UVA self-photosensitized oxygenation of b-ionone

C. D. BORSARELLI; M. MICHNE; A. LA VENIA; F.E. MORÁN VIEYRA

PHOTOCHEMISTRY AND PHOTOBIOLOGY

American Society for Photobiology

Año: 2007 vol. 83 p. 1313 - 1318

0031-8655

The steady-state UVA (350 nm) photolysis of (E)-b-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 a ratio 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 of 0.50 as the precursor for the generation of singlet oxygen 1O2 (Phi = 0.16) and the isomeric a-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 1.0x106 M-1s-1 and 2.5x108 M-1s-1 to yield the oxygenated products 5 and 2, respectively, indicating the relevance of the fixed s-cis configuration in the a-pyran ring in the concerted [2+4] cycloaddition of 1O2.