Elimination of metal fluorides

Reactions of lithium derivatives and Grignard reagents from polyfluoroalkenes or polyfluoroaromatic compounds are often complicated by eliminations but are generally much more useful in synthesis than polyfluoroalkane derivatives. Generation of arynes is discussed later (see Chapter 9). 

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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). 

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... 

Heptafluoro-2-naphthyllithiuni prepared by metalation reaction can thermally decompose to a hexafluoro-l,2-naphlhalyne by elimination of lithium fluoride [36, 37] In this organolithium compound, fluorine elimination can occur from either position 1 or 3, however, no evidence for fluorine elimination from position 3 IS observed... 

The group CF2CHF2 is often completely stable towards alkali metal hydroxides as can be seen in 2,2,3,3-tetrafluoropropanoic acid (CHF2CF2C02H)17,20 or 17/,477-perfluorobutane (CHF2CF2CF2CHF2).17 However, there are exceptions such as the reaction of 2,2,3,3-tetra-fluoropropan-1-ol(CHF2CF2CH2OH)18 with methyllithium. In aryl-substituted aliphatic compounds. elimination of hydrogen fluoride from the side chain is facilitated when a conjugated double bond is formed,21,201 e.g. formation of 3.201... 

Another inexpensive and readily accessible precursor to difluoroenol ethers is trifluoroethanol. Several 0-substituted derivatives of trifluoroethanol have been converted into 1-lithio 2,2-difluoroenolates by elimination of hydrogen fluoride and subsequent metalation with LDA [36]. These building blocks have the advantage... 

Aryne formation by elimination of lithium fluoride limits the stability of 113 a polymer is formed, but attempts to trap the aryne (114) were unsuccessful.352 Decarboxylation of metal salts of tetrafluoroisonicotinic add occurs,357 giving the corresponding metal derivative, but pyrolysis of the disilver salt (115) appears to produce the aryne (114), since small amounts of diazabiphenylenes may be isolated from the reaction product.358 359... 

Whitfield and Sanford isolated di and monochloro Pd(IV) complexes of 2-phenylpyridine which upon heating in acetic acid gave substantial amounts of the chloroarene (Scheme 22) [55], although the fate of the transition metal remained unclear. More recently, Puruya and Ritter presented the first example of a reductive elimination of aryl fluoride from an isolated Pd(IV) center (Scheme 23) [56]. 

Fluorosilane elimination resulting from the reaction of metal fluorides and silicon precursors such as SiH and Si H was first demonstrated with tungsten metal. The basis for these reactions is the formation of the stable Si-F bond, which leads to an exothermic reaction. The overall chemistry is given by [14]... 

The initial step in this reaction is the oxidative insertion of calcium to the carbon-fluorine bond (also named oxidative addition to metals see [8]), followed by rapid elimination of calcium fluoride. 

The known l-cfaloro-2-pentafiuorophenylacetylene has been isolated from reactions between pentafluorophenyl-lithium and the olefins CHChCFj and CX)IF CC1F. The reactions appear to proceed by elimination of lithium fluoride from intermediate alkenyl-lithiums, themselves formed by a nucleophilic addition-elimination-metallation sequence, e.g. Scheme 46. Attempts... 

Alternative paths for decomposition of the metal carboxylate can lead to ketones, acid anhydrides, esters, acid fluorides (1,11,22,68,77,78), and various coupling products (21,77,78), and aspects of these reactions have been reviewed (1,11). Competition from these routes is often substantial when thermal decomposition is carried out in the absence of a solvent (Section III,D), and their formation is attributable to homolytic pathways (11,21,77,78). Other alternative paths are reductive elimination rather than metal-carbon bond formation [Eq. (36)] (Section III,B) and formation of metal-oxygen rather than metal-carbon bonded compounds [e.g., Eqs. (107) (119) and (108) (120). Reactions (36) and (108) are reversible, and C02 activation (116) is involved in the reverse reactions (48,120). 

Industrial Procedure,u Into 500 parts of benzotrichloride cooled to 0° in a copper flask equipped with a stirrer, an outlet tube, and an inlet tube reaching to the bottom, is introduced 200 parts of hydrogen fluoride over a period of seventy-two hours. At first, only hydrogen chloride escapes later, a mixture of hydrogen fluoride and benzotrifluoride distils and is condensed in a metal receiver cooled with ice and salt. After completion of the reaction, the contents of the receiver and the reaction flask are united and gently warmed to drive off most of the hydrogen fluoride. The remainder is eliminated by agitation with powdered... 

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