Copper-Mediated Di- and Monofluoromethanesulfonylation of Arenediazonium Tetrafluoroborates: Probing the Fluorine Effect

Article abstract of DOI:10.1002/cjoc.201700748

A copper-mediated di- and monofluoromethanesulfonylation of arenediazonium tetrafluoroborates using di- and monofluoromethanesulfinate reagents provides aryl difluoromethyl (or monofluoromethyl) sulfones in good yields. It was found that the relative reactivity of these sodium fluoroalkanesulfinates in the present reactions decreases in the following order: CH₂FSO₂Na > CF₂HSO₂Na > CF₃SO₂Na.

Full text of DOI:10.1002/cjoc.201700748

Copper-Mediated Di- and Monofluoromethanesulfonylation of
Arenediazonium Tetrafluoroborates: Probing the Fluorine Effect
Bo Xing,a Chunafa Ni,a and Jinbo Hu*,a
ABSTRACT
A
copper-mediated di- and monofluoromethanesulfonylation of arenediazonium tetrafluoroborates using di- and
monofluoromethanesulfinate reagents provide aryl difluoromethyl (or monofluoromethyl) sulfones in good yields. It was found that the relative reactivity
of these sodium fluoroalkanesulfinates in the present reactions is decreasing in the following order: CH2FSO2Na > CF2HSO2Na > CF3SO2Na.
KEYWORDS difluoromethanesulfonylation; fluoromethanesulfonylation; arenediazonium; sodium sulfinate; fluorine effect
reaction takes place.10 Previously, we reported
a practical
Introduction
preparation of sodium fluoroalkanesulfinates (RfSO2Na) via
NaBH4-mediated reduction of corresponding benzo[d]thiazol-2-yl
sulfones (Scheme 1, eq 3), and we also found that the reactivity of
these sulfinates in radical fluoroalkylations is decreasing in the
following order: PhCF2SO2Na > CF2HSO2Na > CH2FSO2Na.11 As our
continuing effort in probing the unique fluorine effects in organic
reactions,12 we were interested in finding out the reactivity order of
RfSO2Na (Rf = CF3, CF2H, and CH2F) in the fluoroalkanesulfonylations.
Herein, we report our results on the synthesis of di- and
Nowadays, fluorinated organic compounds play increasingly
important roles in pharmaceutical and agrochemical industries,
owing to the fact that fluorine or fluorine-containing groups could
effectively improve the metabolic stability, lipophilicity, and
biological potency of a target drug molecule.1,2 Among various
fluorine-containing groups, the gem-difluoromethylene (CF2) is
known to be isosteric to the ethereal oxygen atom, and has
attracted much attention.3 Difluoromethyl phenyl sulfone,
PhSO2CF2H, as an effective nucleophilic reagent to introduce CF2
motif, has been widely used in organic synthesis.4 The “chemical
chameleon”5 character of the phenylsulfonyl group enables it a
highly useful synthon to difluorinated functionalities such as
monofluoromethyl
aryl
sulfones
via
copper-mediated
fluoroalkanesulfonylation reactions between arenediazonium
tetrafluoroborates13 and HCF2SO2Na or FCH2SO2Na (Scheme 1, eqs 3
and 4), and the relative reactivity of RfSO2Na (Rf = CF3, CF2H, and
CH2F) in these fluoroalkanesulfonylations.
difluoromethylene
(-CF2-),
difluoromethyl
(-CF2H)
and
difluoromethylidene (=CF2).6 In addition to their widespread use in
organic synthesis, difluoromethyl aryl sulfones (such as PT2399)
have recently been reported to show biological activity as HIF-2α
antagonist in preclinical kidney cancer models (Figure 1).7 Despite
the wide applications of difluoromethyl aryl sulfones (ArSO2CF2H) in
organic synthesis and life sciences, synthetic methods for their
preparation have remained largely unexplored since the first
synthesis by Hine and Porter in 1960.8 Difluoromethyl aryl sulfones
are often prepared by oxidation of corresponding difluoromethyl
aryl sulfides.4a,4d
Scheme
1
Fluoromethanesulfonylations
of
arenediazonium
tetrafluoroborates.
Figure 1 PT2399 as HIF-2α antagonist.
