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(SiMe 3)2 (Cp′ = η5 - Me5C5; Cp″ = η5-Me4C5; Ln = lanthanide; R* = chiral auxiliary). Sluggish catalyst initiation processes were first circumvented by hydrogenolysis of the Ln-CH(SiMe3)2 functionality. For α-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)2 effect 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. D2O 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)2 and (R)-Me2SiCp″[(-)-menthylCp]SmCH(SiMe3)2 also 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).
|Number of pages||12|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - Jul 12 1995|
ASJC Scopus subject areas