Soluble cycloannulated tetroxa[8]circulane derivatives: Synthesis, optical and electrochemical properties, and generation of their robust cation-radical salts

Article abstract of DOI:10.1016/j.tetlet.2004.05.019

Syntheses of soluble bicycloalkane-annulated tetraoxa[8]circulane derivatives via Lewis acid-catalyzed tetramerization of readily available cycloannulated benzoquinons is described. The ready availability of the soluble circulane derivatives allows the evaluation of their optical and electrochemical properties. These circulanes form stable (isolable) cation-radical salts upon 1-electron oxidation using antimony pentachloride and various aromatic oxidants.

Full text of DOI:10.1016/j.tetlet.2004.05.019

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 5267–5270
Soluble cycloannulated tetroxa[8]circulane derivatives: synthesis,
optical and electrochemical properties, and generation of their
robust cation–radical salts
Rajendra Rathore* and Sameh H. Abdelwahed
Department of Chemistry, Marquette University, PO Box 1881, Milwaukee, WI53201-1881, USA
Received 2 May 2004; accepted 5 May 2004
Abstract—Syntheses ofsoluble bicycloalkane-annulated tetraoxa[8]circulane derivatives via Lewis acid-catalyzed tetramerization of
readily available cycloannulated benzoquinons is described. The ready availability ofthe soluble circulane derivatives allows the
evaluation oftheir optical and electrochemical properties. These circulanes ofrm stable (isolable) cation–radical salts upon
1-electron oxidation using antimony pentachloride and various aromatic oxidants.
ꢀ 2004 Elsevier Ltd. All rights reserved.
Polycyclic aromatic hydrocarbons (PAH) have found
widespread applications as building blocks for the
preparation ofmaterials that can hold potential usage as
conductors, ferromagnets, wires, liquid crystalline
materials, and in other electronic and optoelectronic
devices.1 The saddle-shaped tetraphenylene derivatives
(see Structure A) are extensively studied for their novel
conformational properties as well as for their usages as
clatherate precursors.2 Interestingly, these nonplanar
polyaromatic hydrocarbons can be transformed to the
corresponding planar structures (see Structure B) by
introduction ofthe uf ran linkages, that is,
PAH could not be readily exploited owing to its insol-
ubility in most common organic solvents.3 The lack of
the solubility ofthe parent circulane is attributed to its
planar structure, which allows it to pack in graphite-like
stacks.
Mullen and co-workers4 have recently demonstrated by
synthesizing a variety ofsoluble (planar) graphitic
fragments by substitution of free aromatic p-positions
with bulky t-butyl groups to prevent graphite-like
stacking. Moreover, Rathore et al.5 and others6 have
recently noted that the structural modification ofsimple
aromatic as well as polycyclic aromatic hydrocarbons
with bulky bicycloalkane frameworks leads to the
derivatives, which allow ready isolation ofthe reactive
intermediates, such as the Wheland intermediates and
cation–radical salts for spectral and structural charac-
terization by X-ray crystallography.
Herein, we report the preparation oftetraoxa[8]circu-
lane derivatives, annulated by bicyclo[2.2.2]octane and
bicyclo[2.2.2]heptane frameworks, in excellent yields,
from the Lewis-acid catalyzed tetramerization of read-
ily-available cycloannulated quinones7 4a and 4b (see
Scheme 1 below). These soluble bicyclo-annulated cir-
culane derivatives allow their optical and electrochemi-
cal properties to be evaluated for the first time as well as
generation oftheir highly robust cation–radical salts
upon 1-electron oxidation using a variety ofoxidants as
follows.
