Abstract
The photocyclodehydrogenation of several mono-meta-substituted 1,2-diphenylcyclopentenes into 2- and 4-substituted phenanthrene derivatives was investigated under various conditions (varying solvent, concentration of oxidant and temperature). As with the para-substituted isomers, the primary photocyclization step into a 4 a,4 b-dihydrophenanthrene (DHP) is not influenced by the substitutents. The two phenanthrenes are not always obtained in a ratio of 1:1. Depending on the substituent, solvent, oxidant and temperature the ratio can reach values larger than 50:1. Deviations from a ratio of 1:1 originate from the competition between oxidation and thermal ring opening of the DHPs which, in the cases studied, are formed in nearly equal amounts. The thermal ring opening of the DHPs is influenced by substituents: substituents at C2 raise the activation energy and those at C4 lower the activation energy. An isokinetic relationship is found for the thermal ring opening. The results are compared with those of substituted stilbenes and with the results of force field calculations.
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