In the 1960s, ruthenium trichloride was found to catalyze olefin metathesis. Processes were commercialized based on these discoveries. These ill-defined but highly active homogeneous catalysts remain in industrial use.[6] The first well-defined ruthenium catalyst was reported in 1992.[7] It was prepared from RuCl2(PPh3)4 and diphenylcyclopropene.
Preparation of the first-generation Grubbs catalyst
The first-generation Grubbs catalyst was the first well-defined Ru-based catalyst. It is also important as a precursor to all other Grubbs-type catalysts.
The second-generation catalyst has the same uses in organic synthesis as the first generation catalyst, but generally with higher activity. This catalyst is stable toward moisture and air, thus is easier to handle in laboratories.
Shortly before the discovery of the second-generation Grubbs catalyst, a very similar catalyst based on an unsaturated N-heterocyclic carbene (1,3-bis(2,4,6-trimethylphenyl)imidazole) was reported independently by Nolan[10] and Grubbs[11] in March 1999, and by Fürstner[12] in June of the same year. Shortly thereafter, in August 1999, Grubbs reported the second-generation catalyst, based on a saturated N-heterocyclic carbene (1,3-bis(2,4,6-trimethylphenyl)dihydroimidazole):[13]
Synthesis of the second–generation Grubbs catalyst
In both the saturated and unsaturated cases a phosphine ligand is replaced with an N-heterocyclic carbene (NHC), which is characteristic of all second-generation-type catalysts.[3]
Both the first- and second-generation catalysts are commercially available, along with many derivatives of the second-generation catalyst.
In the Hoveyda–Grubbs catalysts, the benzylidene ligands have a chelating ortho-isopropoxy group attached to the benzene rings. The ortho-isopropoxybenzylidene moiety is sometimes referred to as a Hoveyda chelate. The chelating oxygen atom replaces a phosphine ligand, which in the case of the 2nd generation catalyst, gives a completely phosphine-free structure. The 1st generation Hoveyda–Grubbs catalyst was reported in 1999 by Amir H. Hoveyda's group,[14] and in the following year, the second-generation Hoveyda–Grubbs catalyst was described in nearly simultaneous publications by the Blechert[15] and Hoveyda[16] laboratories. Siegfried Blechert's name is not commonly included in the eponymous catalyst name. The Hoveyda–Grubbs catalysts, while more expensive and slower to initiate than the Grubbs catalyst from which they are derived, are popular because of their improved stability.[3] By changing the steric and electronic properties of the chelate, the initiation rate of the catalyst can be modulated,[17][18] such as in the Zhan catalysts. Hoveyda–Grubbs catalysts are easily formed from the corresponding Grubbs catalyst by the addition of the chelating ligand and the use of a phosphine scavenger like copper(I) chloride:[16]
The second-generation Hoveyda–Grubbs catalysts can also be prepared from the 1st generation Hoveyda–Grubbs catalyst by the addition of the NHC:[15]
Preparation of the first-generation Hoveyda–Grubbs catalyst from the first-generation Grubbs catalystPreparation of the second-generation Hoveyda–Grubbs catalyst from the second–generation Grubbs catalyst
In one study published by Grubbs and Hong in 2006, a water-soluble Grubbs catalyst was prepared by attaching a polyethylene glycol chain to the imidazolidine group.[19] This catalyst is used in the ring-closing metathesis reaction in water of a diene carrying an ammonium salt group making it water-soluble as well.
The rate of the Grubbs catalyst can be altered by replacing the phosphine ligand with more labile pyridine ligands. By using 3-bromopyridine the initiation rate is increased more than a millionfold.[20] Both pyridine and 3-bromopyridine are commonly used, with the bromo- version 4.8 times more labile resulting in even faster rates.[21] The catalyst is traditionally isolated as a two pyridine complex, however one pyridine is lost upon dissolving and reversibly inhibits the ruthenium center throughout any chemical reaction.
The principal application of the fast-initiating catalysts is as initiators for ring opening metathesis polymerisation (ROMP). Because of their usefulness in ROMP these catalysts are sometimes referred to as the 3rd generation Grubbs catalysts.[22] The high ratio of the rate of initiation to the rate of propagation makes these catalysts useful in living polymerization, yielding polymers with low polydispersity.[23]
Applications
Grubbs catalysts are of interest for olefin metathesis. It is mainly applied to fine chemical synthesis. Large-scale commercial applications of olefin metathesis almost always employ heterogeneous catalysts or ill-defined systems based on ruthenium trichloride.[6]
References
^Grubbs, Robert H. (2003). Handbook of Metathesis (1st ed.). Weinheim: Wiley-VCH. ISBN978-3-527-30616-9.
