Metal-Free Synthesis of (E)-Vinyl Sulfones via An Insertion of Sulfur Dioxide/1,5-Hydrogen Atom Transfer Sequence

Article abstract of DOI:10.1002/adsc.202000778

A metal-free three-component reaction of propargyl alcohols, sodium metabisulfite, and aryldiazonium tetrafluoroborates under mild conditions is described. The reaction proceeds efficiently at room temperature without the need of any catalysts, affording the corresponding (E)-vinyl sulfones in moderate to good yields with excellent functional group tolerance. Preliminary mechanistic studies show that a vinyl radical-induced 1,5-hydrogen atom transfer process and functional group migration are involved, resulting in the cleavage of inert C?H and C?C bonds in sequence. (Figure presented.).

Full text of DOI:10.1002/adsc.202000778

Title: Metal-Free Synthesis of (E)-Vinyl Sulfones via An Insertion of
Sulfur Dioxide/1,5-Hydrogen Atom Transfer Sequence
Authors: Fu-Sheng He, Yanfang Yao, Wenlin Xie, and Jie Wu
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.202000778
Link to VoR: https://doi.org/10.1002/adsc.202000778

10.1002/adsc.202000778
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.202((will be filled in by the editorial staff))
Metal-Free Synthesis of (E)-Vinyl Sulfones via An Insertion of
Sulfur Dioxide/1,5-Hydrogen Atom Transfer Sequence
Fu-Sheng He,a Yanfang Yao,b Wenlin Xie,b and Jie Wu*a,c
a
School of Pharmaceutical and Materials Engineering & Institute for Advanced Studies, Taizhou University, 1139 Shifu
Avenue, Taizhou 318000, China.
E-mail: jie_wu@fudan.edu.cn
School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201,
b
China.
c
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 345 Lingling Road, Shanghai 200032, China.
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract.
A metal-free three-component reaction of
propargyl alcohols, sodium metabisulfite, and aryldiazonium
tetrafluoroborates under mild conditions is described. The
reaction proceeds efficiently at room temperature without the
need of any catalysts, affording the corresponding (E)-vinyl
sulfones in moderate to good yields with excellent functional
group tolerance. Preliminary mechanistic studies show that a
vinyl radical-induced 1,5-hydrogen atom transfer process
and functional group migration are involved, resulting in the
cleavage of inert C-H and C-C bonds in sequence.
Keywords: vinyl sulfones; sulfur dioxide insertion; vinyl
radical; 1,5-hydrogen atom transfer; metal-free
by using the readily available DABCO·(SO2)2 (1,4-
Introduction
diazabicyclo[2.2.2]octane-sulfur
dioxide)
or
potassium/sodium metabisulfite as the sulfur dioxide
surrogates, significant progress has been witnessed in
this area. The synthesis of vinyl sulfones from sulfur
dioxide was developed as well. For instance, we
demonstrated the photoinduced synthesis of vinyl
sulfones through a three-component reaction of
potassium alkyltrifluoroborates, sulfur dioxide, and
alkynes in the presence of copper(II) triflate.[8d] We
also disclosed an approach for the synthesis of (E)-2-
methyl vinyl sulfones via a FeCl3-mediated C-H
methyl sulfonylation of alkenes.[8h] However, these
two sulfonylation protocols suffered from
organometallic reagents or high reaction temperature,
which limited their further applications. Additionally,
only aryl-substituted alkynes or alkenes were suitable
for the above transformations, the alkyl-substituted
alkynes could not participate in the reaction to
produce the corresponding vinyl sulfones. Therefore,
the development of efficient synthetic methodologies
to furnish vinyl sulfones under mild conditions is
highly desirable.
Vinyl sulfones are widely found in many natural
products and pharmaceuticals.[1] For example, vinyl
sulfones have been discovered as a novel class of
neuroprotective agents toward Parkinson’s disease
therapy.[2] Furthermore, they can also serve as
versatile building blocks in organic synthesis.[3] The
high reactivity of the α,β-unsaturated sulfones leads
to various transformations, such as nucleophilic
addition, radical addition and cycloaddition.
Therefore, the establishment of efficient methods for
the synthesis of vinyl sulfones has attracted much
interest from organic chemists. Traditionally, the
generation of vinyl sulfones was accomplished
through Knoevenagel condensation,[4] Wittig
reaction,[5] or oxidation of sulfides,[6] which often
required pre-prepared substrates and harsh reaction
conditions.
