Ruthenium indenylidene catalysts

Scheme 8.2 Transformation of allenylidene-ruthenium into active indenylidene-ruthenium 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. 

Fig. 3 Metathesis-active vinylidene, allenylidene, and indenylidene ruthenium catalysts...

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],... 

From these studies, it was demonstrated that the alkene metathesis activity was not due to the allenylidene precursor, but due to the indenylidene ruthenium catalyst 6, which has a structure analogous to the Grubbs I catalyst [15, 17]. Both complexes generate the same RuCl2(=CFl2) intermediate upon reaction with a terminal alkene. 

Binuclear Indenylidene Ruthenium Catalysts Arising from Ruthenium(arene) Complexes... 

Scheme 14.31 RCM of diene 63 with the chelating indenylidene ruthenium catalyst 57a.

For previous reviews on the preparation and activity of indenylidene-ruthenium catalysts see Ref. 10 and (a) Dragutan,... 

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. 

The allenylidene-mthenium complexes I also catalyze the enyne metathesis to alkenylcydoalkenes with a 1,3-diene stmcture. Initial studies showed the transformation of simple enynes with ether function [37] (Scheme 8.5). This reaction was significantly accelerated by initial catalyst photodiemical activation, which is now understood to favor the rearrangement of the allenylidene- into the active indenylidene-ruthenium moiety and arene displacement. 

Although the in situ generation and use of XV is convenient, the presence of add also favors slow decomposition of XV. It was thus suitable to isolate catalyst XV by filtration of its addic solution on basic alumina [34]. Thus the indenylidene-ruthenium(arene) complexes containing various phosphines were isolated (Scheme 8.12). 

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. 

A class of olefin metathesis catalysts that contains phosphite ligands has advantages over current catalysts for some challenging reactions, such as ring-closing metatheses of hindered dienes. Cazin et al. [184] modified an existing ruthenium indenylidene metathesis catalyst with triisopropyl phosphite groups to form cis and trans phosphite complexes. 

The indenylidene-ruthenium complexes were shown to be the actual alkene metathesis catalysts arising from the addition of propargylic alcohols [15-18]. The Dixneuf group [19, 20] later revealed that the intramolecular rearrangement of allenylidene-ruthenium complexes into indenylidene-ruthenium complexes was... 

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