Electrooxidative Tandem Cyclization of Activated Alkynes with Sulfinic Acids to Access Sulfonated Indenones

Article abstract of DOI:10.1021/acs.orglett.7b01256

An electrooxidative direct arylsulfonlylation of ynones with sulfinic acids via a radical tandem cyclization strategy has been developed for the construction of sulfonated indenones under oxidant-free conditions. This method provides a simple and efficient approach to prepare various sulfonylindenones in good to excellent yields, demonstrating the tremendous prospect of utilizing electrocatalysis in oxidative coupling. Notably, this reaction could be easily scaled up with good efficiency.

Full text of DOI:10.1021/acs.orglett.7b01256

Letter
pubs.acs.org/OrgLett
Electrooxidative Tandem Cyclization of Activated Alkynes with
Sulfinic Acids To Access Sulfonated Indenones
Jiangwei Wen,†,‡ Wenyan Shi,† Fan Zhang,† Dong Liu,† Shan Tang,† Huamin Wang,† Xiao-Min Lin,‡
and Aiwen Lei*,†
†The Institute for Advanced Studies (IAS), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, Hubei,
China
‡Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States
S
* Supporting Information
ABSTRACT: An electrooxidative direct arylsulfonlylation of ynones
with sulfinic acids via a radical tandem cyclization strategy has been
developed for the construction of sulfonated indenones under oxidant-
free conditions. This method provides a simple and efficient approach
to prepare various sulfonylindenones in good to excellent yields,
demonstrating the tremendous prospect of utilizing electrocatalysis in
oxidative coupling. Notably, this reaction could be easily scaled up with good efficiency.
uring the past decades, oxidative tandem chemistry has
Dushered in a new epoch in the evolution of chemistry, and
it has become one of the most powerful and efficient strategies in
organic synthesis.1 Among the diverse oxidative tandem-
mediated synthetic methodologies, strategies utilizing free-
radical tandem reactions have proven to be highly valuable for
the synthesis of complex carbo- and heteropolycyclic skeletons
by avoiding the prefunctionalization process.2 Despite the
promising progress that has been made in recent years, most of
the traditional free-radical cascade reactions usually need
transition-metal catalysts3 or stoichiometric oxidants.4 There-
fore, it is still a challenging but attractive task to develop simple,
efficient, and oxidant-free reactions to afford a variety of
important and structurally diverse compounds.
Indenones, widely existing in many natural products and
biologically active compounds, are among the most important
classes of carbocycles that have unique biological activity.5
Functionalized indenones have attracted considerable synthetic
interest because of their important applications in asymmetric
synthesis, library design, and drug discovery.6 A variety of
synthetic methods have been developed for the construction of
these carbocycles.7 Recently, oxidative radical tandem cyclization
of activated alkynes under transition-metal catalysis or metal-free
conditions provided a good alternative, and different functional
groups such as phosphoryl, cyclohexyl, and thioalcohol groups
have been successfully incorporated into the indenone frame-
work. Even so, it is still urgent to develop a transition-metal-free
and oxidant-free radical tandem cyclization strategy to construct
structurally diverse functional indenones.
via oxidative radical tandem cyclization have been demonstrated
to be efficient methods to synthesize thiolated indenones.10
However, these reported methods normally suffer from some
limitations, such as harsh conditions or the need to use
stoichiometric amounts of oxidants to generate sulfonyl radical.
As a result, the development of a practical and general protocol to
synthesize diverse multisubstituted indenones is still strongly
desired.
In this regard, electrocatalysis provides an ideal alternative
method.11 In particular, electrocatalytic oxidative C−H
functionalization proved to be an efficient alternative method
for classic oxidative coupling methods.12 However, there is no
precedent for the anodic oxidative tandem cyclization of alkynes.
As part of our continuing interest in the synthesis of sulfone-
containing organic compounds,13 herein we report a new and
efficient oxidant-free direct arylsulfonlylation of ynones with
sulfinic acids toward sulfonated indenones in a simple undivided
cell under constant current conditions. To the best of our
knowledge, this is the first example of constructing sulfonated
indenones via electrooxidative tandem cyclization of alkynes.
