Copper-Catalyzed Decarboxylative Disulfonylation of Alkynyl Carboxylic Acids with Sulfinic Acids

Article abstract of DOI:10.1021/acs.orglett.7b03922

A copper-catalyzed decarboxylative disulfonylation of alkynyl carboxylic acids with sulfinic acids in aqueous solution has been developed. The reaction provides a straightforward and practical access to (E)-1,2-disulfonylethenes, which are important building blocks in synthetic organic chemistry, and exhibits a good functional group tolerance and excellent stereoselectivity. A possible mechanism for the transformation is proposed.

Full text of DOI:10.1021/acs.orglett.7b03922

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Copper-Catalyzed Decarboxylative Disulfonylation of Alkynyl
Carboxylic Acids with Sulfinic Acids
Hong Fu, Jia-Qi Shang, Tao Yang, Yuehai Shen, Chuan-Zhu Gao, and Ya-Min Li*
Faculty of Life Science and Technology, Kunming University of Science and Technology, Kunming 650500, People’s Republic of
China
S
* Supporting Information
ABSTRACT: A copper-catalyzed decarboxylative disulfonylation of
alkynyl carboxylic acids with sulfinic acids in aqueous solution has been
developed. The reaction provides a straightforward and practical access to
(E)-1,2-disulfonylethenes, which are important building blocks in synthetic
organic chemistry, and exhibits a good functional group tolerance and
excellent stereoselectivity. A possible mechanism for the transformation is
proposed.
ulfone-containing molecules have broad applications in the
fields of biochemistry, medicinal chemistry, materials, and
Scheme 1. Decarboxylative Sulfonylation of Alkynyl
Carboxylic Acids
S
organic synthesis.1 Among them, 1,2-disulfonylethenes have
attracted significant attention because of their usefulness as
synthetic intermediates.2 In light of their importance, the
development of an efficient approach to 1,2-disulfonylethenes
is of considerable interest. Generally, the preparation of 1,2-
disulfonylethenes relies on several classic methods, the oxidation
of 1,2-dithioethenes or 1-thio-2-sulfonylethenes,3 the condensa-
tion of 1-sulfonyl-2,2-dichloroethanes with sodium sulfonates,4
the nucleophilic substitution of alkenyliodonium salts with
sodium sulfonates,5 and cycloaddition or Michael-type addition
of bis(sulfonyl)acetylene.6 However, these approaches always
involve complex substrates and/or multiple synthetic steps.
Thus, developing a simple and efficient method to construct 1,2-
disulfonylethenes is necessary.
Decarboxylative coupling is one of the most powerful methods
for the construction of C−C and C−heteroatom bonds due to
the easy storage and operability of carboxylic acids, the high
selectivity, and the release of a nontoxic byproduct, CO2.7,8
Recently, the use of alkynyl carboxylic acids as terminal alkyne
surrogates has received increased attention.9−14 Since Lee and
co-workers first demonstrated a palladium-catalyzed decarbox-
ylative coupling reaction of alkynyl carboxylic acids with aryl
halides to afford unsymmetric diarylalkynes,9a numerous
decarboxylative coupling reactions of alkynyl carboxylic acids
have been developed to construct C−C,9 C−N,10 C−P,11 C−
Si,12 C−B,13 and C−S14 bonds. In this context, much effort has
been made to create C−S bonds by decarboxylative sulfonylation
of alkynyl carboxylic acids (Scheme 1). Jiang et al. reported a
palladium-catalyzed coupling reaction for the formation of
internal alkynes and vinyl sulfones from alkynyl carboxylic acids
and sodium sulfonates.14a Mao, Zhang, and co-workers
subsequently developed Cu/Fe-cocatalyzed sulfonylation of
aromatic propiolic acids with sulfonyl hydrazides to construct
vinyl sulfones.14b Brønsted acid14c and base14d promoted
decarboxylative sulfonylations were also achieved (Scheme 1a).