Over the past few years, the preparation of sulfonyl
compounds through the strategy of sulfur dioxide
insertion has received considerable attention in the
synthetic community.[7] We and others have
demonstrated that the in situ generated carbon-
centred radical could react with sulfur dioxide, thus
giving rise to sulfonyl radical intermediate for the
generation of sulfonyl compounds.[8,9,10] As a result,
Recently, the radical migration strategy offers a
new access to the selective and mild C-H bond
functionalizations.[11] Thus, the rapid development of
sustainable remote radical migration induced C-H
bond functionalization has been witnessed. Compared
1
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10.1002/adsc.202000778
Advanced Synthesis & Catalysis
to the well-developed oxygen or nitrogen radical-
mediated 1,5-hydrogen atom transfer (HAT) process,
vinyl radical-induced C(sp3)-H functionalization has
been less studied.[12] For example, Liu and co-
workers demonstrated the first example of using in
situ generated vinyl radicals as the key intermediate
in cascade radical cyclization of alkynyl ketones to
construct carbocyclic and heterocyclic 6(5)-6-5 fused
ring systems.[13a] Remarkably, the vinyl radical-
mediated remote vinylation of the unactivated C(sp3)-
H bond via 1,5-HAT process and intramolecular
olefin migration was achieved by Zhu and co-
workers.[13b-d] Additionally, Xie, Zhu and co-workers
effect of solvent on this reaction was then
investigated, but no better result was obtained (Table
1, entries 3-8). Subsequently, the amount of 4-
methylphenyldiazonium tetrafluoroborate 2a was
examined. The use of 1.2 equivalents of compound
2a led to a decreased yield (Table 1, entry 9), while
an improved yield was obtained when 2.0 equivalents
of compound 2a was employed in this reaction (Table
1, entry 10). Gratifyingly, the use of K2CO3, NaHCO3,
K2HPO4 and Na2HPO4 could facilitate the reaction,
and Na2HPO4 was found to be the best choice,
affording the corresponding product 3a in 74% yield
(Table 1, entries 11-14). Finally, the reaction had
little influence on the efficiency either by increasing
or reducing the amount of Na2HPO4 (Table 1, entries
15-16).
developed
a
visible-light-mediated
hydrotrifluoromethylation of aromatic alkynes and
remote C(sp3)-H benzoyloxylation using Togni
reagent as a bifunctional reagent enabled by vinyl
radical-induced 1,5-HAT strategy.[13e] Although Zhu
Table 1. Initial studies for metal-free sulfonylation of
propargyl alcohol 1a with the insertion of sulfur
dioxide [a]
and co-workers reported
a
photocatalyzed
sulfonylation of propargyl alcohols and sulfonyl
chlorides to deliver vinylsulfones, the use of metal
photocatalyst and sulfonyl chlorides in their
conditions did involve some inherent limitations.
Prompted by the recent advances of vinyl radical-
mediated remote C(sp3)-H bond functionalization and
the achievements of sulfur dioxide chemistry, we
envisioned that the insertion of sulfur dioxide into
propargyl alcohols might be feasible for the synthesis
of vinyl sulfones under metal-free conditions. Herein,
we report a novel sulfonylation reaction of propargyl
alcohols using sodium metabisulfite as the sulfur
dioxide surrogate at room temperature without the
need of any catalysts. This metal-free route to vinyl
sulfone proceeds smoothly and features good
functional group compatibility, affording the (E)-
vinyl sulfones in moderate to good yields. In this
report, (1) the cheap and easily available sodium
metabisulfite is used as the source of sulfur dioxide;
(2) generation of vinylsulfones is accomplished under
metal-free reaction; (3) for the first time, alkyl-
substituted alkynes can be participated in radical
reactions with the insertion of sulfur dioxide enabled
by the 1,5-hydrogen atom transfer process under mild
conditions.
