Scaled up epoxidation of terpenes in microemulsion

Abstract

The dioxirane epoxidation was traditionally carried out in a two-phase medium: an organic solvent to dilute the olefin to be epoxidised and an aqueous medium in which the peroxymonosulphate oxidant is dissolved. This phase incompatibility generally requires a large quantity of oxidant and oxygen yield does not exceed 25 %. Despite the major advantages of this technique, it is still only used at laboratory scale. The aim of this study is to illustrate a dioxirane epoxidation ability to produce terpene epoxides at the scale of hundreds of milliliters. The technique employs an acetone catalyst that reacts with oxone® to in-situ form dimethyldioxirane, which epoxidizes the terpene. Oxone® is an aqueous oxidant. To remedy its incompatibility with hydrophobic terpene the use of a surfactant at concentration above its critical micelle concentration (cmc) enabled the terpene to be highly dispersed in the aqueous medium in absence of any additional organic solvent by forming a microemulsion, thus improving interface reaction rate and reducing oxygen loss. At 25°C and for a t_r of 90 min, several hundred milliliters of limonene, α-pinene, β-pinene, α-terpinene, γ-terpinene, β-myrcene and farnesene were easily epoxidized with both conversion and selectivity of almost 100 %. Oxygen yields of up to 80 % were achieved, against 29 % in absence of microemulsion. The high selectivity of epoxides, the solvent-free aqueous reaction, the ambient conditions, the simple acetone used as catalyst and the easy separation of epoxides from the aqueous medium make this innovative process more appropriate for easy scale-up.