Azimine. I. Bildung und Stereoisomerie von 2, 3‐Diaryl‐ und 2, 3‐Dialkyl‐1‐phthalimido‐aziminen
Authors: Hoesch Lienhard , Karpf Martin , Dunkelblum Esra , Dreiding André S. 
Published: December 2004

Azimines. I. Synthesis and Stereoisomerism of 2, 3‐Diaryl‐ and 2, 3‐Dialkyl‐1‐phthalimido‐azimines

Teilweise vorgetragen in der Versammlung der Schweizerischen Chemischen Gesellschaft in Lausanne am 7./8. Mai 1971 und als Autoreferat veröffentlicht [1].

Special examples of a new class of compounds, the open‐chain azimines (1), have been prepared and their properties examined.

Addition of phthalimido‐nitrene (4), generated by lead tetraacetate oxidation of N‐aminophthalimide (3), to cis‐ and trans‐azobenzene (6 and 5), ‐azo‐p‐toluene (8 and 7), and to trans‐azomethane (9), ‐azoethane (10) and ‐azo‐α‐phenylethane (11) afforded the separable cis‐ and trans‐isomers of 2, 3‐diphenyl‐ (12 and 13), 2, 3‐di‐p‐toyl‐ (14 and 15), 2, 3‐dimethyl‐ (16 and 17), 2, 3‐diethyl‐ (18 and 19) and 2, 3‐di‐(α‐phenylethyl)‐1‐phthalimido‐azimines (20 and 21) in different ratios (see Scheme 1).

The constitution of the nitrene azo compound adducts as azimines was derived from their properties, especially from the conjugation effect (visible in the UV. spectra) of the aryl‐substituted compounds and from the non‐equivalence (shown by the 1H‐NMR. spectra) of the substituents on the two nitrogen atoms derived from the azo compounds. This evidence excluded the triaziridine 22 and an alternative azimine constitution 23 for the adducts.

Of the two stereoisomers obtained for each of the azimines, the aryl‐substituted examples 12/13 and 14/15 were readily interconverted by warming in solution, the cis‐isomers 12 and 14 exceeding the trans‐isomers 13 and 15 in the equilibrium. The dialkyl‐azimines appear to be configurationally more stable, since interconversion of the dimethyl‐azimines 16 and 17 was not possible under the same conditions, and also not before another thermal reaction took place (see below).

The identification of the N(2)‐N(3) bond as the stereogenic center, i.e. that the stereoisomerism of the azimines is due to the difference in relative position at N(2) and N(3) of the substituents derived from the azo compounds, as well as a configurational assignment was possible in the aryl‐substituted examples on the basis of the UV. spectroscopic comparison of the isomeric azimines with the corresponding stereoisomeric azoxy compounds: The cis‐azimines 12 and 14 showed absorptions similar to those of cis‐azoxybenzene and cis‐azoxy‐p‐toluene, and the trans‐azimines 13 and 15 showed absorptions similar to those of the respective trans‐azoxy compounds. With respect to the configuration of the alkyl‐substituted azimines, it was observed that the isomers 17 and 19, which from their formation and chromatographic behaviour are likely to be the trans‐isomers, show a visible coupling (∽ 1 Hz) between the two H (α)'s in the 1H‐NMR. spectrum, whereas the dimethyl isomer 16 (cis) does not exhibit such a coupling.

Thermal treatment of four azimines, namely 12, 14, 16 and 17, in solution for a longer time afforded the corresponding N, N′‐disubstituted N, N′‐phthaloyl‐hydrazines 27, 28 and 29. The order of velocity of this fragmentation with nitrogen extrusion was 12/13 ≈︁ 14/15 > 16(cis) > 17(trans).