Synthesis of Dibenzothiophene and 1,4-Dihydrodibenzothiophene Derivatives via Allylic Phosphonium Salt Initiated Domino Reactions

Article abstract of DOI:10.1021/acs.orglett.8b00028

Two efficient synthetic protocols were developed for the synthesis of dibenzothiophene and 1,4-dihydrodibenzothiophene using thioaurones and allylic phosphonium salt. Mild reaction conditions, a one-pot procedure, and easily accessible starting materials make these protocols powerful tools for the synthesis of these compounds, which are often used in material and pharmaceutical sciences.

Full text of DOI:10.1021/acs.orglett.8b00028

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synthesis of Dibenzothiophene and 1,4-Dihydrodibenzothiophene
Derivatives via Allylic Phosphonium Salt Initiated Domino Reactions
Ke Li,‡ Aimin Yu,‡ and Xiangtai Meng*
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry & Chemical Engineering, Tianjin
University of Technology, Tianjin 300384, P. R. China
S
* Supporting Information
ABSTRACT: Two efficient synthetic protocols were devel-
oped for the synthesis of dibenzothiophene and 1,4-
dihydrodibenzothiophene using thioaurones and allylic
phosphonium salt. Mild reaction conditions, a one-pot
procedure, and easily accessible starting materials make these
protocols powerful tools for the synthesis of these compounds,
which are often used in material and pharmaceutical sciences.
ibenzothiophene (DBT) and its derivatives are common
aromatic tricyclic compounds that naturally exist in coal tar
Scheme 1. Domino Reactions of Thioaurones and Their
Analogues
D
and occur widely in the heavier fractions of petroleum. DBTs are
usually key core motifs in many organic compounds, including
photoactive compounds,1 dyes,2 liquid crystals,3 and pharma-
ceuticals.4 Moreover, the DBT moiety is a building block for
selective estrogen modulators.5 There have been many studies
reported in the literature exploring different ways to synthesize
them.6 We surveyed the synthetic methods described in the
literature and found that there are generally two types of
synthesis of these compounds. The first uses biphenyl derivatives
and forms one or two S−C bonds.7 The second uses diaryl
sulfides and transition-metal catalysts and a dual C−H activation
process.8 To the best of our knowledge, few synthetic methods
use an annulation reaction to construct the second arene ring
under mild reaction conditions.
1,4-Dihydroarenes are an important structural motif often
found in pharmaceutical compounds and natural products.9 The
traditional synthetic approaches for producing them synthetically
include the Birch reduction and aryne Diels−Alder reactions.10
Ma and co-workers have reported a gold-catalyzed reaction of
allenes to synthesize 1,4-dihydroarene derivatives.11 Although
there are synthetic methods described, development of a mild,
efficient, and selective approach is still needed. During our
ongoing investigation of domino reactions, we synthesized a
series of thioaurones and their analogues.12 As shown in Scheme
1, the first [4 + 3] annulation of crotonate-derived sulfur ylides
was developed (Scheme 1a). Furthermore, when an ester group
was placed at the double bond, solvent-controlled switching
domino reactions were developed, producing a broad spectrum
of benzothiophene-fused pyran, 2,3-dihydrooxepine, and oxa-
tricyclodecene derivatives (Scheme 1b). Herein, we describe two
efficient allylic phosphonium salt initiated domino reactions
which afford a facile synthesis of dibenzothiophenes and 1,4-
dihydrodibenzothiophenes under mild conditions (Scheme 1c).
