The π2 + π2 cyclodimerisation of cis-butene is the simplest cycloaddition reaction with stereochemical implications. I here give it the same treatment as I did previously for electrocyclic pericyclic reactions.

Imperial College London

Henry Rzepa

The π~2~ + π~2~ cyclodimerisation of
[*cis*-butene](http://www.ch.imperial.ac.uk/rzepa/blog/?p=5087){target="_blank"}
is the simplest cycloaddition reaction with stereochemical implications.
I here give it the same treatment as I did previously for [electrocyclic
pericyclic
reactions](http://www.ch.imperial.ac.uk/rzepa/blog/?p=5632){target="_blank"}.

[![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/2+2.svg "2+2"){.aligncenter
.size-full .wp-image-5656 decoding="async"
width="300"}](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/2+2.svg)  
The photochemical reaction is
[known](http://dx.doi.org/10.1021/ja01030a066){target="_blank"} to give
a mixture of two tetramethylcyclobutanes in the ratio of 1.3:1.0, with
the all-*cis* isomer apparently predominating. The key geometry is the
***conical intersection***, at which the energies of the S~1~ and S~0~
states coincide. This geometry has a typical trapezoidal appearance,
with ***suprafacial*** addition accross both components. The [*exo*
addition](http://hdl.handle.net/10042/to-10497){target="_blank"} is
calculated to be about 1 kcal/mol lower in total energy than the
[endo](http://hdl.handle.net/10042/to-10387){target="_blank"} (at the
CASSCF(12,8)/6-31G(d) level), which implies that the latter should be
the minor and not the major form. However, these CASSCF energies are not
corrected for thermal (entropic) or dynamic correlation components, and
moreover the active space orbitals are probably not identical either (I
demonstrated in [another
post](http://www.ch.imperial.ac.uk/rzepa/blog/?p=5040){target="_blank"}
how the orbitals of the alkene interact with those of the methyl groups,
and its quite likely that the *endo* and *exo* orientations will result
in slightly different interactions) which makes such comparisons non
trivial. These calculations do support the idea that both isomers should
form (which at first sight might be counter-intuitive given the apparent
steric constraints of the *endo* isomer).

<table class="aligncenter" data-border="0">
<colgroup>
<col style="width: 50%" />
<col style="width: 50%" />
</colgroup>
<tbody>
<tr class="odd">
<td><div id="attachment_5657" class="wp-caption aligncenter"
style="width: 210px">
<img
src="http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/2+2-exo.jpg"
title="2+2-exo" class="size-full wp-image-5657" loading="lazy"
decoding="async" aria-describedby="caption-attachment-5657"
onclick="jmolInitialize(&#39;../Jmol/&#39;);jmolSetAppletColor(&#39;red&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2011/11/2+2-exo.cml;measure 1 2;measure 3 4;measure 2 4;&#39;);"
width="200" height="176" />
<p>2+2 exo addition. click for 3D</p>
</div></td>
<td><div id="attachment_5659" class="wp-caption aligncenter"
style="width: 210px">
<img
src="http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/2+2-endo.jpg"
title="2+2-endo" class="size-full wp-image-5659" loading="lazy"
decoding="async" aria-describedby="caption-attachment-5659"
onclick="jmolInitialize(&#39;../Jmol/&#39;);jmolSetAppletColor(&#39;yellow&#39;);jmolApplet([450,450],&#39;load wp-content/uploads/2011/11/2+2-endo.mol;measure 1 2;measure 3 4;measure 2 4;&#39;);"
width="200" height="188" />
<p>2+2 endo addition. Click for 3D.</p>
</div></td>
</tr>
</tbody>
</table>

The thermally activated reaction is not known for this alkene, and the
calculations support this with an enormous barrier to reaction (\> 68
kcal/mol).

[![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/2a+2s_me.svg "2a+2s"){.aligncenter
.size-full .wp-image-5660 decoding="async"
width="300"}](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/2a+2s_me.svg)

As a 4n-electron pericyclic, the selections rules require there to be
one ***antarafacial*** component present, and the [IRC for this
reaction](http://hdl.handle.net/10042/to-10509){target="_blank"}
illustrates this very nicely. The formation of two pairs of C-C bonds is
very asynchronous. Only when the first bond is almost complete does the
second C=C start to rotate. The second C-C bond only starts to form
after this rotation (the *antarafacial* component) is essentially
complete, forming a product where one methyl group is on the opposite
face of the ring to the other three. Note in particular that the rhs
alkene has the two C-H hydrogens *synplanar* to start with, but that
they are exactly *antiperiplanar* in the product.

::: {#attachment_5691 .wp-caption .aligncenter style="width: 310px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2011/11/2+2a.gif "2+2a"){.size-full
.wp-image-5691 decoding="async"
aria-describedby="caption-attachment-5691"
onclick="jmolInitialize('../Jmol/');jmolSetAppletColor('yellow');jmolApplet([450,450],'load wp-content/uploads/2011/11/2a+2s_me.log;frame 41; zoom 100;measure 3 4;measure 1 2;vectors on;vectors 4;vectors scale 5.0; color vectors green; vibration 20;animation mode loop;');"
width="300"}

2a + 2s cycloaddition showing IRC. Click for 3D
:::

For this small system the two critical points, a conical intersection
and a transition state, could not be more different. But they do capture
the essential features of pericyclic reactions and their selection
rules.

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


The π
<sub>
 2
</sub>
+ π
<sub>
 2
</sub>
cyclodimerisation of
<em>
 cis
</em>
-butene is the simplest cycloaddition reaction with stereochemical implications. I here give it the same treatment as I did previously for electrocyclic pericyclic reactions. The photochemical reaction is known to give a mixture of two tetramethylcyclobutanes in the ratio of 1.3:1.0, with the all-
<em>
 cis
</em>
isomer apparently predominating.

Messaggi di Rogue Scholar

A modern take on pericyclic cycloaddition. Dimerisation of cis-butene