Yield[b]
(%)
41
51
25
50
trace
44
Entry
“SO2”
Additive (eq)
Solvent
1
2
3
4
5
6
K2S2O5
Na2S2O5
Na2S2O5
Na2S2O5
Na2S2O5
Na2S2O5
DCE
DCE
CH3CN
CH2Cl2
DMF
toluene
1,4-
dioxane
EtOAc
DCE
DCE
DCE
DCE
DCE
7
Na2S2O5
trace
8
Na2S2O5
Na2S2O5
12
42
55
71
61
68
74
67
73
9[c]
10[d] Na2S2O5
11[d] Na2S2O5
K2CO3 (1.5)
12[d] Na2S2O5 NaHCO3 (1.5)
13[d] Na2S2O5 K2HPO4 (1.5)
14[d] Na2S2O5 Na2HPO4 (1.5)
15[d] Na2S2O5 Na2HPO4 (1.0)
16[d] Na2S2O5 Na2HPO4 (2.0)
DCE
DCE
DCE
[a]
Reaction conditions: 1a (0.2 mmol), “SO2” (0.4 mmol),
2a (0.3 mmol), solvent (2.0 mL), N2, room temperature, 24
Results and Discussion
[b]
[c]
h.
Isolated yield based on propargyl alcohol 1a.
2a
Initially, we selected propargyl alcohol 1a, potassium
(0.24 mmol). [d] 2a (0.4 mmol).
metabisulfite,
and
4-methylphenyldiazonium
tetrafluoroborate 2a as the model substrates for
screening the reaction parameters. To our delight, the
reaction proceeded smoothly in DCE at room
temperature in the absence of any catalysts or
additives, providing the desired product 3a in 41%
yield (Table 1, entry 1). The structure of compound
3a was confirmed by the X-ray diffraction analysis,
showing excellent stereoselectivity in the outcome.[15]
Interestingly, the yield of product 3a was increased to
51% by using sodium metabisulfite as the source of
sulfur dioxide instead of potassium metabisulfite
(Table 1, entry 2). Encouraged by this result, the
Having identified the optimal conditions, we
further evaluated the reaction scope of this three-
component reaction, and the results are summarized
in Table 2. At the beginning, a variety of
aryldiazonium tetrafluoroborates 2 were examined in
the reaction of propargyl alcohol 1a and sodium
metabisulfite. To our delight, aryldiazonium
tetrafluoroborates 2 bearing electron-donating or
electron-withdrawing groups on the aromatic ring
worked well in this transformation, providing the
2
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10.1002/adsc.202000778
Advanced Synthesis & Catalysis
desired products 3 in moderate to good yields. For
instance, the trifluoromethyl-substituted product 3k
was generated in 76% yield, while the ester-
containing product 3l was obtained in 58% yield.
Additionally, the naphthyl-substituted product 3n was
produced in 37% yield. Subsequently, the substrate
scope of propargyl alcohols 1 was explored. As
expected, reactions of a wide range of propargyl
alcohols 1 worked smoothly to provide the
corresponding products 3o-3y in moderate to good
yields. Functional groups including methoxy and
halogen were well compatible under the standard
conditions. Notably, the vinylation of heteroaryl-
substituted propargyl alcohol was also feasible,
giving rise to the desired (E)-vinyl sulfone 3o in 77%
yield. In addition to the tertiary C(sp3)-H bonds, other
secondary aliphatic C(sp3)-H bonds could be
functionalized as well, leading to the desired products
3w and 3x in 63% and 56% yields, respectively.
Moreover, the regioselective sulfonylvinylation of
cyclic C(sp3)-H bond was achieved, and the
corresponding product 3y was produced in 69% yield.
The less-reactive internal propargyl alcohol was also
suitable for this metal-free process, furnishing the
trisubstituted alkene 3z in 21% yield.
methylphenyldiazonium tetrafluoroborate 2a (4
mmol) provided the corresponding product 3q (0.55
g) in 74% yield.
Scheme 1. Large-scale sulfonylation reaction.
To get more insights into the mechanism, several
control experiments were carried out (Scheme 2).
Since a radical process might be involved in this
metal-free reaction as hypothesized, 2 equivalents of
2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as
radical scavenger was introduced into the model
reaction in Table 1 under the standard conditions. As
expected, the reaction was hampered, and the
formation of desired product 3a was not detected.
Subsequently, when
2
equivalents of 1,1-
diphenylethylene was added to the reaction, product
3a was obtained in 16% yield and (2-tosylethene-1,1-
diyl)dibenzene 4 was isolated in 55% yield.
