Ag-Mediated Radical Cyclization of 2-Alkynylthio(seleno)anisoles: Direct Synthesis of 3-Phosphinoylbenzothio(seleno)phenes

Article abstract of DOI:10.1021/acs.orglett.9b01510

A new method for the direct synthesis of 3-phosphinoylbenzothio(seleno)phenes has been achieved through an Ag-mediated radical addition-cyclization of 2-alkynylthio(seleno)anisoles with secondary phosphine oxides in good yields under mild conditions. In this single reaction, benzenethiophene or benzeneselenophene skeleton, C(sp²)-P and C(sp²)-S bonds can be constructed with the cleavage of the C(sp³)-S bond, highlighting the efficiency and step-economics of this protocol.

Full text of DOI:10.1021/acs.orglett.9b01510

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Ag-Mediated Radical Cyclization of 2‑Alkynylthio(seleno)anisoles:
Direct Synthesis of 3‑Phosphinoylbenzothio(seleno)phenes
,‡
†
†
†
†
†
†
Tao Cai, Jian Liu, Huimiao Zhang, Xiaolong Wang, Jing Feng, Runpu Shen, and Yuzhen Gao*
†
College of Chemistry and Chemical Engineering, Shaoxing University, Shaoxing, 312000, China
‡
Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Fujian Institute of Research on the Structure of Matter,
Chinese Academy of Sciences, Fuzhou, Fujian 350002, China
*
S Supporting Information
ABSTRACT: A new method for the direct synthesis of 3-
phosphinoylbenzothio(seleno)phenes has been achieved through
an Ag-mediated radical addition−cyclization of 2-alkynylthio-
(
seleno)anisoles with secondary phosphine oxides in good yields
under mild conditions. In this single reaction, benzenethiophene or
benzeneselenophene skeleton, C(sp )−P and C(sp )−S bonds can
be constructed with the cleavage of the C(sp )−S bond, highlighting the efficiency and step-economics of this protocol.
2
2
3
enzothiophene moieties are important skeletons that have
between 2-substituted benzothiophenes and trialkyl phosphites
for the formation of 3-phosphinoylbenzothiophenes (Scheme
1b). Nevertheless, a benzothiophene skeleton is required for
the formation of target molecules in these above-mentioned
reports, which may limit the substrate scope to synthesize
more-diverse compounds. Therefore, it still remains a great
challenge to develop new approaches for the synthesis of
phosphinoylbenzothiophenes.
B
been widely found in many pharmaceuticals, natural
1
4b
products, and polymers. On the other hand, organo-
phosphorus compounds have attracted tremendous attention
because of their broad applications in the fields of organic
synthesis, materials science, pharmaceuticals, and ligand
2
chemistry. Therefore, synthesis of diverse phosphinoylbenzo-
thiophenes is highly desirable. However, the reported
approaches for the construction of phosphinoylbenzothio-
Nowadays, radical cascade reactions have become one of the
most efficient routes for the construction of polycyclic and
3
,4
phenes were very limited, much less for 3-phosphinoylben-
5
4
heterocyclic compounds. Therefore, great development was
zothiophenes. In 2016, visible-light-driven phosphonylation of
made for the construction of benzothiophenes via a radical
3
-bromobenzothiophene with triethyl phosphite for the
6
,7
cyclization pathway.
On the other hand, thioanisole
construction of 3-phosphonatebenzothiophene was reported
4a
derivatives have been widely used in the radical cyclization
process with the release of a methyl group for the construction
by the Konig group (Scheme 1a). However, just one example
̈
was given. Subsequently, Cai’s group disclosed a reaction
7
,8
of heterocyclic compounds. As part of our interests in the
8a,9
synthesis of heterocycles through radical cascade processes,
we questioned whether a phosphoryl radical could react with
-alkynylthioanisoles via a radical addition to the sulfur atom,
Scheme 1. Route to 3-Phosphorylbenzothiophenes
2
leading to the formation of 3-phosphinoylbenzothiophenes
2
with the construction of benzenethiophene skeleton, C(sp )−
2
P and C(sp )−S bonds in one step (Scheme 1c). However, Ag-
mediated arylphosphine oxide radical annulation of diary-
lalkynes to synthesize benzo[b]phosphole oxides were well-
10
studied (Scheme 1d). Therefore, considering the competi-
tion between the two reactions, the preparation of 3-
phosphinoylbenzothiophenes through Ag-mediated 5-exo-trig
cyclization was extremely challenging.
