Vinylic fluorine, hydrogenolysis

The conspicuously high speed of hydrogenolysis of allylic and vinylic halogens as compared with those removed further from the multiple bonds or halogens in alkanes implies that the multiple bond participates in a multicenter transition state [63]. Such a mechanism would account even for the surprisingly easy hydrogenolysis of allylic and vinylic fluorine [66, 528, 529, 530]. 

Catalytic hydrogenation of substituted 2-fluoroprop-2-enoates over palladium at room temperature under atmospheric pressure results in saturation and only partial fluorine hydrogenolysis.100 The replacement of vinylic fluorines with hydrogen by catalytic hydrogenation proceeds more readily in fluoromaleic, fluorofumaric, difluoromaleic, and difluorofumaric acid 10.no It is assumed that fluorine hydrogenolysis occurs before saturation of the C = C bond. Yields of defluorinated products are strongly dependent on the catalyst used.110... 

During hydrogenation of vinylic chlorides, both saturation of the double bond and hydrogenolysis of the C-Cl bond take place.433 35 Also, selective hydrogenolysis of chlorine occurs in the presence of fluorine atoms on a Pd/C catalyst (Scheme 4.125).436... 

Hydrogenolysis of vinylic fluorides is hardly ever done with the aim of removing fluorine finom a molecule. Rather, it occurs as a usually undesirable side reaction, accompanying reductions of other functions in fluorinated compounds. " It frequently takes place during catalytic hydrogenation of fluorinated alkenes. A clear-cut saturation of the double bonds in polyfluorinated alkenes is rare and usually requires mild reaction conditions,and hydrogenolysis of fluorine without reduction of the double bond in catalytic hydrogenation is unknown. Numerous examples are quoted in the review literature, " and a few samples are shown in Table 1. 

Vinylic bromine can be replaced fairly easily without reduction of the double bond. This is a characteristic difference from hydrogenolysis of vinylic chlorine and especially fluorine. Catalytic hydrogenation of ethyl a-methyl-3-bromocrotonate resulted in replacement of bromine and formation of ethyl a-methylcrotonate (equation 15). In 14-bromocodeinone (9), replacement of bromine occurs with a double bond shift to give neopinone (10 equation 16).4... 

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