IF5 affects the final stage of the Cl-F exchange fluorination in the synthesis of pentafluoro-λ6-sulfanyl-pyridines, pyrimidines and benzenes with electron-withdrawing substituents

Article abstract of DOI:10.1039/c7cc02802d

A difficult chlorine-fluorine (Cl-F) exchange fluorination reaction in the final stage of the preparation of pentafluoro-λ⁶-sulfanyl-(hetero)arenes having electron-withdrawing substituents has now been elucidated through the use of iodine pentafluoride. A major side-reaction of C-S bond cleavage was sufficiently inhibited by the potential interaction between F and I with a halogen bonding.

Full text of DOI:10.1039/c7cc02802d

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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DOI: 10.1039/C7CC02802D
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IF5 affects the final stage of the Cl-F exchange fluorination in the
synthesis of pentafluoro-λ6-sufanyl-pyridines, pyrimidines and
benzenes with electron-withdrawing substituents
Received 00th January 20xx,
Accepted 00th January 20xx
Benqiang Cuia, Mikhail Kosobokova, Kohei Matsuzakia, Etsuko Tokunagaa and Norio Shibata*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
A difficult chlorine-fluorine (Cl-F) exchange fluorination reaction in derivatives (Scheme 1c).7c Although both methods have a wide
the final stage of the preparation of pentafluoro-λ6-sufanyl- functional group compatibility, they are not applicable for the
(hetero)arenes having electron-withdrawing substituents has now synthesis of o-SF5- pyridines bearing an electron-withdrawing group
been elucidated through the use of iodine pentafluoride. A major (EWG). Dolbier attempted the fluorination of o-SF4Cl-pyridines with
side-reaction of C-S bond cleavage was sufficiently inhibited by the a 5-NO2 or 5-CF3 group with AgF to give little or no SF5-pyridines.7b
potential interaction between F and I with a halogen bonding.
The failure of the Cl-F exchange reaction of o-SF4Cl-pyridines could
be explained as follows: o-Fluoropyridines were obtained by the
competitive nucleophilic aromatic substitution (SNAr) reaction by
fluoride via C-S bond cleavage (route a, Scheme 1c) rather than by
the expected Cl-F exchange reaction with fluoride via the SN pathway
on the sulfur centre (route b, Scheme 1c). The Cl-F exchange
fluorination of electron-deficient SF4Cl-arenes is notoriously difficult.
Umemoto and co-workers reported that the Cl-F exchange of p-NO2-
phenyl-SF4Cl requires ZnF2 under a high reaction temperature to
produce p-NO2-phenyl-SF5 in low yield of 36%.7a We now resolved
this problem in the Cl-F exchange of electron-deficient
(hetero)arene-SF4Cl by using iodine pentafluoride (IF5) (Scheme 1d).
The pentafluorosulfanyl (SF5) group has received increasing attention
in research as futuristic material, particularly in the drug market due
to its unique physiochemical properties1, metabolic stability2, high
electronegativity3, excellent lipophilicity4, relatively hydrolytically
and chemically stable5. These advantages have encouraged
medicinal chemists to investigate the potential of SF5-substituents as
drug candidates,1 such as herbicides6a, insecticides6b, anti-depression
agents6c, and calcimimetic6d. The progress of SF5 chemistry was slow
in the last century but has picked up pace in the past decade due to
the discovery of useful synthetic methods by Umemoto, Welch,
Dolbier, and others.1d, 7, 8 In particular, a practical two-step approach
consisting of the oxidative chlorotetrafluorination of aryl disulfides
followed by a chlorine-fluorine (Cl-F) exchange reaction (SN reaction)
of resulting aryl-sulfur chlorotetrafluorides, aryl-SF4Cl, with fluorides
such as ZnF2, HF, and Sb(III/V) fluorides by Umemoto and co-
Scheme 1. Synthesis of SF5-(hetero)arenes by metal fluorides (previous works) vs
IF5 (this work).
