Catalytic oxidation of cyclic ethers to lactones over various titanosilicates

Various crystalline microporous metallosilicates have been used in the liquid phase catalytic oxidation of different cyclic ethers into their corresponding lactones in the presence of dilute aqueous H2O2 as oxidant. Among the various metallosilicates studied for the oxidation of tetrahydrofuran to γ-butyrolactone, titanosilicates exhibited the best activity than the other metallosilicates such as chromium silicalite-1 (CrS-1), chromium silicalite-2 (CrS-2) and vanadium silicalite-1 (VS-1). The intrinsic activity of TS-1 was found to be marginally higher than the other titanosilicates. Cyclic ethers undergo αC–H oxidation to give the corresponding lactones; whereas open-chain ether produce carboxylic acids by initial αC–H bond oxidation to give ester as an intermediate product, which further undergoes cleavage of –O– linkage to give the final carboxylic acids. The conversion of substituted tetrahydrofuran is decreased with number of –CH3 groups at α- and/or β-position. The lactone formation is hindered when both the α-positions are substituted with methyl substituents. Mechanistically, titanium hydroperoxo complex formed in the titanosilicate/H2O2/H2O system is believed to oxidize the αC–H bond of ethers producing the respective α-hydroxylated product, which undergoes further oxidation to give the lactones (for cyclic ethers) or carboxylic acids (for open-chain ethers).

Graphical abstractCatalytic oxidation over TS-1, TS-2, Ti-β, Ti-MCM-22, VS-1, CrS-1, CrS-2. Partial oxidation of cyclic and open-chain ethers. Synthesis of lactones from ethers. Titanosilicates vis-à-vis other metallosilicates.Figure optionsDownload full-size imageDownload high-quality image (129 K)Download as PowerPoint slideResearch highlights► Catalytic oxidation over TS-1, TS-2, Ti-β, Ti-MCM-22, VS-1, CrS-1, CrS-2. ► Partial oxidation of cyclic and open-chain ethers. ► Synthesis of lactones from ethers. ► Titanosilicates vis-à-vis other metallosilicates.