Fluoride, tetrabutylammonium reaction with silanes

Fluoride ion-catalysed addition of trifluoromethyltrimethylsilane to acyl silanes occurs to give l,l-difluoro-2-trimethylsilyloxyalkenes (silyl enol ethers of difluoromethyl ketones), through nucleophilic addition of trifluoromethyl anion, Brook rearrangement and loss of fluoride. These compounds could be isolated when tetrabutylammonium difluorotriphenylstannate was used as a catalyst use of tetrabutylammonium fluoride gave the product corresponding to subsequent aldol reaction with the difluoromethyl ketone (Scheme 78)m. 

Silyl enol ethers of acyl silanes have been used in Lewis acid-mediated Mukaiyama reactions with acetals. Treatment of the resulting /1-alkoxy acyl silanes with tetrabutylammonium hydroxide or tetrabutylammonium fluoride gave the corresponding a,/J-unsaturated aldehydes (Scheme 99)210. 

The authors found that phenylsilane/anhydrous cesium fluoride was the preferred reagent combination for this reaction sequence. Other silanes were effective, whereas tributyltin hydride afforded low yields (< 20%) and only in the presence of a fluoride source. Tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF) and tetrabutylammonium fluoride (TBAF) destroyed the silane. Acetonitrile was the best solvent among the ethers and halogenated solvents evaluated." In a subsequent study " the authors converted a 2,5-disubstimted oxazole 15 (Rl = R3 = CH3, CeHs, or OC2H5, R2 = H) to the oxazolium salt followed by treatment with trimethylsilylcyanide (Scheme 1.290). The resulting intermediate 2-cyano-4-oxazoline 1107 spontaneously ring opened to the stabilized azomethine... 

The resultant alkenyl(2-pyridyl)silanes can be subjected to further transformations. For example, the treatment of alkenyl (2-pyridyl)silanes with aryl iodides in the presence of tetrabutyl-ammonium fluoride and palladium catalyst affords substituted oleflns in good yields (eq 3). This cross-coupling reaction is suggested to proceed through the intermediacy of a highly reactive alkenylsilanol. The treatment of alkenyl(2-pyridyl)silanes with tetrabutylammonium fluoride effects simple protodesilyl-ation (eq The reaction with acetyl chloride in the presence of aluminum chloride affords the corresponding Q ,/3-unsaturated enones (eq 5). ... 

Reaction of allylic silanes with aldehydes and ketones can also be induced by fluoride ion, which is usually supplied by the THF-soluble salt tetrabutylammonium fluoride (TBAF). Fluoride adds at silicon to form a hypervalent anion with much enhanced nucleophilicity.73 An alternative reagent to TBAF is tetrabutylammonium triphenyldi-fluorosilicate.74... 

Acid halides, such as benzoyl chloride, also react with trimethyl(trifluoromethyl)silane to give a mixture of products 20 and 21. The reaction requires the presence of more than one molar equivalent of tetrabutylammonium fluoride. ... 

No reaction between triiiiethyl(trifluoromethyl)silane and sulfur dioxide occurs in the absence of an anionic initiator when one molar equivalent of tetrabutylammonium fluoride is used, clean foriiiiition of tetrabutylammonium trilluoromethancsullinatc is observed." Although oxidation to the corresponding sulfonate occurred readily upon treatment with 30% hydrogen peroxide, attempts to liberate the free acid from the salt proved unsuccessful. When sodium trimcthylsilanolate was used as the initiator, however, the reaction sequence was successful. The Overall yield of trifluoromethancsulfonic acid (31) was about 30%. 

Hexyl trifluoroacetate reacts with trimethyl(trifluoroniethyl)silane in the presence of a molar equivalent of tetrabutylammonium fluoride to give the silylated hemiketal 29 in 35% yield. Much of the trimethyl(trifluoromcthyl)silanc is converted into the undesired trifluoromethane due to rapid quenching of the incipient trifluoromethide species by the protic impurities in the reaction mixture.71... 

The cyclopropenes were distilled directly from the reaction mixture and were characterized by trapping as Diels-Alder adducts with 1,3-diphenylisobenzofuran (X = Br, 77%, mp 107-109"C X = Cl, 54%, mp 104-106 C). The same method was used to generate the unstable 7-chlorobicyclo[4.1.0]hept-l(7)-ene which was trapped in situ by 1,3-diphenylisobenzofuran. Reaction of the silanes 2 with tetrabutylammonium fluoride in tetrahydrofuran gave 1,2-dibromocyclopropene (3a) or 1-bromo-2-chlorocyclopropene (3b) ... 

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