The 1,3-cycloaddition of phenyl azides to the olefinic double bond of
4-vinylpyridine, yields the 1-phenyl-2-pyridylaziridine as the main pr
oduct with only smaller amounts of the 1-phenyl-5-pyridyl-1,2,3-triazo
line, although the reaction constitutes a general approach to the synt
hesis of Delta(2)-1,2,3-triazolines. Experimental and theoretical evid
ence are provided to explain the results on the basis that the olefini
c bond in 4-vinylpyridine is an electron-deficient bond and that azide
addition is not regiospecific. In the bidirectional addition reaction
, the HOMOazide-LUMOolefin interaction predominates leading to a 1-phe
nyl-4-pyridyl-1,2,3-triazoline, which, unlike the 5-pyridyl compound,
loses nitrogen under thermal conditions to yield the aziridine, and at
room temperature, a mixture of the respective triazole and the azirid
ine.