Singlet oxygen photo-oxidation of some triazapentalenes

Abstract

The self-sensitized photo-oxidation of 1,3-dimethyl-5H-pyrazolo[1′,2′:1,2]-1,2,3-triazolo[3,2-a]phenazin-4-ium inner salt (1) leads to the cleavage of the triazapentalene moiety, yielding an epoxy-ketone. If the two methyl groups are absent, as in compound (2), the photo-oxidation takes place much better in the presence of dyes and yields an αβ unsaturated aldehyde. The first reaction is shown to involve singlet oxygen, the formation of which requires oxygen-promoted intersystem crossing from singlet to triplet (1), followed by energy transfer from the latter state. The rate of the singlet oxygen addition onto (1) was measured in some solvents, and showed no dependence on the polarity of the medium. The mechanism of the addition is discussed, taking into account also the results from the photo-oxidation of the 6,7-dihydro-derivatives of compounds (1) and (2).