Synthesis of NHC Precursors

Precursors for pincer ligands containing only one NHC and two additional donor groups are also known. Phosphines (37), 5-amines (38), pyridyl (39), or phenoxy groups (40) can function as additional donors. A double 

The second type of chiral azolium precursors possesses a chiral centre within the N-heterocyclic ring, which is normally substituted at the C4 and C5 positions, like in compounds 54 or 55.  

The fourth group of chiral imidazolium precursors is made up from planar chiral derivatives. The first representative of this type, 58, was prepared by Bohn et al Togni and co-workers described the C2-symmetrical ferrocenyl-sub-stituted imidazolium salt 59 and related derivatives are also known.The planar-chiral imidazolium salt with A-paracyclophane substituents 60 and some derivatives are known. The enantiomerically pure rra s-l,2-diaminocy-clohexane was an useful starting material for the generation of the chiral diimi-dazolium salts and 62 and of mono-imidazolium salts.  

The last group of chiral NHC ligand precursors is comprised of oxazoline-substituted imidazolium salts. The first derivative of this type 63 was reported by Herrmann et al. in 1998. A slight modification of the ligand backbone and use of the oxazoline C4 atom for bridging gave the imidazolium salt 64. The direct connection of the two heterocycles led to 65. Imidazolium salts built from oxazoline heterocycles like 66 were prepared by Glorius et alP  

The corresponding alcohol elimination from 2-alkoxyimidazolidines 69 to give imidazolin-2-ylidenes of type 70 was reported by Grubbs and co-workers after 

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