Bis(tricyclohexylphosphine)(4-polystyrylmethylidene)ruthenium Dichloride

[223415-64-3]P  · [Cl2(PCy3)2Ru=CHC6H4CHCH2)]m(C6H5CHCH2)nloading ca. 0.1 mmol g-1

(polymer ‘boomerang’ supported alkene metathesis complex that catalyses ring-closing metathesis of dienes, minimizes ruthenium contamination of the ring-closed adduct without chromatography, and can be recycled and re-used by simple filtration)

Alternate Name: vinyl polystyrene polymer-supported Grubbs' catalyst, ‘Boomerang’ catalyst.

Solubility: insoluble in all solvents; swells in CH2Cl2, toluene and THF; does not swell in MeOH.

Form Supplied in: commercially available from NovaBiochem as a lilac powder.

Preparative Methods: Bis(tricyclohexylphosphine)(4-polystyrylmethylidene)ruthenium dichloride (PS-Grubbs' catalyst) is commercially available from NovaBiochem.1 It can also be conveniently prepared by shaking Grubbs' catalyst [Cl2(PCy3)2Ru=CHPh]2 with vinylpolystyrene3 (1-2% cross-linked with divinylbenzene) in dichloromethane solution for 1-2 h, followed by filtration (1).

Handling, Storage, and Precautions: when dry may be stored in a sealed vessel for a period of months.

Ring-Closing Metathesis

PS-Grubbs' catalyst is used for ring-closing metathesis, minimizes the ruthenium contamination of the ring-closed adduct without extensive chromatography, and can be recycled and re-used by simple filtration.4 Specifically, it has been applied for the preparation of five-membered carbocycles (2),4 dihydropyrans (3),4 bicyclic b-lactams (4)4 and sulfonamides (5).5

The advantage of the PS-Grubbs' catalyst over the traditional Grubbs' catalyst is that ruthenium contamination of the product is minimized and its use is simplified. Typically, the PS-Grubbs' catalyst is suspended in dichloromethane, the diene added and the mixture shaken for 40 min, followed by filtration. The product is obtained by simple evaporation of the washings, with minimal ruthenium contamination. The recovered catalyst can be re-used but is less active. The lifetime and activity of a given batch of PS-Grubbs' catalyst can be extended to allow three recycles by the simple addition of hex-1-ene or 3,3-dimethylbut-1-ene (9 mol%) to the mixture.4 These volatile alkenes are removed with the dichloromethane solvent in the evaporation stage to obtain the product.

The mechanism by which the PS-Grubbs' catalyst operates has been dubbed a ‘boomerang’ process. Initial metathesis with a given diene releases the catalyst from the solid support into solution. Homogeneous alkylidene [Cl2(PCy3)2Ru=CHR] catalysed metathesis ensues and the system therefore displays very similar activities to the homogeneous Grubbs' catalyst [Cl2(PCy3)2Ru=CHPh]. It is therefore expected that for any ring-closing metathesis that Grubbs' catalyst [Cl2(PCy3)2Ru=CHPh] can catalyse, PS-supported Grubbs' catalyst will be equally effective. After metathesis is complete, the resin recaptures the catalyst at a vacant vinyl site.

Recent improvements in the catalyst design to allow extended catalyst recycling and re-use (up to five times) have been achieved by the incorporation of an imidazol-2-ylidene ligand in lieu of one the PCy3 ligands (6).6 In this system oct-1-ene (10 mol %) and PPh3 (5 mol %) were used as additives to extend catalyst lifetime in re-cycle mode. Other ‘boomerang’ catalysts for ring-closing metathesis have been reported by Jafarpour & Nolan,7 Yao,8 Garber et al.9 and Dowden.10

1. Novabiochem catalogue no. 01-64-0389, 2001, Switzerland.
2. Schwab, P.; France, M. B.; Ziller, J. W.; Grubbs, R. H., Angew. Chem., Int. Ed. Engl. 1995, 34, 2039.
3. Sylvain, C.; Wagner, A.; Mioskowski, C., Tetrahedron Lett. 1998, 39, 9679.
4. Ahmed, M.; Barrett, A. G. M.; Braddock, D. C.; Cramp, S. M.; Procopiou, P. A., Tetrahedron Lett. 1999, 40, 8657.
5. Long, D. D.; Termin, A. P., Tetrahedron Lett. 2000, 41, 6743.
6. Ahmed, M.; Arnauld, T.; Barrett, A. G. M.; Braddock, D. C.; Procopiou, P. A., Synlett 2000, 1007.
7. Jafarpour, L.; Nolan, S. P., Org. Lett. 2000, 2, 4075.
8. Yao, Q., Angew. Chem., Int. Ed. Engl. 2000, 39, 3896.
9. Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H., J. Am. Chem. Soc. 2000, 122, 8168.
10. Dowden, J.; Savovic, J., Chem. Commun. 2001, 37.

Anthony G. M. Barrett & D. Christopher Braddock

Imperial College, London, UK

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