 (s, 2H), 7.14–7.28 (m, 10H); 13C NMR (CDCl3) δ:
26.20, 37.49, 125.85, 127.83, 124.83, 140.23, 141.02; GC–
MS m/z, calcd. for C20H20: 260; found (M+): 260. Anal.
calcd. for C20H20: C 92.26, H 7.74; found: C 91.98, H 7.78.
2.00 (m) and 2.13 (d, J = 7.2 Hz, 8H), 2.96 and 3.03 (two
singlets, 2H), 7.00–8.12 (m, 14H); 13C NMR (CDCl3) δ: 26.12,
26.64, 27.55, 28.28, 37.49, 38.37, 125.21, 1225.45, 125.53,
125.68, 126.35, 126.46, 126.60, 126.68, 126.78, 128.83, 128.29,
12.56, 132.14, 133.78, 133.84, 140.04, 140.38, 142.23, 143.88;
GC–MS m/z, calcd. for C28H24: 360; found (M+): 360. Anal.
calcd. for C28H24: C 93.29, H 6.71; found: C 93.04, H 6.64.
Dichloromethane (Mallinckrodt analytical reagent) was
repeatedly stirred with fresh aliquots of concentrated sulfuric
acid (~20% by volume) until the acid layer remained color-
less. After separation, it was washed successively with wa-
2,3-Bis(4-methylphenyl)bicyclo[2.2.2]oct-2-ene (1b): Yield
1
92%, oil; UV–vis (CH2Cl2) λmax: 227, 283 nm; H NMR
(CDCl3) δ: 1.70 (d, J = 7.5 Hz, 4H), 1.81 (d, J = 7.5 Hz,
4H), 2.40 (s, 6H), 3.04 (br s, 2H), 7.10 (d, J = 8.4 Hz, 4H),
© 1999 NRC Canada

920
Can. J. Chem. Vol. 77, 1999
ter, aqueous sodium bicarbonate, water, and aqueous sodium
chloride, and dried over anhydrous calcium chloride. The di-
chloromethane was distilled twice from P2O5 under an argon
atmosphere and stored in a Schlenk flask equipped with a
Teflon valve fitted with Viton O-rings. The hexane and tolu-
ene were distilled from P2O5 under an argon atmosphere and
then refluxed over calcium hydride (~12 h). After distillation
from CaH2, the solvents were stored in the Schlenk flasks
under an argon atmosphere.
Fig. 3a). The dark magenta solution was cooled to –50°C in
an Dry Ice – acetone bath, and anhydrous toluene (100 mL)
was added to precipitate the dissolved salt. The dark-red pre-
cipitate was filtered under an argon atmosphere, washed
with hexane (3 × 25 mL), and dried in vacuo. The cation
–
radical 1c+• SbCl6 (vide infra) was obtained in essentially
quantitative yield (550 mg, 0.84 mmol).
–
The purity of the isolated cation radical 1c+• SbCl6 was
determined by iodometric titration as follows. A solution of
–
1c+• SbCl6 (65.5 mg, 0.01 M) in dichloromethane was added
Instrumentation
to a dichloromethane solution containing excess tetra-n-
butylammonium iodide (1 mmol, 0.1 M) at 22°C, under an
argon atmosphere, to afford a dark-brown solution. The mix-
ture was stirred for 5 min and was titrated (with rapid stir-
ring) by slow addition of a standard aqueous sodium
thiosulfate solution (0.005 M) in the presence of a starch so-
lution as internal indicator. Based on the amount of
thiosulfate solution consumed (57.4 mL), purity of the cat-
ion radical was determined to be >97%. With the same pro-
cedure, the crystalline cation radical salts of 2a and 2c were
also isolated in 76 and 81% yields, respectively.
The UV–vis absorption spectra were recorded on HP 8453
diode array and Varian CARY 5 (UV–vis–NIR) spectrome-
ters. The 1H and 13C NMR spectra were obtained on a
General Electric QE-300 FT NMR spectrometer. Electro-
chemical apparatus and the procedures for the determination
of oxidation potentials and for the preparation of cation radi-
cals have been described elsewhere (9).
Oxidation of various stilbenoid donors
General procedure
A stock solution of EA+• (λmax = 486 nm, log ε486 = 3.66
M–1 cm–1) was obtained by dissolving a known quantity of
X-ray crystallography
–
EA+•SbCl6 (4.25 mg, 6.92 × 10–5 mmol) in anhydrous di-
The intensity data for all the compounds were collected
with the aid of a Siemens SMART diffractometer equipped
with a CCD detector using Mo-Kα radiation (λ = 0.71073
Å), at –150°C unless otherwise specified. The structures
were solved by direct methods (26) and refined by full-
matrix least-squares procedure with IBM Pentium and SGI
Indigo computers. The details of the X-ray structure of the
various compounds have been deposited.7
chloromethane (50 mL) at 22°C and under an argon atmo-
sphere. A 5 mL aliquot of the red-orange solution was
transferred to a 1 cm quartz cuvette equipped with a Schlenk
adapter (under an argon atmosphere). A solution of the stil-
benoid donor 2c (1 equiv.) was added at 25°C. The reaction
mixture immediately turned bright green and the UV–vis
spectral analysis confirmed the formation of 2c+• (λ max = 428,
700, and 1315 nm, log ε1315 = 3.98 M–1 cm–1). In a similar man-
ner, various stilbenoid donors in Chart 1 were quantitatively
oxidized to corresponding cation radicals, and the UV–vis
absorption maxima are compiled in Table 3. Various donors
with E0ox<1.2 V vs. SCE were readily oxidized with MA+•
(λmax = 518 nm, log ε518 = 3.86 M–1 cm–1, whereas donors
2,3-Diphenylbicyclo[2.2.2]oct-2-ene (1a) (C20H20)
An X-ray quality crystal (0.20 × 0.20 × 0.20 mm) of diphenyl-
bicyclooctene was obtained from an ethanol–dichloromethane
mixture at –30°C. MW = 260.36, orthorhombic, space group
P212121, a = 6.0645(1), b = 12.1491(3), and c = 19.4396(5)
Å, Dc = 1.207 Mg m–3, V = 1432.28(6) Å3, Z = 4. The total
number of reflections measured were 6492, of which 6492
reflections were symmetrically non-equivalent. Final residu-
als were R1 = 0.0477 and wR2 = 0.1017 for 6492 reflections
with I > 2σ (I).
