Synthesis, x-ray structure, and properties of a tetrabenzannelated 1,2,4,5-cyclophane

Article abstract of DOI:10.1002/1521-3773(20021004)41:19<3688::AID-ANIE3688>3.0.CO;2-Q

Highly strained but thermally stable: Tetrabenzocyclophane 1, which was obtained in a four-step synthesis by two Diels - Alder addition reactions (see scheme) sublimes at high temperature, rather than decomposing. The distorted aromatic rings are more rea

Full text of DOI:10.1002/1521-3773(20021004)41:19<3688::AID-ANIE3688>3.0.CO;2-Q

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direct methods and refined with full-matrix least squares
(SHELX97[12]). CCDC-185351 for [NEt4]3[4], CCDC-185352 for
[NEt4]2[5], and CCDC-185353 for 0.65[NEt4]5[2]/0.35[NEt4]4[3] con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/
retrieving.html (or from the Cambridge Crystallographic Data Centre,
12, Union Road, Cambridge CB21EZ, UK; fax: (þ 44)1223-336-033;
or deposit@ccdc.cam.ac.uk).
Over the last five decades, a substantial number of chemists
have prepared many fascinating, strained saturated and
unsaturated molecules.[2,3] The most notable of the strained
unsaturated molecules are those of the fullerene C60[4] and the
cyclophane families.[5] In the former, the hexagons are
essentially cyclohexatrienes[6] and in the latter, the hexagons,
while considerably distorted, still retain their benzenoid
character. Since the first synthesis of [2.2]paracyclophane
diene by Dewhirst and Cram,[7] a variety of [2.2]paracyclo-
phanes with unsaturated or benzannelated bridges have been
synthesized.[8] The influence of the bridges on the trans-
annular benzene interactions and the geometry of the strained
cyclophanes has been widely investigated.[9] To date, only a
few unsaturated bridged and benzannelated cyclophanes are
known,[9] but no benzannelated [2n]cyclophane with more
than two bridges (n > 2) has been reported.[10] One would
expect that, as the number of o-phenylene bridges increased,
the total strain would also increase.
[11] G. M. Sheldrick, SADABS, Universit‰t Gottingen, Germany.
[12] G. M. Sheldrick, SHELXS-97, Program for the Solution of Crystal
Structures, Universit‰t Gˆttingen, Germany, 1997
[13] Handbook of Chemistry and Physics (Ed.: R. C. Weast), CRC Press,
Cleveland, 1977.
[14] a) A. E. Stevens, C. S. Feigerle, W. C. Lineberger, J. Am. Chem. Soc.
1982, 104, 5026; b) M. R. A. Blomberg, P. E. M. Siegbahn, T. J. Lee,
A. P. Rendell, J. E. Rice, J. Chem. Phys. 1991, 95, 5898.
[15] C. Mealli, D. M. Proserpio, J. Chem. Educ. 1990, 67, 399.
[16] a) B. T. Heaton, C. Brown, D. O. Smith, L. Strona, R. J. Goodfellow, P.
Chini, S. Martinengo, J. Am. Chem. Soc. 1980, 102, 6175; b) C. Allevi,
B. T. Heaton, C. Seregni, L. Strona, R. J. Goodfellow, P. Chini, S.
Martinengo, J. Chem. Soc. Dalton Trans. 1986, 1375.
[17] D. M. P. Mingos, L. Zheniang, J. Chem. Soc. Dalton Trans. 1988, 1657.
To prepare a superparylene and to test the effect of benzo
bridges in place of the alkyl bridges of cyclophanes one needs
a rapid, reasonably high-yield synthetic entry. A priori, based
on existing cyclophane synthetic methodology, the prepara-
tion of a symmetrical tetrabenzannelated [2n]cyclophane
tetraene would appear to be rather difficult and lengthy.
However, careful consideration of the molecular symmetry of
the target revealed that the synthesis could be easily achieved.
Herein, we describe the synthesis, X-ray structure, and some
of the properties of the symmetrically benzannelated [24]cy-
clophane tetraene 1. In future publications we will report on
the results of its pyrolytic decomposition.
