Fluonnation bonds

Perfluorinated organic bromides can be oxidatively fluonnated with elemental fluorine to derivatives containing tn- [124] and pentavalent [/25 126 127] bromine in yields up to 42% Perfluoroheptylbromine tetrafluoride has been used to fluonnate double bonds in halogenated alkenes [127]... 

The high reactivity of N-H bonds has also been exploited to produce N-F denvatives without significant substitution on neighbonng C-H bonds, Diethyl-phosphoramidates of ammonia, alkylammes, and a,polar solvents to produce difluoroamine [57], N,N-difluoroalkylamines, and a,to-bis(At,7V-difluoroamino)alkanes [52] Acetamide undergoes fluonnation to give modest yields of N,N difluoroacetatnide and acetyl fluonde when fluorinated... 

The product from fluonnation of sodium acetate is acetyl hypofluorite [64], which IS isolated and characterized [65] The value of this reagent lies in its relative mildness, because it reacts cleanly with most olefins adding the elements of acetoxyl and fluorine [66] Tnfluoroacetyl hypofluorite adds cleanly only to benzylic or electron-rich double bonds... 

Up to 20 products with different numbers of fluorine atoms and double bonds have been isolated from the fluonnation of benzene with silver difluoride [d], manganese trifluoride [7], potassium tetrafluoroargentate [d], and potassium hexafluoronickelate [24] The composition of the products depends on the fluorinating agents and on the temperature (Table 1)... 

Systems usually fluonnated by electropositive fluorine reagents include acti-vated alkenes (enol ethers, enol acetates, silyl enol ethers, and enamines), activated aromatic systems, certain slightly activated carbon-hydrogen bonds, and selected organometallics. 

Fluorine is well known for its ability to fluonnate seleeti vely terttary carbon-h drogen bonds that are slightly activated by polar substituents Hypofluorite reagents also react with carbon-hydrogen bonds, both secondary and tertiary but tertiary bonds react better [6]... 

On the other hand, oxidizing fluonnating agents like silver difluoride, xenon difluoride, or bromine trifluoride replace one chlorine group and then cleave the sulfur-nitrogen bond [56],... 

Benzyl and alkyl tnalkylsilyl ethers undergo clean fluonnation to give good yields of benzyl and alkyl fluorides, respectively, when reacted with a combination of d quaternary ammonium fluoride and methanesulfonyl orp- toluenesulfonyl fluoride. The reactions are applicable strictly to a primary carbon-oxygen bond, secondary and tertiary alkyl silyl ethers remain intact or, under forcing conditions, aie dehydrated to olefins [29] (equation 22)... 

Oxidations of higly fluonnated alkanes and cycloalkanes are rare because of the resistance of these compounds to oxidation agents Reactive centers include C-H and C-I bonds (oxidations of lodo compounds at lodme atom are descnbed in a special part of this chapter)... 

Hydroxylation, the addition of two hydroxyl groups across a double bond, converts fluonnated alkenes to different products depending on the presence or absence of a fluorine atom at the hydroxylated carbon... 

Fluonnated alkenes with one fluonne atom attached to the double bond are converted to a-hydroxyketones by potassium permanganate [30] (equation 22) a-Diketones are formed by permanganate hydroxylation of double bonds flanked by fluonne atoms [31] (equation 23)... 

Tnorganic and perfluoroorganic hypohalites add across olefimc bonds (equation 11 andTable 1) Thereactionsofhypofluontes have been included in a review on their chemistry [2J], In reactions of perfluoorganic hypofluontes, considerable fluonnation of the C=C bond may be observed With hypochlorites such as... 

Four-membered heterocycles are easily formed via [2-I-2] cycloaddition reac tions [65] These cycloaddmon reactions normally represent multistep processes with dipolar or biradical intermediates The fact that heterocumulenes, like isocyanates, react with electron-deficient C=X systems is well-known [116] Via this route, (1 lactones are formed on addition of ketene derivatives to hexafluoroacetone [117, 118] The presence of a trifluoromethyl group adjacent to the C=N bond in quinoxalines, 1,4-benzoxazin-2-ones, l,2,4-triazm-5-ones, and l,2,4-tnazin-3,5-diones accelerates [2-I-2] photocycloaddition processes with ketenes and allenes [106] to yield the corresponding azetidine derivatives Starting from olefins, fluonnaied oxetanes are formed thermally and photochemically [119, 120] The reaction of 5//-l,2-azaphospholes with fluonnated ketones leads to [2-i-2j cycloadducts [121] (equation 27)... 

The extraordinanly high polaiity of the fluonnated alcohols reflects their strong hydrogen-bonding capability [54] The P values of 10 2 for CF3CH2OH and 11 08 for (CF3)2CHOH compare with 10 6 and 12 1 for 50% formic acid and water, respectively [39]... 

The effects of fluonnation on bond strengths, reactive intermediates, and steric interactions that directly relate to chemical reactivity are summanzed in this section... 

Extensive reviews of the effects of fluonnation on stmeture and bonding are available [75, 76, 77], and only the charactenstic trends in bond strengths will be covered here. The bond energies cited are average values corrected for the revised heats of formation of alkyl radicals [78], but their precision is seldom better than 2 kcal/mol for the fluoro compounds. 

Fluonnation usually strengthens C-C bonds, and the effect of partial fluorma-tion can be especially pronounced. The C-C bonds m poly(tetrafluoroethylene) are about 8 kcal/mol stronger than those in poly(ethylene), and CF CFj has a 10 kcal/mol stronger C-C bond than CFI3CH3, but CF3CFI3 has an even stronger, polar... 

C-C bond Fluonnation increases the bond strengths in cycloalkanes, including cyclobutanes [75, 94], but by contrast, it decreases C-C bond strengths and increases nng strain in cyclopropanes and other three-membered nng compounds [75 94. 9S]... 

P Fluonnation always strongly stabilizes carbamons both by induction and by negative (anionic) hyperconjugaQon, 7 The latter "no-bond resonance has been controversial, but its importance is now well established both theoretically [133, 134] and expenmentally [67] The X-ray crystal structures of salts 8 [fi5] and 9 [136] provide cogent evidence for negative hyperconjugation... 

The word fluorocarbon has been used here in a generic way to indicate completely fluonnated liquids, including ethers and amines, rather than in its stricter meaning of having only carbon and fluorine atoms The definition of the word perfluori-nated is somewhat vague, because it can include compounds that retain double bonds or protons... 

Reactions of the halogen fluorides with other unsaturated functional groups are generally reported only with highly fluonnated substrates Hexafluorobenzene and derivatives [ 1)8] and octafluoronaphthalene [119] react with mixtures of bromine and bromine trifluoride by 1,4 addition of fluorine followed by addition of bromine fluonde across a remaining double bond (equation 22)... 

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 fluonnation of acetyl fluoride produces perfluoro-acetyl fluoride in 36-45% yields [85]. Electrochemical fluonnation of octanoyl chloride results in perfluonnated 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-AIkyl-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... 

Fluoroxytrifluoroinethane effectively fluonnates tertiary carbon-hydrogen bonds in materials with biological applications [2 3] (equations 23 and 24)... 

Acetyl hypofhiorite is very effective in the fluorination of the aryl-metal (Hg, Ge, or Si) bond, but yields are frequently low. With aryl silicon compounds some competition exists for replacement of an aromatic hydrogen [51, 52, 53, 54] (equations 25-27). Fluoroxytrifluoromethane fluonnates p-methoxypheny 1 mercuric acetate to give p-fluoroanisole in 86% yield [52]... 

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