In this earlier post, I described how the stereochemistry of π2+π2 cycloadditions occurs suprafacially if induced by light, and how one antarafacial component appears if the reaction is induced by heat alone. I also noted how Woodward and Hoffmann (WH) explained that violations to their rules were avoided by mandating a change in mechanism requiring stepwise pathways with intermediates along the route. Here I illustrate how the stereochemistry of a thermal π2+π2 cycloaddition can indeed avoid an antarafacial component by performing appropriate gymnastic contortions instead of a mechanistic change (a WH violation certainly in the letter of their law, if not their spirit).
The IRC for the reaction (R=CH=S) shows the two molecules moving towards each other in a trapezoidal motion, with the axes of the C=C bonds parallel and the faces clearly suprafacial. At IRC ~-4, the nature of motion changes into a slide of one C=C axis along the other, converting the trapezoidal geometry into a square. By doing so, a full frontal “violation” is changed into a more subtle sideways insertion.This trapezoidal motif for a thermal reaction does require the two R groups to have electron withdrawing nature, but it is noteworthy that the same motif was found for the photochemically induced π2+π2 cycloaddition. That both the thermal AND the photochemical pathways have essentially the same geometry is indeed a violation of WH!