Ligands imidazoline-type

Phosphino-imidazoline ligands of this type were originally synthesized by Bu-sacca and coworkers and used in an enantioselective Heck reaction [28]. 

Additional factors which lead to an increased stability of the carbene complexes towards reductive ehmination are the type of NHC ligand and the NA -substitution pattern. The stability of NHC complexes depends strongly on the electronic situation at the carbene center. The oxidative addition of p-tolyl chloride to linear Pd° complexes bearing two unsaturated imidazolin-2-ylidenes (type 5, Fig. 6) or two saturated imidazolidin-2-ylidenes (t3q>e 7, Fig. 6) proceeds readily. The Pd complex with the imidazolin-2-yhdene ligands is stable, while the one with the imida-zolidin-2-ylidene ligands reductively ehminates the C2-arylated imidazolidinium salt [134]. 

More recently, imidazole- and pyrazole-type ligands with a phosphorus atom as the bridge have been reported,97,98 as depicted in formulae (46)-(48). Also rings with imidazolines (49)," or bridged by a —C—OH group (SO),100 have been reported. Boron-bridged polypyrazolylborates and related systems are dealt with in Chapter 13.6. 

NHCs have become popular ligands in coordination chemistry owing to the facile access to this type of ligands and to metal-NHC complexes. Most NHC ligands are prepared from azohum compounds such as imidazolium, triazolium, benzimidazo-lium, imidazolidinium, or thi azolium salts [1]. Alternatively, the reductive desulfurization of imidazolin-, benzimidazolin-, and imidazolidin-2-thiones to yield a variety of NHCs has been described. The preparation of suitable azolium salts and imidazolin-2-thiones is presented in Sect. 2.1. This is followed by the description of methods to liberate the free NHCs from these compounds. Today, stable singlet... 

The mechanism for the formation of complexes 63—66 has not been established unambiguously yet. As mentioned before, related complexes bearing bidentate imidazolin-2-ylidene/donor or benzimidazolin-2-ylidene/donor ligands (Scheme 9.18) have been described as formed by tautomerization of the azole to the NR,NH-substituted NHC that then together with the tethered donor coordinates to the metal center. This rather simple type of reaction mechanism is not very likely to be operative. 

While up to 1990 all attempts to isolate a stable N-heterocyclic carbene failed, metal complexes of unsaturated imidazol-2-ylidenes were known as early as 1968. The first complexes of this type were obtained by in situ deprotonation of imidazolium salts using mercury(ll) acetate or dimethylimidazolium hydridopentacarbonylchromate(-II) followed by coordination of the carbene to the metal center (Scheme 1.3). Shortly thereafter, the stabilization of the saturated imidazolin-2-ylidene in a metal complex was described by Lappert who treated electron-rich entetraamines of type 6=6 with coordinatively unsaturated transition metal complexes to obtain complexes with imidazolin-2-ylidene ligands (Scheme 1.3). ... 

Imidazoline-anchored phosphine ligand-Zn(II) complexes promote asymmetric Mannich-type reaction of F2C=C(R )OTMS with hydrazones (R CH=NNHCOR ) under mild conditions. ... 

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