Metal-Catalyzed Photooxidation of Flavones in Aqueous Media.

Soluble model compounds, such as flavones, are frequently employed in initial and mechanistic studies under homogeneous conditions in the search for effective bleaching catalysts for raw cotton. The relevance of model substrates, such as morin and chrysin, and especially their reactivity with manganese catalysts [i.e. in combination with 1,4,7-triazacyclononane (tacn) based ligands] applied in raw cotton bleaching with HO in alkaline solutions is examined. We show that morin, used frequently as a model, is highly sensitive to oxidation with O, by processes catalyzed by trace metal ions, that can be accelerated photochemically, although not involve generation of O. The structurally related chrysin is not susceptible to such photo-accelerated oxidation with O. Furthermore, chrysin is oxidized by HO only in the presence of a Mn-tacn based catalyst, and does not undergo oxidation with O as terminal oxidant. Chrysin mimics the behavior of raw cotton's chromophores in their catalyzed oxidation with HO, and is likely a mechanistically relevant model compound for the study of transition metal catalysts for dye bleaching catalysts under homogeneous conditions.