Abstract
Abstract It is shown that ground state conformations of cyclohexadienes and acyclic hexatrienes play an important role, in addition to orbital symmetry factors, to govern the stereochemistry of the photoisomerization reactions of these compounds. Studying the influence of the wave-length of irradiation allows to attribute to discrete conformations some specific photoreactions (H shift, formation of bicyclo {3.1. 0} hexene). New examples are given, with kinetic data, showing a {4π a + 2π a} stereospecificity for the hexatriene Photo-DIELS-ALDER reaction with a predominent orbital control. The concertedness of the reaction is discussed. Kinetics of Z, E-photoisomerizations around C-C double bonds of hexatrienes are shown, in relation with the ease of terminal compared to central double bonds rotations. Singlet and triplet states have different behaviours in this respect, as well as different wave-lengths of excitation.
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