Chiral organolanthanides designed for asymmetric catalysis. Synthesis, characterization, and configurational interconversions of chiral, C₁-symmetric organolanthanide halides, amides, and hydrocarbyls

This contribution describes the synthesis, structural systematics, absolute configurations, and structural interconversions of a series of C₁-symmetric lanthanide chloro, hydrocarbyl, and amide complexes/precatalysts based on chiral chelating Me₂Si(η⁵-Me₄C₅)(η ⁵-C₅H₃R*)^2- ligands [Me₂SiCp″(R*Cp)]^2-, where R* = (+)-neomenthyl, (-)-menthyl, and (-)-phenylmenthyl. Theligands are prepared in three steps from known chiral cyclopentadienes. Metalation of the chiral dienes followed by condensation with Me₄C₅Si(CH₃)₂Cl 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 Me₂SiCp″(R*Cp)LnCl₂Li(OEt₂) ₂ complexes in high yield. For (R)-Me₂-SiCp″[(+)-neomenthylCp]Lu(μ-Cl) ₂Li(OEt₂)₂: space group P2₁2₁2₁; a= 12.240(2), and b= 12.876(2), and c = 24.387(5) Å (24 °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) ⇌ (S) equilibrium constant in THF yields ΔH= 1.7 ± 0.3 kcal/mol and ΔS = 3.6 ± 0.8 eu for Me₂SiCp″[(+)-neomenthylCp)Lu(μ-Cl)₂Li(OEt ₂)₂ and ÄÇ= 4.8 ±0.5 kcal/mol and ΔS = 13.4 ± 0.5 eu for Me₂SiCp″[(-)-menthylCp]Sm(μ-Cl)₂Li(OEt ₂)₂. The mechanism is proposed to involve reversible ring detachment to an intermediate LiCpR* complex. Alkylation or amidation with ME(SiMe₃)₂ (M = Li or K, E = CH; M = Na or K, E = N) yields the corresponding chiral hydrocarbyls and amides in high yield. For (R/S)-Me₂SiCp″[(+)-neomenthylCp]YCH(SiMe₃) ₂: space group P2₁; a = 19.178(4), b = 8.736(1), and c = 21.391(5) Å; β = 97.62(2); Z = 4; R(F) = 0.071. For (R)-Me₂SiCp″[(-)-menthylCp]SmCH(SiMe₃)₂: space group P1; a = 8.993(3), b = 12.738(2), and c = 16.549(4) Å; α = 86.04(2)°; β= 82.81(2)°, γ= 72.91(2)°; Z = 2; R(F) = 0.026. For (R)-Me₂SiCp″[(-)-menthylCp]YCH(SiMe₃)₂; space group P2₁; a = 12.319(3), b = 15.707(4), and c = 18.693(5) Å; β= 91.59(2)°; Z = 4; R(F) = 0.054. For (S)-Me₂SiCp″[(+)-neomenthylCp]SmN(SiMe₃) ₂: space group P2₁; a = 9.122(2), b = 10.112(3), and c = 18.478(3) Å; β= 90.58(2)°; Z = 2; R(F) = 0.029. For (S)-Me₂-SiCp″[(-)-menthylCp]SmN(SiMe₃)₂: space group P2₁2₁2₁; a = 10.217(3), b = 19.103(6), and c = 19.456(7) Å; Z = 4; R(F) = 0.044. For (R)-Me₂SiCp″[(-)-menthylCp]YN(SiMe₃)₂: space group P1; a = 8.937(3), b = 12.397(6), and c = 16.673(7) Å; α = 85.53(2)°, β = 82.17(2)°, γ = 74.78(2)°; Z = 2; R(F) =0.065. The preferred planar chiral configurations of these complexes can be largely understood on the basis of significant, crystallographically identifiable, nonbonded interactions between R* and the remainder of the molecule. The hydrocarbyl and amide complexes are configurationally stable in toluene at 60 °C for many hours but undergo facile epimerization in the presence of primary alkyl amines, presumably via reversible Cp protonation/detachment. The hydrocarbyl complexes undergo rapid hydrogenolysis at ambient temperature, with retention of configuration, to yield the corresponding hydrides.