Alkoxysilanes fluorides

Vinylsilane to copper transmetallation has entered the literature,93 93a,93b and a system suitable for catalytic asymmetric addition of vinylsilanes to aldehydes was developed (Scheme 24).94 A copper(l) fluoride or alkoxide is necessary to initiate transmetallation, and the work employs a copper(ll) fluoride salt as a pre-catalyst, presumably reduced in situ by excess phosphine ligand. The use of a bis-phosphine was found crucial for reactivity of the vinylcopper species, which ordinarily would not be regarded as good nucleophiles for addition to aldehydes. The highly tailored 5,5 -bis(di(3,5-di-tert-butyl-4-methoxyphenyl)phosphino-4,4 -bis(benzodioxolyl) (DTBM-SEGPHOS) (see Scheme 24) was found to provide the best results, and the use of alkoxysilanes is required. Functional group tolerance has not been adequately addressed, but the method does appear encouraging as a way to activate vinylsilanes for use as nucleophiles. 

It has been well recognized that the hydrolysis of alkoxysilanes and chlorosilanes is effectively catalyzed when fluoride anions are present due to formation of hypercoordinated silicon intermediates.803 More in-depth studies by Bassindale et al. showed that the reaction of PhSi(OEt)3 with stoichiometric amounts of Bu4NF surprisingly yields an encapsulation complex, namely tetrabutylammonium octaphenyloctasilsesquioxane fluoride 830, in which the fluorine atom is situated inside the cubic siloxane cage (Scheme 114). The Si--F distance of average 2.65 A is shorter than the sum of van der Waals radii (3.57 A), which renders the coordination number of the silicon atoms at [4+1]. 

The second method122 uses a polar aprotic medium, such as DMF or N-methylpyrrol idone, a strong base, such as CsOH or KH, and r-BuOOH to facilitate nucleophilic attack at silicon sometimes, the presence of fluoride ion is advantageous. This protocol also cleaves sterically hindered alkoxysilanes. It provided a key step (equation 32) in a total synthesis123 of ( )-lupinine. 

The hydrosilylation of carbonyl compounds with polymethylhydrosiloxane (PMHS) or other alkoxysilanes can be catalyzed by TBAF, at high efficiency [9]. The asymmetric version of this process has been developed by Lawrence and coworkers using chiral ammonium fluoride 7c prepared via the method of Shioiri [10]. The reduction of acetophenone was performed with trimethoxysilane (1.5 equiv.) and 7c (10 mol%) in THF at room temperature, yielding phenethyl alcohol quantitatively with 51% ee (R) (Scheme 4.6). A slightly higher enantioselectivity was observed in the reduction of propiophenone. When tris(trimethylsiloxy)silane was used as a hydride source, the enantioselectivity was increased, though a pro-... 

Alkoxysilanes, however, react with COFj (at 0-20%) in a similar way to that described for COClj to give silyl fluoride and alkyl fluoroformate derivatives [1452] in high yields ... 

Reaction of tiichloroacetaldehyde with hydrogen fluoride in the presence of chromium-based catalysts gives CFs-CHO.HF which, on treatment with an alkoxysilane (RO)4Si or with ROH-SiCU (R = Me, Et, Pr, or n-CsHu), yields the corresponding hemiacetals CF3-CH(OH)OR (69— 92%) the acetal I-----------------------1... 

A challenge for industrial syntheses of perfluoroalkanesulfonate tetraalkylammonium salts is the impurities present in technical perfluorooctanesulfonyl fluoride. These impurities cause side reactions that result in tar formation, thus lowering the production yield. One patent claims the preparation of the desired perfluoroalkanesulfonate tetraalkylammonium salts in excellent yields and good purities by reaction of the sulfonyl fluoride with alkoxysilanes (e.g., triethoxymethylsilane) and trial-kylamines (e.g., triethylamine) in a suitable organic solvent (e.g., chlorobenzene, ether, or toluene) as outlined in Scheme 18.1. Under these conditions, the ammonium salts crystallize from the reaction mixture, whereas the fluorosilane by-products remain dissolved in the organic solvent. 

While the five-membered cyclic alkoxysilane moiety has been found to tolerate a wide range of chemical transformations (OSO4 oxidation, Swern oxidation, Horner-Emmons alkenation, protection conditions such as Tr/py+ BF4 and Mel/NaH, and catalytic hydrogenation), the carbon-silicon bond can be cleaved efficiently by tetra- -butylammonium fluoride in DMF (eq 7). ... 

Startg. m. allowed to react with phenylphosphine tetrafluoride at -10 to 25° 2-chloro-l-fluoro-l-phenylethane (Y 95%) treated at 90° with excess piperidine l-fluoro-2-piperidino-l-phenylethane (Y 79%). F. e. s. R. Guedj, R. Nabet, and T. Wade, Tetrah. Let. 1973, 907 fluorides from alkoxysilanes s. a. D.U. Robert et al., Tetrahedron 29, 1877 (1973). 

Tetraethylammonium fluoride Sulfonic acid esters from alkoxysilanes... 

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