Recently, we were interested in two reports on the preparation
of
trifluoromethyl
aryl
sulfones
from
sodium
trifluoromethanesulfinate (CF3SO2Na).9,10 In 2013, Shekhar and
co-workers reported a copper-catalyzed coupling of aryl iodonium
salts with CF3SO2Na (Scheme 1, eq 1).9 In 2015, Qing and
co-workers
arenediazonium
reported
a
copper-promoted
with
coupling
of
tetrafluoroborates
sodium
trifluoromethanesulfinate (Scheme 1, eq 2).10 In these reactions,
both iodonium salts and arenediazonium tetrafluoroborates are not
able to oxidize CF3SO2Na to generate trifluoromethyl radical. Indeed,
when an external oxidant (such as t-BuOOH) is used, CF3SO2Na can
Results and Discussion
At the outset of our study, we added CF2HSO2Na and Cu2O and
conducted the experiment in DMSO using benzenediazonium
tetrafluoroborate 1a as substrate (Table 1, entry 1). However, no
desired product 2a was detected. Most of the substrate 1a was
be oxidized to give trifluoromethyl radical and
trifluoromethylation (rather than trifluoromethanesulfonylation)
a
a Key Laboratory of Organofluorine Chemistry, Center for Excellence in
Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of
Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Lu,
Shanghai 200032, China
E-mail: jinbohu@sioc.ac.cn
This article has been accepted for publication and undergone full peer review but has not
been through the copyediting, typesetting, pagination and proofreading process, which
may lead to differences between this version and the Version of Record. Please cite this
article as doi: 10.1002/cjoc.201700748
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transformed to the reduced product (benzene) and biphenyl
(detected by GC-MS), while CF2HSO2Na was dominantly recovered
(detected by 19F NMR). Utilizing other copper species such as CuTc
and CuOAc, we were delighted to find that the addition of CuTc
successfully gave 8% yield of the sulfonylation product 2a (entry 2).
Changing the solvent to MeCN, the yield was increased to 64%,
while other copper species displayed low efficiency (entries 4-12).
Instead of mixing the substrate 1a with all the reagents together at
the beginning, slow dropwise adding of the 1a to the acetonitrile
solution of CuTc and CF2HSO2Na could improve the yield to 79%
(entry 13). Furthermore, the amount of CuTc could be reduced to
0.5 equivalent (entry 14). Further reducing the amount of CuTc
led to the loss of yield (Table 1, entry 15-17).
With the optimized conditions in hand (Table 1 entry 18), we
examined the scope of this reaction, which are shown in Scheme 2.
Although the reaction gave the same results for 1a using either 0.5
or 1.0 equivalent of CuTc (Table 1, entries 18 and 19), for most
substrates, reaction with 0.5 equivalent of CuTc delivered 5%-10%
lower yields than those utilizing 1.0 equivalent of CuTc, therefore,
the latter conditions were applied in Scheme 2 (For the results using
0.5 equivalent of CuTc, see Supporting Information).
resulting in undesired coupling reaction with thiophene carboxylate
(Tc). The reaction conditions are compatible with halogens (2h-j),
ketones (2g, 2q), carboxylate esters (2k, 2t), nitrile (2s) and
trimethylsilylacetylene (2u), which makes these compounds ready
for further transformations to more complicated structures. To be
noted, sulfones containing acidic protons, such as carboxylic acid
(2v) and amide (2d), which are difficult to prepare using
conventional methods, were accessible with high yields. For
substrate 1u, the trimethylsilyl group was partially transformed to
hydrogen, which delivered the unmasked acetylene compound
directly (2u’). Finally, to show the synthetic utility of this method in
organic synthesis, the α-Tocopherol derivative was synthesized
using this new method in 54% yield (2x).
This reaction could be expanded to synthesize aromatic
sulfones containing other fluoroalkyls, such as mono- and
trifluoromethyl groups (Schemes 3 and 4). It was found that the
monofluoromethanesulfonylation
of
arenediazonium
tetrafluoroborates gave the products 3 in moderate to good yields
(Scheme 3). Electron-neutral and electron-rich substrates (1a-f)
displayed better reactivity than those bearing electron-withdrawing
groups (1g and 1y), which is consistent with the electronic effect.
