Synthesis of 1-azaspiro[2.4]hepta-1,4,6-trienes and azaspiroconjugation studied by photoelectron spectroscopy.

1-Azaspiro[2.4]hepta-1,4,6-trienes 3a-c have been prepared by photolysis or thermolysis of 6-azidofulvenes 5a-c, which were accessible by nucleophilic substitution reactions of the precursors 4a,b or by nucleophilic addition of hydrazoic acid to ethenylidene-cyclopentadiene (6c). The UV photoelectron spectrum of 2-methyl-1-aza-spiro[2.4]hepta-1,4,6-triene (3c) has been recorded and analyzed by making use of density functional theory (DFT) B3LYP calculations. Substantial homo-conjugative interactions have been determined. The lone-pair orbital n(N) of the 2H-azirine nitrogen atom interacts with the π 1 orbital of the cyclopentadiene ring. The energies of these orbitals are lowered or increased by 0.95 or 0.91 eV with respect to the two parent compounds cyclopentadiene (7) and 3-methyl-2H-azirine (9), respectively. In addition, in compound 3c the π(C=N) orbital of the three-membered ring interacts with a o orbital of the cyclopentadiene unit and is destabilized by 0.47 eV by this effect.