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Issue 42, 2008
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Olefinmetathesiscatalysts bearing a pH-responsive NHC ligand: a feasible approach to catalyst separation from RCM products

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Abstract

Two novel ruthenium-based olefin metathesis catalysts, H2ITap(PCy3)Cl2Ru[double bond, length as m-dash]CH–Ph 12 and H2ITapCl2Ru[double bond, length as m-dash]CH–(C6H4–O–iPr) 13 (H2ITap = 1,3-bis(2′,6′-dimethyl-4′-dimethylaminophenyl)-4,5-dihydroimidazol-2-ylidene), were synthesized bearing a pH-responsive NHC ligand with two aromatic NMe2 groups. The crystal structures of complexes 12 and 13 were determined via X-ray crystallography. Both catalysts perform ring opening metathesis polymerization (ROMP) of cyclooctene (COE) at faster rates than their commercially available counterparts H2IMes(PCy3)Cl2Ru[double bond, length as m-dash]CH–Ph 2 and H2IMesCl2Ru[double bond, length as m-dash]CH–(C6H4–O–iPr) 3 (H2IMes = 1,3-bis(2′,4′,6′-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene) and perform at similar rates during ring closing metathesis (RCM) of diethyldiallylmalonate (DEDAM). Upon addition of 2 equiv. of HCl, catalyst 12 is converted into a mixture of several mono and diprotonated Ru-carbene species 12′ which are soluble in methanol but degrade within a few hours at room temperature. Catalyst 13 can be protonated with 2 equiv. of HCl and the resulting complex 13′ is moderately water-soluble. The complex is stable in aqueous solution in air for >4 h, but over prolonged periods of time shows degradation in acidic media due to hydrolysis of the NHC–Ru bond. Catalysts 12 and 13 perform RCM of diallylmalonic acid in acidic protic media with only moderate activity at 50 °C and do not produce polymer in the ROMP of cationic 7-oxanorbornene derivative 14 under the same conditions. Catalyst 13 was used for Ru-seperation studies when RCM of DEDAM or 3,3-diallypentadione (DAP) was conducted in low-polar organic solution and the Ru-species was subsequently precipitated by addition of strong acid. The Ru-species were removed by (1) filtration and (2) filtration and subsequent extraction with water. The residual Ru-levels could be reduced to as far as 11 ppm (method 2) and 24 ppm (method 1) without the use of chromatography or other scavenging methods.

Graphical abstract: Olefin metathesis catalysts bearing a pH-responsive NHC ligand: a feasible approach to catalyst separation from RCM products

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Supplementary files

Article information


Submitted
11 Jun 2008
Accepted
17 Jul 2008
First published
12 Sep 2008

Dalton Trans., 2008, 5791-5799
Article type
Paper

Olefin metathesis catalysts bearing a pH-responsive NHC ligand: a feasible approach to catalyst separation from RCM products

S. L. Balof, S. J. P'Pool, N. J. Berger, E. J. Valente, A. M. Shiller and H. Schanz, Dalton Trans., 2008, 5791
DOI: 10.1039/B809793C

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