Ruthenium-catalyzed olefin metathesis

It has been shown that ruthenium carbene complex lb developed for olefin metathesis can catalyze RCM of enynes. Using this catalyst lb, five- to nine-membered ring compounds 3 are synthesized from enyne 2 (Scheme j) i Sa-iSc The reaction procedure for RCM of an enyne is very simple. A benzene solution of enyne 2b is stirred in the presence of 1 mol% of ruthenium carbene complex lb at room temperature (RT) under argon gas to give cyclic compound 3b having a diene moiety. 

This chapter is concerned specifically with olefin metathesis reactions catalyzed by ruthenium-carbene complexes, mainly because of their great success during recent years. We begin with an overview of these catalysts, and then focus on mechanistic considerations that are important for understanding the reactivity profiles of various catalyst derivatives. The second part of the chapter deals with applications of ruthenium-catalyzed olefin metathesis, especially RCM, CM, and combination processes in organic synthesis. 

The mechanism of olefin metathesis reaction catalyzed by ruthenium (Ru) carbenoids was intensively studied over last 8 years both experimentally [1] and theoretically involving calculation method [2]. It has been assumed for this reaction to proceed with participant of 14-electix)n-Ru complexes and... 

Grubbs et al. reported that the ruthenium-catalyzed RCM of a conjugated diene proceeds in such a way that the less hindered olefin moieties participate in the reaction. Consequently, RCM of 115 gives exo-methylene compound 116, and not exo-vinyl compound 117 (Scheme 24) [105]. This regioselectivity is complementary to that observed for enyne metathesis of 118, which gives exclusively 117 (Scheme 24) [106a]. 

Bent ansa-metallocenes of early transition metals (especially Ti, Zr, Hf) have attracted considerable interest due to their catalytic activity in the polymerization of a-olefins. Ruthenium-catalyzed olefin metathesis has been used to connect two Cp substituents coordinated to the same metal [120c, 121a] by RCM or to connect two bent metallocenes by cross metathesis [121b]. A remarkable influence of the catalyst on E/Z selectivity was described for the latter case while first-generation catalyst 9 yields a 1 1 mixture of E- and Z-dimer 127, -127 is the only product formed with 56d (Eq. 19). 

The synthesis and olefin metathesis activity in protic solvents of a phosphine-free ruthenium alkylidene bound to a hydrophilic solid support have been reported. This heterogeneous catalyst promotes relatively efficient ring-closing and cross-metathesis reactions in both methanol and water.200 The catalyst-catalyzed cross-metathesis of allyl alcohol in D20 gave 80% HOCH2CH=CHCH2OH. 

Conventionally, organometallic chemistry and transition-metal catalysis are carried out under an inert gas atmosphere and the exclusion of moisture has been essential. In contrast, the catalytic actions of transition metals under ambient conditions of air and water have played a key role in various enzymatic reactions, which is in sharp contrast to most transition-metal-catalyzed reactions commonly used in the laboratory. Quasi-nature catalysis has now been developed using late transition metals in air and water, for instance copper-, palladium- and rhodium-catalyzed C-C bond formation, and ruthenium-catalyzed olefin isomerization, metathesis and C-H activation. Even a Grignard-type reaction could be realized in water using a bimetallic ruthenium-indium catalytic system [67]. 

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. 

This article presents the principles known so far for the synthesis of metal complexes containing stable carbenes, including the preparation of the relevant carbene precursors. The use of some of these compounds in transition-metal-catalyzed reactions is discussed mainly for ruthenium-catalyzed olefin metathesis and palladium-Znickel-catalyzed coupling reactions of aryl halides, but other reactions will be touched upon as well. Chapters about the properties of metal- carbene complexes, their applications in materials science and medicinal chemistry, and their role in bioinorganic chemistry round the survey off. The focus of this review is on ZV-heterocyclic carbenes, in the following abbreviated as NHC and NHCs, respectively. 

The recent applications of NHCs in ruthenium-catalyzed olefin metathesis and palladium/nickel-catalyzed coupling reactions show the value of such a profound understanding about ligand properties before using them for specific catalytic transformations. 

Ruthenium(II)-NHC systems ean be used for atom transfer radical polymerization (ATRP). Generally, similar results as for the analogous phosphine complexes are obtained. For the ATRP of styrene and methyl methacrylate (MMA) [(NHC)2peBr2] was found to rival copper(I)-based systems and to yield poly (MMA) with low polydispersities. Polymerizations based on olefin metathesis that are catalyzed by ruthenium-NHC complexes are discussed separately vide supra). 

Ruthenium(n) systems containing imidazol-2-ylidene or imidazolidin-2-ylidene have been used to catalyze the synthesis of 2,3-dimethylfuran starting at (Z)-3-methylpent-2-en-4-yn-l-ol [Eq. (54)]. The activity of the catalyst strongly depends on the nature of the NHC ligand. Benzimidazolin-2-ylidenes give the best results for this transformation. Similar systems have also been used for olefin metathesis reactions. ... 

Promising applications for metal-NHC compounds in materials science and medicinal chemistry are based on the strong metal-carbon bond and the high donor capability of the NHC. The most extensive investigations have been carried out in the field of homogeneous catalysis. Here, NHCs have to be considered as typical directing or innocent spectator ligands, best comparable to trialkylphosphines. The recent successful applications of NHCs in ruthenium-catalyzed olefin metathesis... 

Sanford, M. S. Love, J. A. Mechanism of Ruthenium-catalyzed Olefin Metathesis Reactions. In Handbook of Metathesis-, Grubbs, R. H., Ed. Wiley-VCH Weinheim, 2003 Vol. 1, pp 112-131. 

Since the discovery of ruthenium and molybdenum carbene complexes that efficiently catalyze olefin metathesis under mild reaction conditions and that are compatible with a broad range of functional groups, olefin metathesis has increasingly been used for the preparation of alkenes on insoluble supports. In particular, the ruthenium complexes Cl2(PCy3)2Ru=CHR, developed by Grubbs, show sufficient catalytic activity even in the presence of air and water [781] and are well suited for solid-phase synthesis. 

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. 

Ruthenium-Catalyzed Olefin-Metathesis Reactions in Aqueous Media. 332... 

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