Ruthenium N-heterocyclic carbene complexes

Haller LJL, Page MJ, Erhardt S, Macgregor SA, Mahon MF, Naser MA, Velez A, Whittlesey MK (2010) Experimental and computational investigation of C-N bond activation in ruthenium N-heterocyclic carbene complexes. J Am Chem Soc 132 18408-18416... 

A synthetic protocol consisting of the reaction of RuCl2(PPh3)2(CHR) with imidazol-2-ylidenes [50-52] was used for the generation of another type of ruthenium-based system. While bis(N-heterocyclic carbene) complexes displayed a comparably low reactivity in ROMP [18, 19], the corresponding mono-N-heterocyclic carbene-monophosphane complexes turned out to be highly active [18, 19, 53]. 

The search for even more active and recyclable ruthenium-based metathesis catalysts has recently led to the development of phosphine-free complexes by combining the concept of ligation with N-heterocyclic carbenes and benzyli-denes bearing a coordinating isopropoxy ligand. The latter was exemplified for Hoveyda s monophosphine complex 13 in Scheme 5 [12]. Pioneering studies in this field have been conducted by the groups of Hoveyda [49a] and Blechert [49b], who described the phosphine-free precatalyst 71a. Compound 71a is prepared either from 56d [49a] or from 13 [49b], as illustrated in Scheme 16. 

Further improvements in activity of the ruthenium carbene complexes were achieved by incorporation of methyl groups in 3,4-position of imidazol-2-ylidene moiety. Introduction of sulfur in the trara-position to the N-heterocyclic carbene leads to increased stability of the resulting ruthenium complexes. The synthesis and the first applications of these new rathenium complexes are described herein. 

By contrast, much of the work performed using ruthenium-based catalysts has employed well-defined complexes. These have mostly been studied in the ATRP of MMA, and include complexes (158)-(165).400-405 Recent studies with (158) have shown the importance of amine additives which afford faster, more controlled polymerization.406 A fast polymerization has also been reported with a dimethylaminoindenyl analog of (161).407 The Grubbs-type metathesis initiator (165) polymerizes MMA without the need for an organic initiator, and may therefore be used to prepare block copolymers of MMA and 1,5-cyclooctadiene.405 Hydrogenation of this product yields PE-b-PMMA. N-heterocyclic carbene analogs of (164) have also been used to catalyze the free radical polymerization of both MMA and styrene.408... 

Fig. 5 Typical half-sandwich ruthenium fragments used in the preparation of allenylidene complexes. Ancillary ligands include CO, mono- and bidentate phosphines or N-heterocyclic carbenes...

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]. 

Alkylidene Complexes of Ruthenium Containing N-Heterocyclic Carbene Ligands... 

HeterobimetalUc ruthenium alkylidenes, which may be prepared by reaction of ClaRuIPCyalafCHR) with [Ru(pactive initiators are the bimetallic complexes (p-cymene)RuCl(p-Cl)2RuCl(CHPh)(NHC) and (Cp )RhCl(p-Cl)2RuCl(CHPh)(NHC) (NHC=N-heterocyclic carbene, Cp =pentamefhylcyclopentadienyl) [236, 237]. 

Let us examine first the principal sites of tagging ruthenium complexes (Scheme 1.41) they include covalent ligands A [phosphines, N-heterocyclic carbenes (NHCs) or pyridine], anionic ligands B, or the carbene ligand C (either in the aromatic moiety or at the alkoxy ligand). 

Synthesis and application of new N-heterocyclic carbene ruthenium complexes in catalysis 06COC203. 

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