Chiral MnIII(Salen) supported on tunable phenoxyl group modified zirconium poly (styrene-phenylvinylphosphonate)-phosphate as an efficient catalyst for epoxidation of unfunctionalized olefins

We have developed a series of chiral MnIII(Salen) (chiral Jacobsen's catalyst) catalysts that were axially supported onto zirconium poly(styrene-phenylvinylphosphonate)-phosphate through tunable phenoxyl linkers of varying steric hindrance, and evaluated their performance as catalysts (Cat1–Cat3) for the epoxidation of unfunctionalized olefins using m-chloroperbenzoic acid as an oxidant. The corresponding non-supported analogues (Cat4–Cat6) were also prepared and tested under similar conditions. The results demonstrated that the incorporation of substituents at the ortho-positions of the tunable phenoxyl linkers had a critical effect on the catalytic activity. Furthermore, the enantioselectivity increased as the steric hindrance imposed by the linker increased, although this also led to a decrease in the conversions under the same reaction conditions, especially for heterogeneous epoxidation. The heterogeneous system also displayed high ee values and conversions in the absence of the axial additive N-methylmorpholine N-oxide, which is commonly required to improve the catalytic activity of epoxidation reactions, especially for the epoxidation of α-methylstyrene. The reusability of the catalyst was also evaluated over 11 catalytic cycles, with no significant reduction observed in the catalytic activity or enantioselectivity after five runs.

Graphical AbstractWe have reported for the first time that the use of tuneable rigid phenoxyl linkers bearing ortho substituents imposing different steric hindrance effects has a critical effect on the catalytic activity of the supported systems towards the epoxidation of olefins, especially heterogeneous epoxidation systems.Figure optionsDownload full-size imageDownload as PowerPoint slide