Metal fluorides eliminations

For larger quantities the above procedure is used with a greater number of silicon(IV) fluoride elimination steps. Fifty-gram batches have been prepared by scaling up this technique and using a larger metal-pipe reactor. 

Nitrosoalkenes.1 Nitrosoalkenes are usually generated by reaction of a-halo oximes with bases, and have been used for [4 + 2] cycloadditions. They are also generated efficiently from a-chloro silyloximes by fluoride ion. This elimination is used to effect an intramolecular cyclization of the nitrosoalkene generated from 1 to give a mixture of the dihydrooxazines 2 and 3. The choice of the metal fluoride is critical for acceptable yields. Highest yields are obtained when the nitrosoalkene is generated slowly by a... 

The pyrolytic decomposition of the sodium salts of various fluorinated carboxylic acids to give isomeric unsaturated compounds has also been reported. The products were identified as alkenes with the C = C bond inside the carbon chain, mainly alk-2-enes. This isomerization may be catalyzed by the coal-like products formed during the pyrolytic decarboxylation of the salts, but the metal fluoride formed in the reaction may also be responsible for the isomerization. When potassium perfluoro(5-chloropentanoate) is heated in a rocking autoclave at 300 C for 2 hours, perfluorobut-2-ene (2b) is isolated in 82% yield.This is only possible by migration of the double bond away from the terminal position after carbon dioxide elimination and halogen exchange to form potassium chloride. ... 

Polyfluoroaryl-lithiums and, to a lesser extent, the Grignard reagents will decompose by a (3-elimination of metal fluoride, leaving an aryne whose properties are considerably affected by the remaining fluorine atoms. The highly electrophilic nature of these species leads to some reactions, e.g. with benzene derivatives, that are not shown by benzyne itself [228] (Figure 9.88). 

Fluorocarbon derivatives of copper have been studied quite widely, probably because there is little evidence for the elimination of metal fluoride being a limitation in these systems. Early work [118] showed that when perfluoroiodoalkanes are heated with copper in DMSO or DMF, then the copper compounds are formed in solution and these have been successfully applied in a variety of coupling reactions. High-dielectric media are essential to the success of these processes (Figure 10.57). 

Besides substitution by amino, a chloro or bromo substituent may also be replaced by iodo, hydroxy, and alkoxy," though it is not possible to prepare 4-fluoroimidazoles by halogen exchange with metal fluorides on activated bromo- or chloroimidazoles." 4-Fluoroimidazoles are relatively resistant to nucleophiles, but the corresponding 2-fluoro compounds are subject to displacement (via an addition-elimination reaction) under mild conditions (see also Section III,B). Undoubtedly, the intermediate formation of a stable symmetrical 2-fluoroimidazolium cation (120) is responsible for this ease of displacement. (Eq. 28). Iodine at all ring carbons... 

It was assumed that an a-fluorosilyl potassium species was formed initially, and that this subsequently underwent a self-condensation reaction. The eventual product (1) displayed both nucleophilic as well as electrophilic character, which was demonstrated in various derivatization reactions. Although it contains fluorine and potassium atoms in close proximity, the compound displayed a remarkable thermal stability. Even at 80 °C, potassium fluoride elimination occurred only sluggishly. Attempted transmetalation reactions with various metal halides, though, caused an immediate elimination of metal fluoride and the formation of tetrakis(trimethylsilyl)disilene. The latter can be trapped in cycloaddition reactions [5] or, in the absence of trapping reagents, it dimerizes to a cyclotetrasilane (Scheme 2) [6]. 

Fluorocarbon organometallic compounds have been discussed more generally elsewhere.351 Lithium derivatives and Grignard reagents352-354,240.270,280,297 may obtained as indicated in Scheme 32. These compounds are unstable relative to elimination of metal fluoride, but can be used at lower temperatures for a variety of transformations. Attempts to generate corresponding lithium or magnesium derivatives of perfluorinated diazines resulted in tars.286,355... 

The conversion of 5,5,5-trifluoro-4-trifluoromethylpent-3-en-2-one with methyl acetoacetate in the presence of sodium hydride opens a new reaction pathway for the preparation of molecules which might show some biological and medical activities. The first step of this reaction is the anti-Michael addition of the nucleophile (a-metallated ester) to the 3-position of 5,5,5-trifluoro-4-trifluoromethylpent-3-en-2-one. In the next step metallation in the /-position yields the enolate which attacks nucleophilically the olefinic CF2-carbon forming 4-acetyl-2-fluoro-6-methyl-5-methoxy-carbonyl-3-trifluoromethylpyran by fluoride elimination. 

The earliest method for synthesizing perfluorinated oxaziridines entailed addition of trifluoromethyl hydroperoxide (287) to a perfluoroimine, followed by elimination of HF mediated by a metal fluoride. The fluoride of choice was the mild reagent KHF2 because it was basic enough to effect the elimination but too weak to ring open the oxaziridine product. Oxidation of imine 288 by this method gave perfluoro-A-methyloxaziridine (289), the first perfluorooxaziridine. In the case of hindered imine 290, no adduct was obtained with hydroperoxide 287, but oxidation to oxaziridine 291 was successful nonetheless with the hydroperoxide in the presence of KF. Presumably the imine was attacked by the hydroperoxide anion, forming the intermediate anion 292. 

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