Geraniol regioselective epoxidation

As a further example of a hydroxyl-assisted epoxidation, geraniol and nerol bearing two isolated C=C double bonds were regioselectively epoxidized with TS-1 at the 2-position (near the OH group), as reported by Kumar et al. (795). On the basis of these results, Kumar et al. (195) proposed that the transition state of the epoxidation of allylic alcohols involves coordination of the alcoholic functional group to the Ti active site and that the double bond interacts with one of the peroxidic oxygen atoms, not with the titanium site (Scheme 9). 

All the polymers of Table III have been applied for the epoxidation of olefins with alkyl hydroperoxides. For example, the polymers with iminodiacetic acid or diethylene triamine groups were used for the regioselective epoxidation of (E)-geraniol with t-BuOOH to the 2,3-epoxide (225), whereas the Mo anchored to the diphenylphosphinopolystyrene catalyst is used in the epoxidation of cyclohexene with t-BuOOH (228). The polymer-supported molybdenyl thioglycolate has also been used for the catalytic oxidation of thiols and phosphines with air or pyridine N-oxide as the oxidant (234). 

Moreover, the catalytic activity of a reused catalyst is completely preserved after several cycles, and the filtrate is catalytically inactive. This supported and reusable P-W catalyst has also been applied for the epoxidation of a series of terpene olefins (356). The productivity of the catalyst for the conversion of these bulky molecules is superior to that reported for Ti-MCM materials, for example. The stereochemical and regioselective characteristics of W catalysis are preserved in the supported P-W catalyst. For instance, geraniol is epoxidized at the 2,3 position, affording the tram product. 

Oxovanadium(V) and oxomolybdenum(VI) were incorporated into crosslinked polystyrene resins functionalized with iminodiacetic acid or diethylenetriamine derivatives 921 The polymer complexes were used as catalysts in the oxidation of olefins with f-butylhydroperoxide. Vanadium(V) complexes promote the epoxidation of allylic alcohols in a highly regioselective manner, e.g., 2,3-epoxide was obtained in 98 % selectivity from e-geraniol at 80 °C. The catalytic activity of the vanadium(V) complexes is generally higher than that of the molybdenium(VI) complexes in the oxidation of allylic alcohols, whereas an opposed trend holds for the epoxidation of cyclohexene. 

Allyl alcohol derivatives proved to be suitable substrates for the asymmetric epoxidation with classical chiral salen Mn(III) complexes, showing both good regioselectivity in the case of geraniol derivatives where only the allylic double bond was converted into the epoxide, as well as moderate enantioselectivities and yields, the latter being obtained with a large excess of oxidant because of the concomitant decomposition of H2O2 induced by the chiral catalysts itself, as commonly observed with catalase (Scheme 23.2). 

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