Silicon oxide fluorides, preparation

A detailed study14 of the stereochemistry of fluoro-desilylation (Table 5) showed that the removal of silicon is a stereospecific process occurring with retention of configuration. Accordingly, from the E-isomers of thioacylsilane 5-oxides 30, the Z-thioaldehyde 5-oxides 40 were obtained. The myrtanyl derivative (entry 9) is the first enantiomerically pure thioaldehyde 5-oxide ever prepared. In contrast, loss of stereochemical integrity was observed during the desilylation of tert-butyl trimethylsilyl sulfine (entries 4 and 5) and mesityl-trimethylsilylsulfine (entries 10, 11, and 12). It was demonstrated that the loss of stereospecificity results from a fluoride-induced equilibration of thioaldehyde 5-oxides after the desilylation. 

The simplest oxide fluoride of silicon, SiOp2, is known it is prepared by the high-temperature reaction of silica with Cap2 in an argon atmosphere. The other known oxide fluorides of silicon are a series of fluorosiloxanes, SiF30(SiF20) Sip3, of which Si2 0F6 is the best known. 

At the turn of the 18th century many scientists believed that silica, or silica earth, contained an unknown chemical element and tried to isolate it in a free state. H. Davy attempted to decompose silica with an electric current—the method by which a number of alkali metals had already been prepared—but without success. The scientist s attempt to prepare free silicon by passing metallic potassium vapour over red-hot silicon oxide also failed. In 1811 L.J. Gay Lussac and L. Thenard applied themselves to the problem. They observed a vigorous reaction between silicon tetra-fluoride and metallic potassium a reddish brown compound was formed in the reaction. The scientists could not reveal the nature of the product most likely, it was contaminated amorphous silicon. 

Our synthesis is based on the hydrolysis of a silicon alkoxide (TEOS Si(OCH2CH3)4) in a diluted solution of nonionic polyethylene oxide-based surfactants. The hydrolysis is then induced by the addition of a small amount of sodium fluoride [5], Depending on the initial mixing conditions, the size of the solubilized objects leads to either a colorless or milky emulsion. Small particles ( 300 nm) with a 3D worm-hole porous structure or small hollow spheres with mesoporous walls, are usually obtained [6]. The synthesis we report herein after exhibits an apparently slight but actually drastic change in the preparation conditions. The main feature of this approach is an intermediate step that utilizes a mild acidity (pH 2 - 4), in which, prior to the reaction, a stable colorless microemulsion containing all reactants is... 

The usual method of preparing boron trifluoride from calcium fluoride, boric oxide, and sulfuric acid is unsatisfactory, since yields are low, much silicon fluoride contaminates the product, and a solid residue which is difficult to remove remains in the generator. 

Hydrofluoric acid — (HF) A solution of hydrogen fluoride in water. The pure hydrogen fluoride is characterized by Mw of 20.0063 gmol-1 m.p. -83.55 °C (1 atm) b.p. 19.5 °C (latm). When concentrated, this colorless fuming liquid is extremely corrosive and can dissolve almost all inorganic oxides such as silicate compounds or oxides of metals like stainless steel, aluminum, and uranium however, it can be stored in casted iron bottles because a corrosion-resistant iron fluoride layer protects the metal. It is used for several purposes such as the preparation of titanium oxide nano tube arrays [i], silicon nanoparticles [ii] and electrochemical etching of silicon [iii], electrochemical deposition of lithium [iv], etc. 

Trimethyl(trifluoromethyl)silane can be converted into tributyl(trifluoromethyl)tin (40) in quantitative yield by treatment with bis(tributyltin) oxide in tetrahydrofuran under fluoride ion initiation. Tributyl(trifluoromethyl)tin readily trifluoromethylates disilyl sulfides at room temperature giving the corresponding trifluoromethylated di- and trialkylsilancs 41 in high yields. This method provides a new approach for the preparation of silicon-based trifluoro-methylating reagents via transtrifluoromethylation of compounds containing Si — S bonds. 

Metallurgy. — The metals of most of the cerium group elements have been prepared, three general methods having been used t (1) fusion of the anhydrous halides with sodium, potassium, calcium, or aluminium (2) electrolysis of the fused chlorides or of a solution of the oxide in the molten fluoride (3) heating the oxides with magnesium, calcium, or silicon. Reduction with aluminium has also been tried, but it is not satisfactory except possibly for cerium itself. Electrolysis has been the most successful, the other methods usually giving at best an alloy. 

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