Synthesis of [6.8]3cyclacene: Conjugated belt and model for an unusual type of carbon nanotube

Article abstract of DOI:10.1021/ja801918n

[6.8]₃Cyclacene as the first hydrocarbon being a fully conjugated molecular belt has been synthesized in an eight-step reaction sequence starting from 4,6-dimethylisophthalaldehyde. It is the smallest and most strained member of the [6.8]cyclacene family, and the synthetic path offers a general route to its higher members. The structural pattern of [6.8]3cyclacene represents a model for a novel type of carbon nanotubes. Copyright

Full text of DOI:10.1021/ja801918n

Published on Web 05/07/2008
Synthesis of [6.8]3Cyclacene: Conjugated Belt and Model for an Unusual Type
of Carbon Nanotube
Birgit Esser, Frank Rominger, and Rolf Gleiter*
Organisch-Chemisches Institut, UniVersita¨t Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Received March 21, 2008; E-mail: rolf.gleiter@oci.uni-heidelberg.de
Hoop shaped molecules consisting of annelated, unsaturated rings
are of interest with respect to their conjugation, their spectroscopic
properties, their cavities and as possible structural motifs for carbon
nanotubes. [6]nCyclacenes1 (2 (n ) 8)) which are built from n
linearly annelated benzene rings have been discussed in the literature
since 19542,3 and can, as well as cyclo[n]phenacenes (3 (n ) 10)),
be regarded as subunits of carbon nanotubes.4 However, their
synthesis is still lacking, although the challenge has been undertaken
by several groups.5–7 Their results have shown that there are two
main obstacles in the synthesis of 2. The bending of a linear chain
of annelated benzene rings into a hoop shape as the first has been
Figure 1. [6.8]3Cyclacene (1), [6]8cyclacene (2), cyclo[10]phenacene (3),
overcome by the preparation of belt-like precursors through
Diels-Alder reactions with 7-oxanorbornane5,7 or cyclohexane
derivatives.6 The anticipated high reactivity of [6]ncyclacenes due
to a small singlet-triplet gap predicted by quantum chemical
calculations8 is the second obstacle to which we also ascribe the
failures to convert the belt-like precursors to fully conjugated
systems. This assumption was supported by a recent synthesis of a
derivative of the cyclo[10]phenacene 3 (Figure 1) for which, unlike
the [6]ncyclacenes, a large singlet-triplet gap is expected. It was
accomplished by selective reduction of the north and south pole of
C60.9
and CpCo-capped hexabenzo[4.8]3cyclacene (4).
Scheme 1a
Both obstacles, the bending problem and the singlet-triplet gap,
can be circumvented by incorporating such conjugated ring systems
that naturally adopt a boat-like conformation.3c Especially favorable
should be eight-membered (cyclooctatetraene) rings in conjunction
with four- and six-membered rings, the former allowing for
conjugation and fulfilling the boat shape requirement. Thus, we
started to explore the synthesis of [4.8]n and [6.8]ncyclacenes3c and
found a rather simple access to the [4.8]3cyclacene derivative 4
(Figure 1).10 In connection with this work and supported by
quantum chemical calculations,11 we report herein the synthesis
of [6.8]3cyclacene 1 (Figure 1) as the first purely hydrocarbon
cyclacene and model for a new type of carbon nanotube.
a Reagents and conditions: (a) NaBH4, EtOH, rt; (b) HBr, HOAc, reflux,
65% (two steps); (c) PPh3, toluene, reflux, 99%; (d) LiOEt/EtOH, DMF,
-10 °C, 7%; (e) hν, benzene, 5 °C, 85%; (f) NBS, dibenzoylperoxide, CCl4,
reflux, 40%; (g) IBX, DMSO, 65 °C, 37%; (h) TiCl3(DME)1.5, Zn-Cu,
DME, reflux, 8%.
Our stepwise synthesis of 1 commenced with the readily available
4,6-dimethylisophthalaldehyde 512 (Scheme 1). Selective reduction
of one of the aldehydic groups with NaBH4 and subsequent
treatment of the monoalcohol with HBr in boiling HOAc afforded
the benzylic bromide 6 in 65% yield (two steps). 6 was quantita-
tively converted to the phosphonium salt 7 by refluxing with tri-
phenylphosphine in toluene. The three benzene rings were as-
sembled in a cyclic fashion by an intermolecular Wittig cyclization
reaction of 7, achieving the hexamethyl[23]-meta-cyclophanetriene
