Deacetalization by photoinduced electron transfer with a pyrylium salt: Effect of limiting the amounts of water, oxygen and sensitizer

The photodeacetalization of 4-(p-isopropylphenyl)-2-nonyl-1,3-dioxolane 1 using 2,4,6-triphenylpyrylium tetrafluoroborate (TPT) as sensitizer has been studied to explore the possibility of achieving a slow sunlight-controlled release of polymer-bound carbonyl functionalized semiochemicals. This reaction has been found to produce mainly n-decanal and p-isopropylbenzaldehyde, together with lower amounts of the corresponding acids. The degree of conversion and the product distribution depended on the presence of water or oxygen and the amount of photosensitizer. The results are justified through generation of the radical cation 1(+.), followed by ring opening and trapping of the radical/cationic sites by oxygen or water. The delocalized 1,3-mdical cations III and the ozonides IV appear to be involved as key intermediates. Formation of n-nonanal as a minor product is explained through cleavage of the C-2-nonyl bond of 1(+.).