Furthermore, the reaction of 1,1-diphenylethylene,
sodium metabisulfite, and 4-methylphenyldiazonium
tetrafluoroborate 2a gave rise to the radical trapped
product 4 in 36% yield. These observations
confirmed that 4-methylphenylsulfonyl radical
Table 2. Scope exploration for the metal-free
sulfonylation of propargyl alcohols with the insertion
of sulfur dioxide [a,b]
generated
from
4-methylphenyldiazonium
tetrafluoroborate 2a and sodium metabisulfite was
involved in the reaction. We also investigated the
reaction of TBS-protected propargyl alcohol 5,
[13b]
sodium metabisulfite, and 4-methylphenyldiazonium
tetrafluoroborate 2a. As a result, the vinyl-migrated
product 3a was obtained in 28% yield, which
suggested that the alkenyl radical was generated for
promoting the 1,5-HAT process, rather than the
alkoxy radical from the hydroxyl group. The
deuterium-labelling experiment (H/D≈6) revealed
that the 1,5-HAT step was selectivity-determining,
which the alkenyl radical preferred to absorb H atom
rather than D.
[a] Reaction conditions: 1 (0.2 mmol), Na2S2O5 (0.4 mmol),
2 (0.4 mmol), DCE (2.0 mL), N2, room temperature, 24 h..
[b] Isolated yields based on propargyl alcohols 1.
The synthetic utility of the present protocol was
further demonstrated by conducting the metal-free
three-component reaction on a preparative scale. As
shown in Scheme 1, the reaction of propargyl alcohol
1b (2 mmol), sodium metabisulfite (4 mmol), and 4-
3
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10.1002/adsc.202000778
Advanced Synthesis & Catalysis
In conclusion, we have developed a metal-free three-
component reaction of propargyl alcohols, sodium
metabisulfite, and aryldiazonium tetrafluoroborates
under mild conditions. This transformation proceeds
smoothly at room temperature without the need of
any catalysts, affording the corresponding (E)-vinyl
sulfones in moderate to good yields with excellent
functional group tolerance. Preliminary mechanistic
studies suggest
a
plausible reaction pathway
including the in situ generation of arylsulfonyl radical,
addition of the radical to the alkyne, 1,5-hydrogen
atom transfer, functional group migration, single
electron transfer, and deprotonation.
Experimental Section
General experimental procedure for the reaction of
propargyl alcohols 1, Na2S2O5, and aryldiazonium
tetrafluoroborates 2.
Scheme 2. Control experiments.
Propargyl alcohols 1 (0.2 mmol) was added to a mixture of
Na2S2O5 (0.4 mmol), aryldiazonium tetrafluoroborates 2
(0.3 mmol) and Na2HPO4 (0.3 mmol) in DCE (2.0 mL)
under N2 atmosphere. The mixture was stirred at room
temperature for 24 h. After completion of reaction as
indicated by TLC, the solvent was evaporated and the
residue was purified directly by flash column
chromatography (n-hexane/ethyl acetate = 8:1) to give the
corresponding products 3.
On the basis of the above results, a plausible
mechanism for the formation of vinyl sulfones is
shown in Scheme 3. According to our previous
reports, arylsulfonyl radical A could be generated in
situ through the combination of aryldiazonium
tetrafluoroborate 2 and sulfur dioxide. Subsequently,
the addition of arylsulfonyl radical A to propargyl
alcohol 1 would give rise to the reactive vinyl radical
species B, followed by 1,5-HAT to deliver the alkyl
radical intermediate C.[13] Then, the radical C would
convert into radical E via the cyclopentanol
intermediate D,[14] which involved an intramolecular
vinyl migration process. Finally, the alkyl radical E
could be oxidized by the excess amount of compound
2 to provide the carbocation F, which would undergo
deprotonation to afford the corresponding vinyl
sulfone 3.
Acknowledgements
Financial support from the National Natural Science Foundation
of China (Nos. 21871053 and 21532001) and the Leading
Innovative and Entrepreneur Team Introduction Program of
Zhejiang (No. 2019R01005) is gratefully acknowledged.
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Advanced Synthesis & Catalysis
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10.1002/adsc.202000778
Advanced Synthesis & Catalysis
FULL PAPER
Metal-Free Synthesis of (E)-Vinyl Sulfones via An
Insertion of Sulfur Dioxide/1,5-Hydrogen Atom
Transfer Sequence
Adv. Synth. Catal. Year, Volume, Page – Page
Fu-Sheng He,a Yanfang Yao,b Wenlin Xie,b and Jie
Wu*a,c
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