Journal of the American Chemical Society, Vol.116, No.22, 10212-10240, 1994
Chiral Organolanthanides Designed for Asymmetric Catalysis - Synthesis, Characterization, and Configurational Interconversions of Chiral, C-1-Symmetrical Organolanthanide Halides, Amides, and Hydrocarbyls
This contribution describes the synthesis, structural systematics, absolute configurations, and structural interconversions of a series of C-1-symmetric lanthanide chloro, hydrocarbyl, and amide complexes/precatalysts based on chiral chelating Me(2)Si(eta(5)-Me(4)C(5)) (eta(5)-C(5)H(3)R*)(2-) ligands [Me(2)SiCp"(R*Cp)](2-), where R* = (+)-neomenthyl, (-)-menthyl, and (-)-phenylmenthyl. The ligands are prepared in three steps from known chiral cyclopentadienes. Metalation of the chiral dienes followed by condensation with Me(4)C(5)Si(CH3)(2)Cl and in situ lithiation provides the dianions in nearly quantitative yield. Transmetalation of the lithiated ligands with anhydrous lanthanide trichlorides followed by ambient temperature ether workup provides Me(2)SiCp"(R*Cp)LnCl(2)Li(OEt(2))(2) complexes in high yield. For (R)-Me(2)SiCp" [(+)-neomenthylCp]Lu(mu-Cl)(2)Li(OEt(2))(2) : space group P2(1)2(1)2(1); a = 12.240(2), b = 12.876(2), and c = 24.387(5) Angstrom, (24 degrees C); Z = 4; R(F) = 0.0509. As established by NMR and circular dichroism, the diastereomerically pure chloro complexes can be epimerized in appropriate donor solvents to afford mixtures of (R)- and (S)-configurational isomers with the isomer ratio dependent on solvent, R*, and lanthanide ion. Selective epimerization allows enrichment in either antipode with diastereomerically pure complexes obtained in a single recrystallization. Li+ sequestering crown ethers inhibit epimerization. The temperature dependence of the (R) reversible arrow (S) equilibrium constant in THF yields Delta H = 1.7 +/- 0.3 kcal/mol and Delta S 3.6 +/- 0.8 eu for Me(2)SiCp"[(+)-neomenthylCp)Lu(mu-Cl)(2)Li(OEt(2))(2) and Delta H = 4.8 +/- 0.5 kcal/mol and Delta S = 13.4 +/- 0.5 eu for Me(2)SiCp"[(-)-menthylCp]Sm(mu-Cl)(2)Li(OEt(2))(2). The mechanism is proposed to involve reversible ring detachment to an intermediate LiCpR* complex. Alkylation or amidation with ME(SiMe(3))(2) (M = Li or K, E = CH; M = Na or K, E = N) yields the corresponding chiral hydrocarbyls and amides in high yield.
Keywords:ANSA-METALLOCENE DERIVATIVES;TRANSITION-METAL COMPLEXES;HIGHLY REACTIVE ORGANOLANTHANIDES;BRIDGED TETRAHYDROINDENYL LIGANDS;ZIEGLER-NATTA POLYMERIZATION;X-RAY STRUCTURE;ORGANOACTINIDE COORDINATIVE UNSATURATION;FUSED CYCLOPENTADIENYL LIGAND;OPTICALLY-ACTIVE ENAMINES;UNPROTECTED AMINO OLEFINS