Photoinduced intramolecular carbosulfonylation of alkynes: access to sulfone-containing dibenzazepines from sulfur dioxide

Article abstract of DOI:10.1039/d0cc07927h

A photoinduced three-component reaction ofN-benzyl-N-(2-ethynylaryl)amides, sulfur dioxide and aryldiazonium tetrafluoroborates is developed, providing an efficient and straightforward approach for the synthesis of diverse vinylsulfonylated dibenzazepine derivatives in moderate to good yields under mild conditions. This transformation proceeds smoothly at room temperature in the presence of visible light, which shows a wide range of substrate scope with good functional group compatibility. The synthetic utility of this methodology is further demonstrated through Suzuki coupling. Mechanistic studies show that this reaction is triggered by the addition of an arylsulfonyl radical to an alkyne from sulfur dioxide, followed by vinyl radical cyclization to afford the desired sulfonated dibenzazepines.

Full text of DOI:10.1039/d0cc07927h

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Photoinduced intramolecular carbosulfonylation
of alkynes: access to sulfone-containing
dibenzazepines from sulfur dioxide†
Cite this: Chem. Commun., 2021,
7, 2883
5
Received 6th December 2020,
Accepted 8th February 2021
ab
a
a
a
b
Yanfang Yao, Ziqing Yin, Fu-Sheng He,* Xuwei Qin, Wenlin Xie* and
acd
Jie Wu
*
DOI: 10.1039/d0cc07927h
rsc.li/chemcomm
A photoinduced three-component reaction of N-benzyl-N-(2-ethynyl-
The importance of sulfonyl compounds has been widely
4
aryl)amides, sulfur dioxide and aryldiazonium tetrafluoroborates is witnessed in organic chemistry and pharmaceutical chemistry.
developed, providing an efficient and straightforward approach for In particular, vinyl sulfones often serve as versatile synthons in
the synthesis of diverse vinylsulfonylated dibenzazepine derivatives organic synthesis, while they can also be used as neuroprotective
5
6
in moderate to good yields under mild conditions. This transforma- agents for Parkinson’s disease therapy. Accordingly, great efforts
tion proceeds smoothly at room temperature in the presence of have been devoted to developing efficient approaches toward the
visible light, which shows a wide range of substrate scope with good synthesis of vinyl sulfones. Among them, vicinal difunctionaliza-
functional group compatibility. The synthetic utility of this methodology tion of alkynes has been demonstrated as an efficient and
is further demonstrated through Suzuki coupling. Mechanistic studies convenient strategy for the generation of multi-substituted
show that this reaction is triggered by the addition of an arylsulfonyl alkenes in recent years, in which two functional groups can be
7
radical to an alkyne from sulfur dioxide, followed by vinyl radical incorporated into alkyne moieties simultaneously. For example,
cyclization to afford the desired sulfonated dibenzazepines.
by employing sulfonyl chlorides and arylboronic acids as reaction
components, Nevado and co-workers reported an elegant nickel-
Owing to their widespread application in the synthesis of catalyzed three-component intermolecular carbosulfonylation of
natural products and bioactive compounds, seven-membered alkynes for the construction of b,b-disubstituted vinyl sulfones
8
heterocycles and derivatives have attracted much attention (Scheme 1a). Additionally, sulfonyl radical-mediated synthesis of
1
from both the synthetic and medicinal community. Among five- or six-membered ring compounds through sulfonylation of
them, the scaffold of dibenzazepine often serves as a privileged alkynes with the simultaneous formation of C–C bonds has been
9
structural motif in biologically active molecules and drugs, widely explored. Despite these achievements, to the best of our
2
such as Mianserin, Mirtazapine and Epinastine. However, knowledge, there is no example of alkyne carbosulfonylation
traditional methods for the synthesis of the dibenzazepine involving intramolecular C–H functionalization to construct
3
skeleton often require harsh conditions or tedious steps.
Therefore, the development of efficient protocols to access
functionalized dibenzazepines is highly desirable.
sulfone-containing seven-membered ring compounds.
