Pericyclic Reactions: Electrocyclic Reactions
Electrocyclic Reactions: Examples for Systems with 2 and 4 Electrons.
2-Electron sytems (cyclopropyl cation/allylic cation)
Opening of a cyclopropyl cation to an allylic cation involves the smallest system capable of undergoing an electrocyclic reaction. With two participating electrons, the reaction is disrotatory.
Presumably, removal of the tosylate (-OTs) and ring opening occur simultaneously, therefore, a cyclopropyl cation is not considered an intermediate.
The observation of stereochemical selection rules is illustrated by the pyrolysis of deuterium-marked (Z,E)-2,3,4,5-tetraphenyl-2,4-hexadiene. An equilibrium between the (E,Z)- and (Z,E)-compounds is reached very fast by conrotatory ringclosure on one side and conrotatory ring opening in the other direction. Even after 51 days, no traces of the (Z,Z)-diene could be found. This compound could only be formed by the forbidden disrotatory pathway.