Onium fluorides

Quaternary onium fluorides, particularly tetraalkylammonium fluorides, have been widely recognized as a convenient, organic-soluble source of naked fluoride ion. Their utiUty in modem organic synthesis, taking advantage of the nucleophilic 

---

Based on the design of chiral quaternary onium bromides and chlorides as chiral phase-transfer catalysts, chiral quaternary onium fluorides [5] and phenoxides [6] have been developed as chiral base catalysts for homogeneous reactions. 

During electrochemical fluorination retention of important functional groups or atoms in molecules is essential. Acyl fluorides and chlorides, but not carboxylic acids and anhydrides (which decarboxylate), survive perfluorination to the perfluorinated acid fluorides, albeit with some cyclization in longer chain (>C4) species [73]. Electrochemical fluorination of acetyl fluoride produces perfluoro-acetyl fluoride in 36-45% yields [85]. Electrochemical fluorination of octanoyl chloride results in perfluorinated cyclic ethers as well as perfluorinated octanoyl fluonde. Cyclization decreases as initial substrate concentration increases and has been linked to hydrogen-bonded onium polycations [73]. Cyclization is a common phenomenon involving longer (>C4) and branched chains. a-Alkyl-substituted carboxylic acid chlorides, fluorides, and methyl esters produce both the perfluorinated cyclic five- and six-membered ring ethers as well as the perfluorinated acid... 

UOP in a joint venture with ChevronTexaco developed an additive technology named Alkad . The additive is based on HF salts of amines, which form liquid onium polyhydrogen fluoride complexes with HF, reducing the vapor pressure of the catalyst 65% to more than 80% aerosol reduction is claimed with this additive. As in the ReVap technology, additional separation columns have to be installed. Both additives are claimed to increase the product octane number, especially when propene, isobutylene, and pentenes are employed in the feedstock. 

It is useful to re-examine Chapter 3, Section VI describing conventional carbocationic polymerizations. Termination reactions are not very common and involve either the formation of too stable carbocations, unreactive alkyl halides, usually alkyl fluorides, or unreactive onium ions via reaction with very strong nucleophiles (impurities or intentionally added compounds) ... 

Destlylation. Anhydrous cesium fluoride desilylates trimethylsilylmethyl-sulfonium, trimethylsilylmethylammonium, and trimethylsilylmethylphosphonium salts at room temperature to produce ylides, which undergo various useful transformations. Use of potassium fluoride-18-crown-6 or a tetraalkylammonium fluoride gives products in low yield in these reactions. The trimethylsilylmethyl-onium salts are prepared by alkylation of sulfides, amines, imines, and phosphines with (trifluoromethanesulfonylmethyl)trimethylsilane (1). 

Review. New synthetic reactions based on the onium salts of aza-arenes have been reviewed (75 references). The reactions discussed involve activation of carboxylic acids or alcohols with 2-haIopyridinium, benzoxazolium, benzothiazolium, and pyridinium salts to afford 2-acyloxy or 2-alkoxy intermediates, which can be transformed into esters, amides, thiol esters, (macrocyclic) lactones, acid fluorides, olefins, allenes, carbodiimides, isocyanates, isothiocyanates, and nitriles under appropriate conditions. 

Olah GA, Welch J (1975) Onium ions. XII. Heterolytic dediazoniation of benzenediazonium ions in halide ions in pyridinium polyhydrogen fluoride solution giving isomeric haloben-zenes reflecting ambident reactivity of benzenediazonium ions and intermediate phenyl cation as well as subsequent aryne formation. J Am Chem Soc 97 208—210... 

In the same way, the solubility of F" cannot be correlated to the added amount of onium salts as indicated in Table 23 and in the literature (ref. 41). Quantitative determinations show that ammonium salts are mainly located on the surface of the undissolved potassium fluoride (ref. 17). 

Inside the -phase, which is mainly constituted by solvent, the rules of homogeneous chemistry can be applied. Thus, the reaction kinetics is dependent on the nature of the solvent and the concentrations, the conversion of the aromatic chloride is a first-order process and oniums salts, which concentrate in the -phase, are effective to enhance the nucleophilicity of fluoride anions. This last hypothesis is consistent with the p value (+ 6.4) given by Smyth (ref. 41) who, on the other hand, also determined activation parameters which are consistent with a Meisenheimer s process (AH 5 = 24.4 4 kcal/mol, AS = -14.3 2.2 kcal/mol). 

To reduce the reaction time between diisocyanates and dianhydrides, catalysts such as alkali metal methoxides, or alkali metal fluoride can be optionally combined with a quaternary onium salt. ° However, the addition of catalysts are not advised in the productions of yarns that should exhibit good characteristics. ... 

Copolymers of propylene and tetrafluoroethylene were introduced in the early 1980s by Asahi Glass Co., Japan (21-26). 3M introduced bisphenol/onium cured copolymers of vinylidene fluoride, tetrafluoroethylene, and propylene in the late 1980s (27-30). 

---