b CAS Key Laboratory of Synthetic Chemistry of Natural Substances, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China;
c School of Management Science & Engineering, Nanjing University of Finance & Economics, Nanjing 210046, China
Fluorinated surfactants are the most of efficient surfactants since they exhibit high surface activity at low critical micelle concentrations (CMC). In addition, fluorinated surfactants generally have high chemical and thermal stability. Commonly, they are composed of a hydrophilic group and a perfluorinated chain [1-3]. The most famous fluorinated surfactants are PFOA (C7F15CO2H) and PFOS (C8F17SO3X, where X = Na, K, H). To date, fluorinated surfactants have been widely used in firefighting, biomedical, textile, daily chemical, and other fields .
However, fluorosurfactants are more difficult to prepare than hydrocarbon surfactants. The previously reported methods are electrolytic fluorination , telomerization of fluoroolefins , oligomerization of fluoroolefins  and etc. Electrolytic fluorination uses inexpensive HF as the main raw material, but the method has many by-products. The telomerization of fluoroolefins customarily results in mixtures with broad chain lengths, so that the reaction conditions need to be strictly controlled. Hexafluoropropylene (HFP) and tetrafluoroethylene (TFE) are frequently applied in the oligomerization of fluoroolefins.
Apart from all the advantages and uses of PFOA and PFOS, the residual of them in the surroundings , combined with their toxicity , has led to global concerns and controls. The development of new environment friendly fluorinated surfactants with excellent surface property is essential. The results showed that hydrocarbon surfactants had low surface energy (γcmc = 23.8 mN/m) close to that of fluorocarbon surfactants after the formation of branched structure on common hydrocarbon surfactants . W. Dmowsi synthesized brached fluorocarbon surfactant with the structure of CF3(CF2)2C(CF3)2CH2CH2COONa and found its surface activity was better than the straight chain sodium perfluorooctanoate . Therefore, the introduction of branched fluorocarbon chain was an effective strategy for the synthesis of alternatives to PFOA. The raw materials of the branched fluorocarbon chain were mainly hexafluoropropylene oligomers, which had been mass-produced in the industry.
Recently, it was found that preparation of fluorocarbon surfactant intermediates using hexafluoropropylene dimer (HFPD) as the raw material boasted low environmental hazard and low raw material cost, which was a very significant direction for the development of substitutes to PFOS/PFOA. According to the current research and development status of alternatives to PFOS/PFOA, a series of branched fluorocarbon surfactants with better performance than sodium perfluorooctanoate were synthesized from HFPD by our group based on fluoro-olefin double bond reactive sites (Scheme 1) [12-19]. Besides, literature reported that perfluoroether chains were easier to degrade than fluorinated chains due to the better flexibility of ether bond [12, 20-22]. So, introducing ether bond based on D2 was an efficient and environmentally friendly method to develop fluorinated surfactants in this concern.
In view of the facts mentioned above, a series of novel fluorocarbon surfactant intermediates with oxy fluorocarbon chain were designed and synthesized with a new method based on the double bond of the HFPD, which further enriched the species of surfactants and provided experimental and theoretical foundations for the development and application of fluorosurfactants. All the chemicals, instruments used in this work, the experimental details and the key spectra are presented in the Supporting information.
In this work, addition of the fluoride to 1 was effected by leading either N2O4 into a cold, well-mixed suspension of 1 and KF in DMAc. If excess N2O4 was apllied, the green nitrosoalkane was slowly oxidized to the light yellow nitrite. The oxidation by N2O4 or O2 probably taked place through the radical-chain mechanism and occurred under milder situations than those reported for the oxidation of CF3CF2CF3(CF3)2CNO . Perfluoro-2-methyl-2- pentanol 2 was successfully prepared in a milder and simpler way. Since 2 possessed the property of acid, compound 3 was obtained by neutralization reaction with the base (Scheme 2).
