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Electrocyclic reactions: examples of 2- and 4-electron systems
2-electron systems (cyclopropyl cation / allyl cation)
The smallest system that an electrocyclic reaction can enter into is the opening of the cyclopropyl cation to the allyl cation. With two electrons involved, the reaction is disrotatory.
The cleavage of the tosylate (OTs) and the ring opening presumably take place simultaneously, so that no free cyclopropyl cation occurs as an intermediate stage.
4-electron systems (cyclobutene / butadiene)
The pyrolysis of deuterium-labeled shows how strictly the stereochemical selection rules are adhered to(Z, E) -2,3,4,5-tetraphenyl-2,4-hexadiene. By conrotatory ring closure and conrotatory ring opening in the other direction, an equilibrium is quickly established between (E, Z) - and(Z, E) -Connection. But even after 51 days there are still no detectable amounts of(Z, Z) -Diene arose, which can only arise through the forbidden disrotatory path: