Metal‐Catalyzed Photooxidation of Flavones in Aqueous Media

Abstract

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 H2O2 in alkaline solutions is examined. We show that morin, used frequently as a model, is highly sensitive to oxidation with O2, by processes catalyzed by trace metal ions, that can be accelerated photochemically, although not involve generation of 1O2. The structurally related chrysin is not susceptible to such photo‐accelerated oxidation with O2. Furthermore, chrysin is oxidized by H2O2 only in the presence of a Mn‐tacn based catalyst, and does not undergo oxidation with O2 as terminal oxidant. Chrysin mimics the behavior of raw cotton's chromophores in their catalyzed oxidation with H2O2, and is likely a mechanistically relevant model compound for the study of transition metal catalysts for dye bleaching catalysts under homogeneous conditions.

Neutralization of Reactive Oxygen Species at Dinuclear Cu(II)-Cores: Tuning the Antioxidant Manifold in Water by Ligand Design

“…As a representative case, the antioxidant versus pro-oxidant capacity of Cu 2 L 1 2 has been evaluated by considering the catalytic bleaching of morin, generally used as the molecular probe for peroxidase screening (Scheme S1). − The oxidative degradation of morin (0.12 mM) catalyzed by Cu 2 L 1 2 (50 μM) in the presence of H 2 O 2 (10–30 mM in BBS buffer 50 mM, pH = 7.8, Figures S78–S80) is conveniently monitored via UV–vis spectroscopy, by registering the absorbance depletion at 390 nm over a time-range superimposable to the oxygen evolution kinetics. The rate of morin bleaching by Cu 2 L 1 2 varies linearly with H 2 O 2 concentration, yielding a second-order rate constants, k b = 6.1 × 10 –4 M –1 s –1 , that is orders of magnitude lower than the k H 2 O 2 associated with the catalase-like activity of the copper complex (cf.…”

Organophotoredox-catalyzed stereoselective reductive dimerization of chromone-2-carboxylic esters

“…[7][8][9] The presence of CQO and CQC chromophoric units in flavones as well as chromones makes them extremely active for photoreactions. Flavones have been shown to undergo a wide variety of photoreactions that include photocycloaddition, 10 photodimerization, 11 photo-tautomerization, 10 photo-oxidation, 12 and photo-reduction reactions. 10 For example, the photoinduced electron transfer reactions of flavones have been reported by Chen and co-workers, which involves the irradiation of flavone 1 in the presence of triethylamine and acetonitrile to obtain a mixture of racemic and meso biflavanones (2a and 2b) along with the reduced flavone 3 (Scheme 1A).…”