Reduction of Ketones with Hydrocarbon‐Soluble Calcium Hydride: Stoichiometric Reactions and Catalytic Hydrosilylation

Abstract

Reactions of the dimeric calcium hydride complex[(DIPP‐nacnac)CaH·thf]2 {1; DIPP‐nacnac = CH[(CMe)(2,6‐iPr2C6H3N)]2} with the α‐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(CH2Ph)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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Ring‐Shaped Phosphinoamido‐Magnesium‐Hydride Complexes: Syntheses, Structures, Reactivity, and Catalysis

“…We were also interested in the stoichiometric reactivity of non‐enolisable ketones with 5 . Previously, Group 2 metal‐hydride complexes have been reported to undergo reductions of ketones,,,,–, including older reports of less well‐defined MgH species. The reaction of four equivalents of 2‐adamantanone (2‐AdO) with 5 mainly yielded the hydromagnesiated complex 14 [(L 2 Mg 2 (2‐AdOH) 2 (2‐AdO)] (Figure ).…”

“…Thus, we attempted the hydrosilylation of our previous two substrates with PhSiH 3 in varying stoichiometries in the presence of catalytic amounts of 5 . Initial studies revealed that treatment of benzophenone with phenylsilane in C 6 D 6 at 70 °C with 1.5 mol % of compound 5 produces the dialkoxysilane, Ph(H)Si(OCHPh 2 ) 2 , as the major product, which was also observed for hydrosilylations of ketones using a calcium‐hydride catalyst . The silane/ketone ratio was therefore chosen to be approximately 1:2 for all further reactions (Table , entries 7–10).…”

Calcium–Amidoborane–Ammine Complexes: Thermal Decomposition of Model Systems

“…Apart from inter-and intramolecular NH···HB interactions, the amidoborane anion NH 2 BH 3 À was also involved in an agostic BH···Ca 2+ interaction. Thermal decomposition of a solution of Ca(DIPPnacnac)- [11] Solvent-free dimers were only observed with larger bridging groups (X = C CPh, [18] NHBn, [19] OCHPh 2 , [20] OSiMe 3 [21] ). The fact that this complex is free of donors demonstrates the relatively weak coordinating ability of the NH 3 ligand.…”

Synthesis and characterization of rare examples of stable potassium and arylcalcium triethylboranate complexes

“…Due to the fact that solid CaH 2 exhibits a very low reactivity numerous attempts were undertaken to prepare soluble calcium derivatives containing a reactive Ca-H bond. Harder and co-workers demonstrated that R-Ca-H is a valuable synthon [5] for the hydrogenation of alkenes [6], an intermediate during hydrosilylation of ketones [7], and reacts also as a strong reducing reagent [8]. Phenylcalcium hydride (R = Ph) was prepared via a cocondensation reaction of calcium with benzene yielding extremely reactive PhCaH [9,10].…”