Thioaurone 1a and allylic phosphonium salt 2a were used as
starting materials, and DBU was used as a base in acetonitrile at
room temperature. No desired product 3a was obtained (Table
1, entry 1). Changing the solvent to ethanol and acetone did not
produce the desired product 3a either (entries 2 and 3). When
THF was used as the solvent, the desired product 3a was formed
in a yield of 36% (entry 4). The structure of 3a was confirmed
using NMR, HRMS, and single crystal X-ray diffraction. The
reaction conditions were optimized to improve the yield. After
screening several other solvents were screened, we found CHCl3
was the best solvent (entries 5−10). The inorganic base effect
was studied using bases such as tBuOK, EtONa, NaOH, K2CO3,
Cs2CO3, with no significant improvement of the yield compared
to DBU (entries 11−15). Furthermore, the organic bases
DABCO and DMAP were also studied. Only 31% and 16%,
Received: January 4, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b00028
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Optimization of the Reaction Conditions
Table 2. Substrates Scope of Reaction 1 and 2a
b
b
entry
R1
R2
time (min)
yield (%)
entry
solvent
base
DBU
time (h)
yield (%)
1
CH3CN
EtOH
acetone
THF
2
0
0
1
5-Cl (1a)
5-Cl (1b)
5-Cl (1c)
5-Cl (1d)
5-Cl (1e)
5-Cl (1f)
5-Cl (1g)
5-Cl (1h)
5-Cl (1i)
5-Cl (1j)
5-Cl (1k)
5-Cl (1l)
5-Cl (1m)
5-Cl (1n)
5-Cl (1o)
5-Cl (1p)
5-Cl (1q)
5-Br (1r)
5-F (1s)
H
60
60
65
60
60
45
40
45
30
65
45
60
70
60
80
70
60
60
45
70
83 (3a)
60 (3b)
80 (3c)
60 (3d)
63 (3e)
83 (3f)
57 (3g)
20 (3h)
20 (3i)
77 (3j)
61 (3k)
51 (3l)
95 (3m)
74 (3n)
78 (3o)
69 (3p)
48 (3q)
51 (3r)
63 (3s)
54 (3t)
2
DBU
1.5
1.5
1
2
4′-CH3
4′-OCH3
4′-Cl
3
DBU
0
3
4
DBU
36
42
40
38
54
0
4
5
toluene
DCE
DBU
1
5
4′-Br
6
DBU
1
6
4′-F
7
DCM
DBU
1
7
4′-CF3
4′-CN
4′-NO2
3′-CH3
3′-OCH3
3′-Cl
2′-CH3
2′-Cl
2′-Br
8
CHCl3
DMSO
DMF
DBU
1
8
9
DBU
2
9
10
11
12
13
14
15
16
17
DBU
2
0
10
11
12
13
14
15
16
17
18
19
20
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
tBuOK
EtONa
NaOH
K2CO3
Cs2CO3
DABCO
DMAP
DBU
2
0
1
14
36
20
42
31
16
60
63
47
83
1
2
1.5
1
1-naphthyl
2-thienyl
H
1
c
18
1
d
H
19
DBU
1
e
7-Cl (1t)
H
20
DBU
1
f
a
21
DBU
1
Reaction conditions: 2a (1.2 mmol), DBU (1.5 mmol) in solvent (2
a
mL), room temperature, 45 min, then 1 (0.3 mmol) was added and
the mixture stirred for a further 30−80 min at 61 °C. Isolated yields.
Reaction conditions: unless otherwise noted, 2a (0.4 mmol), base
b
(0.6 mmol) in solvent (1.5 mL), room temperature, 45 min, then 1a
(0.2 mmol) was added and stirred for a further 1−2 h at room
b
c
d
a
temperature. Isolated yields. 4 equiv of 2a was used. 5 equiv of
Table 3. Substrates Scope of Reaction 1 and 2b
e
DBU and 4 equiv of 2a were used. Reaction temperature was 0 °C.
f
Reaction temperature was 61 °C.
respectively, of the desired product 3a was obtained (entries 16
and 17). When the amount of 2a was increased to 4 equiv, the
yield of 3a was improved to 60% (entry 18). In addition, when
the amount of DBU was increased to 5 equiv, the yield of 3a was
slightly improved (entry 19). Final optimization of the reaction
temperature revealed that raising the temperature to 61 °C led to
a significant improvement in the yield of 3a to 83% (entries 20
and 21).