Such a planar tetraphenylene derivative (e.g. the parent
tetraoxa[8]circulane, structure B) has been known since
the early sixties, however, the potential ofthis planar
* Corresponding author. Tel.: +1-4142882076; fax: +1-4142887066;
e-mail: rajendra.rathore@marquette.edu
0040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.05.019

5268
R. Rathore, S. H. Abdelwahed / Tetrahedron Letters 45 (2004) 5267–5270
800
600
400
200
0
OAc
O
O
O
O
5a
80000
60000
40000
20000
0
5b
Ac2O
Py
5b
EtOH
X
X
X
5a
OAc
2
1
ε
H2/Pd-C
EtOH
a, X = -CH2-
b, X = -CH2-CH2-
O
O
OAc
OAc
250
300
350
400
450
250
350
450
550
650
1. HCl/EtOH
2. NO2/ O2
X
X
(a)
(b)
Wavelength [nm]
Wavelength [nm]
Figure 1. (a) The electronic absorption spectra of 5a and 5b in di-
chloromethane (left). (b) The excitation and emission spectra of 5a and
5b (as indicated) in toluene (right).
4
3
BF3.Et2O/ CH2Cl2
reflux/ 12 h
annulated 5b as well as the larger extinction coefficient
at the maximum of 5a (kmax ¼ 268, 279, 369, 382 nm,
e382 ¼ 73; 200 MÀ1 cmÀ1) as compared to 5b (kmax ¼ 263,
274, 359, 377 nm, e377 ¼ 62; 300 MÀ1 cmÀ1) can be
attributed to the smaller ring size ofthe bicycloalkane
frameworks in 5a.12 The structured emission spectra of
5a (kmax ¼ 507, 541 nm) and 5b (kmax ¼ 482, 530 nm)
recorded in benzene were ofcomparable intensity and
they both show well-defined vibronic bands Dm ꢀ
1300 cmÀ1 (see Fig. 1b). Moreover, keeping with the
rigid structures of 5a and 5b, their excitation spectra
were identical to their corresponding absorption spectra
(compare Fig. 1a and b).
O
O
O
O
O
O
O
O
5b
5a
a mixture of isomers
Scheme 1.
As shown in Scheme 1, the bicycloalkane-annulated
quinones 4a and 4b were obtained by Diels–Alder
cycloaddition of p-benzoquinone with cyclopentadiene
(a-series) and 1,3-cyclohexadiene (b-series), respec-
tively.8 The cycloadducts (1a–b) were aromatized in the
presence ofacetic anhydride and pyridine and the
resulting aromatic diesters (2a–b) were hydrogenated
using catalytic amounts ofpalladium–carbon in ethyl
acetate. The reduced diesters (3a–b) were then hydro-
lyzed in refluxing ethanol/hydrochloric acid mixture to
the corresponding hydroquinones, which were in turn
oxidized to the corresponding quinones (4a–b) using
catalytic amounts ofnitrogen dioxide in presence ofair
in quantitative yields.7 The circulane derivatives 5a and
5b were conveniently obtained by tetramerization of
cycloannulated benzoquinones 4a and 4b, respectively,
in refluxing dichloromethane in the presence ofboron-
trifluoride–etherate complex as a Lewis acid catalyst, in
The electron donor strength of 5a and 5b were evaluated
by electrochemical oxidation at a platinum electrode
as a 2 · 10À3 M solution in chloroform containing
0.2 M tetra-n-butylammonium hexafluorophosphate
(TBAPF6) as the supporting electrolyte. The cyclic vol-
tammogram ofboth 5a and 5b in Figure 2 showed two
1
70% and 76% yields, respectively. The simplicity of H
and 13C NMR spectra together with FAB mass spec-
trometric analysis confirmed the identity ofthe bicyclo-
octane-annulated circulane 5b as a single compound.9
However, both H and 13C NMR data10 indicated that
1
bicyclohepatane-annulated 5a contained an inseparable
mixture ofisomers in which the methanobridges were up
and down in various combinations.11 Repeated attempts
to separate the isomeric mixture of 5a were unsuccessful
and thus it was used as such.
The electronic absorption spectra ofthe yellow 5b and
the isomeric mixture of 5a, obtained as dichloromethane
solutions, are shown in Fig. 1a. The fine-structure in the
UV–vis absorption spectra of 5a and 5b is typical to that
observed in various planar polyaromatic hydrocarbons.
