Pyrene-Tagged Alcoholic Ionic Liquids as Phase Transfer Catalysts for Nucleophilic Fluorination

Article abstract of DOI:10.1002/bkcs.12123

Functional group?activity relationships of pyrene-tagged ionic liquid (PTIL)-based organocatalysts for nucleophilic fluorination using alkali metal fluorides (MFs) are described, which demonstrate that the pyrene, oligoether and alcohol moieties on the imidazolium ring are vital for efficient catalysis. Further investigation of these findings led to the discovery of new strategy, which showed superior catalyst separation process, i.e., catalyst is effortlessly separated from the reaction mixture using reduced graphene oxide. The catalytic efficiency of the PTIL as a phase transfer catalyst was demonstrated by the high conversion of the reactants up to 98% fluorinated yield using MFs in CH₃CN or t-amyl alcohol. Importantly, the catalyst not only enhanced the reactivity of bimolecular nucleophilic substitutions (S_N2) within a short reaction time and reduces the formation of by-products but also affords high yield with easy isolation and separation under mild conditions.

Full text of DOI:10.1002/bkcs.12123

Article
DOI: 10.1002/bkcs.12123
BULLETIN OF THE
A. Taher and D. W. Kim
KOREAN CHEMICAL SOCIETY
Pyrene-Tagged Alcoholic Ionic Liquids as Phase Transfer Catalysts
for Nucleophilic Fluorination
*
Abu Taher and Dong Wook Kim
Department of Chemistry and Chemical Engineering, Inha University, 100 Inha-ro, Nam-gu, Incheon
22212, South Korea. *E-mail: kimdw@inha.ac.kr
Received July 31, 2020, Accepted September 26, 2020
Functional group−activity relationships of pyrene-tagged ionic liquid (PTIL)-based organocatalysts for
nucleophilic fluorination using alkali metal fluorides (MFs) are described, which demonstrate that the
pyrene, oligoether and alcohol moieties on the imidazolium ring are vital for efficient catalysis. Further
investigation of these findings led to the discovery of new strategy, which showed superior catalyst sepa-
ration process, i.e., catalyst is effortlessly separated from the reaction mixture using reduced graphene
oxide. The catalytic efficiency of the PTIL as a phase transfer catalyst was demonstrated by the high con-
version of the reactants up to 98% fluorinated yield using MFs in CH3CN or t-amyl alcohol. Importantly,
the catalyst not only enhanced the reactivity of bimolecular nucleophilic substitutions (SN2) within a short
reaction time and reduces the formation of by-products but also affords high yield with easy isolation and
separation under mild conditions.
Keywords: Ionic liquid, Fluorination, Phase transfer catalyst, Pyrene, Graphene oxide
Introduction
catalyst (PTC) in SN2 reactions (including fluorination).
Unfortunately, most of these catalysts are still difficult to
separate from the reaction mixture.14 In particularly, if the
product and IL show partial mutual solubility, the separa-
tion is much more complicated and satisfactory results were
obtained in only limited cases.16 Importantly, the catalytic
activity, selectivity, and catalyst separation strategies are
also important for the large-scale synthesis. Therefore, the
development of active and typical separable catalysts for
the SN2 fluorination under mild conditions is highly desir-
able and a hot topic in both green chemistry and current
organic synthesis.
