Promotional effect of ionic liquids in the electrophilic fluorination of phenols

Article abstract of DOI:10.24820/ark.5550190.p010.164

The influence of a stoichiometric amount of ionic liquids (IL) on the fluorination of phenols in various solvents has been studied. The fluorination of phenol, 1-naphthol and resorcinol was carried out using 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (F-TEDA-BF₄, Selectfluor) with the formation of 2-fluoro-, 4-fluorophenol, 2-fluoro-, 4-fluoronaphthol and 4-fluoro-, 4,6-difluoro-benzene-1,3-diol as the main products. The use of a stoichiometric amount of ionic liquid as an additive results in acceleration of the reactions. The effect is most significant at low temperatures. It has been found that solvent polarity has an essential effect on the difference in yields of fluoroproducts obtained in the presence of IL and without it.

Full text of DOI:10.24820/ark.5550190.p010.164

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Paper
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Organic Chemistry
Arkivoc 2018, part ii, 60‐71
Promotional effect of ionic liquids in the electrophilic fluorination of phenols
Gennady I. Borodkin,a,b* Innokenty R. Elanov,a and Vyacheslav G. Shubina
aN.N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of the Russian Academy of
Sciences, Academician Lavrent’ev Ave., 9, Novosibirsk 630 090, Russian Federation
bNovosibirsk State University, Pirogov st. 2, Novosibirsk 630 090, Russia
E‐mail: gibor@nioch.nsc.ru
Dedicated to Prof. Kenneth Laali on the occasion of his 65th birthday
Received 05‐10‐2017
Accepted 06‐28‐2017
Published on line 10‐15‐2017
Abstract
The influence of a stoichiometric amount of ionic liquids (IL) on the fluorination of phenols in various solvents
has been studied. The fluorination of phenol, 1‐naphthol and resorcinol was carried out using 1‐chloromethyl‐
4‐fluoro‐1,4‐diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (F‐TEDA‐BF4, Selectfluor™) with the formation
of 2‐fluoro‐, 4‐fluorophenol, 2‐fluoro‐, 4‐fluoronaphthol and 4‐fluoro‐, 4,6‐difluoro‐benzene‐1,3‐diol as the
main products. The use of a stoichiometric amount of ionic liquid as an additive results in acceleration of the
reactions. The effect is most significant at low temperatures. It has been found that solvent polarity has an
essential effect on the difference in yields of fluoroproducts obtained in the presence of IL and without it.
OH
OH
F-TEDA-BF4
F
IL/ArOH 1-15,
Solvent
X
X
Keywords: Fluorination, phenols, ionic liquid, solvent
DOI: https://doi.org/10.24820/ark.5550190.p010.164
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Introduction
Fluorinated phenol units are frequently encountered in both biologically active1‐4 and functional organic
molecules.5,6 Fluoronaphthols are key intermediates for the synthesis of duloxetine metabolites.7 Highly
fluorescent fluorinated fluoresceins were prepared by the reaction of fluororesorcinols with phthalic
anhydride and its derivatives.8 Electrophilic fluorination of phenols is the most common synthetic strategy in
the preparation of fluorinated phenols. In the last two decades a broad range of NF‐reagents has started to be
widely used for electrophilic fluorination of organic compounds.9‐13 Among the NF‐reagents Selectfluor™ is one
of the most widely utilized chemical (Figure 1).13 This reagent is a stable, easy to handle, nonhygroscopic solid
which is commercially available. Reactions are typically performed in MeCN, CH2Cl2, CHCl3 or ClCH2CH2Cl.9‐13
SelectfluorTM was shown to be soluble in ionic liquids (ILs), thus ILs can be expected to act as promoters in the
electrophilic fluorination of organic compounds. One of the disadvantages of ionic liquids is their high cost.
