Disproportionate Coupling Reaction of Sodium Sulfinates Mediated by BF3·OEt2: An Approach to Symmetrical/Unsymmetrical Thiosulfonates

Article abstract of DOI:10.1021/acs.orglett.8b01808

The BF₃·OEt₂-mediated disproportionate coupling reaction of sodium sulfinates was found for the first time. In this reaction, various S-S(O)₂ bonds can be formed, efficiently giving thiosulfonates in moderate to excellent yields. As a convenient protocol for the synthesis of symmetrical and unsymmetrical thiosulfonates, its reaction mechanism involves the formation of a thiyl radical and sulfonyl radical via a sulfinyl radical disproportionation. What is more, this transformation can also be applied practically as a gram-scale reaction and to the two-step synthesis of sulfone and sulfonamide in one pot in situ using thiosulfonate as an intermediate.

Full text of DOI:10.1021/acs.orglett.8b01808

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Disproportionate Coupling Reaction of Sodium Sulfinates Mediated
by BF3·OEt2: An Approach to Symmetrical/Unsymmetrical
Thiosulfonates
Liang Cao,†,∥ Shi-He Luo,†,§,∥ Kai Jiang,† Zhi-Feng Hao,*,‡ Bo-Wen Wang,† Chu-Ming Pang,†
and Zhao-Yang Wang*,†,§
†School of Chemistry and Environment, South China Normal University, Key Laboratory of Theoretical Chemistry of Environment,
Ministry of Education, Guangzhou 510006, P. R. China
‡School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, P. R. China
§Key Laboratory of Functional Molecular Engineering of Guangdong Province, School of Chemistry and Chemical Engineering,
South China University of Technology, 381 Wushan Road, Guangzhou 510640, P. R. China
S
* Supporting Information
ABSTRACT: The BF3·OEt2-mediated disproportionate coupling reaction of sodium sulfinates was found for the first time. In
this reaction, various S−S(O)2 bonds can be formed, efficiently giving thiosulfonates in moderate to excellent yields. As a
convenient protocol for the synthesis of symmetrical and unsymmetrical thiosulfonates, its reaction mechanism involves the
formation of a thiyl radical and sulfonyl radical via a sulfinyl radical disproportionation. What is more, this transformation can
also be applied practically as a gram-scale reaction and to the two-step synthesis of sulfone and sulfonamide in one pot in situ
using thiosulfonate as an intermediate.
oxidation of thiols or disulfides is frequently employed for the
synthesis of symmetric thiosulfonates (Scheme 1a).4 In
contrast, the reductive coupling of sulfonyl chlorides or
hydrazides has also been reported (Scheme 1b).5 Recently,
the cross-coupling reaction of sulfonyl hydrazides with thiols
(or sodium sulfinates with disulfides) has been developed for
the preparation of unsymmetrical thiosulfonates with the help
of transitional metal catalysts (Scheme 1c).6 In short, for the
synthesis of symmetrical/unsymmetrical thiosulfonates in the
reported methods, some special additives (oxidants or
reductants), toxic reagents (e.g., RSH), or harsh conditions
are still demanded for these transformations. Hence, the
development of more efficient and environmentally friendly
metal-free reaction systems to achieve the thiosulfonates is still
highly essential.
rganosulfur compounds have attracted considerable
Oattention because of their interesting medicinal, material,
and agrochemical applications in recent years.1 Among them,
thiosulfonates are of particular interest, for they have shown a
broad spectrum of pharmaceutical and clinical properties such
as bactericidal, antimicrobial and fungicidal activities.2 In
addition, thiosulfonates have been widely used as reagents or
intermediates in laboratory and industrial organic chemistry
due to many advantages, including much better reactivity and
stability than commonly used sulfenyl halides and disulfides.3
Thus, considerable efforts have been made in the development
of synthetic protocols for thiosulfonates (Scheme 1). The
Scheme 1. Different Methods for the Synthesis of
Symmetrical/Unsymmetrical Thiosulfonates
Sodium sulfinates7 and sulfoxides8 are also important
organosulfur compounds. Meanwhile, 2(5H)-furanone chem-
istry has recently become a hot research topic.9 Being
interested in the reaction system of sodium sulfinates (I),
2(5H)-furanones (II), and sulfoxides (e.g., DMSO, III,
Scheme 2),10 we have already reported the construction of
C−C and C−S bonds by the metal-catalyzed reaction of 3,4-
dihalo-2(5H)-furanones with sodium arylsulfinates and sulf-
oxides, respectively. Some metal-free sulfonylations using
Received: June 11, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01808
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 2. Tunable Synthesis by the Combination of
Sodium Sulfinates, 2(5H)-Furanones, DMSO (Solvent), and
Catalyst
Table 1. Optimization of Reaction Conditions
b
entry
acid (x equiv)
temp (°C)
solvent
yield (%)
1
2
3
4
5
6
7
8
2a (1)
2b (1)
BF3·OEt2 (1)
HCl (1)
H2SO4 (1)
50
50
50
50
50
50
50
50
50
50
50
50
50
r.t.
