Fluoride ion as catalyst

Alkynylsilanes will react with bis(tributyltin) oxide in the presence of fluoride ion as catalyst to give the stannylalkynes in excellent yield and hexamethyldisiloxane, which can be removed under reduced pressure (Equation (82)).68 244... 

Coumalic acid, 56, 51 Crotyl fluoride, 57,73 18-CROWN-6,57, 30 Curtius rearrangement, 59, 1 Cyanide ion, as catalyst for conjugate addition of aldehydes, 59, 56 p-Cyanobenzenesulfonyl cyanide, 57, 89 2-( 1 -Cyanocyclohexyljhydrazinecarboxylic acid methyl ester, 58,102 Cy a noferrocene, 56, 30 Cyanogen chloride, 57, 88... 

The addition of nitromethane to chalcones has been studied using N-benzylquininium chloride as the chiral phase-transfer catalyst and fluoride ion as the base (46). The enantiomeric excess was moderate (up to 26%). No conclusions were drawn from this study. 

A stereoselective Henry reaction has also been observed between bicyclic trimethylsilyl nitronates and benzaldehyde with fluoride ion as the catalyst. The reaction results in the formation in high yield of a cyclic hemiacetal (equation 20) and is highly diastereoselective (95% ds)P-... 

The allylation of electrophiles with allylsilanes can also be carried out in the presence of fluoride ion as the catalyst. In this case the reaction is initiated by attack of the fluoride ion on silicon to produce an allyl anion, or at least something akin to it. Two such examples are shown below, one an intermolecular reaction76 (equation 71) and the other intramolecular one77 (equation 72). 

The proton exchange membrane can be a source of fluoride ions as well [143]. Hydroxyl radicals, formed via crossover gases or reactions of hydrogen peroxide with Fenton-active contaminants (e.g., Fe +), could attack the backbone of Nafion, causing the release of fluoride anions these anions in turn promote corrosion of the fuel cell plates and catalyst, and release transition metals into the fuel cell [143]. Transition metal ions, such as Fe, then catalyze the formation of radicals within the Nafion membrane, resulting in a further release of fluoride anions. On the other hand, transition metal ions also can cause decreased membrane and ionomer conductivity in catalyst layers, as discussed in section 2.4 of this chapter. 

Protection of Alcohols.—The acylation of alcohols by polymers (48) of 3-acyl-2-oxazolone has been reported using fluoride ion as the catalyst, selective acylation of primary alcohols in the presence of secondary alcohols has been observed. Transesterification, mediated by titanium alkoxides, has been investigated and shown to be a selective, mild, and inexpensive technique for example, for the deacylation of alcohols. ... 

There is another variation of fluoride ion induced Peterson-type reaction that utilizes two silyl groups. The reactions between bis(trimethylsilyl)methyl derivatives 157 and carbonyl compounds using fluoride ion as a catalyst give the corresponding alkenes (Scheme 2.99) [281-283]. This reaction is usefiil for many bis(trimethylsilyl)methane derivatives, bearing a variety of substituents, with many ketones and aldehydes. As regards the stereoselectivity of the reaction, E- and Z-isomers are mostly produced in equal amounts. 

Fluorides. Tantalum pentafluoride [7783-71-3] TaF, (mp = 96.8° C, bp = 229.5° C) is used in petrochemistry as an isomerization and alkalation catalyst. In addition, the fluoride can be utilized as a fluorination catalyst for the production of fluorinated hydrocarbons. The pentafluoride is produced by the direct fluorination of tantalum metal or by reacting anhydrous hydrogen fluoride with the corresponding pentoxide or oxychloride in the presence of a suitable dehydrating agent (71). The ability of TaF to act as a fluoride ion acceptor in anhydrous HF has been used in the preparation of salts of the AsH, H S, and PH ions (72). The oxyfluorides TaOF [20263-47-2] and Ta02F [13597-27-8] do not find any industrial appHcation. 

