A modern take on pericyclic sigmatropic migrations.

Another common type of pericyclic reaction is the migration of hydrogen or carbon along a conjugated chain, as in the [1,3] migration of a carbon as shown below. As before, I explore the stereochemistry of the thermal and photochemical reactions.

The reaction is known to proceed thermally with inversion of configuration at the migrating carbon, and constitutes a 4n-electron reaction involving one antarafacial component (at that carbon). As usual, the thermal reaction can be rationalised by its transition state and an accompanying IRC.

The photochemical reaction (not hitherto studied as far as I know) has a conical intersection between the S₁ and S₀ states which clearly displays migration of the carbon with retention (suprafacially).

I conclude this trio of pericyclic reactions with a re-affirmation of the general rules for such reactions, which state that changing the reaction conditions from heat to light is likely invert one suprafacial or antarafacial stereochemical component,  located either on the transition state (for the thermal reaction) or the conical intersection (for the photochemical equivalent). A caveat is necessary, in that the pathways for photochemical reactions are generally more complex than thermal ones, and so exceptions to this broad statement may well be easy to find.