Formation and Reactivity of Triplet Vinylnitrenes as a Function of Ring Size.

The photoreactivity of cyclic vinyl azides 1 (3-azido-2-methyl-cyclopenten-1-one) and 2 (3-azido-2-methyl-2-cyclohexen-1-one), which have five- and six-membered rings, respectively, was characterized at cryogenic temperature with electron paramagnetic resonance (EPR), IR, and UV spectroscopy. EPR spectroscopy revealed that irradiating (λ > 250 nm) vinyl azides 1 and 2 in 2-methyltetrahydrofuran at 10 K resulted in the corresponding triplet vinylnitrenes 31N (D/hc = 0.611 cm-1 and E/hc = 0.011 cm-1) and 32N (D/hc = 0.607 cm-1 and E/hc = 0.006 cm-1), which are thermally stable at cryogenic temperature. Irradiation of vinyl azides 1 (310 nm light-emitting diode at 12 K) and 2 (xenon arc lamp through a 310-350 nm filter at 8 K) in argon matrices showed that in competition with intersystem crossing to form vinylnitrenes 31N and 32N, vinyl azide 1 formed a small amount of ketenimine 3, whereas vinyl azide 2 formed significant amounts of azirine 7 and ketenimine 6. Thus, vinyl azide 1 undergoes intersystem crossing more efficiently than vinyl azide 2. Similarly, vinylnitrene 31N is much more photoreactive than vinylnitrene 32N. Quantum chemical calculations were used to support the mechanisms for forming vinylnitrenes 31N and 32N and their reactivity.