workers7a resulted in
a series of SF5-arenes that are now
commercially available (Scheme 1a). However, the synthesis of SF5-
pyridines has long been a standing challenge since the first report of
SF5-arenes in the 1960s.8
In 2015, Dolbier et al. disclosed the first efficient method for the
synthesis of o-SF5-pyridines by the oxidative chlorotetrafluorination
of pyridine disulfides followed by the Cl-F exchange reaction of
resulting o-SF4Cl-pyridines using silver fluoride (AgF) (Scheme 1b).7b
In 2016, our group achieved the synthesis of m- and p-SF5-pyridines
by the extension of Dolbier’s procedure using fluorinated pyridine
The procedure is simple and a variety of electron-deficient SF5-
arenes and heteroarenes have been synthesized in good to high
yields. The key to success is suggested to be a halogen bond
between F and I with the process in the Cl-F exchange
a.Department of Nanopharmaceutical Sciences, Nagoya Institute of Technology,
Gokiso, Showa-ku, Nagoya466-8555, Japan
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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fluorination reaction of arene-SF4Cl via SNi-like substitution. The was very effective, furnishing desired 3a in exceVllieewnAtrtyicilee lOdnlionef
DOI: 10.1039/C7CC02802D
synthesis of o-SF5-pyridines having NO2 or CF3 substituents, and 97% (Table 1, entry 13).
SF5-pyrimides, was achieved for the first time by the IF5 method.
With the optimized conditions in hand, the scope and
limitation of the Cl-F exchange of (hetero)aryl-sulfur
Table 1. Optimizing the Cl-F exchange reaction of 5-NO2-pyridyl-o-SF4Cl 2a to 5-
chlorotetrafluorides, (hetero)aryl-SF4Cl
starting (hetero)aryl-SF4Cl , including pyridines, pyrimidines
and electron-deficient benzenes, were prepared from
corresponding disulfides via oxidative chlorotetrafluorination
employing KF/Cl2/MeCN conditions in a PFA bottle (Scheme 2).7
The yields of this transformation from to are shown in ESI
(Table S2, ESI). All the (hetero)aryl-SF4Cl obtained was
reasonably stable in a PFA bottle at room temperature. The
purities of
lay in the range of 80%-95%, as determined by 19
NMR, and these were used directly for the next Cl-F exchange
step to (hetero)aryl-SF5 3
2, were examined. The
NO2-pyridyl-o-SF5 3a
.
2
1
1
2
Fluorinating
reagents
(equiv)
Temp.
(oC)
Time
(h)
2
Entry
Solvent
Yielda
2
F
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
ZnF2 (0.6 )
CuF2 (0.6)
TiF4 (0.4)
DMPU·HF (3)
Py·HF (3)
AgF (2)
IF5·Py·HF (3)
IF5·Py·HF (3)
IF5 (3)
80
80
80
55
55
60
50
50
r.t.
65
65
65
65
65
40
r.t.
40
65
80
12
12
12
3
neat
neat
neat
neat
neat
neat
CH2Cl2
neat
neat
neat
neat
neat
neat
neat
neat
neat
CH2Cl2
Hexane
neat
0
0
0
0
0
0
0
0
.
Scheme 2. Synthesis of (hetero)aryl-SF4Cl 2
a and (hetero)aryl-SF5 3b
3
48
14
40
40
14
14
14
14
14
14
24
17
17
48
trace
12
43
57
97 (88%)b
IF5 (2)
IF5 (3)
IF5 (4)
IF5 (5)
IF5 (6)
IF5 (5)
IF5 (5)
IF5 (5)
[a] An excess amount of Cl2 (8 mol or more) in the presence of an excess amount
of KF (16 mol) in acetonitrile at ambient temperature. See Table S2 in ESI. [b] See
the details in Scheme 3.
