One-Pot Synthesis of Pentafluorophenyl Sulfonic Esters via Copper-Catalyzed Reaction of Aryl Diazonium Salts, DABSO, and Pentafluorophenol

Article abstract of DOI:10.1021/acs.orglett.1c01056

Pentafluorophenyl (PFP) sulfonic esters were synthesized via a copper-catalyzed one-pot multicomponent reaction of aryl diazonium tetrafluoroborate, DABSO (DABCO·(SO2)2), and pentafluorophenol. The reaction system provided the desired pentafluorophenyl sulfonic esters in good yields and exhibited excellent functional group tolerance. In addition, the generated PFP sulfonic esters were successfully applied in Sonogashira, Suzuki, Chan-Evans-Lam, and decarboxylative coupling reactions.

Full text of DOI:10.1021/acs.orglett.1c01056

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Letter
One-Pot Synthesis of Pentafluorophenyl Sulfonic Esters via Copper-
Catalyzed Reaction of Aryl Diazonium Salts, DABSO, and
Pentafluorophenol
Muhammad Aliyu Idris and Sunwoo Lee*
Cite This: Org. Lett. 2021, 23, 4516−4520
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ABSTRACT: Pentafluorophenyl (PFP) sulfonic esters were
synthesized via a copper-catalyzed one-pot multicomponent
reaction of aryl diazonium tetrafluoroborate, DABSO (DABCO·
(SO2)2), and pentafluorophenol. The reaction system provided the
desired pentafluorophenyl sulfonic esters in good yields and
exhibited excellent functional group tolerance. In addition, the
generated PFP sulfonic esters were successfully applied in
Sonogashira, Suzuki, Chan−Evans−Lam, and decarboxylative
coupling reactions.
Scheme 1. Synthesis of Pentafluorosulfonic Esters
ulfonyl-bearing organic compounds are among the most
Simportant building blocks used for the synthesis of organic
functional molecules such as drugs.1 For example, sulfone,
sulfonamide, sulfonyl ester, and sulfonyl hydrazine moieties are
present in a variety of bioactive molecules and drugs.2 Thus,
numerous synthetic methods for accessing sulfonyl-containing
compounds have been developed to date.3 The oxidation of
sulfides to sulfones is one such reliable method; however,
sulfide preparation often entails multistep processes, wherein
thiols are used as starting materials. In addition, direct
oxidation of thiols to sulfonyl chlorides has been developed
and employed for the synthesis of sulfonyl derivatives.
However, the use of thiols as starting materials presents
several drawbacks, such as strong unpleasant odors.4
The use of sulfonyl chlorides as electrophiles in reactions
with a variety of nucleophiles is the most straightforward
approach because sulfonyl chloride derivatives are commer-
cially available and highly reactive.5 However, their high
reactivity suffers from disadvantages such as narrow functional
group tolerance, diminished stability, and complicated storage
and handling.
To address these issues, several sulfonyl chloride surrogates
have been developed. Among them, pentafluorophenyl (PFP)
sulfonic esters have been used as an alternative to sulfonyl
electrophiles due to their stability and versatility.6
The Caddick group reported the first synthesis of PFP
vinylsulfonate via the reaction of sulfonyl chloride and
pentafluorophenol (PFPOH) (Scheme 1a).7 In 2004, the
same group developed a general tool for the synthesis of PFP
sulfonic esters from sulfonic acid (Scheme 1b). This
methodology initially entailed a two-step process, and was
subsequently modified to a one-pot reaction.8 Furthermore,
2,4,6-trichlorophenol (TCP) sulfonic esters have also been
Received: March 28, 2021
Published: May 12, 2021
https://doi.org/10.1021/acs.orglett.1c01056
Org. Lett. 2021, 23, 4516−4520
© 2021 American Chemical Society
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developed as sulfonyl chloride surrogates by the same group.9
However, the drawback of these approaches is the necessity to
prepare sulfonyl derivatives using expensive materials.
Willis and co-workers developed Pd/Cu and Ni/Cu-
catalyzed sequential reactions for the synthesis of PFP sulfonic
esters from aryl boronic acids, DABSO (DABCO·(SO2)2), and
PFPOH in 2018 (Scheme 1c).10 However, the protocol
involves a sequential two-step process employing an expensive
palladium catalyst, with only one example being reported for
the Ni/Cu catalytic system. Therefore, the development of a
general synthetic method for accessing sulfonyl derivatives is
imperative.
