Ru-based olefin metathesis catalyst

Towards Chiral Ru-Based Olefin Metathesis Catalysts. 229... 

Fig. 11 An example of a new generation of chiral Ru-based olefin metathesis catalysts...

To date, three distinct strategies have been utilized to impose a preference for i yn-metal-lacycles in Ru-based olefin metathesis catalysts, resulting in three families of Z-selective catalysts. Promising initial reactivity has been observed with both thiophenolate- and dithiolate-based catalysts and both frameworks offer many opportunities for further tuning of activity and Z-selectivity. The cyclometallated catalysts have been further developed and have demonstrated high activity and Z-selectivity for a wide variety of substrates. However, in all cases, further improvements will be necessary to achieve Z-selective metathesis across the broad substrate scope demonstrated by previous generations of Ru-based catalysts. 

Scheme 12.1 The rate of initiation of phosphine-containing, Ru-based olefin metathesis catalyst 1 is faster in solvents with higher dielectric constants.

Scheme 12.2 The relative rates of initiation of Ru-based olefin metathesis catalysts 3 and 4.

Accordingly, considerable effort has been dedicated to the development of olefin metathesis catalysts exhibiting kinetic selectivity. As a result, a number of Z-selective tungsten-, molybdenum-, and ruthenium-based olefin metathesis catalysts have been recently developed (For Mo- and W-based Z-selective catalysts [24-41], For Ru-based Z-selective catalysts [42-45], For cyclometalated Ru-based Z-selective catalysts [46-58]). Many of these systems exhibit consistently high levels of activity and selectivity across a broad range of substrates. Herein, we will focus specifically on the cyclometalated ruthenium-based catalysts developed in our laboratory [46-58]. This chapter is intended to provide a comprehensive summary of the evolution of these cyclometalated ruthenium catalysts, from their initial serendipitous discovery to their recent applications in Z-selective olefin metathesis transformations. Current mechanistic hypotheses and limitations, as well as future directions, will also be discussed. 

The outstanding performances of five-membered NHC ligands in organometallic chemistry and catalysis prompted Grubbs and co-workers to develop a novel stable four-membered NHC [64]. Following their interest in developing new ruthenium olefin metathesis catalysts, they synthesised and fully characterised complex 51 to study the impact of the architecturally unique NHC ligand on the activity of the Ru-based catalyst [65] (Fig. 3.20). In the RCM of 1 at 40°C in CH Cl with 51 (5 mol% catalyst), the reaction reached completion within 20 min, whereas less than 10 min are required for standard catalysts 14 and 16. It should be noted that catalysts 14 and 16 are able to complete the RCM of 1 with only 1 mol% catalyst at 30°C. 

On the other hand, late transition metal-based catalyst systems that had been identified by the early 1990s were characterized by low activity but high functional group tolerance, especially toward water and other protic solvents. These features led to reinvestigations of ruthenium systems and, ultimately, to the preparation of the first well-defined, ruthenium-carbene olefin metathesis catalyst (PPh3)2(Cl)2Ru=CHCH=Ph2 (Ru-1) in 1992 [5]. 

Catalyst Structure and Cis-Trans Selectivity in Ruthenium-based Olefin Metathesis 33 2.4 Z-selective Ru-based metathesis catalysts 2.4.1 Thiophenolate-based Z-selective catalysts... 

In 2002, Hoveyda et al. reported the synthesis, structure and reactivity of a chiral bidentate Ru-based catalyst 65, bearing a binaphthyl moiety, for olefin metathesis [33]. Preference for a bidentate chiral imidazolinylidene was based on the hypothesis that such a ligand would induce chirality more efficiently. This catalyst was designed by analogy with similar achiral complexes 66 that... 

Despite the numerous reports concerning NHC-Ru olefin metathesis initiators, a complex incorporating a carbene that has only one exocyclic substituent adjacent to the carbenic centre was not reported until 2008. Studies by Grubbs and co-workers led to the development of ruthenium-based catalysts bearing such carbene ligands, in this case incorporating thiazole-2-ylidenes [63] (Fig. 3.19). 

Olefin metathesis is one of the most important reaction in organic synthesis [44], Complexes of Ru are extremely useful for this transformation, especially so-called Grubbs catalysts. The introduction of NHCs in Ru metathesis catalysts a decade ago ( second generation Grubbs catalysts) resulted in enhanced activity and lifetime, hence overall improved catalytic performance [45, 46]. However, compared to the archetypal phosphine-based Ru metathesis catalyst 24 (Fig. 13.3), Ru-NHC complexes such as 25 display specific reactivity patterns and as a consequence, are prone to additional decomposition pathways as well as non NHC-specific pathways [47]. 

For a review of asymmetric Mo-catalyzed metathesis, see Catalytic Asymmetric Olefin Metathesis, A. H. Hoveyda, R. R. ScHROCK, Chem. Eur. J. 2001, 7, 945-950 for reports on chiral Ru-based complexes, see (b) Enantioselective Ruthenium-Catalyzed Ring-Qosing Metathesis, T.J. Sei-DERS, D.W. Ward, R.H. Grubbs, Org. Lett. 2001, 3, 3225-3228 (c) A Recyclable Chiral Ru Catalyst for Enantioselective Olefin Metathesis. Efficient Catalytic Asymmetric Ring-Opening/Cross Metathesis In Air, J. J. Van Veldhuizen, S. B. 

We note that there are NMR-based kinetic studies on zirconocene-catalyzed pro-pene polymerization [32], Rh-catalyzed asymmetric hydrogenation of olefins [33], titanocene-catalyzed hydroboration of alkenes and alkynes [34], Pd-catalyzed olefin polymerizations [35], ethylene and CO copolymerization [36] and phosphine dissociation from a Ru-carbene metathesis catalyst [37], just to mention a few. 

Keywords Asymmetric synthesis, Chiral catalysis, Mo-based catalysts, Natural product synthesis, Olefin metathesis, Recyclable catalysts, Ru-based catalysts, Supported chiral catalysts... 

Scheme 21. Air-stable chiral Ru-based catalyst for olefin metathesis can be used for highly effective and selective AROM/CM reactions...

Olefin metathesis is a unique reaction and is only possible by transition metal catalysis. In fact only complexes of Mo, W, Re, and Ru are known to catalyze olefin metathesis. Once it was known that metallocarbenes were the actual catalytic species, a variety of metal carbene complexes were prepared and evaluated as catalysts. Two types of catalysts have emerged as the most useful overall. The molybdenum-based catalysts developed by Schrock and ruthenium-based catalysts developed by Grubbs. 

---