The influence of substituents on the photodehydrocyclization of 1,2-diphenylcyclopentenes I: Para substituents

The photodehydrocyclization of several mono-para-substituted 1,2-diphenylcyclopentenes into a phenanthrene derivative has been investigated under several conditions. It was found that the relative quantum yield of phenanthrene formation in methanol depends on the substituent when an oxidant with low efficiency (e.g. atmospheric oxygen) is used. In cyclohexane solution or in the presence of a more efficient oxidant, no significant substituent influence was observed. Furthermore, it has been established that the thermal ring opening of the primary photocyclization product dihydrophenanthrene (DHP) is influenced by substituents: an isokinetic relationship was found for this reaction but a linear Hammett relation was not observed. From these results it was concluded that the primary photocyclization step is not influenced by substituents. The quantum yield of product formation is influenced under conditions where thermal ring opening and oxidation of the intermediate DHP become competitive reactions (i.e. at low oxidation rates). The differences between the photocyclization behaviours of 1,2-diphenylcyclopentenes and stilbenes are discussed.