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

Abstract

Two novel ruthenium-based olefin metathesis catalysts, H₂ITap(PCy₃)Cl₂RuCH–Ph 12 and H₂ITapCl₂RuCH–(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)Cl₂RuCH–Ph 2 and H₂IMesCl₂RuCH–(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.