Details of the fluorination of (hetero)aryl-SF4Cl
shown in Scheme 3. Since the SF5 compounds
2
to
3 are
95
90
83
0
46
0
3
are rather
volatile, both NMR and isolated yields were considered. Strong
electron-withdrawing CF3-substituted-pyridyl-o-SF4Cl 2b was
nicely fluorinated under the same conditions described for NO2-
substrate 2a (condition A, 5 equivalents of IF5) to give 5-CF3-
pyridyl-o-SF5 3b in 89% yield (81% isolated yield). Moderate
EWG-substituted pyridyl-o-SF4Cl, such as chloro 2c and bromo
2d, were charged with a less amount of IF5 (condition B, 3
equivalents, see details in Table S3, S4 ESI) to form desired o-
SF5-chloropyridine 3c and bromopyridine 3d in 70% and 80%
yield, respectively. These results suggest that the Cl-F exchange
reaction of strongly electron-deficient pyridines required
harsher IF5 conditions. We next examined the preparation of m-
SF5-pyridines. Interestingly, m-SF5-fluoropyridine 3e was
obtained in 97% yield from m-SF4Cl-fluoropyridine 2e. This yield
is much higher than that under previous conditions (63% NMR
yield, by 2.0 equiv of AgF, at 100-120 °C, for 48 h).7c 2-Chloro-1-
fluoro-substituted m-SF5-pyridine 3f was also obtained in 53%
NMR yield. Due to the highly volatile nature of p-SF5-
difluoropyridine 3g, the synthesis of 3g required modifications.
Thus, 2,6-difluoropyridyl-p-SF4Cl 2g was transformed into 3g in
20% isolated yield under a large-scale reaction with slightly
different conditions. It should be noted that the IF5 method can
be applied to the synthesis of previously unknown SF5-
pyrimidines. Under the same conditions used for the
preparation of NO2-pyridyl-o-SF5 3a (condition A), two types of
SF5-halo-pyrimidines 3h and 3i were obtained in 48-50% yield.
We next applied the IF5 method for the Cl-F exchange reaction
of non-hetero electron-deficient substrates, NO2-phenyl-SF4Cl
2j and 2k. In condition B, fluorination was smooth, providing p-
and m-NO2-phenyl-SF5 3j and 3k in high yields, 89% and 91%,
respectively. These results suggest that the IF5 method is more
IF5 (5)
IF5 (0.5)
Conditions: 2a (54 mg, 0.2 mmol) for entries 1-8; 2a (192 mg, 0.72 mmol) for
entries 9-19. [a] Yield determined by 19F NMR spectroscopy with fluorobenzene as
the internal standard. [b] Isolated yield.
We examined the effects of fluorinating reagents on the two
competitive fluorination pathways, SNAr vs SN using 5-NO2-
pyridyl-o-SF4Cl 2a as a model substrate (Table 1). We first
7a
conducted the reaction under Umemoto conditions using ZnF2
in a PFA tube without solvent at 80 °C. Initially, 2a disappeared,
and sulfonyl fluoride, 5-NO2-pyridyl-o-SO2F was produced
instead of pyridyl-o-SF5 3a (detected by 19F NMR spectroscopy
and GC-MS) (Table 1, entry 1). The result could not be improved
by using other fluoride reagents, and the decomposition of the
starting material 2a and/or the formation of o-fluoro-pyridine
by C-S bond cleavage via SNAr was a typical result (Table 1,
entries 1-6). We next examined the use of IF5·Py·HF9, which is
an air- and moisture-stable solid. However, in CH2Cl2 or without
solvent, only by-products without the desired pyridyl-SF5 3a
10
formed (Table 1, entries 7 and 8). Interestingly, when liquid IF5
was used, a trace amount of desired pyridyl-SF5 3a was detected
on the 19F NMR spectra. Encouraged by this result, IF5
equivalents, solvents, temperature, and reaction time were
exhaustively screened (Table 1, entries 9-19). Fewer IF5
equivalents or the use of these solvent conditions gave less or
no desired 3a
fluoropyridine
.
4
The competitive formation of 5-NO2-2-
was distinctly inhibited by increasing the IF5
equivalents (Table S1 in ESI). Finally, the reaction with 5
equivalents of IF5 in a closed PFA vial at 65 °C without solvent
2 | J. Name., 2012, 00, 1-3
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advantageous than the ZnF2 method for the preparation of SF5- this transformation from 2n to 3n. On the other hand, 5-NO2-o-
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arenes with an EWG.7a
SF5-Py 3a was obtained only in 6% yiDelOdI: 1u0n.1d0e3r9/C57eCqC.0o28f0I2FD3
generated in situ using similar conditions of IF5 with 2a (Scheme
5b). Thus pyridyl-2a is basically unreactive to IF3.
