Definition of Electrocyclic Reactions:
The Electrocyclic ring closure reaction is defined as the reaction of an acyclic conjugated polyene system in which the terminal carbon atoms interact forming a sigma bond and thus forming a cyclic system. During the course of the ring closure step the pi system is reorganized in a concerted manner. For the sake of convenience the reorganization of the electrons can be represented by curved arrows as shown below for the electrocyclization of 1,3-butadiene and 1,3,5-hexatriene.
The curved arrows can flow either in the clockwise or in the anti-clockwise direction. Electrocyclic reactions are generally reversible and thus it is possible to go from the cyclic system by the breaking of the sigma bond and the reorganization of the pi bonds to an acyclic conjugated polyene. It must be noted here that the polyenes are drawn in the s-cis conformation so that the carbons at the termini of the polyene system are able to approach each other for bond formation. In the s-trans conformation this is not possible because the distance between the terminal carbons is too far apart for bond formation.
Classification of Electrocyclic Reactions:
Electrocyclic reactions are classified according to the number of electrons involved in the cyclic transition state. For example in the electrocyclization of butadiene all four pi electrons are involved In the reverse reaction two pi?electrons and two sigma electrons are involved. Therefore the electrocyclization of butadiene to cyclobutene and the reverse reaction correspond to a 4 electron system, or a 4n electron system where n = 1. Similarly if we analyze the electrocyclization of hexatriene and the reverse reaction we come to the conclusion that the forward reaction involves 6 pi?electrons whereas the reverse reaction involves 4 pi electrons and two sigma electrons making it a 6 electron system or a 4n+2 electron system where n = 1 .
Stereochemistry of Electrocyclic Reactions:
Consider the ring opening reaction of cis-3,4-dimethylcyclobutene During the breaking of the sigma?bond carbon atoms C3 and C4 can either twist in the same direction or in the opposite direction. The stereochemistry of the resulting product will depend upon the mode of twisting of the C1-C4 and C2-C3 sigma bonds. If these two sigma bonds are twisted in the same direction then the stereochemistry of the resulting product is Z,E-hexa-2,4-diene. This mode of ring opening is termed as conrotatory ring opening. On the other hand if the sigma bonds are twisted in the opposite direction then the stereochemistry of the resulting product is either E,E- or Z,Z-hexa-2,4-diene depending upon whether substituents are twisted inward or outward . This mode of ring opening is termed as the disrotatory ring opening. The same processes can take place during the ring closure reaction as well. Thus Z,E-hexa-2,4-diene on conrotatory ring closure will give the cis-3,4-dimethylcyclobutene whereas the E,E- or the Z,Z- isomers on conrotatory ring closure will give trans-3,4-dimethylcyclobutene. Similarly, the Z,E-hexa-2,4-diene on disrotatory ring closure will give trans-3,4-dimethylcyclobutene whereas the E,E- and Z,Z-isomers will give the cis isomer of the 3,4-dimethylcyclobutene .
The Woodward-Hoffmann rules for the electrocyclic reactions tell us whether a system will undergo the conrotatory or the disrotatory mode of reaction under a given condition. It is dictated by the number of electrons involved in the reaction, namely 4n or 4n+2 system of electrons and also on whether the reaction is thermally or photochemically activated.
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