Moreover, the slight red shift of the absorption maxima
in the spectrum ofbicycloheptane-annulated 5a (as an
isomeric mixture) as compared to the bicyclooctane-
Figure 2. Cyclic voltammograms of2 mM 5a and 5b (as indicated) in
dichloromethane containing 0.2 M TBAPF6 at
200 mV sÀ1 and at 22 ꢁC.
a scan rate of

R. Rathore, S. H. Abdelwahed / Tetrahedron Letters 45 (2004) 5267–5270
5269
chemically reversible oxidation waves at a potential of
1.09, 1.38, and 1.10, 1.49 V versus SCE, respectively. A
quantitative evaluation ofthe CV peak currents of
various waves in 5a and 5b (with added ferrocene as an
internal standard) confirmed that all the waves corre-
sponded to the removal ofa single electron in their cyclic
voltammograms. The similarity ofthe oxidation poten-
tials for both 5a and 5b suggests that the redox prop-
erties are not significantly influenced by the size of
bicycloalkane bridges.13
1 equiv of 5a; and the resulting highly-structured
absorption spectrum of 5aþÅ [kmax ¼ 493, 531, 593, 628,
and a broad band extending beyond 1100 nm, e531
¼
10; 800 Æ 500 MÀ1 cmÀ1] remained unchanged upon
further addition of neutral 5a (i.e. Eq. 1).
NAPþÅ þ 5a ! 5aþÅ þ NAP
ð1Þ
A similar redox titration of NAPþÅ cation radical with
neutral 5b yielded 5bþÅ [kmax ¼ 492, 532, 584, 619, and
The reversibility ofthe cyclic voltammograms of 5a and
5b encouraged us to examine the stability ofthe cation
radicals derived from these cycloannulated-circulane
derivatives by 1-electron oxidation as follows. In order
to examine the spectral characteristics ofthe cation
radicals of 5a and 5b, they were generated using two
stable organic oxidants MAþÅ (kmax ¼ 518 nm,
a broad band extending beyond 1100 nm, e532
¼
11; 000 Æ 400 MÀ1 cmÀ1] quantitatively. [It is notewor-
thy that the absorption spectrum ofcation radicals 5aþÅ
and 5bþÅ (see Fig. 3C) obtained above were identical
to that obtained by an oxidation of 5a and 5b using
either MAþÅ SbClÀ6 or an equivalent electrochemical
method.15]
e518 ¼ 7300 MÀ1 cmÀ1 5a
)
and NAPþÅ (kmax ¼ 672 nm,
14
e672 ¼ 9300 MÀ1 cmÀ1
)
potentials that differ by 230 mV (Fig. 3).
with reversible reduction
It is noteworthy that the solutions 5aþÅ and 5bþÅ ob-
tained above are highly persistent at room temperature
and did not show any decomposition during a 24 h
period at 22 ꢁC. The high stability ofthese cation radical
salts allowed their isolation as microcrystalline powders
by a treatment ofa solution of 5a or 5b with SbCl5 in
anhydrous dichloromethane at )20 ꢁC followed by a
precipitation using anhydrous ether, in nearly quanti-
tative yield, according to the stoichiometry in Eq. 2.15
O
.
+
.
+
O
MA+
.
.
NAP+
Eroed
1.11
1.34
(V vs. SCE)
2 5b þ 3 SbCl5 ! 2½5bþÅSbCl6Àꢁ þ SbCl3
ð2Þ
Thus, Figure 3A shows the spectral changes attendant
upon the reduction of1.6 · 10À4 M NAPþÅ SbCl6À
(kmax ¼ 672 nm) by an incremental addition of
2.7 · 10À3 M 5a to its cation radical 5aþÅ in dichloro-
methane at 22 ꢁC. The presence ofwell-defined isosbestic
points at kmax ¼ 558, 588, and 714 nm in Figure 3A
established the clean electron transfer from 5a to NAPþÅ.