50
acetone
acetone
acetone
acetone
acetone
acetone
C2H5OH
THF
CH2Cl2
CH3CN
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
46
46
51
0
0
0
<10
25
83
67
86
88
86
66
88
BF3·OEt2 (1)
BF3·OEt2 (1)
BF3·OEt2 (1)
BF3·OEt2 (1)
BF3·OEt2 (2)
BF3·OEt2 (3)
BF3·OEt2 (4)
BF3·OEt2 (3)
BF3·OEt2 (3)
9
10
11
12
13
14
sodium arylsulfinates as reagents have also been developed.
Herein, we report the construction of S−S(O2) bonds only
starting from sodium sulfinates (I) based on the systematical
investigations on the tunable reaction system of different
compounds (I, II, and III, Scheme 2).
Of course, this work is also a surprising discovery, much like
our previous reports.10b,c During the examination on the scope
of sulfonylation on the sodium arylsulfinates, when using
mucobromic acid as 2(5H)-furanone (II), we did not obtain
the expected C−S(O2) bond product but simply observed a
single byproduct. It is thiosulfonate 3a, which was confirmed
by single-crystal X-ray diffraction analysis (Scheme 2 or Figure
S1, see the Supporting Information (SI) for details).11 This
shows that the substrate mucobromic acid may act as an acid
and does not participate in the reaction, and the product
thiosulfonate 3a should be obtained by a disproportionate
coupling reaction (Scheme 3). Thus, this reaction may be
c
15
a
Reaction conditions: all reactions were performed with 1a (0.50
mmol), acid (x equiv) and solvent (3.0 mL) at the designed oil bath
b
c
temperature under air. Isolated yield. 5 h.
were subsequently screened (entries 7−10). To our delight,
using CH2Cl2 as solvent can give 83% yield (entry 9). We also
examined the amount of BF3·OEt2 (entries 9, 11−13). It can
be found that 3 equiv of BF3·OEt2 can give the best yield
(entry 12). Furthermore, there is no improvement when the
reaction temperature or time is altered (entries 14 and 15). In
short, we can achieve the disproportionate coupling reaction of
sodium sulfinate 1a to synthesize thiosulfonate 3a under mild
conditions (entry 12).
Under the optimized conditions, the disproportionate
coupling reaction of various sodium arylsulfinates was
examined to establish the scope and generality of this protocol.
The results are summarized in Scheme 4. As expected, many
sodium arylsulfinates 1 bearing electron-donating (e.g., alkyl
and methoxy) and electron-withdrawing (e.g., halogen and
trifluoromethyl) substituents can be transformed into the
corresponding symmetrical thiosulfonates 3a−i in moderate to
excellent yields. Generally, the sodium arylsulfinates 1
containing electron-donating groups perform better than
those with electron-withdrawing groups (3a vs 3i). This
reaction is insensitive to the substrates bearing meta-
substituent fluorine groups (3j vs 3f). For methyl-substituted
sodium arylsulfinates 1, though the number or position of
methyl is different, all work well under mild conditions, giving
the desired products in good to excellent yields (3k−m vs 3a).
Furthermore, polycyclic substituted sodium sulfinates 1 can be
transformed into the corresponding products in good yields
(3n and 3o). To our delight, heteroaryl sodium sulfinate may
be also suitable in the protocol with satisfactory results (3p).
