Alkene metathesis complexes

The synthetic utility of the alkene metathesis reaction may in some cases be limited because of the formation of a mixture of products. The steps of the catalytic cycle are equilibrium processes, with the yields being determined by the thermodynamic equilibrium. The metathesis process generally tends to give complex mixtures of products. For example, pent-2-ene 8 disproportionates to give, at equilibrium, a statistical mixture of but-2-enes, pent-2-enes and hex-3-enes ... 

These carbene (or alkylidene) complexes are used for various transformations. Known reactions of these complexes are (a) alkene metathesis, (b) alkene cyclopropanation, (c) carbonyl alkenation, (d) insertion into C-H, N-H and O-H bonds, (e) ylide formation and (f) dimerization. The reactivity of these complexes can be tuned by varying the metal, oxidation state or ligands. Nowadays carbene complexes with cumulated double bonds have also been synthesized and investigated [45-49] as well as carbene cluster compounds, which will not be discussed here [50]. 

Initial reports of cross-metathesis reactions using well-defined catalysts were limited to simple isolated examples the metathesis of ethyl or methyl oleate with dec-5-ene catalysed by tungsten alkylidenes [13,14] and the cross-metathesis of unsaturated ethers catalysed by a chromium carbene complex [15]. With the discovery of the well-defined molybdenum and ruthenium alkylidene catalysts 3 and 4,by Schrock [16] and Grubbs [17],respectively, the development of alkene metathesis as a tool for organic synthesis began in earnest. 

Carbenes are both reactive intermediates and ligands in catalysis. They occur as intermediates in the alkene metathesis reaction (Chapter 16) and the cyclopropanation of alkenes. As intermediates they carry hydrogen and carbon substituents and belong therefore to the class of Schrock carbenes. As ligands they contain nitrogen substituents and are clearly Fischer carbenes. They have received a great deal of attention in the last decade as ligands in catalytic metal complexes [58], but the structural motive was already explored in the early seventies [59],... 

These observations indicate that there is no sharp borderline between cyclopro-panating and metathesis-catalyzing carbene complexes. Fortunately the number of carbene complexes which mediate both cyclopropanation and alkene metathesis is rather small, and in the detailed overview given in the following sections it will become apparent that most carbene complexes are highly selective and thus valuable reagents for organic synthesis. 

Table 3.15. Fischer-type carbene complexes as catalysts for homogeneous-phase alkene metathesis.

Despite this seminal work, it has only been recently that these metallacumulenes have really emerged as useful catalyst precursors or catalyst intermediates in organic synthesis. In particular, significant advances have been made in the field of alkene metathesis and propargylation reactions using mainly ruthenium complexes. A survey of this chemistry is presented in the following section. 

I 7 7 Surface Organometallic Chemistry of d(0) Metal Complexes Table 11.3 Alkene metathesis with surface organometallic and related species. 

Much of the chemistry of vinylidene complexes has been developed with catalytic applications in mind, as detailed later in this volume. Early examples had low activity for alkene metathesis, although complexes containing imidazolylidene ligands showed improved efficiencies [35]. However, in many cases, reactions of the vinylidene ligand have resulted in transformation to other carbon-based ligands which have not been released from the metal fragment. 

In 1998 it was revealed that allenylidene-ruthenium complexes, arising simply from propargylic alcohols, were efficient precursors for alkene metathesis [12], This discovery first initiated a renaissance in allenylidene metal complexes as possible alkene metathesis precursors, then it was observed and demonstrated that allenylidene-ruthenium complexes rearranged into indenylidene-ruthenium intermediates that are actually the real catalyst precursors. The synthesis of indenylidene-metal complexes and their efficient use in alkene metathesis are now under development. The interest in finding a convenient source of easy to make alkene metathesis initiators is currently leading to investigation of other routes to initiators from propargylic derivatives. 

The objective of this chapter is to report on these various aspects allenylidenes in alkene metathesis, their transformation into indenylidenes, alkene metathesis with indenylidene complexes, other propargylic derivatives as alkene metathesis initiators and their application in alkene metathesis. 

Allenylidene-Ruthenium Complexes as Alkene Metathesis Catalyst Precursors the First Evidence... 

Alkene metathesis, promoted by the allenylidene-ruthenium complexes, was revealed in the RCM of diallyl tosylamide [12]. The first studies showed some significant influences [31, 32]. 

Indenylidene-Ruthenium Complexes the Alkene Metathesis Catalytic Species from Allenylidene Ruthenium Complexes... 

The evidence that mthenium-allenylidenes were easy to make and eflEcient alkene metathesis precursors motivated several groups to design new allenylidene metal complexes and to explore their impact on alkene metathesis. Nolan first reported... 

Moreover, Fiirstner shotved that tvhereas complex VIII was inactive for RCM reaction, its PCy3 analog IX was, in contrast, very active in a variety of RCM reactions. The latter is now commercially available and currently used in alkene metathesis (see Section 8.3 for further applications). 

Other closely related ruthenium-allenylidene were made and evaluated in alkene metathesis [32]. Werner et al. [49] also produced allenylidene complexes of analogous structure to that of the Grubbs catalyst, but containing hemilabile phosphine such as complex X (Scheme 8.9). However, the Ru—O bond may be too stable to initiate the rearrangement into indenylidene, the coordination of alkene and to become a catalyst. 

This complex XV was proved to be an active alkene metathesis catalyst even at 0 °C or room temperature. The in situ generated catalyst XV catalyzed the ROMP of cyclooctene at room temperature at high cydooctene/Ru ratio (Table 8.2 entries 4,5) reaching TOF of more than 17000min. ... 

Indenylidene-Ruthenium Catalysts in Alkene Metathesis 265 Table 8.5 Diene and enyne RCM reactions with 2 mol% of complex XXb at room temperature. 

Two observations initiated a strong motivation for the preparation of indenylidene-ruthenium complexes via activation of propargyl alcohols and the synthesis of allenylidene-ruthenium intermediates. The first results from the synthesis of the first indenylidene complexes VIII and IX without observation of the expected allenylidene intermediate [42-44] (Schemes 8.7 and 8.8), and the initial evidence that the well-defined complex IX was an efficient catalyst for alkene metathesis reactions [43-44]. The second observation concerned the direct evidence that the well-defined stable allenylidene ruthenium(arene) complex Ib rearranged intramo-lecularly into the indenylidene-ruthenium complex XV via an acid-promoted process [22, 23] (Scheme 8.11) and that the in situ prepared [33] or isolated [34] derivatives XV behaved as efficient catalysts for ROMP and RCM reactions. 

Actually, applications of indenylidene-ruthenium complexes for alkene metathesis were reported before, at a time when the action mode of their ruthenium allenylidene precursors was not known. These complexes catalyzed a variety of RCM reactions of dienes and enynes [31, 32, 47] (see Section 8.2.2). 

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