When potassium fluoride is combined with a variety of quaternary ammonium salts its reaction rate is accelerated and the overall yields of a vanety of halogen displacements are improved [57, p 112ff. Variables like catalyst type and moisture content of the alkali metal fluoride need to be optimized. In addition, the maximum yield is a function of two parallel reactions direct fluorination and catalyst decomposition due to its low thermal stability in the presence of fluoride ion [5,8, 59, 60] One example is trimethylsilyl fluoride, which can be prepared from the chloride by using either 18-crown-6 (Procedure 3, p 192) or Aliquot 336 in wet chlorobenzene, as illustrated in equation 35 [61],... 

Cordes et al995 carried out alkaline hydrolyses of p-nitrophenylhexanoate 55 (PNPH) in the presence of poly-4-vinylpyridine partially quaternized with dodecyl-bromide and ethylbromide (QPVP). They also found that the polyelectrolytes are increasingly effective as catalysts with an increasing ratio of dodecyl to ethyl groups, and the hydrophobic interactions are important in determining the catalytic efficiency. They observed the inhibitory effects of several gegen-anions fluoride ions are the weakest inhibitor, and nitrate is the strongest (F- < Cl < S04 [Pg.159]

Dichloro monomers can also be polymerized with bisphenols in the presence of fluorides as promoting agents.78 The fluoride ions promote the displacement of the chloride sites to form more reactive fluoride sites, which react with phenolate anion to form high-molecular-weight polymers. Adding 5-10 mol % phase transfer catalysts such as A-alkyl-4-(dialkylamino)pyridium chlorides significantly increased the nucleophilicity and solubility of phenoxide anion and thus shortened the reaction time to one fifth of the uncatalyzed reaction to achieve the same molecular weight.79... 

B. Potassium allyl- and crotyltrifluoroborates undergo addition to aldehydes in biphasic media as well as water to provide homoallylic alcohol in high yields (>94%) and excellent diastereoselectivity (dr >98 2). The presence of a phase-transfer catalyst (e.g., B114NI) significantly accelerates the rate of reaction, whereas adding fluoride ion retards the reaction (Eq. 8.70).165 The method was applied to the asymmetric total synthesis of the antiobesity agent tetrahydrolipstatin (orlistat).166... 

The stannylation and silylation of heteroaryl halides has also been studied. The stannylation of 4-iodo pyrimidine was conducted with hexamethyl and hexa-ra-butyl ditin in the presence of fluoride ion and Pd(PPh3)2(OAc)2 as catalyst. The stannylation of 5-bromopyrimidine occurred in the highest yields in the presence of the ligandless catalyst bis(7r-allylpalladium chloride).175 The silylation of bromopyridines, bromopyrimidines, and bromoquinolines also occurred after 96 h at 160 °C with Pd(PPh3)4 as catalyst in HMPT solvent.162... 

It might be thought that the question whether a particular alkyl halide is a co-catalyst for a particular monomer-catalyst combination could be settled easily, by adding some of the compound in question to a non-reacting mixture of monomer and catalyst. This approach has been used [36, 44], but it must be carried out in a polar solvent which is itself not a co-catalyst, or only a very weak one. The ideal solvent for this kind of work remains to be found it may be that S02 or even CS2 (which behaves like a polar solvent) will provide the answer. If one wants to use alkyl chloride solvents without being troubled by the possibility of solvent co-catalysis, boron fluoride should be used as catalyst, since the ion BF3C1 is not formed under the conditions generally used for polymerisations. 

The stability of dialkylimidazolium cation-containing ionic liquids can be a problem even at moderate temperatures in the presence of some reagents or catalysts. For example, when CsF and KF were used in the ionic liquid [BMIM]PFg to perform a halogen exchange reaction in an attempt to replace Br from bromo-carbons with F , it was found that alkyl elimination from the [BMIM] cation took place, forming methyl imidazole, 1-butene, 1-fluorobutane, and other unidentified products at 150°C overnight 69). The fluoride ion acted as a base that promotes elimination or substitution processes. 

Group transfer polymerization (GTP) requires either a nucleophilic or Lewis acid catalyst. Bifluoride (HF2) and fluoride ions, supplied by soluble reagents such as tris(dimethylamino)-sulfonium bifluoride, [(CH3)2N]3SHF2, and (w-C NF, are the most effective nucleophilic catalysts, although other nucleophiles (CN , acetate, p-nitrophenolate) are also useful. Zinc... 

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