(582ao) Functionalized Metal?Organic Framework As a Biomimetic Heterogeneous Catalyst for Transfer Hydrogenation of Imines

Functionalized Metal−Organic Framework as a Biomimetic Heterogeneous Catalyst for Transfer Hydrogenation of Imines

Jingwen Chen†, Zhiguo Zhang†,* Zongbi Bao†, Huabin Xing†, Qiwei Yang†, and Qilong Ren†

Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Zheda Road 38, Hangzhou 310027, P. R. China

Abstract

Nature enzymes have evolved to carry out diverse chemical transformations with enormous proficiencies and exquisite specificities. Mimicking the action modes of enzymes by artificial materials, whose features are akin to enzymatic active sites, has become a fertile pursuit. Reduced nicotinamide adenine dinucleotide (NADH) is the most abundant electron carrier in cell metabolism, and its capacity as a redox agent has been exploited by numerous enzymes for the production of amines, alcohols, and so on. Inspired by nature occurring hydrogenations, a biomimetic approach employing enzyme-like organocatalysts and Hantzsch ester (HEH) as the NADH mimic for the reduction of C=N has received increasing interest. In this context, Brønsted acids such as diphenyl phosphate and trifluoromethanesulfonic acid have been used as efficient catalysts in the reduction of ketimines with Hantzsch ester as the hydrogen source. However, from a practical point of view, use of the homogeneous Brønsted acids faced detrimental problems, such as severe equipment corrosion, tedious separation process, and serious environmental pollution. As a consequence, the development of a biomimetic solid acid catalyst is desirable to alleviate or eliminate those problems. Metal-organic frameworks (MOFs), a class of hybrid materials constructed via the linkage of metal ions/clusters with various organic ligands, have received much attention in catalysis, adsorption, and so on. In view of their ordered porous structure, hydrophobic confined environment, and fruitful functionalizations, MOFs have been envisioned as one of the most appropriate platforms for the design of biomimetic catalyst.

In the present work, we employed MIL-101(Cr) as a biomimetic platform, and customized a sulfonic group (SO₃H) into its hierarchical pores to generate a heterogeneous catalyst (MIL-101(Cr)-SO₃H) for the transfer hydrogenation of imines with Hantzsch ester as the reductant (Scheme 1). Both aldimines and ketimines were efficiently converted to their hydrogenated counterparts in moderate to high yields. Experimental investigations revealed that the superior catalytic activity of MIL-101(Cr)-SO₃H to MIL-101(Cr) or its homogeneous counterparts may be resulted from the synergistic interaction of Lewis acid sites and Brønsted acid sites within the hydrophobic pores. This protocol much resembles the action mode of metalloenzyme in biology and demonstrates an attractive choice of using MOFs to mimic enzymatic catalysis. Furthermore, MIL-101(Cr)-SO₃H could be recovered and reused several times without considerable loss in reactivity.

Scheme 1. MIL-101(Cr)-SO₃H catalyzed biomimetic transfer hydrogenation of imines

 

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