Grubbs first and second generation catalysts

The mechanism of Ru-alkylidene-catalyzed reactions has been investigated. Note that Grubbs first- and second-generation catalysts are 16-electron species, so if the first step involves complexation of an alkene to the metal, this process could occur in an associative or dissociative manner. Evidence suggests (see Scheme 11.5) that this occurs in a dissociative manner, however, first forming a 14-electron intermediate 25 and then 26a or 26b after complexation of the alkene. Gas-phase mass spectral evidence supports the initial formation of 25. Complexes similar to 26a and b have been isolated from reaction mixtures under appropriate conditions, but ruthenacyclobutane 27 has not been directly observed until quite recently.37... 

Scheme 2.10 Alkene metathesis eatalysed by Grubbs first- and second-generation catalysts.

Metathesis reactions are reported in some reviews [72-75], a handbook [76], etc. [77-82]. The first well-defined metathesis-active mthenium-alkylidene complex is a compound, (Ph3P)2Cl2 Ru=CH-CH=CPh2), but Hoveyda-Grubbs first- and second-generation ruthenium-based catalysts are ruthenium-alkylidene five-membered ring compounds (see Chap. 1). 

The Grubbs second-generation ruthenium catalyst could be inhibited only by the addition of 20 equiv of MIM [59]. Interestingly, the acid-activated 92 and 94 catalysts were more efficient than their precursors (Grubbs first- and second-generation ruthenium catalysts). For example, ROMP of e vo-ONBE dibutylester... 

Figure 16.19. The first and second generation Grubbs catalysts and an asymmetric catalyst...

The most widely used catalysts for RCM are Grubbs ruthenium catalyst 9 and its second generation analogue 10, as well as first and second generation Hoveyda-Grubbs catalysts 11 and 12 (Fig. 6) [38]. The latter have superior stability and reactivity, expanding the applicability of the method considerably. Schrock molybdenum catalyst 13 has also been described for macrocy-clization [38]. 

An efficient method for the preparation of highly functionalized conjugated dienes with the use of silicon-tethered ring-closing enyne metathesis in the presence of first- and second-generation Grubbs catalysts has been described... 

Recent years have seen rapid advancements in development of W, Mo, and Ru carbene complexes that serve not only as catalysts for metathesis of small molecules (Section 11-1-3) but also as ROMP catalysts. Grubbs use of a Ru(III) catalyst for ROMP in aqueous medium helped pave the way for development of his first- and second-generation Ru alkylidenes, which also catalyze ROMP. The Schrock catalysts that we have encountered already (compounds 15... 

XV. Both the first and second generation Grubbs olefin metathesis catalysts have been shown to isomerize allylic ethers to vinyl ethers that are readily hydrolyzed.It is a decomposition product of the catalyst that was shown to be the isomerization catalyst. ... 

ESI-MS was also employed for the study of the intrinsic reactivity of first- and second-generation Grubbs metathesis catalysts 19 [(PCy3)(Cl)2Ru=CHR] (Cy = cyclohexyl, R= -Me3NCH2C6H4) and 20... 

Fig. 27.5 Initial steps in the mechanism of alkene metathesis involving first and second generation Grubbs catalysts. Two possibilities for the formation of the metallocyclobutane intermediates are shown.

Richard Schrock investigated the properties of some of the first catalysts for olefin metathesis. His work included catalysts prepared from tantalum, titanium, and molybdenum. The catalysts predominandy in use today, however, are ruthenium catalysts developed by Robert Grubbs. His so-called first generation and second generation catalysts are shown here. 

In addition to the electronic effects of the ligand, there may be a steric component to the differing stabilities of the phosphine- and NHC-ligated met-allacyclobutanes. Jensen and coworkers [17] have analyzed the steric exchange interactions in the phosphine-bound precatalysts and the metallacyclobutanes derived from the first- and second-generation Grubbs catalysts, among other... 

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