Synthesis, structure, and redox properties of a quinone-bridged calix[6]arene

Article abstract of DOI:10.1016/S0040-4039(99)02171-1

The bridged calix[6]arene 1, where a 1,4-benzoquinone moiety is covalently anchored in the cavity of the calix[6]arene macrocycle with a rigid cone conformation, was synthesized and its structure was determined by X-ray analysis. The cyclic voltammogram showed that the reduction potential of 1 is negatively shifted in comparison with the reference compounds without the calixarene macrocycle, whose shift can be explained in terms of the interaction between the quinone moiety and the oxygen atoms at the lower rim of the calixarene framework. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(99)02171-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 897–901
Synthesis, structure, and redox properties of a quinone-bridged
calix[6]arene
Shigehisa Akine, Kei Goto ∗ and Takayuki Kawashima ∗
Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033,
Japan
Received 12 October 1999; revised 10 November 1999; accepted 12 November 1999
Abstract
The bridged calix[6]arene 1, where a 1,4-benzoquinone moiety is covalently anchored in the cavity of the
calix[6]arene macrocycle with a rigid cone conformation, was synthesized and its structure was determined by X-
ray analysis. The cyclic voltammogram showed that the reduction potential of 1 is negatively shifted in comparison
with the reference compounds without the calixarene macrocycle, whose shift can be explained in terms of the
interaction between the quinone moiety and the oxygen atoms at the lower rim of the calixarene framework. © 2000
Elsevier Science Ltd. All rights reserved.
Keywords: benzoquinones; calixarenes; electrochemistry; X-ray crystal structures.
The inner space of macrocyclic molecules provides a unique microenvironment to the included guest
species.1,2 Recently, the electrochemical behavior of redox-active species such as quinones included in
the cavity of calixarenes and carcerands has been reported.3,4 In these complexes based on the non-
covalent interaction, however, only the averaged properties of the complexed and dissociated guest
species are observable. Furthermore, the geometry of the guest molecule in the cavity of the macrocycle
is often ambiguous. If a redox-active species is covalently anchored in the cavity with well-defined
geometry, it is expected that the effect of the surrounding macrocycle on the properties of the species
can be elucidated much more definitely. Although a large number of macrocyclic quinones such as
calixquinones have been reported so far,5 there has been no example of a compound bearing a quinone
moiety covalently fixed in the cavity. We have been studying applications of calix[6]arenes bridged by
a functionalized m-xylenyl unit,6 whose cavity is expected to serve also as a reaction field for a redox-
active moiety covalently anchored in it. In this communication, we report the synthesis, structure, and
redox properties of the quinone-bridged calix[6]arene 1 with a rigid cone conformation.
∗
Corresponding authors.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02171-1
tetl 16099

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Synthesis of 1 was effected by the route shown in Scheme 1, where a p-bromoanisole derivative was
used as a precursor of the 1,4-benzoquinone moiety. The bridged compound 4 was prepared by the
reaction of p-tert-butylcalix[6]arene (2) and tribromide 3 in 73% yield. Benzylation of the four hydroxyl
groups of 4 afforded the tetrasubstituted product 5 in 93% yield, whose spectral features are essentially
the same as those of other bridged calix[6]arenes fixed in a cone conformation which we have reported so
far.6b–e The bromide functionality of 5 was converted to a hydroxyl group via lithiation followed by the
reaction with trimethyl borate and treatment with an alkaline solution of hydrogen peroxide. Oxidation
of the p-methoxyphenol derivative 6 by iron(III) chloride afforded the target compound 1 as pale yellow
crystals. Compound 1 was also found to adopt a cone conformation,7 indicating that no conformational
change occurred during the chemical modification of the bridging unit.
Scheme 1.
In the infrared spectrum of 1, the C_O absorption was observed at 1653 cm−1, which is almost the
same as those of the reference compounds 7 (1657 cm−1) and 8 (1653 cm−1) without the calixarene
framework. On the other hand, in the 1H NMR spectrum, a strong upfield shift of H1 and H2 was observed
for 1 in comparison with the phenoxymethyl derivative 7 (Table 1), indicating that the quinone moiety of
1 is magnetically shielded by the surrounding aromatic rings of the calixarene framework. This effect was
also found in the 13C NMR spectrum of 1, which shows the upfield shift of C1 and C3. In the electronic
absorption spectra, it was difficult to determine the maximum absorption wavelength and the absorption

