Grubbs ruthenium carbene catalyst

The same authors later expanded the concept and additionaly provided an imidazolium-tagged Howeyda-Grubbs ruthenium carbene catalyst for the RCM reaction [260]. The resulting system proved to be highly active for the conversion of di-, tri- and tetrasubstituted diene and enyne substrates. In the catalyst solvent system [BMIM][PF6]-CH2Cl2 (volume ratios 1 1 to 1 9) the catalyst could be recycled 17 times with only very slight loss in activity. Also in this work it was demonstrated that the imidazolium tag is essential to obtain a stable and recycleable catalyst. 

The ruthenium carbene catalysts 1 developed by Grubbs are distinguished by an exceptional tolerance towards polar functional groups [3]. Although generalizations are difficult and further experimental data are necessary in order to obtain a fully comprehensive picture, some trends may be deduced from the literature reports. Thus, many examples indicate that ethers, silyl ethers, acetals, esters, amides, carbamates, sulfonamides, silanes and various heterocyclic entities do not disturb. Moreover, ketones and even aldehyde functions are compatible, in contrast to reactions catalyzed by the molybdenum alkylidene complex 24 which is known to react with these groups under certain conditions [26]. Even unprotected alcohols and free carboxylic acids seem to be tolerated by 1. It should also be emphasized that the sensitivity of 1 toward the substitution pattern of alkenes outlined above usually leaves pre-existing di-, tri- and tetrasubstituted double bonds in the substrates unaffected. A nice example that illustrates many of these features is the clean dimerization of FK-506 45 to compound 46 reported by Schreiber et al. (Scheme 12) [27]. 

With the development of an analogous ruthenium benzylidene catalyst 17 by Grubbs and co-workers in 1995, a ruthenium carbene catalyst suitable for the cross-metathesis reaction was in place [34]. Benzylidene 17 exhibited the same impressive tolerance of air and moisture, and the same stability towards functional groups as its predecessor 4, but benefited from easier preparation [35,36] and much improved initiation rates. 

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

The Grubbs ruthenium-carbene complexes (2 and 3) exhibit high reactivity (albeit lower than the best molybdenum- and tungsten-based catalysts) in a variety of metathesis processes while showing a remarkable tolerance towards many different organic functionalities. These initiators are stable for weeks and reactions can be carried out in the presence of air and humidity or even in water. 

Recent efforts have achieved Z-selective ruthenium carbene catalysts, providing a catalyst solution to a general problem in stereoselective alkene synthesis Diastereoselectivity in Olefin Metathesis Development of Z-Selective Ru Catalysts Vol 1, Chapter 3 Grubbs, Handbook of Metathesis, 2nd Edition, Volume 2, Chapter 7. As this is a relatively new field, mechanistic studies are... 

Michrowska A, Bujok R, Hamtyunyan S, Sashuk V, Dolgonos G, Grela K. Nitro-Substituted Hoveyda—Grubbs Ruthenium Carbenes Enhancement of Catalyst Activity through Electronic Activation. J Hm Chem Soc. 2004 126(30) 9318—9325. 

Grubb s ruthenium carbene catalyst to afford medium-sized carbocycles (Equation (10))." Iron tricarbonyl systems containing diazoketones tethered to the terminal position of the acyclic coordinated diene have been utilized to effect ring formation, via inter- and intramolecular carbene reactions, affording new cyclohexa-2,4-dienone and cyclopent-2-enone tricarbonyl iron complexes. ... 

A. Michrowska, R. Bujok, S. Harutyunyan, V. Sashuk, G. Dolgonos, K. Grela, Nitro-Substituted Hoveyda-Grubbs ruthenium carbenes enhancement of catalyst activity through electronic activation, J. Am. Chem. Soc. 126 (2004) 9318-9325. 

Despite the numerous reports concerning NHC-Ru olefin metathesis initiators, a complex incorporating a carbene that has only one exocyclic substituent adjacent to the carbenic centre was not reported until 2008. Studies by Grubbs and co-workers led to the development of ruthenium-based catalysts bearing such carbene ligands, in this case incorporating thiazole-2-ylidenes [63] (Fig. 3.19). 

For the last 2 decades ruthenium carbene complexes (Grubbs catalyst first generation 109 or second generation 110, Fig. 5.1) have been largely employed and studied in metathesis type reactions (see Chapter 3) [31]. However, in recent years, the benefits of NHC-Ru complexes as catalysts (or pre-catalysts) have expanded to the area of non-metathetical transformations such as cycloisomerisation. 

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... 

Initial reports of cross-metathesis reactions using well-defined catalysts were limited to simple isolated examples the metathesis of ethyl or methyl oleate with dec-5-ene catalysed by tungsten alkylidenes [13,14] and the cross-metathesis of unsaturated ethers catalysed by a chromium carbene complex [15]. With the discovery of the well-defined molybdenum and ruthenium alkylidene catalysts 3 and 4,by Schrock [16] and Grubbs [17],respectively, the development of alkene metathesis as a tool for organic synthesis began in earnest. 

Ruthenium Carbene-Based Olefin Metathesis Initiators Catalyst Decomposition and Longevity M. Ulman, R.H. Grubbs,/. Org.Chem. 1999, 64, 7202— 7207. 

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