Trisulfur Radical Anion (S3?-) Involved [1 + 2 + 2] and [1 + 3 + 1] Cycloaddition with Aromatic Alkynes: Synthesis of Tetraphenylthiophene and 2-Benzylidenetetrahydrothiophene Derivatives

Article abstract of DOI:10.1021/acs.orglett.8b02066

S₃^?--mediated [1 + 2 + 2] and [1 + 3 + 1] cycloaddition reactions of aromatic alkynes to give tetraphenylthiophene and 2-benzylidenetetrahydrothiophene derivatives via two C-S bond formations are developed. These two protocols provide new, simple, and straightforward strategies to construct tetraphenylthiophene and 2-benzylidenetetrahydrothiophene derivatives under transition-metal-free conditions. This study also expands the application of S₃^?- in organic reactions.

Full text of DOI:10.1021/acs.orglett.8b02066

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
•−
Trisulfur Radical Anion (S3 ) Involved [1 + 2 + 2] and [1 + 3 + 1]
Cycloaddition with Aromatic Alkynes: Synthesis of
Tetraphenylthiophene and 2‑Benzylidenetetrahydrothiophene
Derivatives
Jing-Hao Li, Qi Huang, Shun-Yi Wang,* and Shun-Jun Ji*
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science &
Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, China
S
* Supporting Information
ABSTRACT: S3•−-mediated [1 + 2 + 2] and [1 + 3 + 1] cycloaddition
reactions of aromatic alkynes to give tetraphenylthiophene and 2-
benzylidenetetrahydrothiophene derivatives via two C−S bond formations
are developed. These two protocols provide new, simple, and straightforward
strategies to construct tetraphenylthiophene and 2-benzylidenetetrahydrothiophene derivatives under transition-metal-free
•−
conditions. This study also expands the application of S3 in organic reactions.
catalyzed dual C−S bond formation reactions of 1,4-diiodo-
ubstituted thiophenes and tetrahydrothiophenes have
Semerged as a class of important heterocycles because of 1,3-diene with K2S (Scheme 1a-2).8 However, the limited
reported methods have some drawbacks such as multireaction
steps, low total product yields, long reaction times, and use of
transition metal catalysts. It is more desirable to develop new
simple strategies to construct these compounds in one step
under mild conditions.
their presence in a broad spectrum of natural and synthetic
molecules with diverse biological properties,1 material
characteristics,2 and utility in organic synthesis as versatile
intermediates.3 In particular, poly- and oligoarylthiophenes
have recently attracted much attention due to their useful
electronic and optical properties.4 What is more, tetra-
arylthiophenes can also be used as an important reactant for
the synthesis of special full substituted aromatic benzenes,5
naphthalenes, and anthracenes.6 The general synthetic
approach to such tetra-arylthiophenes involves multiple C−H
coupling reactions from 3-methoxythiophene.5,6 It can also be
synthesized by palladium-catalyzed coupling of 3-thiophene-
carboxylic acid with aryl halides (Scheme 1a-1)7 and copper-
The S3•− species has been known for more than 40 years and
can be very easily formed by the reaction of elemental sulfur
with KOH in DMF at room temperature.9 However,
applications for the synthesis of organosulfur compounds
involving S3 via C−S bond formation are very rare.10
•−
Recently, Lei’s group reported an S8 and base in situ generated
S3•−-mediated [4 + 1] cycloaddition with 1,3-diynes for
synthesis of 2,5-substituted thiophene.10b More recently, our
group developed an in situ generated S3•− from a K2S-initiated
[5 + 1] cycloaddition reaction with enaminones to afford
benzothiazine derivatives (Scheme 1b).10d As part of our
continued efforts on the reactions involving S3•−, herein, we
describe a S3•− in situ generated from S2−-mediated [1 + 2 + 2]
and [1 + 3 + 1] cycloaddition with aromatic alkynes to give
tetraphenylthiophene and 2-benzylidenetetrahydrothiophene
derivatives via two C−S bond formations (Scheme 1c).
