Imperial College London

Henry Rzepa

One thing leads to another. Thus in the [previous
post](http://www.ch.imperial.ac.uk/rzepa/blog/?p=12825 "Using a polar bond to flip: on the knife-edge!"){target="_blank"},
I described a thermal pericyclic reaction that appears to exhibit
**two** transition states resulting in **two** different stereochemical
outcomes. I noted that another such reaction appeared to be a \[1,6\]
carousel migration in homotropylium
cation,\[cite\]10.1134/S1070428007080076\[/cite\] where transition
states for both retention and inversion of the configuration of the
migrating group (respectively formally allowed and forbidden) were
reported (scheme below). Here I explore this system further.
[![homotropylium](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/homotropylium.svg){.aligncenter
.size-full .wp-image-12857 decoding="async"
width="350"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/homotropylium.svg)
Firstly, the pathway leading to
inversion.\[cite\]10.6084/m9.figshare.1134556\[/cite\] The reaction path
(ωB97XD/6-311G(d,p)/SCRF=chloroform) has got a very odd (table-top
mountain) shape, whereby the region of the transition state (IRC = 0.0)
is very flat, and the region close to reactant and (identical) product
is very steep. The gradient norm shows this best, with sharp spikes at
IRC ± 4.2. Something clearly is happening here to cause this behaviour.
Before moving on to analyze this, I want you first to observe the methyl
groups below. Note how one of them rotates at the start of the process,
and the other at the end. I have elsewhere called this behaviour the
[methyl
flag](http://www.ch.imperial.ac.uk/rzepa/blog/?p=5087 "cis-Butene: a reaction coordinate dissected and methyl flags."){target="_blank"},
and it is due to stereoelectronic re-alignments of the C-H groups
accompanying the changes in the conjugated array.
[![htropa](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/htropa.gif){.aligncenter
.size-full .wp-image-12854 decoding="async"
width="400"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/htropa.gif)
[![htrop](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/htrop.svg){.aligncenter
.size-full .wp-image-12855 decoding="async"
width="400"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/htrop.svg)
[![htropG](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/htropG.svg){.aligncenter
.size-full .wp-image-12856 decoding="async"
width="400"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/htropG.svg)
The homotropylium cation is said to be homoaromatic, indicating that
cyclic conjugation can be maintained across a ring in which the σ
framework is interrupted at one point. A NICS probe placed at the ring
critical point of this molecule reveals a chemical shift of -11.3
ppm\[cite\]10.6084/m9.figshare.1135694\[/cite\], very similar to eg that
obtained for benzene itself. The three highest doubly occupied NBOs
(below) show two normal π-type orbitals and one rather different one
that spans the homo-bond (the MOs, before you ask, are a bit of a mess,
with lots of mixed contributions from other parts of the σ framework).

<table>
<colgroup>
<col style="width: 50%" />
<col style="width: 50%" />
</colgroup>
<tbody>
<tr class="odd">
<td>HONBO (two)</td>
<td>HONBO-2</td>
</tr>
<tr class="even">
<td><div id="attachment_12866" class="wp-caption aligncenter"
style="width: 210px">
<img
src="http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/htrop.jpg"
class="size-full wp-image-12866" decoding="async"
aria-describedby="caption-attachment-12866"
onclick="jmolInitialize(&#39;../Jmol/&#39;);jmolSetAppletColor(&#39;white&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2014/08/htrop_mo36.cub.xyz;isosurface wp-content/uploads/2014/08/htrop_mo36.cub.jvxl translucent;&#39;);"
width="200" alt="Click for 3D" />
<p>Click for 3D</p>
</div></td>
<td><div id="attachment_12867" class="wp-caption aligncenter"
style="width: 210px">
<img
src="http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/htrop-2.jpg"
class="size-full wp-image-12867" decoding="async"
aria-describedby="caption-attachment-12867"
onclick="jmolInitialize(&#39;../Jmol/&#39;);jmolSetAppletColor(&#39;white&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2014/08/htrop_mo34.cub.xyz;isosurface wp-content/uploads/2014/08/htrop_mo34.cub.jvxl translucent;&#39;);"
width="200" alt="Click for 3D" />
<p>Click for 3D</p>
</div></td>
</tr>
</tbody>
</table>

At the transition state for the \[1,6\] migration, the same NICS probe
registers a value of +2.6
ppm\[cite\]10.6084/m9.figshare.1135695\[/cite\], now firmly in the
non-aromatic zone. So this reaction is characterised by two zones,
ring-aromatic ones at the start and the end of the process, and a higher
energy non-aromatic one in the middle of the reaction pathway as
\~enclosed by the region of IRC ± 4.2. The homo-bond in the aromatic
zone starts with a length of 1.74Å, reduces to 1.53Å at the transition
state and ends up as a normal aromatic bond of length 1.41Å. Meanwhile,
the relocated homo-bond changes in the opposite sense, starting as a
normal aromatic length of 1.41Å, becoming 1.53Å at the transition state
and ending as a homo-length of 1.74Å. Presumably, virtually full
strength homoaromaticity can be sustained for a homo-bond of 1.74Å,
but as that bond mutates to a σ-bond of 1.53Å, the cyclic conjugation
falls off the edge of the cliff, to be restored only at the end.
Pericyclic reactions are themselves said to sustain aromatic transition
states,\[cite\]10.1021/ed084p1535\[/cite\] and so a simplistic way of
looking at this is that the "aromaticity" relocates (or morphs) from the
reactant to the transition state, and then back again during the course
of the migration. A reaction path from which one can indeed learn a lot.

Now to the pathway in which the migrating group retains
configuration. This is no longer a single step concerted
reaction,\[cite\]10.6084/m9.figshare.1135668\[/cite\] since at the
half-way point we no longer have a transition state but a shallow
intermediate (\~IRC +2, \[cite\]10.6084/m9.figshare.1134559\[/cite\]).
It (formally at least) becomes a two-step non-concerted process, and the
overall barrier is \~5 kcal/mol **lower** than for the inversion path.
The aromaticity changes in a similar manner to before (i.e. IRC
\~-5).[![htrop-ra](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/htrop-ra.gif){.aligncenter
.size-full .wp-image-12869 decoding="async"
width="400"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/htrop-ra.gif)

[![Htrop-ret](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/Htrop-ret.svg){.aligncenter
.size-full .wp-image-12860 decoding="async"
width="400"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/Htrop-ret.svg)[![Htrop-retG](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/Htrop-retG.svg){.aligncenter
.size-full .wp-image-12859 decoding="async"
width="400"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/Htrop-retG.svg)

So this emerges as not quite the example I thought it was, but
nonetheless unusual with the "forbidden" pathway being concerted and the
"allowed" pathway being non-concerted. Molecular dynamics on these two
systems would indeed be interesting to see what proportion of the
trajectories go *via* each pathway.

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


One thing leads to another. Thus in the previous post, I described a thermal pericyclic reaction that appears to exhibit
<strong>
 two
</strong>
transition states resulting in
<strong>
 two
</strong>
different stereochemical outcomes.

Postagens de Rogue Scholar

An unusual [1,6] shift in homotropylium cation exhibiting zones of aromaticity.