To further improve the scope of this protocol, this
disproportionate coupling method was also applied to the
preparation of unsymmetrical thiosulfonates when two differ-
ent sodium sulfinates were subjected to the standard reaction
conditions. The results are shown in Scheme 5. To our delight,
the BF3·OEt2-mediated disproportionate cross-coupling reac-
tion of sodium alkylsulfinates and sodium arylsulfinates is
major, giving unsymmetrical thiosulfonate products in
moderate to good yields (4a−h). It is worth noting that
Scheme 3. Reaction of p-Toluenesulfinate 1a with
Mucobromic Acid 2a (or Mucochloric Acid 2b)
designed to synthesize thiosulfonates without any metal
catalysts and extra oxidants (or reductants). The systematical
investigation proved this indeed.
Initially, we attempted the disproportionate coupling
reaction of p-toluenesulfinate 1a as a model to examine
various reaction parameters. The results are summarized in
Table 1. Not only treating 1a with mucobromic acid 2a (1
equiv) in acetone as solvent at 50 °C for 3 h can give the
product 3a in 46% yield (entry 1), but also using mucochloric
acid 2b (Scheme 3) can obtain the desired product 3a with a
yield of 46% (entry 2). Afterward, we examined the type of
acid (entries 3−6). The results show that BF3·OEt2 can give a
better yield of 51% (entry 3). The control experiments show
that acid is essential for this transformation, and while weak
acid can significantly promote the reaction, strong acid (e.g.,
HCl and H2SO4) is not conducive to the reaction. Various
polar solvents, such as C2H5OH, THF, CH2Cl2, and CH3CN,
B
DOI: 10.1021/acs.orglett.8b01808
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Organic Letters
Letter
coupling reaction was reported to construct unsymmetrical
disulfide via a electrochemical pathway recently.13 On the
other hand, BF3·OEt2 is commonly used as Lewis acid with
high activity in many synthetic procedures.14 However, the
BF3·OEt2-mediated disproportionate coupling reaction of
sodium sulfinates for the synthesis of thiosulfonates has been
rarely reported. Thus, in order to understand the reaction
mechanism, a series of control experiments were carried out
(Scheme 6).
Scheme 4. Synthesis of Symmetrical Thiosulfonates from
Sodium Sulfinates 1
a,b
Scheme 6. Control Experiments
a
Reaction conditions: all reactions were performed with 1 (0.50
mmol), BF3·OEt2 (3 equiv), and CH2Cl2 (3.0 mL) at 50 °C for 3 h
b
under air unless otherwise noted. Yields of isolated products are
given.
Scheme 5. Synthesis of Unsymmetrical Thiosulfonates from
a,b
Sodium Sulfinates 1 and 2
Initially, we conducted the radical trapping experiments by
adding 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) into
the model reaction under the standard conditions. As expected,
the reaction is completely suppressed (eq 1 in Scheme 6),
while the capture of thiyl radical (IV) and sulfinyl radical (III)
by TEMPO can be detected by LC−MS. Similarly, the
formation of product 3a is inhibited upon the addition of the
radical inhibitor 2,6-di-tert-butyl-4-methylphenol (BHT) to the
reaction (eq 2), and the BHT adduct (BHT-IV) is also
detected by LC−MS (see Figure S2). Therefore, both cases
indicate that a radical pathway should be involved in this
process with the existence of thiyl and sulfinyl radical
intermediates. The control experiment also implies that BF3·
OEt2 is essential for this transformation (eq 3). In addition, it
can be found that an N2 atmosphere is markedly disadvanta-
geous for the reaction (eq 4) compared with the air
atmosphere. If I2 is added under the N2 atmosphere, a yield
of 46% can be obtained (eq 5). These results suggest that air
may play an important role in the transformation.
On the basis of the above experimental results and previous
reports,15−18 a possible reaction pathway is proposed, as shown
in Scheme 7 (for the unsymmetrical thiosulfonate 4, its
plausible mechanism based on Scheme 7 can be seen in the
SI). First, an oxygen-centered radical I is produced through the
oxidation of sodium sulfinate 1 by air upon heating, and radical
I can be resonance-stabilized with a sulfonyl radical II.15,16
Self-coupling of the radical II can generate the intermediate
disulfone B16 (some intermediates B, e.g., 5a and 5b, have
been isolated, and their characterization data, especially some
a
Reaction conditions: all reactions were performed with 1 (0.25
mmol), 1′ (0.25 mmol), BF3·OEt2 (3 equiv), and CH2Cl2 (3.0 mL) at
b
50 °C for 3 h under air unless otherwise noted. Yields of isolated
products are given.
sodium cyclopropanesulfinates are also a suitable reaction
partner for this novel transformation, and they can react with
different sodium arylsulfinates (4i−k). On the other hand, two
different sodium arylsulfinates also can be subjected to the
desired cross-coupling products in moderate yields (4l−n).
