HETARYNE INTERMEDIATES

The electronic structures of all of the possible 1,2-, 1,3-, and 1,4-didehydroaromatic intermediates derived from pyridine and the diazines (hetaryne intermediates) have been calculated using the extended Htickel theory (EHT). For the didehydropyridines, it is found that of the six possible isomers, the 3,4-didehydropyridine is the most stable, and the 2,6-didehydropyridine the least stable. Great relative stability is also predicted for 4,5-dide-hydropyrazine and 4,6-didehydropyrimidine. The complex computational trends in the hetaryne stabilities can be rationalized very well by simple molecular orbital considerations of the orbital interactions among nonbonding radical lobes and lone pair orbitals. A dominant effect is shown to be a nitrogen lone pair destabilization of nearby radical lobes. The calculated stability sequences and total electron distributions provide an excellent correlation of the available experimental data on relative stability and orientation effects in the hetaryne intermediates. © 1969, American Chemical Society. All rights reserved.