Synthesis of GO-SalenMn and Asymmetric Catalytic Olefin Epoxidation

Fengqin Wang,Tiankui Huang,Qian Chen,Xiyue Ding,Cheng Huang,Shurong Rao,Xiaochuan Zou,Zhiwen Tan

doi:10.3390/catal9100824

Fengqin Wang, Tiankui Huang + Show 6 more

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https://doi.org/10.3390/catal9100824

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Abstract

Graphene oxide (GO) was used as a catalyst carrier, and after the hydroxyl group in GO was modified by 3-aminopropyltrimethoxysilane (MPTMS), axial coordination and immobilization with homogeneous chiral salenMnCl catalyst were carried out. The immobilized catalysts were characterized in detail by FT–IR, TG–DSC, XPS, EDS, SEM, X-ray, and AAS, and the successful preparation of GO-salenMn was confirmed. Subsequently, the catalytic performance of GO-salenMn for asymmetric epoxidation of α-methyl-styrene, styrene, and indene was examined, and it was observed that GO-salenMn could efficiently catalyze the epoxidation of olefins under an m-CPBA/NMO oxidation system. In addition, α-methyl-styrene was used as a substrate to investigate the recycling performance of GO-salenMn. After repeated use for three times, the catalytic activity and enantioselectivity did not significantly change, and the conversion was still greater than 99%. As the number of cycles increased, the enantioselectivity and chemoselectivity gradually decreased, but even after 10 cycles, the enantiomeric excess was 52%, which was higher than that of the homogeneous counterpart under the same conditions. However, compared to fresh catalysts, the yield decreased from 96.9 to 55.6%.

Highlights

Chiral salenMnCl catalyst (Jacobsen’s catalyst) has been proved to be one of the most effective catalysts for asymmetric catalytic epoxidation of alkenes [1,2]
The results demonstrated that the NMO could coordinate with oxo-salenMn (V) and stabilize the generated intermediate oxo-salenMn (V) complex [30], so that the substrates and catalysts could fully react
Atomic absorption spectroscopy (AAS) was used to determine the Mn content of the catalysts using a TAS-986G (Pgeneral, Beijing, China), where 0.02 g of Graphene oxide (GO)-salenMn was calcined at 700 ◦ C for 3 h, dissolved in 1:1 hydrochloric acid for 30 min, and the volume was adjusted

Summary

Introduction

Chiral salenMnCl catalyst (Jacobsen’s catalyst) has been proved to be one of the most effective catalysts for asymmetric catalytic epoxidation of alkenes [1,2]. The chiral epoxides obtained are widely used in the synthesis of fine chemicals including pesticides, flavorings, and pharmaceuticals, such as the key intermediates for anti-hypertensive drugs and for the side chain of the anti-cancer drug paclitaxel. Because it is difficult to separate and recycle the expensive chiral catalysts after the reaction, it is impossible to realize continuous flow reactors and large-scale production, which increases operating costs and wastes limited resources. Heterogeneity of homogeneous catalysts is an important strategy to enable the reuse of expensive chiral catalysts and large-scale synthesis [3]. Immobilized catalysts still have some deficiencies such as low Catalysts 2019, 9, 824; doi:10.3390/catal9100824 www.mdpi.com/journal/catalysts

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