Therefore, it would be attractive to use a small amount of ionic liquids with organic solvents to minimize their
consumption (cf. refs. 14‐18). Recently, we have studied promotional effect of ionic liquids in electrophilic
fluorination of methylated uracils with SelectfluorTM in alcohols.19 The use of a stoichiometric amount of ionic
liquid as an additive results in acceleration of the reaction. As a part of our program aimed at exploring the
potential use of a stoichiometric amount of ionic liquid we have tried to use similar catalytic strategy for
fluorination of phenol, 1‐naphthol and resorcinol in various solvents. In this work, we present details of the
positive effect of IL additives on the electrophilic fluorination of phenols with SelectfluorTM focusing on the
role of ionic species in reactions. The increasingly popular imidazolium‐ and pyrrolidinium based ionic liquids
[Emim][X] (Emim = 1‐ethyl‐3‐methylimidazolium), [Bmim][X] (Bmim = 1‐butyl‐3‐methylimidazolium) and
[Pyr][X] (Pyr = pyrrolidinium), especially those with less nucleophilic anions X (Figure 1), have the potential to
be ideal promoters of ionic reactions.
X = BF4
[Bmim][BF4]
CH2Cl
Bu
Et
N
X = OTf
[Emim][OTf]
X = PF6
[Bmim][PF6]
OTf-
[Pyr][OTf]
N
N
+
X-
+
X-
-
+
N
+
(BF4 )2
X = NTf2
[Emim][NTf2]
X = OTf
[Bmim][OTf]
+
N
N
N
Bu
Me
Me
Me
X = [HSO ]
[Bmim][HSO ]
4
F
4
F-TEDA-BF4
(Selectfluor™)
Figure 1. Chemical structures of F‐TEDA‐BF4 and the ionic liquids used in this study.
Results and Discussion
The fluorination of phenol (1) and 1‐naphthol (2) with F‐TEDA‐BF4 was carried out in various solvents in the
presence of a small amount of 1‐butyl‐3‐methylimidazolium ionic liquids ([Bmim][Y]). The reaction of phenols
1 and 2 with F‐TEDA‐BF4 (molar ratio F‐TEDA‐BF4/phenol 1.1) gives 2‐fluoro‐(3), 4‐fluorophenol (4), and 2‐
fluoro‐(5) and 4‐fluoronaphthol (6) as the main products (Schemes 1, 2, Tables 1, 2). Minor quantities of
difluorides 7‐9 are also formed. Quantities of other difluorides in the reaction mixtures were less 0.5%.
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OH
OH
OH
OH
F
F
F-TEDA-BF4
IL/Solvent
+
+
F
F
3
4
1
7
Scheme 1. Reaction of phenol with F‐TEDA‐BF4.
O
F
F
OH
OH
OH
OH
F
F
F-TEDA-BF4
IL/Solvent
+
+
+
F
F
2
5
6
9
8
Scheme 2. Reaction of 1‐naphthol with F‐TEDA‐BF4.
Table 1. IL effect on the yield of 3, 4 and 7 in fluorination of phenol with F‐TEDA‐BF4 in various solventsa
Yield, % d
Total
Entry
Solvent
εb
[Bmim][OTf]/1c
3
0
33
4
40
0
41
0
4
2
20
12
20
3
24
3
25
2
27
0.5
16
0.2
20
0
10
0
22
7
0
6
1
7
0
7
0
6
0
5
0
7
0
6
0
2
0
7
yield, % e
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
MeNO2
MeNO2
MeCN
38.6
38.6
36.2
36.2
32.6
32.6
24.3
24.3
18.3
18.3
10.4
10.4
9.18
9.18
8.2
0
4
0
4
0
4
0
4
0
4
0
4
0
4
0
4
0
4
2
59
17
67
3
72
3
70
4
74
1
65
1
58
0
23
0
MeCN
MeOH
MeOH
EtOH
EtOH
39
2
i‐PrOH
i‐PrOH
(ClCH2)2
(ClCH2)2
PhCF3
42
0.6
42
0.4
32
0
11
0
33
PhCF3
(Cl2CH)2
(Cl2CH)2
heptane
heptane
8.2
1.92
1.92
62
a
o
Reaction conditions: phenol (0.213 mmol), F‐TEDA‐BF4 (0.234 mmol), solvent (5 mL), 80 C, 5 h. b Dielectric
constant of solvent.20 c Molar ratio. Yields determined by 19F NMR. e Total yield of fluoro‐products.
