Imperial College London

Henry Rzepa

The four-electron thermal cycloaddition (in reverse a cheletropic
elimination) of dichlorocarbene to ethene is a classic example of a
forbidden pericyclic process [taking a roundabout
route](http://www.ch.imperial.ac.uk/rzepa/blog/?p=6977){target="_blank"}
to avoid directly violating the Woodward-Hoffmann rules. However, a
thermal six-electron process normally does take the direct route, as in
for example the Diels-Alder cycloaddition as Houk and co have recently
showed using molecular dynamics\[cite\]10.1073/pnas.1209316109\[/cite\].
So can one contrive a six-electron cycloaddition involving
dichlorocarbene?

![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/09/carbene-b.svg "carbene-b"){.aligncenter
.size-full .wp-image-7743 decoding="async"}

Surely, it should now form the two new C-C bonds at the same time
(synchronously)? Well, here comes a
[ωB97XD/6-311G(d,p)/SCRF=dichloromethane](http://hdl.handle.net/10042/20328){target="_blank"}
intrinsic reaction coordinate calculation:

::: {#attachment_7745 .wp-caption .aligncenter style="width: 331px"}
![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/09/butadiene+carbene.gif "butadiene+carbene"){.wp-image-7745
loading="lazy" decoding="async"
aria-describedby="caption-attachment-7745" width="321" height="322"}

Butadiene + dichlorocarbene.
:::

1.  The reaction starts at IRC -5, 

2.  and proceeds with only a small barrier to the transition state (IRC
    =0.0) 

3.  At IRC +4, the potential flattens out and the gradients drop, with
    formation of the first C-C bond completed. But the gradients do not
    quite go to zero, which would have implied the formation of a
    discrete intermediate such
    as:![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/09/carbene1.svg "carbene1"){.aligncenter
    .size-full .wp-image-7749 decoding="async"}

4.  The concerted reaction continues and by IRC \~ +11, the two chlorine
    atoms now exhibit quite different C-Cl lengths. The one that is
    orthogonal to the second forming C-C bond is normal (1.815Å),
    whereas the one antiperiplanar to the C-C bond is 1.92Å. There are
    some interesting stereoelectronic alignments involved.

5.  Coincidentally perhaps, but these phenomena of an intermediate
    almost forming in a system containing a CCl~2~ group with
    concomitant lengthening of one C-Cl bond compared to the other, was
    also observed [in my IRC for the addition of thiolate to a
    dichlorobuteneone](http://www.ch.imperial.ac.uk/rzepa/blog/?p=7495&cpage=1#comment-10361){target="_blank"}.
    For that system,  Dan Singleton's work had shown that molecular
    dynamics is necessary to obtain a more complete picture, and that
    may well be also true for the example here!  Perhaps Ken Houk might
    give it a go!

6.  The second C-C bond then completes at around IRC +16.

    ![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/09/butadiene+carbene.svg "butadiene+carbene"){.aligncenter
    .size-full .wp-image-7746 decoding="async"}

    ![](http://www.ch.imperial.ac.uk/rzepa/blog/wp-content/uploads/2012/09/butadiene+carbeneg.svg "butadiene+carbeneg"){.aligncenter
    .size-full .wp-image-7747 decoding="async"}

Well, this shows that a reaction only modestly removed from the
classical six-electron Diels-Alder can change character dramatically
from the synchrony expected of the latter. I am hunting for a simple
explanation of this phenomenon, but perhaps participation of the C-Cl
bonds makes this different from a simple cycloaddition. Or possibly, the
explanation will only properly emerge when the molecular dynamics is
studied?

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


The four-electron thermal cycloaddition (in reverse a cheletropic elimination) of dichlorocarbene to ethene is a classic example of a forbidden pericyclic process taking a roundabout route to avoid directly violating the Woodward-Hoffmann rules.

Messages de Rogue Scholar

The direct approach is not always the best: butadiene plus dichlorocarbene