The influence of the catalyst preparation protocol and silane structure on the rate and enantioselectivity of ansa-titanocene catalysed hydrosilation of prochiral ketones

The hydrosilation of prochiral ketones using catalysts prepared by alkylation of [1,2-bis(tetrahydroindenyl)ethane]titanium(IV)(1,1'-binaphth-2,2'-diolate) with MeLi and n-BuLi, and (EtO)(3)SiH, Me(EtO)(2)SiH, [MeSi(H)O](4), Me3SiO[MeSi(H)O](n)SiMe3 and MeSiH3 as the hydrosilane is described. The rates obtained with the MeLi based catalyst are one to two orders of magnitude faster than previously observed with MeLi based catalysts in the presence of MePhSiH2 and Ph2SiH2 and about the same as those observed with n-BuLi based catalysts. Me(EtO)(2)SM, [MeSi(H)O](4) and Me3SiO[MeSi(H)O](n)SiMe3 all undergo rapid redistribution in the presence of the catalyst to give MeSiH3, the actual hydrosilating agent in all three cases. Likewise, (EtO)(3)SiH redistributes to SiH4. The ee's for the hydrosilation product of acetophenone are consistently much higher (similar to 99%) for the n-BuLi based catalyst than for the MeLi based catalyst (similar to 40-50%). The hydride complex [(BTHLE)TiH](2) gives essentially the same enantioselectivity as the MeLi based catalyst. The ee's for a test set of dialkylketones are relatively insensitive to either the catalyst or the hydrosilane. Some possible mechansims that are consistent with the experimental results are discussed. (C) 1998 Elsevier Science S.A.