Azimines IV. Kinetics and Mechanism of the Thermal Stereoisomerization of 2,3‐Diaryl‐1‐phthalimido‐azimines^1^)
Mixtures of (1__E__, 2__Z__)‐ and (1__Z__, 2__E__)‐2‐phenyl‐1‐phthalimido‐3‐p‐tolyl‐azimine (3a and 3b, resp.) and (1__E__, 2__Z__)‐ and (1__Z__, 2__E__)‐3‐phenyl‐1‐phthalimido‐2‐p‐tolylazimine (4a and 4b, resp.) were obtained by the addition of oxidatively generated phthalimido‐nitrene (6) to (E)‐ and (Z)‐4‐methyl‐azobenzene (7a and 7b, resp.). Whereas complete separation of the 4 isomers 3a, 3b, 4a and 4b was not possible, partial separation by chromatography and crystallization led to 5 differently composed mixtures of azimine isomers. The spectroscopic properties of these mixtures (UV., ^1^H‐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 azimines 3a, 3b, 4a and 4b within the 5 mixtures at various concentrations by ^1^H‐NMR.‐spectroscopy at room temperature revealed that only stereoisomers are interconverted (3a ⇄ 3b; 4a ⇄ 4b) and that the (1__E__, 2__Z__) ⇄ (1__Z__, 2__E__) stereoisomerization is a unimolecular reaction. These observations exclude an isomerization mechanism via an intermediate 1‐phthalimido‐triaziridine (2) or via dimerization of 1‐phthalimido‐azimines (1), respectively. The 3‐p‐tolyl substituted stereoisomers 3a and 3b isomerized slightly slower than the 3‐phenyl substituted ones 4a and 4b, an effect which is consistent with the assumption that the rate determining step of the interconversion of (1__E__, 2__Z__)‐ and (1__Z__, 2__E__)‐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 occur via the thermodynamically less stable (1__Z__, 2__Z__)‐ and (1__E__, 2__E__)‐isomers 1c and 1d, respectively, as intermediates in undetectably low concentrations which stay in rapidly established equilibria with the observed, thermodynamically more stable (1__E__, 2__Z__)‐ and (1__Z__, 2__E__)‐isomers 1a and 1b, respectively.
At higher temperatures, the azimines 3 and 4 are transformed into N‐phenyl‐N,N′‐phthaloyl‐N′‐p‐tolyl‐hydrazine (8) with loss of nitrogen.