Azimines. VIIP. Contribution to the Preparation and Thermolysis of Alkyl (2Z)-2,3-Diisopropyl-azimine-l-carboxylates+

After improving the preparation of the azimines 9 (a = methyl, b = ethyl ester), we reinvestigated the mild thermolysis of 9 to 13 in CDCh: Unlike previously postulated, the azimines 9 first undergo a 1,4-H migration to the triazones 14 (step a), which suffer an elimination to yield the diazenes 16 (step b), which, in turn, are finally transformed by a 1,3-H migration to the carbazates 13 (step c). Step a is base catalyzed while step b is not (or less), so that 14, example of a relatively rare class of compounds, could be isolated after thermolysis of 9 in t-butylamine. During the thermolyses of pure 14 in CDCla, the diazenes 16 accumulated transiently (step b) before passing to 13. This accumulation of 16 was less (10%, as compared to 40%) when starting with 10% (as compared to 1%) solution of 14, which suggest that the conversion of 16 to 13 (step c) is catalyzed by 14. With pure 16, prepared in another way, step c occured in a 1% CDCh solution at 500C with ti" = 21 hrs. Step c also proved to be catalyzed by silica.