Reduction of ketones with hydrocarbon-soluble calcium hydride: Stoichiometric reactions and catalytic hydrosilylation

Reactions of the dimeric calcium hydride complex [(DIPP-nacnac)CaH center dot thf](2) {1; DIPP-nacnac = CH[(CMe)(2,6-iPr(2)C(6)H(3)N)](2)} with the a-hydrogen containing ketones acetophenone, acetone, dibenzylketone and 2-adamantone are smooth. In most cases not only addition but also substantial enolization is observed as a side reaction and in some cases also aldol condensation was found. Despite this unselectivity, the addition products could be isolated crystalline pure. Crystal structures of [(DIPP-nacnac)CaOCH(Me)Ph](2) (3), [(DIPP-nacnac)CaOCH(CH(2)Ph)(2)](2) (6) and [(DIPP-nacnac)Ca(2-adamantoxide)](2) (7) have been determined. The calcium hydride complex 1 is an effective catalyst in the hydrosilylation of ketones. Independent from the silane/ketone ratio, a strong preference for formation of bis-alkoxy silanes [PhSiH(OR)(2)] is observed. In most cases no enoxy groups have been found in the product. This indicates that the mechanism does not involve addition of the calcium hydride to the ketone functionality. A concerted addition of silane to ketone through a six-coordinate hypervalent silicon intermediate is proposed. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).