Alkaline aqueous solution promoted debromination of 1,2 dibromo-fluorocarbons – A convenient method for electron deficient perfluorovinyl ethers

Article abstract of DOI:10.1016/j.tetlet.2017.04.025

A facile and efficient base-mediated protocol for debromination of vic-dibromides in perfluoroalkyl(aryl) compounds in aqueous medium has been demonstrated. With mild reaction conditions, the developed strategy has a good substrate scope and electron-deficient olefin products were obtained in good yields. A mechanistic explanation of the debromination is offered with three key experimental observations: (1) the reactions are accelerated by the more electron-rich nucleophiles, (2) the reactions are promoted by the more electron poor vic-dibromides in perfluoroalkyl compounds, and (3) the nucleophilic side reaction is preventable. It is evident that the electronic factors strongly dictate vic-dibromides elimination to the perfluorovinyl ethers, which are the precursors for various perfluorinated polymers. The different reaction conditions were tested in implicit solvent (water) conditions, which helped to confirm the E2-like mechanism.

Full text of DOI:10.1016/j.tetlet.2017.04.025

Accepted Manuscript
Alkaline Aqueous Solution Promoted Debromination of 1, 2 Dibromo-fluoro-
carbons – A Convenient Method for Electron Deficient Perfluorovinyl Ethers
Hua Mei, Rebecca McCloud, Faisal Ibrahim, Chimaroke Nworie, Anna Musket
PII:
S0040-4039(17)30455-0
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.04.025
TETL 48817
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
22 February 2017
3 April 2017
5 April 2017
Please cite this article as: Mei, H., McCloud, R., Ibrahim, F., Nworie, C., Musket, A., Alkaline Aqueous Solution
Promoted Debromination of 1, 2 Dibromo-fluorocarbons – A Convenient Method for Electron Deficient
Perfluorovinyl Ethers, Tetrahedron Letters (2017), doi: http://dx.doi.org/10.1016/j.tetlet.2017.04.025
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Alkaline Aqueous Solution Promoted Debromination of 1, 2
Dibromo-fluorocarbons – A Convenient Method for Electron
Deficient Perfluorovinyl Ethers
Hua Mei*, Rebecca McCloud, Faisal Ibrahim, Chimaroke Nworie, Anna Musket
Department of Chemistry, East Tennessee State University, Johnson City, TN 37604, United
States
meih01@etsu.edu
Abstract
A facile and efficient base-mediated protocol for debromination of vic-dibromides in
perfluoroalkyl(aryl) compounds in aqueous medium has been demonstrated. With mild reaction
conditions, the developed strategy has a good substrate scope and electron-deficient olefin
products were obtained in good yields. A mechanistic explanation of the debromination is
offered with three key experimental observations: (1) the reactions are accelerated by the more
electron-rich nucleophiles, (2) the reactions are promoted by the more electron poor vic-
dibromides in perfluoroalkyl compounds, and (3) the nucleophilic side reaction is preventable. It
is evident that the electronic factors strongly dictate vic-dibromides elimination to the
perfluorovinyl ethers, which are the precursors for various perfluorinated polymers. The different
reaction conditions were tested in implicit solvent (water) conditions, which helped to confirm
the E2-like mechanism.
Introduction
It is well known that protection/deprotection of double bonds via bromination-debromination
is widely utilized to prevent the undesirable side reaction to the olefins.1 Except for the
traditional methods with metal zinc/acetic acid or sodium iodide/acetone, recent reports on
debrominations include reactions with strong nucleophiles, such as sodium dithionite and
selenium tellurides, and low-valent metals, such as lithium, sodium, copper, iron, samarium,
indium, and the organic solvent DMF.2 The different debrominating agents and methods were
explored depending on the substituent patterns and other functionality in the substrates being
inert to the conditions of debromination.3 However, none of them efficiently provided electron-
deficient poor olefins, such as perfluorovinyl ethers, from vic-dibromides under mild conditions.
Perfluorovinyl ethers (RfOCF=CF2) are main components of monomers to be copolymerized
in fluoroelastomers and to be used to modify the crystallinity in fluoroplastics.4 At elevated
temperature, those copolymers exhibit excellent chemical and mechanical properties for wide
industrial applications.5 During the past several years, our research groups have been actively
involved in preparing various of diazonium perfluoroalkyl(aryl) sulfonimides (PFSIs)
compounds for proton exchange membrane (PEM) fuel cells and photo acid generators.6 Among
them, a series of diazonium PFSI zwitterionic monomers, containing the polymerizable
1