Scheme 3. Cl-F exchange reaction of electron-deficient (hetero)aryl-SF4Cla.
Scheme 5. Cl-F exchange fluorination reaction by IF3 generated in-situ by the
reaction of Me3SiN3 and IF5.
Although the reaction mechanism is not clear, we
hypothesized that a halogen bond11 between IF5 and PySF4Cl
should be involved to suppress the SNAr reaction. Structure of
IF5 is revealed to be a square based pyramid with the unpaired
electrons sitting in an axial position of an octahedral by X-ray
crystallographic analysis at –80°C.12a Kirshenboim and Kozuch
investigated the electrostatic potential surfaces (EPS) and other
methods from the computational chemistry of IF5 to have the σ-
holes on iodine acting as electron acceptor.12c We thus
examined the EPS of IF5 and Py-o-SF4Cl by DFT computation
based on B3LYP/6-311+G(d,p) level of theory (Figure 1 and S3,
ESI).12b As can be seen in Figure S3a and S3b (ESI), the σ-holes
on iodine in IF5 which are dispersed into 4 corners between 4
equatorial groups. This fact is in good agreement with the
reported data.12c The ESP of Py-o-SF4Cl was next examined. As
can be seen in Fig S3c (ESI), the most electron negative region
of the molecule is the top of fluorine and chlorine follows next.
Based on these results, the computation of the complex of
[IF5···Py-o-SF4Cl] was attempted. While the initial structure with
slightly low level of DFT calculations (B3LYP/6-31G(d)) gave the
linear, symmetry complex structure with one point interaction
of I···Cl (Figures 1a, also S3d in ESI), a higher level of theory
(B3LYP/6-311+G(d,p)) finally resulted in two points interaction
of a stronger bonding of I···F (I-F: 3.169 Å ) and a weaker
bonding of I···Cl (I-Cl: 3.948 Å) (Figures 1b-c, also S3e-f, in ESI).
This symmetry breaking of halogen bond in Figure 1b is
reasonable and could be explained by the splitting of the σ-
holes forming a quadruple site system above mentioned (Figure
S3a, ESI). The similar phenomena was also observed for the
interaction of IF5 and Ph-SF4Cl (Figure S3g-I, ESI).
[a] Isolated yields are given and 19F NMR yields are in parentheses (fluorobenzene
as internal standard). [b] Purities of 2 in the range of 80%-95% determined by 19
F
NMR, which were not considered in the calculation of the yields of 3. [c] Condition
A: 2 (0.72 mmol), IF5 (5 equiv), 65 oC, 14 h. [d] Condition B: 2 (0.72 mmol), IF5 (3
equiv), 65 oC, 14 h. [e] 2 (4.9 mmol), IF5 (1.5 equiv), 85 °C, 24 h.
During the expansion of the utility of the IF5 method, we
noticed that regular aryl-SF4Cl substrates 2l-p having electron-
donating and halogen substituents did not require much IF5 for
the Cl-F exchange reaction. Surprisingly, only 0.2 equivalents of
IF5 were sufficient (Table S6 in ESI), even at room temperature,
to support the desired products 3l-p in moderate to good yields
(Scheme 4), although chlorination was problematic for the
substrate 2l that was substituted with a methyl group (detected
by GC-MS analysis). This side reaction observed for 2l was
suppressed by controlling reaction time to short (Table S7 in ESI).
The starting aryl-SF4Cl substrates 2l-p were prepared from
corresponding aryl-thiols by oxidative chlorotetrafluorination
under the same conditions indicated in Scheme 2 (see Table S2,
in ESI).
Scheme 4. Cl-F exchange reaction of aryl-SF4Cl having an electron-donating or
halogen substituent.a
Figure 1. DFT calculations of (a), a complex [IF5/Py-o-SF4Cl] (b), PhSF4Cl (c) by
B3LYP/6-311+G(d,p) except a complex [IF5/Py-o-SF4Cl] (a) by B3LYP/6-31G(d). See
also Figure S3 in ESI.