For trifluoromethanesulfonylation of arenediazonium salts, we
chose 1c as the model compound owing to its high reactivity
towards sulfonylation as exhibited in Schemes 1 and 2. Although
the desired trifluoromethanesulfonylation product 4 was obtained
only in moderate yield (Table 2, entry 11), it is encouraging for us to
develop a complementary procedure to synthesize sulfones with
electron-rich groups (Scheme 4), which were not easily accessible
using known methods.10
Table 1 Optimization of the reaction conditions.
entrya
1
“Cu”
Cu2O
solvent
DMSO
DMSO
DMSO
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
time
12 h
12 h
12 h
12 h
12 h
12 h
12 h
12 h
12 h
12 h
12 h
12 h
12 h
12 h
12 h
12 h
12 h
0.5 h
0.5 h
yield
none
8%
2
CuTc
Scheme
2
Copper-mediated
difluoromethanesulfonylation
of
3
CuOAc
CuTc
none
64%
none
none
48%
none
16%
19%
none
35%
79%
80%
58%
47%
34%
80%
80%
arenediazonium tetrafluoroborates.a
4
a
.
5
Cu2O
6
CuOAc
Cu(OAc)2
CuSO4
Cu(acac)2
CuCN
7
8
9
10
11
12
13b
14b,c
15b,d
16 b,e
17 b,f
18 b
19b,c
CuF2
Cu(MeCN)4(PF6)2
CuTc
CuTc
CuTc
CuTc
CuTc
CuTc
CuTc
a The reaction was conducted on 0.2 mmol scale: 1a (1.0 equiv), “Cu” (1.0
equiv), CF2HSO2Na (1.5 equiv) was mixed together and 2.0 mL solvent was
added. The reaction was stirred for the time in the Table. The yield was
b
determined by 19F NMR with trifluoromethylbenzene as internal standard.
1.0 mL of solution of 1a (1.0 equiv) was added via syringe to 1.0 mL of
c
solution of CuTc (1.0 equiv) and CF2HSO2Na (1.5 equiv). 0.5 equiv of CuTc
d
was used. 0.4 equiv of CuTc was used. e 0.3 equiv of CuTc was used. f 0.2
equiv of CuTc was used.
As shown in Table 2, our method has a good compatibility with
various functional groups, giving the corresponding products 2 in
moderate to good yields. The substrates bearing electron-donating
groups give better yields than the electron-deficient ones, which
might attribute to the latter’s higher reactivity with CuTc, thus
a The reaction was conducted on a 0.5-mmol scale: a solution of substrate 1
(1.0 equiv) was added dropwise to the solution of CuTc (1.0 equiv) and
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CF2HSO2Na (1.5 equiv). Isolated yields are given. b 0.5 equiv of CuTc was used
instead. c More solvent was used to dissolved the substrate.
1
2
Cu2O
CuTc
DMSO
DMSO
DMSO
DMSO
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
12 h
12 h
12 h
12 h
12 h
12 h
12 h
12 h
0.5 h
0.5 h
0.5 h
23%
15%
19%
21%
7%
.
Scheme
3
Copper-mediated monofluoromethanesulfonylation of
3
Cu(OAc)2 H2O
CuOTf
arenediazonium tetrafluoroborates.a
4
5
Cu2O
6
CuTc
37%
16%
9%
.
7
Cu(OAc)2 H2O
8
CuOTf
CuTc
CuTc
CuTc
9
10b
11c
34%
33%
40%
a The reaction was conducted on 0.2 mmol scale: 1c (1.0 equiv), CuTc (1.0
equiv), CF3SO2Na (1.5 equiv), solvent (2.0 mL). Yield was detected by 19F
o
c
NMR. b The reaction was heated at 50 C. The reaction was conducted on
a The reaction was conducted on 0.5 mmol scale: a solution of substrate 1
0.5 mmol scale. Isolated yield was given.
(1.0 equiv) was added dropwise to the solution of CuTc (1.0 equiv) and
CH2FSO2Na (1.5 equiv). Isolated yields are given. More amount of solvent
was used to dissolve the substrate.
