Silicon reaction with fluoride

Finally, alcohols can also be protected as silyl ethers. For example, the reaction of the alcohol with trimethylsilyl chloride in the presence of triethylamine (to react with the HC1 that is produced) produces the trimethylsilyl ether of the alcohol as shown in the following equation. (This reaction is a nucleophilic substitution by the oxygen on the silicon.) The silyl group can be removed in high yield by reaction with fluoride anion. 

Charge-transfer processes involving electrochemical reactions offer unique opportunities for the control of atomic scale structure, including pore formation. Peter Searson discusses recent advances in understanding the interaction of silicon surfaces with fluoride ions, processes on which the most important methods for surface modification and fabrication are based. 

Even if silicon chemistry is new to you, you should by now have a picture of stable compounds with C-Si bonds and selective reaction with fluoride. You are already familiar with silyl enol ethers as nucleophilic enolate equivalents and allyl silanes resemble these in many ways. The missing link is the i-silyl effect. A Si atom stabilises a cation in the p-posit ion by overlap of the populated and relatively high energy C-Si c-orbital with the empty p orbital of the cation. This overlap is already present in the preferred conformation 95a of the allyl silane 95 as an anti-bonding interaction 95b between the C-Si c-orbital and the n orbital of the double bond. The resultant molecular orbital (the new HOMO) 95c increases the nucleophilic reactivity of the carbon atom in the y-position. 

Tetrabutylammonium fluoride (TBAF) is usually used in the form of the trihydrate or as a solution in tetrahydrofuran (THF). The pure form is difficult to isolate, owing to decomposition to FFF, tributylamine, and but-l-ene [18, 19] on dehydration. It has been used for a variety of reactions, including as a catalyst for various reactions with silicon compounds [20, 21]. One of its main uses is in the cleavage of silyl ether protecting groups [22]. 

Because allyltrimethylsilane 82 or benzyltrimethylsilane 83 can be regarded as combinations of the hard trimethylsilyl cation and the soff allyl or benzyl anions, pyridine N-oxide 860 reacts with excess 82 or 83 in the presence of catalytic amounts of tetrabutylammonium fluoride di- or trihydrate in THF to give 2-allyl-or 2-benzylpyridines 948 and 950 [60]. The general reaction of silicon reagents such as 82 and 83 or of trimethylsilyl cyanide 18 with fluoride to generate allyl or... 

The covalently bonded solids such as silica cannot be easily broken by aqueous solutions. For example, the strong Si-O bonds silica is not dissolvable by boiling with concentrated acids except hydrofluoric acid because of the formation of silicon fluoride which is a gas and expels otherwise else it may form fluosilicic acid by reaction with water. 

Two procedures were developed for efficient isomerization (229—>230—>232). One procedure is based on the reaction of fluoride anion, which has pronounced selectivity with respect to the silicon atom (189, 207). Another procedure involves treatment of nitroso acetals (229) with methanol containing a catalytic amount of triethylamine (174, 216). 

Carreira and Kruger reported facile transmetallation of silicon enolates to other soft metal enolates including Gu derivatives.499 They reasoned that the use of soft metal fluoride complexes enabled silyl metal transmetallation with catalytic use of a soft metal source. The concept is illustrated in Scheme 103. Normal Lewis acid-catalyzed reactions of silicon enolates with aldehydes proceed via activation of aldehydes by carbonyl oxygen coordination to Lewis acids, as shown in the upper equation of Scheme 103. A key step for catalytic turnover is the desilyation of 233 by the... 

The sensing of fluoride with a reaction-based indicator can be based on its unique reactivity with silicon. As shown in Fig. 20, the O-Si bond cleavage with fluoride can be used to produce the coumarin dye 53. For this system, a 100-fold sensitivity increase was seen after incorporation of the reaction scheme into a conjugated polymer [143]. 

In acidic electrolytes with fluoride, silicon is stable at OCP, while electrochemical dissolution takes place for anodic potentials. For anodic current densities below the critical current density JPS PS is formed and the electrolyte-electrode interface is found to be Si-H covered. Species active in the dissolution process are HF, (HF)2 and HF2. A dissolution reaction proposed for this regime is ... 

These lithiooxiranes can be trapped by various electrophiles with retention of the configuration. The addition to aldehydes occurs with a low diastereoselectivity [but this can be enhanced by adding ClTi(OPr-/)3]. The reaction with enones occurs in a 1,2 fashion only. Intramolecular 1,4-silicon shift has also been reported. The reaction of the enantiomerically pure TMS-stabilized lithiooxirane 189 (Scheme 80) with an aldehyde has been used in a total synthesis of (-l-)-cerulenine. It must be noted that protodesi-lylation using TBAF (tetrabutylammonium fluoride) occurs with conservation of the oxirane stereochemistry. 

