D A. Evans Introduction to Photochemistry Chem 206 Photochemical reactions Hexatriene. Frontier MO Description derotation Fleming I. Frontier Orbitals and Organic Chemical Reactions", Chapter 6. " Photochemical reactio Reactions of olefins Consider [2+ 2] cycloaddition: Photochemical activation MO HOMO 中1e 中3 199 The rotatory motion of ning closure may be reversed by photo-activation One of the first cases where heat and light induced electrocyclization t concerted followed different pathways energy Havinga, Tetrahedron, 1961, 16, 1 The(2+ 2 cycloaddition of two olefins exhibits the option of being concerted if one of the olefins reacts out of its photochemically excited state Excited State Geometry disputation connotation 84 xcited state('s and TD. A. Evans Introduction to Photochemistry Chem 206 Photochemical Reactions Frontier MO Description Fleming, I. "Frontier Orbitals and Organic Chemical Reactions", Chapter 6, "Photochemical Reactions" Reactions of Olefins p* p concerted The [2 + 2] cycloaddition of two olefins exhibits the option of being concerted if one of the olefins reacts out of its photochemically excited state. ✻ bonding bonding HOMO + energy ✻ + light ✻ p p* new HOMO light Consider [2 + 2] cycloaddition: Photochemical activation LUMO C C C C C C C C C C C C C C C C C C The rotatory motion of ring closure may be reversed by photo-activation controtation 1e￾Y4 (hexatriene HOMO) 2e- 2e- 1e- 2e- 2e- 2e￾light Hexatriene: disrotation Y3 (hexatriene HOMO) H Me H Me H Me H Me Me H H Me Me H Me H Y4 Excited State Geometry p* p p p* new HOMO light C R H R H Excited state (1S and 3T) Rotation Me Me RO R H Me Me RO R heat disrotation light controtation Me Me RO R H One of the first cases where heat and light induced electrocyclizations followed different pathways. Havinga, Tetrahedron, 1961, 16, 146. Me Me RO R H Me Me RO R H Y4 Y1 Y3 Y3 Y1