Alkenes difluoromethyl

The most common situation where this effect is seen is in a comparison of E- and Z- isomers of trifluoromethyl- or difluoromethyl-substituted alkenes, but as the naphthalene examples indicate, the effect is not unique to that situation. 

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. 

Substituted fluorocarbenes, for example, arylfluorocarbenes 1, (alkoxycarbonyl)fluorocar-benes 2. fluoro(trichlorovinyl)carbene (3). fluoro(trifluoromethyl)carbenc (4). and (difluoromethyl)fluorocarbene (5), have been reported. Their addition to alkenes to give fluo-rinated cyclopropanes has been studied (see Table 2). 

Analogously, the gas-phase thermal decomposition of trifluoro(l,l,2,2-tetrafluoroethyl)silane (at 140-200 C) or trimethyl(l,l,2,2-tetrafluoroethyI)silane (at 300-370°C) generated di-fluoromethyl(fluoro)carbene which underwent addition to alkenes to give 1-difluoromethyl-l-fluorocyclopropane 8. ° (Z)- and ( )-But-2-ene were cyclopropanated stereospecifically, and allylic C-H insertion was not observed. With dimethyl(vinyl)silane or allyl(dimeth-yl)silane, however, the carbene underwent both double bond addition and Si-H insertion. ... 

When a mixture of trifluoro(l,l,2,2-tetrafluoroethyl)silane and an alkane was thermolyzed, 1-difluoromethyl-l-fluorocyclopropanes were formed as byproducts in 7-11% yield. It is assumed that the generated carbene intermediate dehydrogenates the alkane, and that the resulting alkene is then cyclopropanated vide supra). 

Difluoromethyl-l-fluorocyclopropanes 3 were synthesized via pyrolysis of the corresponding silane in the presence of an alkene " (see Houben-Weyl, Vol. E19b, pp 1214-1215). 

The stereospecific addition to alkenes is consistent with (difluoromethyl)fluorocarbene being generated in the singlet state, Adducts of this carbene with alkenes were observed during the pyrolysis of trimethyl(l,l,2,2-tetrafluoroethyl)silane. ... 

The mixture SmI2/HMPA has been applied to the conversion of vicinal bis (sulfonyl) derivatives into the corresponding alkenes.100 An application of this reaction to the synthesis of a difluoromethylene nucleoside, where conventional difluoromethylation strategies (e.g., Wadsworth-Emmons and Wittig reactions) failed, is shown in Eq. 132.234... 

The iron-catalyzed aryldifluoromethylation of activated alkenes with PhS02CF2l and aryl C(sp )-H bonds to difluoromethylated oxindoles has also been established by Wang and coworkers (Scheme 9.19) [22]. This method provides a useful route to fluorine-containing oxindoles. 

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