Fluorine addition to, alkenes

Fluorine addition to alkenes is a violent reaction difficult to control and accompa med by substitution of hydrogens by fluorine Vicinal diicxlides on the other hand tend to lose I2 and revert to alkenes making them an infrequently encountered class of compounds... 

It is not normally possible to add fluorine directly to alkenes as the reaction is so exothermic that bond fission occurs. Many alkenes will not add iodine directly either, and when the reaction does occur it is usually readily reversible. Alkynes are also found to undergo preferential, though not exclusive, ANTI addition of halogens, e.g. with butyne-l,2-dioic acid (17) ... 

Perfluorination of unsaturated hydrocarbons such as alkenes, allenes (Fig. 7) [53] and aromatics (Fig. 8) [54,55] is also possible since the total energy released upon fluorine addition to a carbon-carbon double bond (typically between 251.4-292.9 kjmol-1) is not sufficient to break carbon-carbon single bonds. 

Fluorinated radicals play a significant role in synthetic organo-fluorine chemistry, for example, in electrophilic radical addition to alkenes, single-electron transfer reactions (SET), telomerization of fluoroalkenes with perfluoroalkyl iodides, polymerization to fluoropolymers and copolymers, and thermal, photochemical and radiation destruction of fluorocarbons. Furthermore, such free radicals are of interest for studying structures, reaction kinetics and ESR spectroscopic parameters.38... 

Bromine monofluoride or iodine monofluoride can be prepared from the corresponding elements in trichlorofluoromethane at — 781 and — 45 C,2,3 respectively. The compounds are unstable and decompose at low temperatures, iodine monofluoridc at — 14 C2,3 and bromine monofluoride, in part, even at — 78 C.1 Therefore, they are used, without separation and purification, in Freon solutions at low temperatures or, most frequently, as stoichiometric mixtures bromine trifluoride/bromine and iodine pentafluoride/iodine. A solution of iodine monofluoride, obtained by bubbling nitrogen-diluted fluorine gas into a suspension of iodine in trichlorofluoromethane at — 75 C, was used for addition to alkene C = C bonds. The reaction is regioselective and in most cases obeys the Markovnikov rule.4 Iodine monofiuoride prepared in situ reacts quite efficiently with steroidal alkenes.4... 

Owing to the high electronegativity of fluorine atoms, perfluoroalkyl radicals show an electrophilic character moreover, these radicals are usually much more reactive than the nucleophilic alkyl radicals in the addition to alkenes, aromatic rings, and quinones for enthalpic reasons (Scheme 14.5b). 

The synthesis of fluorinated cyclopropanes has been achieved via the addition of fluorinated carbenes to alkenes and via the addition of nonfluorinated carbenes to fluorinated alkenes. 

It is increasingly apparent that polar characteristics of radicals are important in organic synthesis [122] and the effect of fluorine on the polarity of radicals is very significant. Reactions of perfluoroalkyl radicals with a series of substituted p-styrenes [123] (Figure 4.51) shows that the rate constant for radical addition to alkenes increases as the alkene becomes more electron-rich (Table 4.16) and, in similar additions, perfluoroalkyl radicals reacted 40000 times faster with 1-hexene than the corresponding alkyl radicals. 

IS the most popular, one-step method for m ag fluorinated cyclopropanes and cyclopropenes a-Fluorocarbenes are particularly well behaved, because they all have singlet ground states [/, 2] and therefore usually add stereospecifically to alkenes and do not insert into C-H bonds competitively with addition Moreover, quantitative competition studies of carbene additions to alkenes near room temperature show that a-fluorocarbenes are more selective than other a-halocarbenes, with difluorocarbene being the most selective electrophihc carbene known [3, 4] The relative selectivities, however, can be quite temperature dependent [5, d] The numerous preparations and cycloaddmons of fluorocarbenes have been reviewed thoroughly [7, 8 9,10 ... 

We noted in this chapter that nucleophilic addition to alkenes occurs when groups capable of electron withdrawal by both induction and resonance are attached to the alkene. The addition of alkoxide anions to 2,2-dichloro-l,l-difluoroethylene occurs on the carbon with the fl uorines. Why does the addition occur to place the negative charge of the carbanion intermediate on the carbon with the chlorines, rather than on the carbon with the more electronegative fluorines ... 

The reaction of perfluoroalkyl iodides with alkenes affords the perfluoro-alkylated alkyl iodides 931. Q.a-Difluoro-functionalized phosphonates are prepared by the addition of the iododifluoromethylphosphonate (932) at room temperature[778], A one-electron transfer-initiated radical mechanism has been proposed for the addition reaction. Addition to alkynes affords 1-perfluoro-alkyl-2-iodoalkenes (933)[779-781]. The fluorine-containing oxirane 934 is obtained by the reaction of allyl aicohol[782]. Under a CO atmosphere, the carbocarbonylation of the alkenol 935 and the alkynol 937 takes place with perfluoroalkyl iodides to give the fluorine-containing lactones 936 and 938[783]. 

Polymer-bound phenyliodine difluoride, which also has been used as a reagent to add fluorine to alkenes, can be prepared by the addition of xenon difluoride to the polymer [134, 135 136] Methyl iodide is converted to trifluoro methyliodine difluoride by treatment with fluorine at -110 C [137] Perfluoro-alkyliodine tetrafluorides could be synthesized from the perfluoroalkyliodine difluorides and fluorine [138] or chlorine trifluoride [139] Perfluoroalkyl [140] and perfluoroaryl [141] iodides are oxidized to the corresponding iodine difluorides by chlorine trifluoride. 

Radical addition of dibromodifluoromethane to alkenes followed by sodium borohydride reduction is a convenient two-step method for the introduction of the difluoromethyl group.5 Either one or both carbon-bromine bonds in the intermediate dibromides may be reduced, depending on the reaction conditions. In the case of acyclic dibromodifluoromethane-alkene adducts, the reduction occurs regioselectively to yield the relatively inaccessible bromodifluoromethyl-substituted alkanes. The latter are potential building blocks for other fluorinated compounds. For example, they may be dehydrohalogenated to 1,1-difluoroalkenes an example of this methodology is illustrated in this synthesis of (3,3-difluoroallyl)trimethylsilane. 

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). 

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