The Rearrangements of Naphthylnitrenes: UV/Vis and IR Spectra of Azirines, Cyclic Ketenimines, and Cyclic Nitrile Ylides

Ar matrix photolysis of 1- and 2-naphthyl azides 3 and 4 at 313 nm initially affords the singlet
naphthyl nitrenes, 11 and 12. Relaxation to the corresponding lower energy, persistent triplet nitrenes 31
and 32 competes with cyclization to the azirines 15 and 18, which can also be formed photochemically
from the triplet nitrenes. On prolonged irradiation, the azirines can be converted to the seven-membered
cyclic ketenimines 10 and 13, respectively, as described earlier by Dunkin and Thomson. However, instead
of the o-quinoid ketenimines 16 and 19, which are the expected primary ring-opening products of azirines
15 and 18, respectively, we observed their novel bond-shift isomers 17 and 20, which may be formally
regarded as cyclic nitrile ylides. The existence of such ylidic heterocumulenes has been predicted previously,
but this work provides the first experimental observation of such species. The factors which are responsible
for the special stability of the ylidic species 17 and 20 are discussed.