d
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It is observed that a small amount of IL strongly influences the selectivity and efficiency of the fluorination
reaction (Tables 1, 2). The addition of IL to solvent usually results in acceleration of the reaction. The phenols 1
and 2 react faster with F‐TEDA‐BF4 in the presence of the IL providing better yields of fluoro derivatives in
comparison to the same reaction without IL. It seems likely that the F‐TEDA‐BF4, being more soluble in the IL,
is transferred by it into the solvent and compounds 1 and 2 react under these conditions more rapidly. The
influence of neat solvent and solvent‐IL mixture on total yields of the fluorinated products in the reaction of
phenol and 1‐naphthol is similar and the solvent polarity usually has a profound effect on the difference in
yields of the products obtained in the presence of IL and without it (Tables 1, 2; Figure 2).
o
It was found that fluorination of 1‐naphthol at 80 C when using [Bmim][BF4] instead of [Bmim][OTf]
proceeds with the same efficiency (Table 2).
Table 2. IL effect on the yield of 5, 6, 8 and 9 in fluorination of 1‐naphthol with F‐TEDA‐BF4 in various solventsa
Yield, %d
Total
yield, %e
42
67
44
68
52
71
49
65
23
60
3
Entry Solvent
εb
[Bmim][Y]/2c
Y
5
6
8
0.5
1
9
0.3
7
5
7
6
9
4
6
0.8
5
0
4
1
3
0
4
0.5
2
1
2
MeNO2 38.6
MeNO2 38.6
0
4
0
4
0
4
0
4
0
4
0
4
0
4
0
4
0
4
4
4
4
‐
23
34
19
34
26
35
25
34
12
32
2
27
4
28
3
29
4
25
28
32
32
18
25
20
25
16
21
17
20
10
19
1
OTf
‐
OTf
‐
OTf
‐
OTf
‐
OTf
‐
OTf
‐
OTf
‐
OTf
‐
3
4
5
6
MeCN
MeCN
MeOH
MeOH
EtOH
36.2
36.2
32.6
32.6
24.3
24.3
18.3
18.3
0
2
4
6
3
5
7
8
EtOH
9
i‐PrOH
i‐PrOH
(ClCH2)2 10.4
(ClCH2)2 10.4
PhCF3
PhCF3
(Cl2CH)2
(Cl2CH)2
heptane 1.92
heptane 1.92
heptane 1.92
MeNO2 38.6
0.5
4
10
11
12
13
14
15
16
17
18
19
20
21
0
3
0
3
0
3
0
1
7
1
41
6
46
4
43
6
40
50
59
9.18
9.18
8.2
12
1
8.2
7
2
OTf
BF4
BF4
BF4
12
17
21
20
3
1
5
2
5
6
EtOH
24.3
63
a
o
Reaction conditions: 1‐naphthol (0.138 mmol), F‐TEDA‐BF4 (0.152 mmol), solvent (5 mL), 80 C, 5 h.
bDielectric constant of solvent.20 Molar ratio. Yields determined by 19F NMR. Total yield of fluoro‐
c
d
e
products.
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Polar solvents
75
70
65
60
55
50
45
40
MeOH
MeCN
MeNO2
EtOH
i-PrOH
r = 0.97, s = 3.4
PhCF3
(Cl2CH)2
heptane
(ClCH2)2
0
10
20
30
40
50
60
Yield, Solv, %
Figure 2. The influence of solvents and solvent‐IL mixtures on total yields of the fluorinated products in the
reaction of 1‐naphthol (2:F‐TEDA‐BF4:[Bmim][OTf]) = 1 : 1.1 : 4; 80 oC, 5 h).