Recently, reactions involving DABSO have garnered
considerable attention, as this reagent can be employed as a
surrogate for SO2, which is a toxic gas, whereas DABSO is a
commercially available air-stable solid. The use of sulfinate salts
as intermediates readily provides a variety of sulfonyl
compounds, such as sulfonyl fluorides, sulfones, sulfonamides,
and sulfonic esters.11
The transition-metal-catalyzed sulfination of aryl substrates,
such as aryl halides and boronic acids, has been developed,
entailing the use of DABSO as a sulfur dioxide surrogate to
access the corresponding arene sulfinate salts.12
Aryl diazonium salts have been widely utilized as starting
materials for the synthesis of aryl derivatives, as they are readily
prepared from a plethora of commercially available aryl
amines.13 Han and co-workers reported that copper-catalyzed
reactions of aryl diazonium salts, DABSO, and alkyl alcohols
provide the corresponding sulfonic esters. However, they failed
to obtain the corresponding aryl sulfonic esters from aromatic
alcohols.14
Table 1. Optimization of Conditions for the Synthesis of
PFP Sulfonic Esters
a
Entry
Change from the standard conditions
Yield (%)
1
2
No changes
No CuI
98
0
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
CuCl instead of CuI
CuBr instead of CuI
CuCl2 instead of CuI
CuBr2 instead of CuI
Na2S2O5 instead of DABSO
K2S2O5 instead of DABSO
1,4-Dioxane instead of MeCN
DMF instead of MeCN
DMSO instead of MeCN
Toluene instead of MeCN
10 mol % CuI instead of 20 mol %
50 °C instead of 85 °C
6 h instead of 12 h
67
86
50
55
0
0
trace
trace
35
30
60
45
96
trace
trace
0
In the presence of CF3CO2H (1 equiv)
In the presence of CF3CO2H (0.1 equiv)
Under N2 atmosphere
a
Reaction conditions: 1a (0.15 mmol), DABSO (0.2 mmol), HOPFP
(0.1 mmol), and CuI (0.02 mmol) were reacted in MeCN (1.0 mL)
under air at 85 °C for 12 h.
We envisaged that the use of aryl diazonium salts and
DABSO would be an efficient system for the synthesis of PFP
sulfonic esters. Herein, we report the copper-catalyzed
synthesis of diverse PFP sulfonic esters entailing a multi-
component reaction of aryl diazonium salts, DABSO, and
pentafluorophenol. This method employs an inexpensive Cu
catalyst in a single-step operation.
steric properties were well-tolerated. Phenyl diazonium
provided a 98% isolated yield of 2a. Aryl diazonium salts
bearing alkyl substituents on the phenyl ring delivered
excellent yields of the corresponding products (2b, 2c, and
2d; Scheme 2). Halide substitutions with chloride, bromide,
iodide, and fluoride were well-tolerated, and moderate to good
yields of the corresponding products 2e−2j were obtained.
Aryl diazonium salts disubstituted with methyl and iodide as
well as methyl and bromide groups also reacted well to
produce 2k, 2l, and 2m in 53%, 98% and 48% yield,
respectively. 2-Naphthyl- and 2-phenoxyphenyl diazonium
salts gave 2n and 2o, respectively, in good yields; however,
2-biphenyl diazonium provided a low yield due to the steric
hindrance of the ortho-substituted phenyl group. Alkoxy-,
thiomethyl-, and amine-substituted aryl diazonium salts
afforded the corresponding products 2q, 2r, 2s, 2t, and 2u in
moderate to excellent yields. 3-Pyridyl-, 3-acetyl-, and 3-ethyl
ester-substituted aryl diazonium salts produced 2v, 2w, and 2x
in 45%, 77%, and 98% yields, respectively. In the case of aryl
diazonium salts containing electron-withdrawing groups at the
para-position, such as a cyano or nitro, the corresponding
products (2y, 2z, and 2ab) were also isolated in moderate
yields (45%−53%). It is noteworthy that no desired sulfonic
esters were formed when phenol and methanol were employed
instead of HOPFP (see Scheme S1 in the Supporting
Information).
After extensive screening of the reaction parameters, the
optimized conditions for the reaction of 1a and HOPFP were
established as follows: CuI (20 mol %) in CH3CN in the
presence of DABSO in air at 85 °C for 12 h afforded the
desired product 2a in 98% yield (Table 1, entry 1).
Modification of the reaction parameters provided the following
results. No product was formed in the absence of copper
catalyst (entry 2). In addition to copper(I), copper(II) likewise
provided 2a, albeit in lower yields (entries 2−6). When
inorganic sulfur dioxide surrogates, such as Na2S2O5 and
K2S2O5, were employed, no product was formed (entries 7 and
8). Conducting the reaction in 1,4-dioxane or DMF afforded a
trace amount of product (entries 9 and 10). When DMSO or
toluene was employed as the solvent, the product was obtained
in 35% and 30% yield, respectively (entries 11 and 12).
