On the mechanism of the photocyclization of azadienes

The photocyclization of azadienes to yield quinolines has been studied by means of CASPT2//CASSCF calculations. In order to study this extensive family of compounds, a minimal model system keeping the fundamental key features of this kind of compounds have been analyzed with the aim of explaining the general behavior of azadienes found experimentally. We show that, although several conformers of the reacting molecule are populated on the ground-state at room temperature, there is only one capable of successfully yield the products. It is shown that photocyclization takes place with fast relaxation in both S2 and S1 excited states, in agreement with the experimental lack of fluorescence. This fact, together with the presence of reaction paths that allow effective recovery of the starting molecule, explains not only the experimental photocyclization of azadienes under UV radiation, but also accounts for the low efficiency of the process. The mechanistic information obtained allows to provide an explanation of the found reaction outcome and to suggest some modifications to improve the synthetic utility of these reactions.