Catalysts alkene metathesis

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.

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. ... 

The enantiomerically-pure intermediate 1 was prepared from the dioxolanone 4, available in three steps from L-malic acid. Lewis acid-mediated homologation converted 4, a 4 1 mixture of diastereomers, into 5 as a single diastereomer. After establishment of the alkenyl iodide, it necessary to maintain the lactone in its open form. A solution was found in the formation of the Weinreb amide. The final stereogenic center was established by Brown allylation of the derived aldehyde. The alkene metathesis to form 1 was carried out with the commercially-available Schrock Mo catalyst. The authors did not comment on the relative efficacy of alternative alkene metathesis catalysts. 

The reaction products are those expected if cyclobutanes were intermediates, but formation and cleavage of cyclobutanes is not the correct mechanism because cyclobutanes generally are not converted to alkenes over alkene-metathesis catalysts. 

Exercise 31-9 Explain how an alkene-metathesis catalyst might convert a cyclo-alkene into (a) a long-chain unsaturated polymer, (b) a mixture of large-ring polymers, and (c) a catenane (interlocking carbon rings like two links in a chain). 

Several catalysts have been introduced since the discovery of alkene metathesis. Catalysts generated by combinations of MoC15 and WClg with alkylaluminium (RnAlCl3 ) were the first-generation catalysts, which were not suitable for functionalized alkenes. 

The expense of ruthenium and the current rise of importance attached to environmentally friendly and cost-effective synthetic protocols gives the search for recyclable alkene metathesis catalysts ever-increasing impetus. Since the... 

The first attempt seems to have been promoted by binuclear ruthenium alkene metathesis catalyst for the ROMP of cyclooctene followed by hydrogenation (50 °C, 30 psi H2) in the presence of 10 equiv. of triethylamine [72] (Scheme 31). It was motivated by the observed capability of [RuCl2(arene)]2... 

A similar approach was performed using the alkene metathesis catalyst RuCl2(= CHPh)(PCy3)2 [76]. It was known that this complex reacts with hydrogen in THF to give ruthenium hydride complexes capable of catalytic alkene hydrogenation [77] (Scheme 32). 

The synthesis of alkylidenes incorporating late transition metals has resulted in alkene metathesis catalysts having unprecedented functional group tolerance. In particular, the discovery that complexes of Group VIII transition metals were efficient ROMP catalysts introduced several advantages. Relative to their early transition metal counterparts, these classical catalysts functioned well in the presence of a variety of polar and protic functional groups (Table 2), and they functioned homogeneously in water. 

For a number of 1,1-disubstituted alkenes metathesis catalysts first bring about conversion to isomers which then undergo cross-metathesis with the remaining substrate (Strel chik 1976 Usov 1983 Kawai 1990). 

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