Epoxidation by layered double hydroxide-hosted catalysts. Catalyst synthesis and use in the epoxidation of R-(+)-limonene and (-)-a-pinene using molecular oxygen

A novel chiral sulphonato-salen-manganese(III) complex has been intercalated into a Zn(II)-Al(III) layered double hydroxide (LDH) host to produce a stable heterogeneous epoxidation catalyst. Powder X-ray diffraction, IR and UV-Visible ( solid) spectroscopy and TGA confirmed the successful intercalation of the Mn complex within the LDH gallery height. The intercalated sulphonato-salen-manganese(III) complex was found to be an effective heterogeneous catalyst for the stereoselective epoxidation of R-(+)-limonene and (-)-alpha-pinene at room temperature and using molecular oxygen at atmospheric pressure as oxidant. At close to 100% conversion, R-(+)- limonene was converted into the corresponding epoxide with 93% selectivity and 43% de (diastereomeric excess), whereas (-)-alpha-pinene was converted with 93% selectivity and 98% de. The catalyst could be recycled without loss of efficiency.