Synthesis, X-ray Structure, and Properties of a
Tetrabenzannelated 1,2,4,5-Cyclophane**
Michael Brettreich, Michael Bendikov,
Sterling Chaffins, Dmitrii F. Perepichka,
Olivier Dautel, Hieu Duong, Roger Helgeson, and
Fred Wudl*
In Scheme 1 we depict the retrosynthetic analysis with a
rather unusual disconnection leading to two dibenzocyclooc-
tadiene-diynes and four methine units. As shown, the latter
can originate from a meso-ionic precursor.
Parylene is the most frequently used material in the
protective encapsulation of modern electronic components
and medical implants.[1] This high-performance polymer is
produced by the pyrolytic decomposition of [2.2]paracyclo-
phane.[1c] Another high-performance organic material with
ꢀ
even stronger C C bonds would be produced if another highly
strained, all-aromatic cyclophane could be pyrolyzed, thus
resulting in a ™superparylene∫. However, contrary to the
mode of pyrolytic decomposition of [2.2]paracyclophane,
+
"4 C-H"
ꢀ
where scission of the Csp3 Csp3 bond is the important first step
leading to a p-xylylene monomer, in the case of a molecule
such as 1 (Scheme 2), cleavage of a biaryl bond would produce
a very reactive diradical monomer.
Angle and bond strain in organic molecules and their effect
on properties also continue to be an active field of research.[2]
O
O
"4 C-H"
2
N
N
Ph
Ph
Scheme 1. Retrosynthetic analysis of 1.
[*] Prof. F. Wudl, Dr. M. Brettreich, Dr. M. Bendikov, S. Chaffins,
Dr. D. F. Perepichka, Dr. O. Dautel, H. Duong, Dr. R. Helgeson
Department of Chemistry and Biochemistry and Exotic Materials
Institute, University of California
The synthesis of the tetrabenzocyclophane 1 is based on the
known Diels Alder reactivity of diene 3[11] to afford 4 (as a
mixture of syn and anti isomers),[12] followed by thermal
extrusion of phenyl isocyanate, which leads to a new reactive
diene 5 (Scheme 2).[13] The twofold addition of 2[14] to 3
afforded the bisadduct 4 in 70% yield; the monoadduct does
not form even with a 1:1 ratio of reagents. Pyrolysis of 4 gave
reactive diene 5 quantitatively. Compound 5 has absorption
maxima at 252 and 333 nm, and exhibits a weak blue
fluorescence at 406 nm (in THF). It reacts with an excess of
Los Angeles, CA 90095-1569 (USA)
Fax : (þ 1)310-825-0767
E-mail: wudl@chem.ucla.edu
[**] We thank the National Science Foundation for support through DMR-
9796302 and an FRG with Rice University (NSF-FRG 0073046). We
thank the Navy for support of a MURI through ONR N00014-01-1-
0757
Supporting information for this article is available on the WWW under
http://www.angewandte.org or from the author.
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Figure 1. The X-ray structure of 1. ORTEP view and numbering scheme of
the independent molecule located on an inversion center (with 50%
probability displacement ellipsoids). Selected bond angles [8]: C10-C15-
C16 116.3(3), C17-C16-C18 118.5(4), C17-C16-C15 117.8(3), C18-C16-C15
119.6(3), C18’-C9-C8-C7 12.7(3).
Scheme 2. Synthesis of cyclophane 1. a) o-dichlorobenzene (ODCB),
1108C, 1 day; b) neat, 2508C, 1 h; c) 2, ODCB, 1508C, 3 d; d) neat,
3508C, 4 h.[13]
benzo bridges are distorted to 115.3 116.38 (calcd 116.28) and
dienophile 2 to give compound 6 in 25% yield, without
polymerization but accompanied by decomposition products
of diyne 2. The absence of polymerization is the result of:
1) the high dilution conditions and 2) the preorganization,
arising from the boat conformation of the intermediate that
results from the intramolecular cycloaddition to form 6.
Pyrolysis of 6 afforded a mixture of 1 (50% isolated) and 7
(30% isolated), which co-sublime at this temperature. In
contrast to 4, cyclophane precursor 6 is less willing to extrude
its phenylisocyanate bridges. Thermogravimetric analysis
(TGA) of 4 showed that the loss of the isocyanate occurred
between 200 and 2508C, whereas with 6 a temperature of
3508C is required (see Supporting Information).
