Fluorine reactions with alkenes

Table 1 Results of the alkene epoxidation reactions with fluorinated (salen)Mn complexes under biphasic conditions ...

Fullerenes can be derivatized by various means. For example, reaction with fluorine gas proceeds stepwise to the formation of colorless CeoFeo, which, according to the 19F nuclear magnetic resonance (NMR) spectrum, contains just one type of F site and so evidently retains a high degree of symmetry.9 In view of the low adhesion typical of fluorocarbons, this spherical molecule is expected to have extraordinary lubricant properties. Curiously, bromination of Ceo is reversible on heating otherwise, the reactions of fullerenes resemble those of alkenes or arenes (aromatic hydrocarbons). 

Fluorine-containing aldehydes, ketones and acid fluorides undergo photoinitiated cycloaddition reactions with fluorinated alkenes to give oxetanes.The addition of hexafluoroacetone... 

Fluorine-containing aldehydes, ketones and acid fluorides undergo photoinitiated cycloaddition reactions with fluorinated alkenes to give oxetanes.90 91 The addition of hexafluoroacetone (11) to fluoroalkenes can also be performed in the presence of the Lewis acid, aluminum chloride fluoride.92-1 22-1 23 Unlike the photochemical cycloaddition of hexafluoroacetone with trifluoroethene, the Lewis acid catalyzed addition is regioselective.92 Fluorooxetanes (c. g., 14) have also been synthesized by the addition of formaldehyde to fluoroalkenes in hydrogen fluoride.94 Examples of the formation of fluorooxetanes by [2 + 2]-cycloaddition reactions arc-given in Table 8. 

A range of perfluoroalkyl derivatives of highly fluorinated heterocyclic compounds may be obtained in fluoride-induced reactions with fluorinated alkenes " (Eq. 76), but these will be described in more detail later. Copyrolysis of thiophenethiol with tetrafluoroethylene gives a fused ring system (Eq. 77)." ""... 

Halogenation of alkenes is only practical for the addition of chlorine or bromine. The reaction with fluorine is too violent, and the reaction with iodine often produces very low yields. 

Mixtures of anhydrous hydrogen fluoride and tetrahydrofuran are successfully used as fluorinating agents to convert 1,1,2-trifluoro-l-allcen-3-ols, easily prepared from bromotrifluoroethene via lithiation followed by the reaction with aldehydes or ketones, to 1,1,1,2-tetrafluoro-2-alkenes The yields are optimal with a 5 1 ratio of hydrogen fluoride to tetrahydrofuran The fluorination reaction involves a fluonde lon-induced rearrangement (Sf,j2 mechanism) of allylic alcohols [65] (equation 40)... 

Reactions of fluorinated dipolarophiles. Electron-deficient unsaturated species generally make better dipolarophiles, therefore, fluonnated alkenes become better dipolarophiles when vinylic fluonnes are replaced by perfluoroalkyl groups For example, perfluoro-2-butene is unreactive with diazomethane, but more highly substituted perfluoroalkenes, such as perfluoro-2-methyl-2-pentene, undergo cycloadditions in high yields [5] (equation 2) Note the regiospecificity that IS observed in this reaction... 

Fluorinated alkenes and alkynes are highly activated toward nucleophilic attack and reaction with bifunctional nucleophiles is a fruitful area for the synthesis of heterocycles. A review on perfluoroalkyl(aryl)acety-lenes contains many examples (91RCR501). 

Other nitrones (21-23) having the chiral moiety located at the carbon atom have been applied in reactions with various alkenes (Scheme 12.10) (33-35). Nitrone 21 offered poor discrimination in 1,3-dipolar cycloadditions with benzyl crotonate, as all four diastereomers were obtained in both reactions (33). The fluorinated nitrone... 

The reaction with ethylene is visualized as an alkene exchange to give the unstable parent /3-sultone, which undergoes ring opening and concomitant proton shift to give ethylenesul-fonic acid. The latter reacts with the fluorinated /3-sultone with ultimate formation of ethylenebis-sulfonic acid (Scheme 53) (70BAU574). 

