Azimines IV. Kinetics and Mechanism of the Thermal Stereoisomerization of 2,3‐Diaryl‐1‐phthalimido‐azimines1)Mixtures of (1E, 2Z)‐ and (1Z, 2E)‐2‐phenyl‐1‐phthalimido‐3‐p‐tolyl‐azimine (3aand3b, resp.) and (1E, 2Z)‐ and (1Z, 2E)‐3‐phenyl‐1‐phthalimido‐2‐p‐tolylazimine (4aand4b, resp.) were obtained by the addition of oxidatively generated phthalimido‐nitrene(6)to (E)‐ and (Z)‐4‐methyl‐azobenzene (7aand7b, resp.). Whereas complete separation of the 4 isomers3a, 3b, 4aand4bwas not possible, partial separation by chromatography and crystallization led to 5 differently composed mixtures of azimine isomers. The spectroscopic properties of these mixtures (UV.,1H‐NMR.) were used to determine the ratios of isomers in the mixtures, and served as a tool for the assignment of constitution and configuration to those isomers which were dominant in each of these mixtures, respectively.Investigation of the isomerization of the azimines3a, 3b, 4aand4bwithin the 5 mixtures at various concentrations by1H‐NMR.‐spectroscopy at room temperature revealed that only stereoisomers are interconverted (3a ⇄ 3b; 4a ⇄ 4b)and that the (1E, 2Z) ⇄ (1Z, 2E) stereoisomerization is a unimolecular reaction. These observations exclude an isomerization mechanismviaan intermediate 1‐phthalimido‐triaziridine(2)orviadimerization of 1‐phthalimido‐azimines(1), respectively. The 3‐p‐tolyl substituted stereoisomers3aand3bisomerized slightly slower than the 3‐phenyl substituted ones4aand4b, an effect which is consistent with the assumption that the rate determining step of the interconversion of (1E, 2Z)‐ and (1Z, 2E)‐1‐phthalimido‐azimines(1a ⇄ 1b)is the stereoisomerization of the stereogenic center at N(2), N(3), either by inversion of N(3) or by rotation around the N(2), N(3) bond. The total isomerization process is assumed to occurviathe thermodynamically less stable (1Z, 2Z)‐ and (1E, 2E)‐isomers1cand1d, respectively, as intermediates in undetectably low concentrations which stay in rapidly established equilibria with the observed, thermodynamically more stable (1E, 2Z)‐ and (1Z, 2E)‐isomers1aand1b, respectively.At higher temperatures, the azimines