Dichlorobis[2-(dicyclohexylphosphino-P)-N,N,N-trimethylethanaaminium](phenylmethylene) Ruthenium Chloride
· (MW 901.87)
(reagent used as a metathesis catalyst in protic solvents)
Solubility: soluble in water and polar protic organic solvents such as methanol. Insoluble in acetone, THF and benzene. Also insoluble in Lewis acidic, basic, and neutral chloroaluminate-imidazolium ionic liquid melts.
Form Supplied in: purple solid; literature preparation (see below).
Preparative Methods: alkylidene 1 was prepared via phosphine ligand exchange of (PPh3)2Ru(=CHPh)(Cl2) (Grubb's catalyst) with a water soluble phosphine (1).1,2
Purification: iterative precipitation from CH2Cl2/pentane and CH2Cl2.
Handling, Storage, and Precautions: solid alkylidene 1 is unstable to air atmosphere and should be handled and stored under an inert atmosphere. In addition, alkylidene 1 is highly labile to oxygen in solution; therefore all solvents must be rigorously degassed and handled using standard Schlenk techniques in order to avoid oxidative decomposition of the catalyst. A methanol solution of alkylidene 1 is stable up to 3 weeks while an aqueous solution of alkylidene 1 shows signs of decomposition after 2 days.
Ring-opening Metathesis Polymerization (ROMP)3
Alkylidene 1 has been shown to initiate ring-opening metathesis polymerization (ROMP) in both methanol and water.1,2,4 However, polymerization was found to be incomplete due to destruction of the propagating alkylidene species. For instance, water soluble norbornene-derived monomers displayed conversions of 45-80% in the presence of alkylidene 1 (2).2,4 The presence of hydroxide anions in solution was determined to be the cause of the untimely decomposition of the propagating species. Addition of small amounts (0.3 to 1 equiv) of Brønsted acid, such as DCl, not only stabilized the propagating alkylidene species but increased reaction rates. Conversions greater than 95% were observed for the ROMP of norbornene derived monomers in the presence of DCl.2,4 Moreover, in the presence of a Brønsted acid, alkylidene 1 was found to initiate a living polymerization of these water-soluble monomers.2,4 The nature of this living polymerization with alkylidene 1 allowed for a block copolymerization of two different monomers in aqueous media. Emulsifiers such as dodecyltrimethylammonium chloride (DTAC) can also be used to encourage complete consumption of water-soluble monomers in ROMP reactions mediated by alkylidene 1.2 Note that the choice of Brønsted acid or emulsifier is limited to reagents with chloride counterions due to potential ligand exchange with alkylidene 1.5
Ring-Closing Metathesis (RCM)6
Alkylidene 1 initiates ring-closing metathesis (RCM) on a series of acyclic a,o-dienes in methanol or water. Treatment of diethyl diallylmalonate (R = H) with alkylidene 1 yielded no desired product (3); however, if one of the double bonds were internal (R = Me or Ph), then metathesis occurred with 30-80% conversion in methanol.7 The increased substitution on one of the olefins served to increase the stability of the propagating alkylidene species. Note that in the case of the phenyl-substituted malonate (R = Ph), alkylidene 1 is regenerated during the RCM reaction, making it a true catalytic species. Geometry of the internal olefin plays a vital role in both reaction rate and conversion of RCM with alkylidene 1. Finally, modest conversion was observed for the RCM of ammonium-based dienes with alkylidene 1 in aqueous media (4).7
Dichlorobis[2-dicyclohexylphosphino-P)-N,N-dimethylpiperidinium](phenylmethylene) ruthenium chloride;
dichlorobis(tricyclohexylphosphino-P)(phenylmethylene) ruthenium chloride.
- 1. Mohr, B.; Lynn, D. M.; Grubbs, R. H., Organometallics 1996, 15, 4317.
- 2. Lynn, D. M.; Mohr, B.; Grubbs, R. H.; Henling, L. M.; Day, M. W., J. Am. Chem. Soc. 2000, 122, 6601.
- 3. Hillmyer, M. A.; Lepetit, C.; McGrath, D. V.; Novak, B. M.; Grubbs, R. H., Macromolecules 1992, 25, 3345 (and references therein).
- 4. Lynn, D. M.; Mohr, B.; Grubbs, R. H., J. Am. Chem. Soc. 1998, 120, 1627.
- 5. In addition to counterion ligand exchange, alkylidene 1 undergoes non-destructive exchange with deuterated solvents, see: Lynn, D. M.; Grubbs, R. H., J. Am. Chem. Soc. 2001, 123, 3187.
- 6. Grubbs, R. H.; Chang, S., Tetrahedron 1998, 54, 4413 (and references therein).
- 7. Kirkland, T. A.; Lynn, D. M.; Grubbs, R. H., J. Org. Chem. 1998, 63, 9904.
Wayne State University, Detroit, Michigan, USA
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