Indenylidene-ruthenium

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. 

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

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. 

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

Scheme 8.11 Formation of indenylidene ruthenium complex accelerated bythe protonation of ruthenium allenylidene complex.

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). 

Applications of Isolated Indenylidene-Ruthenium Complexes in ROMP... 

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. 

Scheme 8.15 Preparation of indenylidene ruthenium complexes containing a NHC ligand.

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). 

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],... 

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... 

From Allenylidene to Indenylidene Ruthenium Complexes and Catalysts... 

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. 

A few weeks later, Nolan [16] reported the indenylidene-ruthenium complexes 7-10, which contain a NHC ligand analogous to the Grubbs II catalyst. Using catalysts 6 and 7 -10, excellent activities were shown in the RCM reaction of various... 

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

Once the NHC was introduced onto the ruthenium center, the addition of pyridine (10 equiv.) led to the 16-electron, monopyridine indenylidene complexes 54a-f (Scheme 14.24) [55, 56, 65, 66]. Interestingly, from a synthetic point of view, this preparation can also be achieved starting from (triphenylphos-phine)(indenylidene)ruthenium complexes, thereby avoiding the use of the costly and sensitive tricyclohexylphosphine [50,66]. The monopyridine complexes bearing mesityl substituents on the nitrogens were not very active at room temperature for the RCM of dienes and enynes, nor in the CM of methyl acrylate [66]. However, the controlled ROMP of norbornene-type monomers was allowed [65]. In contrast, the indenylidene complexes 54b and 54f, which bear 2,6-diisopropylphenyl substituents on the NHC and either a pyridine or 3-bromopyridine ligand, were... 

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,... 

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