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

Abstract

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(C5H4SiPh(OBn)2) (2), was prepared and fully characterized from the ferrocenylsilane, FeCp(C5H4SiPhH2) (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 [(Ph3CO)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(C5H4SiPhH2)2 (4) with diols 1,4‐(HOCH2)2‐(C6H4) and 1,4‐(CH(CH3)OH)2‐(C6H4), 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.

Molecular Main Group Metal Hydrides

“…The dehydrocoupling of silanes and alcohols mediated by heavier alkaline-earth catalysts, [M­{N (SiMe 3 ) 2 } 2 ·(THF) 2 ] and [M­{CH­(SiMe 3 ) 2 } 2 ·(THF) 2 ] (M = Ca, Sr, and Ba) has recently been described . Primary, secondary, and tertiary alcohols were coupled to phenylsilane or diphenylsilane, whereas tertiary silanes were less tolerant towards bulky substrates, while turnover frequencies were found to increase on descending Group 2, with the barium amide pre-catalyst displaying an apparent first-order dependence in both silane and alcohol.…”

Phosphinoborane interception at magnesium by borane-assisted phosphine-borane dehydrogenation

“…Our own interest in this topic is motivated by the use of alkaline-earth (Ae) elements as non-toxic and inexpensive catalysts for the synthesis of inorganic polymers. 15,16 Following previous investigations into Ae-mediated catalytic and stoichiometric amine-borane dehydrogenation and dehydrocoupling, [17][18][19][20][21][22][23][24][25][26] we recently reported a series of β-diketiminate (BDI) supported alkaline-earth (Ae) phosphidoborane complexes, which are prepared by exposing diphenylphosphine-borane to readily accessible BDI-Ae hydride, alkyl, or amide precursors (Scheme 2a). 27 Attempts to achieve complete phosphine-borane dehydrogenation were unsuccessful, however, and the use of super-stoichiometric quantities of phosphine-borane resulted in BH 3 transfer to provide the phosphinodiboronate complex 2 and uncomplexed diphenylphosphine (Scheme 2a).…”

Organocalcium Complex-Catalyzed Selective Redistribution of ArSiH3 or Ar(alkyl)SiH2 to Ar3SiH or Ar2(alkyl)SiH

“…The development of calcium catalysts has lagged far behind that of transition-metal catalysts, which is mainly associated with the following features: (1) the divalent calcium ion lacks the d-electrons, which are perceived as crucial for catalytic reactions and (2) the calcium alkyl and hydrido complexes, which are the best candidates for catalysis, are rare. − The groups of Harder, Hill, Okuda, and others have made some pioneer contributions in developing organocalcium complexes as catalysts. They synthesized several calcium alkyl, hydrido, and amido complexes and demonstrated their efficiency as catalysts in organic synthesis. − However, the reported reaction types catalyzed by organocalcium complexes remain limited and are mainly focused on the hydrogenation or hydroelementation of alkenes (or alkynes), dehydrocoupling of aminoboranes, and dehydrocoupling of silanes with terminal alkynes (or amines).…”