Electrophilic fluoride reagents

The electrophilic fluoridation reagent Selectfluor [152, l-chloromethyl-4-flu-oro-l,4-diazabicyclo[2.2.2]octane bis(tetrafluoroborate)] was allowed to react with tii-O-acetyl-D-galactal, and the resulting salt 153 l-(3,4,6-tri-0-acetyl-2-deoxy-2-fluoro-a-D-galactopyranosyl)-4-chloromethyl-l,4-diazabicyclo[2.2.2]octane bis(tet-rafluoroborate) reacted with NaNs alTording the 1,2-trans fluoro azide 154. ... 

One of the most convenient synthetic routes for the preparation of vinyl fluorides 2 is by treatment of the corresponding vinyltin compounds 1 with electrophilic fluorinating reagents. This procedure is also used for the preparation of aryl fluorides, since it is well-established that fluorination with electrophilic reagents is facilitated by ipso substitution of a suitable, usually weakly bonded, metal function. In the case of aryl fluorides, the reaction is also carried out using elemental fluorine as the fluorinating agent. ... 

Xenon difluoride (XeF ) is the electrophilic fluorinating reagent whose reactions with vinyltin compounds have been studied most exhaustively. " A series of vinyl fluorides prepared according to this procedure is listed in Table 1. 

Table 4. Synthesis of Aryl Fluorides by Reaction of Aryltrialkyltin Derivatives with Electrophilic Fluorinating Reagents and with Elemental Fluorine...

A considerable effort is still devoted to the preparation of halogeno-steroids. A new family of electrophilic fluorinating reagents (hypofluorites) can now replace the less powerful and explosive perchloryl fluoride (C103F). A review of the fluorination of cholestane derivatives with CF3OF has appeared. ... 

The first electrophilic fluorination reagent with industrial relevance was perchloryl fluoride FCIO3 [172] (Scheme 2.77) a gas (m. p. —147.8 °C, b. p. —46.7 °C) which is thermally stable up to 500 °C [173], It was used commercially from the beginning of the 1960s for production of fluoropharmaceuticals, in particular fluorosteroids. 

While Yu et al. [15] reported the palladium-catalyzed ortho trifluoromethylation of arenes with an electrophilic trifluoromethylation reagent (Scheme 19.9), Liu and coworkers [69] developed a nondirected C-H trifluoromethylation method using the nucleophilic Ruppert-Prakash (TMSCFj) reagent (Scheme 19.43). Thus, indoles underwent trifluoromethylation in moderate yields at the C-2 position or, when the C-3 position was free, at the C-3 position in the presence of a Pd /bis-oxazoline catalyst, cesium fluoride, PhI(OAc)2 as the stoichiometric oxidant, and TEMPO as a radical trap. Similar to Yu s method, the mechanism was proposed to involve the formation of an Ar-Ph -CEj intermediate that reductively eliminates to form the Ar-CFj bond. 

The enhancement of the electrophilic properties of thaHium(III) ttifluoroacetate makes it a very important thaHation reagent. The products of thaHation, eg, arylthaHium bis(ttifluoracetate), undergo a variety of substitution reactions, yielding iodides, fluorides, nitriles, thiophenols, phenols, and biaryls. 

Enol ethers are readily attacked in buffered medium by electrophilic reagents such as halogens, A -haloamides, perchloryl fluoride and organic peracids to give a-substituted ketones. Similarly, electrophilic attack on... 

A variety of reagent combinations add fluorine and sulfur to fluonnated olefins. Typically the olefin is reacted with fluoride ion and a source of electrophilic sulfur, such as sulfur tetrafluonde[757,178,179, 7S0], alkyl or amino sulfur tnfluondes [757, 787],... 

Tnflic acid is an excellent catalyst for the nitration of aromatic compounds [.S7]. In a mixture with nitnc acid, it forms the highly electrophilic nitronium inflate, which can be isolated as a white crystalline solid Nitronium inflate is a powerful nitrating reagent in inert organie solvents and in tnflic acid or sulfuric acid. It nitrates benzene, toluene, chlorobenzene, nitrobenzene, m-xylene, and benzotn-fluoride quantitatively in the temperature range of-110 to 30 °C with exeeptionally high positional selectivity [87],... 

Scheme 2.23 provides some examples of conjugate addition reactions. Entry 1 illustrates the tendency for reaction to proceed through the more stable enolate. Entries 2 to 5 are typical examples of addition of doubly stabilized enolates to electrophilic alkenes. Entries 6 to 8 are cases of addition of nitroalkanes. Nitroalkanes are comparable in acidity to (i-ketocslcrs (see Table 1.1) and are often excellent nucleophiles for conjugate addition. Note that in Entry 8 fluoride ion is used as the base. Entry 9 is a case of adding a zinc enolate (Reformatsky reagent) to a nitroalkene. Entry 10 shows an enamine as the carbon nucleophile. All of these reactions were done under equilibrating conditions. 

As a starting material, tetrahydrofuran has little to recommend it except in rare cases, one of which is the preparation of fluorofurans. With cobalt(III) fluoride, tetrahydrofuran yields a mixture of polyfluoro derivatives from which alkali fusion removes HF leaving various fluorofurans including tetrafluorofuran and 2,3,4-trifluorofuran (1). Potassium tetrafluorocobaltate acts on tetrahydrofuran giving 2 as the main product and alkali fusion converts this into 2,5-difluorofuran. The fluorofurans all polymerize readily and are rather unresponsive to electrophilic reagents.17... 

---