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37.2.1.1.1 Method 1: Titanium-Mediated Epoxidation with Alkyl Hydroperoxides

DOI: 10.1055/sos-SD-037-00140

Adolfsson, H.Science of Synthesis, (200837228.

One of the routes for the industrial production of 2-methyloxirane is based on a heterogeneous titanium-on-silica catalyst that employs ethylbenzene hydroperoxide as the terminal oxidant.[‌4‌] This process represents one of the commercial routes for the manufacturing of 2-methyloxirane using metal-catalyzed epoxidation.[‌5‌] The heterogeneous catalyst used in one of the processes, the so-called SMPO process (styrene monomer propylene oxide) is made in a four-step sequence starting with impregnating silica gel with gaseous titanium(IV) chloride, followed by calcination, hydrolysis, and finally silylation of the material. The obtained supported titanium-on-silica catalyst efficiently mediates the epoxidation of propene to 2-methyloxirane (selectivity between 9598%) and at the same time converts ethylbenzene hydroperoxide to 1-phenylethanol, which is used as a precursor for the production of vinylbenzene. Alkenes other than propene are also tolerated by the catalyst. A drawback of this process is the sensitivity of the heterogeneous titanium compounds, which easily become deactivated by moisture; strict anhydrous conditions are therefore required. There are other examples of more moisture-tolerant heterogeneous titanium catalysts (e.g., TS-1, see Section 37.2.1.1.1.1) used in combination with alkyl hydroperoxides; however, such systems demonstrate poor substrate generality. The development of a significantly more stable heterogeneous titanium-based catalyst system was originally presented by Mayoral and coworkers[‌6‌] and more recently Alper and coworkers investigated the synthetic scope of the system.[‌7‌] The catalyst, formed by gas-phase grafting of titanium(IV) isopropoxide onto aerosol silica, is employed in the epoxidation of various alkenes to yield the corresponding epoxides with high selectivity and in medium to good conversions, using tert-butyl hydroperoxide as the terminal oxidant. With a catalytic loading of 0.1mol%, based on the amount of titanium, (2E)-3,7-dimethylocta-2,6-dien-1-ol is selectively epoxidized in the allylic position (94% conversion, 96% selectivity) at room temperature. Increasing the temperature results in epoxidation also at the C6 position.

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