The thermally stable cyclophane 1 was isolated by column
chromatography and characterized by 1H and 13C NMR
spectroscopy. Slow evaporation of a solution of 1 in CS2 at
408C afforded colorless platelike crystals suitable for X-ray
structural analysis (Figure 1).[15] To assess the electronic
structure of 1 we performed quantum mechanical density
functional theory calculations[16] at the B3LYP/6-31G(d) level
of theory[17] (used for discussion of structural features) as well
as ab initio calculations at the HF/3-21G* level[18] (used for
discussion of orbital energies).[19,20]
ꢀ
the C C bond in the bridges is elongated to 1.419 1.424 ä
ꢀ
(calcd 1.427 ä). The C9 C17 bond between the central
carbon atoms of the cyclophane rings is 2.996 ä (calcd
3.060 ä), while the calculated distance between the two
strained ring protons is shorter,[21] at 2.912 ä.[22] Consequently,
in NMR experiments the C9 carbon atoms of the strained
rings resonate at low field (d: 139.3 ppm)[23] and the adjacent
protons resonate at higher field (d: 6.57 ppm).
The UV/Vis spectrum, with two absorptions at 225 and
285 nm, is comparable to that of previously reported cyclo-
phanes,[24] but with an important exception: the broad ™cyclo-
phane band∫ appears at 330 nm, which is the most red-shifted
position of this band recorded to date. Electrochemical
experiments show an irreversible oxidation at Ep.c.
¼
þ 1.49 V versus the ferrocene/ferrocenium couple[25] and a
reversible reduction wave at E0 ¼ ꢀ2.80 V for 1. This obser-
vation should be contrasted with the oxidation wave for
[2.2]paracyclophane (Ep.c ¼ 1.10 V, no reduction peak was
observed down to ꢀ3.0 V) under the same conditions. At first
sight, the large negative value observed for 1 is contrary to
expectations based on strain arguments in fullerene chemistry,
where the electron-accepting property was related to strain
(hybridization).[26] However, results of ab initio calculations
indicate that although the LUMO (2.45 eV, Figure 2b) is
lower lying than that of benzene (4.12 eV), it is higher than
that of anthracene (1.81 eV), which reduces at a less negative
potential (E0 ¼ ꢀ2.47 V). On the other hand, the larger value
for the oxidation wave of 1 relative to [2.2]paracyclophane is
consistent with increased strain in 1 compared with [2.2]para-
cyclophane; C60 is also easier to reduce than to oxidize.[27] The
HOMO of cyclophane 1 (ꢀ8.21 eV Figure 2a) is relatively
high, lying between that of benzene (ꢀ9.22 eV) and anthra-
The independent unit in the crystal is located at an
inversion center and forms layers. The benzene moiety is
significantly bent with deviation from planarity for the carbon
skeleton of 0.133 0.139 ä (calcd 0.153 ä) and a C18’-C9-C8-
ꢀ
C7 dihedral angle of 12.7 13.78 (calcd 14.68) between the C C
bonds in the benzene moiety. A very significant strain is
ꢀ
reflected in a large angle between the C6 C7 bond and the
C7-C8-C17’ plane (19.8 21.48). The sum of three linear angles
around C7 is 355.4 356.08 (calcd 355.88), which reflects its
partial sp3 character. The C-C-C angles (e.g., C1-C6-C7) in the
Angew. Chem. Int. Ed. 2002, 41, No. 19
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In conclusion, we have described a simple, symmetry-driven
synthesis of a strained, very thermally stable cyclophane. The
distorted aromatic rings are more reactive with TCNE than
those in [2.2]paracyclophane. Another consequence of height-
ened strain is a bathochromic shift of the cyclophane band to
330 nm, the longest wavelength absorption observed for this
band to date. By using this methodology other benzo-ring-
substituted cyclophanes should now be accessible through
functionalized dibenzocyclooctadienediyne.[32]
Experimental Section
All syntheses were performed under argon. See Supporting Information for
detailed characterization of 4 6 and details of the reaction of 1 with TCNE.