Due to the increase in environmental consciousness in
chemical synthesis, the challenge for a sustainable environ-
ment calls for clean procedures that avoid the use of harm-
ful chemicals. In recent years, ionic liquids (ILs) have
attracted increasing interest and been successfully used for
different purposes in various branches of pure and applied
sciences as environmentally benign solvents and catalysts
such as gas absorption,1 liquid separation,2 heat transfer
fluids,3 lubricants,4 electrolytes,5 and catalysts.6 The most
attractive features of ILs are organic salts that are composed
entirely of ions and their negligible vapor pressure, as well
as the ability to tune their physicochemical properties by
varying the ion structure. These key features along with
other characteristics make them excellent candidates for
organocatalysis such as fluorination,7 esterification,8 aldol
condensation,9 polymerization,10 dimerization,11 oxidation,12
and Knoevenagel condensation.13 In particular, much atten-
tion is currently being focused on organic reactions with ILs
as organocatalysts and many chemical reactions including
bimolecular nucleophilic substitutions (SN2) have been per-
formed in ILs with excellent outcomes.14,7 Recently, Chi
et al. reported a t-alcohol-functionalized IL. Also, our group
developed oligoether-functionalized imidazolium-based IL,
which provided remarkable reactivity and selectivity in
nucleophilic fluorination.15
As the unique properties of the fluorine atom,17 synthesis
of fluorinated biomolecules has attracted
a lot of
attention, and make it significant in pharmaceutical,18,19,7b
agrochemical,20 and material sciences.21 However, nature
produces only a limited number of structurally simple
fluorine-containing natural molecules22-24 that can poten-
tially be used in medicinal chemistry and material sciences
as fluorinated starting materials. Therefore, developments
in these fields are closely related to the development of
practical, selective, cost-effective, and efficient methodolo-
gies for the formation of C F bonds onto biomolecules in
the appropriate procedure. In this study, oligoethylene gly-
col or t-alcohol functionalized PTILs were synthesized and
used as PTC for the nucleophilic fluorination of various
substrates in CH3CN or t-amyl alcohol. We studied in
detail the roles of activity of pyrene, oligoether, and alcohol
moieties attached to PTIL1 as well as the role of pyrene
moiety for the separation of catalyst after reaction. It is well
known that metal cations can bind with pyrene moiety
Various ILs can be synthesized in many ways and most
consist of heterocyclic organic molecules, such as imidaz-
ole, pyrrolidine, piperidine, pyridine, etc. Moreover,
imidazolium-based ILs perform well as phase transfer
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through the cation-π interaction25,26 and they can form che-
late with oligoether,15b and at the same time fluoride can
make H-bonding with alcohol moiety,15 thus metal fluoride
(MF) Coulomb (electrostatic) attraction becomes weak and
fluoride can work easily in fluorination reactions (Figure 1).
On the other hand, the presence of pyrene moiety is inter-
esting and desirable from a separation point of view due to
noncovalent bonding (NCB) effect with graphene.26,27
to afford 0.79 g (90%) of PTIL1 as a light yellow thick
liquid: H NMR (400 MHz, CDCl3) δ 1.86–2.00 (m, 4H),
1
2.77 (s, 3H), 3.43–3.61 (m, 22H), 3.78 (t, J = 4.0 Hz, 2H),
4.22 (t, J = 8.0 Hz, 2H), 4.45 (t, J = 4.0 Hz, 2H), 7.10 (s,
1H), 7.55 (s, 1H), 7.81–8.19 (m, 9H), 9.72 (s, 1H); 13C
NMR (100 MHz, CDCl3) δ 28.0, 29.8, 32.4, 39.5, 49.5 (d,
J = 9.0 Hz), 61.2, 69.9, 70.0, 70.0, 70.1, 70.2, 70.2, 70.3,
70.4, 70.4, 70.4, 70.5, 121.0, 123.1, 123.5, 124.7, 124.7,
124.8, 124.9124.9, 125.8, 126.7, 127.3, 127.3, 127.4,
128.5, 129.9, 130.7, 131.3, 135.4, 137.5; IR (KBr) cm−1
:
Experimental
3436 (w), 3033 (w), 2926 (w), 1603 (m), 1559 (s), 1194
(s), 1057 (s), 841 (s), 770 (m), 535 (m). HRMS (FAB
TOF): m/z calcd for C35H45N2O6: (M-X)+ 589.3278, found
589.3282; (X = [MsO]).
Synthesis of Pyrene-Tagged Ionic Liquid PTIL1.