To test this hypothesis, methyl(2-(phenylethynyl)phenyl)-
sulfane (1a) and diphenylphosphine oxide (2a) were chosen as
reaction partners to optimize the reaction conditions (Table
1
). When 3 equiv of AgOAc was employed as the initiator with
HOAc as the solvent, 3a was obtained in 56% yield after being
Received: April 30, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01510
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Optimization of Reaction Conditions
Scheme 2. Scope for the Formation of 3-
Phosphinoylbenzothiophenes
a
yield of 3a/3a′
entry
initiator (equiv)
AgOAc (3.0)
solvent
HOAc
(%)
1
2
3
4
5
56/15
35/39
38/48
trace/0
19/20
42/8
51/15
47/10
58/6
AgOAc (3.0)
AgOAc (3.0)
AgOAc (3.0)
AgOAc (3.0)
AgOAc (3.0)
AgOAc (3.0)
CH CN
3
DMF
DMSO
dioxane
HOAc
HOAc
HOAc
HOAc
HOAc
c
6
d
7
8
9
AgNO (3.0)
3
Ag CO (3.0)
2
3
10
11
12
13
14
Ag O (3.0)
73/5
2
e
Ag O (3.0)
TFA
81/0
2
e
b
Ag O (1.0)
TFA
78 /0
2
f
e
Ag O (1.0)
TFA
63/0
24/0
2
e
Ag O (0.2) + Zn(NO ) ·6H O
TFA
2
3
2
2
(
2.0)
1
5
AgNO (0.1) + Mg(NO ) ·6H O
CH CN
25/0
3
3
2
2
3
(
0.5)
1
6
AgNO (0.1) + Na S O (0.5)
CH CN
trace/0
3
2
2
8
3
a
Reaction conditions: 1a (0.15 mmol), 2a (0.3 mmol), initiator in
solvent (1.5 mL) stirring at 70 °C under argon for 7 h. The yield was
1
determined via H NMR, using CH Br as the internal standard.
2
2
a
b
c
d
e
Reaction conditions: 1 (0.15 mmol), 2 (0.3 mmol), Ag O (1.0
Isolated yield. At 90 °C. At 50 °C. TFA = trifluoracetic acid.
2
b
f
equiv), in TFA (1.5 mL) stirring under argon at 70 °C for 7 h. Two
Under air.
mmol scale.
heated at 70 °C for 7 h under argon (Table 1, entry 1).
Subsequently, various solvents such as CH CN, dimethylfor-
fluorine, chlorine, and bromine have little influence under the
optimized reaction conditions to afford the corresponding
products 3g−3j in moderate to good yields, which could allow
for further synthetic transformations. Moreover, the structure
of 3h was confirmed by X-ray crystal structure analysis. Note
3
mamide (DMF), dimethylsulfoxide (DMSO), and 1,4-dioxane
were explored, revealing that HOAc was the best choice (Table
1
, entries 2−5). The yield of product 3a decreased when the
temperature was increased to 90 °C or reduced to 50 °C
Table 1, entries 6 and 7). By screening other silver salts, such
as AgNO , Ag CO , and Ag O, Ag O was revealed to be the
(
that electron-withdrawing substituents such as CF on the
3
benzene ring A influenced the radical cyclization significantly
by providing product 3k only in 35% yield. Compounds with
3
2
3
2
2
most effective and gave 3a in 73% yield (Table 1, entries 8−
1
0). Note that a small amount of benzo[b]phosphole oxides
a′ were detected under these reaction conditions through the
functional groups such as halogen atoms and CF on the
3
3
benzene ring B were also examined and afforded the
corresponding products 3l−3n in good yields. In addition,
compounds with the thienyl group could also provide the
expected products 3o and 3p in acceptable yields. Moreover,
the reactions between substituted phenylphosphine oxides 2
and 1a were also examined and gave the corresponding
products 3q−3s in yields of 62%−56%. No desired products
were obtained when aliphatic alkynes was employed as
exemplified by 3t.