Scheme 1. Approaches to Tetra-arylthiophenes and Our
•−
Works on C−S Bond Construction by S3
Initially, we investigated the reaction of 1,2-diphenylethyne
1a and K2S 2a in DMF at 120 °C for 12 h under Ar
atmosphere. To our delight, the [2 + 2 + 1] cycloaddition
product tetraphenylthiophene 3a was obtained in 65% liquid
chromatography yield (LC-yield). The structure of 3a was
confirmed by NMR, IR, HRMS, and X-ray analyses. With this
promising result in hand, we further screened the reaction
conditions to optimize the reaction. It was found that the
reaction was very messy without argon protecting. Next, other
Received: July 2, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02066
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
sulfur reagents such as cyclo-S8, Na2S·9H2O, and Na2S2O3
were applied in the reaction (Table 1, entries 2−4). The
a
Table 1. Optimization of the Reaction Conditions
ab
,
entry
S reagent
K2S
S8 + NaOtBu
Na2S·9H2O
Na2S2O3
K2S
K2S
K2S
K2S
K2S
solvent
temperature (°C)
3a (%)
c
1
2
3
4
5
6
7
8
9
DMF
DMF
DMF
DMF
DMSO
DMA
toluene
DMF
DMF
DMF
DMF
120
120
120
120
120
120
120
80
65(48)
<5
36
0
52
40
<5
0
c
c
130
140
130
70(63)
68
78(69)
10
11
K2S
K2S
d
a
Reaction conditions: 1a (0.6 mmol), 2a (0.36 mmol), dry DMF (2.5
b
mL), Ar, 12 h. LC yield using biphenyl as the internal standard.
Figure 1. Substrate scope of arylacetylene derivatives.
c
d
Isolated yield. 2a (0.6 mmol).
conditions, 2,5-diarylthiophene 3m and 3n could also be
observed in 27% and 36% yields, respectively. It is worth
noting that the reaction of trimethyl(phenylethynyl)silane 1o
with K2S could also afford 2,5-diphenylthiophene 3o (3m =
3o) in 50% yield via C−Si bond cleavage.
To verify the potential applications of this [1 + 2 + 2]
cycloaddition reaction, the reaction of 1a was scaled up to 2
mmol. The desired product 3a was observed in 65% yield.
Pleasingly, following further oxidation and Itami’s [4 + 2]
reaction, 1,2,3,4-tetraphenylnaphthalene 4 could be obtained
in total 32% yield (see Supporting Information (SI) for more
details).
To our delight, the reaction of (cyclopropylethynyl)benzene
5a with K2S produced the E/Z mixture of tetrahydrothiophene
product 6a instead of tetrasubstituted thiophene 6a′. The
reaction of 5a with K2S showed a different pathway, and [1 + 3
+ 1] cycloaddition product 6a was formed via a ring-opening
reaction of 5a and following cyclization (Scheme 2a).
reaction of 1a with Na2S·9H2O could also react smoothly to
give 3a in 36% LC-yield. Only a trace amount of 3a was
detected when 1a reacted with Na2S2O3 and cyclo-S8 (with
NaOtBu) under similar reaction conditions, respectively. Then,
different solvents such as dimethyl sulfoxide (DMSO), N,N-
dimethylacetamide (DMA), and toluene were tested for the
reaction of 1a and 2a at 120 °C for 12 h under an argon
atmosphere. The reaction in DMSO, DMA resulted in 3a with
moderate yields (Table 1, entries 5−6), and only a trace
amount of 3a was detected when the reaction was carried out
in toluene (Table 1, entry 7). When the reaction temperature
was decreased to 80 °C, no desired product was observed
(Table 1, entry 8). When the temperature was raised to 130
°C, the LC-yield of 3a increased to 75% (Table 1, entry 9). It
should be noted that the yield of 3a decreased to 68% when
the reaction temperature increased to 140 °C (Table 1, entry
10). When 2.0 equiv of K2S was applied in this reaction, 3a was
obtained in 78% LC yield and 69% isolated yield (Table 1,
entry 10). Therefore, the optimal reaction conditions were 1a
(0.6 mmol) and K2S 2a (0.6 mmol) in dry DMF (2.5 mL) at
130 °C for 12 h.