Initially, 1,3-diphenylprop-2-yn-1-one 1a and benzenesulfinic
acid 2a were chosen as the model substrates to optimize the
reaction conditions (Table S1). By optimizing key reaction
parameters, we obtained the best results by performing the
reaction under a constant current electrolysis at 10 mA cm−2 in
an undivided cell using tetrabutylammonium iodide (TBAI) as
redox catalyst, LiClO4 as electrolytes, and two platinum plates as
Sulfone groups represent one of the most important organic
functionalities and are widely found in a number of biologically
active compounds, drug molecules, and versatile synthetic
intermediates.7d,8 Therefore, the incorporation of sulfone groups
into organic molecules is an important task.4c,9 Recently,
reactions of sulfur-containing substrates with activated alkynes
Received: April 25, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b01256
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Letter
the working electrode and cathode in CH3CN/DCE at room
temperature (Table S1, entry 1). Both increasing and decreasing
the constant current would lead to decreased reaction yields
(Table S1, entries 2 and 3). Subsequently, replacing TBAI with
NH4I or tetrabutylammonium bromide (TBAB) diminished the
reaction yield significantly or suppressed it entirely (Table S1,
entries 4 and 5).The variation of the mixed solvent also led to
significantly lower yields (Table S1, entries 6 and 7). Addition-
ally, inferior results were observed when the TBAI equivalent was
decreased or increased (Table S1, entries 8−10). As for the
choice of electrolyte, ammonium salts decreased the reaction
efficiency compared with lithium perchlorate (Table S1, entries
11−13). Moreover, the effect of the electrode material was also
explored. Lower reaction yields were obtained when the
platinum anode or cathode was replaced by carbon (Table S1,
entries 14 and 15). Control experiments also showed that no
desired product was generated without electricity (Table S1,
entry 16).
interesting discovery is that electron-donating or electron-
withdrawing groups on the 3-positions of aryl rings were suitable
for this protocol, and the corresponding products were obtained
in moderate to good yields (3oa−ra). Unfortunately, lack of
access to the desired products may be affected by the conjugation
effect when the 3-alkyl-substituted phenylprop-2-yn-1-ones as
substrates react with 2a.
In the next step, the tolerance of the sulfinic acid moiety was
also studied (Scheme 2). Both the electron-rich and electron-rich
Scheme 2. Results for Electrocatalytic Arylsulfonylation of
a
Ynones with Sulfinic Acids 2
With the optimized conditions described above, the scope and
limitations of the reaction of various ynones with sulfinic acids
were investigated, and the results are shown in Scheme 1. First,
Scheme 1. Results for Electrocatalytic Arylsulfonylation of
a,b
Ynones with Benzenesulfinic acids 2a
a
Standard conditions: Pt anode, Pt cathode, constant current = 10 mA,
1a (0.25 mmol), 2 (0.75 mmol), LiClO4 (3 mmol), TBAI (0.8 equiv),
b
c
MeCN/DCE (10.0 mL, 9/1), 25 °C, 2 h. Isolated yields. Sodium
methylsulfite, H2SO4 (0.75 mmol), 60 °C, 3 h.
and electron-deficient sulfinic acids were all tolerated in this
reaction, thereby facilitating possible further modifications
(3ab−ah). Moreover, other aromatic sufinic acids such as
naphthalene- or thiophene-2-sulfinic acid can also generate the
desired products in good yields (3ai−aj).
Notably, a product with relatively high yield was also obtained
when the alkanesulfinates, such as sodium methylsulfinates, were
used as the substrates (3ak). Moreover, the faradaic efficiency is
also selectively calculated in 60 5% yield, respectively (Figure
S1).
