Name: heterogeneous removal of water-soluble ruthenium olefin metathesis catalyst from aqueous media via host-guest interaction
Uploaded: 2018-08-23T14:00:00
Description: Watch this Scientific Journal Video about Heterogeneous Removal of Water-Soluble Ruthenium Olefin Metathesis Catalyst from Aqueous Media Via Host-Guest Interaction at JoVE.com

This work was supported by the Florida State University Energy and Materials Hiring initiative and the FSU Department of Chemical and Biomedical Engineering.

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, &#946;-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. Sy...

<ol>
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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...

The removal of the homogeneous organometallic catalysis from the product is an important issue in modern chemistry1,2. 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 heterogeneous catalyst, i.e., homogeneous reaction and heterogeneous removal, represents important concept for highly reactive and easily removable organometallic catalyst. Figure 1 illustrated the working principle for homogeneous reaction and heterogeneous removal of the catalyst via host-guest interaction.
Host-guest chemistry is noncovalent bonding molecular recognition between host molecules and guest molecules in supramolecular chemistry4,5,6,7,8. Cyclodextrins (CDs), cyclic oligosaccharides, are representative host molecules9,10,11,12, and they have been applied in broad fields of science such as, polymer science13,14, catalysis15,16, biomedical applications6,10, and analytical chemistry17. A guest molecule, adamatane, binds strongly to the hydrophobic cavity of &#946;-CD (host, 7-membered cyclic saccharide) with high association constant, Ka (log Ka = 5.04)18. This supramolecular binding affinity is strong enough to remove residual catalyst complex from the aqueous reaction solution with solid supported &#946;-CD.
Among many catalysts that are eligible for the host-guest removal, Ru olefin metathesis catalyst was studied due to high practical utilities and high stability against air and moisture. The olefin metathesis reaction is an important tool in synthetic chemistry to form a carbon-carbon double bond in the presence of a transition metal catalyst19,20,21,22. The development of stable Ru olefin metathesis catalyst trigged the metathesis as a major field in synthetic chemistry (e.g., RCM and cross metathesis (CM)) as well as polymer science (e.g., ROMP and acyclic diene metathesis (ADMET)). In particular, the RCM synthesizes macrocycles and medium-sized rings that have been hard to construct23.
In spite of synthetic utilities of Ru catalyzed olefin metathesis, complete removal of used Ru catalyst from the desired product is a major challenge for many practical applications24. For example, 1912 ppm of Ru residue was observed in ring-closing metathesis product after silica gel column chromatography25. Residual Ru may cause problems such as olefin isomerization, decomposition, colorization, and toxicity of pharmaceutical products26. International Conference on Harmonization (ICH) published a guideline of residual metal reagents in pharmaceuticals. The maximum allowed Ru level in pharmaceutical product is 10 ppm27. For these reasons, various approaches were tried to remove Ru residue from the product solution28,29,30,31,32,33. Also, the developments of removable Ru catalysts have been studied for purification without any special treatment after the reaction. Among various purification methods, catalyst ligand modifications were tried to improve efficiency of silica gel filtration and liquid extraction. For example, highly efficient silica gel filtration can be achieved by introduced ion tag on benzylidene34 or backbone of NHC ligand35,36. The catalyst bearing poly(ethylene glycol)37 or ion tag35 on a NHC ligand can improve the efficiency of aqueous extraction for Ru catalyst removal.
Recently, we reported a highly water soluble Ru olefin metathesis catalyst, which demonstrated not only high reactivity, but also high catalyst removal rate. Moreover, the metathesis and catalyst removal occurred in both water and dichloromethane34,35,36,37. The key feature of new catalyst is that the new NHC bears adamantyl tethered oligo(ethylene glycol). Oligo(ethylene glycol) provides high water solubility of the entire catalyst complex. In addition, the oligo(ethylene glycol) possesses adamantyl end group that can be used in host-guest interaction with external &#946;-CD.
Herein, we described the protocols for catalyst synthesis, metathesis reactions, and catalyst removal in both water and dichloromethane.

<table border="0" cellpadding="0" cellspacing="0" style="width:767px;" width="767">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Hoveyda-Grubbs Catalyst 1st Generation</td>
			<td style="width:160px;">Sigma-Aldrich</td>
			<td style="width:157px;">577944</td>
			<td style="width:167px;">Air sensitivie. Light sensitivie.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Diethyl diallylmalonate</td>
			<td style="width:160px;">Sigma-Aldrich</td>
			<td style="width:157px;">283479</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Ethyl vinyl ether</td>
			<td style="width:160px;">Sigma-Aldrich</td>
			<td style="width:157px;">422177</td>
			<td>Air sensitive.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Aluminum oxide</td>
			<td style="width:160px;">Sigma-Aldrich</td>
			<td style="width:157px;">06300</td>
			<td>Activated, neutral, Brockmann Activity I</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:283px;">Potassium bis(trimethylsilyl)amide solution (0.5 M in toluene)</td>
			<td style="width:160px;">Sigma-Aldrich</td>
			<td style="width:157px;">277304</td>
			<td>Moisture sensitive.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Etyhl acetate</td>
			<td style="width:160px;">VWR</td>
			<td style="width:157px;">BDH1123</td>
			<td>Flammable liquid.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Methanol</td>
			<td style="width:160px;">VWR</td>
			<td style="width:157px;">BDH1135</td>
			<td>Flammable liquid. Toxic.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Deuterium Oxide 99.8%D</td>
			<td style="width:160px;">TCI</td>
			<td style="width:157px;">W0002</td>
			<td></td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:283px;">Methylene Chloride-D2 (D, 99.8%)</td>
			<td style="width:160px;">Cambridge Isotope Laboratories, Inc.</td>
			<td style="width:157px;">DLM-23</td>
			<td>Flammable liquid. Toxic.</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Activated carbon</td>
			<td style="width:160px;">Sigma-Aldrich</td>
			<td style="width:157px;">242276</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Magnesium sulfate</td>
			<td style="width:160px;">EMD Millipore</td>
			<td style="width:157px;">MX0075</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:283px;">Ethyl ether</td>
			<td style="width:160px;">EMD Millipore</td>
			<td style="width:157px;">EX0190</td>
			<td>Flammable liquid.</td>
		</tr>
	</tbody>
</table>

heterogeneous removal of water-soluble ruthenium olefin metathesis catalyst from aqueous media via host-guest interaction

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 &#946;-cyclodextrin (&#946;-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 &#946;-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.

Figure 2&#160;describes the ligand exchange reaction for our catalyst 1. The 1H NMR spectrum is shown in Figure 3.
Figure 4&#160;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 me...

Watch this Scientific Journal Video about Heterogeneous Removal of Water-Soluble Ruthenium Olefin Metathesis Catalyst from Aqueous Media Via Host-Guest Interaction at JoVE.com

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

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.

A highly efficient transition metal catalyst removal method is developed. The water-soluble catalyst contains a newly-designed NHC ligand for the catalyst ...

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