Coordinative unsaturation in chiral organolanthanides. Synthetic and asymmetric catalytic mechanistic study of organoyttrium and -lutetium complexes having pseudo-meso Me(2)Si(eta(5)-RC(5)H(3))(eta(5)-R(*)C(5)H(3)) ancillary ligation

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 = eta(5)-C5H3) 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(mu-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-2v symmetry, evidenced by a significant deviation of angle Si-bridge-Lu-Li from linearity (158 degrees). 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(H2)/v(D2) = 2.2 +/- 0.1. The apparent rate constant for 2-phenyl-1-butene hydrogenation (12(1) x 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.