Kuhakarn et al. reported an I2-catalyzed decarboxylative coupling
of arylacetylenic acids for the synthesis of arylacetylenic sulfones
(Scheme 1b).14e Very recently, copper(I)-catalyzed decarbox-
ylative sulfonylation of arylpropiolic acids have also been
developed to construct β-keto sulfones by the Wu group
(Scheme 1c).14f Despite these advances, to the best of our
knowledge, the decarboxylative disulfonylation of alkynyl
carboxylic acids has not been well developed up to now. On
the basis of our continuing interest in decarboxylative coupling
and sulfonylation,15 herein we report a novel, efficient, and
practical copper-catalyzed decarboxylative disulfonylation of
alkynyl carboxylic acids with sulfinic acids to give (E)-1,2-
disulfonylethenes.
At the outset of our investigation, phenylpropiolic acid (1a)
and benzenesulfinic acid (2a) were chosen as the model
substrates to optimize reaction conditions for the decarboxylative
disulfonylation. When phenylpropiolic acid was treated with 3.0
equiv of benzenesulfinic acid in the presence of 10 mol % of
Cu(ClO4)2·6H2O and 3.0 equiv of K2S2O8 in CH3CN/H2O (2/
Received: December 17, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03922
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
1 (v/v)) at 80 °C for 12 h under an argon atmosphere, to our
delight, the reaction proceeded smoothly and afforded the
desired disulfonylated product 3aa in 52% yield with 98/2 E/Z
selectivity. Subsequently, various oxidants were screened to
improve the reaction efficiency, and (NH4)2S2O8 proved to be
better than the other oxidants (see the Supporting Information).
In addition to Cu(ClO4)2·6H2O, other Cu catalysts including
CuCl, CuO, Cu(OAc)2, and CuSO4·5H2O were also tested, and
the results indicate that Cu(ClO4)2·6H2O was still the best
choice (Table 1, entries 1−5). Different solvents were
evaluate the scope of the decarboxylative disulfonylation, and the
results are summarized in Table 2. The electronic properties of
a b
,
Table 2. Substrate Scope
a
Table 1. Screening of Reaction Conditions
entry
catalyst
solvent (v/v)
yield (%)
E/Z
1
Cu(ClO4)2·6H2O
CuCl
CH3CN/H2O (2/1)
CH3CN/H2O (2/1)
CH3CN/H2O (2/1)
CH3CN/H2O (2/1)
CH3CN/H2O (2/1)
acetone/H2O (2/1)
DMF/H2O (2/1)
55
48
53
trace
50
39
50
77
35
93
88
84
90
96
trace
0
99/1
>99/1
98/2
2
3
CuO
4
Cu(OAc)2
5
CuSO4·5H2O
99/1
6
Cu(ClO4)2·6H2O
Cu(ClO4)2·6H2O
Cu(ClO4)2·6H2O
Cu(ClO4)2·6H2O
Cu(ClO4)2·6H2O
Cu(ClO4)2·6H2O
Cu(ClO4)2·6H2O
Cu(ClO4)2·6H2O
Cu(ClO4)2·6H2O
Cu(ClO4)2·6H2O
99/1
7
>99/1
>99/1
94/6
8
DMSO/H2O (2/1)
toluene/H2O (2/1)
DMSO/H2O (2/1)
DMSO/H2O (3/1)
DMSO/H2O (1/1)
DMSO/H2O (2/1)
DMSO/H2O (2/1)
DMSO/H2O (2/1)
DMSO/H2O (2/1)
DMSO/H2O (2/1)
9
b
b
b
10
11
12
13
14
15
16
17
>99/1
>99:1
>99:1
>99:1
>99/1
bc
,
b
−
−
d
e
b
a
bcd
, ,
All of the reactions were carried out in the presence of 0.3 mmol of 1,
4.0 equiv of 2, 10 mol % of Cu(ClO4)2·6H2O, and 3.0 equiv of
(NH4)2S2O8 in 3.0 mL of DMSO/H2O (2/1) at room temperature for
b−df
,
Cu(ClO4)2·6H2O
0
a
Reaction conditions unless specified otherwise: 1a (0.3 mmol), 2a
b
6 h under Ar. Isolated yield.
