Olefin metathesis has emerged as a unique and powerful transformation for the interconversion of olefinic hydrocarbons, namely due to the development of well-defined catalysts. See Grubbs, R. H. Handbook of Metathesis, Wiley-VCH: Weinheim, Germany (2003). The use of ruthenium alkylidene complexes has greatly expanded the scope of this process due to increased tolerance of organic functionality, moisture and oxygen. However, even with these advances, catalyst lifetime and efficiency represent the major limiting factors in the further development of this technology. Thus, the development of methods to reduce catalyst decomposition and increase the overall efficiency is highly desired. One such approach is to anchor the catalyst onto a solid support, such as silica gel, which is known to prevent bimolecular decomposition via site isolation. See, e.g. Collman, J. P. et al. J. Am. Chem. Soc., 105, 7288-7294 (1983); Drago, R. S. et al., Inorg. Chem., 24, 1983-1985 (1985); Tollner, K. et al., Science, 278, 2100-2102 (1997); and Annis, D. A. et al., J. Am. Chem. Soc., 121, 4147-4154 (1999). Likewise, the resulting supported catalysts have the added benefit of being recyclable which will increase the overall catalytic efficiency, as well as the production of materials that are free of ruthenium contamination.
A number of reports have been published employing various strategies to obtain solid supported olefin metathesis catalysts. See, e.g. Buchmeiser, M. R. New. J. Chem., 28, 549-557 (2004); Coperet, C.; Basset, J.-M. Adv. Synth. Catal., 349, 78-92 (2007); Clavier, H.; Grela, K.; and Kirschning, A.; Mauduit, M.; Nolan, S. P. Angew. Chem. Int. Ed., 46, 6786-6801 (2007). These consist of anchoring the catalytic moiety, via a number of positions within the catalyst framework, to a variety of solid supports, such as organic polymers or inorganic oxides. Of the various strategies, immobilization through a chelating alkylidene ligand has been the most widely employed [see Garber, S. B.; Kingsbury, J. S.; Gray, B. L.; Hoveyda, A. H. J. Am. Chem. Soc. 122, 8168, (2000); A. H. Hoveyda U.S. Pat. No. 6,921,735; Chen, S.-W.; Kim, J. H.; Song, C. E.; Lee, S.-g Organic Letters. 9, 3845 (2007); Connon, S.; Dunne, A. M.; Blechert, S. Angew. Chem. Int. Ed., 39, 3898-3901 (2000); Lee, B. S.; Namgoong, S. K.; Lee, S.-g Tetrahedron Letters 46, 4501 (2005); Elias, X.; Pleixats, R.; Man, M. W. C.; Moreau, J. J. E. Adv. Syn. Catal. 348, 751, 2006. These catalysts operate via a release/return phenomenon with all the catalytic activity arising from a homogeneous species, which is susceptible to the same bimetallic decomposition pathways. Likewise, such systems cannot realize all the benefits of solid-phase catalysis, such as desirable continuous flow processes. Other approaches to catalyst immobilization have been carried out by exchanging the phosphine ligands of a Grubbs first generation catalyst with phosphines incorporated on a polystyrene-divinylbenzene polymer (PS-DVB) [see, e.g., S. T. Nguyen; R. H. Grubbs J. Organomet. Chem. 497, 195 (1995).]. Catalysts immobilized by such techniques, however, have been generally reviewed as providing reduced performance compared with homogeneous equivalents and as not being adaptable to flow-through technologies due to catalyst leaching [see, e.g., Coperet, C.; Basset, J.-M. Adv. Synth. Catal., 349, 78-92 (2007)]. Still other strategies involve immobilization via alternative X-type ligands that replace the ancillary chlorides, such as fluorinated carboxylates, [see, e.g. Halbach, T. S.; Mix, S.; Fischer, D.; Maechling, S.; Krause, J. O.; Sievers, C.; Blechert, S.; Nuyken, O.; Buchmeiser, M. R. J. Org. Chem., 70, 4687-4694 (2005)], or via functionalized NHC ligands. See, e.g. Schürer, S. C.; Gessler, S.; Buschmann, N.; Blechert, S. Angew. Chem. Int. Ed., 39, 3898-3901 (2000); Mayer, M.; Buchmeiser, M. R.; Wurst, K. Adv. Synth. Catal., 344, 712-719 (2002); Prühs, S.; Lehmann, C. W.; Fürstner, A. Organometallics, 23, 280-287 (2004); Koehler WO 2007/017047; WO 2007/017041; WO 2005/016522; and WO 2005/016524]. The latter is a very attractive approach as NHC ligands generally form strong bonds to the ruthenium center and are often the most substitutionally inert ligand within the catalyst coordination sphere.
However, anchoring the catalyst on a support can be problematic since various factors may affect the performance of such supported catalysts, including the reactivity of the functional group on the catalyst, the stability of the catalyst once anchored on the support, and the ability of the catalyst to perform as an effective catalyst after it has been grafted onto a support.
Despite the advances achieved in preparing olefin metathesis catalysts, including supported catalysts, a continuing need in the art exists for improved supported catalyst systems, as well as precursor complexes that are capable of being used in such systems.