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Mnbius Aromatic Hydrocarbons Challenges for Theory and Synthesis.

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Mbius Aromatic Hydrocarbons: Challenges for Theory
and Synthesis
Takeshi Kawase* and Masaji Oda*
annulenes · aromaticity · carbocycles · hydrocarbons
ince the beginnings of organic
chemistry the concept of aromaticity
has been studied by both theoretical and
synthetic chemists, and their interaction
has resulted in exciting developments in
this field.[1] In the nineteenth century it
was recognized that benzene exhibits
unusual stability for a highly unsaturated hydrocarbon and that all six carbon
atoms are equivalent. In 1865 Kekul%
put forward the hexagonal formula now
used and introduced the hypothesis of
valence oscillation between two cyclohexatriene structures to explain the
absence of double-bond isomers for
1,2- and 1,3-disubstituted benzenes.
However, the unusually high stability
of benzene was not explained satisfactorily for a long time.
In the 1930s H+ckel published his
detailed treatise on the theoretical foundations of aromaticity which led to the
formulation of his now famous “4n+2”
rule: for ground-state molecules with a
cyclic array of p orbitals, (4n+2) p
electrons, where n is zero or any positive
number, lead to special stability based
on the presence of a closed electronic
shell. Calculations also predicted that
systems having 4n p electrons are unstable, namely “antiaromatic” owing to
open-shell electronic structures. The
H+ckel rule not only accounted for the
unusual stability of benzene but it also
stimulated synthetic organic chemists to
construct various carbocyclic conjugat-
[*] Prof. Dr. T. Kawase, Prof. Dr. M. Oda
Department of Chemistry
Graduate School of Science
Osaka University
Toyonaka, Osaka 560-0043 (Japan)
Fax: (+ 81) 6-6850-5387
In memory of Masazumi Nakagawa
ed systems. Annulenes (CH)n were the
main targets of synthesis. The scope and
limitation of the H+ckel rule have thus
been established: the rule is applicable
to planar or nearly planar cyclic systems
without any apparent twist of the p
frameworks interrupting the conjugation.
However, while the development of
synthetic methodology has expanded
the chemistry of conjugated systems,
some difficulties have emerged. It is
not easy to determine experimentally
whether a compound is aromatic, particularly for non-benzenoid systems. In
this context, computational analyses of
bond-length equalization and magnetic
properties such as the harmonic oscillator model of aromaticity (HOMA)[2]
and nucleus-independent chemical
shifts (NICS)[3] are now used as probes
of aromaticity.
In 1964 Heilbronner conceived of
“M:bius aromaticity” based on simple
H+ckel molecular orbital theory and
predicted that [4n] M:bius systems
would be stable because they possess a
closed-shell structure in contrast to [4n]
H+ckel systems (Figure 1).[4] His idea
was extended in the formulation of a
selection rule for inferring the aromaticity of the transition states of pericyclic
reactions: Zimmerman generalized the
idea as the “M:bius–H+ckel” concept.[5]
However, M:bius annulenes conforming to Heilbronner>s concept have not
been realized up until recently, probably
because it is difficult for even large
macrocyclic annulenes to adopt M:bius
geometries without any apparent angle
or steric strain.
In 1998 Schleyer and co-workers
reported that cyclononatetraenyl cation
1 with 8 p electrons is M:bius aromatic
rather than nonaromatic on the basis of
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200460050
Figure 1. The p system labeled “Hckel” is a
destabilized [4n] system, whereas that labeled
“Mbius” is a stabilized [4n] system.
calculated NICS values (Figure 2). The
cation 1 is formed easily in spite of the
anticipated antiaromaticity of the planar
structure.[6] According to calculations
(B3LYP 6-311 + G*), the M:bius conformation (C2) 1 a is 21.6 kcal mol 1
Figure 2. Conformations of (CH)9+.
Angew. Chem. Int. Ed. 2004, 43, 4396 –4398
lower in energy than a second minimum
conformation, 1 b, which has strong
bond alternation. The energy difference
between 1 a and the planar H+ckel
conformation (C2v) 1 c is 26.3 kcal mol 1.
