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. This was followed up by exploring what happens at the half way stage, i.e. X=CH2, 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.
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.‡