1
0
process mentioned above (Table 1, entries 1−10).
Pleasingly, the side reaction was totally restrained by
employing TFA as the solvent with a slight improvement of
the yield of 3a (Table 1, entry 11). Moreover, 3a was isolated
in 78% yield when the loading of Ag O was reduced to 1 equiv
2
(
Table 1, entry 12). In addition, the yield of 3a was decreased
to 61% when the reaction was conducted under air (Table 1,
entry 13). Finally, the efforts to use catalytic amounts of Ag O
2
in the presence of oxidants such as Zn(NO ) ·6H O,
Pyridyl-based compounds also were examined; however, no
desired product 3u was isolated, which might be due to the
formation of ammonium salt in the strong acid TFA to restrain
the reaction.
3
2
2
Mg(NO ) ·6H O, and Na S O have not been successful
3
2
2
2
2
8
(
Table 1, entries 14−16).
With the optimized reaction conditions in hand, we
investigated the scope of this Ag-mediated radical cyclization
of 2-alkynylthioanisoles, and the results are summarized in
Scheme 2. First, various functional groups on the benzene ring
A were examined and most of the functional groups were
tolerated under the optimized conditions. With electron-
donating substituents on benzene ring A, such as methyl,
methoxy, ethyl, and phenyl groups, these compounds were
obtained in yields of 85%−70%. Halogen atoms such as
Moreover, benzoselenophenes have also been applied in
medicinal chemistry and organic synthesis, as well as materials
11
science. For this reason, we extended the scope of application
of this protocol by preparing benzoselenophenes stemming
from 2-alkynylselenoanisoles 4 and secondary phosphine
oxides 2 under the optimal reaction conditions (see Scheme
3). In the beginning, some substituted phenylphosphine oxides
were examined and afforded the corresponding desired
B
DOI: 10.1021/acs.orglett.9b01510
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3. Scope for the Formation of 3-
Phosphinoylbenzoselenophenes
yields of 60% and 34%, respectively (see Schemes 4b and 4c).
Moreover, benzene was detected by gas chromatography−
mass spectrometry (GC-MS) from the reaction mixtures when
a
1
ac was employed as the substrate. In addition, to further
support the mechanism, the labeling experiment by using
deuterated TAF (d-TAF) as the solvent was performed under
the optimized reaction conditions (see Scheme 4d). Gladly, 3a
was obtained in 38% yield and d -benzene was detected by
1
GC-MS. The results indicated the benzene might be produced
from phenyl radical via H-abstraction from the solvent.
In accordance to the control experiments and corresponding
literature reports, a tentative pathway was proposed for the Ag-
mediated transformation (see Scheme 5). Initially, diphenyl-
Scheme 5. Proposed Mechanism
a
Reaction conditions: 4 (0.15 mmol), 2 (0.3 mmol), Ag O (1.0
2
equiv), in TFA (1.5 mL) stirring under argon at 70 °C for 7 h.
products in good yields (5a−5c). Moreover, ethyl phenyl-
phosphinate and 9,10-dihydro-9-oxa-10-phosphaphenanthrene
phosphine oxide 2a reacts with Ag O to form the phosphoryl
2
1
0-oxide (DOPO) also could be used in the transformation
radical I, which selectively attacks the triple bond of 1a to give
the vinyl radical II. The resulting vinyl radical II participates in
a 5-exo-trig cyclization with SMe moiety to generate the
product 3a, along with the release of a methyl radical. Finally,
methane is generated from the methyl radical via a H-
abstraction process from the reaction mixture.
(
5d, 5e). With electron-donating (methyl) or electron-
withdrawing (fluorine) substituent groups on the aromatic
rings of 2-alkynylselenoanisoles, these compounds reacted
smoothly and provided products 5f−5h in good to moderate
yields. However, no desired product (5i) was afforded when
the Se atom in compound 4a was replaced by an O atom.