The NICS value of 1 a ( 13.4) is more
negative than that of 1 b (+ 8.6) and
even than that of benzene ( 9.7). Although formed easily, 1 a is a short-lived
species; it isomerizes readily to the more
stable bicyclo[4.3.0]nonatrienyl cation 2,
a bishomoaromatic cation ( 11.8),
through an electrocyclic ring closure
(Scheme 1).
have rigid frameworks that enforce a
smoothly twisted conjugation. Moreover, the molecular strain should be
dispersed over the entire molecule. Almost the same concept was applied in
the molecular design of “beltshaped” conjugated systems.[9] The systems, in which the p orbitals are aligned
horizontally on a rigid surface, have also
been sought as attractive synthetic targets. Since the discovery of fullerenes
and carbon nanotubes, these belt-shaped conjugated systems have been regarded as good models for the new
carbon materials. The first examples,
picotubes 3 and 4 and carbon nanorings
5 and 6, were synthesized as relatively
Scheme 1. Electrocyclization of 1 to 2.
obtained from the [2+2] adduct of 7
with benzene (Scheme 3).[10a] The steric
interaction of the inner ortho protons of
Scheme 3. Reaction of benzene with 7.
the anthracene units forces the molecule
to assume a tubelike structure. This
interesting synthetic strategy was also
applied to the synthesis of the twisted
[16]annulene 9 using syn-tricyclooctadiene, a valence-bond isomer of cyclooctatetraene, in place of benzene
(Scheme 4). In this case, the first meta-
The electronic properties of smallring compounds containing a strained
allene bond or a trans double bond have
also been discussed by theoreticians in
connection with M:bius aromatic properties.[7] Although these computational
studies predicted the importance of
M:bius conjugation, the twisted cyclic
molecules are destabilized by strong
ring strain. In larger cyclic systems,
however, ring strain is less pronounced.
Rzepa>s and Schleyer>s groups have
explored various isomers and conformations of [4n]annulenes (n = 3–5) computationally. These annulenes exist in
equilibrium mixtures of geometrical isomers because of their structural flexibility. The calculations predict that some
isomers of these systems adopt M:bius
aromatic structures as energy minima,
but the structures are very flexible and
tend to flip back to the less strained
H+ckel structures with low activation
To display M:bius aromaticity,
therefore, the target molecules should
stable substances by Herges et al. and
by our research group in 1996.[10, 11] It is
notable that a M:bius structure can be
regarded as a combination of a “nor- Scheme 4. Reaction of syn-tricyclooctadiene with 7.
mal” aromatic structure and a beltshaped aromatic structure as illustrated
thesis proceeded thermally, but the
graphically in Scheme 2.[10]
Recently Herges and co-workers second one proceeded photochemically.
have synthesized the first stable M:bius Although it contains rigid anthraquinomolecule based on this consideration.[12] dimethane moieties, annulene 9 has a
They found that tetradehydrodianthra- number of geometrical and conformacene (TDDA, 7) reacts with a number of tional isomers. According to calculaalkenes upon photoirradiation to form tions of all 108 isomers, the six most
rigid macrocyclic compounds by meta- stable isomers, including the global
thesis of the corresponding [2+2] cyclo- minimum, possess M:bius-type strucadducts. The twisted[14] annulene 3 was tures.[12]
Five isomers were separated by
HPLC and fully characterized by spectroscopic and crystallographic analysis.
The orange crystalline Z,E,Z isomer 9 a
(C2) exhibits relatively little bond alternation (HOMA value = 0.50), whereas
the colorless crystalline Z,Z,Z isomer
9 b exhibits strong bond alternation
(HOMA value = 0.05) (Figure 3). The
authors concluded that 9 a is M:bius
Scheme 2. Schematic construction of a Mbius structure.
Angew. Chem. Int. Ed. 2004, 43, 4396 –4398
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 3. Molecular structures of isomers of
aromatic and that 9 b is a H+ckel
structure but nonaromatic. The orange
color of 9 a clearly indicates the extension of conjugation.
Schleyer>s group have predicted that
“[4n+2] trannulenes” having in-plane
cyclic conjugation are aromatic like
H+ckel annulenes;[13] however, it is not
yet clear how orbital pyramidalization
can cause a change in cyclic conjugation.
