UVA self-photosensitized oxygenation of β-ionone

Abstract

The steady-state UVA (350 nm) photolysis of (E)-β-ionone (1) in aerated toluene solutions was studied by 1H NMR spectroscopy. The formation of the 1,2,4-trioxane (2) and 5,8-endoperoxide (5) derivatives in the ratio of 4:1 was observed. Time-resolved laser induced experiments at 355 nm, such as laser-flash photolysis, photoacoustic and singlet oxygen 1O2 phosphorescence detection, confirmed the formation of the excited triplet state of 1 with a quantum yield ΦT = 0.50 as the precursor for the generation of singlet oxygen 1O2 (ΦΔ = 0.16) and the isomeric α-pyran derivative (3), which was a reaction intermediate detected by NMR. In turn, the reaction of 1O2 with 1 and 3 occurred with rate constants of 1.0 × 106 and 2.5 × 108 m-1s -1 to yield the oxygenated products 5 and 2, respectively, indicating the relevance of the fixed s-cis configuration in the α-pyran ring in the concerted [2+4] cycloaddition of 1O2. © 2007 The Authors.