A New Generation of Catalyst Ligands

Since the first edition of this book went into print (1996), many new catalyst ligands have been discovered. They very closely resemble one another in terms of their structural features and their coordination behavior. However, there has also come along a completely new generation of ligands that in part substitutes and in part supplements the ubiquitous organophosphanes (Structure 22) A-het-erocyclic carbenes 23 (cf. also Section 3.1.10) [121]. 

Whereas phosphanes have a conical shape, much of which is decisive in stereoselective catalysis, the A -heterocyclic carbenes (NHCs) exhibit flat core structures. More importantly, phosphanes dissociate from metal centers in common catalysts, whereas the C-coordination of the new ligands is much more stable under catalysis conditions, thus preventing the catalytically active metal from aggregating and precipitating. Theoretical studies (DFT cf. Section 3.1.2) showed that the phosphanes in the Ru metathesis catalyst 24 dissociate much more easily (ca. 27 kcal/mol) from the metal than the carbene in 25 (ca. 21 kcal/mol) the 

---