Moreover, it is well known that a
benzene nucleus in polycyclic conjugated systems drastically reduces the peripheral conjugation in the molecules.
Actually, the families of picotubes and
carbon nanorings do not exhibit much
effect of peripheral conjugation.[11b] Despite these disadvantages, the hydrocarbon prepared by the Herges group is
the first stable molecule having a M:bius structure. Thus, synthetic organic
chemists have now realized Heilbron-
ner>s idea about 40 years after it was
proposed. Theoretical studies on other
M:bius conjugated systems have been
advanced recently, and the synthesis of
such compounds will be a highly interesting subject.[14] Thus, the synthesis of
aromatic compounds has now entered a
new stage, and bent cyclophanes, twisted
polycyclic aromatic hydrocarbons, and,
notably, fullerenes are just some examples of new targets.
[1] For recent reviews, see: P. von R.
Schleyer (special editor), Chem. Rev.
2001, 101, 1115 – 1566.
[2] T. M. Krygowski, J. Chem. Inf. Comput.
Sci. 1993, 33, 70 – 78.
[3] P. von R. Schleyer, C. Maerker, A.
Dransfield, H. Jiao, N. J. R. van Eikema Hommes, J. Am. Chem. Soc.
1996, 118, 6317 – 6318.
[4] E. Heilbronner, Tetrahedron Lett. 1964,
1923 – 1926.
[5] H. E. Zimmerman, Acc. Chem. Res.
1971, 4, 272 – 280.
[6] M. Mauksch, V. Gogonea, H. Jiao,
P. von R. Schleyer, Angew. Chem. 1998,
110, 2515 – 2517; Angew. Chem. Int. Ed.
1998, 37, 2395 – 2397.
[7] S. Martin-Santamaria, B. Lavan, H. S.
Rzepa, Chem. Commun. 2000, 1089 –
1090; S. Martin-Santamaria, H. S. Rzepa, J. Chem. Soc. Perkin Trans. 2 2000,
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2372 – 2377; C. J. Kastrup, S. P. Oldfield,
H. S. Rzepa, Chem. Commun. 2002,
642 – 643.
S. Martin-Santamaria, B. Lavan, H. S.
Rzepa, J. Chem. Soc. Perkin Trans. 2
2000, 1415 – 1417; C. Castro, C. M. Isborn, W. L. Karney, M. Mauksch,
P. von R. Schleyer, Org. Lett. 2002, 4,
3431 – 3434.
L. T. Scott, Angew. Chem. 2003, 115,
4265 – 4267; Angew. Chem. Int. Ed. 2003,
42, 4133 – 4135, and references therein.
a) S. Kammermeier, P. G. Jones, R.
Herges, Angew. Chem. 1996, 108, 470 –
472; Angew. Chem. Int. Ed. Engl. 1996,
35, 417 – 419; b) S. Kammermeier, P. G.
Jones, R. Herges, Angew. Chem. 1996,
108, 2834 – 2836; Angew. Chem. Int. Ed.
Engl. 1996, 35, 2669 – 2671.
a) T. Kawase, H. R. Darabi, M. Oda,
Angew. Chem. 1996, 108, 2803 – 2805;
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2664 – 2666; b) T. Kawase, N. Ueda, K.
Tanaka, Y. Seirai, M. Oda, Tetrahedron
Lett. 2001, 42, 5509 – 5511.
D. Ajami, O. Oeckler, A. Simon, R.
Herges, Nature 2003, 426, 819 – 821.
A. A. Fokin, H. Jiao, P. von R. Schleyer,
J. Am. Chem. Soc. 1998, 120, 9364 –
Recent theoretical studies of a molecular M:bius strip with a figure-eight
structure; J. Dobrowolski, J. Chem. Inf.
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Balaban, M. Randic, J. Chem. Inf. Comput. Sci. 2004, 44, 50 – 59.
Angew. Chem. Int. Ed. 2004, 43, 4396 –4398
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challenge, synthesis, mnbius, hydrocarbonic, theory, aromatic
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