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Mesoporous titanium-silicate Ti-MMM-2 catalyzes oxidation of alkanes (cyclooctane, n-heptane, n-octane, isooctane, methylcyclohexane, cis- and trans-1,2-dimethylcyclohexane) and olefins (cyclooctene, 1-decene, (S)-limonene) by H2O2 in acetonitrile solution at 60 °C. The catalytic reaction is truly heterogeneous in nature. The oxidation occurs via the formation of a ‘Ti–OOH’ species on the catalyst surface which either epoxidizes a nucleophilic double bond or generates, after O–O bond splitting, hydroxyl radical. The HO radical attacks an alkane or olefin C–H bond producing alkyl radical. The reaction R + O2 → ROO leads to the formation of alkyl hydroperoxide as the main product of the alkane oxidation. In the case of alkenes this reaction leads to allylic oxidation products. The composition of products of the olefin oxygenation (the epoxide/alkyl hydroperoxide or epoxide/diol ratios, etc.) strongly depends on the nature of the substrate. In the oxidation of (S)-limonene, isomeric diepoxides are formed along with monoepoxides even at the early stage of the reaction and (SRR)-diepoxide predominates among other products. This can be rationalized by suggesting the epoxidation of the two double bonds to occur simultaneously on two adjacent ‘Ti–O–OH’ centers on the catalyst surface. |
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