Isomerization of Hydrofluorocyclopentenes Promoted by Fluoride Anion

The isomerization of hydrofluorocyclopentenes promoted by fluoride anion was investigated. It was found that two processes were responsible for interconversion of the isomers: an allylic syn-addition/elimination of fluoride anion that does not change the mutual positions of hydrogen atoms but is responsible for transfers of fluorine atoms, and a fluoride anion-assisted deprotonation/protonation which does not change the mutual positions of fluorine atoms but is responsible for transfers of hydrogen atoms. In the deprotonation, HF can easily capture excess fluoride anion to form 2 HF − anion which can probably inhibit the protonation.


Introduction
In order to fulfill the Montreal Protocol and the Kyoto Protocol, which mandate to phase out the applications of both ozone depleting substances (ODS) and high greenhouse substances in the field of chlorofluorocarbons (CFCs), many countries have looked for alternatives of CFCs.Hydrofluoroolefins (HFOs) have short atmospheric lifetimes, leading to distinct environmental benefits.Thus, HFOs have been considered as alternatives to HCFCs and HFCs.Heptafluorocyclopentenes, including 1H-heptafluorocyclopentene (F7E-1H), 3H-heptafluorocyclopentene (F7E-3H) and 4H-heptafluorocyclopentene (F7E-4H), are one kind of alternatives that have zero ODP and low GWP.It is used as dry etching gas, fluorine-containing medicine intermediate, and hydrofluorocarbon-based solvent [1].
The above isomerization technologies focused mainly on the isomerization between geometric isomers of HFOs [6] [7] [8] [9], while the isomerization between positional isomers of HFOs was rarely reported.The latter type of isomerization of HFOs always occurred at high temperature in the presence of special catalysts [10] [11], such as Cr 2 O 3 , which are always considered as business secrets, and for competitive reasons, details of these catalysts are probably not public.Until now, few literatures reported that the isomerization between positional isomers of HFOs promoted by simple catalysts occurred under mild conditions.
Here, we reported the isomerization of hydrofluorocyclopentenes (c5-HFOs) promoted by fluoride anion under mild conditions.The structures of c5-HFOs were confirmed by GC-MS, 1 H NMR and 19 F NMR. Based on the results of our experiments, the mechanisms of the isomerizations of c5-HFOs in liquid-phase were proposed.

Instrument
The mass spectrometer was a GC-MS-QP2010 Ultra (Shimadzu).

Experiment Procedure
Preparation of raw materials F7E-1H and F7E-4H: (NH 4 ) 2 CO 3 (0.50 mol), tetra-n-butylammonium bromide (0.05 mol) and 500.0ml of H 2 O were placed into a 1000 mL, three-necked, round-bottomed flask equipped with a thermometer and an agitating device.Then cis-F8A (0.50 mol) was added by drops into the above solution.Under magnetic stirring for 6 h at 50˚C, the organic phase of the products from the above system was dried with 4A molecular sieve.Then, the organic phase was detected by 19  The products were washed by water, which separated the liquid-phase organic product from HCl. Next, the liquid organic products were dried by a molecular sieve and then detected by 19 F-NMR.The result was shown as follows: the total conversion of F6-13 and F6-14 was 87.3%, and the yield of F6E-33H, F6E-13H, F6E-14H and F6E-44 was 16.8%, 45.1%, 10.6% and 2.5%, respectively.The above Green and Sustainable Chemistry organic phase was directly rectified by a distillation tower with 3 meters long, F6E-33H, F6E-14H and F6E-13H were obtained, respectively.
Preparation of raw materials F6E-12H and F6E-15H: (NH 4 ) 2 CO 3 (0.50 mol), tetra-n-butylammonium bromide (0.05 mol) and 500.0ml of H 2 O were placed into a 1000 mL, three-necked, round-bottomed flask equipped with a thermometer and an agitating device.Then F7A (0.50 mol) was added by drops into the above solution.Under magnetic stirring for 6 h at 50˚C, the organic phase of the products from the above system was dried with 4A molecular sieve.Then, the organic phase was detected by 19 F-NMR.The conversion of F7A was 70.5%, and the yield of F6E-12H, and F6E-15H was 64.0%, and 6.5%, respectively.The above organic phase was directly rectified by a distillation tower with 3 meters long, F6E-12H and F6E-15H were obtained, respectively.
Liquid-phase isomerization of isomers of heptafluorocyclopentene: metal fluoride (0.015 mol) and 20.0 ml of N.N-dimethylformamide were placed into a 50 mL, three-necked, round-bottomed flask equipped with a thermometer, and an agitating device.Then F7E-3H, F7E-1H or F7E-4H (0.03 mol) was added by drops into the above solution.Under magnetic stirring for a certain time at room temperature (25˚C), the products from the above system were scrubbed with 100 mL H 2 O to remove metal fluoride and N.N-dimethylformamide. Later, the products were dried with 4A molecular sieve to obtain the organic phase of the product, which was detected by 19 F-NMR.The results were shown in Table 1.
Table 1.Impact of catalyst on the isomerization of heptafluorocyclopentene a .  1F NMR versus a calibrated internal standard.

