Triazolines. XXXIII . Nonregiospecific 1,3‐cycloaddition of aryl azides to vinylpyridines: A unique route to the synthesis of 2‐pyridyl substituted aziridines via unstable 4‐pyridyltriazoline intermediates

The 1,3‐cycloaddition of aryl azides to the olefinic bonds of 4‐ and 2‐vinylpyridines has been found to yield pyridyl substituted aziridines as the main reaction products with only smaller amounts of the normally expected l‐aryl‐5‐pyridyl‐1,2,3‐triazolincs. Theoretical and experimental evidence are provided to explain the results: based on the fact that the olefinic bonds in 4‐ and 2‐vinylpyridines are electron‐deficient, azide addition can be expected to be not regiospecific. In the bidirectional addition reaction, the HOMO azide‐ LUMO olefin interaction predominates leading to unstable l‐aryl‐4‐pyridyl‐1,2,3‐triazolines, which, unlike the more stable 5‐pyridyl compounds, lose nitrogen under thermal conditions to yield the aziridines. At room temperature, the reactions yield the aziridine along with the l‐aryl‐4‐pyridyltriazole, providing evidence for the formation of the 4‐pyridyltriazoline intermediate. Reaction of the vinylpyridines with variously substituted phenyl azides, clearly indicates that the electron donating methyl and methoxy groups on the phenyl azide facilitate reaction, while the electron withdrawing nitro group has a retarding effect. This is consistent with an increase in the HOMO azide energy and hence in azide reactivity. According to the FMO model, the 1,3‐cycloaddition of aryl azides to vinylpyridines appears to be predominantly, but not exclusively, a HOMO azide ‐LUMO olefin interaction and provides a unique route to the synthesis of 2‐pyridyl substituted aziridines.