4-(1-pyrenyl)butyl mesylate (2) was synthesized according
to the literature procedure.28 Typically, a solution of
1-pyrene-butanol (1) (0.78 g, 2.85 mmol), methylene chlo-
ride (20 mL) and triethylamine (1.0 g, 9.89 mmol) were
cooled in an ice bath, then methane sulfonyl chloride
(0.5 g, 4.38 mmol) was added dropwise under constant stir-
ring. The mixture was stirred overnight and filtered off,
then the solution was washed with sodium bicarbonate and
brine, respectively. The organic layer was dried over
sodium sulfate and evaporated to dryness at reduced pres-
sure. The crude product was purified by column chromatog-
raphy on silica gel using 50% mixture of dichloromethane
and petroleum ether to afford the resulting compound 2 as
a colorless oil (0.97 g, 98%). Further, 17-(1H-imidazol-
1-yl)-3,6,9,12,15-pentaoxaheptadecan-1-ol (3) was synthe-
sized from 17-bromo-3,6,9,12,15-pentaoxaheptadecan-1-ol
and imidazole in acetone as described our earlier reported7a
and was used as a precursor for synthesizing PTIL1. Subse-
quently, 3 (0.66 g, 2.0 mmol) was added drop-wise to the
solution of 2 (0.70 g, 2.0 mmol) in CH3CN (18 mL). The
reaction mixture was stirred at 90ꢀC for 24 h and evapo-
rated under reduced pressure to remove CH3CN. The resi-
due was repeatedly washed with diethyl ether (10 mL × 5)
and dried under high vacuum for 24 h at room temperature
Synthesis of Pyrene-Tagged Ionic Liquid PTIL2. 1-(1H-
imidazol-1-yl)-2-methylpropan-2-ol (4) was synthesized
according to the literature procedure15a and was used as a
precursor for synthesizing PTIL2. Subsequently, 4 (0.35 g,
2.5 mmol) was added dropwise to the solution of 2 (0.88 g,
2.5 mmol) in CH3CN (20 mL). The reaction mixture was
stirred at 90ꢀC for 36 h and evaporated under reduced pres-
sure to remove solvent. Finally, the residue was repeatedly
washed several times with diethyl ether and dried under
high vacuum for 24 h at 25ꢀC to afford 0.90 g (76%) of
PTIL2 as a colorless thick liquid. 1H NMR (400 MHz,
CDCl3) δ 1.16 (s, 6H), 1.77–2.12 (m, 4H), 2.74 (s, 3H),
3.31 (t, J = 7.2 Hz, 2H), 4.05–4.11 (m, 4H), 5.00 (s, 1H),
6.94 (s, 1H), 7.15 (s, 1H), 7.79 (s, 1H), 7.95–8.14 (m, 8H),
9.57 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 26.4, 26.4,
27.8, 29.5, 32.3, 39.6, 49.6, 59.5, 68.4, 120.2, 121.4,
122.6, 123.0, 123.6, 124.8, 125.0, 125.9, 126.7, 127.2,
127.4, 127.5, 128.5, 129.9, 130.7, 131.3, 135.2, 138.1,
138.5; IR (KBr) cm−1: 3417 (w), 3033 (w), 2925 (w), 1602
(m), 1558 (s), 1193 (s), 1058 (s), 840 (s), 771 (m),
536 (m); HRMS (FAB TOF): m/z calcd for C27H29N2O:
(M-X)+ 397.2280, found 397.2281; (X = [MsO]).
General Procedure for the SN2 Fluorination Reaction.
Cesium fluoride (CsF, 456 mg, 3 mmol) was added to the
mixture of a substrate (1.0 mmol) and PTIL1 (274 mg,
0.4 mmol) and solvent (4 mL) were added in a reaction
vial. The reaction mixture was stirred at 100ꢀC, and the
reaction time was determined by monitoring thin layer
chromatography (TLC). After completion of the reaction it
was diluted with CH2Cl2 and then added reduced graphene
oxide (rGO). The mixture was sonicated for 20 min and
stirred for 1 h at 25ꢀC. The organic layer was separated
from mixture by simple filtration, and evaporated under
reduced pressure. The crude reaction product was purified
using column chromatography on silica-gel (230–400
mesh) afforded the corresponding products.