Scheme 2. [1 + 3 + 1] Cycloaddition and Crystal Structure
of 7
With the optimized reaction conditions in hand, we explored
the substrate scope of arylacetylene derivatives. The results are
summarized in Figure 1. The reactions of symmetric
diarylethynes 1b, 1d, and 1e with K2S resulted in the
corresponding products 3b, 3d, and 3e in 81%, 86%, and
73% yields, respectively. It should be noted that the reaction of
1,2-bis(4-fluorophenyl)ethyne 1c afforded 3c in 28% yield for
the effect of the F substituent. When unsymmetric diary-
lethynes 1f−i were subjected to the reaction with K2S under
the optimized conditions, inseparable mixtures of 3f−i, 3f′−i′,
and 3f″−i″ were obtained in up to 75% total yield,
respectively. It should be noted that the unsymmetric 3-
(phenylethynyl)pyridine 1j reacted smoothly to give the only
product 3j′ in 51% yield. The reaction of 1,2-di(pyridin-3-
yl)ethyne 1k and 1,2-di(pyridin-4-yl)ethyne 1l with K2S could
gave 3k and 3l in 85% and 73% yields, respectively. When
ethynylbenzene 1m and 1-ethynyl-4-methylbenzene under 1n
were applied to the reaction with K2S under identical
Oxidation of 6a by H2O2 afforded the corresponding (Z)-2-
benzylidenetetrahydrothiophene 1-oxide 7 in 85% yield. The
structure of 7 was further confirmed by X-ray diffraction
analysis (Scheme 2b).
Then, we further optimized the reaction conditions to
improve the yield of 6a. It was found that LC yield of 6a was
dramatically increased without argon pretection. When Na2S·
9H2O along with DMA were applied in this reaction instead of
K2S along with DMF, the reaction resulted in 6a (E/Z =
B
DOI: 10.1021/acs.orglett.8b02066
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Letter
24:76) in 78% LC yield (69% isolated yield) at 150 °C for 9 h
(see SI for more details).
With the optimized conditions in hand, we explored the
scope of (cyclopropylethynyl)benzenes 5, and the results are
listed in Figure 2. The reaction of (cyclopropylethynyl)-
To explore the plausible mechanism of these two reactions,
radical-trapping experiments were performed (see SI for more
details). When 1.0 equiv of TEMPO was added to the
reactions, the yields of 3a and 6a were reduced to 42% and
35%. As the amount of TEMPO was increased to 4.0 equiv,
these two reactions were almost fully suppressed. This
observation indicated that a radical pathway might be involved
in the reactions.
•−
Based on our previous study, it was known that S3 could
be in situ generated by the mixture of K2S in DMF.10d It was
•−
found that Na2S·9H2O in DMA could also in situ generate S3
based on an electron paramagnetic resonance (EPR) experi-
ment. A strong single EPR signal was observed in DMA
solution of Na2S·9H2O at room temperature (see SI for more
details), and our EPR parameter (g = 2.015) is very similar to
the reported one.10b,d,11 Based on the above results, we suspect
•−
that the in situ generated S3 might be involved in these two
cycloaddition reactions.
As shown in Scheme 3, 2-(phenylmethylene-d)-
tetrahydrothiophene-3,3-d2 8 was formed in 58% yield when
Scheme 3. Investigation of the Proton Source in [1 + 3 + 1]
Cycloaddition Reaction
Figure 2. Substrate scope of (cyclopropylethynyl)benzene. The
reaction conditions were (cyclopropylethynyl)benzenes 5 (0.5 mmol,
1.0 equiv) and Na2S·9H2O 2b (2.0 equiv) in DMA (2.5 mL) at 150
°C.
Na2S·9H2O was replaced by K2S and D2O.12 In addition, when
the reaction system was without any water, only trace amounts
of target product 6a could be detected. This result indicated
that water was the hydrogen donor in the transformation.
Based on the reported literature10 and the above results, two
reaction mechanisms were proposed in Scheme 4. For the [1 +
benzenes 5d, 5h, and 5j bearing weak electron-donating
substituent CH3 on the aryl ring with 2b gave the ring
expansion products 6d (E/Z = 13:87), 6h (E/Z = 18:82), and
6j (E/Z = 17:83) in 52%, 68%, and 68% yields, respectively.
