Amido-cyclopentadienyl -based

Brintzinger and coworkers produced a titanium complex with a binaphthyl linked ansa metallocene ligand [56[. Upon activation with nBuLi, this complex catalyzed the reductions of B and 2 phenyl 1 pyrroline in 76 and 98% ee, respectively. Titanium catalysts with chiral amido cyclopentadienyl ligands have also been reported, but were considerably less stereoselective than the metallocene based ones [57]. 

The broader subject of the interaction of stable carbenes with main-group compounds has recently been reviewed. Accordingly, the following discussion focuses on metallic elements of the s and p blocks. Dimeric NHC-alkali adducts have been characterized for lithium, sodium, and potassium. For imidazolin-2-ylidenes, alkoxy-bridged lithium dimer 20 and a lithium-cyclopentadienyl derivative 21 have been reported. For tetrahydropyrimid-2-ylidenes, amido-bridged dimers 22 have been characterized for lithium, sodium, and potassium. Since one of the synthetic approaches to stable NHCs involves the deprotonation of imidazolium cations with alkali metal bases, the interactions of alkali metal cations with NHCs are considered to be important for understanding the solution behavior of NHCs. 

A different, possibly more productive, research direction has been the development of less syndioselective catalysts, based on modifications of the silyl-bridged cyclopentadienyl-amido titanium complex. 

The single-crystal X-ray molecular structure of the complex p-(Me2Si) (3,6di- BuFlu)( BuN)TiCl2 10, is depicted in Fig. 17. It shows a striking similarity to crystal structure of complex 9 with respect to its overall symmetry, despite the exchange of one of the aromatic rings, the cyclopentadienyl, in the molecule and its replacement with an amido group. Complex 10 exemplifies one of the rare examples of a titanium-based syndiotactic-specific metallocene catalyst systems. 

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