4: A solution of 2 (100 mg, 0.500 mmol) and 3 (315 mg, 1.05 mmol) in o-
dichlorobenzene (ODCB, 6 mL) was stirred at 1108C until full conversion
of 2 was indicated by thin-layer chromatography (TLC, 1 day). The red
reaction mixture was evaporated in vacuo, and purified by column
chromatography on silica (eluting with hexane/ethyl acetate, 1:1 v/v) to
yield compound 4 (250 mg, 70%), which can be further purified by
recrystallization from chloroform, m.p. 2508C (decomp).
5: Compound 4 (50 mg, 0.069 mmol) was heated to 2508C for 1 h in vacuo
and then purified by column chromatography on silica (eluting with
hexanes/ethyl acetate, 1:1 v/v) to give 5 as a light-orange powder (32 mg,
95%), m.p. > 3508C.
6: A solution of 5 (50 mg, 0.10 mmol) in ODCB was slowly added (16 h,
using syringe pump) to a solution of 2 (20 mg, 0.10 mmol) in ODCB (5 mL)
at 1508C. The reaction mixture was stirred at 1508C until full consumption
of 5 (3 days, TLC monitoring), then ODCB was distilled in vacuo and the
solid residue was purified by column chromatography on silica (eluting
with chloroform) to give 6 as a powder (17 mg, 25%), m.p. 3508C
(decomp).
Figure 2. a) The HOMO and b) LUMO of 1 as calculated at HF/3-21G*//
HF/3-21G* level, generated with Spartan 5.0.3.
1: Compound 6 (30 mg, 0.043 mmol) was heated in a sublimation apparatus
to 3508C for 4 h and the sublimed material was purified by column
chromatography on silica (eluting with chloroform). The first fraction (Rf
ca. 0.8) gave the target product 1 (10 mg, 50%) as a white powder,
cene (ꢀ7.16 eV). These low LUMO and high HOMO energies
are further evidence of molecular strain.
1
m.p. > 3508C (sublim). H NMR (CDCl3, 400 MHz): d ¼ 6.57 (s, 4H), 7.21
(q, J ¼ 3.5 Hz, 4H), 7.38 ppm (q, J ¼ 3.5 Hz, 4H); 13C NMR (Cl2(CD)2Cl2,
As a result of the high HOMO energy, 1 is expected to be
very reactive as a diene in Diels Alder reactions. Indeed, the
unusual electronic properties of the strained rings in 1 are
manifested in their high reactivity towards tetracyanoethylene
(TCNE).[28] The published cyclophane (closest in structure to
1), [24](1,2,4,5)cyclophane,[29] forms a blue charge-transfer
(CT) complex which lasts ™a few seconds∫,[30] followed by
formation of the Diels Alder adduct. In addition, a highly
strained pyrene belt cyclophane[28] is converted at first into a
green CT complex with a lifetime of minutes.[31] In compar-
ison, 1 reacts with TCNE directly, without the detection of a
CT band (either visual or spectrophotometric) to produce
adduct 8 in almost quantitative yield (Scheme 3). The
disappearance of the ™cyclophane∫ band, without the appear-
ance of a CT band, can be followed spectroscopically (see
Supporting Information).
125 MHz): d ¼ 124.6, 127.0, 139.3, 143.4, 144.3 ppm; UV/Vis (c-C6H12, lmax
,
(e)) ¼ 225 (30300), 285 (3020), 330 nm (110); IR (DRIFT): n˜ ¼ 3060, 3024,
2924, 1444, 1234, 916, 750, 516 cmꢀ1; m/z (EI): 452 (Mþ, 100). HRMS (EI):
calcd for [C36H20]: 452.1565; found: 452.1556. The second fraction afforded
an intermediate 7 (7.4 mg, 30%), m.p. > 3508C, which can be converted
into 1 under the conditions described for 6.
Received: May 14, 2002
Revised: July 17, 2002 [Z19293]
[1] a) J. C. Wittman, P. Smith, Nature 1991, 352, 414; b) M. F. Nichols, Crit.
Rev. Biomed. Eng. 1994, 22, 39; c) K. M. Vaeth, K. F. Jensen, Chem.
