Alkene ruthenium indenylidene catalysts 389,

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

The control of alkene geometry in RCM reactions has been an area of intense research and interest since the process was first developed. While a general solution to this challenge has not yet been developed, intriguing observations of E Z control in macrocyclizations continue to be reported. For example, in the course of their studies on the synthesis of herbarumin I and II, Fiirstner and co-workers reported the selective formation of either of the two isomeric alkene products 16 or 17 via RCM of diene 15 <02JA7061> (Scheme 8). The diene 15 was transformed into the -alkene 17 using the ruthenium indenylidene catalyst (Fiirstner Metathesis Catalyst FMC, <01MI4811>) while use of the MC2 led to clean formation of the Z-isomer 16. 

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... 

The same reaction (RCM) was used as the key step for the formation of a family of potent herbicidal 10-membered lactones. An important aspect from the preparative point of view is the control of stereochemical outcome of the RCM by the choice of catalyst. Thus, the use of the ruthenium indenylidene complex IX always leads to the corresponding ( )-alkenes, whereas the second generation of Grubbs catalyst bearing a N-heterocyclic carbene ligand affords the isomeric (Z)-olefin with good selectivity (Scheme 8.19) [64]. 

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

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. 

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. 

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. 

The strategic sequential RCM/hydrogenation reactions have been used for the synthesis of (R)-(+)-muscopyridinc. The RCM of the protonated pyridine bearing two alkene chains was performed with an indenylidene-ruthenium catalyst that also allows hydrogenation under 50 atm of hydrogen, and mus-copyridine was obtained in 57% yield [84] (Scheme 38). Normuscopyridine was similarly prepared in 68% yield [84],... 

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