To develop a more efficient procedure for preparing fluorophenols, we examined the effects of different
factors on the fluorination of 2 in EtOH‐[Bmim][OTf]. At 80 oC, the rate of the reaction is rather high and the
total yield of fluoro‐products 5, 6, 8 and 9 reaches 65%. When the temperature is reduced to 40 oC, the yield
decreases (Table 2, entry 8, Table 3, entry 1), the regioselectivity being unchanged, the ratio of 5/6 remaining
practically constant.
Table 3. IL effect on the yield of 5, 6, 8 and 9 in fluorination of 1‐naphthol with F‐TEDA‐BF4 in EtOH at 40 oC a
Yield, % d
Entry
IL
εb
IL/2 c
Total yield, % e
5
6
8
9
1
2
3
4
5
6
7
8
[Bmim][OTf]
[Bmim][OTf]
[Bmim][BF4]
[Bmim][PF6]
[Bmim][HSO4]
[Emim][OTf]
[Emim][NTf2]
[Pyr][OTf]
12.9
12.9
13.9
14.0
‐
4
2
2
2
2
2
2
2
29
17
3
3
52
38
18
25
62
41
43
43
23
11
15
34
24
24
24
13
7
1
0.2
0.5
5
1
0.2
0.6
7
9
16
14
15
15
16.5
12.0
‐
1
2
2
2
2
2
a Reaction conditions: 1‐naphthol (0.138 mmol), F‐TEDA‐BF4 (0.152 mmol), solvent (5 mL), 5 h.
b
d
e
Dielectric constant of IL.21 c Molar ratio. Yields determined by 19F NMR. Total yield of fluoro‐
products.
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In further experiments, the ratio of IL/2 was varied under otherwise similar conditions in various solvents
(Figures 3‐5).
30
25
20
15
10
5
5
6
9
8
0
0
2
4
6
8
10
12
14
16
IL/2
Figure 3. Yields of compound 5, 6, 8 and 9 as a function of molar ratio IL/2 (heptane, 80 oC, 5 h); ([Bmim][OTf] :
2 : F‐TEDA‐BF4 = 0‐15 : 1 : 1.1).
40
35
5
6
30
25
20
15
10
5
9
8
0
0
2
4
6
8
10
12
14
16
IL/2
Figure 4. Yields of compound 5, 6, 8 and 9 as a function of molar ratio IL/2 (MeNO2, 80 oC, 5 h) ([Bmim][OTf] :
2 : F‐TEDA‐BF4 = 0‐15 : 1 : 1.1).
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5
35
30
25
20
15
10
5
6
9
8
0
0
2
4
6
8
10
12
14
16
IL/2
Figure 5. Yields of compound 5, 6, 8 and 9 as a function of molar ratio IL/2 (EtOH, 80 oC, 5 h) ([Bmim][OTf]:2:F‐
TEDA‐BF4 = 0‐15:1:1.1).
The effect of the ionic liquid is already significant when a molar ratio of IL/2 is 1. Increase of the IL/2 ratio
over 2 does not seem to result in further essential changes in the yield of 5, 6, 8 and 9. When IL/2 ratio
changes from 2 to 4, reduction of the yields of 5 and 6 is usually observed, probably owing to their further
fluorination forming 8 and 9 (Scheme 3).
OH
OH
OH
F
F
F-TEDA-BF4
IL/Solvent
+
F
F
5
6
8
OH
OH
O
F
F
F
F
F-TEDA-BF4
IL/Solvent
+
F
-H+
5
9
Scheme 3. Reactions of 5 and 6 with F‐TEDA‐BF4.