In order to investigate the reaction mechanism, several
control experiments were performed. In the beginning, radical
trapping experiments were conducted to validate our original
design of the radical cyclization process (Scheme 4a). The
In summary, we have successfully developed an efficient
protocol for the direct synthesis of 3-phosphinoylbenzothio-
(
seleno)phenes through an Ag-mediated radical addition−
cyclization of 2-alkynylthio(seleno)anisoles with secondary
phosphine oxides involving the formation of thiophene or
2
2
selenophene skeleton, C(sp )−P and C(sp )−S bonds and the
Scheme 4. Control Experiments
3
cleavage of the C(sp )−S bond. This protocol features good
functional group tolerance and easy handling with good yields
under mild conditions.
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.9b01510.
General procedure, characterization data, H, 13C 31P,
1
1
9
and F NMR spectra (PDF)
Accession Codes
CCDC 1892103 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.
reaction was totally inhibited when 1.5 equiv of 2,2,6,6-tetra-
methylpiperidinooxy (TEMPO) was added to the reaction,
and the yield of 3a was shut down when butylated
hydroxytoluene (BHT) was employed. These results indicated
the Ag-mediated annulation might proceed via a radical
pathway. Notably, when the methyl group at the S atom was
changed to ethyl and phenyl groups, 3a was also obtained in
AUTHOR INFORMATION
Corresponding Author
■
*
E-mail: gyz@fjirsm.ac.cn.
ORCID
Yuzhen Gao: 0000-0002-2690-7407
C
DOI: 10.1021/acs.orglett.9b01510
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Notes
(Arylsulfonyl)benzothiophenes and Benzoselenophenes via TBHP-
Initiated Radical Cyclization of 2-Alkynylthioanisoles or -Selenoani-
soles with Sulfinic Acids. Org. Lett. 2017, 19, 6292−6295. (c) Yan, J.;
Xu, J.; Zhou, Y.; Chen, J.; Song, Q. Photoredox-Catalyzed Cascade
Annulation of Methyl(2-(phenylethynyl)phenyl)sulfanes and Methyl-
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(
2-(phenylethynyl)phenyl)selanes with Sulfonyl Chlorides: Synthesis
We gratefully thank the financial support from NSFC (Grant
No. 21801240). T.C. thanks the Youth Foud of Shaoxing
University (No. 20185019). Y.G. also thanks the National
Postdoctoral Program for Innovative Talents (No.
BX201700244).
of Benzothiophenes and Benzoselenophenes. Org. Chem. Front. 2018,
5
, 1483−1487. (d) Liu, W.; Hu, Y.-Q.; Hong, X.-Y.; Li, G.-X.; Huang,
X.-B.; Gao, W.-X.; Liu, M.-C.; Xia, Y.; Zhou, Y.-B.; Wu, H.-Y. Direct
Synthesis of 3-Acylbenzothiophenes via the Radical Cyclization of 2-
Alkynylthioanisoles with α-Oxocarboxylic Acids. Chem. Commun.
2
018, 54, 14148−14151. (e) Staples, M. K.; Grange, R. L.; Angus, J.
A.; Ziogas, J.; Tan, N. P. H.; Taylor, K. T.; Schiesser, C. H. Tandem
Free-Radical Addition/Substitution Chemistry and Its Application to
REFERENCES
■
(
1) (a) Flynn, B. L.; Hamel, E. M.; Jung, K. One-Pot Synthesis of
the Preparation of Novel AT Receptor Antagonists. Org. Biomol.
1
Benzo[b]furan and Indole Inhibitors of Tubulin Polymerization. J.
Med. Chem. 2002, 45, 2670−2673. (b) Bettinetti, L.; Schlotter, K.;
Huebner, H.; Gmeiner, P. Interactive SAR Studies: Rational
Discovery of Super-Potent and Highly Selective Dopamine D3
Receptor Antagonists and Partial Agonists. J. Med. Chem. 2002, 45,
Chem. 2011, 9, 473−479. (f) Shaaban, S.; Jolit, A.; Petkova, D.;
Maulide, N. A Family of Low Molecular-Weight, Organic Catalysts
for Reductive C−C Bond Formation. Chem. Commun. 2015, 51,
1
3902−13905. (g) McDonald, F. E.; Burova, S. A.; Huffman, L. G., Jr
Sulfur-Alkyne Cyclizations for Formation of Dihydrothiophenes and
4
594−4597. (c) Pieroni, M.; Azzali, E.; Basilico, N.; Parapini, S.;
Annulated Thiophenes. Synthesis 2000, 2000, 970−974. (h) Hari, D.