Mater. 2000, 12, 1305, and references therein.
[2] V. V. Kane, W. H. De Wolf, F. Bickelhaupt, Tetrahedron 1994, 50,
4575; I. V. Komarov, Russ. Chem. Rev. 2001, 70, 991; J. Reinbold, E.
Sackers, T. Osswald, K. Weber, A. Weiler, T. Voss, D. Hunkler, J.
Worth, L. Knothe, F. Sommer, B. von Issendorff, H. Prinzbach, Chem.
Eur. J. 2002, 8, 509.
[3] A. Nickon, E. F. Silversmith, Organic Chemistry: the Name Game,
Pergamon, New York, 1988.
NC
NC
CN
CN
[4] ™Fullerenes and Related Structures∫: Top. Curr. Chem. 1999, 199.
[5] ™Cyclophanes I∫: Top. Curr. Chem. 1983, 113; ™Cyclophanes II∫: Top.
Curr. Chem. 1983, 115; Cyclophanes: Organic Chemistry, Vol. 45
(Eds.: P. M. Keehn, S. M. Rosenfeld), Academic Press, New York,
1983; F. Diederich, Cyclophanes, Royal Society of Chemistry, Cam-
bridge, UK, 1991 (Monographs in Supramolecular Chemistry); T.
Tsuji, M. Ohkita, H. Kawai, Bull. Chem. Soc. Jpn. 2002, 75, 415.
[6] S. Liu, Y. J. Lu, M. M. Kappes, J. A. Ibers, Nature 1991, 254, 408.
[7] K. C. Dewhirst, D. J. Cram, J. Am. Chem. Soc. 1958, 80, 3115.
TCNE
1
8
Scheme 3. Addition of TCNE to cyclophane 1.
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[8] C. W. Chan, H. N. C. Wong, J. Am. Chem. Soc. 1985, 107, 4790; M.
Psiorz, H. Hopf, Angew. Chem. 1982, 94, 639; Angew. Chem. Int. Ed.
Engl. 1982, 21, 623; N. Jacobson, V. Boekelheide, Angew. Chem. 1978,
90, 49; Angew. Chem. Int. Ed. Engl. 1978, 17, 46; M. Stˆbbe, O. Reiser,
R. N‰der, A. de Meijere, Chem. Ber. 1987, 120, 1667; T. Wong, S. S.
Cheung, H. N. C. Wong, Angew. Chem. 1988, 100, 716; Angew. Chem.
Int. Ed. Engl. 1988, 27, 705; Erratum: T. Wong, S. S. Cheung, H. N. C.
Wong, Angew. Chem. 1988, 100, 1242; Angew. Chem. Int. Ed. Engl.
1988, 27, 1200; C. W. Chang, H. N. C. Wong,J. Am. Chem. Soc. 1988,
110, 462; H. Buchholz, A. de Meijere, Synlett 1993, 253; O. Reiser, S.
Reichow, A. de Meijere, Angew. Chem. 1987, 99, 1285; Angew. Chem.
Int. Ed. Engl. 1987, 26, 1277; H.-F. Gr¸tzmacher, W. Husemann,
Tetrahedron Lett. 1985, 26, 2431; Y.-H. Lai, S.-M. Lee, J. Org. Chem.
1988, 53, 4472; M. Wittek, F. Vˆgtle, G. St¸hler, A. Mannschreck,
B. M. Lang, H. Irngartinger, Chem. Ber. 1983, 116, 207.
[9] Z.-Z. Yang, B. Kova, E. Heilbronner, J. Lecoultre, C. W. Chan,
H. N. C. Wong, H. Hopf, F. Vˆgtle, Helv. Chim. Acta, 1987, 70, 299.
[10] The largest number of unsaturated bridges reported is three ethenes:
V. Boekelheide, R. A. Hollins, J. Am. Chem. Soc. 1973, 95, 3201; V.
Boekelheide, R. A. Hollins, J. Am. Chem. Soc. 1970, 92, 3512; V.
Boekelheide, W. Schmidt,Chem. Phys. Lett. 1972, 17, 410; H. Hopf, C.
Mlynek, J. Org. Chem. 1990, 55, 1361.
