N-heterocyclic carbenes metal complexes

Biologically Active N-Heterocyclic Carbene-Metal Complexes... 

Visbal R, Gimeno MC. N-heterocyclic carbene metal complexes photoluminescence and applications. Chem Soc Rev. 2014 43 3551-3574, and references cited therein. 

Abstract The manuscript describes the methods that are most often used in the preparation of N-heterocyclic carbene (NHC) complexes. These methods include (1) insertion of a metal into the C = C bond of bis(imidazolidin-2-ylidene) olefins (2) use of carbene adducts or protected forms of free NHC carbenes (3) use of preformed, isolated free carbenes (4) deprotonation of an azolium salt with a base (5) transmetallation from an Ag-NHC complex prepared from direct reaction of an imidazolium precursor and Ag20 and (6) oxidative addition via activation of the C2 - X (X = Me, halogen, H) of an imidazolium cation. 

Direct C-H activation of 2-imidazolines in the addition to alkenes has been observed under rhodium catalysis as shown for the formation of 586 (Scheme 139) <20040L1685>. The proposed intermediate was thought to be similar to that involved in metal-N-heterocyclic carbene (NHC) complexes <2002AGE1290>. 

The aim of this chapter is to examine the application of well-defined N-heterocyclic carbene (NHC) complexes, as well as the systems prepared in situ which involve free NHCs or the precursor salt, for the reduction of unsaturated organic molecules such as alkynes, alkenes and carbonyl compounds. The most active complexes for such reactions are based on electron-rich, late transition metals in low oxidation states. Herein, reductions useful for organic synthesis will be classified into four types according to the hydride source used (i) hydrogenations, (ii) transfer hydrogenation, (iii) hydrosilylation and (iv) hydroboration. For examples of reduction reactions with systems containing non-classical NHC ligands, the reader is referred to Chapter 5. 

The surprising stability of N-heterocyclic carbenes was of interest to organometallic chemists who started to explore the metal complexes of these new ligands. The first examples of this class had been synthesized as early as 1968 by Wanzlick [9] and Ofele [10], only 4 years after the first Fischer-type carbene complex was synthesized [2,3] and 6 years before the first report of a Schrock-type carbene complex [11]. Once the N-heterocyclic ligands are attached to a metal they show a completely different reaction pattern compared to the electrophilic Fischer- and nucleophilic Schrock-type carbene complexes. 

During the last decade N-heterocyclic carbene complexes of transition metals have been developed for catalytic applications for many different or-... 

C. S. J. Cazin (ed.), N-Heterocyclic Carbenes in Transition Metal Catalysis and Organocatalysis, Catalysis by Metal Complexes 32,... 

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