Styrene epoxidation phenylacetaldehyde formation

In some cases, oxidation of double bonds does not stop at the epoxide, but proceeds further to oxidative cleavage of the double bond. It was reported that the reaction of a-methyl styrene with H2O2 in the presence of TS-1 or TS-2 produces a-methyl styrene epoxide (15%), a-methyl styrene diol (10-40%) and acetophenone (40-60%) (Reddy, J. S. et al., 1992). However, results similar to those obtained with titanium silicates were obtained for many other catalysts, such as HZSM-5, H-mordenite, HY, A1203, HGa-silicalite-2, and fumed Si02. These materials have much different properties and differ significantly from titanium silicates thus, the results cast some doubt on the role of the catalyst in this reaction. Furthermore, the oxidation of styrene is reported to proceed with C=C cleavage and formation of benzaldehyde, in contrast to previous reports of the formation of phenylacetaldehyde with 85% selectivity (Neri et al., 1986). 

FIGURE 19.9 Electronic energy as a function of the reaction coordinate for (a) epoxidation of styrene and (b) formation of phenylacetaldehyde. Results are shown for the quintet spin state in vacuum and in dichloromethane. 

However, we have demonstrated the formation of a metallacycle [(dipy)(Cl)Rh-(0-CH2-CHPh) or (dipy)(Cl)Rh-(0-CHPh-CH2)] from styrene and dioxygen. These intermediates could give rise to both styrene oxide and the carbonate. The higher reaction rate when starting from styrene and dioxygen with respect to the epoxide can be, thus, justified. High temperature (> 353 K) often cause decomposition of the catalyst. Two mutually free cis positions are necessary for the formation of the metallacycle, that interacts with carbon dioxide and yields the carbonate so, in the presence of Rh(diphos)2Cl and Rh(dipy)2Cl, no conversion at all into the carbonate has been observed, either starting from styrene or from styrene oxide. In the latter case, only a minor isomerization into acetophenone and phenylacetaldehyde has been observed. 

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