Photochemistry of 2,3-Di(1'-naphthyl)oxiranes. Spectral and Kinetic Behaviour of Carbonyl Ylides in Condensed Media

Article abstract of DOI:10.1021/jo00039a036

The photochemistry of cis- and trans-2,3-di(1'-naphthyl)oxiranes (c- and t-1) has been studied by nanosecond laser flash photolysis in fluid solution.Direct photolysis (λ_exc = 248, 266, 308 nm) produces carbonyl ylides which are characterized by broad, structureless absorption bands in the visible region (450-650 nm, λ_max ca. 545) and unusually short lifetimes (14-22 ns) in a variety of solvent systems.Triplet energy transfer to the oxiranes results in formation of naphthalene-like triplets (λ_max = 410-420 nm, τ_T < 80 ns) which act as precursors tocarbonyl ylides that exhibit spectra (λ_max ca. 540-570) similar to those formed via direct excitation but are longer lived (τ = 40-70 ns).In contrast to triplet sensitization, there is no evidence of triplet oxiranes formed via direct excitation.Direct photolysis of the oxiranes appears to result in singlet-mediated photochemistry which initially gives rise to one-bond fragmentation yielding the ultra-short-lived ground-state yieldes.Direct photolysis also leads to the formation of 1-naphthaldehyde and 1-naphthylcarbene via either two-bond fragmentation of the singlet excited oxirane or fragmentation of the ground-state carbonyl ylides.A portion of the fragmentation process appears to occur adiabatically as evidenced by the observation of triplet 1-naphthaldehyde in the transient absorption phenomena.In acetonitrile, the transient absorption spectra from direct laser photolysis (308 nm) reveal evidence for photoionization of the oxiranes giving radical cations (λ_max ca. 620 nm).