Dichloro-bis(tricyclohexylphosphine) methyleneruthenium

[171368-36-8]  · [(C6H11)3P]2Ru(CH2)Cl2  · (MW 746.86)

(metathesis catalysis, ring-closing metathesis (RCM), olefin cross-metathesis, ruthenium alkylidene complex, presumed active catalyst in many ruthenium-catalyzed metathesis reactions)

Alternate Name: Grubbs' catalyst.

Solubility: dichloromethane, benzene.

Form Supplied in: burgundy microcrystalline solid.

Preparative Methods: catalyst 1 is most commonly generated in situ upon the first turnover of the more stable and more easily handled catalysts, 2 or 3, in their reaction with terminal alkene substrates. However, a number of general methods have been used to synthesize 1, two in particular have been used most frequently. In the first of these two methods, 1 is formed by a double oxidative addition of [RuH2(H2)2(PCy3)2] or [RuH2(N2)2(PCy3)2] directly to dichloromethane1,2 or to a higher homolog RC(H)Cl2 (R = Ph, CO2Me) in the presence of ethylene.3 For example,1,2 CH2Cl2 (0.60 mmol) was added via syringe to a suspension of [RuH2(H2)2(PCy3)2] (0.15 mmol) in pentane (7 mL), and the reaction was stirred under argon at room temperature for 3 h or, alternatively, heated to 60 °C for 15 min. A color change occurs during the course of the reaction; the suspension changing from white to brown-red. The solid was recovered by filtration, washed with pentane, and dried in vacuo to give 1 (63-67%). In a variation of this procedure,2 [RuH2(H2)2(PCy3)2] is first converted to [RuH2(N2)2(PCy3)2] by bubbling N2 through the suspension of [RuH2(H2)2(PCy3)2] in pentane for 15 min, upon which dichloromethane is added. The resulting mixture is stirred for 20 min at room temperature to give 1 (70%) as above. In the second method,4,5 a cooled (-50 °C) solution of [RuCl2 (PPh3)2] (2.47 mmol) in dichloromethane or pentane (3 mL) is treated with phenyldiazomethane (4.94 mmol). The cold bath is removed and, after 5 min of stirring, the solution is concentrated to ~3 mL. A color change, orange-brown to brown-green, was observed. Pentane (20 mL) was added to precipitate the solid, which was separated from the mother-liquor via cannula. The solid was redissolved in dichloromethane, and reprecipitated with pentane; this procedure repeated until the mother liquor is nearly colorless. The resulting gray-green microcrystalline solid of [(PPh3)2Cl2Ru = CHPh] was dried under vacuum (89%). The complex was then dissolved in dichloromethane, and 2.2 equiv of tricyclohexylphosphine (PCy3) was added. The solution was stirred for 30 min at room temperature, and then filtered and dried in vacuo. The residue was washed repeatedly with acetone or methanol and dried in vacuo to give 2 (89%) as a purple microcrystalline solid. Compound 2 was then converted into 1 by stirring a solution of 2 in dichloromethane under an atmosphere of ethylene (1 atm) for 15 to 30 min at room temperature. The solvent was removed under vacuum, and the residue washed repeatedly with acetone or pentane (5 mL) to give 1 as a burgundy colored microcrystalline solid in quantitative yield. It was reported that phenyldiazomethane is used to form the intermediate alkylidene [(PPh3)2Cl2Ru = CHPh], because the direct reaction of [RuCl2(PPh3)2] with diazomethane gives a complex mixture of products.5 Ruthenium alkylidenes can also be formed by the reaction of [RuHCl(H2)(PCy3)2] with terminal alkynes6 or with propargyl or vinyl chlorides.7

Purification: repeated washings with pentane or acetone; dried under vacuum for several hours.

Handling, Storage, and Precautions: air-stable solid; toxic; decomposition of the complex is observed after 12 h in solution (CH2Cl2 or C6H6) at ambient temperature; faster decomposition rates are observed in heated solutions, for example, the thermolytic half-life of 1 in C6D6 solution at 55 °C is 40 min.

