Imperial College London

Henry Rzepa

Woodward and Hoffmann published their[ milestone
article](http://dx.doi.org/10.1021/ja01080a054){target="_blank"} 
"Stereochemistry of Electrocyclic Reactions" in 1965. This brought
maturity to the electronic theory of organic chemistry, arguably started
by the [proto-theory of
Armstrong](http://www.ch.imperial.ac.uk/rzepa/blog/?p=5411){target="_blank"}
some 75 years earlier. Here, I take a modern look at the archetypal
carrier of this insight, the ring opening of dimethylcyclobutene.

[![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/electrocyclic.svg "electrocyclic"){.aligncenter
.size-full .wp-image-5633 decoding="async"
width="200"}](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/electrocyclic.svg)

The thermal (Δ) reaction is defined by the ***transition state***. The
remarkable feature noted by Woodward and Hoffmann was the
stereospecificity of pericyclic reactions. In this example, the breaking
**σ-bond** in the transition state is defined by its connectivity to the
top face of one terminus (the red arrow) and the bottom face of the
other terminus (the green arrow). The technical name for this is
***antarafacial***, and this is also associated with a C~2~ axis of
symmetry for (this particular) example. The modern theoretical
explanation for this is a
[Möbius-aromatic](http://dx.doi.org/10.1021/ed084p1535){target="_blank"} transition
state resulting from a total of **4n** circulating electrons (note the
[methyl
flags](http://www.ch.imperial.ac.uk/rzepa/blog/?p=5087){target="_blank"}
waving).

<table data-border="0">
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<tr class="odd">
<td><a
href="http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/open.svg"><img
src="http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/open.svg"
title="open" class="aligncenter size-full wp-image-5636"
decoding="async" width="200" /></a></td>
<td><a
href="http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/openg.svg"><img
src="http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/openg.svg"
title="openg" class="aligncenter size-full wp-image-5637"
decoding="async" width="200" /></a></td>
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<tr class="even">
<td><div id="attachment_5635" class="wp-caption aligncenter"
style="width: 210px">
<img
src="http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/con-open.gif"
title="con-open" class="size-full wp-image-5635" loading="lazy"
decoding="async" aria-describedby="caption-attachment-5635"
onclick="jmolInitialize(&#39;../Jmol/&#39;);jmolSetAppletColor(&#39;yellow&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2011/11/cyclobutene-con.log;frame 17; zoom 100;vectors on;vectors 4;vectors scale 5.0; color vectors green; vibration 20;animation mode loop;&#39;);"
width="200" height="121" />
<p>IRC for Electrocylic ring opening of dimethyl cyclobutene. Click for
3D.</p>
</div></td>
<td><img
src="http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/cyclobutene-con.jpg"
title="cyclobutene-con" class="aligncenter size-full wp-image-5639"
loading="lazy" decoding="async"
onclick="jmolInitialize(&#39;../Jmol/&#39;);jmolSetAppletColor(&#39;yellow&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2011/11/cyclobutene-con.log;frame 17; zoom 100;vectors on;vectors 4;vectors scale 5.0; color vectors green; vibration 20;animation mode loop;&#39;);"
width="200" height="136" /></td>
</tr>
</tbody>
</table>

The concept of aromatic (or anti-aromatic) transition states is a very
useful one for thermal reactions, but this transition state becomes a
little less helpful for the photochemical (hν) version. Instead, a new
concept is introduced of a [conical
intersection](http://dx.doi.org/10.1021/ja00161a013){target="_blank"}
between the (thermal) ground state and the (photochemical) excited
state. Think of it in terms of the famous painting showing God (in an
exalted state) touching Adam (very much on the ground).

::: {#attachment_5640 .wp-caption .aligncenter style="width: 310px"}
[![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/500px-Creación_de_Adán_Miguel_Ángel.jpg "500px-Creación_de_Adán_(Miguel_Ángel)"){.size-full
.wp-image-5640 loading="lazy" decoding="async"
aria-describedby="caption-attachment-5640" width="300"
height="136"}](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/500px-Creación_de_Adán_Miguel_Ángel.jpg)

A schematic conical intersection.
:::

The conical intersection is the geometry at which a photochemically
excited molecule leaves the S~1~ state and returns to the ground S~0~
state. It is this point that determines the resulting stereochemistry.
That for dimethyl-cyclobutene ([casscf(12,8)/6-31g(d,p)
model](http://hdl.handle.net/10042/to-10495){target="_blank"}) is shown
below. On the right hand side of the molecule, the σ-bond region looks
very similar to that of the thermal transition state shown above; the
bond is associated with the **bottom** face of the molecule. However,
the left hand side is rotated clockwise relative to the thermal
reaction, and this rotation now presents the bottom face for connection
to the σ-bond (rather than the top face as for the thermal case).
The  σ-bond is thus connected ***suprafacially***.

::: {#attachment_5641 .wp-caption .aligncenter style="width: 269px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/cyclobutene-dis.jpg "cyclobutene-dis"){.size-full
.wp-image-5641 loading="lazy" decoding="async"
aria-describedby="caption-attachment-5641"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('green');jmolApplet([450,450],'load wp-content/uploads/2011/11/cyclobutene-conical.mol;');"
width="259" height="175"}

The conical intersection for the photochemical reaction of
dimethylcyclobutene. Click for 3D
:::

When the rules are presented to students, the photochemical case is
often defined as the inverse of the thermal rule. Thus for the same
electron count (4n in this case), a thermal reaction is
***antarafacial*** and the photochemical one ***suprafacial***. A more
fascinating question is whether the aromaticity associated with the key
geometry should also be inverted. Thus the thermal reaction corresponds
to a ***Möbius-aromatic*** ([topological linking
number](http://www.ch.imperial.ac.uk/rzepa/blog/?p=2046){target="_blank"}
=1π) transition state. Should the photochemical reaction rule be
inverted to refer to a ***Hückel-aromatic***(topological linking number
= 0π) conical intersection?

I am unaware of any formal studies of the *aromaticity of conical
intersections* specifically, but it would be nice to know if this
analogy has any reality. Watch this space.

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


Woodward and Hoffmann published their milestone article  “Stereochemistry of Electrocyclic Reactions” in 1965. This brought maturity to the electronic theory of organic chemistry, arguably started by the proto-theory of Armstrong some 75 years earlier. Here, I take a modern look at the archetypal carrier of this insight, the ring opening of dimethylcyclobutene.

Rogue Scholar Beiträge

A modern take on the pericyclic electrocyclic ring opening of cyclobutene.