Results and Discussion
The multifunctional organocatalysts, PTILs were designed
and prepared as shown in Scheme 1. First, 4-(1-pyrenyl)
butyl mesylate (2) was synthesized from 4-(pyren-1-yl)
Figure 1. Concept of pyrene-tagged hexaethylene glycolic ionic
liquids (PTIL1) as a phase transfer catalyst.
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Pyrene-Tagged Alcoholic Ionic Liquids as Phase Transfer Catalysts
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Scheme 1. Synthesis of pyrene-tagged ionic liquids: PTIL1 and
PTIL2.
Figure 3. Nucleophilic fluorination of 5 with different MFs in
CH3CN catalyzed by PTIL1. R = 4-nitrophenyl.
pyrene hardly proceeded, whereas the same reaction con-
dition in the presence of the commercial IL [bmim][OMs]
(bmim
=1-n-butyl-3-methylimidazolium)
proceeded
smoothly. Importantly, the use of pyrene substituted
alcohol-functionalized ILs such as PTIL1 and PTIL2
showed much faster reaction rates than the use of [bmim]
[OMs] under the defined same reaction condition.
According to literatures this may be due to the presence of
pyrene moiety26 and alcohol functional group15 on ILs.
Furthermore, PTIL1 contains oligoether moiety with same
IL core and showed higher catalytic activity than PTIL2.
These results were highly consistent with our hypothesis
depicted in Figure 1, demonstrating that metal-cation-π
interactions, the formation of “flexible” fluoride,15 and
the metal cation chelation along with oligoethers play a
crucial role in enhancing the rate of reaction.
Figure 2. Nucleophilic fluorination of 5 with CsF in CH3CN cata-
lyzed by PTIL1, PTIL2, [bmim][OMs], pyrene, or without any
catalyst. R = 4-nitrophenyl.
In order to obtain the optimized catalytic performance,
SN2 fluorination reactions were carried out with various
MFs and different molar ratios of catalyst in a wide range
of solvent systems. Initially, we performed the fluorination
of 5 with various amounts of catalyst for screening the
loading. As shown in Figure S1 (see Supporting informa-
tion), the yield of product gradually increased with an
increase of loading amount of catalyst (0–0.4 equiv). In
addition, the reaction was completed within 1 h in the pres-
ence of 0.4 equiv of PTIL1 in CH3CN. Therefore, 0.4 equiv
loading of catalyst was determined to be the optimal load-
ing for the SN2 fluorinations. Subsequently, in a quick sur-
vey of MFs, CsF gave the best result whereas other MFs
such as rubidium fluoride (RbF) and potassium fluoride
(KF) demonstrated very low (29% or less) activities. More-
over, NaF was fully inactive (Figure 3).
butan-1-ol (1) with methanesulfonyl chloride according to
literature procedure.28 Simultaneously, 17-(1H-imidazol-
1-yl)-3,6,9,12,15-pentaoxaheptadecan-1-ol (3) was treated
with 2 to afford pyrene-tagged ionic liquid, PTIL1, in 90%
yield. Further, the treatment of 1-(1H-imidazol-1-yl)-
2-methylpropan-2-ol (4) with 2 at 90ꢀC for 36 h to obtain
PTIL2 in 76% yield.
The catalytic activity of the prepared catalyst PTIL1
was tested with a representative set of primary and sec-
ondary mesylate and halide substrates. Initially, to investi-
gate the catalytic activity of PTIL1 and PTIL2, a series of
controlled experiments were carried out under exactly the
same reaction conditions. Typically, the SN2 fluorination
of 3-(4-nitrophenoxy)propyl mesylate (5) was performed
by using 3 equiv of CsF in the presence of the catalyst
(0.4 equiv) at 100ꢀC for 1 h in CH3CN and the results are
summarized in Figure 2. The nucleophilic fluorination in
the absence of any catalyst or in the presence of only
Furthermore, we investigated the effect of solvent to
obtain higher efficiency for the SN2 fluorination reactions
(Figure 4). PTIL1 showed poor catalytic activity (11% or
less) when 1,4-dioxane was used as the solvent.
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sugar molecule is known to be difficult to extract from
the ILs due to their solubility in the liquid salts. The
reaction of a triflate sugar substrate using CsF in the
presence of PTIL1 not only proceeded in a high yield,
affording the fluoro-sugar products, but also allowed the
products to be isolated and purified easily (entry 13).
