Selective Oxidation of Monoterpenes with Hydrogen Peroxide Catalyzed by Peroxotungstophosphate (PCWP)

Abstract

Catalytic epoxidation of monoterpenes with aqueous hydrogen peroxide catalyzed by peroxotungstophosphate (PCWP) under biphase conditions using chloroform as the solvent was examined. A variety of terpenes was oxidized to the corresponding monoepoxides or diepoxides in good yields under mild conditions. For example, limonene (1) was converted into limonene oxide (1a) in which the cyclohexene double bond was selectively epoxidized in almost quantitative yield. The oxidation of γ-terpinene (2) with 2.2 equiv of 35% H2O2 took place with high stereoselectivity to give cis-diepoxide 2c. In terpenes bearing electron-withdrawing groups such as neryl acetate (3), geranyl acetate (4), citral (5), and geranyl nitrile (6), the double bonds remote from the substituents were epoxidized in preference to the others. The epoxidation of linalool (9) by the present catalyst−oxidant system produced the cyclic products, hydroxy furan 9a and hydroxy pyran 9b, rather than epoxide. tert-Butyl alcohol was successfully employed as the solvent by treating a hydrogen peroxide solution of tert-butyl alcohol with MgSO4 prior to use. The regioselectivities in the epoxidation of monoterpenes can be favorably explained from the electron densities of the double bonds which were estimated using the CAChe system.