3-fluoro-2-methylpropene

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... 

The addition of hydrogen fluoride to but-l-ene or but-2-ene gives 2-fluorobutane " - ° the hydrofluorination of 2-methylpropene leads to 2-fluoro-2-methylpropane. ... 

However, in most cases ionic polymerization of the alkene is observed. Only a few examples of the formation of fluoroalkanes have been described. Thus, l-fluoro-3,3-dimethylbutane (2) is obtained in 26 /o yield from the reaction of ethene and 2-methylpropene in hydrogen fluoride. 

When perflluoro(2-methylpropene) reacts with tributylphosphane in acetonitrile at — 30"C, the reaction does not stop with the formation of fluoro-2 -phosphanes, and, as a result of a series of transformations, perfluorodiene 3 and conjugated cross-triene 4 are formed. When the reaction is carried out with triphenylphosphane in acetonitrile without cooling triene 4 is obtained in 67 % yield.On the other hand, the reaction of perfluoro(2-methylpropcnc) with tributylphosphane in diethyl ether at — 70 C leads to the corresponding fluoro-A -phos-phane treating this compound with potassium fluoride dihydrate in tetraglyme gives 1,1,1,3-tetrafluoro-2-(trifluoromethyl)hept-3-ene in 75% yield,... 

Perfluoroalkenes react readily with tertiary phosphanes to form various products depending on the position of the multiple bond in the alkene. Internal alkenes (perfluorobut-2-ene and perfluorocyclobutene) undergo addition of phosphane at the multiple bond with subsequent 1,2-migration of the fluorine atom to form stable phosphanes. Terminal fluoroalkenes under similar conditions form the corresponding fluoro(perfluorovinyl)-/5-phosphanes. The exception is perfluoro(2-methylpropene) which reacts with triphenylphosphane to form the triene... 

The cleavage of silane into alkylfluorocarbene requires temperatures of at least 140°C. A competitive reaction is the rearrangement of carbene to fluoroalkene. 1-Fluoro-l-fluoromethyl-2,2-dimethylcyclopropane was prepared from trifluoro(l,l,2-trifluoroethyl)silane and 2-methyl-propene see Houben-Weyl, Vol. E19b, pp 1466-1467. Similarly, fluoro(methyl)carbene was generated and underwent addition to 2-methylpropene to give 2. ... 

In a novel variation, l,3-dehydrofluorination of 3,3-difluoro-2-methylpropen-l-ol leads to 2-fluoro-l-hydroxymethylcyclopropenes. ... 

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