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