Furthermore, a plot ofthe depletion of NAPþÅ (i.e.
decrease ofthe absorbance at 672 nm) and formation of
5aþÅ (i.e. increase in the absorbance at 532 nm) against
the increments ofadded 5a (Fig. 3B), established that
NAPþÅ was completely consumed after the addition of
The purity ofthe hexachloroantimonate salts of 5aþÅ
and 5bþÅ were determined by iodometric titrations15 and
was found to be of greater than 99% purity. The identity
ofcation radicals 5aþÅ and 5bþÅ was further confirmed by
the oxidation ofa solution ofcycloannulated cyclo-
octatetraene16 (COT, Ered ¼ 0:8 V vs SCE) with 1 equiv
of[ 5aþÅ SbCl6À] or [5bþÅ SbClÀ6 ] in dichloromethane to a
emerald green cation radical COTþÅ (kmax ¼ 442,
16
745 nm, e745 ¼ 4600 MÀ1 cmÀ1
) in quantitative yield, as
determined by the UV–vis spectral analysis (Eq. 3).
Figure 3. (A) Spectral changes attendant upon the reduction of1.6 · 10À4 M naphthalene cation radical NAPþÅ (thick line) by incremental addition of
2.7 · 10À3 M 5a to its cation radical 5aþÅ in dichloromethane at 22 ꢁC. (B) A plot ofdepletion ofabsorbance (data from panel A) of NAPþÅ (circles,
monitored at 672 nm) and an increase ofthe absorbance of 5aþÅ (diamonds, monitored at 532 nm) against the equivalent ofadded 5a showed a
complete consumption of NAPþÅ after addition of 1 equiv of 5a. (C) UV–vis absorption spectra ofcation radicals 5aþÅ (thick line) and 5bþÅ (thin line)
in dichloromethane at 22 ꢁC.

5270
R. Rathore, S. H. Abdelwahed / Tetrahedron Letters 45 (2004) 5267–5270
Le Magueres, P.; Lindeman, S. V.; Kochi, J. K. Angew.
Chem., Int. Ed. 2000, 39, 809, and references cited therein.
5a+.
+
+
.
+
5a
€
3. (a) Erdtman, H.; Hogberg, H.-E. Tetrahedron Lett. 1970,
38, 3389; (b) Hogberg, H.-E. Acta. Chem. Scand, 1972, 26,
ð3Þ
€
.
COT+
309; Also see: (c) Eskildsen, J.; Reenberg, T.; Christensen,
J. B. Eur. J. Org. Chem. 2000, 1637.
COT
In a similar vein, a variety ofaromatic and olefinic
donors such as octamethylbiphenylene, octamethylan-
thracene, and tetraanisylethylene were quantitatively
oxidized to their brightly colored cation radicals14;16
using dichloromethane solutions ofeither [ 5aþÅ SbCl6À]
or [5bþÅ SbClÀ6 ], according to the stoichiometry in Eq. 3.
[Note that our repeated attempts to obtain the single
crystals ofthe cation radical salt ofbicyclooctane-
annulated 5b thus far have been unsuccessful.]
€
4. Reviews: (a) Berresheim, A. J.; Muller, M.; Mullen, K.
Chem. Rev. 1999, 99, 1747; (b) Watson, M. D.; Fech-
€
€
tenkotter, A.; Mullen, K. Chem. Rev. 2001, 101, 1267, and
references cited therein.
€
5. (a) Rathore, R.; Burns, C.L. Org. Synth., in press; (b)
Rathore, R.; Loyd, S. H.; Kochi, J. K. J. Am. Chem. Soc.
1994, 116, 8414; (c) Rathore, R.; Kochi, J. K. J. Org.
Chem. 1995, 60, 4399; Also see: (d) Rathore, R.; Kochi, J.
K. Adv. Phys. Org. Chem. 2000, 35, 193, and references
cited therein.