Halides are known to be more challenging substrates for
nucleophilic fluorination and demonstrated low or negligible
reactivity under identical reaction conditions. Hence, we also
performed the SN2 fluorination of various iodide and bromide
substrates using CsF in the presence of PTIL1 (entries 6–9
and 12). According to literature, the transformation of sec-
ondary alkyl bromide into the fluoride using the conventional
PTC is very difficult because of a competing β-elimination
side reaction.29,30 Therefore, the fluorination of a secondary
alkyl bromide such as 2-(2-bromopropoxy)naphthalene
afforded the desired secondary alkyl fluoride product in only
27% with alkene (73%) in CH3CN (entry 6; method A). The
primary alkyl bromide substrates such as 2-(bromomethyl)
naphthalene and 3-O-(3-bromopropyl)estrone proceeded
smoothly to give the corresponding product in good yields
(entries 9 and 12, respectively) whereas the same reaction of
2-(3-bromopropoxy)naphthalene showed a somewhat a low
chemoselectivity and poor yield in CH3CN (61%, entry 7;
method A). The reaction of a primary iodo-substrate
showed similar trends (50% yield, entry 8; method A).
The synergistic effect of PTIL catalyst with protic t-
alcohol solvent using t-amyl alcohol allowed the fluorina-
tion (in particular, using base-sensitive substrates) to
proceeded highly chemoselectively enhancing reaction
rate significantly (Table 1, method B).29,30,15b Under t-
amyl alcohol medium, the reactions of all substrates
proceeded much more selectively and faster, affording the
corresponding fluoro-products (up to 98% isolated yield)
in the presence of PTIL (Table 1, entries 1–8; method B).
In particular, 2-(2-fluoroethyl)naphthalene could prepared
in 94% yield along with only 5% alkene in t-amyl alcohol
from the base-sensitive 2-(2-mesyloxyethyl)naphthalene. In a
similarly manner, the primary and secondary halides could be
converted to the corresponding fluoro-products in excellent
yield by this fluorination protocol (entries 6–8, method B). In
addition, PTIL1 was successfully removed by the interaction
with commercially available rGO from the reaction mixture
including the fluoro-product (even fluoro-products containing
naphthalene).
Figure 4. Nucleophilic fluorination of 5 using CsF in different
solvent in the presence of PTIL1. R = 4-nitrophenyl.
Dimethylformamide (DMF) was not good solvent in this
reaction using PTIL1 compared with CH3CN. On the other
hand, in t-amyl alcohol medium, the fluorination had a sig-
nificantly faster rate, affording the desired product 6 in excel-
lent yield without the formation of any by-products under
the defined reaction condition. The results showed that t-
amyl alcohol along with CH3CN were the best solvents for
the catalytic activity of the solvent used. With these opti-
mized results in hand, the reactions of various substrates
with CsF were performed.
