Well-Defined Ruthenium Catalysts

First-Generation Grubbs Cataiysts RuCi2(PR3)2CHRi 

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Fig. 10 Structure of well-defined ruthenium catalysts employed in the dehydrogoiative amidatitm of alcohols and experimental conditions...

Noyori and coworkers reported well-defined ruthenium(II) catalyst systems of the type RuH( 76-arene)(NH2CHPhCHPhNTs) for the asymmetric transfer hydrogenation of ketones and imines [94]. These also act via an outer-sphere hydride transfer mechanism shown in Scheme 3.12. The hydride transfer from ruthenium and proton transfer from the amino group to the C=0 bond of a ketone or C=N bond of an imine produces the alcohol or amine product, respectively. The amido complex that is produced is unreactive to H2 (except at high pressures), but readily reacts with iPrOH or formate to regenerate the hydride catalyst. 

Well-Defined Ruthenium Olefin Metathesis Catalysts Mechanism and Activity, ... 

These observations led to the catalytic application of well-defined ruthenium alkyUdenes, some of them freely soluble and sufficiently stable in water (Scheme 7.9) although their stability was found somewhat less in aqueous solutions than in methanol [21,27,28], With these catalysts a real living ROMP of water-soluble monomers could be achieved, i.e. addition of a suitable monomer to a final solution of a quantitative reaction resulted in further polymerization activity of the catalyst [28], This is particularly important in the preparation of block copolymers. 

Since its discovery more than 50 years ago, olefin metathesis has evolved from its origins in binary and ternary mixtures of the Ziegler-Natta type into a research area dominated by well-defined molecular catalysts. Surveys of developments up to 1993 were presented in COMC (1982) and COMC (1995). Major advances in ROMP over the last 10 years include the development of modular, stereoselective group 6 initiators, and easily handled, functional-group tolerant ruthenium initiators. The capacity to tailor polymer functionality, chain length, and microstructure has expanded applications in materials science, to the point where ROMP now constitutes one of the most powerful methods available for the molecular-level design of macromolecular materials. In addition to an excellent and comprehensive text on olefin metathesis, a three-volume handbook s has recently appeared, of which the third volume focuses specifically on applications of metathesis in polymer synthesis. 

B. Mohr, D.M. Lynn, and R.H. Grubbs, Synthesis of water-soluble, aliphatic phosphines and their application to well-defined ruthenium olefin metathesis catalysts, Organometallics, 15(20) 4317 4325,1996. 

Well-defined ruthenium vinylidene complexes are efficient catalyst precursors for the ring-opening metathesis polymerization (ROMP) of cyclic olefins (Fig. 9). Most of them are neutral 16-electron complexes of the type RuC12(L)2(=C=CHR) (LI) [65-68] and the more active precursors contain... 

S. T. Nguyen, and T. M. Trnka, The Discovery and Development of Well-Defined, Ruthenium-Based Olefin Metathesis Catalysts, in Handbook of Metathesis, Vol. 1 (Ed. R. H. Grubbs, Wiley-VCH, Weinheim, 2003, Chap. 1.6). 

E. L. Dias, S. T. Nguyen, and R. H. Grubbs, Well-Defined Ruthenium Olefm Metathesis Catalysts Mechanism and Activity, J. Am. Chem. Soc. 119, 3887-3897 (1997). 

Grubbs first well-defined ruthenium carbene catalyst ([Ru]) was introduced in the early 1990s as the first air stable metathesis catalyst allowing for manip-... 

On the other hand, late transition metal-based catalyst systems that had been identified by the early 1990s were characterized by low activity but high functional group tolerance, especially toward water and other protic solvents. These features led to reinvestigations of ruthenium systems and, ultimately, to the preparation of the first well-defined, ruthenium-carbene olefin metathesis catalyst (PPh3)2(Cl)2Ru=CHCH=Ph2 (Ru-1) in 1992 [5]. 

Since 1957 and the discovery of the Speir s catalyst H2PtCl6/ PrOH, considerable efforts have been made to find new catalysts with high activity and selectivity. Along with the platinum-based catalysts, the Wilkinson s complex [103] Rh(Ph3P)3Cl is one of the most popular hydrosilylation catalysts. Ruthenium catalysts are also able to promote the addition of silanes to unsaturated carbon-carbon bonds, and several reports have shown during the past decade that the well-defined ruthenium complexes of type Ru(H)(Cl)(CO)L can provide excellent activity and selectivity [104—... 

Olefin metathesis has been extensively written on in both books and journals [1-10]. This chapter will focus on ADMET. Of particular interest are the issues of catalysis, mainly functional group tolerance, kinetics, and mechanistic details. The development of late-transition metal catalysts has enormously expanded the scope of ADMET, so particular attention will be given to the well-defined ruthenium-based olefin metathesis catalysts. Pertinent information pertaining to catalysts of Group VI metals will also be provided. Important procedural aspects of ADMET will be presented in conclusion. 

Grubbs-type initiators are well-defined ruthenium aUcylidenes. Compared to molybdenum- or tungsten-based Schrock catalysts, the reactivity of ruthenium-based Grubbs catalysts is somewhat different. In terms of polymer structure, ROMP of norbom-2-enes and norbomadienes using ruthenium-based systems generally results in the formation of polymers that, in most cases, predominantly contain frany-vinylene units. Polymerizations initiated by Grubbs-type initiators are best terminated by the use of ethyl vinyl ether, yielding methylidene-terminated polymers. 

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