Zolkiewski, M.; Beato, C.; Annunziato, G.; Bruno, A.; Vacondio, F.;
Costantino, G. Accepting the Invitation to Open Innovation in
Malaria Drug Discovery: Synthesis, Biological Evaluation, and
Investigation on the Structure−Activity Relationships of Benzo[b]-
thiophene-2-carboxamides as Antimalarial Agents. J. Med. Chem.
P.; Hering, T.; Konig, B. Visible Light Photocatalytic Synthesis of
̈
Benzothiophenes. Org. Lett. 2012, 14, 5334−5337. (i) Zang, H.; Sun,
J.-G.; Dong, X.; Li, P.; Zhang, B. Preparation of Benzothiophenes and
Benzoselenophenes from Arylamines and Alkynes via Radical Cascade
Reactions. Adv. Synth. Catal. 2016, 358, 1746−1752.
2
017, 60, 1959−1970. (d) Osaka, I.; Shinamura, S.; Abe, T.;
(8) (a) Gao, Y.; Zhang, P.; Li, G.; Zhao, Y. Cascade Annulation of 2-
Takimiya, K. Naphthodithiophenes as Building Units for Small
Molecules to Polymers; A Case Study for In-depth Understanding of
Structure-Property Relationships in Organic Semiconductors. J. Mater.
Chem. C 2013, 1, 1297−1304.
Alkynylthioanisoles with Unsaturated α-Bromocarbonyls Leading to
Thio-Benzobicyclic Skeletons. J. Org. Chem. 2018, 83, 13726−13733.
3
(
b) Yang, W.-C.; Wei, K.; Sun, X.; Zhu, J.; Wu, L. Cascade C(sp )−S
Bond Cleavage and Imidoyl C−S Formation: Radical Cyclization of
-Isocyanoaryl Thioethers toward 2-Substituted Benzothiazoles. Org.
(
2) (a) Queffelec, C.; Petit, M.; Janvier, P.; Knight, D. A.; Bujoli, B.
2
Surface Modification Using Phosphonic Acids and Esters. Chem. Rev.
Lett. 2018, 20, 3144−3147. (c) Xu, J.; Zhang, F.; Zhang, S.; Zhang,
2
012, 112, 3777−3807. (b) George, A.; Veis, A. Phosphorylated
2
L.; Yu, X.; Yan, J.; Song, Q. Radical Promoted C(sp )−S Formation
Proteins and Control over Apatite Nucleation, Crystal Growth, and
3
and C(sp )−S Bond Cleavage: Access to 2-Substituted Thiochro-
Inhibition. Chem. Rev. 2008, 108, 4670−4693.
(
Bond Formation Using Aromatic Esters and Organophosphorus
Compounds. Org. Lett. 2018, 20, 1150−1153. (b) Sun, W.-B.; Xue, J.-
F.; Zhang, G.-Y.; Zeng, R.-S.; An, L.-T.; Zhang, P.-Z.; Zou, J.-P. Silver-
mones. Org. Lett. 2019, 21, 1112−1115.
3) (a) Isshiki, R.; Muto, K.; Yamaguchi, J. Decarbonylative C−P
(9) (a) Gao, Y.; Lu, G.; Zhang, P.; Zhang, L.; Tang, G.; Zhao, Y. A
Cascade Phosphinoylation/Cyclization/Desulfonylation Process for
the Synthesis of 3-Phosphinoylindoles. Org. Lett. 2016, 18, 1242−
1245. (b) Gao, Y.; Zhang, P.; Ji, Z.; Tang, G.; Zhao, Y. Copper-
Catalyzed Direct C −H Phosphorylation of Indoles Leading to
sp2
Catalyzed Cascade Radical Addition-Cyclization Halogen Atom
Transfer between Alkynes and Unsaturated α-Halogenocarbonyls.
ACS Catal. 2017, 7, 186−190.
Phosphoindoles. Adv. Synth. Catal. 2016, 358, 1753−1758. (c) Lin,
W.; Su, F.; Zhang, H.-J.; Wen, T.-B. K S O -Promoted Direct C−H
2
2
8
Phosphorylation of (Benzo)thiazoles. Eur. J. Org. Chem. 2017, 2017,
(10) For representative examples, see: (a) Unoh, Y.; Hirano, K.;
1
(
757−1759.