b
Scheme
4
Copper-mediated
trifluoromethanesulfonylation
of
arenediazonium tetrafluoroborates.a
In 2015, we compared the reactivity of different sodium
sulfinates in radical fluoroalkylation reactions.11 In the oxidative
radical fluoroalkylations, the reactivity of sodium sulfinates
decreases in the following order: PhCF2SO2Na > CF2HSO2Na >
CH2FSO2Na. Since arenediazonium tetrafluoroborates were not able
to oxidize the sulfinates to the fluoroalkyl radicals,10 it is reasonable
to presume that the sodium fluoromethanesulfinates in our
reactions react as nucleophiles rather than radical precursors. As
our continuing effort in probing the unique fluorine effects in
organic reactions,12 we were interested in probing the reactivity
order of RfSO2Na (Rf = CF3, CF2H, and CH2F) in the current
fluoroalkanesulfonylations (Scheme 5). To rule out the influence of
solubility of different sulfinates, a mixture of MeCN/H2O was
utilized as a co-solvent system (in MeCN, the solubility of these
sodium sulfinates decreases as follows: CF3SO2Na > HCF2SO2Na >>
FCH2SO2Na). When 1c was subjected to a mixture of CF2HSO2Na and
CH2FSO2Na, 3c was formed as the major product with a ratio of
3c/2c = 100/13 (Scheme 5, eq A). When 1c was exposed to a
mixture of CH2FSO2Na and CF3SO2Na, the ratio of 3c to 4c was
about 100/0.6 (Scheme 5, eq B). As for the reaction between 1c and
CF2HSO2Na/CF3SO2Na, the ratio of compound 2c to 4c was 100/6
(Scheme 5, eq C). Based on the results of these competition
experiments, it can be concluded that the relative reactivity of
sodium fluoroalkanesulfinates is decreasing in the following order:
CH2FSO2Na > CF2HSO2Na > CF3SO2Na, which can be explained by
their relative nucleophilicity (more electron-rich sulfinates possess
higher nucleophilicity). However, this trend is remarkably different
from their reactivity in oxidative radical fluoroalkylations.11 In the
radical process, CF3SO2Na usually displays high tendency to extrude
SO2 to generate CF3 radical once oxidized. However, in our present
reaction as shown in Table 2 and Scheme 4, trifluoromethylation
was not observed when no external oxidant was added. Indeed,
most of the CF3SO2Na remained unreacted under our reaction
conditions. Furthermore, considering the relatively higher reactivity
of CH2FSO2Na and CF2HSO2Na (compared to CF3SO2Na) in our
reactions, it is reasonable to hypothesize that the sodium sulfinates
RfSO2Na (Rf = CF3, CF2H, and CH2F) do not undergo oxidation to
generate radicals but probably react as nucleophiles.
a The reaction was conducted on 0.5 mmol scale: a solution of substrate 1
(1.0 equiv) was added dropwisely to the solution of CuTc (1.0 equiv) and
RfSO2Na (1.5 equiv). Isolated yields are given. bNot isolatable from the
by-product. Yield was detected by 19F NMR.
Scheme 5 Competition experiments of sodium fluoromethanesulfinates
reacting with compound 1c.
Scheme 6 Proposed mechanism.
Table 2 Optimization of conditions for trifluoromethanesulfonylation of
substrate 1c.
Based on the aforementioned information, we propose a
plausible reaction mechanism as shown in Scheme 6. In the first
step (Scheme 6, eq a), a copper species CuX, either X = Tc or RfSO2,
entrya
“Cu”
solvent
time
yield
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serves as an initiator to generate aromatic radical through single
electron reansfer (SET), and the latter species react with
fluoroalkylsulfinate to a radical anion (Scheme 6, eq b). Then the
radical anion can be quenched by either another molecule of
substrate (nucleophilic radical chain reaction) or the cuprous
compound (redox process).
O’Hagan D.; Wang Y.; Skibinski, M.; Slawin, A. M. Z. Pure Appl. Chem.
2012, 84, 1587.
[4] (a) Ni, C.; Hu, M.; Hu J. Chem. Rev. 2015, 115, 765; (b) Li, X.; Zhao, J.; Hu,
M.; Chen, D; Ni C.; Wang, L.; Hu J. Chem. Commun. 2016, 52, 3657; (c)
Xiao, P.; Rong, J.; Ni, C.; Guo, J.; Li, X.; Chen, D.; Hu, J. Org. Lett. 2016,
18, 5912; (d) Hu, J. J. Fluorine Chem. 2009, 130, 1130.
[5] Trost, B. M. Bull. Chem. Soc. Jpn. 1988, 61, 107.
Conclusions
[6] (a) Prakash, G. K. S.; Hu, J.; Olah, G. A. J. Org. Chem. 2003, 68, 4457; (b)
Prakash, G. K. S.; Hu, J.; Mathew, T.; Olah, G. A. Angew. Chem. Int. Ed.
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Eur. J. Org. Chem. 2005, 2218; (g) Zhu, J.; Wang, F.; Huang, W.; Zhao,
Y.; Ye, W.; Hu, J. Synlett, 2011, 899.