with E0 >1.2 V were oxidized with EA+• (λmax = 518 nm,
ox
log ε 486 = 3.66 M–1 cm–1). Oxidation procedures with chloranil
– methanesulfonic acid (method B) (15) and triethyloxonium
hexachloroantimonate (method C) (17) have been described
in detail.
Preparative isolation of stilbenoid cation radical salts
2,3-Bis(pentamethylphenyl)bicyclo[2.2.2]oct-2-ene (2a)
(C30H40 )
–
using Et3O+ SbCl6
A 200 mL flask equipped with a Schlenk adapter was
charged with triethyloxonium hexachloroantimonate (657 mg,
1.5 mmol), and a solution of 1c (320 mg, 1 mmol) in anhy-
drous dichloromethane (25 mL) was added under an argon
atmosphere at –20°C. The heterogeneous mixture immedi-
ately took on magenta coloration, which intensified with
time. The dark-red mixture was stirred for 1 h to yield a
magenta-red solution of 1c+• (λmax (nm) = 546, 880; see
An X-ray quality crystal (0.3 × 0.2 × 0.1 mm) of 2a was
obtained from an ethanol–dichloromethane mixture at –30°C.
MW = 400.62, monoclinic, space group P21/n, a = 8.6608(7),
b = 13.223(1), and c = 20.401(2) Å, β = 91.519(2)°, Dc =
1.139 Mg m–3, V = 2335.6(3) Å3, Z = 4. The total number of
reflections measured were 8416, of which 3237 reflections
were symmetrically non-equivalent. Final residuals were R1 =
0.0730 and wR2 = 0.1259 for 3073 reflections with I > 2σ(I).
7 Supplementary material, including tables of crystal data and structure refinement, atomic coordinates, isotropic and anisotropic displacement
parameters, and bond lengths and angles for 2,3-diphenylbicyclo[2.2.2]oct-2-ene (1a), bis(pentamethylphenyl)bicyclooct-2-ene (2a),
bis(2,4,6-trimethyl-phenyl)bicyclooct-2-ene (2b), and 2,4,2′,4′-tetramethoxy-m-cyclophane (3)·chloranil molecular complex may be
purchased from: The Depository of Unpublished Data, Document Delivery, CISTI, National Research Council Canada, Ottawa, Canada,
K1A 0S2. With the exception of the anisotropic displacement parameters, this material has also been deposited with the Cambridge Crystal-
lographic Data Centre and can be obtained on request from: The Director, Cambridge Crystallographic Data Centre, University Chemical
Laboratory, 12 Union Road, Cambridge, CB2 1EZ, U.K.
© 1999 NRC Canada

Rathore and Kochi
921
297 (1978); (c) Y.-S. Chen, J.W. Kampf, and R.C. Lawton.
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2,3-Bis(2,4,6-trimethylphenyl)bicyclo-[2.2.2]oct-2-ene (2b)
(C26H32 )
An X-ray quality crystal (0.5 × 0.4 × 0.2 mm) of 2b was
obtained from an ethanol–dichloromethane mixture at –30°C.
MW = 344.52, orthorhombic, space group Pbcn, a =
21.6486(1), b = 8.7822(1), and c = 21.7995(2) Å, Dc = 1.104
Mg m–3, V = 4144.57(6) Å3, Z = 8. The total number of reflec-
tions measured were 50 440, of which 9646 reflections were
symmetrically non-equivalent. Final residuals were R1 = 0.0584
and wR2 = 0.1342 for 9638 reflections with I > 2σ (I).
2,4,2′,4′-Tetramethoxy-[3.3]-m-cyclophane (3)–chloranil
molecular complex (C22H28O4·C6Cl4O2 ).
X-ray quality crystals (0.35 × 0.30 × 0.20 mm) of the
charge-transfer complex of 3·chloranil were obtained by
slow evaporation of a equimolar solution of 3 and chloranil
in dichloromethane at 25°C. MW = 602.30, orthorhombic,
space group Pnma, a = 13.464(3), b = 20.144(4), and c =
10.033(2) Å, Dc = 1.470 Mg m–3, V = 2721.1(9) Å3, Z = 4.
The total number of reflections measured were 6618, of
which 2161 reflections were symmetrically non-equivalent.
Final residuals were R1 = 0.0388 and wR2 = 0.0948 for
2153 reflections with I > 2σ (I). (Note that the molecular
structure of the cyclophane moiety in the 3·chloranil com-
plex is quite similar to that previously observed for the
uncomplexed cyclophane (21).)
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Org. Chem. 63, 5847 (1998).
Acknowledgments
We thank S.V. Lindeman and J. Hecht for the X-ray
crystallography and the National Science Foundation and
Robert A. Welch Foundation for financial support.
18. T. Shida. Electronic absorption spectra of radical ions. Elsevier,
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© 1999 NRC Canada