UVA Self-Photosensitized Oxygenation of b-Ionone

CLAUDIO D. BORSARELLI; MIRTA P. MISCHNE; AGUSTINA LA VENIA; CLAUDIO D. BORSARELLI*1, MIRTA MISCHNE2, AGUSTINA LA VENIA2 AND FAUSTINO E. MORA´N VIEYRA1

PHOTOCHEMISTRY AND PHOTOBIOLOGY

WILEY-BLACKWELL PUBLISHING, INC

Año: 2007 vol. 83 p. 1 - 6

0031-8655

1The steady-state UVA (350 nm) photolysis of (E)-beta-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 field FT = 0.50 as the precursor for the generation of singlet oxygen 1O2 ( D = 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 1.0 · 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 alpha-pyran ring in the concerted [2+4] cycloaddition of 1O2.