In 1988, Wilke[cite]10.1002/anie.198801851[/cite] reported molecule 1 It was a highly unexpected outcome of a nickel-catalyzed reaction and was described as a 24-annulene with an unusual 3D shape. Little attention has been paid to this molecule since its original report, but the focus has now returned!

Imperial College London

Henry Rzepa

In 1988, Wilke\[cite\]10.1002/anie.198801851\[/cite\] reported molecule
**1**

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A 24-annulene. Click for 3D.
:::

  
It was a highly unexpected outcome of a nickel-catalyzed reaction and
was described as a 24-annulene with an unusual 3D shape. Little
attention has been paid to this molecule since its original report, but
the focus has now returned! The reason is that a 24- annulene belongs
formally to a class of molecule with 4n (n=6) π-electrons, and which
makes it **antiaromatic** according to the (extended) Hückel rule. This
is a select class of molecule, of which the first two members are
[cyclobutadiene](http://www.ch.ic.ac.uk/rzepa/blog/?p=2355){target="_blank"
rel="noopener noreferrer"} and cyclo-octatetraene. The first of these is
exceptionally reactive and unstable and is the archetypal anti-aromatic
molecule. The second is not actually unstable, but it is reactive and
conventional wisdom has it that it avoids the undesirable
antiaromaticity by adopting a highly non-planar tub shape and hence
instead adopts reactive non-aromaticity. Both these examples have
localized double bonds, a great contrast with the molecule which
sandwiches them, cyclo-hexatriene (i.e. benzene). The reason for the
resurgent interest is that a number of crystalline, apparently stable,
antiaromatic molecules have recently been discovered, and ostensibly,
molecule **1** belongs to this select class!

So is **1** actually anti-aromatic? Let us look at some of the ways in
which this might be estimated.

1.  One can inspect the bond lengths, measured from X-ray analysis. The
    longest is 1.463Å, labelled *a* above, and it corresponds to a
    single bond (value from the crystal structure).
2.  If the molecule had a bond alternating structure, the adjacent bonds
    would be expected to be much shorter, in the region of 1.32Å. In
    fact, they are rather longer, at 1.37Å. Indeed, in the
    cycloheptatriene part of the molecule, the alternation is much less
    than one might expect of an anti-aromatic molecule, oscillating
    between 1.37 and 1.43Å.
3.  One can also inspect aromaticity via a variety of magnetic indices.
    The simplest of these is the NICS probe. Placed at a ring centroid,
    a negative value of this index of around -10ppm indicates
    aromaticity (this is the value for benzene), whilst a strongly
    positive value (of up to +20 ppm) indicates anti-aromaticity.
    Molecule **1** has two potential centroids, one placed at the
    absolute centre of the system, and one placed at the centroid of the
    \~6-membered ring completed using bond *b* (in reality, the
    centroids were computed from the positions of ring critical points
    obtained from an AIM analysis). The NICS values at these two
    positions are both \~-4.4 ppm (See DOI:
    [10042/to-2156](http://dx.doi.org/10042/to-2156){target="new"
    rel="noopener noreferrer"} for details of the calculation). These
    values does not indicate antiaromaticity! They could even be
    described as mildly aromatic. So what is going on?
4.  What about the chemical shifts of the other protons? All the
    hydrogens attached to sp2 carbons are predicted to resonate at
    around 6.7ppm (unfortunately Wilke does not report the experimental
    spectrum), which is typical of an aromatic system (anti-aromatic
    systems have high upfield shifts for such protons, at around 2ppm or
    even -2 ppm, see \[cite\]10.1021/ol703129z\[/cite\] for examples).
    The two protons of the methylene bridges are also quite different;
    2.9 and 0.8 ppm. The latter is the proton *endo* to the
    cycloheptatrienyl ring, and is typical of a proton placed in the
    anisotropic magnetic shielding region of e.g. benzene. Thus the
    cycloheptatrienyl ring is itself behaving as if it were
    **aromatic**, whereas the overarching 24-annulene ring is certainly
    not behaving as if it were **antiaromatic**.

One possible explanation involves a concept known as
**homoaromaticity**. The bond marked as *b* could be regarded as
completing the 6π-electron local aromaticity of that ring (it would be
formally considered as a 1π-electron bond, with no underlying
σ-framework, see \[cite\[10.1021/ct8001915\[/cite\] for further detail).
Well characterised examples of such neutral homoaromatics are in fact
very rare indeed; the phenomenon is thought to manifest mostly through
cationic homoaromatics (i.e. the homotropylium cation, see
\[cite\]10.1021/jo801022b\[/cite\] for discussion). So has the case been
made for **1** being the first clear cut example of a neutral
homoaromatic molecule, containing no less than four rings exhibiting
this type of aromaticity?

There is one further concept that can be introduced.
[Clar](http://www.ch.ic.ac.uk/rzepa/blog/?p=1292){target="_blank"
rel="noopener noreferrer"} (for a discussion, see DOI:
\[cite\]10.1021/cr0300946\[/cite\] proposed that benzenoid 6π-electron
local aromaticity is preferred to less local or more extended cyclic
conjugations, if the two compete. Many examples in a type of compound
known as polybenzenoid aromatics are known where the most favourable
resonance structure is that which maximises the number of Clar rings.
More recently, quite a few ostensibly **antiaromatic** molecules have
been shown to attenuate this unfavourable effect by forming instead
groups of aromatic Clar *islands* containing delocalized benzene like
rings (discussion of this point can be found at
\[cite\]10.1039/b810147g\[/cite\]. In molecule **1**, we could have a
new phenomenon; a **homoClar** ring, formed to avoid antiaromaticity.

For further discussion, see the [comment
posted](http://hackberry.chem.trinity.edu/blog/?p=231#comments){target="new"
rel="noopener noreferrer"} to Steve Bachrach's blog.

::: {.wp_orcid_field}
[https://orcid.org/0000-0002-8635-8390](https://orcid.org/0000-0002-8635-8390){target="_blank"
rel="author"}
:::


In 1988, Wilke[cite]10.1002/anie.198801851[/cite] reported  molecule
<strong>
 1
</strong>
A 24-annulene. Click for 3D. It was a highly unexpected outcome of a nickel-catalyzed reaction and was described as  a 24-annulene with an unusual 3D shape. Little attention has been paid to this molecule since its original report, but the focus has now returned!

Rogue Scholar Beiträge

A molecule with an identity crisis: Aromatic or anti-aromatic?