Heavier Alkaline-Earth Catalyzed Dehydrocoupling of Silanes and Alcohols for the Synthesis of Metallo-Polysilylethers

The dehydrocoupling of silanes and alcohols mediated by heavier alkaline-earth catalysts, [Ae{N(SiMe₃ )₂ }₂ ⋅(THF)₂ ] (I-III) and [Ae{CH(SiMe₃ )₂ }₂ ⋅(THF)₂ ], (IV-VI) (Ae=Ca, Sr, Ba) is described. Primary, secondary, and tertiary alcohols were coupled to phenylsilane or diphenylsilane, whereas tertiary silanes are less tolerant towards bulky substrates. Some control over reaction selectivity towards mono-, di-, or tri-substituted silylether products was achieved through alteration of reaction stoichiometry, conditions, and catalyst. The ferrocenyl silylether, FeCp(C₅ H₄ SiPh(OBn)₂ ) (2), was prepared and fully characterized from the ferrocenylsilane, FeCp(C₅ H₄ SiPhH₂ ) (1), and benzyl alcohol using barium catalysis. Stoichiometric experiments suggested a reaction manifold involving the formation of Ae-alkoxide and hydride species, and a series of dimeric Ae-alkoxides [(Ph₃ CO)Ae(μ₂ -OCPh₃ )Ae(THF)] (3 a-c, Ae=Ca, Sr, Ba) were isolated and fully characterized. Mechanistic experiments suggested a complex reaction mechanism involving dimeric or polynuclear active species, whose kinetics are highly dependent on variables such as the identity and concentration of the precatalyst, silane, and alcohol. Turnover frequencies increase on descending Group 2 of the periodic table, with the barium precatalyst III displaying an apparent first-order dependence in both silane and alcohol, and an optimum catalyst loading of 3 mol % Ba, above which activity decreases. With precatalyst III in THF, ferrocene-containing poly- and oligosilylethers with ferrocene pendent to- (P1-P4) or as a constituent (P5, P6) of the main polymer chain were prepared from 1 or Fe(C₅ H₄ SiPhH₂ )₂ (4) with diols 1,4-(HOCH₂ )₂ -(C₆ H₄ ) and 1,4-(CH(CH₃ )OH)₂ -(C₆ H₄ ), respectively. The resultant materials were characterized by NMR spectroscopy, gel permeation chromatography (GPC) and DOSY NMR spectroscopy, with estimated molecular weights in excess of 20,000 Da for P1 and P4. The iron centers display reversible redox behavior and thermal analysis showed P1 and P5 to be promising precursors to magnetic ceramic materials.

A Modern Twist to a Classic Synthetic Route: Ph3Bi-Based Redox Transmetalation Protolysis (RTP) for the Preparation of Barium Metalorganic Species

This paper reports advances in redox transmetalation/protolysis (RTP) utilizing the readily available Ph₃Bi for the synthesis of a series of barium metal-organic species. On the basis of easily available starting materials, an easy one-pot procedure, and workup, we have obtained BaL₂ compounds (L = bis(trimethylsilyl)amide, phenyl(trimethylsilyl)amide, pentamethylcyclopentadienide, fluorenide, 2,6-di-isopropylphenolate, and 3,5-diphenylpyrazolate) quantitatively by sonication of an excess of barium metal with triphenylbismuth and HL in perdeuterotetrahydrofuran, as established by NMR measurements. Rates of conversion are affected by both pK_a and bulk of HL. Competition occurs from direct reaction of Ba with HL, thereby enhancing the overall conversion, the effect being pronounced for the less bulky and more acidic ligands. Overall, the method significantly adds to the synthetic armory for barium metal-organic/organometallic compounds.