The reaction of 1-chloro-4-(cyclopropylethynyl)benzene with
2b afforded the desired product 6b in 72% yield, and the E/Z
ratio of 6b is 5:95. Halogen-substituted (cyclopropyl-ethynyl)-
benzenes 5g and 5i also underwent reaction to give the desired
products E/Z-6g and E/Z-6i in similar LC yields. However,
only Z-6g and Z-6i could be isolated by column chromatog-
raphy. The reactions of 5c and 5n with 2b tolerated the bromo
substituent to give the desired products 6c (E/Z = 23:77) and
6n (E/Z = 21:79) in 66% and 65% yields, respectively. It
should be noted that 1-(cyclopropylethynyl)-4-(trifluoro-
methyl)benzene 5e bearing an electron-withdrawing group
was proven to be suitable for the reaction and furnished the
desired products 6e (E/Z = 30:70) in 68% yield. The reactions
of 1-(cyclopropylethynyl)naphthalene and 4-(cyclopropyle-
thynyl)-1,1′-biphenyl with 2b resulted in the desired products
6k (E/Z = 24:76) and 6l (E/Z = 15:85) in 60% and 76%
yields, respectively. When 3-(cyclopropylethynyl)thiophene
5m was applied to reaction with 2b, the desired product 6m
(E/Z = 12:88) was observed in 68% yield. The reaction of ((1-
methylcyclopropyl)ethynyl)benzene 5p with 2b could also give
the Z-6p in 36% yield. It is worth noting that CN-
functionalized (cyclopropylethynyl)benzene 5f resulted in 4-
((dihydrothiophen-2(3H)-ylidene)methyl)benzamide 6f in
48% yield with 100% (Z)-selectivity after further nitrile
hydrolysis.
Scheme 4. Plausible Mechanisms
•−
2 + 2] cycloaddition reaction, S3 in situ generated from K2S
attacks the alkyne 1a to generate the radical anion intermediate
A. Subsequently, the intermediate A adds to another molecular
1a to furnish the intermediate B. The intramolecular radical
coupling of B gave the [1 + 2 + 2] cycloaddition product 3a via
•−
S−S bond homolysis and releasing S2
.
13 For the [1 + 3 + 1]
cycloaddition reaction, S3•− is in situ formed by the mixture of
C
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•−
Brock, T. A.; Dixon, R. A. F. Discovery, Modeling, and Human
Pharmacokinetics of N-(2-Acetyl-4,6-dimethylphenyl)-3-(3,4-dime-
thylisoxazol-5-ylsulfamoyl)thiophene-2-carboxamide (TBC3711), a
Second Generation, ETA Selective, and Orally Bioavailable Endothelin
Antagonist. J. Med. Chem. 2004, 47, 1969. (b) Dore, K.; Dubus, S.;
Ho, H. A.; Levesque, I.; Brunette, M.; Corbeil, G. V.; Boissinot, M.;
Boivin, G.; Bergeron, M. G.; Boudreau, D.; Leclerc, M. Fluorescent
Polymeric Transducer for the Rapid, Simple, and Specific Detection
of Nucleic Acids at the Zeptomole Level. J. Am. Chem. Soc. 2004, 126,
4240. (c) Amspacher, D. R.; Blanchard, C. Z.; Fronczek, F. R.;
Saraiva, M. C.; Waldrop, G. L.; Strongin, R. M. Synthesis of a
Reaction Intermediate Analogue of Biotin−Dependent Carboxylases
via a Selective Derivatization of Biotin. Org. Lett. 1999, 1, 99.
(2) (a) Funahashi, M.; Zhang, F.; Tamaoki, N. High Ambipolar
Mobility in a Highly Ordered Smectic Phase of a Dialkylphenylter-
thiophene Derivative That Can Be Applied to Solution−Processed
Organic Field−Effect Transistors. Adv. Mater. 2007, 19, 353.
