Indenylidene catalysts

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... 

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

The indenylidene catalysts XXVIII a-d, containing a bidentate ligand, were obtained on reaction of complex IX or XXI with the corresponding iminophenol thalium derivative (Scheme 8.16) [61]. 

Scheme 8.21 RCM reactions with pyridine indenylidene catalyst XXVI.

The bulk ethenolysis of methyl oleate was performed by Forman et al. using a phoban-indenylidene catalyst [38]. As for the SM of methyl oleate, this readily available catalyst demonstrated to be a suitable alternative to C3, affording the desired products in 64% conversion with a catalyst load of 0.005 mol%, at 50°C and 10 bar of ethylene. Using the same conditions, C3 led to a conversion of 43%. 

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. 

The synthesis of the first ruthenium indenylidene catalysts arose from a 1998 report by the Fiirtsner and Dixneuf groups [13] that identified ruthenium-allenylidene complexes, easily prepared by the activation and dehydration of propargylic alcohols, as efficient catalysts for RCM reactions. These catalyst precursors were made from readily available arene-ruthenium(II) complexes containing a bulky and electron-donating ligand in the presence of the non- or weakly coordinating anion salt NaX or AgX (Scheme 14.1). 

Preparation of Ruthenium Indenylidene Catalysts from RuCI IPPhj) ... 

First-Generation Ruthenium Indenylidene Catalysts Bearing Two Phosphine Ligands... 

The iButylPhoban complex 20, which is commercially available, has been used to prepare a second-generation, ruthenium indenylidene catalyst [37]. 

A similar procedure for the two-step synthesis of the first-generation ruthenium indenylidene catalysts has been patented by Umicore (Scheme 14.11) [38]. The main differences from the previously reported route lie in the utilization of diox-ane as the reaction solvent at 90 C in the first step, followed by the addition of HCl for the acid-promoted formation ofthe indenylidene ligand via the alkenyl carbyne [19]. The preparation of the tricyclohexylphosphine complex was carried out in one pot without isolating the intermediate triphenylphosphine complex 5 [38]. 

First-Generation Ruthenium indenylidene Catalysts Bearing a Chelating Ligand... 

Scheme 14.14 Preparation of ruthenium indenylidene catalysts bearing a bidentate Schiff base ligand.

Scheme 14.23 Preparation of the third-generation ruthenium indenylidene catalyst 53.

In addition to their catalytic evaluation using classical model compounds, ruthenium indenylidene catalysts have been employed in RCM for the synthesis of natural products [15, 83-85], as well as in CM [49, 87, 88], acyclic diene metathesis (ADMET) [89], ROMP [21f, 65], and dienyne metathesis [90]. 

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