Coordinative unsaturation in chiral organolanthanides. synthetic and asymmetric catalytic mechanistic study of organoyttrium and -lutetium complexes having pseudo-meso Me₂Si(η⁵-RC₅H₃)(η ⁵-R*C₅H₃) ancillary ligation

As established by NMR, circular dichroism, and X-ray diffraction, organolanthanide complexes of the new chelating ligand Me₂Si(3-Me₃SiCp)[3-(-)-menthylCp]^2- (Cp = η⁵-C₅H₃) preferentially adopt a single planar chiral configuration of the asymmetric metal-ligand template. Chloro complexes (S,R)-Me₂Si(Me₃SiCp)[(-)-menthylCp]Ln(μ-Cl) ₂Li(OEt₂)₂ (Ln = Y, Lu) were isolated diastereomerically pure by crystallization from diethyl ether. The unusual pseudo-meso configuration leads to a gross distortion from ideal C_2v symmetry, evidenced by a significant deviation of 〈Si_bridge-Lu-Li from linearity (158°). At least two additional epimers are detected in THF solution. Alkylation of the (S,R) epimers with LiCH-(SiMe₃)₂ proceeds with retention of configuration, affording chiral hydrocarbyl complexes in quantitative yield. In solution, the hydrocarbyls exhibit temperature-dependent conformational exchange processes in the NMR ascribable to restricted rotation about the Ln-CH-(SiMe₃)₂ bond. These complexes are effective precatalysts for asymmetric hydrogenation of unfunctionalized olefins and for the reductive cyclization of 1,5-dienes. The highest enantioselectivities are obtained when the Lu complex is used for hydrogenation of 2-phenyl-1-butene (45% ee) and deuteration of styrene (10% ee) and 1-pentene (30% ee). The hydrogenation of 2-phenyl-1-butene with the Y catalyst (yielding exclusively 2-phenylbutane-1,2-d₂ under D₂) obeys a rate law of the approximate form v = (k[olefin]¹[lanthanide]^1/2-[H₂]¹)/(K + [olefin]), suggesting rapid, operationally irreversible olefin insertion at a putative hydride, a rapid preequilibrium involving an alkyl or alkyl/hydride dimer, and turnover-limiting hydrogenolysis of an intermediate yttrium alkyl with v_H2/v_D2 = 2.2 ± 0.1. The apparent rate constant for 2-phenyl-1-butene hydrogenation (12(1) × 10^-3 M^1/2 atm^-1 s^-1) is ca. 1 order of magnitude lower than for chiral Me₂Si(Me₄C₅)(3-R*Cp)Ln-based systems (R* = (-)-menthyl, (+)-neomenthyl; Ln = Y, La, Nd, Sm, Lu), principally reflecting diminished Ln-C bond hydrogenolytic reactivity.