|Scheme 2. Synthetic route of the perfluoro-2-methyl-2-pentanol potassium.|
Initially, perfluoro-2-methyl-2-pentanol potassium 3 and methyl 4-(bromomethyl) benzoate 4a were used as the model substrates to optimize reaction conditions. The results were summarized in Table 1. When Pd(OAc)2 (0.01 equiv.) and P(t-Bu)3 (0.03 equiv.) were used as the catalyst and ligand at 110 ℃ in xylene or toluene, the desired product 5a could only be obtained in 9% yield, and the yields could not be promoted obviously in the presence of Pd(OAc)2 and P(t-Bu)3 under other conditions (Table 1, entries 1–3). When Pyridine (1.2 equiv.) was used as the catalyst, the desired product could be obtained in about 18% yield in DMF and 20% yield in DMAC at 110 ℃, the yield could be promoted to 59% by reducing the temperature to room temperature (Table 1, entries 4–7). To our delight, 5a was obtained in 97% yield without catalyst in DMAc at room temperature (Table 1, entry 8). The reason may be that potassium perfluoro-2-methyl-2- pentanol had excellent nucleophilicity due to the strong electron withdrawing effect of the multiple perfluoromethyl groups. It can directly react with active benzyl bromide without adding alkali as catalyst at room temperature (Table 1, entry 8). The presence of strong alkali caused transesterification in the system, resulting in a decline in the yield of target products (Table 1, entry 7). High temperature was favorable for transesterification (Table 1, entry 6). Lowering temperature inhibited the occurrence of partial transesterification.
With the optimized conditions in hand, the scope of substrates was investigated. As shown in Scheme 3, a series of fluorocarbon surfactant intermediates could be synthesized smoothly from perfluoro-2-methyl-2-pentanol potassium 3 and the corresponding 4 in moderate to good yields. The substituted reaction between 3 and mono benzyl bromide derivatives proceeded well under the optimized conditions and gave the desired products 5a-5e in high yields. The reaction of compound 3 with polybenzylic carbon– bromine bonds of aromatic can also obtain the product 5f-5i in medium yields, which was greatly improved compared with the reported method . For the bromomethylbenzene sulfonate, such as methyl bromomethylbenzene sulfonate, we got the hydrolysis product of the fluorinated benzene sulfonate 5j directly. The surface tension of its aqueous solution could reduce to 19.2 mN/m. Besides, for the ethyl bromoacetate was used under the standard conditions, desired product (CH3CH2OCOCH2OC (CF3)2CF2CF2CF3) could also be obtained in 92% yields. Based on our results and the previous reports, we demonstrated that the potassium salt of perfluoro-2-methyl-2-pentanol can react well with benzylic bromides or 1-bromoalkanes to form perfluoro-2- methyl-2-pentyl ponytails at room temperature and under air atmosphere.
|Scheme 3. Synthesis of intermediates of novel branched fluorinated surfactants. Isolated yield. For 5 j, the final product had undergone hydrolysis.|
The results of conversion of surfactants converted from some intermediates and their surface activity were shown in the Table 2 . The results showed that these surfactants had excellent surface properties (PFOA, CMC = 3.1 ×10-2 mol/L, γcmc = 24.7 mN/m) .
In conclusion, we had developed a novel and efficient method for the synthesis of the critical intermediates of branched fluorinated surfactants with CF3CF2CF2C(CF3)2- group using HFPD as starting material. The reaction conditions were mild and easy to handle, which was promisingly applied to the industrial production. Since these compounds had the branched fluorocarbon chain and the total number of fluorocarbons was 6, their degradability was better . This way was a simple, environmentally friendly and economical to develop PFOA/PFOS alternatives.
We believe that they can be applied to the substitute for PFOA. Compounds 5a-5k can be used as perfect intermediate for the development of new fluorinated surfactants. On the basis of these results, we will further develop new surfactants which have the branch-chained group CF2CF2CF2C(CF3)2-, characterize their behavior in solution and use them in special applications.Acknowledgment
This project was supported by the National Natural Science Foundation of China (No. 2167020782).Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cclet.2018.11.014.
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