Azimine. V. . Untersuchungen zur stereoisomerie an der N(2), N(3)‐bindung von 2, 3‐dialkyl‐1‐phthalimido‐aziminen

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.