Azomethinimine. VI. Zum thermischen Verhalten des Photochromiesystems Azomethinimin/Diaziridin,untersucht an Pyrazolidon-(3)-azomethiniminen und deren Photoprodukten

Azomethineimine. VI. On the Thermic Behaviour of the Photochromic System Azomethineimine/Diaziridine, Investigated by Means of the Photoproducts of Pyrazolidone-(3)-Azomethineimines The photochemically obtained cycloproducts 2 of pyrazolidone-(3)-azomethineimines 1 react thermically to 1 in aprotic solvents. This backreaction was investigated by following up the growth of the intensive long wave absorptionband of 1 . The kinetic analysis was carried out according to SWINBOURNE [5, 6]. The thermic backreaction is accelerated by factors favouring a polarization of the C N(2)-bond (C^δ+  N^δ−) of diaziridine 2 . Such factors are polar solvents, donors in the substituent R and extensive, easily polarizable π-electronic systems in R. The influence of substituents on the backreaction can be correlated by means of the σ⁺-Hammett-constants. For most compounds the values of E_A ≈ ΔH^≠, Δ^≠ and ΔG^≠ were determined. The possible reaction mechanism is discussed. The influence of steric factors on the thermic backreaction is regarded as a first indication of an exo-form of the bicyclic diaziridines 2 .