perfluorovinyl ether moiety, are employed to yield desired polymer electrolytes in PEM fuel
cells.7
Interestingly, the debromination of vic-dihalides occurred while refluxing one intermediate
of diazonium PFSI monomer, ArSO2N-(M)SO2CF2CF2OCF(CF3)CF2OCFBrCF2Br (compound
5b in Figure 1) with Cs2CO3 water/acetone solution.8 This observation intrigues the exploration
of more convenient and efficient routes toward protection/deprotection versatile perfluorovinyl
ethers in PFSI compounds (Figure 1).
Here, we wish to report the reflux-assisted or sonication assisted debromination of vic-
dibromides in the case of PFSIs with moderate base/nucleophiles, which provides the
perfluorovinyl ether moiety with excellent yield.
Figure 1. Nitro Substituted Aromatic PFSIs
Results and Discussion
Although the debromination of vic-dibromides generally proceeds smoothly to provide the
olefins with high yields, the regular methods encounter quite a few challenges towards
brominated perfluorovinyl ether pendant in perfluoroalkyl(aryl) compounds.9 In general, highly
fluorinated alkenes are very electron deficient species and therefore susceptible to attack by a
wide range of nucleophiles.10 Therefore, the nucleophiles promoted debromination, which often
used for regular olefins, is not tried for perfluorovinyl ethers. On the other hand, due to the
specific electronic and steric properties of F-alkyl groups, perfluorinated olefins, particularly
those containing one or two perfluoroalkyl groups, are extremely resistant towards electrophilic
attack.
As part of our research project to prepare polymer electrolytes for PEM fuel cells from
diazonium PFSI monomers, perfluorovinyl ether was protected with bromination first (Step 2 in
Scheme 1) against the strong nucleophiles (2a-d in Scheme 1) at high temperature. The
debromination (Step 5 in Scheme 1) was anticipated to recover the perfluorovinyl ether group
after the nucleophilic substitution reaction (Step 3 in Scheme 1).
2

Scheme 1. Part of Synthetic Route for Diazonium PFSI Monomers
Initially, NaI/acetone, the metallic zinc, and iron in acid were attempted to debrominate
brominated perfluorovinyl ethers in PFSIs because of low cost and availability. Except for the
first one, the rest of transition metals/acid systems worked but often offered moderate yields
around 50-75% with unavoidable hydrogenated by-products.11 For example, during the zinc
redox reaction, the maximum conversion was achieved via refluxing with more than the tenfold
excess of activated zinc powder in dry acetonitrile overnight.9 Due to strong electron-
withdrawing perfluoroalkyl pendant, the zinc insertion product (ArOSO2N(M)SO2RfOCF2CF2-
ZnBr) exhibits unusual stability in these sluggish redox reactions. Care must be taken that the
PFSI vic-dibromides do not debromination completely with a small amount of moisture. Thus,
those traditional methods suffer from harsh reaction conditions, tedious purification procedures,
and moderate yields in addition to characterization and waste handling problems.11
As reported earlier, the work up the process to purify the crude nitro-substituted aromatic
PFSI compound 5b involved first acidification, followed by neutralization with Cs2CO3 aqueous
solution and recrystallization with acetone/water. The unusual partial debromination took place
during the recrystallization process.5 Encouraged by this result, our investigation started with an
effort to discover and optimize the reaction conditions for debromination of vic-dibromides to
perfluorovinyl ether with inexpensive reagents and environmental benign reaction conditions.
The cesium salt of vic-dibromide of 4-nitrobenzene PFSI compound (5a in Scheme 1), because
of the accessibility, was first chosen as a probe for the optimization process. The initial trials
were carried out as the similar process of the last two steps to work-up the coupling product of
3

5a, which involved neutralization by Cs2CO3 and next refluxing in the co-solvent of
acetone/water. Entries 1 to 4 in Table 1 represent the examples of debromination of 5a via
refluxing.
These examples illustrate the issues that must be considered in optimizing reaction protocol.
The major factors are the refluxing time, the amount of base, and solvents ratio. Entry 1 shows
that the debromination will not happen if just stirring the base with the dibromides 5a.
Particularly, heating or sonication is essential for the completion of debromination. Comparing
Entries 2 and 3, the required minimum refluxing time is around 30 minutes. It is also suggested
that less amount of water for recrystallization can lead to higher final yield according to Entries 3
and 4, which represents the optimization condition for this method.
Meanwhile, the resulting purified 6a can be simply reduced to aromatic amino compounds
with the ultrasound assistance. It was contemplated to pursue the sonicated debromination for a
more efficient and general method. Unlike the refluxing, sonication procedure can take place at
room temperature. Moreover, extraction for purification of product replaced the recrystallization,
which was expected to increase the overall yield. Some examples of sonicated debromination of
5a (Entry 5-15) are shown in Table 1.
Table 1. Debromination of Vic-Dibromides of Perfluorovinyl Ether in 4-Nitrobenzene
PFSI Compound (5a).
Entry
Base
n (base): n
Time C
(mins)
SolventB
Debr%D
Yield %E
(dibromides)A
1
2
Cs2CO3
Cs2CO3
9.2 : 1
9.2:1
2:1
1:5
0
15
0H
58.5
0
35.1
3
4
Cs2CO3
Cs2CO3
9.2:1
1.9:1
1:5
5:1
30
30
73.4
100
18.4
74.0
5
6
7
Cs2CO3
Na2CO3
Na2CO3
9.2:1
9.2:1
7.7:1
0.8:1
0.8:1
0.8:1
30
30
40
100
100
94.2
83.3
66.7
60.2
Na2CO3
&
8
9.2:1
0.8:1
30
100
77.6
DIEA
9
Na2CO3
Na2CO3
Na2CO3
NaOH
2.3:1
9.2:1
9.2:1
9.2:1
20:3
0.8:1F
1 :4F
0.8:1
30
30
30
30
1.4
0
1.4
0
10
11
12
0
0
100 G
45.5
4

