Imperial College London

Henry Rzepa

Mauksch and Tsogoeva have recently published an article illustrating how
a thermal electrocyclic reaction can proceed with distoratory ring
closure, whilst simultaneously also exhibiting 4n electron
Möbius-aromatic character\[cite\]10.1002/anie.200806009\[/cite\]. Why is
this remarkable? Because the *simple* Woodward-Hoffmann rules state that
a disrotatory thermal electrocyclic reaction should proceed via a
Hückel-aromatic 4n+2 electron transition state. Famously, Woodward and
Hoffmann stated there were no exceptions to this rule. Yet here we
apparently have one! So what is the more fundamental? The disrotatory
character, or the 4n/Möbius character in the example above? Mauksch and
Tsogoeva are in no doubt; it is the former that gives, and the latter is
correct.

So inevitably one has to ask; are there other examples? Well, during the
annual updating of my own lecture notes on [pericyclic
reactions](http://www.ch.ic.ac.uk/local/organic/pericyclic/), I had
decided to revisit a fascinating reaction, which we had first looked at
years ago (below).

::: {#attachment_24 .wp-caption .alignnone style="width: 473px"}
![Electrocylization of \[14\]
annulene](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2009/04/p322.jpg "Electrocylization of [14] annulene"){.wp-image-24
loading="lazy" decoding="async" aria-describedby="caption-attachment-24"
onclick="jmolApplet([450,450],'load wp-content/uploads/2009/04/4_2_bq.mol;select atomno=29;color magenta;label %A NICS -16.4;select atomno=30;color magenta; label %A NICS 4.8;select atomno=31;color magenta; label %A NICS 4.8;spin 3;','c1');"
width="463" height="277"}

Electrocylization of 14 annulene. Click above to obtain model
:::

Let us focus specifically on the last reaction, which involves
cyclization of a \[14\] annulene. The pedagogic interest was to
challenge the students as to whether this was a π4+π2 *endo*
cycloaddition, or two 6-electron electrocyclizations. The answer of
course was that either way of considering this reaction was equally
valid and both modes were presumed to proceed via Hückel-aromatic
transition states. I had said as such in my lectures for many years.
This year, I finally decided to evaluate the NICS index to verify this
long stated hypothesis. NICS is a magnetic index which yields a negative
value (of -10 to -16 ppm) for aromatic rings, and positive values for
antiaromatic rings.

![](https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2020/09/Screenshot-263.jpg){.aligncenter
.size-full .wp-image-22748 decoding="async"
onclick="jmolApplet([450,450],'load wp-content/uploads/2009/04/4_2_bq.mol;select atomno=29;color magenta;label %A NICS -16.4;select atomno=30;color magenta; label %A NICS +4.8;select atomno=31;color magenta; label %A NICS +4.8;spin 3;','c2');"
width="540"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2020/09/Screenshot-263.jpg 762w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2020/09/Screenshot-263-300x217.jpg 300w"
sizes="(max-width: 762px) 100vw, 762px"}

I was fully expecting to get negative values for all three rings at the
transition state. Of course, this result was not obtained! Instead,
whilst the central ring did have a negative value (of -16.4 ppm), the
two outer rings had the initially mystifying value of +4.8! In other
words they were **antiaromatic**. The electron count was in no doubt, ie
4n+2 (six). The transition state stereochemistry was clearly
disrotatory. But the resulting transition state ring was antiaromatic
and not aromatic! Well, close (and this is why it helps to have
[models)](http://www.ch.ic.ac.uk/local/organic/pericyclic/p1_electro_7.html){target="new"
rel="noopener noreferrer"} inspection reveals that despite the
disrotation, the bond which is being formed actually does so
**antarafacially**, and that the topology of this transition state is
actually Möbius and not Hückel. A little more thought reveals that a
thermal Möbius transition state with 4n+2 electrons must be
antiaromatic. So we conclude by saying that a second example of how a
disrotatory reaction can actually have Möbius character has now been
revealed. Unlike the example shown by Mauksch and Tsogoeva, this Möbius
transition state is actually *anti-aromatic*, the first example of such
which has been postulated.

That the disrotatory mode is not the fundamental here is shown by the
alternative
*[exo](http://www.ch.ic.ac.uk/local/organic/pericyclic/p1_electro_21.html){target="new"
rel="noopener noreferrer"}* cycloaddition isomer of this transition
state. The *antarafacial* nature of the bond formation is retained in
the electrocyclisation region, but now the disrotation is replaced by
***conrotation***. The two electrocyclic rings however retain their
***anti-aromaticity***, as appropriate for a 4n+2 conrotatory
electrocyclic reaction. Notice also how for both the examples, the bond
lengths in the aromatic central cycloaddition ring are fully delocalized
(equalized, \~1.4Å), whereas for the two outside rings, they are full
localized (with long \~1.480Å and short \~1.345Å bond lengths). These
two properties are of course characteristic for aromatic and
antiaromatic rings.

![](https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2020/09/Screenshot-264.jpg){.aligncenter
.size-full .wp-image-22762 decoding="async"
onclick="jmolApplet([450,450],'load wp-content/uploads/2020/09/42-endo-Bq.mol;select atomno=29;color green;label %A NICS -10.9;select atomno=30;color red; label %A NICS +9.9;select atomno=31;color red; label %A NICS +9.9;spin 3;','c3');"
width="450"
srcset="https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2020/09/Screenshot-264.jpg 802w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2020/09/Screenshot-264-300x186.jpg 300w, https://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2020/09/Screenshot-264-768x475.jpg 768w"
sizes="(max-width: 802px) 100vw, 802px"}

Let's also take a look at the example preceeding the  \[14\] annulene,
which is the analogous reaction of a \[16\] annulene. [This
reaction](http://www.ch.ic.ac.uk/local/organic/pericyclic/p1_electro_20.html){target="new"
rel="noopener noreferrer"} takes the phenomenon one stage further. The
central ring is formed by a ~π~4~a~ + ~π~4~s~ cycloaddition via a Möbius
aromatic transition state, whilst the two outer rings are again Möbius
antiaromatic transition states, but now with conventional conrotation
rather than disrotation.

Oh, there are other reactions in the above scheme. They, it turns out,
are equally fascinating. But I will leave analysis of that to another
day.

(See also [Steve
Bachrach's](http://hackberry.chem.trinity.edu/blog/){target="new"
rel="friend colleague noopener noreferrer"} blog)

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


Mauksch and Tsogoeva have recently published an article illustrating how a thermal electrocyclic reaction can proceed with distoratory ring closure, whilst simultaneously also exhibiting 4n electron Möbius-aromatic character[cite]10.1002/anie.200806009[/cite]. Why is this remarkable?

Messaggi di Rogue Scholar

A Disrotatory 4n+2 electron anti-aromatic Möbius transition state for a thermal electrocyclic reaction.