Purification of a metathesis mixture can give you all sorts of problems (especially when using high catalyst loadings). Sometimes you need multiple columns to get a colorless product. Sometimes residual ruthenium can cause the product to isomerize during distillation or decompose over time. Numerous methods have been reported for the removal of metathesis catalysts after a reaction. In general, you need a ligand or adsorbent that binds ruthenium better than your product, or a reagent that will react with the residual ruthenium preferentially. Unfortunately the optimal process tends to vary on a case by case basis.
- P(CH2OH)3 (Grubbs, Tetrahedron Lett. 1999, 40, 4137). The most versatile technique out there. This water-soluble phosphine lets you wash the ruthenium away with an aqueous workup. At least 20 equiv. are necessary for efficient ruthenium removal.
- P(CH2OH)4Cl + KOH (Pederson, Adv. Synth. Catal. 2002, 344, 728). This generates P(CH2OH)3 from the inexpensive salt.
- CNCH2CO2K (Diver, Org. Lett. 2007, 9, 1203). This polar isocyanide is much faster than P(CH2OH)3 in quenching the catalyst (~30min) and the complex can be removed by filtration through silica. Unfortunately the isocyanide is not commercially available and takes two steps to prepare.
- Hydrogenation (Wang, Org. Process Res. Dev. 2008, 12, 226). Hydrogenation with Pd/C following an RCM reaction resulted in removal of residual ruthenium, which is presumably adsorbed onto the solid catalyst.
- Basic solution of cysteine (Wang, Tetrahedron 2009, 65, 6291). After a RCM in EtOAc, the crude mixture was washed with an aqueous solution of cysteine and NaOH.
- Activated carbon (Kim, Org. Lett. 2003, 5, 531). Sequential treatments with silica gel, activated carbon, and column chromatography.
- Amine-functionalized mesoporous silicates (Crudden, Org. Lett. 2006, 8, 2663). This method doesn’t require an aqueous wash or chromatography but a large amount of the adsorbent is used.
- DMSO or Ph3PO (Georg, Org. Lett. 2001, 3, 1411). Stirring overnight with 50 equiv. followed by column chromatography.
- Pb(OAc)4 (Paquette, Org. Lett. 2000, 2, 1259). A slight excess (1.5 equiv. per ruthenium) was used followed by silica filtration.