Satoh, T.; Miura, M. An Approach to Benzophosphole Oxides
through Silver- or Manganese-Mediated Dehydrogenative Annulation
Involving C−C and C−P Bond Formation. Angew. Chem., Int. Ed.
4) (a) Shaikh, R. S.; Dusel, S. J. S.; Konig, B. Visible-Light Photo-
̈
̈
Arbuzov Reaction of Aryl Bromides and Trialkyl Phosphites Yielding
Aryl Phosphonates. ACS Catal. 2016, 6, 8410−8414. (b) Wang, Y.;
Yang, Y.; Jie, K.; Huang, L.; Guo, S.; Cai, H. Copper-Catalyzed C2
and C3 Phosphonation of Benzofuran and Benzothiophene with
Trialkyl Phosphites. ChemCatChem 2018, 10, 716−719.
2
013, 52, 12975−12979. (b) Chen, Y.-R.; Duan, W.-L. Silver-
Mediated Oxidative C−H/P−H Functionalization: An Efficient
Route for the Synthesis of Benzo[b]phosphole Oxides. J. Am. Chem.
Soc. 2013, 135, 16754−16757.
(
5) For selected reviews on cascade reactions, see: (a) Gao, Y.;
(
11) (a) Arsenyan, P.; Paegle, E.; Belyakov, S.; Shestakova, I.;
Tang, G.; Zhao, Y. Recent Progress toward Organophosphorus
Compounds Based on Phosphorus-Centered Radical Difunctionaliza-
tions. Phosphorus, Sulfur Silicon Relat. Elem. 2017, 192, 589−596.
Jaschenko, E.; Domracheva, I.; Popelis, J. Synthesis, Structure and
Cytotoxicity of 3-C, N, S, Se Substituted Benzo[b]selenophene
Derivatives. Eur. J. Med. Chem. 2011, 46, 3434−3443. (b) Ebata, H.;
Miyazaki, E.; Yamamoto, T.; Takimiya, K. Synthesis, Properties, and
Structures of Benzo[1,2-b:4,5-b′]bis[b]benzothiophene and Benzo-
(
b) Huang, M.-H.; Hao, W.-J.; Li, G.; Tu, S.-J.; Jiang, B. Recent
Advances in Radical Transformations of Internal Alkynes. Chem.
Commun. 2018, 54, 10791−10811. (c) Xuan, J.; Studer, A. Radical
Cascade Cyclization of 1,n-Enynes and Diynes for the Synthesis of
Carbocycles and Heterocycles. Chem. Soc. Rev. 2017, 46, 4329−4346.
[
(
1,2-b:4,5-b′]bis[b]benzoselenophene. Org. Lett. 2007, 9, 4499−4502.
c) Takimiya, K.; Kunugi, Y.; Konda, Y.; Ebata, H.; Toyoshima, Y.;
Otsubo, T. 2,7-Diphenyl[1]benzoselenopheno[3,2-b][1]-
benzoselenophene as a Stable Organic Semiconductor for a High-
Performance Field-Effect Transistor. J. Am. Chem. Soc. 2006, 128,
(
6) Liu, K.; Jia, F.; Xi, H.; Li, Y.; Zheng, X.; Guo, Q.; Shen, B.; Li, Z.
Direct Benzothiophene Formation via Oxygen-Triggered Intermo-
lecular Cyclization of Thiophenols and Alkynes Assisted by
Manganese/PhCOOH. Org. Lett. 2013, 15, 2026−2029.
3044−3050.
(
7) (a) Leardini, R.; Pedulli, G. F.; Tundo, A.; Zanardi, G. Reaction
pathways for the cyclization of ortho-thioalkyl and ortho-thioaryl
substituted phenyl radicals with alkynes. Reaction of o-Methylthioar-
enediazonium Tetrafluoroborates with Alkynes to Give 2-Substituted
Benzo[b]thiophenes. J. Chem. Soc., Chem. Commun. 1985, 1390−
1
391. (b) Xu, J.; Yu, X.; Yan, J.; Song, Q. Synthesis of 3-
D
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