Regioselection and Enantioselection in Organolanthanide-Catalyzed Olefin Hydrosilylation. A Kinetic and Mechanistic Study

This contribution describes a study of scope, regioselection, enantioselection, metal and ancillary ligand effects, and kinetics in the catalytic PhSiH₃ hydrosilylation of olefins using the organolanthanide precatalysts Cp’₂- LnCH(SiMe₃)₂, Me₂SiCp”₂LnCH(SiMe₃)₂, and Me₂SiCp”(R*C₅H4)LnCH(SiMe₃)₂ (Cp’ = η⁵-Me₅C₅; Cp” = η⁵- Ln = lanthanide; R* = chiral auxiliary). Sluggish catalyst initiation processes were first circumvented by hydrogenolysis of the Ln—CH(SiMe₃)₂ functionality. For a-olefins, hydrosilylation turnover frequency and selectivity for 2,1 addition regiochemistry are enhanced by openness of the metal ligation sphere (Cp’₂Ln → Me₂SiCp”₂, Me₂-SiCp”(R*C₅H₄)) and increasing Ln³⁺ ion radius. For styrenic olefins, complete 2,1 regioselectivity (Si delivery to the benzylic position), rate enhancement by para electron-releasing substituents, and turnover frequencies as high as 400 h^-1 (60 °C) are observed. For 1-hexene, 2,1 addition regioselectivities as high as 76% and turnover frequencies >1000 h^-1(90 °C) are observed. For 2-phenyl-1-butene, (R)-Me₂SiCp“[(—)-menthyl Cp]SmCH(SiMe₃)₂ and (S)-Me₂SiCp”[(-)-menthylCp]SmCH(SiMe₃)₂effect asymmetric hydrosilylation with ee values of 68% and 65%, respectively (25 °C). The former reaction obeys the rate law v = k[Sm]¹[olefin]⁰[PhSiH₃]¹. D₂0 quenching of the reaction yields PhCD(CH₃)(CH₂CH₃) and PhSiH₂D as mechanistically informative products. The hydrosilylation of 1,5-hexadiene effected by Cp’₂SmCH(SiMe₃)₂ affords predominantly cyclopentylCH₂SiH₂Ph, while Me₂SiCp”₂SmCH-(SiMe₃)₂and (R)-Me₂SiCp^,/[(-)-menthylCp]SmCH(SiMe₃)₂also yield hydrosilylation products derived from 1,5-hexadiene skeletal rearrangement. The hydrosilylation mechanism is discussed in terms of a hydride/alkyl cycle involving rapid, exothermic olefin insertion into an Ln-H bond followed by turnover-limiting Si-H/Ln-alkyl transposition (delivery of the alkyl group to Si).