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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry