Kinetics of Thermolysis of Vinyl Azides. Empirical Rules for Formation of Azirines and Rearranged Nitriles

An evaluation of reported thermolyses of acyclic and cyclic vinyl azides R(N₃)C=CR’ leads to the empirical generalization that when R is aryl, alkyl, a heteroatom, or even alkoxycarbonyl, 1-azirines are usually formed upon thermolysis. When R is H or keto carbonyl (and depending on the β-substituents R’), nitriles or heterocycles are usually isolated. The kinetics of the thermolysis of α-azidostyrene (10) were followed by NMR, both by disappearance of 10 and appearance of azirine 11, and are consistent with a concerted loss of nitrogen and ring closure. A reexamination of the kinetics of α-azidochalcone (15) thermolysis, which normally leads to rearranged nitrile 19, revealed the intermediacy of azirine 16 which was isolated and thermolyzed. For the first time we were able to compare kinetic and thermodynamic data for vinyl azide to azirine conversion with those of azirine to nitrile rearrangement. The latter process appears to proceed via the dipolar form of a vinyl nitrene intermediate. A general scheme explaining the empirical findings is discussed.