Titanocenes enantiomerically pure

Early transition-metal complexes have been some of the first well-defined catalyst precursors used in the homogeneous hydrogenation of alkenes. Of the various systems developed, the biscyclopentadienyl Group IV metal complexes are probably the most effective, especially those based on Ti. The most recent development in this field has shown that enantiomerically pure ansa zirconene and titanocene derivatives are highly effective enantioselective hydrogenation catalysts for alkenes, imines, and enamines (up to 99% ee in all cases), whilst in some cases TON of up to 1000 have been achieved. 

Scheme 12.6. Synthesis of enantiomerically pure allylic alcohols with titanocene chloride.

More recently, Doris et al. have described the reductive ring-opening of a-keto epoxides [16]. In this manner, p-hydroxy ketones can be obtained in high yields. The synthesis of enantiomerically pure compounds can easily be realized. The titanocene] 111) reagents are distinctly superior to samarium diiodide, which is also known to induce this transformation. 

Enantiomerically pure bis-Gp derivatives with chiral Gp ligands have been used with success in the catalytic enantioselective opening of meso-epoxides via electron transfer (see Section 4.05.8). The structural features are of relevance for the understanding of activity and selectivity of these complexes in diastereoselective reactions and for the design of novel catalysts. A comparison of the structure of three of these bis-Gp Ti derivatives (Scheme 481) in the solid state and in solution determined by X-ray crystallography and NMR methods indicated that the structures in the crystal and in solution are the same, and that applications of these complexes in catalysis can de discussed on the basis of crystallographic data.1114 In a similar study, the 1-methylcyclohexyl-Cp, 1-butyl-1-methylbutyl-Cp, and cyclohexyl-Cp titanocene dichlorides (Scheme 481) have been prepared and their molecular structures compared. The use of these three compounds in radical addition reactions has been studied.1115... 

Biphenyl-bridged bis-Cp titanocene dichloride and dimethyl complexes have been synthesized, and the kinetic resolution of the racemic final mixture of the products has been carried out. A mixture of diastereomers is obtained by treatment of the dimethyl compound with O-acetyl-mandelic acid, while enantiomerically pure products result in the reaction of the dichloro derivative with (i )-binaphthol and 1 equiv. of LiBun (Scheme 661).1050... 

Under pressure of CO, cyclopentenones can be obtained in good yields in the presence of a catalytic amount of the titanocene Cp2Ti(CO)2 (Eq. 42) [41]. By using an enantiomerically pure analogue, Buchwald was able to perform a highly enantioselective catalytic Pauson-Khand type reaction (Eq. 42) [42]. 

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