Epoxidation of olefins catalyzed by manganese(III) salen complexes grafted to porous heterostructured clays

Two chiral manganese(III) salen complexes were immobilized onto functionalized porous clay heterostructures (PCHs). The Jacobsen complex was anchored onto the PCH derivatized with phenoxyl groups (CAT_1) by axial coordination between the metal center and the support. The manganese(III) salen complex containing vinyl functionalities was grafted onto the mercapto-derivatized PCH through the ligand (CAT_2). All the materials were characterized by nitrogen adsorption–desorption isotherms at −196 °C, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR); the Mn content of CAT_1 and CAT_2 was determined by atomic absorption spectroscopy (AAS). The heterogeneous catalysts were tested in the asymmetric epoxidation of olefins using m-chloroperoxybenzoic acid (m-CPBA)/N-methylmorpholine-N-oxide (NMO) as the oxygen source, at 0 °C in dichloromethane, for two cycles. The catalyst CAT_1 led to almost similar epoxide selectivities and slightly higher enantioselectivities than those obtained with the homogeneous counterpart. Furthermore, in the epoxidation of styrene, it presented higher catalytic performance and stability upon reuse than CAT_2. In contrast, in the α-methylstyrene epoxidation, CAT_2 was the most active, selective and stable catalyst.

Graphical AbstractFigure optionsDownload as PowerPoint slideResearch Highlights
► The Jacobsen complex was anchored onto phenoxyl-functionalized PCH (CAT_1).
► A vinyl-salen Mn(III) complex was grafted onto a mercapto-functionalized PCH (CAT_2).
► CAT_1 showed higher catalytic performance and stability in the styrene epoxidation.
► CAT_2 was more active, selective and stable in the α-methylstyrene epoxidation.
► CAT_1 led to similar selectivity and slightly higher %ee than the Jacobsen complex.