Coordinative unsaturation in chiral organolanthanides: A synthetic and asymmetric catalytic mechanistic study of yttrium and lutetium Ansa-metallocene complexes having pseudo-meso ancillary ligation

As established by NMR, circular dichroism, and x-ray diffraction, organolanthanide complexes of the new chelating cyclopentadienyl-based ligand Me{dollar}sb2{dollar}Si(3-Me{dollar}sb3{dollar}SiCp)(3-({dollar}-{dollar})-menthylCp{dollar}rbrack sp{lcub}2-{rcub}{dollar} (Cp = {dollar}etasp5{dollar}-{dollar}rm Csb5Hsb3){dollar} (1) preferentially adopt a single planar chiral configuration of the asymmetric metal-ligand template. The chloro complexes (S,R)-Me{dollar}sb2{dollar}Si(Me{dollar}sb3{dollar}SiCp) (({dollar}-{dollar})-menthylCp) Ln({dollar}mu{dollar}-Cl){dollar}sb2{dollar}Li(OEt{dollar}sb2)sb2{dollar} (Ln = Y, Lu) (2) were isolated diastereomerically pure by crystallization from diethyl ether solution. For (S,R)-Me{dollar}sb2{dollar}Si(Me{dollar}sb3{dollar}SiCp) (({dollar}-{dollar})-menthylCp) Lu({dollar}mu{dollar}-Cl){dollar}sb2{dollar}Li(OEt{dollar}sb2)sb2{dollar}: space group P{dollar}sb1;{dollar} {dollar}a=11.580(4), b=11.756(2),{dollar} and {dollar}c=16.678(4){dollar} A; {dollar}alpha=108.93(2)spcirc, beta=94.52(2)spcirc, gamma=110.50(2)spcirc; Z=2; Rsb{lcub}rm w{rcub}=0.051.{dollar} The unusual pseudo-meso configuration of the complex leads to a gross distortion of the ideal C{dollar}sb{lcub}2v{rcub}{dollar} symmetry, evidenced by a significant deviation of {dollar}anglerm Sisb{lcub}bridge{rcub}{dollar}-Lu-Li from linearity {dollar}(158spcirc).{dollar} At least two additional epimers are observed in THF solution. Alkylation of the (S,R) epimers with LiCH(SiMe{dollar}sb3)sb2{dollar} proceeds with retention of configuration as determined by NMR and circular dichroism, affording chiral hydrocarbyl complexes (3) 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{dollar}sb3)sb2{dollar} bond. These alkyl complexes are effective precatalysts for asymmetric hydrogenation of unfunctionalized olefins and for the reductive cyclization of unfunctionalized 1,5-diolefins. The highest enantioselectivities are obtained when the lutetium complex is used for hydrogenation of 2-phenyl-1-butene (45% ee) and deuteration of 1-pentene (30% ee) and styrene (10% ee). The hydrogenation of 2-phenyl-1-butene with the yttrium catalyst (yielding exclusively 2-phenylbutane-1,2-d{dollar}sb2{dollar} under D{dollar}sb2){dollar} obeys a rate law of the approximate form {dollar}v=(klbrackrm olefinrbracksp1lbrack lanthaniderbracksp{lcub}1/2{rcub}lbrack Hsb2rbracksp1)/({lcub}it K/{rcub} + lbrack olefinrbrack),{dollar} suggesting rapid, operationally irreversible olefin insertion at a putative hydride (4), a rapid preequilibrium involving an alkyl or alkyl/hydride dimer, and turnover-limiting hydrogenolysis of an intermediate yttrium alkyl with {dollar}vrmsb{lcub}Hsb2{rcub}/vsb{lcub}Dsb2{rcub}=2.2pm0.1.{dollar} The apparent rate constant for 2-phenyl-1-butene hydrogenation in the present case {dollar}(12(1)times10sp{lcub}-3{rcub}{dollar} M{dollar}sp{lcub}1/2{rcub}{dollar} atm{dollar}sp{lcub}-1{rcub}{dollar} s{dollar}sp{lcub}-1{rcub}){dollar} is ca. one order of magnitude lower than those found for chiral Me{dollar}sb2{dollar}Si(Me{dollar}sb4{dollar}C{dollar}sb5{dollar})(3-R*Cp)Ln-based systems (R* = ({dollar}-{dollar})-menthyl, (+)-neomenthyl; Ln = Y, La, Nd, Sm, Lu), reflecting diminished Ln-C bond hydrogenolytic reactivity, which is apparently a result of decreased ligand electron-donating ability.