It was found that the rate of formation of products 5, 6, 8 and 9 at lower temperature while using
hydrophilic ionic liquids [Bmim][HSO4], [Emim][OTf], [Emim][NTf2], [Bmim][OTf], [Pyr][OTf] and EtOH as
solvent is much higher than that with [Bmim][BF4] and [Bmim][PF6], the efficiency of the anions decreasing in
the order of [HSO4 ] > [OTf‐] [NTf2 ]> [PF6 ] > [BF4 ] (Table 3). This row generally corresponds to an order of
‐
‐
‐
‐
~
acceptor ability of anions to form H‐bond: [HSO4 ] > [OTf‐] [NTf2 ]> [PF6 ].22 The change of the cation part of IL
‐
‐
‐
>
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has a small influence on the yield of fluorides (Table 3, entries 2, 6, 8). F‐TEDA‐BF4 completely dissolves at 80
oC in MeCN, MeNO2 and only partially in other solvents (MeOH – 0.0125, EtOH – 0.0029, i‐PrOH – 0.0001,
PhCF3 – 0.0009, (ClCH2)2 – 0.0026, (Cl2CH)2 – 0.0002, n‐heptane – <0.0001 mol/L). In the case of ionic liquids
[Bmim][HSO4], [Emim][OTf], [Emim][NTf2], [Bmim][OTf], [Pyr][OTf] and [Bmim][PF6] dissolution of F‐TEDA‐BF4
is accompanied obviously by counterion exchange, but this does not create any limitation relative to
electrophilic fluorination and even can accelerate reaction by formation of H‐bond between OH‐group of σ‐
complex and counterion. Exception of EtOH as cosolvent results in a small change in the order of anions:
‐
‐
‐
‐
[HSO4 ] > [OTf‐] > [NTf2 ] > [BF4 ] > [PF6 ] (Table 4). The results clearly show that total yields poorly correlate
with the polarity of ionic liquids.
Table 4. IL effect on the yield of 5, 6, 8 and 9 in fluorination of 1‐naphthol with F‐TEDA‐BF4 at 40 oC a
Yield, % d
Total yield,
Entry
IL
εb
IL/2 c
e
%
5
6
8
9
1
2
3
4
5
6
7
[Bmim][OTf]
[Bmim][BF4]
[Bmim][PF6]
[Bmim][HSO4]
[Emim][OTf]
[Emim][NTf2]
[Pyr][OTf]
12.9
13.9
14.0
‐
2
2
2
2
2
2
2
22
13
2
3
40
25
11
44
38
31
27
14
7
9
0.6
0.2
3
1
0.2
5
4
25
21
17
15
11
11
11
10
16.5
12.0
‐
2
4
1
2
1
1
a
b
Reaction conditions: 1‐naphthol (0.138 mmol), F‐TEDA‐BF4 (0.152 mmol), 5 h. Dielectric
constant of IL.21 c Molar ratio. d Yields determined by 19F NMR. Total yield of fluoro‐products.
e
In all cases, the rate of the reaction of 1‐naphthol with F‐TEDA‐BF4 is rather high even with the ratio IL/2
equal to 2.
Further, we have examined resorcinol (10) as a model for IL‐mediated fluoro functionalization with
F‐TEDA‐BF4 and established that dihydroxybenzenes can also be readily converted into fluoro‐derivatives in
this way (Scheme 4, Table 5).
OH
OH
OH
OH
F
F
F-TEDA-BF4
IL-EtOH
+
+
OH
OH
OH
OH
F
F
10
13
11
12
Scheme 4. Reaction of resorcinol with F‐TEDA‐BF4.
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Table 5. IL effect on the yield of 11, 12 and 13 in fluorination of resorcinol with F‐TEDA‐BF4 in EtOH a
Yield, % c
Total
yield, % d
18
Temperature,
oC
Entry
IL
IL/10 b
11
17
27
40
51
57
66
70
28
45
43
59
62
61
64
61
62
68
68
67
12
0.4
1
13
0.2
1
1
2
‐
24
24
24
24
24
24
24
40
40
40
80
80
80
80
80
80
80
80
80
0
1
[Bmim][OTf]
[Bmim][OTf]
[Bmim][OTf]
[Bmim][OTf]
[Bmim][OTf]
[Bmim][OTf]
‐
29
3
2
1
2
43
4
4
1
3
55
5
6
2
5
64
6
10
15
0
2
8
76
7
2
10
1
82
8
1
30
9
[Bmim][OTf]
[Bmim][OTf]
‐
1
1
2
48
10
11
12
13
14
15
16
17
18
19
2
1
2
46
0
3
10
12
11
12
11
12
11
10
9
72
[Bmim][OTf]
[Bmim][OTf]
[Bmim][OTf]
[Bmim][OTf]
[Bmim][OTf]
[Bmim][OTf]
[Bmim][BF4]
[Bmim][BF4]
1
3
77
2
3
75
4
3
79
6
3
75
10
15
1
3
77
3
82
4
82
2
4
80
a Reaction conditions: resorcinol (0.182 mmol), F‐TEDA‐BF4 (0.200 mmol), EtOH (5 mL), 5 h. b Molar ratio.
