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2.2.. The use of perfluoroalkyl halides  in the synthesis of perfluoroalkanesulfonic acids

One of the methods of perfluoroalkanesulfonic acids obtaining extensively used in the chemistry of organofluorine compounds is the transformation of perfluoroalkyliodides. As the iodine movability in these compounds is not high enough, at first lithium salts are obtained, which react with sulfur dioxide, producing lithium salts of perfluoroalkansulfinic acid. The last mentioned oxidize in acetic acid at temperature of 100 ^oC by forming lithium salt of perfluoroalkanesulfonic acid. Usually such acids is obtained by reaction of lithium salt with concentrated sulphuric acid [24].

However this method is used only for lower acids, even perfluoropropyliodide doesn't produce the base product. Here the limiting stage is the obtaining of trifluoromethyl-lithium and pentafluoroethyl-lithium. The oxidization of perfluoroalkansulfinic acid salts  can be conducted by hydrogen peroxide. However in this case the formation of perfluoroalkancarboxylic acid is registered with limited for one unit carbon backbone chain [25].

The use of perfluoralkyl iodides in the synthesis of sulfinic acids is inseparably linked with works of Chinese researchers on sulfinatehalogenation of  halogen containing hydrocarbons. Thus they were the first to state, that perfluoralkyl iodides are able to react with Na₂S₂O₄  in the presence of Lewis' basis (NaHCO₃, Na₂CO₃) in aprotic solvents (DMF, N-methylpyrrolidone, HMTPA) at 40-120 ^oC and at the reaction period 1-10 hours, the result of this was the synthesis of sulfinic acid derivatives.  The review of perfluoroalkanesulfinate chemistry, including their obtaining , characteristics, reactions, and application is given in the works  [33-35].

Using the cheap reducer (for example Na₂S₂O₄ ) one can obtain salts of perfluoroalkansulfinic acids from perfluoroalkyl halides (R_FX, where X=Br,I , R_FCCl₃) in mild conditions. The fact, that this method is extensively used for carrying out the reactions of mentioned salts with olefins, dienes, acetylene derivatives and aromatic compounds is more important. Along with that the present system is effective mainly for polyfluorinated alkyl halides [27]. Bromine and iodine situated in perfluorinated carbon backbone chain are easily replaced and base product is formed with high yield. In future such works were carried out by other researchers, who confirmed the effectiveness of this methodology  [36-50].

Later it was shown that another sulfur compounds, possessing nucleofilic properties, are able to react with perfluoralkyl iodides forming salts of perfluoroalkansulfinic acid (table 1) [42,51-54]. Obviously, these reactions pass according to one-electron transfer mechanism (SET), the catching of perfluoralkyl radical by olefins and nitrosocompounds is the confirmation of this fact [26].

The process of sulfonylization by Na₂S₂O₄ appeared to be important, because it allows to obtain the corresponding perfluoroalkanesulfonic acids, which there are important intermediate components for surfactant synthesis, omitting the electrochemical fluorination stage and using available perfluoroalkyliodides [28,42,55,56]. For example, X(CF₂)_2nO(CF₂)₂SO₃Na (X = Br, I, n =1,2), I(CF₂)₂O(CF₂)COONa, I(CF₂)_nO(CF₂)_nSO₃Na can be obtained from corresponding perfluoroalkyl halides Cl(CF₂)_nI (n = 2,4,6), H(CF₂)₈X (X = Br, I); Br(CF₂)₄Cl and I(CF₂)₂O(CF₂)₂I in the presence of phase-transfer catalyst in polyethylene glycols 200 and -600 or in acetonitrile, ethanol, diglyme.

Also, this reaction can be carried out with perfluoroalkylbromides, for example CF₃Br, which produces sodium salt of trifluoromethansulfinic acid with 90% yield. 1,1,1-Trichloroperfluoroalkanes appeared adequate for these purposes (product yields 90% and more) [37-31, 57,58].

Triethylamine was used as a base [30]. 

The reactions of ,-diiodoperfluoroalkanes [41] and ,-dibromoperfluoroalkanes  with equivalent amount of Na₂S₂O₄ in the media MeCN-H₂O in the presence of sodium bicarbonate result in mixtures of sodium salts of perfluoroalkane-,-sulfinates (yields 66-90%)(see table 2).

I(CF₂)_nSO₂Na and NaSO₂(CF₂)_nSO₂Na at chlorine action at  0^oC produce appropriate sulfochlorides I(CF₂)_nSO₂Cl and ClSO₂(CF₂)_nSO₂Cl [n = 3,4,6]. 

Table 1. Synthesis of  perfluoroalkansulfinic acid sodium salts

Table 2. Sulfinilizing  and subsequent chlorination of ,-diiodoperfluoroalkanes I(CF₂)_nI [41,47].

^a mole ratio
^bin parentheses the yields from NMR ¹⁹F information

The mechanism of this process can be presented as follows: 

The sulfinatedehalogenation reaction had opened a new way to the synthesis of perfluoroalkane-sulfinic and -sulfonic acids and their derivatives. It is interesting, because perfluoroalkyl halides is transformed directly into perfluoroalkylsulfinate.  In this case there is no need of intermediate section – the synthesis of organometallic derivative. This process is surely interesting for extensive application in the technology [27-30,36-39,58].
 

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