Reducing the amount of catalyst and decreasing the reaction
temperature provided inferior results (entries 13 and 14).
However, shortening the reaction time to 6 h did not
significantly affect the yield (entry 15). No product was
formed in the presence of CF3CO2H, employed as a promoter
in CuBr2-catalyzed syntheses of sulfonic esters from alkyl
alcohols (entries 16 and 17).
Next, we evaluated the efficiency of this method for large-
scale reactions, and the applicability of the generated PFP
sulfonic esters in further transformations. Products 2i and 2t
were chosen for gram scale preparation and as substrates in
further transformations. As shown in Scheme 3, 2i and 2t were
successfully obtained in 92% and 63% yield, respectively.
With the optimized conditions in hand, we next investigated
the substrate scope with respect to aryl diazonium salts. Aryl
diazonium salts bearing substituents of varying electronic and
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a
Scheme 2. Cu-Catalyzed Synthesis of PFP Sulfonic Esters
Scheme 4. Transformation of PFP Sulfonic Esters
TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy), BHT (2,6-di-
tert-4-methylphenol), and DPE (1,1-diphenylethylene). As
shown in Scheme 5, no product was formed in the presence
Scheme 5. Control Experiments
a
Reaction conditions: 1 (1.5 mmol), DABSO (2.0 mmol), HOPFP
(1.0 mmol), and CuI (0.2 mmol) were reacted in MeCN (8.0 mL)
under air at 85 °C for 12 h.
of TEMPO or BHT. When the standard reaction was
performed in the presence of DPE, 2a was formed in 20%
yield and the arylsulfonyl-bearing alkene byproduct was
obtained in 44% yield.16
Scheme 3. Gram Scale Synthesis of PFP Sulfonic Esters
Based on the results from the control experiments and
previous reports, a plausible reaction mechanism is outlined in
Scheme 6. The aryldiazonium salt generates the aryl radical,
and it reacts with DABSO to provide the corresponding
arylsulfonyl radical and a DABCO radical cation, which in turn
We then investigated the suitability of PFP sulfonic esters in
several reactions, as shown in Scheme 4. PFP sulfonic ester 2i
was employed as a coupling partner in palladium-catalyzed
cross-coupling reactions. In the case of Sonogashira coupling,
the reaction of 2i and phenyl acetylene afforded the coupled
product in 72% yield. When phenyl propiolic acid was
employed in the reaction with 2i under decarboxylative
coupling conditions, the desired product was formed in 61%
yield. In addition, Suzuki coupling of 2i with phenylboronic
acid provided 4 in 88% yield. The PFP sulfonic ester bearing
an amine group displayed good activity in the Chan−Evans−
Lam reaction15 providing 5 in 67% yield, and N-acylation
proceeded smoothly to afford the corresponding product 6 in
91% yield.
Scheme 6. Proposed Mechanism
To study the reaction pathway, the standard reaction was
conducted in the presence of radical trapping reagents
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oxidizes Cu(I) to Cu(II). In addition, it is suggested that
molecular oxygen may be involved in the oxidation of Cu(I) to
Cu(II) because the reaction fails under a nitrogen atmosphere.
PFPOH reacts with Cu(II) to produce the corresponding
alkoxy copper complex, which subsequently reacts with the
arylsulfonyl radical to generate the Cu(III) intermediate.
Finally, reductive elimination produces the corresponding PFP
sulfonyl ester and regenerates the Cu(I) species.
In summary, we have developed a copper-catalyzed synthesis
of PFP sulfonyl esters via a multicomponent one-pot reaction
of aryl diazonium tetrafluoroborates, DABSO, and PFPOH.
CuI was found to be the optimal copper catalyst source and
CH3CN was the best solvent. The protocol exhibits a broad
substrate scope, providing the corresponding PFP sulfonyl
esters in good yields. PFP 4-iodobenzenesulfonate (2i)
successfully participated in Sonogashira, Suzuki, and decar-
boxylative coupling reactions. In addition, PFP 4-amino-
benzenesulfonate (2t) participated in Chan-Evans-Lam re-
action.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01056.
Experimental procedures and spectral data for the
products (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Sunwoo Lee − Department of Chemistry, Chonnam National
University, Gwangju 61186, Republic of Korea; orcid.org/
0000-0001-5079-3860; Email: sunwoo@chonnam.ac.kr
Author
Muhammad Aliyu Idris − Department of Chemistry,
Chonnam National University, Gwangju 61186, Republic of
Korea
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01056
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the National Research
Foundation of Korea (NRF) grant funded by the Korea
government (MSIT) (NRF-2021R1A2C1005169). The spec-
tral and HRMS data were obtained from the Korea Basic
Science Institute, Gwangju Center and Daegu Center.
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