Chiral lanthanocene complex

Scheme 11.7 Amine induced epimerization of chiral lanthanocene complexes.

The first chiral rare earth metal-based hydroam-ination catalysts were reported in 1992 using chiral lanthanocene. Organolanthanide complexes catalyze regios-elective intermolecular hydroamination of alkenes, alkynes. 

Unfortunately, the chiral lanthanocenes undergo facile epimerization under the conditions of catalytic hydroamination via reversible protolytic cleavage of the metal cyclopentadienyl bond [27, 72, 213, 214] leading to an equilibrium mixture of the two possible diastereomeric complexes. Thus, the enantioselectivity of product formation is limited by the catalyst s epimeric ratio in solution and the absolute configuration of the hydroamination product is independent of the diastereomeric purity of the precatalyst. 

Chiral Ci-symmetric lanthanocenes (see Fig. 4) have achieved enantioselectivities of up to 74% (36) in various intramolecular hydroamination reactions [27,146,147]. While complexes 1-3 are better suitable for the formation of five-membered rings. 

With the aim of developing chiral organolanthanide catalysts for enantioselective transformations, the same group modified the aw a-lanthanocene framework in eomplexes 27 with a chiral group. The new catalytie precursors consist of Ci-symmetric octahydrofluorenyl complexes bearing an enantio-merically pure ( )-menthyl moiety in the lower Cp ring. Figure 6.2 depicts their structure. 

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