Pericyclic Reactions Dr. Tripti Gangwar
Pericyclic ReactionsDr. Tripti Gangwar
Electrocyclic Reactions
• Concerted reactions in which polyolefins containing pi bonds cyclize to give product with one less pi bond and formation of one new sima bond or vice versa.
• These are the first concerted reactions discovered by Woodward and Hoffmann in 1965
• Ring opening or ring closing take place in two modes:
• Conrotation: Rotation of p orbital in the same direction by the angle of 900 in order to form sigma bond.
• Disrotation: Rotation of p orbital in opposite direction
Frontier MO
• Electrocyclic reactions are possible in two types pi system
• (4n)pi and (4n+2) pi
• Reaction Thermal Photochemical
• 4n Conrotation (Even Disrotation odd
• 4n+2 Disrotation odd Conrotation even
Allyl System:
CH2=CH-CH3 Electron=32,1 Free Radical2 Cation2,2 Anion
Electrocyclic Reaction: One componentHOMO is considered
Hexatriene: Photochemical HOMO=ψ4 ( Odd): Disrotation
Thermal Cyclisation: 4n) pi, HOMO ψ2 ( Even) , Terminals Opposite Phase, Symmetry allowed by Conrotation
Photochemical cyclisation : HOMO ψ3,(odd), terminal in same phase: Symmetry allowed by Disrotation
• 4n Pi
• HOMO odd Disrotation Photochemical 4n Thermal 4n+2
• HOMO even Conrotation thermal 4n Photochemical 4n+2
• 4n+2 ethene n=0, Hexatriene n=1
Photochemical Cyclisation 4
Thermal Cyclisation
Butadiene: 4pi eThermal HOMO ψ2 (even) Terminal orbital in different phase