Olefin metathesis catalyst structures

Poly (acetylenes) [16], There are several catalysts available for polymerization of substituted acetylenes. Whereas Ziegler-Natta catalysts are quite effective for polymerization of acetylene itself and simple alkylacetylenes, they are not active towards other substituted acetylenes, e.g. phenylacetylenes. Olefin-metathesis catalysts (Masuda, 1985 Masuda and Higashimura, 1984, 1986) and Rh(i) catalysts (Furlani et al., 1986 Tabata, 1987) are often employed. In our experience, however, many persistent radicals and typical nitrogen-containing functional groups serve as good poisons for these catalysts. Therefore, radical centres have to be introduced after construction of the polymer skeletons. Fortunately, the polymers obtained with these catalysts are often soluble in one or other organic solvent. For example, methyl p-ethynylbenzoate can be polymerized to a brick-coloured amorph- See the Appendix on p. 245 of suffixes to structural formula numbers. 

Despite the remarkable success of olefin metathesis catalysts in organic applications, one major challenge that remains is the diastereomeric control of olefin geometry. Olefin stereoselectivity is an issue in all metathesis reactions. However, prior to the widespread use of CM processes, it was only pertinent to the RGM of large rings (>8 carbons) and in the backbone structure of ROMP-derived polymers. 

These recent findings indicate overall that the ligand that remains on the metal affects the energetics of the catalytic cycle, specifically olefin coordination, and the accessibility of the metallocyclobutane structure, the properties of the phosphane ligand control initiation rates, and thus how much of the catalyst can enter the catalytic cycle. The results of these careful analyses (Table 2) are sure to germinate the next generation of efficacious olefin metathesis catalysts. 

A few examples of polymer supported olefin metathesis have been reported recently. Hoveyda formed styrenyl ether complexes of several ruthenium-based olefin metathesis catalysts that were then incorporated into a dendrimer structure (273) (Scheme 23), reporting good conversions over 6 cycles albeit with diminishing ruthenium content. Yao used a similar chelating ligand to incorporate olefin... 

During the past few decades, a wide variety of molecules with transition metal-carhon mulhple bonds have been studied. The chemistry of doubly bonded species - carbenes - is particularly interesting because it leads to several synthetically important transformations, and for this reason, metal carbenes are the main subject of this chapter. Our discussion begins with a classification of metal-carbene complexes based on electronic structure, which provides a way to understand their reactivity patterns. Next, we summarize the mechanistic highlights of three metal-carbene-mediated reactions carbonyl olefinafion, olefin cyclopropanafion, and olefin metathesis. Throughout the second half of the chapter, we focus mainly on ruthenium-carbene olefin metathesis catalysts, in part because of widespread interest in the applications of these catalysts, and in part because of our expertise in this area. We conclude with some perspectives on the chemistry of metal carbenes and on future developments in catalysis. 

Tebbe found that titanocene complexes promoted olefin metathesis in addition to carbonyl olefination. Despite the fact that these complexes have low activity, they proved to be excellent model systems. For example, the Tebbe complex exchanges methylene units with a labeled terminal methylene at a slow rate that can be easily monitored (Eq. 4.6) [54]. This exchange is the essential transformation of olefin metathesis. When reactions with olefins are performed in the presence of a Lewis base, the intermediate titanium metallacycle can be isolated and even structurally characterized (Eq. 4.7) [61] These derivatives were not only the first metathesis-active metallacyclobutane complexes ever isolated, but they were also the first metallacyclobutanes isolated from the cycloaddition of a metal-carbene complex with an olefin. These metallacycles participate in all the reactions expected of olefin metathesis catalysts, especially exchange with olefins... 

The potential importance of many of these types of conformational interconversion may be found in the asymmetric olefin metathesis catalysts recently developed by Grubbs. A key intermediate formed after loss of 3-bromopyridine or CysP would have structure 35. The orientation of the aromatic rings could be influenced by other substituents (R ) on the A -heterocyclic carbine. Those rings would, in turn, select a preference of location of in the... 

Fig. 16. Structure of the Grubbs heterocyclic carbene olefin metathesis catalyst.

Extensive studies on the coordination sphere of the metal using Mo-bisdiphenylamido and dipyrrolyl complexes for olefin metathesis revealed how to improve the activity and selectivity of these catalysts [62-68]. This seminal work led to the synthesis of a new generation of highly active olefin metathesis catalysts [69, 70]. Using a similar strategy, various surface-metal alkylidene and alkylidyne complexes have also been tested in alkane metathesis to determine if they can be employed as catalyst precursors [11, 71]. This section describes the structure-activity relationship of different metal-alkyl complexes with oxide surfaces in alkane metathesis. 

Hong SH, Wenzel AG, Salguero TT, Day MW, Grubbs RH. Decomposition of Ruthenium Olefin Metathesis Catalysts. /Chem Soc. 2007 129(25) 7961—7968. Vougioukalakis GC, Grubbs RH. Ruthenium-Based Olefin Metathesis Catalysts Coordinated with Unsymmetrical N-HeterocycHc Carbene Ligands Synthesis, Structure, and Catalytic Activity. Chem EurJ. 2008 14(25) 7545—7556. 

On treatment with a Grubbs olefin metathesis catalyst, the compound shown reacted with styrene to give a 95% yield of a product with the molecular formula C25H30O3, which was later used in the synthesis of a metabolite isolated from a species of mollusk. Suggest a reasonable structure for the metathesis product. 

The "equibinary" l,4-cis/l,4-trans polybutadiene prepared using a n-allyl nickel trifluoroacetate catalyst [70,71,96,97] and the polybutadiene obtained by polymerization of cyclooctadiene using an olefin metathesis catalyst system where shown by nmr to have random distributions of cis- and trans-ianits, although there is some indication that "equibinary" copolymers with non-Bernoullian structures are obtained in some cases [96]. Polybutadienes prepared using alkyl lithium initiators in hydrocarbon solvents have also been shown to have random distributions of 1,4-cis and 1,4-trans units [20,23,71,90]. 

J. R. Missert, and W. J. Youngs, Tungsten-oxo alkylidene complexes as olefin metathesis catalysts and the crystal structure of W(0)(CHCMe3)(PEt3)Cl2, J. Amer. Chem. Soc. 102 4515 (1980). 

---