^Grubbs, R. H.; Trnka, T. M. (2004). "Ruthenium-Catalyzed Olefin Metathesis". In Murahashi, S. (ed.). Ruthenium‐Catalyzed Olefin Metathesis. Ruthenium in Organic Synthesis. Weinheim: Wiley-VCH. pp. 153–177. doi:10.1002/3527603832.ch6. ISBN978-3-527-60383-1.
^ abcVougioukalakis, G. C.; Grubbs, R. H. (2010). "Ruthenium-Based Heterocyclic Carbene-Coordinated Olefin Metathesis Catalysts". Chemical Reviews. 110 (3): 1746–1787. doi:10.1021/cr9002424. PMID20000700.
^Schwab, P.; France, M. B.; Ziller, J. W.; Grubbs, R. H. (1995). "A Series of Well-Defined Metathesis Catalysts – Synthesis of [RuCl2(=CHR′)(PR3)2] and Its Reactions". Angew. Chem. Int. Ed.34 (18): 2039–2041. doi:10.1002/anie.199520391.
^Schwab, P.; Grubbs, R. H.; Ziller, J. W. (1996). "Synthesis and Applications of RuCl2(=CHR′)(PR3)2: The Influence of the Alkylidene Moiety on Metathesis Activity". J. Am. Chem. Soc. 118 (1): 100–110. doi:10.1021/ja952676d.
^Huang, J.-K.; Stevens, E. D.; Nolan, S. P.; Petersen, J. L. (1999). "Olefin Metathesis-Active Ruthenium Complexes Bearing a Nucleophilic Carbene Ligand". J. Am. Chem. Soc. 121 (12): 2674–2678. doi:10.1021/ja9831352.
^Scholl, M.; Trnka, T. M.; Morgan, J. P.; Grubbs, R. H. (1999). "Increased Ring Closing Metathesis Activity of Ruthenium-Based Olefin Metathesis Catalysts Coordinated with Imidazolin-2-ylidene Ligands". Tetrahedron Letters. 40 (12): 2247–2250. doi:10.1016/S0040-4039(99)00217-8.
^Ackermann, L.; Fürstner, A.; Weskamp, T.; Kohl, F. J.; Herrmann, W. A. (1999). "Ruthenium Carbene Complexes with Imidazolin-2-ylidene Ligands Allow the Formation of Tetrasubstituted Cycloalkenes by RCM". Tetrahedron Lett. 40 (26): 4787–4790. doi:10.1016/S0040-4039(99)00919-3.
^Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. (1999). "Synthesis and Activity of a New Generation of Ruthenium-Based Olefin Metathesis Catalysts Coordinated with 1,3-Dimesityl-4,5-dihydroimidazol-2-ylidene Ligands". Org. Lett.1 (6): 953–956. doi:10.1021/ol990909q. PMID10823227.
^ abGessler, S.; Randl, S.; Blechert, S. (2000). "Synthesis and metathesis reactions of phosphine-free dihydroimidazole carbene ruthenium complex". Tetrahedron Letters. 41 (51): 9973–9976. doi:10.1016/S0040-4039(00)01808-6.
^ abGarber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H. (2000). "Efficient and Recyclable Monomeric and Dendritic Ru-Based Metathesis Catalysts". Journal of the American Chemical Society. 122 (34): 8168–8179. doi:10.1021/ja001179g.
^Engle, Keary M.; Lu, Gang; Luo, Shao-Xiong; Henling, Lawrence M.; Takase, Michael K.; Liu, Peng; Houk, K. N.; Grubbs, Robert H. (2015). "Origins of Initiation Rate Differences in Ruthenium Olefin Metathesis Catalysts Containing Chelating Benzylidenes". Journal of the American Chemical Society. 137 (17): 5782–5792. doi:10.1021/jacs.5b01144. PMID25897653.
^Walsh, Dylan J.; Lau, Sii Hong; Hyatt, Michael G.; Guironnet, Damien (2017-09-25). "Kinetic Study of Living Ring-Opening Metathesis Polymerization with Third-Generation Grubbs Catalysts". Journal of the American Chemical Society. 139 (39): 13644–13647. doi:10.1021/jacs.7b08010. ISSN0002-7863. PMID28944665.
^Choi, T.-L.; Grubbs, R. H. (2003). "Controlled Living Ring-Opening-Metathesis Polymerization by a Fast-Initiating Ruthenium Catalyst". Angewandte Chemie International Edition. 42 (15): 1743–1746. doi:10.1002/anie.200250632. PMID12707895.