Green and Sustainable Chemistry
Liquid-phase isomerization of isomers of hexafluorocyclopentene: metal fluoride (0.015 mol) and 20.0 ml of N.N-dimethylformamide were placed into a 50 mL, three-necked, round-bottomed flask equipped with a thermometer and an agitating device.Then F6E-33H, F6E-13H, F6E-14H, F6E-15H or F6E-12H (0.03 mol) was added by drops into the above solution.Under magnetic stirring for a certain time at room temperature (25˚C), the products from the above system were scrubbed with 100mL H 2 O to remove metal fluoride and N.N-dimethylformamide and were dried with 4A molecular sieve to obtain the organic phase of the product.The organic phase of the product was detected by 19 F-NMR.The results were shown in Table 2.
Table 2. Impact of catalyst on the isomerization of hexafluorocyclopentene a .   F NMR versus a calibrated internal standard.

Results and Discussion
In DMF, promoted by various alkali metal fluoride such as LiF, NaF, KF, RbF or CsF at 25˚C for 6 hours, F7E-3H was isomerized into F7E-1H and F7E-4H (See Table 1).The conversion of F7E-3H increased with the increasing atomic number of alkali metal.The larger the atomic number of alkali metal, the stronger the reactivity of alkali metal fluoride in halogen exchange [14].And the efficacy of the alkali fluorides with respect to replacement reactions ranked in the sequence: CsF > RbF > KF > NaF > LiF [15] [16] [17].Therefore, CsF owned the highest activity in the isomerization of F7E-3H.In addition, the solubility of the alkali metal fluoride in DMF affect the reactivity at a certain, but not only reason.In DMF, RbF (1.05 mM) has a higher solubility than CsF (0.60 mM) in DMF [18], but its reactivity is weaker than CsF in the isomerization of F7E-3H.The isomerizations of c5-HFOs belong to a kind of special halogen exchange (fluorine-fluorine exchange), which are probably not simply solution reactions but probably occurring on the surface.The system usually involve an excess of the solid metal fluoride present and, indeed, in some reactions it has been observed that the amount of solid metal fluoride is important [14].Therefore, alkali metal fluoride was always excess in the isomerizations of c5-HFOs.

As listed in
1) Alkali metal fluoride was partially soluble in DMF [18].This provided the naked fluoride anions to catalyze the isomerizations of c5-HFOs.
2) In DMF, the C=C of F7E-4H underwent nucleophilic attack by the incoming fluoride anion to give a tetrahedral intermediate I-34, which proceeded to the product F7E-3H by elimination of fluoride anion.The whole process belonged to an allylic syn-1-addition/3-elimination of fluoride anion [12] [19] [20], which was reversible.This is because the syn-elimination reaction of the five membered ring is easier to form a co-planar transition state, which reduces the energy barrier and makes the reaction easy to occur [21].

Conclusion
In the isomerization of c5-HFOs promoted by fluoride anion, two main processes are responsible for the interconversion of the isomers.An allylic syn-addition/ elimination of fluoride anion that does not change the mutual positions of hydrogen atoms but is responsible for transfers of fluorine atoms, and a fluoride anion-assisted deprotonation/protonation which does not change the mutual positions of fluorine atoms but is responsible for transfers of hydrogen atoms.It was also revealed that HF 2 − anion in the deprotonation can inhibit the protonation.This is probably the reason that neither F6E-33h nor F6E-44H was found in the isomerization of F6E-13H or F6E-14H.
number (n) of Fn or Hn in NMR data is in agreement with the number in the molecular structure.

4. Analytic Results of and by-Products
Zhang et al.

Table 2
12H is the absolutely preferred isomer among the various isomers of hexafluorocyclopentene.In addition, the selectivity of various c5-HFOs can be Green and Sustainable Chemistry controlled when the different alkali metal fluoride is chosen as a catalyst.

Table 3 )
, once HF captured excess fluoride anion, with the formation of a very stable anion HF 2 − , the protonation of the intermediate was terminated because the anion HF 2 − was incapable of neither catalyzing isomerizations nor returning the proton back.Thus, the reaction was "frozen".When quenched with H 2 O in the experiments, the intermediate returned to its raw material.Therefore, neither F6E-33H nor F6E-44H was found in the isomerization of F6E-14H or F6E-13H.

Table 3 .
Gibbs free energies calculations for the formation of HF 2 − and the ionization of