In summary, we have developed copper-mediated di- and
monofluoromethanesulfonylation of arenediazonium salts using
CF2HSO2Na and CH2FSO2Na reagents, which can be readily prepared
from the corresponding benzo[d]thiazol-2-yl sulfones.11 Various
structurally diverse di- and monofluoromethyl aryl sulfones can be
readily synthesized in good yields. This method also offers an
alternative protocol for the synthesis of electron-rich
trifluoromethyl sulfones. Furthermore, it was found that the
relative reactivity of these fluoroalkanesulfinates in the current
fluoroalkanesulfonylations is decreasing in the following order:
CH2FSO2Na > CF2HSO2Na > CF3SO2Na. This reactivity order is
consistent with their innate nucleophilicity, but different from their
relative reactivity in oxidative radical fluoroalkylations.11
[7] (a) Cho, H.; Du, X.; Rizzi, J. P.; Liberzon, E.; Chakraborty, A. A.; Gao, W.;
Carvo, I.; Signoretti, S.; Bruick, R. K.; Josey, J. A.; Wallace, E. M.; Kaelin
Jr, W. G. Nature, 2016, 539, 107; (b) Chen, W.; Hill, H.; Christie, A.; Kim,
S. M.; Holloman, E.; Pavia-Jimenez, A.; Hoyamoun, F.; Ma, Y.; Patel, N.;
Yell, P.; Hao, G.; Yousuf, Q.; Joyce, A.; Pedrosa, I.; Geiger, H.; Zhang, H.;
Chang, J.; Gardner, K. H.; Bruick, R. K.; Reeves, C.; Hwang, T. H.;
Courtney, K.; Frenkel, E.; Sun, X.; Zojwalla, N.; Wong, T.; Rizzi, J. P.;
Wallace, E. M.; Josey, J. A.; Xie, Y.; Xie, X.-J.; Kapur, P.; McKay, R. M.;
Brugarolas, J. Nature, 2016, 539, 112.
Supporting Information
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2018xxxxx.
[8] (a) Hine, J.; Porter, J. J. J. Am. Chem. Soc. 1960, 82, 6718; (b) Zheng, Q.-T.;
Wei, Y.; Zheng, J.; Duan, Y.-Y.; Zhao, G.; Wang, Z.-B.; Lin, J.-H.; Zheng,
X.; Xiao, J.-C. RSC Adv. 2016, 6, 82298.
Acknowledgement
This work was supported by the National Key Research and
Development Program of China (2015CB931900, 2016YFB0101200),
the National Natural Science Foundation of China (21632009,
21421002, 21372246, 21302206), the Key Programs of the Chinese
Academy of Sciences (KGZD-EW-T08), the Key Research Program of
Frontier Sciences of CAS (QYZDJ-SSW-SLH049), Shanghai Science
and Technology program (15XD1504400 and 16QA1404600), Youth
Innovation Promotion Association CAS (2014231).
[9] Cullen, S. C.; Shekhar, S.; Nere, N. K. J. Org. Chem. 2013, 78, 12194.
[10] Zhang, K.; Xu, X.-H.; Qing, F.-L. J. Org. Chem. 2015, 80, 7658.
[11] He, Z.; Tan, P.; Ni, C.; Hu, J. Org. Lett. 2015, 17, 1838.
[12] (a) Ni, C.; Hu, J. Chem. Soc. Rev. 2016, 45, 5441; (b) Zhang, W.; Ni, C.;
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[13] For selected recent examples of using arenediazonium
tetrafluoroborates in synthetic fluorine chemistry, see: (a) Danoun, G.;
Bayarmagnai, B.; Grünberg, M. F.; Goossen, L. J. Angew. Chem. Int. Ed.
2013, 52, 7972. (b) Bayarmagnai, B.; Matheis, C.; Jouvin, K.; Goossen, L.
J. Angew. Chem. Int. Ed. 2015, 54, 5753. (c) Matheis, C.; Jouvin, K.;
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G.; Qiu, D.; Feng, J.; Ye, Y.; Zhang, S.; Zhang, Y.; Wang, J. J. Am. Chem.
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Y.; Leng, X.; Shen, Q. Angew. Chem. Int. Ed. 2015, 54, 7648.
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Manuscript received: XXXX, 2017
Revised manuscript received: XXXX, 2017
Accepted manuscript online: XXXX, 2017
Version of record online: XXXX, 2017
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Entry for the Table of Contents
Page No.
Copper-Mediated Di- and
Monofluoromethanesulfonylation of
Arenediazonium Tetrafluoroborates:
Probing the Fluorine Effect
Bo Xing, Chunafa Ni, Jinbo Hu*
This article is protected by copyright. All rights reserved.