d
c Yields determined by 19F NMR. Total yield of fluoroproducts.
The effect of the ionic liquid is already significant at room temperature when a molar ratio of IL/10 is 1.
Increase of the IL/10 ratio results in further growth of the yield of 11‐13 (Figure 6).
At 80 oC, the rate of the reaction is rather high and the yield of fluoro‐products 11‐13 reaches 82% (Table
5). When the temperature is reduced, the total yield of the products decreases but the difference in their
yields obtained in the presence of an IL, and without it increases.
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70
60
50
40
30
20
10
0
11
13
12
0
2
4
6
8
10
12
14
16
IL/10
Figure 6. Yields of compound 11‐13 as a function of molar ratio IL/10 (EtOH, 24 oC, 5 h) ([Bmim][OTf]:10:F‐
TEDA‐BF4 = 0‐15:1:1.1).
Conclusions
In summary, we have developed a simple and efficient protocol for the fluorination of phenols with
SelectfluorTM employing a stoichiometric amount of ionic liquid. Addition of a small amount of IL to a solvent
enhanced the efficiency of the fluorination. The effect of the nature of the anion of the ionic liquid is more
pronounced compared to that of the cation. It has been found that solvent polarity has an essential effect on
the difference in yields of fluoro‐products obtained in the presence of IL and without it. The cost economy and
environmental benefits of using green solvents represent an important contribution to green chemistry.
Experimental Section
General. SelectfluorTM and ILs were purchased from Sigma‐Aldrich and Acros Organics and used without
further purification. The PhOH, 1‐naphthol, resorcinol, n‐heptane and nitromethane were used as obtained
commercially. Other organic solvents were dried by standard methods.20
1
19
The H and F NMR spectra were recorded on a Bruker AV‐300 spectrometer using the residual proton of
deuterated dimethylsulfoxide (δH 2.50 ppm) or chloroform (δH 7.24 ppm) and PhCF3 (δF ‐63.73 ppm) as
internal references. The structures of the compounds obtained were confirmed by 1H and19F NMR and GC‐MS.
The spectral characteristics were consistent with the published data for 2‐fluoro‐, 4‐fluoro‐, 2,4‐
difluorophenol23,24 2‐fluoro‐, 4‐fluoro‐, 2,4‐difluoronaphthalene, 2,2‐difluoronaphthalen‐1(2H)‐one,25,26
2‐fluoro‐, 4‐fluoro‐ and 4,6‐difluoro‐resorcinol.8,27
General procedure for study of influence of Ils. A mixture of phenol (20 mg), F‐TEDA‐BF4 (1.1 equivalents), IL
(0‐15 equivalents) and the organic solvent (5 mL) was stirred for 5 h at various temperatures under an argon
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atmosphere (Tables 1‐5). The mixture was evaporated at a reduced pressure and analysed by 1H, 19F NMR as
solution in CDCl3 or CDCl3‐DMSO‐d6. Cl2CHCHCl2 and PhCF3 were used as internal standards for peak
integration.
Acknowledgements
Spectral and analytical studies were carried out at the Chemical Service Center, Siberian Branch of the Russian
Academy of Sciences.
Financial support from the Chemistry and Material Science Department of the Russian Academy of Sciences
(project no. 5.1.4) is gratefully acknowledged.
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