Introduction

The development of well-defined ruthenium carbene catalysts such as 1, 2, 3, and 4, along with molybdenum alkylidene catalysts such as 5 (Schrock's catalyst), has greatly enhanced the utility of metathesis chemistry in organic synthesis. Ring-closing metathesis (RCM), ring-opening metathesis, polymerization reactions, and a variety of cross-metathesis strategies have become ubiquitous in their application to the synthesis of organic compounds and polymers.8-11 [(PCy3)2Cl2Ru = CH2] (1), was reported by Grubbs to be ‘the first insoluble methylidene complex which is an active metathesis catalyst.’4 This summary focuses on the use of 1 as the initiating catalyst in RCM and cross-coupling metathesis reactions. However, it should be noted that 2 is the complex commonly known as ‘Grubb’s catalyst' and, to date, it has been the most frequently used of the ruthenium catalysts. It shows higher activity12 and better stability in solution than 1. For example, the thermolytic half-life of 1 in solution at 55 °C (C6D6) is 40 min, as compared to 8 days for 2.13 Nonetheless, upon the first turnover in the catalytic cycle involving complexes 2 or 3 and a mono- and gem-disubstituted alkene, the ruthenium alkylidene 2 or 3 is converted into the methylidene species, (1), and it is 1 that is the propagating species in such metathesis reactions.14,15

The ruthenium catalysts 1-4 are remarkably tolerant of dioxygen, trace amounts of water, protic media, and the presence of many common organic functional groups. Recently, ruthenium complex 4 has been shown to catalyze many metathesis reactions that fail with complexes 1, 2 or 3, for example, reactions involving sterically demanding alkenes.10

Ring-Closing Metathesis (RCM)

Complex 1 has been used to convert atactic polybuta-1,2-diene into a polymer containing cyclopentene repeat units (1).16 The RCM of random adjacent dienes at a constant rate is thought to be followed by catalyst migration up and down the chain to remove unreacted vinyl groups. Analysis of the final polymer finds that over 97% of the vinyl substituents are cyclized.

The rate of the RCM reaction catalyzed by complex 1 is significantly depressed by the addition of 0.25-1.0 equiv of PCy3, while the addition of CuCl or CuCl2 accelerates RCM reaction.14

Formation of Allylsilanes (via Cross-Alkene Metathesis)

A mixture of allylsilanes is obtained in moderate yield by an unusual stoichiometric reaction of complex 1 with vinylsilane in a cross-alkene metathesis reaction.17 The reaction presumably proceeds via formation of a metallocyclobutene followed by b-silyl elimination (2). The b-silicon elimination pathway is apparently favored over the alternative b-hydride elimination, with the substituents on silicon strongly influencing the portioning between the two pathways. The observed allylsilane product is then formed via reductive elimination. The rate of the reaction is faster using 2, but slower using the ethylideneruthenium complex, (PCy3)2Cl2Ru = CHCH3. Substrate-independent decomposition of 1 competes with the formation of the allylsilane product.13 While not preparatively useful, the chemistry gives insight into the possible chain termination steps in ruthenium-catalyzed cross-metathesis reactions with vinylsilanes.18

Conversion of Dichloro-bis(tricyclohexylphosphine) methyleneruthenium to Ruthenium Alkylidene Complexes

The conversion of methylidene complex 1 to a homologous ruthenium alkylidene complex tends to proceed slowly and give low yields of product, accompanied by high levels of decomposition. For example, the methylidene complex 1 can be converted into the ethoxycarbeneruthenium complex by reacting with a 100-fold excess of ethyl vinyl ether; 40% yield at 60% conversion (3).19 The reaction of 1 with styrene, silylstyrene, or siloxystyrene proceeds only when the styrene is used in a 50-fold excess, and then, only slowly. This is in stark contrast with complex 2, which reacts rapidly with these alkenes.18

Reactions with Cycloproparenes

Litosh et al. has reported that an unstable 3-ruthenacyclopentene complex can be formed by the stoichiometric reaction of ruthenium complex 1 with benzocyclopropene.20 This metallocycle decomposes to form o-xylylene, which can be trapped as the Diels-Alder adduct when the reaction is carried out in the presence of dimethylene acetylenedicarboxylate (4). The cycloadduct is isolated in 45% yield. Similar results have been seen using cyclopropa[b]naphthalene.

