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 3 1N (D/hc = 0.611 cm–1 and E/hc = 0.011 cm–1) and 3 2N (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 3 1N and 3 2N, 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 3 1N is much more photoreactive than vinylnitrene 3 2N. Quantum chemical calculations were used to support the mechanisms for forming vinylnitrenes 3 1N and 3 2N and their reactivity.