Olefin metathesis catalysts bearing a pH-responsive NHC ligand: a feasible approach to catalyst separation from RCM products
Two novel ruthenium-based olefin metathesis catalysts, H(2)ITap(PCy(3)) Cl(2)Ru=CH-Ph 12 and H(2)ITapCl(2)Ru=CH-(C(6)H(4)-O-iPr)(13) (H(2)ITap = 1,3-bis(2',6'-dimethyl-4'-dimethylaminophenyl)-4,5-dihydroimidazol- 2-ylidene), were synthesized bearing a pH-responsive NHC ligand with two aromatic NMe(2) 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 H(2)IMes(PCy(3))Cl(2)Ru=CH-Ph 2 and H(2)IMesCl(2)Ru=CH-(C(6)H(4)-O-iPr)(3) (H(2)IMes = 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 degrees 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.