[a] 2 (0.72 mmol), IF5 (0.2 equiv) at rt. 5 h for 3l, 24 h for 3m, 12 h for 3n-3p Isolated
yield is shown. 19F NMR yield (fluorobenzene as internal standard) is shown in
parentheses. [b] Starting 2l-p were prepared from corresponding thiols via the
oxidative chlorotetrafluorination employing KF/Cl2/MeCN conditions in a PFA
bottle. Details and yields are shown in Table S2 in ESI. [c] Purities of 2 in the range
of 80%-95% determined by 19F NMR, which were directly used for the IF5 reaction,
were not considered in the calculation of the yields of 3. [d] 12 h, 65 oC.
To gain insight into the difference of the Cl-F exchange
reaction between pyridyl-2 and aryl-2 by IF5, we next carried out
the Cl-F exchange reaction of 2a to 3a and 2n to 3n by in-situ
generated IF3. First, the Cl-F exchange reaction of 2n was carried
out using IF5 (0.4 equiv) in the presence of trimethylsilyl azide
(Me3SiN3, 0.8 equiv). Vigorous gas evolution was initially
observed, suggesting the reduction of IF5 by Me3SiN3 to IF3.9c
Even so, desired 3n was obtained in 70% yield (Scheme 5a). This
result suggests that the IF3 acts at least two fluoride sources for
Based on the calculations and literature information,9-13 the
reaction pathways of 2a, 2n are to 3a, 3n tentatively proposed,
as shown in Scheme 6. The reaction starts with the Cl-F
exchange reaction of (Het)ArSF4Cl 2a,n by IF5 to generate
desired compounds (Het)ArSF5 3a,n via the halogen bond
intermediates TS-I followed by TS-II. In the TS-II, SNi-like Cl-F
exchange reaction could proceed to provide desired (Het)ArSF5l
3a,n with IClF4, which is a key process to suppress the
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6
a) D. S. Lim, J. S. Choi, C. S. Pak, J. T Welch, J. Pestic. Sci. 2007,
competitive SNAr reaction. Similar SNi-like O-F exchange
reaction was suggested by the reaction of BrF5 and NO3 anion
by Christe.13 The generated IClF4 would further react with
ArSF4Cl 2n to provide 3n and ICl2F3 and/or IF3 and Cl2, which also
continuously contribute the Cl-F exchange reaction of ArSF4Cl
2n to furnish ArSF5 3n, while HetArSF4Cl 2a has lower reactivity
to them. All the process induced by IF5 is an ionic reaction
process as mentioned in the literature.9,10 The side-reaction of
the chlorination of 2l having methyl substituent may involve a
reaction with generated Cl2.
View Article Online
32, 255. b) P. J Crowley, G. MitchDeOlIl:,10R.1.03S9a/lCm7CoCn,02P80. 2AD
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a) T. Umemoto, L. Garrick, N. Saito, Beilstein J. Org. Chem.
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K. Matsuzaki, E. Tokunaga, N. Saito, N. Shibata, Angew. Chem.
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2 to 3.
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In summary, we have discovered that IF5 is a very effective
reagent for the Cl-F exchange reaction of (hetero)aryl sulfur
chlorotetrafluorides 2, which is the final stage of the synthesis
9
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of SF5-arenes. The synthesis of o-SF5-pyridines bearing an NO2
or CF3 group was achieved for the first time by the IF5 method.
SF5-Pyrimidines were also accessed. IF5 fluorination is also
advantageous for the synthesis of SF5-benzenes with an NO2
group. The effect of the IF5 method can be explained by a SNi-
like fluorination process via halogen bonding which reduces a
dominant side SNAr fluorination reaction. Since the IF5 reagent
is available in ton-scales and is already popular in several
industrial processes, our method will also support the suppliers
of SF5-compounds on the market.
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This research is partially supported by the Kobayashi
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Catalytic
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International
Foundation,
the
Advanced
Transformation (ACT-C, JPMJCR12Z7) from the JST Agency. B.
Cui was supported by a CSC fellowship. We also thank Kanto
Denka Kogyo Co Ltd for the gift of IF5, and to Ube Industries, Ltd
for support.
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Notes and references
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