Thermolysis of γ-Fluorosulfonyl perfluorobutanoic acid derivatives: New synthetic routes for perfluorinated cyclic sulfones

https://doi.org/10.1016/j.jfluchem.2020.109500Get rights and content

Highlights

  • Thermolysis of γ-fluorosulfonyl perfluorobutanoic acid derivatives.

  • First synthesis of cyclic perfluorinated sulfone through intramolecular cyclization.

  • Novel synthetic method for the preparation of hexafluoropropene.

  • Preparation of perfluorinated carbonyl fluoride as a main product by thermolysis of alkali carboxylate.

  • Optimized reaction conditions.

Abstract

The thermal decarboxylation of previously unknown γ-fluorosulfonyl perfluoro-butanoic acid derivatives and a novel method for the synthesis of perfluorinated cyclic sulfones are described. For the first time, parallel processes of decarboxylation and desulfonylation in one molecule were observed. In addition, the reversibility of the alkaline alcoholysis of perfluorocarbonyl fluorides and the possibility of their generation by the reaction of alkali metal carboxylates with inorganic fluorides under higher temperatures are presented. The reaction conditions, factors affecting reactivity and regioselectivity of the thermolysis process, as well as the pathways of main and side reactions are discussed.

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Introduction

The thermal decarboxylation of sodium and potassium salts of perfluorinated carboxylic and oxy-carboxylic acids is a convenient method to obtain perfluoroalkenes [1] and perfluorovinylethers (PVE) [2] which are important monomers in the synthesis of fluorinated polymers [3]. As an alternative for the preparation of PVE, so far two examples for the decarboxylation of β-alkoxy-carboxylates and their TMSO esters in the presence of TMS-OK [4a] or CsF [4b] have been described. These methods have some advantages, namely the preparation of salts is avoided and moreover it is not necessary to use the energy-consuming drying process of potassium or sodium salts. In its turn, the boiling point of the trimethylsilyl derivatives should be higher than the temperature of the process. Therefore, this method is preferred for high-boiling trimethylsilylesters (b.p. > 120–140 °C). In contrast, the PVE should have lower boiling points for effective separation. Thus, it is difficult to conduct the thermolysis in vacuo. The presence of a fluorosulfonyl group in terminal perfluoroalkenes (PFA) or in perfluorovinyl ethers (PVE) allows them to be used as monomers to produce highly effective ionomers [5a], polymer electrode membrane [5b] or phase carrier in catalytic hydrogenation processes [5c]. However, only a few representatives of this class of compounds are known in literature [5a,[6]] and the methods of their synthesis are rather limited. For the terminal FSO2-PFA only one example is known, namely FSO2CF2CF2CF = CF2. Its synthesis includes the substitution of iodine in a fluoroalkane by SO3Na, followed by chlorination and fluorination of the sulfuryl group and dechlorination of the substrate upon treatment with zinc [5a]. In contrast to that, terminal FSO2-containing perfluorovinyl ethers are described much more extensively [[6]]. Their preparation is based on the reaction of fluorosulfonyl perfluoroalkyl carbonyl fluoride with CsF [6a] and the subsequent reaction with hexafluoropropene oxide (HFPO) [6a], perfluorovinyl chlorosulfate [6b] or 1,2-difluoro-1,2-dichloroethylene [6c]. This kind of reaction involves multistep synthesis often accompanied with low yields. The use of gaseous and highly toxic reagents as well as special methods, such as electrochemical fluorination is also required in this case [6d]. It is known, that the decarboxylation of perfluorocarboxylates proceeds through the formation of CO2 and the formation of a thermodynamically unstable perfluorocarbanion. Its subsequent stabilization is accompanied by the elimination of fluoride as sodium or potassium fluorides and by the formation of a stable perfluorinated double bond [[7]]. Nevertheless, one example is described with carbanion stabilization which proceeds not through the elimination of F and formation of terminal FSO2-PVE, but via intramolecular nucleophilic fluorine substitution in the FSO2 group. Subsequently, the formation of the corresponding five-membered cyclic perfluoro 1,3-oxathiolane-3,3-dioxide occurs [7c]. During the synthesis of FSO2-containing terminal perfluoroalkenes similar results were not obtained and the only cyclic perfluorinated sulfone described in literature, namely perfluorosulfolane, was for the first time obtained upon hydrolysis of perfluorotetramethylenesulfuroxy difluoride [8a]. The synthesis of perfluorosulfolane was also achieved via direct fluorination of sulfolane with elemental fluorine in 28 % yield [8b] or electrochemically in a solution of liquid aHF (Fig. 1) [8c]. In the latter case, β-sulfolene could be also used as a substrate, but regardless of the choice of the starting compound, perfluorosulfolane was only registered as a by-product (5–10%) during the synthesis of n-nonafluorobutanesulf​onyl fluoride (Fig. 1) [8d]. Due to its complete inertness in radical processes and due to its good dissolving power for perfluorinated monomers, perfluorosulfolane (A, Fig. 2) was found to be a valuable solvent in the preparation of low viscosity fluoroelastomers under radical process [9a].

