Alkene hydrofluorination

The hydrofluorination of alkenes with anhydrous hydrogen fluoride has been already described extensively in Houben-Weyl, Vol. 5/3, pp 100-101. In the case of ethene, the yield of fluoroethane increases on raising the temperature (90°C, 20-25 atm), however, the procedure should be carried out at lower temperatures with higher alkenes because of their tendency to polymerize thus, 2-fluoropropane is formed in 60-75% yield at 0-45 C. Similar procedures have been described for 2-fluorobutane, 2-fluoro-2-methylpropane and 2-fluoro-2-methyl-butane from but-l-ene, 2-methylpropene and 2-methylbut-2-ene, respectively.63 Cyclohexene reacts at — 78 C with hydrogen fluoride to give fluorocyclohexane (70%) at 100 C polymerization is observed.59,60 Two equivalents of hydrogen fluoride to allene are taken up at — 70 C, to form 2,2-difluoropropane (50%).64... 

Branched alkenes and cycloalkenes react more readily. The hydrofluorinations may be carried out advantageously with 70 % hydrogen fluoride/pyridine (Olah s reagent) using tetrahydro-furan as a solvent,10,31 but a homogeneous system forms in this case. Therefore, the workup procedure was carried out by quenching the reaction mixture with ice water and neutralization with sodium hydrogen carbonate hydrofluorinations of various alkenes in this way are described in ref 40. 

The 5 (or 6)HF Et3N system reacts with a variety of branched alkenes to give tertiary alkyl fluorides. Hydrofluorination of dienes proceeds at the more reactive C = C bond selectively. Thus, ethyl geranate (1) and ethyl nerolate (2) react with 5HF EtaN at the noncon-... 

In accordance with the Markovnikov rule, terminal alkynes are converted to 2-halo-l-alkenes. The reagents prepared in the reaction of anhydrous HF with organic bases and used in hydrofluorination of alkenes can add to the carbon-carbon triple bond producing Markovnikov products.94-96,98 99 However, 1-bromo-l-alkenes and... 

Stable dialkyl ether poly(hydrogen fluoride) complexes have been shown to be convenient and effective fluorinating agents.553 Various open-chain and cyclic alkenes undergo hydrofluorination with dimethyl ether-5 HF (DMEPHF) atroom temperature to furnish the corresponding fluoro derivatives in high yields (73-94%) with excellent selectivities. The fluorination of secondary and tertiary alcohols exhibit similar features. Bromofluorination of alkenes can also carried out with DMEPHF in combination with /V-bromosuccinimide. The homologous diethyl ether and dipropyl ether complexes are also suitable for fluorinations. 

Olah and Li2 have prepared a solid form of this reagent by reaction of anhydrous HF with cross-linked poly-4-vinylpyridine. It is comparable to pyridinium poly (hydrogen fluoride) for hydrofluorination of alkenes and alkynes and fluorination of alcohols, but is easier to handle. Work-up of reactions requires only a simple filtration, and the spent reagent can be regenerated with HF for further use. 

Table 1. Hydrofluorination of Alkenes with HF at Different Temperatures ...

Another highly effective and convenient hydrogen fluoride/amine reagent for the hydrofluorination of alkenes is the solution of 14% melamine in 86% hydrogen fluoride (Table... 

Hydrofluorination of alkenes is also possible with solid potassium hydrogen fluoride in the presence of silicon tetrafluoride, for example, the formation of 2-fluoro-2,3-dimcthylbutane, ... 

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. ... 

Table 5. Hydrofluorination of Halogenated Alkenes with Tantalum(V) Fluoride as Catalyst ...

Additional catalysts for the hydrofluorination of halogenated alkenes are reviewed in Table 7. 

Table 8. Hydrofluorination of Alkenes with Potassium Fluoride/Formamide...

The reaction of polyfluorinated alkenes with silver fluoride and hydrogen fluoride is another hydrofluorination method. Under certain conditions, perfluoroalkylsilvcr compounds can be isolated. For example, 11 % silver fluoride in acetonitrile reacts with hexafiuoropropene at 25 °C under nitrogen to yield a clear light-yellow solution of [tetrafluoro-l-(trifluoromethyl)-ethyl]silver. The addition of hydrogen chloride in acetonitrile at 0"C yields 1,1.1,2,3,3,3-hepta-fluoropropane. ... 

Another highly effective and convenient hydrogen fluoride/amine reagent for the hydrofluorination of alkenes is the solution of 14% melamine in 86% hydrogen fluoride (w/w)46,47 (Table 3). The two-phase mixture of this reagent with pentane or carbon tetrachloride is a suitable system for repeated use47 (Table 4). 

Olah GA, Nojima M, Kerekes I (1973) Synthetic methods and reactions. II. Hydrofluorination of alkenes, cyclopropane, and alkynes with poly-hydrogen fluoride pyridine (trialkylamine) reagents. Synthesis 1973 (12) 779-780... 

HF-pyridine reagent, however, can also be used for the hy-drofluorination of alkenes,7 alkynes,7 cyclopropanes,7 and diazo compounds,8 the halofluorination of alkenes,9 the preparation of fluoroformates from carbamates,10 the preparation of a-fluorocarboxylic acids from a-amino acids,11 and as a deprotecting reagent in peptide chemistry.12 Examples of the hydrofluorination of alkenes with HF-pyridine are given in Table II. 

Hydrofluorination of Alkenes with Hydrogen Fluoride-Pyridine... 

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