Heterogeneous Removal of Water-Soluble Ruthenium Olefin Metathesis Catalyst from Aqueous Media Via Host-Guest Interaction

Summary

A removable water-soluble N-heterocyclic carbene (NHC) ligand in aqueous media via host-guest interaction has been developed. We demonstrated representative olefin metathesis reactions in water as well as in dichloromethane. Via either host-guest interaction or extraction, the residual ruthenium (Ru) catalyst was as low as 0.14 ppm after the reaction.

Abstract

A highly efficient transition metal catalyst removal method is developed. The water-soluble catalyst contains a newly-designed NHC ligand for the catalyst removal via host-guest interactions. The new NHC ligand possesses an adamantyl (guest) tethered linear ethylene glycol units for hydrophobic inclusion into the cavity of a β-cyclodextrin (β-CD) host compound. The new NHC ligand was applied to a Ru-based olefin metathesis catalyst. The Ru catalyst demonstrated excellent activity in representative ring-closing metathesis (RCM) and ring-opening metathesis polymerization (ROMP) reactions in aqueous media as well as organic solvent, CH₂Cl₂. After the reaction was complete, the lingering Ru residue was removed from the aqueous solution with the efficiency of more than 99% (53 ppm of Ru residue) by simple filtration utilizing a host-guest interaction between insoluble silica-grafted β-CD (host) and the adamantyl moiety (guest) on the catalyst. The new Ru catalyst also demonstrated high removal efficiency via extraction when the reaction is run in organic solvent by partitioning the crude reaction mixture between layers of diethyl ether and water. In this way, the catalyst stays in aqueous layer only. In organic layer, the residual Ru amount was only 0.14 ppm in the RCM reactions of diallyl compounds.

Introduction

The removal of the homogeneous organometallic catalysis from the product is an important issue in modern chemistry^1,2. Residual catalyst causes not only a toxicity problem from its heavy metal element, but also an undesired transformation of product from its potential reactivity. Homogeneous catalyst provides many advantages, such as high activity, rapid reaction rate, and chemoselectivity³, however, its removal from the product is much more difficult than heterogeneous catalyst which is simply removed by filtration or decantation. The combination of the advantages of homogeneous and he....

Protocol

Note: We presented the synthesis of 4-(97-(adamantan-1-yloxy)-2,5,8,11,14,17,20,23,26,29,32,35,38,41,44,47,50,53,56,59,62,65,68,71,74,77,80,83,86,89,92,95-dotriacontaoxaheptanonacontyl)-1,3-dimesityl-4,5-dihydro-1H-imidazol-3-ium tetrafluoroborate (imidazolium salt A) and host complex, β-CD grafted silica, in our previous paper³⁸. In the protocol, we describe a synthesis of our water-soluble Ru olefin metathesis catalyst and metathesis reactions (RCM and ROMP).

1. Synthesis of (4-(97-((adamantan-1-yl)oxy)-2,5,8,11,14,17,20,23,26,29,32,35,38,41,44,47,50,53,56,59,62,65,68,71,74,77,80,83,86,89,92,95-dotriacontaoxahep....

Results

Figure 2 describes the ligand exchange reaction for our catalyst 1. The ¹H NMR spectrum is shown in Figure 3.

Figure 4 shows the RCM in aqueous solution and subsequent removal of used catalyst from the reaction mixture via host-guest interaction, and Table 1 summarizes RCM in aqueous media. Quate.......

Discussion

We described the synthesis of removable homogeneous Ru olefin metathesis catalyst and its removal from both aqueous and organic solutions. Homogeneous catalysis provides many benefits compared to heterogeneous catalysts, such as high reactivity and rapid reaction rate; however, the removal of the used catalyst from the product is more difficult than heterogeneous catalyst³. The key feature of synthesized catalyst is the NHC ligand, which bears adamantyl tethered water soluble oligo(ethylene glycol.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Hoveyda-Grubbs Catalyst 1st Generation	Sigma-Aldrich	577944	Air sensitivie. Light sensitivie.
Diethyl diallylmalonate	Sigma-Aldrich	283479
Ethyl vinyl ether	Sigma-Aldrich	422177	Air sensitive.
Aluminum oxide	Sigma-Aldrich	06300	Activated, neutral, Brockmann Activity I
Potassium bis(trimethylsilyl)amide solution (0.5 M in toluene)	Sigma-Aldrich	277304	Moisture sensitive.
Etyhl acetate	VWR	BDH1123	Flammable liquid.
Methanol	VWR	BDH1135	Flammable liquid. Toxic.
Deuterium Oxide 99.8%D	TCI	W0002
Methylene Chloride-D2 (D, 99.8%)	Cambridge Isotope Laboratories, Inc.	DLM-23	Flammable liquid. Toxic.
Activated carbon	Sigma-Aldrich	242276
Magnesium sulfate	EMD Millipore	MX0075
Ethyl ether	EMD Millipore	EX0190	Flammable liquid.