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
Zw. Lin et Pk. Kadaba, 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, Journal of heterocyclic chemistry, 34(6), 1997, pp. 1645-1650
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.