Photochemical reactions of nitroso oxides at low temperatures: The first experimental evidence for dioxaziridines

Several singlet nitroso oxides (3a-e) were generated by the thermal reactio n of triplet nitrenes (2a-e) with triplet oxygen at. 95 K in 2-methyltetrah ydrofuran. After photolysis of the nitroso oxides at 77 K using strong ligh t intensities, the formation of intermediates was observed for the first ri me. From spectroscopic, kinetic, and chemical arguments, we postulate the f ormation of the following dioxaziridines: 4-(dioxaziridin-yl)stilbene (4a), 4-(dioxaziridin-yl)-4'-nitrostilbene (4b), 4'-(dioxaziridin-yl)-4-(dimethy lamino)st (4c), 4'-(dioxaziridin-yl)-4-aminobiphenyl (4d), and 4-(dioxaziri din-yl)-4'-(nitrene-substituted)stilbene (4f). All dioxaziridines observed are highly reactive species. At 77 K, they react thermally to form the corr esponding nitro compounds (5). The velocity of the ring opening reaction of the dioxaziridines (4 --> 5) is not significantly influenced by substituen ts; the rate constants at 77 K are all equal to 0.0030 +/- 0.0005 s(-1). Th e transients were characterized by stationary UV/vis and/or ESR spectroscop y. Ab initio calculations of the thermal reaction of the nonsubstituted dio xaziridine (6) and N-phenyldioxaziridine (9) were performed. From this, it follows that dioxaziridines are experimentally observable species which are separated from the corresponding nitro products by an orbital symmetry-for bidden barrier.