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 PhSiH(3) hydrosilylation of olefins using the organolanthanide precatalysts Cp'(2)-LnCH(SiMe(3))(2), Me(2)SiCp''(2)LnCH(SiMe(3))2, and Me(2)SiCp''(R*C5H4)LnCH(SiMe(3))2 (Cp' = eta(5) - Me(5)C(5); Cp'' = eta(5)-Me(4)C(5); Ln = lanthanide; R* = chiral auxillary). Sluggish catalyst initiation processes were first circumvented by hydrogenolysis of the Ln-CH(SiMe(3))(2) functionality. For alpha-olefins, hydrosilylation turnover frequency and selectivity for 2,1 addition regiochemistry are enhanced by openness of the metal ligation sphere (Cp'(2)Ln --> Me(2)SiCp''(2), Me(2)-SiCp''(R*C5H4)) and increasing Ln(3+) 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 degrees C) are observed. For 1-hexene, 2,1 addition regioselectivities as high as 76% and turnover frequencies > 1000 h(-1) (90 degrees C) are observed. For 2-phenyl-1-butene, (R)-Me(2)SiCp''[(-)-menthyl Cp]SmCH(SiMe(3))2 and (S)-Me(2)SiCp''[(-)-menthylCp]SmCH(SiMe(3))(2) effect asymmetric hydrosilylation with ee values of 68% and 65%, respectively (25 degrees C). The former reaction obeys the rate law v = k[Sm](1) [olefin](0)[PhSiH(3)](1). D2O quenching of the reaction yields PhCD(CH3)(CH2CH3) and PhSiH(2)D as mechanistically informative products. The hydrosilylation of 1,5-hexadiene effected by Cp'2SmCH(SiMe(3))(2) affords predominantly cyclopentylCH(2)SiH(2)Ph, while Me(2)SiCp''2SmCH(SiMe(3))a and (R)-Me2SiCp''[(-)-menthylCp]SmCH(SiMe(3))(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).