G. S. Serullas treated potassium chlorate with an excess of hydrofluosilicic acid the clear liquid was decanted from the sparingly soluble potassium fluosilicate, the soln. evaporated below 30°, and filtered through glass powder J. J. Berzelius evaporated the acid liquid mixed with finely divided silica below 30° in air, or over cone, sulphuric acid and potassium hydroxide in vacuo. The excess of hydrofluoric acid was volatilized as silicon fluoride, and the clear liquid was then filtered from the excess of silica. R. Bottger treated sodium chlorate with oxalic acid whereby sparingly soluble sodium oxalate was formed J. L. Wheeler, and T. B. Munroe treated sodium chlorate with hydrofluosilicic acid and M. Brandau treated potassium chlorate with aluminium sulphate and sulphuric acid and precipitated the alum so formed with alcohol. Chloric acid is formed in many reactions with hypochlorous and chlorous acid for example, it is formed when an aq. soln. of chlorine or hypochlorous or chlorous acid decomposes in light. It is also formed when an aq. soln. of chlorine dioxide stands in darkness or in light. A mixture of alkali chlorate and chlorite is formed when an aq. soln. of an alkali hydroxide is treated with chlorine dioxide. 

Aliphatic, aromatic, and alicyclic alkenes have been converted into the corresponding bromo fluorides 2 by reaction with l,3-dibromo-5,5-dimethylhydantoin (DBH) and silicon tetrafluoride in 1,4-dioxane at room temperature in a highly regio-, stereo- and chemoselective manner.9... 

Several aromatic and heterocyclic acyl trimethylsilanes have been used as acyl anion equivalents by treatment with fluoride ion (Scheme 81, path a)23 133 154b160191192. Provided that the acyl substituent is electron-withdrawing, and that there are no aryl substituents on the silicon atom, acyl anions can be trapped by various electrophiles in moderate to good yields indeed, acyl anions and pentacoordinate silicon anionic species have both been detected in gas-phase reactions of acyl silanes with fluoride ion193. 

Compounds 4.75 and 4.76 formally contain tetravalent silicon, which is not electron-deficient. It is, however, a strong Lewis acid, and on reaction with F , five-coordinate silicon compounds are obtained. In the case of 4.75, the fluoride anion is localised mainly on the boron atom, although it does display dynamic behaviour involving hopping between boron and silicon. Compound 4.76 as the KF adduct contains two five-coordinate silicon atoms that chelate the F anion. The coordination sphere of the K+ counter-ion is completed by a molecule of [18] crown-6 (Figure 4.32a). 

The photoanodic dissolution of n-silicon in acidic fluoride media provides an example of the complexity of multistep photoelectrochemical reactions [33, 34]. The reaction requires the transfer of four electrons, but it is clear that not all of the steps involve photogenerated holes because the photocurrent quantum efficiency is between 2 and 4. The explanation of the high quantum efficiencies is that the initial hole capture step can be followed by a series of steps in which intermediates with low electron affinity inject electrons into the conduction band. These intermediates can be assigned nominal oxidation states as shown in the following scheme. 

We should compare the S reaction at silicon with the S 2 reaction at carbon. There are some iportant differences. Alkyl halides are soft electrophiles but silyl halides are hard electrophiles. Alkyl halides react only very slowly with fluoride ion but silyl halides react more rapidly with fluoride [than with any other nucleophile. The best nucleophiles for saturated carbon are neutral and/or based on elements down the periodic table (S, Se, I). The best nucleophiles for silicon are charged and based on highly electronegative atoms (chiefly F, Cl, and O). A familiar example is the reaction of enolates at carbon with alkyl halides but at oxygen with silyl chlorides (Chapter 21). 

There are two fluorination reactions with ester substrates reported in the literature which consist of only a single example each. One is the formation of fluoroethene from acetoxyethene and lithium fluoride in the presence of palladium acetate. The other is formation of fluoro-ethane from ethyl orthoformate with silicon tetrafluoride. - ... 

Ammonio- and phosphoniohexafluorocyclobutanide ylides 11, prepared from pcrfluorocy-clobutene and tertiary amines or phosphanes, are soluble in a wide range of (even nonpolar) organic solvents and transfer fluoride, and therefore can be used as an easily soluble source of fluoride anions. Although they contain no ionic fluoride they react with fluoride anion acceptors such as silicon derivatives with elimination of thermodynamically favored fluoro-trimethylsilane to produce carbanions which can be quenched with suitable electrophiles. The reaction proceeds upon slightly heating the reactants in tetrahydrofuran or dimethylform-amide solution. 

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