Title Pericyclic Reactions Cycloaddition Electrocyclization

Ethylene p-Molecular Orbitals 171 nm 167 kcal/mol 400 nm 800 nm 200 nm visible UV IR

1,3-Butadiene from Ethylene Ethylene/Butadiene 1,3-Butadiene from Ethylene

1,3-Butadiene p-Molecular Orbitals 214 nm 133 kcal/mol 400 nm 800 nm 200 nm visible UV IR

1,3,5-Hexatriene p-Molecular Orbitals 258 nm 109 kcal/mol 400 nm 800 nm 200 nm visible UV IR

Butadiene Orbital Coeffients Butadiene: Orbital Coefficients Frontier Molecular Orbitals FMOs

The Allylic System: Allyl Cation

The Allylic System: Allyl Radical

The Allylic System: Allyl Anion

Cycloaddition: Diels-Alder Reaction An Allowed [4+2] Cycloaddition

Diels-Alder Reaction: The Effect of Electron Withdrawing Groups

Allyl Cation DA [4+2]-Cycloaddiitions

Diels-Alder Mechanism Diels-Alder Reaction: Mechanism

Diels-Alder Endo Effect Diels-Alder Reaction: The Endo Effect Secondary Effect

What About a [2+2] Cycloaddition? 2+2? What About a [2+2] Cycloaddition? light (hn)

Thermally Allowed Cycloadditions 4n+2 Rule Thermally Allowed Cycloadditions 4n+2 Rule HOMOs LUMOs A S 2 S A 4 S A 6 [4+2]=6 S A 8 [6+4]=10 [8+6]=14 4n+2; n=1,2,3,4…..

Photochemically Allowed Cycloadditions 4n Rule Photochemically Allowed Cycloadditions 4n Rule HOMOs LUMOs A S 2 4 6 8 [2+2]=4 [4+4]=8 [6+6]=12 [8+8]=16 4n; n=1,2,3,4….. and [2+6]=8; [8+4]=12

Summary of Cycloadditions Thermal 4n+2 Photochemical 4n

Electrocyclizations Electrocyclizations

Butadiene-Cyclobutene

(E,E)-Butadiene-Cyclobutene

(E,Z)-Butadiene-Cyclobutene

(E,Z,E)-Hexatriene-Cyclohexadiene

(E,Z,E)-Hexatriene-Cyclohexadiene Orbitals

Electrocyclization Summary thermal photochemical 4n con dis 4n+2

Violations “There are none!” Violations Woodward and Hoffmann,The Conservation of Orbital Symmetry