b
entry
R1
R2
time (min)
yield (%)
1
5-Cl (1a)
5-Cl (1b)
5-Cl (1c)
5-Cl (1d)
5-Cl (1e)
5-Cl (1f)
5-Cl (1g)
5-Cl (1h)
5-Cl (1j)
5-Cl (1k)
5-Cl (1l)
5-Cl (1n)
5-Cl (1o)
5-Cl (1u)
5-Cl (1q)
5-CH3 (1v)
5-Br (1r)
5-F (1s)
H
45
45
60
50
60
50
30
30
65
80
70
60
70
70
45
60
45
45
60
86 (4a)
81 (4b)
88 (4c)
74 (4d)
74 (4e)
79 (4f)
76 (4g)
66 (4h)
76 (4i)
80 (4j)
72 (4k)
79 (4l)
88 (4m)
92 (4n)
77 (4o)
92 (4p)
86 (4q)
72 (4r)
69 (4s)
2
4′-CH3
4′-CH3O
4′-Cl
4′-Br
4′-F
4′-CF3
4′-CN
3′-CH3
3′-CH3O
3′-Cl
2′-Cl
2′-Br
2′,4′-Cl
2-thienyl
H
3
4
5
Having established the optimal reaction conditions, a wide
range of different thioaurones 1 were explored. The results are
summarized in Table 2. The C5 chloro-substituted (on the blue
benzene ring) substrates were studied. For the red aromatic ring,
both electron-withdrawing and electron-donating groups were
substituted at the para-, meta-, ortho-positions, and the
corresponding products were produced in good-to-excellent
yields. For example, when methyl and methoxyl groups were
substituted onto the para position of the red benzene ring, the
corresponding products 3b and 3c were obtained in yields of
60% and 80%, respectively (entries 2 and 3). Halogen groups
were also substituted at the para position of the red benzene ring,
and the corresponding products 3d−f were formed in yields of
60% to 83% (entries 4−6). However, when the strong electron-
withdrawing groups CF3 (1g), CN (1h), and NO2 (1i) were
substituted, only yields of 20−57% of 3g−i were obtained
(entries 7−9). For the meta-substituted substrates, the results
were similar to those of the para-position substrates (entries 10−
12). Moreover, the ortho-substituted substrates were tolerated as
well, regardless of the electron-donating or electron-withdrawing
groups that were used (entries 13−15). The reactions with 1-
6
7
8
9
10
11
12
13
14
15
16
17
18
19
H
H
7-Cl (1t)
H
a
Reaction conditions: 2b (1.2 mmol), DBU (1.5 mmol) in solvent (2
mL), room temperature, 45 min, then 1 (0.3 mmol) was added and
the mixture stirred for a further 30−80 min at room temperature.
b
Isolated yields.
naphthyl and 2-thienyl also worked well, giving the desired
products 3p and 3q in yields of 69% and 48%, respectively
B
DOI: 10.1021/acs.orglett.8b00028
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Organic Letters
Letter
a
(entries 16 and 17). The substitution effect on the blue benzene
ring was also studied. We found that the reaction of substituted
thioaurone 1 with halogen groups at the C5 or C7 positions led
to the corresponding products in yields of 51−63% (entries 18−
20).
Table 4. Unexpected Reaction between 1 and 2b
In view of the above results, the studies were extended to allylic
phosphonium salt 2b. We hypothesized that the reaction of
thioaurones 1 with allylic phosphonium salt 2b would produce
1,4-dihydroarene derivatives. After careful optimization of the
reaction conditions (see the Supporting Information for details),
the best reaction conditions utilized thioaurones 1 (1 equiv), the
allylic phosphonium salt 2b (4 equiv), and DBU (5 equiv) as the
base at room temperature. A series of thioaurones 1 with allylic
phosphonium salt 2b was also examined using the optimized
reaction conditions. When R1 was a chloride at the C5 position, a
variety of R2 groups were investigated, including different
positions and different electronic properties. For example, when
R2 was located at the para position, electron-rich substrates
reacted better than electron-deficient substrates (Table 3, entries
2−8). Methyl- and methoxyl-substituted substrates reacted well
with 2b to give 4b and 4c in yields of 81% and 88%, respectively
(entries 2 and 3). In contrast, CF3- and CN-substituted
substrates were also used successfully, and the desired products
4d−h were produced in yields of 66−74% (entries 4−8). Similar
results were found when R2 was at the meta position (entries 9−
11). The efficiency of the reaction was not affected by steric
hindrance (entries 12 and 13). Furthermore, 2′,4′-dichloro (1u)
and 2-thienyl (1q) substrates also performed well and produced
the corresponding products 4n and 4o in 92% and 77% yields,
respectively (entries 14 and 15). In addition, when the R1 was a
methyl, bromo, or fluoro group at the C5 position or chloro at
the C7 position, the reaction also proceeded well, producing the
desired products in yields of 69−92% (entries 16−19).