(b) Koumura, N.; Wang, Z.- S.; Mori, S.; Miyashita, M.; Suzuki, E.;
Hara, K. Alkyl-Functionalized Organic Dyes for Efficient Molecular
Photovoltaics. J. Am. Chem. Soc. 2006, 128, 14256. (c) Yasuda, S.;
Yoshida, S.; Sasaki, J.; Okutsu, Y.; Nakamura, T.; Taninaka, A.;
Takeuchi, O.; Shigekawa, H. Bond Fluctuation of S/Se Anchoring
Observed in Single−Molecule Conductance Measurements using the
Point Contact Method with Scanning Tunneling Microscopy. J. Am.
Chem. Soc. 2006, 128, 7746.
(3) (a) Lipshutz, B. H. Five-membered heteroaromatic rings as
intermediates in organic synthesis. Chem. Rev. 1986, 86, 795.
(b) Rassu, G.; Zanardi, F.; Battistini, L.; Casiraghi, G. The synthetic
utility of furan-, pyrrole- and thiophene-based 2-silyloxy dienes. Chem.
Soc. Rev. 2000, 29, 109. (c) Zambianchi, M.; Maria, F. D.; Cazzato, A.;
Gigli, G.; Piacenza, M.; Sala, F. D.; Barbarella, G. Microwave−
Assisted Synthesis of Thiophene Fluorophores, Labeling and
Multilabeling of Monoclonal Antibodies, and Long Lasting Staining
of Fixed Cells. J. Am. Chem. Soc. 2009, 131, 10892. (d) Yu, H.-H.;
Pullen, A. E.; Buschel, M. G.; Swager, T. M. Charge−Specific
Interactions in Segmented Conducting Polymers: An Approach to
Selective Ionoresistive Responses. Angew. Chem., Int. Ed. 2004, 43,
3700. (e) Vriezema, D. M.; Hoogboom, J.; Velonia, K.; Takazawa, K.;
Christianen, P. C. M.; Maan, J. C.; Rowan, A. E.; Nolte, R. J. M.
Vesicles and Polymerized Vesicles from Thiophene−Containing
Rod−Coil Block Copolymers. Angew. Chem., Int. Ed. 2003, 42, 772.
(4) (a) Roncali, J. Synthetic Principles for Bandgap Control in
Linear π-Conjugated Systems. Chem. Rev. 1997, 97, 173. (b) Mitschke,
U.; Bauerle, P. The electroluminescence of organic materials. J. Mater.
Chem. 2000, 10, 1471. (c) Perepichka, I. F.; Perepichka, D. F.; Meng,
H.; Wudl, F. Light−Emitting Polythiophenes. Adv. Mater. 2005, 17,
2281.
Na2S·9H2O in DMA. S3 attacks 5a to give the intermediate
C via radical ring-opening reaction. Intermediate C gets an
electron to give D. Intermediate D captures proton from water
or solvent to furnish the intermediate E. The tautomerization
of E gives F via H shift. The newly generated sulfur radical via
S−S bond homolysis and releasing S2•− adds to the C−C triple
bond formation of intermediate G. Intermediate G gets an
electron to lead to H. Intermediate H captures proton from
water or solvent to afford the desired [1 + 3 + 1] cycloaddition
reaction product 6a.
In summary, we have developed S3•− in situ generated from
S2−-mediated [1 + 2 + 2] and [1 + 3 + 1] cyclaoaddition
reactions with arylacetylene derivatives for the synthesis of
tetraphenylthiophene and 2-benzylidenetetrahydrothiophene
derivatives. These two protocols provide new, environmentally
friendly, and simple strategies to construct tetraphenylth-
iophene and 2-benzylidenetetrahydrothiophene derivatives via
•−
two C−S bond formations involving S3 under transition-
metal-free conditions. The presence of the trisulfur radical
anion was proven by EPR spectroscopy, and reasonable
mechanisms have been proposed.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02066.
■
S
Detailed experimental procedures and characterization
data (PDF)
Accession Codes
CCDC 1840038−1840039 contain the supplementary crys-
tallographic 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
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: shunyi@suda.edu.cn.
*E-mail: shunjun@suda.edu.cn.
(5) Suzuki, S.; Segawa, Y.; Itami, K.; Yamaguchi, J. Synthesis and
characterization of hexaarylbenzenes with five or six different
substituents enabled by programmed synthesis. Nat. Chem. 2015, 7,
227.
