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).
ASJC Scopus subject areas
- Colloid and Surface Chemistry