Enantiomerically pure organolanthanides for asymmetric catalysis. Synthesis, structures, and catalytic properties of complexes having pseudo-meso-Me₂Si(η⁵-C₅H ₃R)(η⁵-C₅H₃R*) ancillary ligation

As established by NMR, circular dichroism, and X-ray diffraction, organolanthanides of the new chelating ligand Me₂Si(^tBuCp)[(+)-neo-Men-Cp]^2- (Men = menthyl; Cp = η⁵-C₅H₃) preferentially adopt a single chiral configuration of the asymmetric metal-ligand template. Metallocene dichloro complexes (R,S)-Me₂Si(^tBuCp)[(+)-neo-Men-Cp]Ln(μ-Cl ₂)Li(OEt₂)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(SiMe₃)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)-Me2Si(^tBuCp)[(+)-neo-Men-Cp]LuH}₂, an active catalyst for asymmetric olefin hydrogenation. X-ray diffraction reveals a pseudo-C₂-symmetric dimer with a pseudo-meso Cp ring substituent arrangement. The two Me₂Si(^tBu-Cp)[(+)-raeo-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.