In order to extend the scope of the SN2 fluorination
catalyzed by PTIL1, we performed fluorination of various pri-
mary/secondary mesylate and halide substrates using CsF at
100ꢀC (Table 1). In most cases, the fluorination reactions of
primary mesylate substrates, such as 3-(4-nitrophenoxy)propyl,
3-(3-nitrophenoxy)propyl, 3-(4-methoxyphenoxy)propyl, and
3-(naphthalen-2-yloxy)propyl mesylate, proceeded smoothly to
give the corresponding products in good yield in polar aprotic
CH3CN (entries 1–3 and 10; method A). However, the reac-
tion of a secondary mesylate substrate demonstrated somewhat
poor yield and chemoselectivity in CH3CN (entry 5; method
A). For further study on the influence of PTIL1 on the
chemoselectivity in the SN2 fluorination in CH3CN, the fluori-
nation was conducted using the base-sensitive substrate such
as 2-(2-mesyloxyethyl)naphthalene. This reaction afforded a
fluorinated product in only 50% yield along with 48% alkene
by-product in CH3CN (entry 4; method A). Furthermore, a
bioactive molecule such as fluoropropyl estrone was prepared
from the fluorination reaction of the corresponding estrone-
mesylate substrate using CsF in excellent yield (97%) in the
presence of PTIL1 (entry 11; method A) whereas the same
reaction in the presence of a pyrene-tagged methylimidazolium
mesylate (PIL), which has no hexaethylene glycolic moiety,
proceeded slightly slow, affording the product in 91% yield
with 6% of alkene by-product (entry 11; method C).26
This result showed the hexaethylene glycolic moiety of
PTIL1 could enhanc its catalytic activity as well as
chemoselectivity in the reaction. In addition, the fluoro-
In the view of green chemistry, we then turned our atten-
tion to the separation of catalyst after reaction under mild
condition. The crucial issue in an IL-based catalyst is the
possibility that some ILs can mixed with products that can
be difficult to separate. Herein, we report an efficient and
convenient procedure for separation of ILs using rGO. The
PTIL1 was anchor on the surface of rGO through
noncovalent interaction.26 Initially, we studied the PTIL1
grafting capacity on the graphitic surface. The exact
grafting amount of PTIL1 onto rGO was 0.37 mmol/g mea-
sured by elemental analysis (Scheme 2). This result
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Table 1. Nucleophilic fluorinations of the various substrates using CsF in the presence of PTIL1 as a catalyst.a
Entry
SM
Methoda
Time (h)
Yield (%)
TMb
Alkenec
1
2
A
B
1
0.5
91
98
8
—
O2N
O
OMs
A
B
1
0.5
87
98
11
—
O
OMs
O2N
MeO
3
4
A
B
1
0.5
93
97
5
—
O
OMs
A
B
2
0.6
50
94
48
5
OMs
5
A
B
2.5
1.5
85
97
13
—
O
OMs
6
A
B
4
1.5
25
98
73
—
O
Br
7
A
B
2.5
1
61
97
37
—
O
Br
8
A
B
2.5
0.6
50
90
48
9
O
I
9
A
A
1.5
1.5
98
98
—
—
Br
10
11
O
OMs
A
3.5
4
97
91
—
H
H
Cd
6
H
OMs
O
O
12
13
A
A
6.5
3.5
93
98
6
H
H
H
O
O
—
OTf
O
O
O
O
O
a All reactions were carried out on a 1.0 mmol scale of substrate with 3.0 equiv of CsF at 100ꢀC; method A in CH3CN; method B in t-amyl
alcohol; method C with 0.5 equiv of a pyrene-tagged methylimidazolium mesylate (PIL) instead of 0.4 equiv of PTIL1 in CH3CN.
b Isolated yield.
c Determined by 1H NMR integration.
d Ref. 26.
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Pyrene-Tagged Alcoholic Ionic Liquids as Phase Transfer Catalysts
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O
HO
N
5
O
HO
N
5
-
N
s
M
O
N
-
s
OM
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OH
rGO (100 mg)
25 oC, 1 h,
CH2Cl2
HO
rGO@PTIL1
O
HO
PTIL1 grafting = 0.37 mmol/g
O
OH
PTIL1
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In conclusion, we have designed and developed oligoether-
functionalized pyrene-tagged PTIL1 that act as highly efficient
multifunctional organocatalysts for nucleophilic fluorination.
This PTIL1 catalyst was demonstrated that a wide range of
substrates can be successfully fluorinated under defined reac-
tion condition. This PTC system could significantly enhance
the reactivity of MFs. Moreover, the synergistic effect of PTIL
catalyst with protic t-alcohol solvent allowed the fluorination
to proceed highly chemoselectively. In particular, the pyrene
moiety of PTIL1 played crucial roles in enhancing the rate of
reaction and in separation of catalyst through noncovalent
interaction. We strongly believe that this catalytic system, in
combination with its efficiency in fluorinated reactions for dif-
ferent substrates, may inspire future research in the field of
organic synthesis and catalytic engineering.
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Pyrene-Tagged Alcoholic Ionic Liquids as Phase Transfer Catalysts
KOREAN CHEMICAL SOCIETY
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