Oxidations by the system “hydrogen peroxide–[Mn 2L 2O 3][PF 6] 2 (L = 1,4,7-trimethyl-1,4,7-triazacyclononane)–oxalic acid”. Part 6. Oxidation of methane and other alkanes and olefins in water

Abstract

Oxidation of alkanes with hydrogen peroxide in water solution at 10–50 °C is efficiently catalyzed by the cationic dinuclear manganese (IV) derivative [Mn 2L 2O 3] 2+ ( 1, with L = 1,4,7-trimethyl-1,4,7-triazacyclononane, TMTACN) in the form of the hexafluorophosphate salt ([ 1][PF 6] 2) if oxalic acid is present as a co-catalyst. Methane gives methanol and formaldehyde (turnover numbers, TONs, were 7 and 2, respectively, after reduction of the reaction mixture with ascorbic acid) whereas cyclohexane was oxidized with TONs up to 160 affording cyclohexyl hydroperoxide, cyclohexanone and cyclohexanol (the ketone was the main product, although at room temperature almost pure alkyl hydroperoxide was formed). In contrast to the oxidation in acetonitrile, the reaction with linear n-alkanes in water exhibits an unusual distribution of oxygenates. For example, in the oxidation of n-heptane the normalized reactivity of the methylene group in position 4 of the chain is 3–7 times higher than that of the CH 2 group in position 2. Dec-1-ene is epoxidized by hydrogen peroxide in water (a biphasic system) catalyzed by [ 1][PF 6] 2 and oxalic acid in the presence of a small amount of acetonitrile with TONs up to 1000 (no epoxidation has been detected in the absence of MeCN).