6. (a) Komatsu, K.; Aonuma, S.; Jinbu, Y.; Tsuji, R.;
Hirosawa, C.; Takeuchi, K. J. Org. Chem. 1991, 56, 195;
For a review, see: (b) Komatsu, K. Bull. Chem. Soc. Jpn.
2001, 74, 407.
7. (a) Rathore, R.; Bosch, E.; Kochi, J. K. Tetrahedron Lett.
1994, 35, 1335; (b) Bosch, E.; Rathore, R.; Kochi, J. K.
J. Org. Chem. 1994, 59, 2529.
8. (a) Oda, M.; Kawase, T.; Okada, T.; Enomoto, T. Org.
Synth. CV IX, 186; (b) Schmid, G. H.; Rabai, J. Synthesis
1988, 332.
9. Compound 5b: Yellow solid, mp >300 ꢁC; 1H NMR
(CDCl3) d 1.67 (d, 16H), 2.03 (d, 16H), 3.96 (s, 8H); 13C
NMR d 26.93, 28.72, 114.51, 126.60, 147.95. FAB Mass:
m=z 680.8 (Mþ), 680.8 calcd for C48H40O4. Anal. Calcd for
C48H40O4: C, 84.68; H, 5.92. Found: C, 84.52; H, 5.78.
10. Compound 5a: Yellow solid, mp >300 ꢁC; 1H NMR
(CDCl3) d 1.42 (m, 8H), 1.79 (m, 4H), 2.02 (m, 4H), 2.13
(m. 8H), 4.10 (br s, 8H); 13C NMR d 27.84, 41.52, 50.57,
115. 68, 130.04, 146.90. (Note that in the 13C spectrum of
5a all signals appear to be singlets, however they are
consisted ofmultiple peaks as can be seen upon expansion
ofthe spectrum. 11) FAB Mass: m/z 624.7 (Mþ), 624.7
calcd for C44H32O4. Anal. Calcd for C44H32O4: C, 84.59;
H, 5.16. Found: C, 84.32; H, 5.04.
In summary, we have developed an efficient synthesis of
soluble circulane derivatives from readily available
precursors and have shown that their stable cation–
radical salts can be obtained in pure form. We are
presently attempting the crystallization of 5bþÅ with
different donors (to obtain mixed-valence salts) as well
as different counter anions to obtain single crystals for
X-ray crystallography and for the study of their solid-
state properties.
Acknowledgements
We thank Petroleum Research Fund (AC12345),
National Science Foundation (Career Award), and
Marquette University for financial support; and Dr. F.
A. Khan (State University ofWest Georgia, Carrolton)
for mass spectrometry.
11. Note that the four different isomers of 5a are possible.
12. The annulation ofbenzene by bicycloalkane rings induces
a significant bond alternation, for example, see: Baldridge,
K. K.; Siegel, J. S. J. Am. Chem. Soc. 1992, 114, 9583.
13. It is noteworthy that the electron-donor properties of
various tris-annulated benzenes were singularly constant
within 30 mV irrespective ofthe ring size ofthe
bicycloalkane annulation; see: Rathore, R.; Lindeman,
S. V.; Kochi, J. K. J. Am. Chem. Soc. 1998, 120, 6012.
14. Rathore, R.; Burns, C. L.; Deselnicu, M. I. Org. Lett.
2001, 3, 2887.
15. For a general procedure for the isolation of organic
cation–radical salts, see: Rathore, R.; Burns, C. L. J. Org.
Chem. 2003, 68, 4071.
16. Rathore, R.; Lindeman, S. V.; Kumar, A. S.; Kochi, J. K.
J. Am. Chem. Soc. 1998, 120, 6931, and references cited
therein.
References and notes
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€
1993, 93, 449; (e) Baumgarten, M.; Mullen, K. Top. Curr.
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N. C. Top. Curr. Chem. 1987, 140, 141; (c) Rajca, A.;
Safronov, A.; Rajca, S.; Ross, C. R., II; Stezowski, J. J. J.
Am. Chem. Soc. 1996, 118, 7272; Also see: (d) Rathore, R.;