As a consequence of its high reactivity towards nucleophiles, perfluorosulfolane is an important synthon in the synthesis of some substituted fluorinated sulfonic acids [8d,9b] and β-sulfoketones [9c]. It is also known that some linear perfluorosulfones are useful components in fire extinguisher (B, Fig. 2) [10a], in electrochemical device electrode (C, Fig. 2) [10b] or as an electrolyte solvent for lithium-ion batteries (D, Fig. 2) [10c].

Based on those data and expanding our latest research in the field of perfluoroallyl derivates [10d] we believe, that the study of the thermolysis of compounds like 2,2,3,3,4,4-hexafluoro-4-(fluorosulfonyl)butanoyl fluoride, which are usually obtained by electrochemical fluorination of the corresponding 1,4-butane sultone in liquid aHF [6d], can have practical significance for the production of new functionalized monomers in the polymer industry. In addition, it could be an important contribution to the area of fluoroorganic chemistry. In this work, we have for the first time systematically investigated the thermal decarboxylation of mono and disodium and potassium salts, obtained from of 2,2,3,3,4,4-hexafluoro-4-(fluorosulfonyl)butanoyl fluoride, and its TMSO-carboxylic derivative in the presence of fluoride anion or a base.

Section snippets

Thermolysis of sodium and potassium 2,2,3,3,4,4-hexafluoro-4-(fluorosulfonyl)butanoate

It is well established that alkali carbonates react selectively with carbonyl fluorides in polar aprotic solvents even in the presence of fluorosulfonyl groups giving rise to the corresponding salts and formation of CO2 [11]. Moreover, in this case the fluorosulfonyl substituent does not participate in the reaction. We observed that 2,2,3,3,4,4-hexafluoro-4-(fluorosulfonyl)butanoyl fluoride (1) reacted with freshly dried sodium carbonate at 22–29 °C in polar, aprotic solvent to give monosodium

Conclusions

For the first time, the thermolysis of several derivatives of hexafluoro-4-(fluorosulfonyl)butanoyl fluoride 1 is reported, namely of its monosalts 2 and 6 and salts 8 & 9, and trimethylsilyl ester 13. Decarboxylation was studied in various high-boiling point solvents (TG, TriG, NMP, DMI, DMF, PhCN) at atmospheric pressure and without solvents in vacuuo, in the presence of additives (Ph2SO2, DMPU, 18-C-6) and catalysts (TMSOK, KF and CsF), as well as under gas-phase conditions on the surface of

General

All thermolysis reactions were carried out in glass reaction vials under an atmosphere of dry Ar. Before use, MeCN, PhCN, DMF, NMP, and DMI were freshly distilled from CaH2. TG, TriG, and MG were distilled from sodium / benzophenone and used immediately. NaF, KF, CsF, Na2CO3, and K2CO3 were carefully dried in vacuo (0.5 mmHg) at 240 °C for 12 h. Unless otherwise noted, all reagents and starting materials were purchased from commercial sources and used without further purification. The

Declaration of Competing Interest

We declare no conflict of interests.

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