Unsaturated compounds, fluorination

Fluorinated carbocations play an important role as intermediates in electrophilic reactions of unsaturated compounds, although the highly electron-deficient double bond in fluoroalkenes is not usually susceptible to electrophilic attack. Only the most powerful electrophilic reagents... 

Dehydrobromation of halogenated phosphonium salts is currently used in the synthesis of unsaturated compounds, (cf. Section II.C) provided that a labile carbanion is created in a suitable position relative to the halogen atom (reaction 261). Such bases as Et3N (for / - or (5-halogenated phosphonium salts)288 or bromide anion357 may induce this elimination. Fluorine atoms of the cationic part of 3-methyl-1,1-difluorocyclopent-2-ene-1-phosphonium hexafluorophosphate were substituted when the compound was treated with absolute methanol under mild conditions412. 

Preparation of Unsaturated Organo-Fluorine Compounds by Intramolecular Dehydrohalogenation and Dehalogenation Reactions... 

The simultaneous elimination of fluorine and chlorine from a-fluoroacyl halides results in the formation of a,/i-unsaturated compounds 2.192... 

Additions of elemental halogens to unsaturated compounds are among the most common preparations of halogenated fluoroorganics. The transformations are usually fairly clean and proceed in good yields. Besides the numerous examples of halogen addition tofluoroalkenes and fluoroalkyl-substituted alkenes, additions to perfluoropropyl vinyl ether [2] and fluorinated styrenes [5, 4] have been reported. Both ionic and free-radical processes occur (equations 1 and 2)... 

Fluorinated carbocations play an important role as intermediates in electrophilic reactions of fluoroolefins and other unsaturated compounds. For example, F-allyl cation 1 was proposed as a reactive intermediate in reactions of HFP with fluoroolefins catalyzed by Lewis acids [7]. The difference in stability of the corresponding allylic cations was suggested as the explanation for regio-specific electrophilic conjugated addition to CF2=CC1CF=CF2 [11]. Allylic polyfluorinated carbocations were proposed as intermediates in the reactions of terminal allenes with HF [53] and BF3 [54], ring-opening reactions of cyclopropanes [55], Carbocations are also an important part of the classic mechanism of electrophilic addition to olefins (see Eq. 2). This section deals with the questions of existence and stability of poly- and perfluorinated carbocations. 

The proposed mechanism of oxidative fluorination of unsaturated compounds by halogen fluorides [84-86] and VF5 [33] includes electron transfer from substrate to halogen fluoride or VF5 as a first step, followed by addition of F" to a radical-cation, leading to formation of a radical and its further oxidation to carbocation (see Eq. 12, pathways A,B). It should be pointed out that this is not the only direction, and the actual mechanism may depend strongly on the substrates and reaction conditions. Other mechanisms, such as a radical process (pathway C), cannot be ruled out. 

In a few isolated instances it has been possible to treat an unsaturated compound with a derivative capable of giving off free fluorine, namely, lead tetrafluoride or iodobenzenedifluoride, which are formed, respectively, from lead tetraacetate or iodosobenzene and hydrogen... 

The addition of fluorine to unsaturated compounds can be achieved by the use of a mixture of lead(IV) oxide and anhydrous hydrogen fluoride or lcad(IV) oxide and sulfur... 

Table 12. Fluorination of Unsaturated Compounds with Hydrogen Fluoride/ Lcad(IV) Oxide...

The electrochemical fluorination of alkenes in anhydrous hydrogen fluoride is another possibility for the addition of fluorine to unsaturated compounds, but the yields are poor to moderate and the difluoro addition products are only byproducts. 

The pyrolytic decomposition of the sodium salts of various fluorinated carboxylic acids to give isomeric unsaturated compounds has also been reported. The products were identified as alkenes with the C = C bond inside the carbon chain, mainly alk-2-enes. This isomerization may be catalyzed by the coal-like products formed during the pyrolytic decarboxylation of the salts, but the metal fluoride formed in the reaction may also be responsible for the isomerization. When potassium perfluoro(5-chloropentanoate) is heated in a rocking autoclave at 300 C for 2 hours, perfluorobut-2-ene (2b) is isolated in 82% yield.This is only possible by migration of the double bond away from the terminal position after carbon dioxide elimination and halogen exchange to form potassium chloride. ... 

The addition of fluorine to unsaturated compounds can be achieved by the use of a mixture of lead(IV) oxide and anhydrous hydrogen fluoride120 - 122131,136 or lead(IV) oxide and sulfur tetrafluoride.123 132 The combination of lcad(IV) acetate with anhydrous hydrogen fluoride7,124-130133,134 also results in addition of fluorine to unsaturated derivatives. The active reagent in this case is lead diacetatc difluoride. A mixture of lead(IV) fluoride and acetic acid is also effective in the same way.133 Some examples of these reactions are summarized in Tables 12 and 13. and arc given by the formation of 10.123 11,123 and 12.7125126... 

Fluorination of Unsaturated Compounds with Sulfur Tetrafluoride/Lead(IV) Oxide General Procedure 123... 

Table 14. Fluorination of Unsaturated Compounds with Cobalt(IIl) Fluoride or Potassium Tetrafluorocobaltate...

Saturated fluorocarbons do not polymerize under normal conditions, although their pyrolysis at high temperatures (500-800 ) is well-known. However, they can exhibit surprising reactivity in a glow discharge. The rate of polymer deposition decreased with increasing number of fluorine atoms in unsaturated compounds. However the rate of fluorinated film deposition increased with increasing fluorine content in saturated compounds. The presence of C-H inter-... 

---