It is known that DABCO (SO and potassium/sodium
metabisulfite are efficient sulfur dioxide surrogates, which have
2 2
)
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, 0618006@zju.edu.cn
b
School of Chemistry and Chemical Engineering, Hunan University of Science and
Technology, Xiangtan 411201, China. E-mail: xwl2000zsu@163.com
c
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032,
China
d
School of Chemistry and Chemical Engineering, Henan Normal University,
Xinxiang 453007, China
†
Electronic supplementary information (ESI) available: Experimental details and
1
13
spectral data, and copies of H and C NMR spectra. CCDC 2046979. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
d0cc07927h
Scheme 1 Intermolecular and intramolecular carbosulfonylation of
alkynes through sulfonyl radical addition.
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been successfully applied to the synthesis of sulfonyl com- Table 1 Initial studies for the sulfonylation of N-benzyl-N-(2-ethynylphenyl)-
a
1
0
2,2,2-trifluoroacetamide 1a with the insertion of sulfur dioxide
pounds. Consequently, the chemistry of sulfur dioxide inser-
tion has become an attractive strategy for the generation of
highly functionalized sulfonyl-containing molecules in organic
1
1–13
synthesis.
four-component fluoroalkylsulfonylation of terminal alkynes
with fluoroalkyl radicals, DABCO (SO and hydrazines, giving
rise to (E)-ethyl 2,2-difluoro-4-aryl-4-sulfamoylbut-3-enoates.
Prompted by the advances in the construction of seven-
membered ring compounds via radical processes and our
continuous interest in sulfonylation from sulfur dioxide,
herein, we report a photoinduced three-component reaction
of N-benzyl-N-(2-ethynylaryl)amides, DABCO (SO ) and aryldia-
For instance, our group recently described a
2 2
)
1
4
b
Entry
Photocatalyst
Solvent
Yield (%)
1
2
3
4
5
6
—
Eosin Y
fac-Ir(ppy)
Ru(bpy)
Ru(bpy)
DMSO
DMSO
DMSO
DMSO
DMF
21
60
69
78
63
68
52
60
57
60
57
1
5
3
3
Cl
2
Cl
2
Á6H
2
2
O
O
3
Á6H
2
2
Ru(bpy) Cl Á6H O
DMA
3
3
3
3
3
3
2
2
2
2
2
2
2
2
2
2
2
2
c
zonium tetrafluoroborates. This protocol provides a range of
sulfonated dibenzazepines in moderate to good yields under
mild conditions (Scheme 1b). Additionally, this sulfonyl
7
Ru(bpy)
Ru(bpy)
Ru(bpy)
Ru(bpy)
Ru(bpy)
Cl
Cl
Cl
Cl
Cl
Á6H
Á6H
Á6H
Á6H
Á6H
O
O
O
O
O
DMSO
DMSO
DMSO
DMSO
DMSO
d
8
e
9
f
1
0
g
radical-mediated multicomponent reaction represents a rare 11
example of carbosulfonylation of alkynes involving the insertion
of sulfur dioxide and intramolecular C–H bond functionalization 1a (0.2 mmol), 4-methylphenyldiazonium tetrafluoroborate 2a (0.3 mmol),
a
Reactions conditions: N-benzyl-N-(2-ethynylphenyl)-2,2,2-trifluoroacetamide
DABCO (SO ) (0.2 mmol), photocatalyst (2 mol%), solvent (2.0 mL), stirred at
room temperature for 24 h under blue LED (30 W) irradiation. Isolated
2 2
to furnish sulfone-containing dibenzazepines. Driven by the
importance of dibenzazepines and sulfones, we envisioned that
such a transformation might be beneficial for organic synthesis
and drug discovery.
b
yield based on N-benzyl-N-(2-ethynylphenyl)-2,2,2-trifluoroacetamide 1a.
c
d
e
Compound 2a (0.4 mmol). Compound 2a (0.2 mmol). DABCO (SO2)2
f
g
(
0.3 mmol). DMSO (4.0 mL). White LED (35 W) instead of blue LED.