b
entry
R1
R2
time (min)
yield (%)
1
2
3
4
5-Cl (1w)
5-Cl (1x)
5-Cl (1i)
5-Cl (1y)
2′-NO2
3′-NO2
4′-NO2
2-pyridyl
20
30
20
45
55 (5a)
47 (5b)
45 (5c)
67 (5d)
a
Reaction conditions: 2b (1.2 mmol), DBU (1.5 mmol) in solvent (2
mL), room temperature, 45 min, then 1 (0.3 mmol) was added and
the mixture stirred for a further 20−45 min at room temperature.
b
Isolated yields.
Scheme 2. Application of Our Reaction To Synthesize 7
Scheme 3. Synthetic Transformations
When R2 is a nitro group at the para (1i), meta (1x), and ortho
positions (1w), or with 2-pyridyl (1y) substituted substrates, the
reaction also proceeded with good yields. Surprisingly,
dibenzothiophene derivatives were produced instead of 1,4-
dihydroarene derivatives according to the NMR spectra (Table 4,
entries 1−4). Similar aromatizations have also been documented
using DBU as base.13 A possible mechanism for the formation of
5 is included in the SI.
To apply our method to material science, we synthesized 1z
and reacted it with 2b. The corresponding 1,4-dihydroarene
derivative 6 was obtained in a yield of 61%. Reaction 6 with DDQ
in PhCl formed compound 7 at 46% yield and can be used as an
organic semiconductor (Scheme 2). Moreover, two gram-scale
reactions were carried out. The desired products 3a and 4a were
obtained in yields of 82% and 86%, respectively (see the SI).
Treatment of compounds 3a with 2.2 equiv of m-CPBA in
CH2Cl2 at room temperature produced product 9 in a yield of
83%. However, when 1.05 equiv of m-CPBA was used,
compound 8 was obtained in a yield of 88% (Scheme 3a).
Similarly, oxidation of 4a in the presence of different amounts of
m-CPBA produced compounds 10 and 11 in yields of 83% and
91%, respectively (Scheme 3b).12 Finally, the reaction of 4b with
DDQ in PhCl under reflux produced dibenzothiophene
derivative 12 in a yield of 94% (Scheme 3c).
Scheme 4. Proposed Mechanism
The full mechanistic details of these reactions are yet to be
determined. A possible mechanism for these reactions, based on
our experimental results and some related literature, is shown in
Scheme 4. Initially, DBU accepts a proton from 2 to produce the
intermediate A. Intermediate B, which is the resonance structure
of A, subsequently reacts with 1a to give the intermediate C via a
C
DOI: 10.1021/acs.orglett.8b00028
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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structure of D, undergoes an intramolecular Wittig reaction to
form intermediate F. When 2a was used (n = 1), an aromatization
of F produced 3a. In contrast, when 2b was used (n = 0), an olefin
isomerization reaction of F produced 4a.
In conclusion, we have developed two domino reactions for
the efficient synthesis of dibenzothiophene and 1,4-dihydrodi-
benzothiophene derivatives. The reaction conditions are mild,
and a broad scope of substrates are tolerated. Future work will
include investigating the application of these reactions in the
synthesis of molecules to make materials and pharmaceuticals.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b00028.
́
Lopez-Casas, P. P.; Hidalgo, M.; de Pascual-Teresa, B.; Ramos, A. Org.
Biomol. Chem. 2008, 6, 3486. (b) Jepsen, T. H.; Larsen, M.; Jørgensen,
M.; Solanko, K. A.; Bond, A. D.; Kadziola, A.; Nielsen, M. B. Eur. J. Org.
Chem. 2011, 2011, 53.
Full experimental details, characterization, and NMR
spectra of all new compounds (PDF)
(6) Andrews, M. D. Sci. Synth. 2001, 10, 211.