(6) Suzuki, S.; Itami, K.; Yamaguchi, J. Synthesis of Octaaryl
Naphthalenes and Anthracenes with Different Substituents. Angew.
Chem., Int. Ed. 2017, 56, 15010.
ORCID
Shun-Yi Wang: 0000-0002-8985-8753
Shun-Jun Ji: 0000-0002-4299-3528
Notes
The authors declare no competing financial interest.
(7) You, W.; Yan, X.-Y.; Liao, Q.; Xi, C.-J. Cu-Catalyzed Double S-
Alkenylation of Potassium Sulfide: A Highly Efficient Method for the
Synthesis of Various Thiophenes. Org. Lett. 2010, 12, 3930.
(8) Nakano, M.; Tsurugi, H.; Satoh, T.; Miura, M. Palladium-
Catalyzed Perarylation of 3-Thiophene- and 3-Furancarboxylic Acids
Accompanied by C−H Bond Cleavage and Decarboxylation. Org. Lett.
2008, 10, 1851.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the National Natural Science
Foundation of China (21772137, 21542015, and 21672157),
PAPD, the Project of Scientific and Technologic Infrastructure
of Suzhou (SZS201708), the Major Basic Research Project of
the Natural Science Foundation of the Jiangsu Higher
Education Institutions (No.16KJA150002), and Soochow
University for financial support. We thank Yue Fang in this
group for reproducing the results of 3a, 3b, 6a, 6f, and 6n.
−
(9) (a) Chivers, T. Ubiquitous trisulphur radical ion S3 ·. Nature
1974, 252, 32. (b) Chivers, T.; Drummond, I. Characterization of the
−
trisulfur radical anion S3 · in blue solutions of alkali polysulfides in
hexamethylphosphoramide. Inorg. Chem. 1972, 11, 2525.
(10) (a) Shields, T. C.; Kurtz, A. N. Active sulfuration of
norbornenes. J. Am. Chem. Soc. 1969, 91, 5415. (b) Zhang, G.-T.;
Yi, H.; Chen, H.; Bian, C.-L.; Liu, C.; Lei, A.-W. Trisulfur Radical
Anion as the Key Intermediate for the Synthesis of Thiophene via the
REFERENCES
■
(1) (a) Wu, C.-D.; Decker, E. R.; Blok, N.; Bui, H.; You, T. J.; Wang,
J.-M.; Bourgoyne, A. R.; Knowles, V.; Berens, K. L.; Holland, G. W.;
D
DOI: 10.1021/acs.orglett.8b02066
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
Interaction between Elemental Sulfur and NaOtBu. Org. Lett. 2014,
16, 6156. (c) Ding, K.-L.; Adam, P. Discussion on “The action of
elemental sulfur plus water on 1-octene at low temperatures” by Said-
Ahmad et al. Org. Geochem. 2015, 87, 133. (d) Gu, Z.-Y.; Cao, J.-J.;
Wang, S.-Y.; Ji, S.-J. The involvement of the trisulfur radical anion in
electron-catalyzed sulfur insertion reactions: facile synthesis of
benzothiazine derivatives under transition metal-free conditions.
Chem. Sci. 2016, 7, 4067. (e) Liu, W.-B.; Chen, C.; Liu, L. Synthesis
of Polysubstituted Thiophenes via Base−Induced [2 + 2+1]
Cycloaddition Reaction of Alkynes and Elemental Sulfur. Adv.
Synth. Catal. 2015, 357, 4050.
(11) Pinon, V.; Levillain, E.; Lelieur, J. P. ESR study of solutions of
sulfur in liquid ammonia. J. Phys. Chem. 1991, 95, 6462.
(12) According to the experimental results (see SI part 7 for more
details), 2-(phenylmethylene-d)tetrahydrothiophene-3,3-d2 8 was
generated through the following pathway.
(13) Based on the above results, we thought that the radical
intermediates’ stabilities contributed to the selectivity for the
formation of tetraphenylthiophene derivatives such as 3j′, 3m, and 3n.
E
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