(3.0 equiv), catalyst (10 mol %), and (NH4)2S2O8 (3.0 equiv) in
solvent (3.0 mL) at 80 °C for 12 h under Ar. Isolated yield. E/Z ratios
b
were determined by 1H NMR of the crude products. Using 4.0 equiv
of 2a. At 25 °C. For 6 h. In air. The reaction was carried out in the
absence of (NH4)2S2O8.
the substituents had no apparent effect on the reaction.
Arylpropiolic acids bearing electron-donating (Me, MeO, OH,
Me3Si, and AcNH) or electron-withdrawing groups (F, NO2, Ac,
and CN) were compatible with the reaction conditions, affording
the desired products 3aa−3oa in satisfactory yields. Halogen
groups on the aromatic ring such as Cl and Br also could be well
tolerated in the copper-catalyzed reaction, which provided
opportunities for further functionalization. However, vinyl-
substituted phenylpropiolic acid was not a suitable substrate.
Steric hindrance in the ortho-substituted arylpropiolic acids does
not have a considerable influence on the transformation, and
good to excellent yields were provided in all cases. In addition, 2-
naphthyl-substituted phenylpropiolic acid also underwent the
reaction smoothly, giving the product 3qa in 61% yield.
Thienylpropiolic acid, a heteroaromatic substrate, was applicable
under the standard conditions as well and afforded the desired
product 3ra in reasonable yield. Notably, alkylpropiolic acids,
such as 2-butynoic acid, 2-hexynoic acid, and 2-octynoic acid,
were not suitable substrates due to their low reactivity (3sa−
3ua).
c
d
e
f
investigated, and among them DMSO/H2O exhibited un-
matched efficacy for the decarboxylative disulfonylation (Table
1, entries 6−9). With regard to the amount of benzenesulfinic
acid 2a, when the amount of 2a was increased to 4.0 equiv, the
yield of 1,2-disulfonylethene improved to 93% (Table 1, entry
10). The catalyst loading and effect of oxidant stoichiometry
were also examined, and the results indicate that 10 mol % of
catalyst with 3.0 equiv of oxidant was the best choice (see the
Supporting Information). The role of H2O might be to improve
the solubility of the oxidant. Among the ratios of DMSO and
H2O examined, the 2/1 ratio was preferred (Table 1, entries 10−
12). Importantly, it was found that the reaction operated equally
well at room temperature (Table 1, entry 13). The effect of
reaction time was also investigated, and the best choice was 6 h
(Table 1, entry 14). Only a trace amount of 3aa was observed
when the reaction was carried out in an atmosphere of air (Table
1, entry 15). Control experiments showed that no reaction
occurred in the absence of either Cu(ClO4)2·6H2O or
(NH4)2S2O8 (Table 1, entries 16 and 17).
The scope of sulfinic acids was also examined. Both electron-
rich and -poor benzenesulfinic acids could be transferred to the
1,2-disulfonylethenes 3ab−3al in good to excellent yields, and a
series of functional groups, such as alkyl, halides, nitrile, and
trifluoromethyl, were compatible with the reaction conditions.
With the optimized reaction conditions, a variety of alkynyl
carboxylic acids were subjected to the optimized conditions to
B
DOI: 10.1021/acs.orglett.7b03922
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
Ortho-substituted benzenesulfinic acids also exhibited a high
reactivity (3ab,ac), indicating that steric effects on the phenyl
ring are not evident in this transformation. Moreover, 2-
naphthylsulfinic acid was also a suitable substrate, albeit in
moderate yields. It was pleasant to find that heterocycle-
substituted sulfinic acids were compatible with this catalytic
system, leading to the expected products 3an,ao in 96% and 86%
yields, respectively. Additionally, this transformation is not
limited to aromatic sulfinic acids; cyclopropylsulfinic acid also
reacted well with phenylpropiolic acid, giving the corresponding
disulfonylethene 3ap in 64% yield.