The only stable 1,3,2-dioxathietanes known are fluorinated sulfate derivatives formed by addition of sulfur trioxide to bis(trifluoromethyl)ketene. These structures are fairly well characterized from spectral data and from reactions with nucleophiles. Hexafluoroisopropy-lidene-l,3,2-dioxathietane 2,2-dioxide acts as a sulfur trioxide transfer agent to alkenes and is in equilibrium with a dimeric form as indicated by 19F NMR (Scheme 138) (71KGS1645, 72KGS306, 73KGS178, 132l). 

The related 5-trifluoromethyl-l,3-dioxin-4-ones underwent high pressure Diels-Alder reactions with Danishefsky s diene (Eq. 114), and [2 + 2] photocycloadditions with alkenes [305]. The former reaction failed entirely when attempted in the absence of the fluorine atoms. 

Af,Ar,AOV -Tetramethylnaphthalene-l,8-diamine described as Proton Sponge 38 (see Section 1.2) has the ability to act as a fluoride ion donor when its hydrofluoridc is used. Furthermore, it may be used to promote oligomerization reactions of fluorinated alkenes, e.g. 3, and poly-fluoroalkylation reactions with activated perfluoroaromatic compounds, e.g. 4.38... 

To overcome problems associated with the removal of iodobenzene and its derivatives formed upon fluorination of arylalkenes and arylalkynes with (difluoroiodo)arenes, polymer-supported (difluoroiodo)arenes were proposed.139 With these agents, the separation procedures are reduced to filtration of the iodinated polymer. For this purpose popcorn polystyrene is io-dinated and then transformed into the difluoroiodide by treatment with xenon difluoride in the presence of hydrogen fluoride in dichloromelhane at 25 C. The amount of active fluorine bonded to iodine atoms on the polymer support is estimated by iodometric titration. The reactions with phenyl-substituted alkenes result in rearranged gew-difluorides. The procedure provides the same fluorination products as with (difluoroiodo)benzenc (see Section 4.13.) but in much higher yields, e.g. PhCF2CH2Ph (96%), PhCF2CH(Me)Ph (95%). PhCH2CF2H (86%), and l,l-difluoro-2-phenylcyclopentanc (91 %). 

Acetyl hypofluorite appears to be a milder reagent than trifluoroacetyl hypofluorite in the fluorination of alkenes and acetylenes. Acetyl hypofluorite undergoes addition to the C = C bond in the fluorination of cyclohexene the ij f7-adduct 4 is obtained in 60% yield, while the reaction with dodec-l-ene gives 2-acctoxy-l-fluorododccanc (5) in 30% yield.10... 

The regioselectivity in the fluorination of phenyl-substituted alkenes 9 has been reported and in the case of an electron-withdrawing group (C02Me, Ac) and a donating group (4-OMe) a more stable carbenium ion is formed and complete regioselectivity is observed, but it is completely lost in the reaction with the chloro derivative.2... 

The reaction of fluorine with water, producing hypofluorous acid (HOF) and subsequently oxygen difluoride (OF2), has been the subject of intensive study over the last fifty years.1 Additionally, hypofluorous acid in acetonitrile has been characterized.5 The reagents are potentially explosive and present a toxicity hazard similar to fluorine therefore, work should be carried out in an efficient hood and proper safety equipment is required. It was established in the earliest investigations that the reaction of hypofluorous acid with alkenes in nonpolar solvents gives fluoro hydrins, e.g. 1, in high yield.6 However, it is now clear that epoxides are initially formed in acetonitrile7 and 1,2-epoxycyclohexane (2) is formed in reactions with cyclohexene in various solvents.5... 

The effect of monofluorination on alkene or aromatic reactivity toward electrophiles is more difficult to predict Although a-fluonne stabilizes a carbocation relative to hydrogen, its opposing inductive effect makes olefins and aromatics more electron deficient. Fluorine therefore is activating only for electrophilic reactions with very late transition states where its resonance stabilization is maximized The faster rate of addition of trifluoroacetic acid and sulfuric acid to 2-fluoropropene vs propene is an example [775,116], but cases of such enhanced fluoroalkene reactivity in solution are quite rare [127] By contrast, there are many examples where the ortho-para-dueeting fluorine substituent is also activating in electrophilic aromatic substitutions [128]... 

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