Though the homocoupling of sodium sulfinates to symmetrical
thiosulfonates 3 was unavoidable, their yields are very low, and
the maximum yield of homocoupling products was no more
than 20%. Thus, this method may provide a rare example for
the selective synthesis of unsymmetrical thiosulfonates 4 from
the same type of sodium sulfinates to form new S−S(O2)
bonds.
The disproportionate reaction is one of the most useful and
powerful methods for organic synthesis.12 Encouragingly,
starting from only thiols, a simple and efficient radical cross-
C
DOI: 10.1021/acs.orglett.8b01808
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
the first time. The simple and mild conditions make it a
convenient procedure for the synthesis of thiosulfonates in
good yields. This practical protocol has good tolerance to
different functional groups and can also be applied for the
preparation of unsymmetrical thiosulfonates. In addition, the
gram-scale protocol can be further developed as a one-pot,
two-step reaction avoiding the isolation and purification of the
intermediate thiosulfonate.
Scheme 7. Proposed Reaction Mechanism for the
Formation of Thiosulfonate 3
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01808.
Experimental section, characterization data of all
1
compounds, H, 13C, and 19F NMR and MS spectra
for all compounds, data for single-crystal X-ray analysis,
experimental spectra used in discussions, and discussions
on the reaction mechanism (PDF)
discussion on the verification experiments based on these
intermediates B can be seen in the SI). On the other hand,
BF3·H2O can be produced in situ from BF3·OEt2 in the case of
a trace amount of water17 under open-flask conditions (the
discussions on air as a dual role can be seen in the SI). Then
BF3·H2O generates sodium sulfinate 1, which is neutralized to
form intermediate sulfinic acid A. Subsequently, B may be
reduced by A to give the corresponding disulfoxide D and
sulfonic acid C.18 In the circle, intermediate C can oxidize
reactant 1 through free-radical intermediates to produce
intermediate B participating in the next reaction.16,18b Finally,
the homolytic cleavage of unstable intermediate D takes place
under heating, and the generated radical III may easily
disproportionate into radical II and radical IV, which can be
combined into the desired thiosulfonate.15
Accession Codes
CCDC 1849386 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: wangzy@scnu.edu.cn.
*E-mail: haozhifeng3377@163.com.
ORCID
To demonstrate the practicability of this protocol, a scaled-
up experiment was performed with p-toluenesulfinate 1a. As
shown in Scheme 8, the disproportionate coupling reaction of
Zhao-Yang Wang: 0000-0002-2391-5938
Author Contributions
∥L.C. and S.-H.L. contributed equally to this work.
Notes
Scheme 8. Practical Application Experiments
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the Guangdong Natural Science
Foundation (No. 2014A030313429), the Guangzhou Science
and Technology Project Scientific Special (General Items, No.
201607010251), Applied Science and Technology Research,
Development Special Foundation of Guangdong Province
(No. 2016B090930004), the Open Fund of the Key
Laboratory of Functional Molecular Engineering of Guang-
dong Province in SCUT (2017kf01), and Guangdong
Provincial Science and Technology Project (No.
2017A010103016) is greatly appreciated.
1a on gram-scale can give thiosulfonate 3a in a yield 83% (eq
1). More importantly, the versatile synthetic utility of the
obtained thiosulfonate as intermediate3,19 in situ was also
investigated. For example, once p-toluenesulfinate 1a in
dichloromethane is treated with BF3·OEt2 for 3 h, only simple
removal of the solvent gives a residue, and then the addition of
benzyl bromide can afford the corresponding sulfone product
6a in a yield of 73% (eq 2). Similarly, a one-pot, two-step
synthesis of sulfonamide 6b can also be achieved (eq 3), and
the remaining halogen may be further derivatized.
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D
DOI: 10.1021/acs.orglett.8b01808
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DOI: 10.1021/acs.orglett.8b01808
Org. Lett. XXXX, XXX, XXX−XXX