| Abstract |
The binuclear manganese(IV) complex [Mn2L2O3][PF6](2) (L = 1,4,7-trimethy1-1,4,7-triazacyclononane) (compound 1) catalyzes very efficiently the oxygenation of alkanes with H2O2 only if a carboxylic (acetic, oxalic) acid is present as a co-catalyst. The oxidation produces mainly alkyl hydroperoxide which slowly decomposes in the course of the reaction to afford the corresponding alcohol and ketone. Real concentrations of the three products (alkyl hydroperoxide, alcohol and ketone) were estimated at each given moment by comparing the chromatograms before and after reduction of the samples with PPh3. Selectivity parameters and kinetics of the cyclohexane oxidation are described. The mode of the initial rate W-0 dependence on the initial concentration of cyclohexane should reflect a concurrence between the alkane and acetonitrile for the oxidizing species generated in the H2O2 decomposition. It was concluded that hydroxyl radicals do not participate in this process. The cyclohexane oxidation by the systems O-18(2)-hydrogen peroxide-1-oxalic acid and O-16(2)-hydrogen peroxide-H-2 O-18-1-oxalic acid were carried out. Cyclohexanol produced in the reaction in an O-18(2) atmosphere after reduction with PPh3 (which was transformed into O-18=PPh3) was found to contain up to 56\% of O-18 pointing that the respective cyclohexyl hydroperoxide also contained labeled oxygen. No traces of labeled cyclohexanol were detected in the reaction under O-16(2) in the samples reduced with PPh3. All experimental data are in an excellent agreement with an unusual catalytic cycle proposed for the alkane oxygenation. The transformation of alkane into the corresponding alkyl hydroperoxide proceeds via generation of alkyl radicals R which rapidly react with atmospheric molecular oxygen. Unlabeled alkyl hydroperoxide can be partially formed via the rebound mechanism R + HOO-Mn -> R-OOH + Mn, where Mn is a fragment of the binuclear intermediate species. (C) 2016 Elsevier B.V. All rights reserved. |
