Chromium oxide, epoxidation

The proposed mechanism includes a reductive epoxide opening, trapping of the intermediate radical by a second equivalent of the chromium(II) reagent, and subsequent (3-elimination of a chromium oxide species to yield the alkene. The highly potent electron-transfer reagent samarium diiodide has also been used for deoxygenations, as shown in Scheme 12.3 [8]. 

Sodium perborate oxidation of alcohols by is aided by Aliquat, but also requires the addition of chromium oxide [17]. However, the long reaction times at 60-80°C and the variable yields do not make the procedure particularly attractive. In contrast, direct epoxidation of a,p-unsaturatcd ketones has been conducted with moderate success using sodium perborate catalysed by tetra-n-hexylammonium hydrogen sulphate [18, 19]. 

Sometimes, an alcohol via the corresponding chromate ester may direct a chromium-promoted epoxidation of an aJkcne. This side reaction, which can happen with other chromium-based oxidants,83 depends on very exacting stereoelectronic factors to occur. 

Alcohols may be oxidized in a similar way. However, these reactions strongly resemble those reported for Cr molecular sieves, and a small concentration of Cr in solution may well account for most of the observations of catalysis. Binary molybdenum-chromium oxides supported on alumina have been used in the autoxidation of cyclohexene with 02 and r-BuOOH as an initiator (62). This is a complex reaction in which uncatalyzed and Cr-catalyzed oxidation combine to yield 2-cyclohexen-l-one, 2-cyclohexen-l-ol, and 2-cyclohexenyl hydroperoxide the Mo compound can use the hydroperoxide formed in situ as an oxidant for the epoxidation of cyclohexene. Although much lower oxygen consumption was observed for the reaction filtrate than for the suspension, it is unclear how the Cr is held by the oxide. 

Cobalt-catalyzed epoxidation of alkenes has been carried out with the cobalt derivative of (174), employing iodosylbenzene as the oxidant. Epoxidation of cfa- -methylstyrene furnishes exclusively the cis-epoxide (equation 62). The reaction proceeds through an active oxo-cobalt(IV) species, and is mote selective than reactions proceeding through oxo-chromium or oxo-manganese species. The catalyst can be recovered unchanged by simple filtration. 

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