Nucleophilic substitution on the gem-difluoromethylene double bond

It should be noted that some highly fluorinated alkenes are poisonous [20]. Perfluo-roisobutene (PFIB 63), a by-product of Teflon manufacture, is one of the most toxic among them and is more toxic than phosgene. Other fluorinated alkenes such as hexafluorocy-clobutene (HFCB 64) and tetrafluorocyclopropene (73) are toxic by inhalation. PFIB has 

Fluorine-accelerated Claisen rearrangement and unusual behavior in aza-Claisen rearrangement 

Fluorine substituents on the C—C double bond greatly accelerate Claisen rearrangement. For instance, difluorovinyl prenyl ether 83 rearranges at 100°C, while nonfluorinated vinyl prenyl ether rearranges at 200°C [ 1 ]. The chemoselective transformation of 76 to 75 reveals also a favorable fluorine-directed rearrangement [2]. 

The temperature required for aza-Claisen rearrangement is about 100-150°C higher than those for the corresponding ethers [3]. The trend is the same in the rearrangment of fluorinated compounds as shown below. The totally unique products formation observed in aza-Claisen rearrangement of fluorinated enamines as compared with those of ethers suggests a different mechanism via, not a concerted six-membered transition state, but a stepwise radical pathway [4]. 

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