Imperial College London

Henry Rzepa

In my [first
post](http://www.ch.imperial.ac.uk/rzepa/blog/?p=12782 "Using a polar bond to flip the (stereochemical) outcome of a pericyclic reaction."){target="_blank"}
on the topic, I discussed how inverting the polarity of the C-X bond
from X=O to X=Be (scheme below) could flip the stereochemical course of
the electrocyclic pericyclic reaction of a divinyl system. This was
[followed
up](http://www.ch.imperial.ac.uk/rzepa/blog/?p=12810 "Using a polar bond to flip: a follow up project."){target="_blank"}
by exploring what happens at the half way stage, *i.e.* X=CH~2~, the
answer being that one gets an ***antarafacial*** pathway as with X=O.
Here I fill in another gap, X=BH to see if a metaphorical microscope can
be used to view the actual region of the "flip" to a ***suprafacial***
mode.[![divinylketon](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/divinylketon.svg){.aligncenter
.size-full .wp-image-12783 decoding="async"
width="350"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/divinylketon.svg)
This time, uniquely, it proved possible to locate TWO transition states
for this process, one
*suprafacial*\[cite\]10.6084/m9.figshare.1133933\[/cite\] and one
*antarafacial*\[cite\]10.6084/m9.figshare.1133934\[/cite\], this latter
being 10.5 kcal/mol lower in ΔG^†^
(ωB97XD/6-311G(d,p)/SCRF=dichloromethane). It is quite rare to be able
to find BOTH stereochemical outcomes of a thermal pericyclic
reaction.^‡^

First, the antarafacial IRC
(X=BH)\[cite\]10.6084/m9.figshare.1133936\[/cite\]. There are several
interesting features. Note at IRC = -8, the divinyl compound appears as
a ***Hidden Intermediate*** (HI), having formed from a compound where
the HB=C substituent has ring opened from a cyclobutene-like precursor
(initial electrocyclic). If you watch the animation, you can see the
antarafacial bond forming from the **bottom** face of the vinyl group on
the left to the **top** face of the vinyl group in the HI on the right
(antarafacial=conrotation). Because the entire process is concerted (no
real intermediates participate), we have here an unusual *pericyclic
cascade* where one electrocyclic reaction is immediately followed by
another quite different
one.[![BH-antaraa](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/BH-antaraa.gif){.aligncenter
.size-full .wp-image-12839 decoding="async"
width="400"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/BH-antaraa.gif)
[![BH-antara](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/BH-antara.svg){.aligncenter
.size-full .wp-image-12841 decoding="async"
width="400"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/BH-antara.svg)
[![BH-antaraG](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/BH-antaraG.svg){.aligncenter
.size-full .wp-image-12840 decoding="async"
width="400"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/BH-antaraG.svg)
Now for the suprafacial IRC\[cite\]10.6084/m9.figshare.1134015\[/cite\].
It is pretty similar to the previous path, but again if you inspect very
carefully you will see that it is the **TOP** face of the vinyl group on
the left forming the bond to the **TOP** face of the vinyl group on the
right (suprafacial/disrotation).
[![BH-supraa](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/BH-supraa.gif){.aligncenter
.size-full .wp-image-12842 decoding="async"
width="400"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/BH-supraa.gif)[![BH-supra](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/BH-supra.svg){.aligncenter
.size-full .wp-image-12849 decoding="async"
width="400"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/BH-supra.svg)[![BH-supraG](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/BH-supraG.svg){.aligncenter
.size-full .wp-image-12850 decoding="async"
width="400"}](http://www.ch.ic.ac.uk/rzepa/blog/wp-content/uploads/2014/08/BH-supraG.svg)
You might ask if the molecules used here are realistic, *i.e.* could
they form the basis of real reactions to be conducted in a laboratory?
Well, the C=B-C fragment has 9 hits in the CCDC crystal database (none
for C=B-H). One example is cited
here.\[cite\]10.1002/anie.199513401\[/cite\]. So, yes, possibly a
realistic system, except the barriers do look too high. Perhaps suitable
substituents might help? But even if this could not be carried out in a
test-tube, it does teach one about pericyclic reactions and how one
might manipulate them.

------------------------------------------------------------------------

^‡^One such is the \[1,6\] sigmatropic shift in homotropylium cation
involving migration of a Me~2~C^+^ group, where the "allowed" process in
which the migrating group retains its configuration has a barrier of
17.7 kcal/mol and the "forbidden" route where the migrating group
inverts its configuration with a barrier of 38.6 kcal/mol (thanks to
Alex Genaev; I will strive to make the coordinates available *via* the
repository shortly).

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[https://orcid.org/0000-0002-8635-8390](https://orcid.org/0000-0002-8635-8390){target="_blank"
rel="author"}
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In my first post on the topic, I discussed how inverting the polarity of the C-X bond from X=O to X=Be (scheme below) could flip the stereochemical course of the electrocyclic pericyclic reaction of a divinyl system.

Messaggi di Rogue Scholar

Using a polar bond to flip: on the knife-edge!