Ruthenium NHC complexes

Fig. 2 Ruthenium-NHC complexes active in catalytic olefin metathesis...

Scheme 5.2 Cyclopropanation of styrene and ethyl diazoacetate using ruthenium-NHC complexes...

The use of stoichiometric ruthenium-NHC complexes generated in situ from [Ruljd-COCKp-cymene)], an imidazohnm salt [4] or an imidizol(idin)ium-2-carboxylate [4] has been applied in the cyclopropanation of styrene 5 with ethyl diazoacetate (EDA) 6 (Scheme 5.2). No base was necessary when imidazolium-2 carboxylate were employed. The diastereoselectivity was low and the cis/trans ratio was around 50/50 (Table 5.1). Although the diastereoselectivity was moderate, the reaction was highly chemoselectivity as possible side reactions (homologation, dimerisation and metathesis) were totally or partially suppressed. 

The ruthenium NHC complex 14 readily donates hydrogen to suitable acceptors and forms the C-H activated complex 15 [6]. The reversible nature of the C-H activation is established by the return of the hydrogen to restore the original... 

Madsen and co-workers have reported an important extension to the amine alkylation chemistry, in which oxidation takes place to give the amide product [13]. A ruthenium NHC complex is formed in situ by the reaction of [RuCl Ccod)] with a phosphine and an imidazolium salt in the presence of base. Rather than returning the borrowed hydrogen, the catalyst expels two equivalents of H. For example, alcohol 31 and benzylamine 27 undergo an oxidative coupling to give amide 32 in good isolated yield (Scheme 11.7). 

The isomerisation of aUylic alcohols to saturated ketones usually has a strong thermodynamic driving force. The ruthenium NHC complex 62 has been used to catalyse the isomerisation of allylic alcohol 59 which gives ketone 60 as the principal product along with some of the reduction product 61 [32]. The catalyst was water-soluble and the aqueous phase could be re-used for several runs (Scheme 11.15). NHC analogues of Crabtree s catalyst, [IrlPCyjKpyridineXcod)] PFg, were found to be less efficient for the isomerisation of allylic alcohols than... 

Ruthenium-NHC complexes exhibit activity in a very wide field of applications. Due to their unique ability to break and reassemble olefin bonds under reaction conditions very favourable to design simple processes, applications in nearly any chemical discipline can be foreseen. This field may span from manufacturing of specialty polymers and rabbers to pharmaceuticals, pharmaceutical intermediates, agrochemicals, fragrances, dyes, specialty chemicals for electronic applications or fine chemicals from natural feedstock and many more. Below are described Ru-NHC catalysed reactions applied from pilot to full commercial scale. 

Ruthenium(II)-NHC systems ean be used for atom transfer radical polymerization (ATRP). Generally, similar results as for the analogous phosphine complexes are obtained. For the ATRP of styrene and methyl methacrylate (MMA) [(NHC)2peBr2] was found to rival copper(I)-based systems and to yield poly (MMA) with low polydispersities. Polymerizations based on olefin metathesis that are catalyzed by ruthenium-NHC complexes are discussed separately vide supra). 

It is interesting to note that C-H activation on ruthenium NHC complexes is not limited to intramolecular protons located in the N-sidechain of the carbene, but occurs inter-molecularly as well. Leimer et al. reacted [MesIRuH PCyj] with toluene-dg at ambient temperature and observed a rapid H/D exchange reaction involving the four hydride hydrogen atoms on ruthenium, the methyl protons of the mesityl substituents of the carbene ligand and the deuterium atoms on the meta positions of toluene-dg. The ortho-, para- and methyl-deuterium atoms of the solvent did not participate [145]. 

The ruthenium NHC complex (53) has been reported to catalyse the enantioselective ring-opening cross-metathesis of norbomenes with allyltrimethylsilane (<98% ee)P ... 

The activity of rhodium (and ruthenium) NHC complexes in hydrosilylation of ketones was already reported in 1977 by Hill and Nile (300). Recently, several new rhodium NHC complexes allowing enantioselective hydrosilylation of ketones and j8-ketoesters were developed (for review, see (220)). 

Complexes 125-131 with thiazole-2-ylidene ligands (Figure 11.28) were the first reported ruthenium NHC complexes that contained just one exocyclic substituent adjacent to the carbenic center [108]. 

Figure 12.17 Ruthenium-NHC complexes cationic (70-73) and neutral complexes (74, 75).

Scheme 38 Ruthenium-NHC complex-catalyzed dehydrogenative Guerbet reaction...

Scheme 2.26 Migratory insertion reactions in ruthenium-NHC complexes.

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