Halogen fluorides, addition alkenes

The combination of A/-halosuccinimides with polyfluorinated ammonium salts, in particular A/-halosuccinimide-triethylamine trisfhydrogen fluoride)42 (Method C), is convenient and effective for the halofluorination of alkenes. These easy-to-handle reagents allow work to be carried out at room temperature in conventional glass apparatus, while Olah s reagent requires polyethylene equipment. The reaction of various alkenes with A -halosuccinimide-triethylamine trisfhydrogen fluoride) in dichloromethane or diethyl ether at 0 C is complete within 3-5 hours giving the halogen fluoride addition products in 70-95% yield. 

Treatment of perhalogenated and highly halogenated alkenes with sulfur tetrafluoride in the presence of lead(IV) oxide, which is most likely converted in situ into lead(IV) fluoride, results in addition of fluorine to the C = C bond of the alkenes. The addition proceeds at 100 C to afford per- and polyhalogenated alkanes 5 in yields ranging from a few percent to almost quantitative.190 Representative examples are given. 

Highly halogenated alkenes, e.g. 6, react with sulfur tetrafluoride at elevated temperatures in the presence of Lewis acid type fluorides to afford halofluoroalkanes, which result from both addition of fluorine to the C = C bond and substitution of halogen atoms.191... 

Boron trifluoride-diethyl ether complex is used as catalyst in addition reactions of hydrogen fluoride to double bonds. If the alkene is halogenated on the C = C bond, fluorine adds to the carbon that bears the halogen.51... 

Boron trifluoride has been applied as catalyst in addition of hydrogen fluoride to asymmetrically halogenated alkenes in the liquid phase.52... 

Table 2. Addition of Hydrogen Fluoride to Halogenated Alkenes with Aluminum Trifluoride Catalysis...

Several groups have employed N-halosuccinimdes for the halofluorination of alkenes. Laurent and co-workers have found that triethylamine tris(hydrofluoride) is a suitable fluoride source for the halofluorination of a number of alkenes (Scheme 70).137 Other examples of halofluorinations include NBS and tetrabutylammonium fluoride as the halogen sources for the formal addition of BrF across funtionalized cyclohexenes138 and the halofluorination of alkenes and alkynes by all three N-halosuccinimides and polymer-supported HF.139... 

As a rule, in nucleophilic additions to halogenated alkenes, the nucleophile always becomes attached to that carbon of the double bond that has lower electron density. In this case, back-donation of electrons by fluorine is stronger in the difluoromethylene group, and nitrogen therefore attacks the difluoromethylene group while proton adds to the other carbon. Because of the basicity of the reaction mixture, hydrogen fluoride is eliminated, and an imide fluoride, 3-chloro-2,3-difluoro-l-phenyl-1-aza-propene is formed ... 

The addition of hydrogen fluoride to halogenated alkenes is no less important than the addition to olefinic hydrocarbons. The replacement of chlorine by fluorine often accompanies the hy-drofluorination of halogenated alkenes. ... 

The reactivity of halogenated alkenes in the electrophilic hydrofluorination reaction decreases with an increasing number of halogen atoms. Thus polyhaloalkenes are rather resistant to the electrophilic addition of hydrogen fluoride.Therefore, the hydrofluorination needs catalysis, with boron trifluoride(Table 6), tantalum(V) fluoride (Table 5), antimony(V) chloride, niobium(V) fluoride,titanium(IV)chloride or molybdenum(V) chloride. ... 

There are only a few examples of the addition of fluorine to halogenated alkenes using silver(II) fluoride in the literature. Two examples 16 and 17 are given. ... 

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