NHCs , applications asymmetric catalysis

Very recently, Stahl et al. reported the first synthesis of a 7-membered NHC ligand [98]. Despite substantial effort, the isolation of the free carbene 21 was not successful. However, palladium complexes of 21 could be formed and structurally characterized. Ligand 21 is C2 symmetric as a result of a torsional twist which is thought to attenuate the antiaromatic character of the 87r-electron carbene heterocycle [101,102]. It will be interesting to see, if the synthesis of conformationally stable analogues and their application in asymmetric catalysis will be feasible. 

This volume provides the reader with the most important and exiting results pertaining the use of NHC complexes in transition-metal catalysis. Following an introductory chapter, which deals with the properties of NHC compounds and discusses some insightful examples, routes to NHC complexes will be described, a prerequisite for doing catalysis. Chapters on NHC complexes in oxidation chemistry and in metathesis reactions are accompanied by a chapter on palladium-catalyzed reactions and another on catalysis by other metals. Finally, this book would be incomplete without treating applications in asymmetric catalysis, which rounds out this volume. 

Although phosphinite- and NHC-based iridium catalysts show very similar enantiose-lectivities in the hydrogenation of various olefins, replacement of the phosphinite group by an A-heterocyclic carbene (NHC) unit results in particularly effective catalysts which are much better suited for the hydrogenation of acid-sensitive substrates beeause of the lower acidity of iridium hydride intermediates produced. The new NHC-pyridine ligands are also likely to prove useful for other applications in asymmetric catalysis." ... 

Pd isocyanides and chiral diamines [eqn (9.11)]/ Complex 98 gave modest activity and enantioselectivity in aza-Claisen rearrangement, however this new family of complexes has great potential for asymmetric catalysis, owing to the large variety of available chiral diamines. Finally, Xia reported the application of a number of simple NHC-Pd complexes to the synthesis of ureas from the corresponding amines and High activities... 

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