Synthesis and application of layered double hydroxide-hosted catalysts for stereoselective epoxidation using molecular oxygen or air

A novel chiral sulfonato-salen manganese(III) complex has been intercalated into a Zn(II)–Al(III) layered double hydroxide (LDH) host to produce a stable, active, and selective heterogeneous epoxidation catalyst. Powder X-ray diffraction, TGA, and IR and UV–visible spectroscopies confirm the successful intercalation of the Mn complex within the LDH structure. Molecular modeling calculations predict the orientation of the complex within the layered structure. The intercalated sulfonato-salen–manganese(III) complex was found to be effective in the stereoselective epoxidation of R-(+)-limonene and (−)-α-pinene at room temperature. At close to 100% conversion, R-(+)-limonene was converted to the corresponding epoxide with good selectivity and de (diastereomeric excess), whereas (−)-α-pinene was converted with 91.7% selectivity and 98.1% de. A significant advantage of this catalyst over the equivalent homogeneous counterpart was the ability to activate oxygen or air at atmospheric pressures in place of NaOCl. The catalyst could be recycled without loss of efficiency.