Grubbs second generation precatalyst

Dissociation of PCy3 from the Grubbs first-generation 16-electron precatalyst complex (I-A) proceeds with AE = 21.89 and 25.92 kcal/mol for the second-generation precatalyst (II-A). This is in agreement with the experimental kinetic studies by Grubbs in which AH values of 23.6 0.5 kcal/mol and 27 2 kcal/ mol were obtained for I and II, respectively. 

The mechanistic investigations presented in this section have stimulated research directed to the development of advanced ruthenium precatalysts for olefin metathesis. It was pointed out by Grubbs et al. that the utility of a catalyst is determined by the ratio of catalysis to the rate of decomposition [31]. The decomposition of ruthenium methylidene complexes, which attribute to approximately 95% of the turnover, proceeds monomolecularly, which explains the commonly observed problem that slowly reacting substrates require high catalyst loadings [31]. This problem has been addressed by the development of a novel class of ruthenium precatalysts, the so-called second-generation catalysts. 

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... 

Initiation of the second-generation Grubbs catalysts is slower than that of the first-generation catalysts because the precatalyst is more stable relative to the 14-electron intermediate, which preferentially adopts an inactive alkylidene conformation. 

Further evaluation of these new Z-selective catalysts showed that, for the bidentate anionic hgand complexes, initiation rates similar to the second-generation Grubbs-Hoveyda complex 3a could be achieved through modification of the complex s electronic and steric environment. Solvent was shown to play a role in overall reaction yields, but its effects were shown to be related to catalyst decomposition, not the precatalyst initiation. 

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