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.
First, the antarafacial IRC (X=BH)[3]. 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.
‡One such is the [1,6] sigmatropic shift in homotropylium cation involving migration of a Me2C+ 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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