Coordinative Unsaturation in Chiral Organolanthanides. Synthetic and Asymmetric Catalytic Mechanistic Study of Organoyttrium and -lutetium Complexes Having Pseudo-mesoMe2Si(η5-RC5H3)(η5-R*C5H3) Ancillary Ligation1

As established by NMR, circular dichroism, and X-ray diffraction, organolanthanide complexes of the new chelating ligand Me 2 Si(3-Me 3 SiCp)[3-(−)-menthylCp] 2 - (Cp = η 5 -C 5 H 3 ) preferentially adopt a single planar chiral configuration of the asymmetric metal−ligand template. Chloro complexes ( S, R )-Me 2 Si(Me 3 SiCp)[(−)-menthylCp]Ln(μ-Cl) 2 Li(OEt 2 ) 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 2 v 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(SiMe 3 ) 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(SiMe 3 ) 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- d 2 under D 2 ) obeys a rate law of the approximate form v = ( k [olefin] 1 [lanthanide] 1/2 [H 2 ] 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 H 2 / v D 2 = 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 Me 2 Si(Me 4 C 5 )(3-R*Cp)Ln-based systems (R* = (−)-menthyl, (+)-neomenthyl; Ln = Y, La, Nd, Sm, Lu), principally reflecting diminished Ln−C bond hydrogenolytic reactivity.