Azimines. V. Investigation on the Stereoisomerism Around the N (2), N (3) Bond in 2, 3‐Dialkyl‐1‐phthalimido‐azimines
2, 3‐(cis‐1, 3‐Cyclopentylene)‐1‐phthalimido‐azimine (7) and isomerically pure (2 Z)‐ and (2 E)‐2, 3‐diisopropyl‐1‐phthalimido‐azimine (9a and 9b) were prepared by the addition of phthalimido‐nitrene (1) to 2, 3‐diazabicyclo [2.2.1]hept‐2‐ene (6) and to (E)‐ and (Z)‐1, 1′‐dimethylazoethane (8a and 8b), respectively. Comparison of their UV. spectra with those of two stereoisomeric azimines of known configuration, namely (1 E, 2 Z)‐ and (1 Z, 2 E)‐2, 3‐dimethyl‐1‐phthalimido‐azimine (5a and 5b), reveals that 2, 3‐dialkyl‐1‐phthalimido‐azimines with (2 Z)‐configuration are characterized by a shoulder at about 258 nm (ϵ ≈︂ 14,000) and those with (2 E)‐configuration by a maximum at 270–278 nm (ϵ ≈︂ 10,000). The (2 E)‐azimine 9b isomerizes under acid catalysis as well as thermally and photochemically into the more stable (2 Z)‐isomer 9a. Under the last two conditions the isomerization is accompanied by a slower fragmentation with loss of nitrogen into N, N′‐diisopropyl‐N, N′‐phthaloylhydrazine (4, R = iso‐C~3~H~7~). The same fragmentation was also observed on thermolysis and photolysis of the (2 Z)‐isomer 9a. The kinetic parameters for the thermal isomerization of 9b (they fit first‐order plots) and for the fragmentation of 9a and 9b were determined by ^1^H‐NMR. spectroscopy in benzene, trichloromethane and acetonitrile. In the photolysis of 9a or 9b the fragmentation is accompanied by dissociation into the azo compounds 8a or 8b and the nitrene 1, the latter being subject to trapping by cyclohexene. With the azimine 7, an analogous thermal fragmentation was observed to give N, N′‐(cis‐1, 3‐cyclo‐pentylene)‐N, N′‐phthaloylhydrazine (15), but more energetic conditions were required than with 9. Photolysis of 7 led exclusively to dissociation into the azo compound 6 and the nitrene 1, perhaps because the fragmentation of 7 is prevented by ring strain.