&
DIEA
13
NaOH
9.2:1
0.8:1
30
100 G
22.2
A: 0.33 mmol of 5a;
B: The solvent ratio (acetone: water);
C: Entries 1-4: refluxing time; Entries 5-14: Sonication time;
D: The calculated debromination percentage base on the fluorine NMR spectra;
E: The isolated recovery weight of debromination product
F: The solvent ratio (Methanol: water)
G: The hydrolysis occurs.
H: The solution was only stirred for 30 mins without reflux.
Owing to the solubility issues, slightly more water was used in the sonication (acetone: water
= 0.8:1) than the refluxing procedure. The higher overall percentage yields resulted from
extraction in Entries 5-11 after sonication. Because of the cost and availability, the regular
inorganic bases, such as Cs2CO3, Na2CO3, NaOH, organic base N, N-diisopropylethylamine
(DIEA), were tried for sonication process (Entries 5-13 in Table 1). For example, under the same
condition in Entries 5 and 6, sonicated debromination resulted in higher percentage yield and full
debromination with Cs2CO3 compared to Na2CO3. The possible reasons can be summarized in
the following. Due to high polarizability and size, Cs2CO3 is the most basic of the alkali
carbonates of the group I metals, referred as the "cesium effect". The preference of cesium
carbonate also rests on practical considerations, particularly low hygroscopicity relative to the
hydroxide and better organic solvent solubility.12 Furthermore, the alkali metal hydroxide not
only can initiate the debromination but also immediately attack the recovery double bond in
Entries 12 and 13. Such phenomenon can attribute to the perfluorovinyloxyl ether moieties,
which are unexpectedly susceptible to the aqueous bases at high temperature or sonication but
resistant to acids compared to other regular alkenes.13 Interestingly, the reaction was high
yielding when assisted by an inorganic base and DIEA (Entry 8), which represented the optimal
purification procedure: omitting the acidification step during work-up of crude product 5a.
Among the alkali carbonates, Na2CO3 was further investigated due to the low cost (Entries 6, 7-
11). Even with longer time consumed in Entry 8, the debromination was not completed with less
than around 9 equivalent of the base catalyst. Replacing the aprotic polar solvent acetone with
protic polar solvent methanol, Entries 11 and 12 reduced the solubility of base and dibromides
and led to no debromination at all. To our satisfaction, the best-optimized procedures were
established as indicated in Table 1, Entries 4, 5, 8 for compound 5a with Cs2CO3 or Na2CO3
catalysts.
Our investigations for the envisaged debromination protocol continued by examining
electronic factors on other vic-dibromides, including compound 5c (Scheme 1) and meso-1, 2-
dibromo-1, 2-diphenyl ethane (e). Several exemplary results are chosen to list in Table 2.
Table 2. Debromination of Other Vic-Dibromides (5c and e) via Sonication Procedure
5

