Summary of Ligand Effects

The difference in the activities ofthe first- and second-generation Grubbs catalysts arises from several interrelated stereoelectronic effects that have been extensively studied. The current view is that, relative to the phosphine ligand, the NHC better stabilizes the active alkylidene conformation, which aligns the p orbital of the alkylidene with those ofthe bottom-bound olefin. This conformational preference has been attributed to the molecular orbital interactions discussed by Straub [23] in his 2005 report, persists throughout the catalytic cycle, and explains a number of correlated effects  

A major goal in olefin metathesis, as in any alkene synthesis, has been the control of the stereochemistry of the alkene product. The overall stereoselectivity observed in an olefin metathesis reaction can be a function of both kinetic and thermodynamic selectivity. This is because the primary metathesis products, which are produced under kinetic selectivity, can often reenter the metathesis cycle to give thermodynamically controlled stereoselectivity [27]. For many catalysts, including the first- and second-generation Grubbs catalysts, the 

Z-selective catalysts (see Section 7.3, Chapter 3, and see Grubbs, Handbook of Metathesis, 2nd Edition, Volume 2, chapter 7). 

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D. Summary of ligand effects on copper-dioxygen reactivity 161... 

D. Summary of Ligand Effects on Copper-Dioxygen Reactivity... 

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