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 normal ly expected 1-aryl-5-pyridyl-1,2,3-triazolines. Theoretical and experi mental evidence are provided to explain the results: based on the fact that the olefinic bonds in 4- and 2-vinylpyridines are electron-defic ient, azide addition can be expected to be not regiospecific. In the b idirectional addition reaction, the HOMOazide-LUMOolefin interaction p redominates leading to unstable 1-aryl-4-pyridyl-1,2,3-triazolines, wh ich, unlike the more stable 5-pyridyl compounds, lose nitrogen under t hermal conditions to yield the aziridines. At room temperature, the re actions yield the aziridine along with the 1-aryl-4-pyridyltriazole, p roviding evidence for the formation of the 4-pyridyltriazoline interme diate. Reaction of the vinylpyridines with variously substituted pheny l azides, clearly indicates that the electron donating methyl and meth oxy groups on the phenyl azide facilitate reaction, while the electron withdrawing nitro group has a retarding effect. This is consistent wi th an increase in the HOMOazide energy and hence in azide reactivity. According to the FMO model, the 1,3-cycloaddition of aryl azides to vi nylpyridines appears to be predominantly, but not exclusively, a HOMOa zide-LUMOolefin interaction and provides a unique route to the synthes is of 2-pyridyl substituted aziridines.