Novel chiral sulphonato-salen-manganese (III)-pillared hydrotalcite catalysts for the asymmetric epoxidation of styrenes and cyclic alkenes

A novel chiral sulphonato-salen-manganese (111) complex has been prepared and intercalated into a Zn-Al layered-double hydroxide (LDH) structure. The resulting catalyst was found to be highly active and enantioselective in the epoxidation of various styrenes and cyclic alkenes when using a combination of pivalaldehyde and molecular oxygen at atmospheric pressure and room temperature. At 94% conversion, 1-methyl-1-cyclohexene could be converted to epoxide with 68% ee and 90% selectivity with a turnover frequency (TOF) of 234 h(-1), whereas 4-methyl-styrene was converted with 62% ee and 70% selectivity with a TOF of 327 h-1. In the case of styrenes and cyclic alkenes, TOF decreased as follows: alpha-methylstyrene > 4-methylstyrene > styrene and 1-methyl-1-cyclohexene > 1-phenyl-1-cyclohexene > cyclohexene. The catalyst could be recycled without detectable loss of efficiency.