Photoisomérisation des quelques cyclohexadiènes et heptatriènes. Photocycloaddition des hexatriènes

Photolytic ring opening of cis and trans‐1,5‐diphenyl‐6‐méthylcyclohexa‐1,3‐dienes yields a unique primary triene with a geometry determined by the cyclodiene preferred conformation in the ground state. Irradiation of 3 isomers of 1,5‐diphenylhepta‐1,3,5‐triene with a cis‐central double bond (Z, Z, E; E, Z, E and E, Z, Z) yields with a complete stereospecificity 3 bicyclo[3.1.0]hexenes (respectively endo‐Me‐endo‐Ph; endo‐Me‐exo‐Ph and exo‐Me‐exo‐Ph). Kinetic measurements have shown that these internal photocycloadditions are of the type [4πa + 2πa], which is forbidden by Woodward‐Hoffmann rules. In all of the three cases studied by us, orbital factors seem anyway to play a major role compared to steric constraints (an endo‐endo photoproduct is formed). A kinetic study of the relative cis‐trans photoisomerization rates of carbon double bonds of the eight isomers of 1,5‐diphenylhepta‐1,3,5‐triene has been performed.