Coordinative Unsaturation in Chiral Organolanthanides. Synthetic and Asymmetric Catalytic Mechanistic Study of Organoyttrium and -lutetium Complexes Having Pseudo-<i>meso</i>Me₂Si(η⁵-RC₅H₃)(η⁵-R*C₅H₃) Ancillary Ligation¹

Abstract

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 C2v symmetry, evidenced by a significant deviation of ∠Sibridge−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 vH2/vD2 = 2.2 ± 0.1. The apparent rate constant for 2-phenyl-1-butene hydrogenation (12(1) × 10-3 M1/2 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.