Anionic tethered N-heterocyclic carbene chemistry

Abstract

Since the discovery of a stable "bottleable" N-heterocyclic carbene (NHC), there has been a spectacular explosion of interest in this ligand class. This interest stems from a desire to understand the fundamentals of the structure and bonding of these systems, but also because of their numerous and emerging applications in small molecule activation, homogeneous catalysis and Lewis acid-catalysed reactions. In this Tutorial Review, we introduce the reader to NHCs, cover general synthetic methods to prepare anionic tethered NHCs and their metal complexes, and discuss emerging applications in reactivity and catalytic studies.

Isolation of Free Phenylide-like Carbanions with N-Heterocyclic Carbene Frameworks

“…21e This type of low-field shift has been correlated with a decrease in the occupancy of the p orbital (π-electron density) of the respective carbon atom. 22 No other remarkable changes were observed upon deprotonation in the 1 H, 13 This behavior, which was mirrored by the structure of the lithium salt 3'a featuring a Li-C contact, is typical of deprotonation reactions at the benzene ring 23 and the imidazolium anions, and can be correlated with the aforementioned decrease of π-electron density on the carbon atom upon the increase in negative charge and consequent charge redistribution. 22 An indication for the resulting increase in π-electron density on the ring backbone upon deprotonation is the slight increase (0.02 -0.05 Å) in the bond lengths involving the exocyclic substituent to boron.…”

“…12 Numerous examples of pendant-arm carbenes containing an anionic functionality such as amido and alkoxi have been reported, in these cases the negative charge being usually unmistakably associated to the tethered functionality. 13 A few methods have been delineated for the separation of carbanions from an ion pair, with the most generally applicable and successful method being the use of crown ethers, pioneered in 1984 by Power.…”

Polymer precursors from catalytic reactions of natural oils

“…The direct NHC transfer from one metal centre to another, in one reaction, appears most practical and straightforward. As an example of this transmetallation approach, the silver-mediated reaction is the most common method for NHC transfer, 279,280 and in this manner, transmetallation can be achieved successfully with numerous metals. 281,282 However, the NHC transfer from silver does not always proceed correctly.…”

Chelating N-heterocyclic carbene–carboranes offer flexible ligand coordination to IrIII, RhIII and RuII: effect of ligand cyclometallation in catalytic transfer hydrogenation

“…pyridyl, oxazoline) [7][8][9][10][11][12][13][14][15] or anionic (e.g. alkoxide, amide) [16][17][18][19][20] donor functionalities often being incorporated into the substituents. These tethering groups enable ligands to chelate a metal centre, allowing fine control over stability, steric and electronic properties.…”