Af-heterocyclic carbene

Af-Heterocyclic carbene complexes of sulfur or, more simply, imidaz-ole-2-thiones, have been known for many years (51, 52) and have... 

A -Heterocyclic carbene complexes of Ir(I) and Ir(III) have also demonstrated high reactivity in transfer hydrogenation reactions of ketones (Scheme 2) [4]. Complex 4 catalyzed the reduction of a range of ketones into the corresponding alcohols, including the reduction of pinacolone 7 into alcohol 8 with a low catalyst loading and short reaction time [5]. The chelating bis(Af-heterocyclic carbene) complex 5 was shown to catalyze the reduction of ketones, and in the case of the reduction of benzophenone 9 to alcohol 10, the reaction was complete within 4 min [6]. 

Recently a new type of metal bonded IV-heterocycUc carbenes named remote Af-heterocyclic carbenes (rNHCs) has been described [138]. In contrast to normal NHCs, rNHCs possess no heteroatom adjacent to the carbene carbon atom. The less efficient stabilization of the carbene carbon atom has so far prevented the isolation of a free rNHC which consequently have to be stabilized by coordination to a suitable metal center. 

Recently, electrocarbonylation of phenol with CO to DPC was achieved in MeCN solution at ambient CO pressure using a heterogeneous Pd/C [102] and a homogeneous Af-heterocyclic carbene Pd electrocatalyst, the activity of which increased with the electron-donating ability of the ligand (Eq. (12.4)) [103] ... 

Af-Heterocyclic carbenes are good cr-donors. The M carbenc bond distances (typically >210 pm) are longer than those in Fischer- or Schrock-type carbene complexes. This implies that the metal-to-carbene back-bonding that is characteristic of Fischer- and Schrock-type carbenes is not as important in A -heterocyclic carbene complexes. 

Scholten et al. [401] reported that Af-heterocyclic carbenes are potent catalysts for group transfer polymerization of acrylic esters. The reaction is illustrated as follows ... 

Also shown in Scheme 8.4 are the CO substitution reactions that typically proceed in bimolecular, thermal processes, thus adding hgands such as cyanide, phosphines, Af-heterocyclic carbenes, and various N-donors. Sequential additions of monodentate ligands proceed with substitutions on alternate iron atoms. This is the observation with cyanide, readily producing the dicyano complex... 

The fate of the benzylidene fragment in these ruthenium complexes is another matter of debate [20a, 25]. The thermal stability of benzylidene complexes 4-6 was tested at 85 °C, under conditions mimicking polymerisation of methyl methacrylate. As monitored by H NMR, complete disappearance of the benzylidene fragment of the mixed phosphine/Af-heterocyclic carbene complex 5 (R = Cy, R = (5j-CHMePh) was observed within 20 min, whereas the Grubbs complex, RuCl2(=CHPh)(PCy3)2, showed only 55 % decomposition, and the bis-A/ -heterocyclic carbene ruthenium complex 6 (R = Cy) 88 % decomposition over the same time interval (Figure 6). 

It is crucial to find an appropriate combination of a catalyst and an ionic liquid which has to be harmonized with the hydrophilicity/hydrophobicity of the product. For example, hydrosilylation reactions in the presence of ionic liquids based rat 1,3-dialkylimidazolium cations did not give the desired polyethersiloxanes, as the 2-H position of the cation was too reactive. The authors assumed that formation of NHC (Af-heterocyclic carbenes) which coordinates to the Pt center is responsible of the catalyst deactivation. 

Af-Heterocyclic carbenes and phosphine ligands can bind metal centers in a somewhat similar feshion via dative coordination using a lone pair of electrons. Therefore, phosphines are often considered to be the closest neighbors to NHCs in terms of organometallic chemistry. However, the properties of these two classes of compounds can be quite different, and can yield, for example, catalysts with quite different reactivity. The use of NHCs in olefin metathesis [25] and in palladium catalysis [26], for example, has allowed the preparation of highly active and stable species that are now at the forefront of their fields. Various studies have been conducted to explore the nature of bonding between NHCs and metal centers. Diez-Gonzalez and Nolan reviewed some aspects of NHC coordination to metal centers in 2007 [27]. 

Af-Heterocyclic Carbenes in the Context of Other Stable Carbenes... 

Very recently, Ma and coworkers prepared a planar-chiral Af-heterocyclic carbene silver complex based on [2.2]-paracyclophane (Scheme 3.18) [37]. The... 

With ruthenium, palladium is arguably the most utilized transition metal for the synthesis of Af-heterocyclic carbene (NHC) complexes and definitely the most versatile for applications in catalysis. This chapter comprises the literature on NHC-Pd complexes since 2006, when the previous edition of this book was released, until the end of 2012. A search in Scopus on the topic Af-heterocyclic carbene palladium in this period yielded almost 550 references, including a number of specifically dedicated reviews [1-6]. We have focused in well-defined complexes, although some relevant in situ systems with interesting catalytic activity are also included. 

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