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coefficient of the quinone moiety of 1 due to the strong absorption of ten benzene rings of the bridged
calix[6]arene framework.
Table 1
1H and 13C NMR chemical shifts (δ)a of quinones 1, 7 and 8
Fig. 1. Cyclic voltammograms of 5×10−4 M 1 and 7 in dichloromethane containing 0.1 M tetrabutylammonium perchlorate on
a glassy carbon electrode. Reference electrode, Ag/Ag+, scan rate, 100 mV·s−1
The cyclic voltammograms of quinones 1 and 7 are shown in Fig. 1. For the quinone-bridged
calix[6]arene 1, two quasi-reversible waves were observed, the second one being considerably broadened.
These two waves are considered to correspond to the formation of a monoanion radical and a dianion of
the quinone moiety, respectively, similarly to other benzoquinone derivatives including 7 and 8. The
reduction/oxidation potentials of these quinones are summarized in Table 2. The reduction potential of
1 is shifted to a more negative region than that of the methyl derivative 8, indicating that the quinone-
bridged calix[6]arene 1 is more difficult to be reduced. Electrons are known to be one of the particles for
which the tunneling is very important and, therefore, the steric factors of the macrocyclic framework
consisting of six benzene rings are unlikely to interfere with the access of electrons to the central
quinone moiety. In fact, sterically hindered 2,5- and 2,6-di-tert-butyl-1,4-benzoquinone are known to
have reduction potentials not so different from those of the corresponding methyl derivatives.8 Therefore,
it can be reasonably concluded that the calixarene framework electronically affects the redox properties
of the quinone moiety.

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Table 2
Oxidation/reduction potentials (E1/2/V)a of 1, 7 and 8
On considering the fact that the potential of the phenoxymethyl substituted quinone 7 is more positive
than that of the methyl derivative 8, the two CH2OAr substituents at the 2,6-positions of the quinone
moiety of 1 would seem not to be responsible for the observed negative shift. Instead, the through-space
interaction between the quinone moiety and the calixarene framework is considered to be more important.
For clarification of the three-dimensional arrangements of the quinone moiety and the calixarene, the
crystal structure of 1 was determined by X-ray analysis (Fig. 2).9 It was revealed that the quinone moiety
is located inside the cone-shaped calixarene framework and surrounded by the benzyloxy groups at the
lower rim. The two carbonyl groups of the quinone moiety are not so close to the six aromatic rings
of the calixarene macrocycle and the benzyl groups at the lower rim. On the other hand, there are
significant non-bonded contacts observed between the olefinic carbon atoms of the quinone moiety and
the oxygen atoms of the calixarene lower-rim (C2–O4: 3.172(3) Å, C6–O7: 3.218(3) Å). This interaction
is considered to raise the LUMO level of the quinone, which results in the more negative reduction
potential of 1.
Fig. 2. X-Ray structure of 1. Solvent molecules and hydrogen atoms are omitted for clarity
Acknowledgements
We thank Prof. H. Nishihara and Ms. M. Yamada of the University of Tokyo for the measurement
of the cyclic voltammograms. This work was partly supported by the Sumitomo Foundation (K.G.). We
thank Tosoh Akzo Co. Ltd. for gifts of alkyllithiums.