8 in 7% yield as the first key intermediate. In the choice of method,
we followed Wennerstro¨m13 who had obtained the unsubstituted
[23]-meta-cyclophanetriene in 28% yield. The lower yield in our
case was presumably caused by the steric hindrance stemming from
the methyl groups. The corresponding tetra- up to octameric meta-
cyclophane-n-enes could be isolated as minor products.
be converted to pure all-Z isomer 8a in 85% yield by irradiation
of a benzenic solution of 8a/8b with a mercury high pressure lamp
(λ ) 254 nm). The NMR spectrum of 8a points toward a C3V
symmetrical structure, although in the solid state the molecular
structure of 8a is twisted to a (noncrystallographic) C2 symmetry
(Figure 2). In solution, a fast equilibrium between these twisted
conformations is observed as mentioned above with no coalescence
down to -90 °C. NBS bromination of 8a in CCl4 afforded the
hexakis(bromomethyl) derivative 9 in 40% yield. X-ray crystal-
lographic investigation of single crystals of 9 revealed a similar C2
symmetrical conformation to that of 8a, while in solution a fast
equilibrium resulting in an average C3V symmetry is seen from the
NMR spectrum. This species opened the door to the second key
intermediate, the hexaaldehyde 10. Oxidation of the bromomethyl
groups in 9 was achieved in 37% yield by reaction with 2-iodoxy-
benzoic acid (IBX) in DMSO.14 The conformation of 10 in the
solid state (X-ray) and its fast equilibrium in solution (NMR) was
The hexamethyl[23]-meta-cyclophanetriene 8 was obtained as a
mixture of the all-Z, 8a, and E,Z,Z isomer 8b. This mixture could
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6716 J. AM. CHEM. SOC. 2008, 130, 6716–6717
10.1021/ja801918n CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
to its unsaturated character, it offers conjugation around the belt-
like torus consisting of annelated six- and eight-membered rings.
It is the smallest and most strained member of the [6.8]n series.11
Our synthesis offers a general path to [6.8]ncyclacenes, thus the
higher and as anticipated less strained and stronger conjugated
members of the [6.8]ncyclacene series are within reach. Furthermore,
our work gives first insight into the molecular structures of [23]-
meta-cyclophanetrienes.
Acknowledgment. We are grateful to the Deutsche Forschungs-
gemeinschaft for financial support. B.E. thanks the Studienstiftung
des deutschen Volkes for a graduate fellowship, and M. Do¨rner
for preparative assistance.
Figure 2. X-ray structure of 8a (50% probability ellipsoids; front and side
view; apart from the olefinic ones, hydrogen atoms are omitted for clarity).
Supporting Information Available: Experimental procedures,
characterization of all key compounds, and X-ray crystallographic data.
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
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Figure 3. X-ray structure of 1 (50% probability ellipsoids; in the left-
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again found to be similar to those of 8a and 9. The final 3-fold
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indicates about 31% conjugation for the cyclacene torus.17 The UV/
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substituted all-Z-[2n]-meta-cyclophane-n-enes.
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Structural alternatives to classical fullerenes and carbon nano-
structures comprising ring sizes of three- to nine-membered have
been considered and theoretically investigated.18 Slanina calculated
a stable cuboctahedron-like C48 structure composed of four-, six-,
and eight-membered rings.18c An expansion of the [6.8]ncyclacenes
in the direction of the principal molecular axis leads to carbon
nanotubes whose molecular pattern comprises four-, six-, and eight-
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In conclusion, we accomplished the synthesis of [6.8]3cyclacene
1 as the first fully conjugated purely hydrocarbon cyclacene. Due
JA801918N
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J. AM. CHEM. SOC. VOL. 130, NO. 21, 2008 6717