Scheme 4. Control Experiments
We further explored the deesterification disulfonylation of
ethyl 3-phenylpropiolate 4 with benzenesulfinic acid 2a under
the optimal reaction conditions. However, poor conversion was
observed, and the disulfonylethene 3aa was obtained in only 17%
yield (Scheme 2). Additionally, cinnamic acid was inert under the
standard conditions.
Scheme 2. Deesterification Disulfonylation of Ethyl 3-
Phenylpropiolate
terminal alkyne and acetylenic sulfone are not involved in the
process of decarboxylative disulfonylation of alkynyl carboxylic
acids. Additionally, only trace amounts of disulfonylated product
were detected and the starting material phenylpropiolic acid was
recovered from the stoichiometric reaction in the absence of an
oxidant, thus implying that sulfinic acid is oxidized by
(NH4)2S2O8 to generate a sulfonyl radical and not Cu(ClO4)2·
6H2O.
To show the potential applications of this protocol, a gram-
scale reaction was carried out. As shown in Scheme 3, the
Scheme 3. Gram-Scale Synthesis
A possible mechanism for this transformation is proposed, as
shown in Scheme 5, on the basis of the above experimental
Scheme 5. Proposed Mechanism
reaction of 10.0 mmol of phenylpropiolic acid with benzene-
sulfinic acid under the standard reaction conditions gave 3.26 g of
the desired product in 85% yield. The result indicates that the
decarboxylative disulfonylation could be readily scaled up with
similar efficiency.
In order to elucidate the reaction mechanism, several control
experiments were carried out (Scheme 4). Initially, when 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO) was added to the
standard reaction conditions, inhibition of the reaction was
observed, and the disulfonylethene 3aa was obtained in only 16%
yield. This reaction was also found to be remarkably suppressed
by 1,1-diphenylethylene, and only trace amounts of 3aa were
detected. These two results suggest that the transformation
should proceed through a radical pathway. Furthermore, when
phenylpropiolic acid was replaced by (phenylethynyl)copper 5,
the desired product 3aa was observed in 32% yield in the absence
of Cu(ClO4)2·6H2O. Meanwhile, the reaction proceeded
smoothly and afforded product 3aa in 80% yield when
(phenylethynyl)copper was employed as the catalyst instead of
Cu(ClO4)2·6H2O. The reaction of benzenesulfinic acid with
ethynylbenzene or ((phenylethynyl)sulfonyl)benzene was also
performed under the standard conditions, and only trace
amounts of 3aa were detected. When diphenylacetylene was
employed in the reaction with benzenesulfinic acid, no reaction
was observed. These results indicate that alkynyl copper is
probably a key intermediate in this transformation, while
results and the precedent literature.9i,11c,16 Initially, sulfinic acid
2a is oxidized by ammonium persulfate to generate sulfonyl
radical I. Meanwhile, the decarboxylation of alkynyl carboxylic
acid with the assistance of a copper salt gives the alkynyl copper
species 5. Subsequently, addition of the sulfonyl radical to the
alkynyl copper species 5 leads to alkenyl radical II, which further
interacted with the second sulfonyl radical to afford intermediate
III. Finally, intermediate III was quenched by proton to yield the
desired disulfonylethene 3aa.
In conclusion, we have developed a novel and practical copper-
catalyzed decarboxylative disulfonylation of alkynyl carboxylic
acids with sulfinic acids in aqueous solution. This transformation
is characterized by its wide substrate scope, high stereoselectivity
C
DOI: 10.1021/acs.orglett.7b03922
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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for E isomers, mild reaction conditions, and utilization of readily
available reagents, thus providing an efficient and practical
approach to form (E)-1,2-disulfonylethenes. Preliminary mech-
anistic studies revealed that this reaction might involve a radical
process. Further mechanistic investigation and the synthetic
applications of this reaction are underway.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b03922.
Experimental procedures, characterization data of all new
compounds, and 1H and 13C NMR spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for Y.-M.L.: liym@kmust.edu.cn.
ORCID
Ya-Min Li: 0000-0003-4233-5274
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the National Natural Science
Foundation of China (Nos. 21662021 and 21402071) and the
Science and Technology Planning Project of Yunnan Province
(No. 2017FB016).
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