Entry Sub A Amount
(mmol)
Base
Debr
%
Ratio B
Solvent
Time C
Yield % E
D
14
15
16
5c
5c
5c
5c
5c
5c
e
0.16
0.16
0.32
0.46
Acetone: water
1:7.3
Acetone: water
1:1.1
Acetone: water
1:1.8
Acetone-water
1.3:1
Acetone-water
1:2.4
Methanol:
water 2.3:1
Acetone-water
0.8:1
Na2CO3 12:1
Na2CO3 14:1
Na2CO3 28:1
90 min 85.7
61.2
42.8
85.7
66.7
5 h
56.6
100
48 h
12 h
17
18
19
20
NaOH
2 : 1
67.8F
2.35
0.46
NaOH
NaOH
1.3:1
2.6:1
7 h
100
0
90.8
0
12 h
0.33
0.33
Na2CO3 9.2:1
Na2CO3 9.2:1
Na2CO3 9.2:1
60 min
60 min
0
0
0
0
e
Methanol:
water
0.8:1
DMSO: Water
0.8:1
21
e
e
0.33
0.33
22
23
60 min
60 min
0
0
0
0
Acetone-water
0.8:1
NaOH
9.2:1
A: Substrate: the dibromides
B: n (base): n (dibromides)
C: Sonication time;
D: The calculated debromination percentage base on the fluorine NMR spectra;
E: The isolated recovery weight of debromination product
F: The hydrolysis occurs.
After some experimentation, it was found that the PFSI group, as a strong electron-
withdrawing group, strongly affected the reaction rate. A unique feature of 5c is that the shorter
PFSI group, compared to 5a, potentially carries less electron withdrawing effect onto the vic-
dibromides. As a consequence, longer time and more bases are desirable for the planned
debromination procedure for 5c (Entries 14-16). Similar to 5a, the stronger base NaOH
promotes the debromination of 5c in Entry 17, and the following nucleophilic addition or
addition-elimination also is often accompanied for producing perfluorovinly olefin 6c. The
mechanism was earlier reported by Dr. Feriring et al..10 The fluorocarbon anions readily
6

generated in situ (I in Scheme 2) can be trapped by electrophiles, such as protons or bromine
cation (I to II or III in Scheme 2).14 Or the substituted fluoroolefin was isolated (I to IV in
Scheme 2) after loss of fluoride ion. Unexpectedly, such side reaction is preventable for 6c if the
debromination was controlled with short reaction time and a small amount of NaOH, as outlined
in Entry 18. It is suggested to avoid the prolonged contact between the dehalogenated product
and the strong base, as the secondary reactions take place in the reaction raw product with
consequent detriment of the yields. Compared to 6c, 6a is more reactive toward the nucleophiles
since the stronger EW groups stabilize the corresponding fluorocarbon anion. Similar to Entries
10 and 11, either the solubility problem or the side reaction (I to II in Scheme 2) prohibits the
debromination to perfluorovinyl ether of 5c in the protic organic solvent, such as methanol in
Entry 19.
Scheme 2. The Possible Side Reactions of Vic-Dibromides Debromination in PFSIs
Besides, the debromination method is ineffective for the debromination of meso-1, 2-
dibromo-1, 2-diphenyl ethane (e) as Entries 20-23 show. Even after dissolving in the polar
solvent, DMSO/water, Entries 20-23 still give the negative result.
From the above results, it is evident that the electronic factors strongly dictate vic-dibromides
elimination to the perfluorovinyl ethers, resulting in the observed relatively better reactivity
towards more electron poor dibromides. It is believed that the nearby strong electron
withdrawing group, such as PFSI group, is crucial for such debromination of vic-dibromides.
Among the two mechanisms proposed by Detty et al.15 (Scheme 3), nucleophilic attack bromides
(E2-like) model is believed to be accountable for the observations: (1) the reaction was
accelerated by the more electron-rich nucleophiles, and (2) the reaction was promoted by the
7

more electron poor vic-dibromides in perfluoroalkyl compounds. On the other hand, since the
intermediate bromonium carbocation is extremely unstable with neighboring fluorine and
perfluoroalkyl groups, the bromonium ion model is not a likely pathway. Hydroxide anion acts
as nucleophile is the basic aqueous condition in our case.
Rf = perfluoroalkyl groups
Scheme 3. Possible Mechanisms for Debromination of Vic-Dibromides in PFSIs
Conclusion
Our experiments outlined in the experimental section indicate a versatile, direct method for
dehalogenation of vic-dibromide in perfluoroalkyl compounds. Notably, ultrasound technology
can be used in the process, which avoids heating and results in a much shorter reaction time and
simpler operation. The method provides a straightforward synthesis of electron-deficient olefins,
such as perfluorovinyl ethers in PFSI compounds. Those compounds are not only troublesome to
prepare from debromination by classical means but also likely to have further
nucleophilic/nucleophilic-elimination reactions. To our delight, the electron withdrawing PFSI
group sufficiently promotes the debromination under the moderate basic conditions, which also
proves the reaction progress through nucleophilic attack bromides (E2 like) model.
Acknowledgement
For financial support, we gratefully acknowledge the NASA Space Grant program from
Tennessee Space Grant Consortium under grant 3806-019687, and the start-up fund from East
Tennessee State University.
A Supplementary Material
Supplementary data associated with this article can be found, in the online version, at xxxxx.
8

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Graphical Abstract