Ruthenium catalysts alkene metathesis

Considerable progress has been made in the characterization and applications of the ruthenium catalysts that will function in aqueous and protic media. A number of approaches have been made to produce conditions and ligand systems that will allow ruthenium based alkene metathesis in aqueous and protic media to reach the same generality as metathesis in organic solvents. 

Another piece of mechanistic evidence was reported by Snapper et al. [14], who describe a ruthenium catalyst caught in action . During studies on ring opening metathesis, these authors were able to isolate and characterize carbene 5 in which a tethered alkene group has replaced one of the phosphines originally present in Id. Control experiments have shown that compound 5 by itself is catalytically active, thus making sure that it is a true intermediate of a dissociative pathway rather than a dead-end product of a metathetic process. 

To investigate if the ruthenium catalyst first reacts with the alkyne part or with the alkene part, the reaction of 64 with ruthenium catalyst 52a was carried out (Eq. 30). Two metathesized products, enyne metathesis product 65 and diene metathesis product 66, were obtained in 19% and 5% yields, respectively. This result indicates that the reaction of the alkyne part with 52a is faster than that of the alkene part with 52a. 

The use of ill-defined catalysts for the cross-metathesis of allyl- and vinylsi-lanes has also received considerable attention, particularly within the past decade. Using certain ruthenium catalysts, allylsilanes were found to isomerise to the corresponding propenylsilanes prior to metathesis [5]. Using rhenium- or tungsten-based catalysts, however, successful cross-metathesis of allylsilanes with a variety of simple alkenes was achieved [6,7] (an example typical of the results reported is shown in Eq. 3). 

In contrast, ruthenium catalysts gave the best results for the cross-metathesis reactions of vinylsilanes with a range of unfunctionalised alkenes [8] (a typical example is shown in Eq. 4). 

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. 

Although the Grubbs ruthenium benzylidene 17 has a significant advantage over the Schrock catalyst 3 in terms of its ease of use, the molybdenum alkylidene is still far superior for the cross-metathesis of certain substrates. Acrylonitrile is one example [28] and allyl stannanes were recently reported to be another. In the presence of the ruthenium catalyst, allyl stannanes were found to be unreactive. They were successfully cross-metathesised with a variety of alkenes, however, using the molybdenum catalyst [39] (for example Eq. 20). 

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. 

Another example is the ionic tagging of a ruthenium carbene catalyst for ringclosing alkene metathesis. A [-(CH2)4-MIM] PF6 moiety was built into the structure of the catalyst, which enabled the long-term retention of the catalyst in the [BMIM]PF6 ionic liquid for multiple recycles (188). 

Ruthenium Allenylidenes and Indenylidenes as Catalysts in Alkene Metathesis... 

Among the R2C(=C) =Ru homologs promoting alkene metathesis the most recent discoveries deal vhth the allenylidene-ruthenium and related pre-catalysts. This chapter is devoted to the class of ruthenium-allenylidene metathesis precatalysts, their intramolecularly rearranged indenylidene catalysts, and their use in... 

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. 

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

Scheme 8.1 Initial pathways for the synthesis of allenylidene-ruthenium alkene metathesis catalysts.

Scheme 8.8 Direct formation of alkene metathesis ruthenium-indenylidene catalyst IX.

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. 

Propargylic Ethers as Alkene Metathesis Initiator Precursors Generation of Alkenyl Alkylidene-Ruthenium Catalysts... 

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. 

---