[11] T. Kappe, D. Pocivalnik, Heterocycles 1983, 20, 1367; T. Kappe, W.
Lube, Angew. Chem. 1971, 83, 967; Angew. Chem. Int. Ed. Engl. 1971,
10, 925; W. Friedrichsen, T. Kappe, A. Bˆttcher,Heterocycles 1982, 19,
1083; K. T. Potts, M. Sorm, J. Org. Chem. 1972, 37, 1422.
[12] Compound 4 should be formed as a mixture of syn and anti isomers;
the relative ratio of syn:anti cannot be determined on the basis of
NMR spectroscopy (see Supporting Information). We speculate the
anti isomer for compound 4 to be preferentially formed as a result of
1) steric repulsion between two phenyl substituents (located on
opposite sides of the saddle-shaped dibenzocyclooctatetraenene ring
and 2) minimization of dipole dipole interactions of the lactam
carbonyls.
M. C. Holthausen, A Chemist©s guide to density functional theory,
Wiley-VCH, Weinheim, 2000.
[17] a) C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785; b) A. D.
Becke, J. Chem. Phys. 1993, 98, 5648.
[18] HF/3-21G* was recently proposed to be very accurate for predicting
geometries of series-strained cyclophanes: R. A. Pascal, Jr.,J. Phys.
Chem. A 2001, 105, 9040.
[19] All calculations used: Gaussian98 (RevisionA.7), M. J. Frisch, G. W.
Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman,
V. G. Zakrzewski, J. A. Montgomery, R. E. Stratmann, J. C. Burant, S.
Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, O.
Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C.
Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y.
Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K.
Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, B. B. Stefanov,
G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L.
Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A.
Nanayakkara, C. Gonzalez, M. Challacombe, P. M. W. Gill, B. G.
Johnson, W. Chen, M. W. Wong, J. L. Andres, M. Head-Gordon, E. S.
Replogle, J. A. Pople, Gaussian, Inc., Pittsburgh, PA, 1998.
[20] All molecules were fully optimized and frequency calculations were
performed at the same level for all stationary points to check that all
frequencies are real and, consequently, all the calculated structures
are minima on the potential energy surface. 1 has molecular D2h
symmetry.
[21] The perspective in Figure 1b, which shows a wider distance between
the protons at C9’ and C17’ than the carbon atoms to which they are
bonded, is misleading since the actual positions were not determined
from the data and are an artifact resulting from hydrogen atom
placement by the program.
[22] The known X-ray structure of [24](1,2,4,5)cyclophane (A. W. Hanson,
Acta Crystallogr. Sect. B 1977, 33, 2003) is very similar to 1: the
separation of the central carbon atoms of the two rings is 2.950 ä, and
that between the two strained ring protons is 2.919 ä (calcd value).
ꢀ
[13] Although a single isomer is shown for compounds 5 and 6, one cannot
say with certainty if it is the anti isomer (as shown). Chromatographic
isolation of 5 and 6 produces a single isomer, as evidenced by NMR
spectroscopy. However, final structural determination will have to rest
with X-ray crystallography. It is possible that recrystallization of 4,
prior to pyrolysis, allowed selection of only one isomer, which could
then only produce a single isomer of 5 (and hence 6).
[14] H. N. C. Wong, P. J. Garratt, F. Sondheimer, J. Am. Chem. Soc. 1974,
96, 5604; A. Orita, D. Hasegawa, T. Nakano, J. Otera, Chem. Eur. J.
2002, 8, 2000. Some new routes for the synthesis of the dibenzocy-
clooctadiyne are published elsewhere (S. Chaffins, M. Brettreich, F.
Wudl, Synthesis 2002, 1191).
The dihedral angle between the C C bonds in the benzene moiety is
12.28.
[23] GIAO-B3LYP/6-311 þ G(d,p)//B3LYP/6-31(d) calculations corrected
for the chemical shift of benzene (d ¼ 134.8 calcd, 128.0 ppm exptl)
predict chemical shifts of d ¼ 141.4, 147.8, 148.3, 125.0, and 126.6 ppm
for C9, C8, C6, C5, and C4, respectively, which allows an assignment of
the experimentally observed resonance at d ¼ 139.3 ppm to the C9
carbon atom.
