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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.

Fig.1: Disrotatory ring opening of a three-membered ring

2-D Animation of three-membered ring opening

Presumably, removal of the tosylate (-OTs) and ring opening occur simultaneously, therefore, a cyclopropyl cation is not considered an intermediate.

4-Electron systems (cyclobutene / butadiene)

Fig.2: Thermal conrotatory ring opening of cis-cyclobutenes

2-D Animation of conrotatory ring opening

Fig.3: Thermal conrotatory ring opening of trans-cyclobutenes

2-D Animation of conrotatory ring opening

Fig.4: Photochemical disrotatory cyclization leading to cis-cyclobutenes

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.