Accession Codes
(7) (a) You, W.; Yan, X.; Liao, Q.; Xi, C. Org. Lett. 2010, 12, 3930.
(b) Tobisu, M.; Masuya, Y.; Baba, K.; Chatani, N. Chem. Sci. 2016, 7,
2587. (c) Pandya, V. B.; Jain, M. R.; Chaugule, B. V.; Patel, J. S.; Parmar,
B. M.; Joshi, J. K.; Patel, P. R. Synth. Commun. 2012, 42, 497. (d) Luo, B.;
Cui, Q.; Luo, H.; Hu, Y.; Huang, P.; Wen, S. Adv. Synth. Catal. 2016, 358,
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2010, 49, 4751. (f) Black, M.; Cadogan, J. I. G.; McNab, H. Org. Biomol.
Chem. 2010, 8, 2961.
CCDC 1811214−1811220 contain the supplementary crystallo-
graphic 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.
(8) (a) Che, R.; Wu, Z.; Li, Z.; Xiang, H.; Zhou, X. Chem. - Eur. J. 2014,
20, 7258. (b) Chen, J.; Murafuji, T.; Tsunashima, R. Organometallics
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: xtmeng@tjut.edu.cn.
ORCID
Angew. Chem., Int. Ed. 2011, 50, 5217. (e) Sanz, R.; Fernan
́
dez, Y.;
Castroviejo, M. P.; Perez, A.; Fananas, F. J. J. Org. Chem. 2006, 71, 6291.
́
́
̃
Xiangtai Meng: 0000-0003-2713-0078
Author Contributions
‡K.L. and A.Y. contributed equally.
Notes
(9) (a) Li, Y.; Tur, F.; Nielsen, R. P.; Jiang, H.; Jensen, F.; Jørgensen, K.
A. Angew. Chem., Int. Ed. 2016, 55, 1020. (b) Milde, B.; Reding, A.;
Geffers, F. J.; Jones, P. G.; Werz, D. B. Chem. - Eur. J. 2016, 22, 14544.
(c) Mohanakrishnan, A.; Ramalingam, B.; Moorthy, N.; Vellaichamy, E.
Synlett 2017, 28, 133.
(10) (a) Birch, A. J. Pure Appl. Chem. 1996, 68, 553. (b) Hook, J. M.;
Mander, N. L. Nat. Prod. Rep. 1986, 3, 35. (c) Rao, G. S. R. S. Pure Appl.
Chem. 2003, 75, 1443. (d) Urban, S.; Ortega, N.; Glorius, F. Angew.
Chem., Int. Ed. 2011, 50, 3803. (e) Kuwano, R.; Morioka, R.;
Kashiwabara, M.; Kameyama, N. Angew. Chem., Int. Ed. 2012, 51,
4136. (f) Dockendorff, C.; Sahli, S.; Olsen, M.; Milhau, L.; Lautens, M. J.
Am. Chem. Soc. 2005, 127, 15028. (g) Pellissier, H.; Santelli, M.
Tetrahedron 2003, 59, 701. (h) Bhunia, A.; Yetra, S. R.; Biju, A. T. Chem.
Soc. Rev. 2012, 41, 3140. (i) Gampe, C. M.; Carreira, E. M. Angew. Chem.,
Int. Ed. 2012, 51, 3766.
(11) Qiu, Y.; Zhou, J.; Li, J.; Fu, C.; Guo, Y.; Wang, H.; Ma, S. Chem. -
Eur. J. 2015, 21, 15939.
(12) (a) Zhang, Y.; Yu, A.; Jia, J.; Ma, S.; Li, K.; Wei, Y.; Meng, X. Chem.
Commun. 2017, 53, 10672. (b) Jia, J.; Yu, A.; Ma, S.; Zhang, Y.; Li, K.;
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Natural
Science Foundation of China (Grant No. 21403154), the Natural
Science Foundation of Tianjin (Grant No. 13JCYBJC38700),
and the Tianjin Municipal Education Commission (Grant No.
20120502). X.M. is grateful for the support from the 131 Talents
Program of Tianjin.
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