[24] V. Boekelheide, Top. Curr. Chem. 1983, 113, 110.
[25] To access the large positive and negative potentials of the cyclophanes,
the cyclic voltammetry experiments were performed in anisole (0.25m
Bu4NPF6) for reduction wave and in nitrobenzene (0.1m Bu4NPF6) for
oxidation wave measurements. The reference electrode was Ag/
AgNO3 and all the potentials are quoted versus the ferrocene/
ferrocenium couple (internal standard) with E0 ¼ þ 0.22 V and
þ 0.23 V versus Ag/AgNO3 in anisole and nitrobenzene, respectively.
[26] R. C. Haddon, Science 1993, 261, 1545.
[15] Crystal data for 1: Single crystals were obtained by slow evaporation
from CS2. Compound 1 (C36H20, Mr ¼ 452.52) crystallizes in the
ꢀ
triclinic space group P1, a ¼ 8.541(6), b ¼ 8.651(6), c ¼ 9.114(6) ä, a ¼
83.671(12), b ¼ 68.044(12), g ¼ 62.543(10)8, V¼ 552.6(6) ä3, Z ¼ 1,
1calcd ¼ 1.360 gcmꢀ3, F(000) ¼ 236, T¼ 100(2) K. Data collection: Bru-
ker AXS CCD with graphite-monochromatic MoKa radiation (l ¼
0.71073 ä). Crystal dimensions: 0.10 î 0.15 î 0.20 mm; 4.8 ꢁ 2q ꢁ
56.048; 3102 measured reflections, of which 2136 [Rint ¼ 0.047] were
independent and were used for the structure refinement of 163 pa-
rameters. An empirical absorption correction (SADABS) was applied.
The structure was solved by direct methods using SHELXS-97 and
refined by the full-matrix least-squares method on F2 using SHELXL-
97 (G. M. Sheldrick, SHELXS-98, Program for the Solution of Crystal
Structures, University of Gˆttingen, Gˆttingen (Germany), 1997) with
anisotropic thermal parameters for all non-hydrogen atoms. The
hydrogen atoms were introduced at calculated positions (riding
[27] Q. Xie, E. Pÿrez-Cordero, L. Echegoyen, J. Am. Chem. Soc. 1992, 114,
3978.
[28] G. J. Bodwell, J. N. Bridson, T. J. Houghton, J. W. J. Kennedy, M. R.
Mannion, Chem. Eur. J. 1999, 5, 1823; G. J. Bodwell, J. J. Fleming,
M. R. Mannion, D. O. Miller, J. Org. Chem. 2000, 65, 5360.
[29] It is claimed by Boekelheide et al. that superphane {[26](1,2,3,4,5,6)cy-
clophane} is the most strained cyclophane (see ref. [30]). However, by
symmetry, its benzene rings are constrained to be nearly planar (for X-
ray analysis, see A. W. Hanson, T. S. Cameron,J. Chem. Res. (M) 1980,
10, 4201) and its reactivity with TCNE was not reported.
[30] Y. Sekine, V. Boekelheide, J. Am. Chem. Soc. 1981, 103, 1777.
[31] G. J. Bodwell, private communication.
ꢀ
model) with Csp2 H bond lengths of 0.95 ä. At convergence, R1 ¼
0.0888 (I > 2s(I)) and wR2 ¼ 0.2345; min/max residual electron
density: ꢀ0.371/ þ 0.490 eä2. CCDC-184166 contains the supplemen-
tary crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from
the Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge CB21EZ, UK; fax: (þ 44)1223-336-033; or deposit
@ccdc.cam.ac.uk).
[32] T.-L. Chan, T. C. W. Mak, C.-D. Poon, C. Wong, J. H. Jia, L. L. Wang,
Tetrahedron 1986, 42, 655; Y.-M. Man, T. C. W. Mak, H. N. C. Wong,J.
Org. Chem. 1990, 55, 3214.
[16] a) R. G. Parr, W. Yang, Density-Functional Theory of Atoms and
Molecules, Oxford University Press, New York, 1989; b) W. Koch,
Angew. Chem. Int. Ed. 2002, 41, No. 19
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