Terpenic substrates, epoxidation

The epoxidation of terpenic substrates is of interest in the flavor and fragrance industry [55,56]. Terpenes are derivatives of isoprene, which has formula CsHg (2-methyl-frans-butadiene). There are tens of examples of terpenes, including limonene, a-pinene, geraniol, citronellol, myrcene, ocimene, camphene, a-terpin-eol, menthol, and isopugelol. Limonene is an abundant monoterpene extracted from citrus oil, which can be epoxidized to obtain fragrances, perfumes, and... 

Molybdenum hexacarbonyl [Mo(CO)6] has been vised in combination with TBHP for the epoxidation of terminal olefins [44]. Good yields and selectivity for the epoxide products were obtained when reactions were performed under anhydrous conditions in hydrocarbon solvents such as benzene. The inexpensive and considerably less toxic Mo02(acac)2 is a robust alternative to Mo(CO)6 [2]. A number of different substrates ranging from simple ot-olefms to more complex terpenes have been oxidized with very low catalytic loadings of this particular molybdenum complex (Scheme 6.2). The epoxidations were carried out with use of dry TBHP (-70%) in toluene. 

Electron withdrawing groups on the alkene considerably retard the epoxidation. For example, acrylic esters and acrylonitrile are not reactive, while allyl chloride is about one tenth as reactive as propylene towards TBHP/MoVI.240 The epoxidation of alkenes is completely stereoselective eri-alkenes are exclusively transformed into ds-epoxides and (runs-alkenes into trans-epoxides. Oxygen addition to the double bond preferentially occurs from the less shielded face of the substrate, e.g. in the selective epoxidation of terpenes by r-pentyl hydroperoxide (t-amyl hydroperoxide, TAHP) (equations 67 and 68).225... 

The reaction of the mixture 112 and 113 leads to the corresponding azido-product 115 together with 1,2-epoxy-terpene 113. Therefore, 112 was selectively transformed and epoxide 113 was recovered. On the other hand, the authors could conclude that the dia-stereoselectivity observed was due to the presence of the C4-substitnent at the starting epoxides, which forces the substrate into the more stable conformation and consequently blocks the approach of the Cr-Ns species from one side. 

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