We began our studies by choosing N-benzyl-N-(2-ethynylphenyl)-
2 2
2,2,2-trifluoroacetamide 1a, DABCO (SO ) and 4-methylphenyl-
diazonium tetrafluoroborate 2a as the model substrates for the ring of aryldiazonium tetrafluoroborates 2, leading to the
optimization of the reaction conditions. Initially, the reaction was corresponding products 3a-3g in moderate to good yields.
performed in DMSO at room temperature without the addition of Different functional groups including methyl, methoxy,
any catalysts. To our delight, the desired sulfonated dibenzazepine methylthio, fluoro, bromo and ester units were found to be
3
a was obtained in 21% yield (Table 1, entry 1). The structure of compatible under the standard conditions. Subsequently, we
16
compound (Z)-3a was confirmed by X-ray diffraction analysis.
investigated the substitution effect on the aromatic ring of the
However, the yield could not be improved when the solvent or N-(2-ethynylaryl) moiety of substrate 1. As expected, the desired
temperature was changed (data not shown in Table 1). We envi- sulfone-containing dibenzazepines 3h-3k were obtained effi-
sioned that an oxidative single electron transfer might be involved ciently with respect to electron-donating and electron-
during the reaction process. Thus, we considered to introduce a withdrawing substituents. Additionally, diverse substrates 1
photocatalyst, and this reaction was performed under visible light with substituents on the aryl ring of the N-benzyl moiety were
irradiation of 30 W blue LEDs. Gratifyingly, the corresponding evaluated, affording the desired products in moderate to good
product 3a was afforded in 60% yield by employing Eosin-Y as the yields. For instance, sulfonated products 3l and 3n bearing a
photocatalyst (Table 1, entry 2). Encouraged by this result, other methyl or a bromo group on the para-position of the N-benzyl
photocatalysts were evaluated, and the results showed that ring were generated in 69% and 55% yield, respectively. The
Ru(bpy)
3
Cl
2
Á6H
2
O was the best choice, providing the corresponding bromo-substituent in 3n was further applied in the Suzuki
product 3a in 78% yield (Table 1, entries 3 and 4). Subsequently, we coupling reaction for further transformation (see the ESI†).
investigated the solvent effect in this transformation. As a result, no The reaction of the 2-(bromomethyl)quinolone-derived sub-
better yields were obtained when the reaction was performed in strate also proceeded smoothly to deliver the product 3p in
DMF or DMA (Table 1, entries 5 and 6). Additionally, the yield 72% yield. Moreover, N-benzyl-N-(2-ethynylphenyl)amide bearing
of product 3a was decreased when the amount of 4-methyl- an ester group at the benzylic position could be employed as the
2 2
phenyldiazonium tetrafluoroborate 1a, DABCO (SO )
or the concen- substrate, and the corresponding product 3q was produced in
tration of the reaction was changed (Table 1, entries 7–10). We also 50% yield. Notably, the replacement of a trifluoroacetyl group by
explored the reaction by using a white LED (35 W) instead of a blue an acetyl group on the nitrogen resulted in the formation of
LED (30 W) as the light source, and the corresponding product products 3r-3t in 55–84% yields. Unfortunately, the ether sub-
3a was generated in 57% yield (Table 1, entry 11).
strate 1-(benzyloxy)-2-ethynylbenzene was not suitable for this
With the above optimized conditions in hand, we further reaction to provide the dibenzoxepine 3u. However, from the
1
evaluated the reaction scope of this three-component reaction
of N-benzyl-N-(2-ethynylaryl)amides 1, DABCO (SO
and aryl- rotamers would be present.
diazonium tetrafluoroborates 2. As shown in Table 2, the
Since the combination of DABCO (SO
reaction could tolerate various substituents on the aromatic tetrafluoroborate would provide an arylsulfonyl radical, to
H NMR spectra, it seemed that a small amount of amide
2 2
)
2 2
) and aryldiazonium
1
0
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Table 2 Substrate scope for the sulfonylation of N-benzyl-N-(2-ethyny-
a
laryl)amides 1 with the insertion of sulfur dioxide
Scheme 2 Control experiments.
to form 4-methylphenyl sulfonyl radical A. Then, intermediate
A would attack the triple bond of N-benzyl-N-(2-ethynylphenyl)-
2,2,2-trifluoroacetamide 1a to provide vinyl radical B, which would
undergo 7-ortho cyclization to form a radical intermediate C.
