Ruthenium indenylidene complex catalysts

For a thorough review of Ru-NHC-catalysts for metathesis, see Samojlowicz C, Bieniek M, Grela K (2009) Chem Rev 109 3708-3742 for ruthenium indenylidene-complexes in cross metathesis, see Boeda F, Bantreil X, Clavier H, Nolan SP (2008) Adv Synth Catal 350 2959-2966 For Hll-types systems, see Schrodi Y, Pederson RL (2007) Aldrichimica Acta 40 45-52... 

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]. 

Arene)ruthenium-indenylidene complex XV (Scheme 8.11), developed by Dixneuf et al. [33, 34] was also a very efficient catalyst for RCM of dienes and enyne metathesis (Table 8.4). 

Another application of ruthenium indenylidene complexes was the atom transfer radical addition of carbon tetrachloride to vinyl monomers reported by Verpoort [61]. This Kharasch reaction afforded good yields for all substrates tested, especially with the catalyst VIII (Equation 8.11, Table 8.8). 

The outcome of the RCM experiments employing different catalysts was consistent with the analysis outlined above (Scheme 3). Exposure of diene 29 to the ruthenium indenylidene complex 4 15), a readily available alternative to the classical Grubbs catalyst, afforded the desired ( >configured lactone ( >30 in 69% isolated yield, together with only 9% of the corresponding (Z)-isomer. Importantly, the E Z ratio does not evolve with time, indicating that the observed... 

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. 

Cavallo and Nolan [53] reported a combined experimental and computational study on the competing ene-then-yne and yne-then-ene pathways in the metathesis of dienynes catalyzed by ruthenium indenylidene complexes. The preferred pathway was found to be dependent on the catalyst as well as the... 

The ruthenium indenylidene complex 6 has also been used as precursor for the first-generation Grubbs- and Hoveyda-type catalysts, as shown in Scheme 14.13. It was shown by Nolan [39] that the reaction of styrene with 6 afforded the first-generation Grubbs catalyst 21 in excellent yield. This procedure benefits from the use of commercially available starting materials and avoids the use of diazo compounds. 

The thermal stabihty of complex 57a was found to be improved relative to the corresponding first-generation ruthenium-indenylidene complex 6 and Hoveyda catalysts 56 [70]. At 110 °C, complex 57a showed only 50% decomposition after 6 d, whereas 50% decomposition ofthe Hoveyda-I complex 56 was reached within 2 d. The non-chelated ruthenium-indenylidene 6 survived only a few hours at 80 C (Scheme 14.29) [70]. 

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. 

As already mentioned, the development of metathesis catalysts that can be easily accessed from simple precursors is necessary if a large-scale application is desired. With this in mind, Forman et al. developed a robust ruthenium-based phoban-indenylidene complex through a simple and relatively inexpensive procedure, if compared to the preparation of C3 [40]. This mthenium alkylidene was tested in the bulk SM of methyl oleate. As a result, they could reach up to 50% conversion with 0.005 mol% catalyst at 50°C. 

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). 

While carbyne complex 12a did not effect the ROMP of unstrained cyclooctene, the cationic indenylidene complex 13a was found to catalyze this reaction with a turnover frequency (TOP) of more than 17000min [20]. In s/ta-generated arene-ruthenium indenylidenes 13a-b were also shown to be excellent catalysts for a variety of RCM reactions employing functional dienes and enynes [20]. 

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

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]. 

Indenylidene complexes have also made their mark in ruthenium olefin metathesis complexes. First noticed by Nolan and coworkers [48] and Furstner et al. [49], these complexes were developed by Nolan and coworkers [50,51] to afford the very stable second-generation complexes 14 (with NHC ligands). More recently, Schrodi and coworkers [52] and Bruneau and coworkers [53,54] presented chelated indenylidene complexes 15 and 16 (Figure 11.5). The relatively fecile methodology for their synthesis and their increased stability certainly makes the indenylidene complexes another well-studied family in the rutheniiun olefin metathesis catalyst field. 

Furstner, A., Guth, O., Duffels, A., Seidel, G., Liebl, M., Gabor, B., Mynott, R., Indenylidene complexes of ruthenium optimized synthesis, structure elucidation, and performance as catalysts for olefin metathesis—application to the synthesis of the ADE-ring system of Nakadomarin A, Chem. Eur. J. 2001, 7 4811-4820. 

Some ruthenium vinylidene complexes have been found to serve as good catalyst precursors for olefin metathesis [160]. Although the efficiency of the vinylidene complexes as initiators is lower than those of the well-known Grubbs alkylidene and indenylidene complexes, the polymerization rate is fast enough for practical use and, more importantly, they are readily prepared from conventional terminal... 

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