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References
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Oxford, 1996; p. 103. (b) Gutsche, C. D. Calixarenes in Monographs in Supramolecular Chemistry; Stoddart, J. F., Ed.; The
Royal Society of Chemistry: Cambridge, 1989. (c) Calixarenes: a Versatile Class of Macrocyclic Compounds; Vicens, J.;
Böhmer, V., Eds.; Kluwer Academic Publishers: Dordrecht, 1991. (d) Gutsche, C. D. Calixarenes Revisited in Monographs
in Supramolecular Chemistry; Stoddart, J. F., Ed.; The Royal Society of Chemistry: Cambridge, 1998.
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E. Chem. Commun. 1998, 1457.
5. For examples, see: (a) Morita, Y.; Agawa, T.; Kai, Y.; Kanehisa, N.; Kasai, N.; Nomura, E.; Taniguchi, H. Chem. Lett. 1989,
1349. (b) Suga, K.; Fujihara, M.; Morita, Y.; Agawa, T. J. Chem. Soc., Faraday Trans. 1991, 87, 1575. (c) Morita, Y.; Agawa,
T.; Nomura, E.; Taniguchi, H. J. Org. Chem. 1992, 57, 3658. (d) Casnati, A.; Comelli, E.; Fabbi, M.; Bocchi, V.; Mori, G.;
Ugozzoli, F.; Lanfredi, A. M. M.; Pochini, A.; Ungaro, R. Recl. Trav. Chim. Pays-Bas 1993, 112, 384.
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Macrocycl. Chem. in press.
7. Compound 1: Pale yellow crystals; mp 187–198°C (dec). 1H NMR (500 MHz, CDCl3) δ 1.00 (s, 36H), 1.46 (s, 18H), 2.47
(s, 4H), 3.23 (d, J=16.5 Hz, 2H), 3.31 (d, J=14.7 Hz, 4H), 4.48 (d, J=14.7 Hz, 4H), 4.50 (d, J=16.5 Hz, 2H), 4.63 (d, J=11.4
Hz, 4H), 4.68 (d, J=11.4 Hz, 4H), 6.36 (s, 2H), 6.86 (s, 4H), 7.02 (s, 4H), 7.17–7.25 (m, 20H), 7.38 (s, 4H); 13C NMR (125
MHz, CDCl3) δ 26.78 (t), 29.80 (t), 31.41 (q), 31.75 (q), 34.19 (s), 34.39 (s), 67.38 (t), 75.35 (t), 125.19 (d), 125.43 (d),
127.44 (d), 127.72 (d), 127.76 (d), 127.79 (d), 128.38 (d), 130.99 (s), 133.90 (s), 134.45 (s), 137.31 (s), 143.58 (s), 145.41 (s),
146.16 (s), 152.71 (s), 153.54 (s), 182.99 (s), 187.28 (s); IR (KBr) 1653 cm−1 (C_O); HRMS (FAB) m/z found: 1465.8427;
calcd. for C102H113O8: 1465.8435 [M+H]+. Anal. calcd for C102H112O8·0.5H2O: C, 83.06; H, 7.71. Found: C, 82.92; H, 7.71.
8. Chambers, J. Q. The Chemistry of the Quinonoid Compound; Patai, S. Ed.; John Wiley & Sons: London, 1974; Chapter 14.
9. Crystallographic data for 1: 0.5C6H6: C105H115O8, M=1505.06, monoclinic, C2/c, a=47.782(2) Å, b=11.969(1) Å,
c=30.850(1) Å, β=99.895(1)°, V=17381(1) Å3, Z=8, Dcalcd=1.150 g cm−3. The intensity data were collected at 120 K
on a MAC Science DIP-2030 imaging plate area detector with MoKα radiation (λ=0.71069 Å). Of the 47445 reflections
which were collected, 16813 were independent. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were
included but not refined. The final cycle of full-matrix least-squares refinement on F was based on 11856 observed reflections
[I>3.00σ(I)]. R=0.068, Rw=0.073 for 1018 parameters.