Subsequently, radical intermediate C would proceed through a
single-electron oxidation by Ru(III) to produce cation intermediate
D and regenerate the photocatalyst. Finally, the resulting cation D
would undergo deprotonation by DABCO leading to the desired
product 3a.
In summary, we have developed an efficient and straightforward
approach for the construction of sulfonylated dibenzazepines
through a visible-light induced three-component reaction of
N-benzyl-N-(2-ethynylaryl)amides, sulfur dioxide and aryldiazonium
tetrafluoroborates. This protocol features mild reaction conditions,
broad substrate scope and good functional group compatibility,
giving rise to the corresponding products in moderate to good
yields. Notably, by employing this method, both the vinyl sulfone
moiety and the dibenzazepine scaffold can be introduced in the
a
Isolated yield based on N-benzyl-N-(2-ethynylaryl)amide 1.
gain further insights into the mechanism of this three-component one-pot process, which represents a valuable strategy to access
reaction, several control experiments were carried out under the sulfonated seven-membered rings. Preliminary mechanistic studies
standard conditions. As expected, the model reaction of N-benzyl- show that this transformation proceeds through a radical addition/
N-(2-ethynylphenyl)-2,2,2-trifluoroacetamide 1a, DABCO (SO2)2 cyclization cascade with the insertion of sulfur dioxide.
and 4-methylphenyldiazonium tetrafluoroborate 2a was comple-
tely hampered in the presence of 2 equivalents of 2,2,6, dation of China (No. 21871053), Natural Science Foundation
-tetramethyl-1-piperidinyloxy (TEMPO), which suggested that a of Zhejiang Province (LQ21B020002), the Leading Innovative
The financial support from the National Natural Science Foun-
6
radical process was involved in this reaction (Scheme 2, eqn (a)). and Entrepreneur Team Introduction Program of Zhejiang (No.
Additionally, to confirm the radical intermediates generated in 2019R01005) and the Open Research Fund of School of Chemistry
this transformation, 2 equivalents of 1,1-diphenylethylene were
added to the model reaction (Scheme 2, eqn (b)). As a result,
no desired product 3a was observed, and (2-tosylethene-1,
1
4
-diyl)dibenzene 4 was obtained in 64% yield, revealing that the
-methylphenylsulfonyl radical was trapped by 1,1-diphenylethylene.
With the above experimental results in hand and inspired by
10
previous reports, a plausible reaction mechanism was proposed
for this photoinduced alkyne carbosulfonylation process. As
shown in Scheme 3, 4-methylphenyl diazonium tetrafluoroborate
2 2
2a would react with DABCO (SO ) to generate 4-methylphenyl
sulfonyl radical A and a tertiary amine radical cation with the
release of molecular nitrogen. In the meantime, photoexcitation
of Ru(II) would give rise to the excited-state Ru(II)*, which could be
oxidized to Ru(III) by the tertiary amine radical cation. Alterna-
tively, 4-methylphenyl diazonium tetrafluoroborate 2a would
undergo a single-electron transfer by the photoexcited Ru(II)* to
give a 4-methylphenyl radical, which would capture sulfur dioxide Scheme 3 Proposed mechanism.
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and Chemical Engineering, Henan Normal University (2020ZD04) is
gratefully acknowledged.
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Conflicts of interest
There are no conflicts to declare.
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2886 | Chem. Commun., 2021, 57, 2883À2886
This journal is The Royal Society of Chemistry 2021