Carbanions fluoro

Several years ago, there was much debate concerning the mechanism of the Darzens condensation.2.3 The debate concerned whether the reaction employed an enolate or a carbene intermediate. In recent years, significant evidence that supports the enolate mechanism has been obtained, wherein the stabilized carbanion (11) of the halide (10) is condensed with the electrophile (12) to give diastereomeric aldolate products (13,14), which subsequently cyclize via an internal Sn2 reaction to give the corresponding oxirane (15 or 16). The intermediate aldolates have been isolated for both a-fluoro- and a-chloroesters 10. 

A more general method for preparing carbenes often involves the a elimination of halides from carbanions.1-57 PAC can be used to examine the rates and energetics of the reverse reactions, the complexation of halides with carbenes (Fig. 5).58 Plots of A//com versus the proton affinities (PA) of the halides are linear for the two carbenes studied. Although the slopes of the plots are similar, complexation of the halides with phenylchlorocarbene is more exothermic than phenylfluorocarbene. This indicates that fluoro substitution stabilizes the carbene relative to the carbanion more than chloro substitution. The rate of complexation of carbenes with salts has also been examined by nanosecond absorption spectroscopy.59... 

Formation of the carbanion of diethyl 1-fluoro-1-(phenylsulfonyl)-methanephosphonate is followed by gas chromatography, by quenching a small aliquot of the reaction in ether/saturated aqueous ammonium chloride. The carbanion forms in ca. 85% to 95% yield after 1 hr. 

The reaction of the carbanion derived from diethyl methylphosphonate with perhalogenated aromatics may result in substitution of halide to yield perhaloaryl(hetaryl)methylphosphonates, which can be converted into tris- or bis-(perhaloaryl)methanes. Displacement of fluoride ion has been reported in the reaction of dimethoxycarbene with l-fluoro-2,4-dinitrobenzene and with hexafluorobenzene. The hydrodehalogenation of halogenated aryl ketones may be facilitated using hydrogen over a Pt/C catalyst. " ... 

Carbanions in the form of organomagnesium, -sodium, or -lithium salts have been fluorinated with various reagents, including perchloryl [ F]fluoride, 1-[ F]fluoro-2-pyridone, A/-[ F]fluoropyridinium triflate and A/-[ F]fluoro-A/-alkylsulphonamides. 

A/-[ F]Fluoro-A/-alkylsulphonamides react with a variety of carbanions and organometallic compounds [84]. For example, A/-[ F]fluoro-A/-enc/o-norbornyl-paratoluenesulphonamide has been successfully used for the preparation of [ F]fluorobenzene (40 and 61% radiochemical yields from phenylmagnesium chloride and phenyllithium, respectively), 1-f F]fluoronaphthalene (53%, from 1-naphthylmagnesium chloride) and [ F]fluorocyc/ohexane (29%, from cyclo-hexylmagnesium chloride) (Scheme 24). 

Table 12. Fluorination of Enolates and Carbanions with 2-Fluoro-l,3,2-ben-zodithiazole 1,1,3,3-Tetraoxide (2) in Diethyl Ether134135...

Chambers RD, Bryce MR (1987) Fluoro-carbanions. In Buncel E, Durst T (eds) Comprehensive carbanion chemistry, part c, vol 6. Elsevier, Amsterdam, p 271... 

Fluoride ion is not a good leaving group in SRN1 reactions. For instance / - fluoroiodobenzene [66c] and 2-fluoro-3-iodopyridine [67] react with carbanions with retention of fluorine. Synthesis of anti-inflammatory drugs, such as fluorobiprophen 39 can be achieved by reaction of 4-bromo-... 

Selective fluorination of a wide variety of carbanions has been carried out very effectively with iV-fluoro-Y-alkyl sulfonamides by Barnette63. Treatment of carbanions of malonates, enolates of ketones, acids and amides, and alkyl and aryl organometallics results in the transfer of fluorine from nitrogen to carbon (equation 43). Yields are fair to... 

The reaction of perfluoro-4-methyl-2-pentene with ethylene glycol also leads to 2-fluoro-2-trifluoromethyl-3-(2,2,2-trifluoro-1 -trifluoromethylethy-lidene)-l,4-dioxane 106 (96ZOB1995). The formation of the six-membered heterocycle occurs via generation of the intermediate carbanion and fluoride ion elimination from the y-position. Subsequent intramolecular nucleophilic cyclization involving the O-nucleophilic center and the internal double bond leads to a 1,4-dioxane derivative (route e). 

Ammonio- and phosphoniohexafluorocyclobutanide ylides 11, prepared from pcrfluorocy-clobutene and tertiary amines or phosphanes, are soluble in a wide range of (even nonpolar) organic solvents and transfer fluoride, and therefore can be used as an easily soluble source of fluoride anions. Although they contain no ionic fluoride they react with fluoride anion acceptors such as silicon derivatives with elimination of thermodynamically favored fluoro-trimethylsilane to produce carbanions which can be quenched with suitable electrophiles. The reaction proceeds upon slightly heating the reactants in tetrahydrofuran or dimethylform-amide solution. 

When perfluoro(2-methylpent-2-ene) reacts with phosphanes, carbanion 5 is formed. It is known that internal alkenes isomerize into the terminal alkenes in the presence of basic reagents, so that 5 reacts with this isomeric form and the end product of the reaction is per-fluoro(3-ethyl-2.5-dimcthylocta-2.4-dieiie) (6). 

The enhanced stability of (3-fluoro carbanions (Figure 4.37) has been attributed to fluorine negative hyperconjugation (FNHC see Section IIIB). For instance, negative hyperconjugation (see Section 111) has been invoked to explain the enhanced reactivity (100-fold) [27] and the higher gas-phase acidity (by 5.4 pKa units) [91] of 4.38A over the... 

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