An efficient chiral porous catalyst support – Hypercrosslinked amino acid polymer

Highlights

• Synthesis of a new chiral catalyst support, porous hypercrosslinked amino acid (HCP-AA).

• A new heterogeneous asymmetric catalyst.

• High enantioselectivity of HCP-AA-Rh for asymmetric hydrosilylation of ketones and olefins (ee% up to 99%).

• HCP-AA-Rh can be easily recycled and well maintain the catalytic performance after recycle.

Abstract

The application of asymmetric catalysis in industrial processes is limited by the high cost of chiral ligands and the difficulty associated with recovering precious metal catalysts. Here, based on a FeCl₃-catalyzed Friedel–Crafts alkylation reaction, an aromatic amino acid was crosslinked with 4,4′-bis(chloromethyl)-1,1′-biphenyl to produce a novel porous chiral catalyst support, hypercrosslinked amino acid polymer (HCP-AA). Subsequently, metal catalysts were loaded on HCP-AA through the coordination of the metal ion with amino acid residues to generate heterogeneous asymmetric catalysts, HCP-AA-M. The resulting catalysts were characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. The HCP-AA-M samples were employed to catalyze the heterogeneous asymmetric reaction, and they achieved high enantioselectivities (up to 99% ee). Meanwhile, the catalysts could be readily recovered through filtration, and they exhibited good recycling performances. The results suggested that the utilization of inexpensive amino acids as raw materials for producing porous chiral supports for recyclable high-performance asymmetric catalysts is an effective strategy.

Introduction

With the increasing demand for chiral compounds [1], [2], [3], [4], [5], [6], [7], the development of efficient synthetic methods for enantiomerically pure compounds has attracted significant interest in both academia and industries [8], [9], [10], Asymmetric catalysis is a common strategy that has been extensively studied in the last few decades [11], [12]. Generally, a transition-metal complex containing a chiral ligand catalyzes the direct transformation of an achiral substrate into enantioenriched products [13], [14]. Numerous catalysis reactions with high enantioselectivity and efficiency have been realized by either a homogeneous or a heterogeneous approach; however, some notable problems still exist. For instance, chiral ligands are still very expensive and difficult to obtain, and the expensive noble-metal asymmetric catalysts are difficult to recycle; therefore, the cost of asymmetric catalysis is still very high, which has driven researchers to find a more economical strategy [15], [16], From the viewpoint of catalyst separation and recovery, separable and recyclable heterogeneous asymmetric catalysts are better alternatives to homogeneous catalysts; thus, they are suitable for large-scale synthesis [17]

Polymer-immobilized chiral catalysts have attracted significant interest and become one of the important technologies for the organic synthesis of optically active compounds. In some cases, a polymer-immobilized catalyst accelerates the reaction rate; in other cases, a polymer- immobilized chiral catalyst realizes enhanced stereoselectivity compared with its low-molecular-weight counterpart. [18] These demonstrate that the design of polymer catalysts is important for developing efficient catalysts and understanding their catalytic processes. Crosslinked polystyrene is the most widely used catalyst support, mainly because of its easy preparation [19], [20]. Meanwhile, various kinds of polymers, including crosslinked, branched, and dendritic polymers, have recently been used as supports for chiral catalysts [21], [22], [23], Porous hypercrosslinked polymers (HCPs) have a large specific surface area, stable physical and chemical properties, and numerous surface functional groups [11]. They are widely employed in the fields of separation [24], catalysis [25], gas storage [26] adsorption [27] etc. The HCPs can be readily prepared by simple reactions, such as Friedel–Crafts alkylation [28], and Scholl reaction on aromatic rings [29]. Their porous structures are conducive for the effective mass transfer of the reaction substrate. Therefore, porous HCPs can be economical, highly efficient, reusable catalyst supports for heterogeneous catalysts [25], [30], [31].

Most chiral ligands are expensive and difficult to obtain; however, as a class of natural chiral compounds, amino acids are widely found in nature and have been industrially produced. Further, the nitrogen and oxygen in their molecular structure can coordinate with metal ions.[32] These make amino acids suitable chiral ligand candidates for obtaining economical asymmetric catalysts. In addition, the crosslinkable aromatic ring in aromatic amino acids facilitates the production of porous hypercrosslinked amino acid polymers that can be used as chiral catalyst supports.

Herein, three aromatic amino acids (L-phenylalanine (L-Phe), L-tryptophan (L-Trp), and L-phenylglycine (L-Phg)) were separately crosslinked with 4,4′-bis(chloromethyl)-1,1′-biphenyl (BCMP; the crosslinker) by a simple FeCl₃-catalyzed Friedel − Crafts alkylation to produce the corresponding hypercrosslinked amino acid polymers (HCP-AA: HCP-(L)Phe, HCP-(L)Trp, and HCP-(L)Phg). Subsequently, an achiral metal catalyst was loaded on HCP-AA as a chiral catalyst support through the coordination between the amino acid residues with the metal species to generate heterogeneous asymmetric catalysts (HCP-AA-M: HCP-(L)Phe-Rh(Ⅰ), (HCP-(L)Phg-Rh(Ⅰ), HCP-(L)Trp-Rh(Ⅰ), HCP-Trp-Ru(Ⅱ), HCP-Trp-Co(Ⅱ), HCP-Trp-Ni(Ⅱ), HCP-Trp-Cu(Ⅰ), and HCP-Trp-Cu(Ⅱ)). The catalytic activity of HCP-AA-M was systematically investigated.