Jacobson's catalyst

Abstract

Different cyclic and acyclic doubly substituted cis-olefins were chosen to demonstrate the enantioselective efficiency of the catalyst. Trans-disubstituted olefins were found to be epoxidised with low enantioselectivity. However, inclusion of chiral quaternary ammonium salts of chincona alkaloids proved useful to provide trans-epoxides from cis-olefins. Enantioselective epoxidation of several 2,2-dimethylchromenes has been performed in high yields, providing a straightforward route to biologically active building blocks. Since the first results of cis-olefin epoxidation, an investigation into cinnamate epoxidation was carried out, as it represented an opportunity for constructing important chiral building blocks. Steric properties of the ester group have a large influence on enantioselectivity, whereas electronic properties of the aromatic ring show no relationship to the observed ee. Addition of pyridine N-oxide derivatives was found to increase the reaction turnover, keeping the same level of stereoselectivity. Jacobsen's catalyst proves less effective when applied to the epoxidation of a variety of tetrasubstituted olefins. For highly hindered double bonds, tuning of the ligand was required to afford both high levels of ee and yields. Following an earlier study on styrene, the combination m-CPBA/NMO proved successful for the preparation of epoxides at low temperature from a wide range of mono, di- and tri-substituted olefins. The combination of molecular oxygen and pivaldehyde has been used as an alternative oxidation system to activate the manganese core of the catalyst. Differently substituted olefins were successfully epoxidised enantioselectively, with alkyl imidazole as superior axial ligands for the catalyst.