To further show the utility of this strategy, we attempted to
apply benzenesulfonyl hydrazide as a coupling partner to react
with 1a. To our delight, the desired product can be obtained in
66% yield when the current is increased to 20 mA (Scheme 3a).
a
Standard conditions: Pt anode, Pt cathode, constant current = 10 mA,
1 (0.25 mmol), 2a (0.75 mmol), LiClO4 (3 mmol), TBAI (0.8 equiv),
MeCN/DCE (10.0 mL, 9/1), 25 °C, 3 h. Isolated yields.
b
ynones with different substituents on the 1-positions of the aryl
groups were applied as substrates to react with benzenesulfinic
acid 2a. As expected, substrates with electron-donating groups at
the para position on the 1-positions of aryl rings were suitable for
this protocol, and the corresponding products were obtained in
good yields (3aa−ea). Halide substituents such as F, Cl, and Br
were well tolerated under electrocatalytic conditions, furnishing
the sulfonated indenones in high yields (3fa−ha). Furthermore,
product 3ia with a strong electron-withdrawing trifluoromethyl
group was isolated in 48% yield, which might be due to electronic
effects. Subsequently, the substituent group at the meta position
of the phenyl ring afforded a single product in moderate yield
(3ja−ka). o-Methyl ynones gave a slightly decreased reaction
yield, which might be due to the steric effect of the methyl group
(3la). Other aromatic ynones were also applied as substrates in
this transformation. Both 1m and 1n furnished the desired
product in good yields (3ma−na). Subsequently, the functional
group tolerance of the 3-positions of the aryl groups were applied
as substrates to react with the benzenesulfinic acid 2a. An
Scheme 3. Substrate Replacement Experiments
Interestingly, the desired product also could be obtained in 50%
yield when the chalcone was used as the substrate to react with
benzenesulfinic acid 2a under the electrooxidation conditions
(Scheme 3b). Moreover, the gram-scale reaction between 1-(4-
fluorophenyl)-3-phenylprop-2-yn-1-one and benzenesulfinic
acid 2a afforded the desired product in 51% yield (Scheme 4),
further highlighting the great potential application of this
transformation.
B
DOI: 10.1021/acs.orglett.7b01256
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Letter
Scheme 4. Gram-Scale Synthesis
Scheme 6. Postulated Reaction Pathway
To further understand the role of iodine in this oxidative
coupling/annulation process, we applied different stoichiometric
amounts of iodine sources in the model reaction. To our surprise,
an iodine radical source, N-iodosuccinimide (NIS), showed
reactivity similar to that of TBAI (Scheme 5a,b).
Scheme 5. Control Experiment
obtained in good yields from readily available starting materials
with excellent functional group tolerance. Importantly, this
reaction can be scaled up in good reaction efficiency, which is
beneficial for practical applications.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b01256.
Experimental procedures, optimization of reaction con-
ditions, characterization data, and NMR spectra of the
products (PDF)
At the same time, no desired product could be obtained by
using a stoichiometric amount of I2 as the oxidant in the absence
of current. These results indicated that the I+1 species were likely
to be involved in the reaction process. Subsequently, radical-
trapping experiments were carried out to confirm whether this
reaction went through a radical reaction pathway. When a
commonly used radical-trapping reagent, TEMPO or BHT, was
added into the reaction system, no desired product could be
observed even when the reaction time was prolonged to 6 h for
the oxidative coupling/annulation of 1a with 2a (Scheme 5c). All
of the results described above indicate that the reaction probably
involves a radical process. At last, the reaction of 1a with 2b under
the standard conditions without TBAI and current also could
give 3ab in 69% yield (Scheme 5d), which indicates that sulfonyl
iodine ought to be the reaction intermediate.
On the basis of the experimental results and previous
reports,14 a plausible reaction pathway is illustrated in Scheme
6. The reaction sequence begins with the anodic oxidation of the
iodide ion to form I+1 species and its subsequent reaction with
sulfinic acid 2a to afford sulfonyl radical 4 and iodine radical.
Subsequently, the addition of sulfonyl radical 4 to 1,3-
diphenylprop-2-yn-1-one 1a generates radical 6. Intramolecular
cyclization of intermediate 6 gives the radical intermediate 7.
Finally, the oxidation of 7 generated the corresponding product
3.
AUTHOR INFORMATION
■
Corresponding Author
* E-mail: aiwenlei@whu.edu.cn.
ORCID
Aiwen Lei: 0000-0001-8417-3061
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (21390402, 21520102003), the Ministry of
Science and Technology of China (2012YQ120060), the
Fundamental Research Funds for the Central Universities, and
the Program of Introducing Talents of Discipline to Universities
of China (111 Program). Use of the Center for Nanoscale
Materials, an Office of Science user facility, was supported by the
U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences, under Contract No. DE-AC02-06CH11357.
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