Metal-carbene complexes Ruthenium

Keywords Metathesis Alkenes Catalysis Ruthenium Metal carbene complexes... 

It has been demonstrated that group 6 Fischer-type metal carbene complexes can in principle undergo carbene transfer reactions in the presence of suitable transition metals [122]. It was therefore interesting to test the compatibility of ruthenium-based metathesis catalysts and electrophilic metal carbene functionalities. A series of examples of the formation of oxacyclic carbene complexes by metathesis (e.g., 128, 129, Scheme 26) was published by Dotz et al. [123]. These include substrates where double bonds conjugated to the pentacarbonyl metal moiety participate in the metathesis reaction. Evidence is... 

Thus the reactivity of transition metal-carbene complexes, that is, whether they behave as electrophiles or nucleophiles, is well explained on the basis of the frontier orbital theory. Studies of carbene complexes of ruthenium and osmium, by providing examples with the metal in either of two oxidation states [Ru(II), Os(II) Ru(0), Os(O)], help clarify this picture, and further illustrations of this will be found in the following sections. 

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. 

In addition to copper and rhodium catalysts commonly used in the generation of metal carbene complexes, other transition metals have also been explored in the diazo decomposition and subsequent ylide generation.Che and co-workers have recently studied ruthenium porphyrin-catalyzed diazo decomposition and demonstrated a three-component coupling reaction of a-diazo ester with a series of iV-benzylidene imines and alkenes to form functionalized pyrrolidines in excellent diastereoselectivities (Scheme 20). ... 

R.H. Grubbs, P. Schwab, and S.T. Nguyen, High metathesis activity ruthenium and osmium metal carbene complexes, US Patent 6 806 325, assigned to California Institute of Technology (Pasadena, CA), October 19, 2004. 

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. 

Transition-metal catalysis, especially by copper, rhodium, palladium and ruthenium compounds, is another approved method for the decomposition of diazo compounds. It is now generally accepted that short-lived metal-carbene intermediates are or may be involved in many of the associated transformations28. Nevertheless, these catalytic carbene transfer reactions will be fully covered in this chapter because of the close similarity in reaction modes of electrophilic carbenes and the presumed electrophilic metal-carbene complexes. 

In connection with these catalytic cyclopropanation reactions, it should be mentioned that the isolable ruthenium-carbene complex 162, which is obtained from 19, [RuCMp-cymene)]2 and 2,6-bis(4-isopropyl-l,3-oxazolin-2-yl)pyridine, reacts with styrene at elevated temperature in a carbene transfer reaction83 (equation 41). Since complex 162 is also catalytically active for (alkoxycarbonyl)carbene transfer to olefins, this reaction represents one of the few connecting links between catalytic and stoichiometric carbene transfer reactions of metal-carbene complexes. 

Olefin metathesis has proved to be a powerful synthetic tool in organic synthesis.5 The advent of well-defined metal carbene complexes with remarkable functional group tolerance has rendered metathesis as an efficient route to the synthesis of new C-C bonds. Examples of widely used ruthenium metathesis catalysts include [Ru-1],6 [Ru-2]7 and [Ru-3] 8 (Figure 1). 

RCM has attracted much attention and has seen a tremendous increase in synthetic applications over the last decade <2000CR2963, 2006JOM(691)5129>. In this reaction, two C-C multiple bonds, such as double and double, or double and triple in the same molecule, are converted to unsaturated carbocycles or heterocycles in the presence of a metal carbene complex. The versatility of Schrock s molybdenum catalyst and Grubbs ruthenium complexes 68 and 69 (Scheme 10) in carbo- and heterocyclizations, respectively, of very different ring sizes were demonstrated <2000CR2963, 2006JOM(691)5129>. 

Grubbs [3] prepared high activity metathesis ruthenium metal carbene complexes, (IV), that were effective as depolymerization catalysts of unsaturated polymers and synthetic agents in preparing telechelic and alkene polymers. Other high activity metathesis ruthenium carbene metal complexes, (V), were prepared by Fogg [4]. 

Another example for methoxy functionalised imidazolium salts comes from the group of Cetinkaya [185,186] featuring an -alkyl tether. Cetinkaya etal.me the traditional route to transition metal carbene complexes employing the electron-rich olefins as carbene source [57-59], Thermal cleavage of the olefinic double bond in the presaice of the metal precursor complex yields the desired transition metal carbene complex (see Figure 3.66). Using this method, Cetinkaya et al. synthesised rhodium(l) [185,186] and ruthenium(ll) [185] complexes. 

From a methodological point of view, it should be pointed out the formation of 51, which is a result of the addition of acetone to an allenylidene ligand. Heteroatom-containing cyclic metal-carbene complexes [24] have been conveniently prepared via metal co-haloacyl, carbamoyl, alkoxycarbonyl, or imido intermediates [25], opening of epoxides by deprotonated Fischer-type carbene complexes [26], and activation of homopropargylic alcohols with low-valent d complexes [27], including ruthenium(II) derivatives [28]. In general, the preparation of unsaturated cyclic carbene complexes requires the previous preparation of functional carbenes to react with P-dicarbonyl derivatives, acrylates, and enol ethers [29]. 

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. 

Better control of the reaction is achieved by using stable, isolable metal-carbene complexes, such as the Schrock initiators based on molybdenum and ffingsten (LXIV, Mt = W, Mo) and the Grubbs initiators based on ruthenium (LXV). The Schrock initiators... 

The generally accepted mechanism for olefin cross-metathesis is outlined for the case of propene in Mechanism 14.4. The catalyst belongs to a class of organometallics known as a metallocarbene, carbene complex, or alkylidene complex. Its structure is characterized by a carbon-metal double bond. In olefin metathesis the metal is typically ruthenium (Ru), tungsten (W), or molybdenum (Mo). Transition-metal carbene complexes were first prepared by Ernst O. Fischer (Munich) who shared the 1973 Nobel Prize in Chemistry with Geoffrey Wilkinson. 

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