The role of salicylic acid, l-ascorbic acid and oxalic acid in promoting the oxidation of alkenes with H2O2 catalysed by [MnIV2(O)3(tmtacn)2]2+

Abstract

The role played by the additives salicylic acid, L-ascorbic acid and oxalic acid in promoting the catalytic activity of [Mn^IV₂(O)₃(tmtacn)₂](PF₆)₂ {1(PF₆)₂, where tmtacn = N,N′,N″-trimethyl-1,4,7-triazacyclononane} in the epoxidation and cis-dihydroxylation of alkenes with H₂O₂ and in suppressing the catalysed decomposition of H₂O₂ is examined. Whereas aliphatic and aromatic carboxylic acids effect enhancement of the catalytic activity of 1 through the in situ formation dinuclear carboxylato bridged complexes of the type [Mn^III₂(μ-O)(μ-RCO₂)₂(tmtacn)₂]²⁺, for L-ascorbic acid and oxalic acid notable differences in reactivity are observed. Although for L-ascorbic acid key differences in the spectroscopic properties of the reaction mixtures are observed compared with carboxylic acids, the involvement of carboxylic acids formed in situ is apparent. For oxalic acid the situation is more complex with two distinct catalyst systems in operation; the first, which engages in epoxidation only, is dominant until the oxalic acid additive is consumed completely at which point carboxylic acids formed in situ take on the role of additives to form a second distinct catalyst system, i.e. that which was observed for alkyl and aromatic carboxylic acids, which yield both cis-diol and epoxide products.