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Mnbius Aromatic Hydrocarbons Challenges for Theory and Synthesis.

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Highlights
Aromaticity
Mbius Aromatic Hydrocarbons: Challenges for Theory
and Synthesis
Takeshi Kawase* and Masaji Oda*
Keywords:
annulenes · aromaticity · carbocycles · hydrocarbons
S
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
E-mail: tkawase@chem.sci.osaka-u.ac.jp
moda@chem.sci.osaka-u.ac.jp
4396
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
Angewandte
Chemie
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
energies.[8]
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
www.angewandte.org
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4397
Highlights
Figure 3. Molecular structures of isomers of
9.
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-
4398
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,
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
[8]
[9]
[10]
[11]
[12]
[13]
[14]
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;
Angew. Chem. Int. Ed. Engl. 1996, 35,
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 –
9365.
Recent theoretical studies of a molecular M:bius strip with a figure-eight
structure; J. Dobrowolski, J. Chem. Inf.
Comput. Sci. 2002, 42, 490 – 499; A. T.
Balaban, M. Randic, J. Chem. Inf. Comput. Sci. 2004, 44, 50 – 59.
Angew. Chem. Int. Ed. 2004, 43, 4396 –4398
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