Purification of Ruthenium-Contaminated Materials

Trace amounts of the dark-colored ruthenium metal can contaminate the desired product even after multiple purifications by chromatography. This is especially problematic since the ruthenium by-products are toxic. A number of solutions to this problem have been reported. The ruthenium metal can be transferred to the aqueous layer during the work-up by the addition of 10 equiv (relative to ruthenium) of tris(hydroxymethyl)phosphine and two equiv of triethylamine during the extraction,21 or by treating the reaction mixture with 1.5 equiv (relative to ruthenium) of lead tetraacetate (stirred overnight at room temperature under an inert atmosphere) prior to aqueous work-up.22 It has also recently been reported that a high percentage of the ruthenium catalyst can be removed by treating the crude reaction mixture with 50 equiv (relative to ruthenium) of DMSO or PPh3 for 12 h followed by filtration through silica.23

Related Reagents.

Dichlorobis(tricyclohexylphosphine)benzylideneruthenium (2) (Grubbs' catalyst); dichlorobis(tricyclohexylphosphine) diphenylvinylalkylideneruthenium (3); dichloro-1,3-dimesityl-4,5-dihydroimidazole tricyclohexylphosphine benzylideneruthenium (4); [2,6-bis(1-methylethyl)benzenaminato] bis(1,1,1,3,3,3-hexafluoro-2-methyl-2-propanolato)(2-methyl-2-phenylpropylidene)molybdenum (5) (Schrock’s catalyst).


1. Olivan, M.; Caulton, K. G., Chem. Commun. 1997, 1733.
2. Olivan, M.; Caulton, K. G., Inorg. Chem. 1999, 38, 566.
3. Belderrain, T. R.; Grubbs, R. H., Organometallics 1997, 16, 4001.
4. Schwab, P.; France, M. B.; Ziller, J. W.; Grubbs, R. H., Angew. Chem., Int. Ed. Engl. 1995, 34, 2039.
5. Schwab, P.; Grubbs, R. H.; Ziller, J. W., J. Am. Chem. Soc 1996, 118, 100.
6. Wolf, J.; Stuer, W.; Grunwald, C.; Werner, H.; Schwab, P.; Schulz, M., Angew. Chem., Int. Ed. 1998, 37, 1124.
7. Wilhelm, T. E.; Belderrain, T. R.; Brown, S. N.; Grubbs, R. H., Organometallics 1997, 16, 3867.
8. Grubbs, R. H.; Chang, S., Tetrahedron 1998, 54, 4413.
9. Grubbs, R. H.; Miller, S. J.; Fu, G. C., Acc. Chem. Res. 1995, 28, 446.
10. Furstner, A., Angew. Chem., Int. Ed. 2000, 39, 3012.
11. Schuster, M.; Blechert, S., Angew. Chem., Int. Ed. Engl. 1997, 36, 2037.
12. Ulman, M.; Grubbs, R. H., Organometallics 1998, 17, 2484.
13. Ulman, M.; Grubbs, R. H., J. Org. Chem. 1999, 64, 7202.
14. Dias, E. L.; Nguyen, S. T.; Grubbs, R. H., J. Am. Chem. Soc. 1997, 119, 3887.
15. Kirkland, T. A.; Grubbs, R. H., J. Org. Chem. 1997, 62, 7310.
16. Coates, G. W.; Grubbs, R. H., J. Am. Chem. Soc. 1996, 118, 229.
17. Pietraszuk, C.; Fischer, H., Chem. Commun. 2000, 2463.
18. Pietraszuk, C.; Marciniec, B.; Fischer, H., Organometallics 2000, 19, 913.
19. Marciniec, B.; Kujawa, M.; Pietraszuk, C., New J. Chem. 2000, 24, 671.
20. Litosh, V. A.; Saini, R. K.; Guzman-Jimenez, I. Y.; Whitmire, K. H.; Billups, W. E., Org. Lett. 2001, 3, 65.
21. Maynard, H. D.; Grubbs, R. H., Tetrahedron Lett. 1999, 40, 4137.
22. Paquette, L. A.; Schloss, J. D.; Efremov, I.; Fabris, F.; Gallou, F.; Mendez-Andino, J.; Yang, J., Org. Lett. 2000, 2, 1259.
23. Ahn, Y. M.; Yang, K.; Georg, G. I., Org. Lett. 2001, 3, 1411.

James M. Takacs & Jeffery M. Atkins

University of Nebraska-Lincoln, Lincoln, Nebraska, USA



Copyright 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.