Enantiomerically Pure Organolanthanides for Asymmetric Catalysis. Synthesis, Structures, and Catalytic Properties of Complexes Having Pseudo-<i>meso</i>-Me₂Si(η⁵-C₅H₃R)(η⁵-C₅H₃R*) Ancillary Ligation

Abstract

As established by NMR, circular dichroism, and X-ray diffraction, organolanthanides of the new chelating ligand Me 2 Si( t BuCp)[(+)- neo -Men-Cp] 2 - (Men = menthyl; Cp = η 5 -C 5 H 3 ) preferentially adopt a single chiral configuration of the asymmetric metal−ligand template. Metallocene dichloro complexes ( R,S )-Me 2 Si( t BuCp)[(+)- neo -Men-Cp]Ln(μ-Cl 2 )Li(OEt 2 ) 2 (Ln = Y, Lu) are synthesized by alkylation of the corresponding lanthanide trichlorides with the ligand dilithium salt and are isolated isomerically pure by crystallization from diethyl ether. Alkylation of the ( R,S )-epimers with MCH(SiMe 3 ) 2 (M = Li, K) proceeds with retention of configuration at the lanthanide center, affording chiral hydrocarbyl complexes in high yield. Reaction of the Lu−hydrocarbyl with hydrogen affords diastereomerically pure {( R,S )-Me 2 Si( t BuCp)[(+)- neo -Men-Cp]LuH} 2, an active catalyst for asymmetric olefin hydrogenation. X-ray diffraction reveals a pseudo- C 2 -symmetric dimer with a pseudo- meso Cp ring substituent arrangement. The two Me 2 Si( t Bu-Cp)[(+)- neo -Men-Cp]LuH fragments are slightly twisted with respect to each other. These hydrocarbyls are effective precatalysts for asymmetric hydrogenation of unfunctionalized olefins. The ee values obtained for deuteration of 1-pentene (up to 63%) are the highest reported to date for this reaction, and in all reactions, the ( R )-product enantiomer is favored. These results together with those obtained using other chiral organolanthanides provide better insight into lanthanocene stereochemical preferences and chirality transfer mechanisms.