Well‐Defined Silica‐Supported Calcium Reagents: Control of Schlenk Equilibrium by Grafting

Abstract

Calcium reagents Ca(alpha-Me(3)Si-2-Me(2)N-benzyl)(2) x 2 thf (1) and Ca[N(SiMe(3))(2)](2) x 2 thf (2) reacted with silica partially dehydroxylated at 700 degrees C to afford materials that bear ([triple bond]SiO)Ca(alpha-Me(3)Si-2-Me(2)N-benzyl) x 1.6 thf (SiO(2)-1) and ([triple bond]SiO)Ca [N(SiMe(3))(2)] x 1.3 thf (SiO(2)-2) fragments, respectively. Due to the bulk of the supported complexes, the silanol groups are only partially metalated: 50% in SiO(2)-1 and 70% in SiO(2)-2. In the case of SiO(2)-2, a parallel SiMe(3)-capping side reaction affords in fine a silanol-free surface. The materials were characterized by IR spectroscopy, 1D and 2D solid-state high-field NMR spectroscopy, and elemental analysis. Reaction of 2 with one equivalent of the bulky silanol (tBuO)(3)SiOH, a silica-surface mimic, afforded the homoleptic bis-silyloxide calcium derivative through ligand exchange (Schlenk equilibrium), and a derivative was isolated and structurally characterized. Preliminary studies have shown that both grafted benzyl and amide derivatives are active in olefin hydrosilylation, intramolecular hydroamination, and styrene polymerization, with evidence showing that catalysis occurs through supported species. In styrene polymerization, a marked influence of the surface acting as a ligand on the stereoselectivity of the reaction was observed, as syndiotactic-rich polystyrene (88% of r diads) was obtained. These results illustrate that grafting of calcium benzyl or amide compounds on a silica surface is a new concept to prevent ligand exchange through the Schlenk equilibrium. Heteroleptic calcium complexes that cannot be synthesized as stable molecular species in solution can be obtained as silica-supported species which have been shown to be catalytically active.

Surface Organobarium and Organomagnesium Chemistry on Periodic Mesoporous Silica MCM‐41: Convergent and Sequential Approaches Traced by Molecular Models

“…The solid-state structure features the trinuclear heteroleptic silylamido/siloxide barium cluster Ba 3 Figure 6). Contrary to the synthesis of CaA [15] dissolved complex 4 is not subject to the Schlenk equilibrium. Cluster 4 has a cube-like core with alternating barium (3 ) and nitrogen(silylamido, 1 )/oxygen(siloxide, 3 ) atoms in the apices; one of the metal sites is vacant.…”

“…[3,4] More recently, Gauvin et al reported on the equimolar reaction of www.chemeurj.org 3 ] 2 , due to the Schlenk equilibrium prevailing in solution. [15] They also emphasized the importance of grafting as a tool for the synthesis of heteroleptic calcium surface complexes of the type…”

“…[15] The resulting heterogeneous catalysts were successfully employed for the hydrosilylation of styrene, cyclohexadiene, and 1,1-diphenylethylene with PhSiH 3 .…”

Heterofunctionalization catalysis with organometallic complexes of calcium, strontium and barium

“…Characterized by IR spectroscopy and one-and two-dimensional solid-state NMR spectroscopy, these materials proved catalytically active for the hydrosilylation of styrene, 1,3-cyclohexadiene and 1,1-diphenylethylene with PhSiH 3 , albeit with lower activities than those reported for homogeneous catalysis, particularly for the hydrosilylation of 1,1-diphenylethylene. While it is unclear whether the active catalyst is bound to the silica surface, the authors suggested that the sensitivity of the reaction to substrate size is an indication of a heterogeneous process (Gauvin et al 2009). …”

Nützliche Alkalimetallamide für die Synthese: Lithium‐, Natrium‐ und Kaliumhexamethyldisilazide, ‐diisopropylamide und ‐tetramethylpiperidide

“…[95] Unterdessen freuen wir uns auf Fortschritte auf dem nahe verwandten, wichtigen Gebiet der Erdalkalimetallderivate, die zurzeit auch im Hinblick auf Anwendungen von mehreren Gruppen weltweit erforscht werden, allen voran denjenigen von Harder, [96] Hill [97] und Westerhausen. [98] Viele hervorragende Chemiker, deren Namen in den relevanten Literaturzitaten erscheinen, haben zur Entwicklung und Ausarbeitung dieser unschätzbaren Reagentien beigetragen.…”