| Popis: |
The photochemical reactions of a number of 2,5-dialkyl-1,3,5-hexatrienes are described. Variations in the selectivity of the reactions are observed, which can be rationalized by considering differences in the composition of the equilibrium mixture of conformational isomers, brought about by steric substituent effects. This result is in agreement with the NEER principle. In addition, examples are given of changes in efficiency of light-induced isomerizations, which are not restricted by the NEER principle. A reaction mechanism, involving a biradical intermediate, is proposed, which accounts for the formation of all of the reaction products containing a three-membered ring. MO correlation diagrams show that the formation of the three-membered ring as a primary step must occur in a conrotatory fashion. |
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