Oxidation of Reactive Alcohols with Hydrogen Peroxide Catalyzed by Manganese Complexes

Abstract

Two manganese-containing catalysts have been employed in the oxidation with hydrogen peroxide of two reactive alcohols (1-phenylethanol and glycerol): soluble catalyst [LMn(μ-O)3MnL](PF6)2 (1a) and heterogenized catalyst [LMn(μ-O)3MnL]2[SiW12O40] (1b) (L is 1,4,7-trimethyl-1,4,7-triazacyclononane, TMTACN). Oxidation of 1-phenylethanol catalyzed by 1a in acetonitrile solution proceeds at room temperature in the presence of a small amount of oxalic acid; the turnover number attains 15,000 after 3 h. It has been proposed on the basis of the kinetic study that an oxidizing species is a manganyl species containing fragment “Mn=O” rather that hydroxyl radical. This species reacts competitively with the alcohol, acetonitrile and hydrogen peroxide. In the case of 1b dependences of the initial rates of acetophenone accumulation on concentration of the alcohol and amount of 1b have plateau. Both homogeneous and heterogeneous catalysts are efficient in the oxidation of glycerol to produce dihydroxyacetone (DHA) as the main product. The oxidation catalyzed by 1a is one of the first examples of the glycerol oxidation by a catalytic homogeneous system. The yield of valuable products attained 45%. The oxidation of DHA in the absence of glycerol afforded mainly glycolic acid in yield 60% based on the starting DHA. The oxidation on 1b represents the first example of the glycerol transformation catalyzed by a heterogenized metal complex. Under certain conditions yields of products of deeper oxidation (glyceric, glycolic and hydroxypyruvic acids) are somewhat higher than the yield of dihydroxyacetone. Special experiments demonstrated that no leaching of active species occurs from catalyst 1b to the solution and that this catalyst can be re-used at least four times without substantial loss of activity. Manganese-containing complexes are very efficient catalysts in the oxidation of reactive alcohols (1-phenylethanol and glycerol) with H2O2: soluble [LMn(O)3MnL](PF6)2 and heterogenized [LMn(O)3MnL]2[SiW12O40] (L is 1,4,7-trimethyl-1,4,7-triazacyclononane).