Exploring the Reactivity of Melanins as Photocatalysts for Reductive Dehalogenations

Four melanin polymers prepared from different precursors (5,6-dihydroxyindole, 5,6-dihydroxyindole 2-carboxylic acid, 1,8-dihydroxynaphthalene and dopamine) have been adopted as photocatalysts in the reductive dehalogenation of a series of model alpha-halogen carbonyl derivatives. The best performing melanin was that obtained from 5,6-dihydroxyindole 2-carboxylic acid, which allowed to dehalogenate efficiently both bromomalonate esters and phenacyl bromide; on the other hand, chloromalonate esters were almost unreactive under the same reaction conditions. Electrochemical and control experiments enabled to elucidate the reaction mechanism, demonstrating the key role of electrons as charge carriers in the observed photocatalytic process. The reductive dehalogenation of alpha-halogenated carbonyl compounds has been performed by exploiting four different melanin polymers in the role of photocatalyst. The reaction smoothly took place under visible light irradiation in the presence of sodium ascorbate as an additive. The melanin obtained from 5,6-dihydroxyindole 2-carboxylic acid offered the best performance, especially in the reduction of brominated substrates. image