Green Preparation of Dibenzothiophene Derivatives Using 2-Biphenylyl Disulfides in the Presence of Molecular Iodine and Its Application to Dibenzoselenophene Synthesis

Article abstract of DOI:10.1002/ejoc.201701155

A protocol for the direct preparation of dibenzothiophenes from 2-biarylyl disulfides in the presence of an economic and ecological oxidant, molecular iodine, was explored. This protocol was used for the direct preparation of dibenzoselenophene.

Full text of DOI:10.1002/ejoc.201701155

A Journal of
Accepted Article
Title: Green Preparation of Dibenzothiophene Derivatives Using 2-
Biphenylyl Disulfides in the Presence of Molecular Iodine and Its
Application to Dibenzoselenophene Synthesis
Authors: Norio Sakai, Kota Nishino, and Yohei Ogiwara
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201701155
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10.1002/ejoc.201701155
European Journal of Organic Chemistry
COMMUNICATION
Green Preparation of Dibenzothiophene Derivatives Using 2-
Biphenylyl Disulfides in the Presence of Molecular Iodine and Its
Application to Dibenzoselenophene Synthesis
Kota Nishino, Yohei Ogiwara and Norio Sakai*
Abstract: Described herein is a novel protocol for the direct
preparation of dibenzothiophenes from 2-biaryl disulfides in the
presence of an economic and ecological oxidant, molecular iodine.
This protocol could be applied to the direct preparation of
dibenzoselenophene.
either an inexpensive metal-catalyst or a metal-free system
would be welcome. In this context, Shimizu group and Jiang
group independently demonstrated a double S-arylation system
via the reaction between a cyclic diaryliodonium salt and
external sulfur sources, such as AcSK and S8 (Scheme 1g).[8,9]
These procedures can allow to use moisture-stable, readily
available, and highly electrophilic iodonium salts as a starting
reagent, and therefore, the use of an expensive transition-metals
can be avoided.
Dibenzothiophene is
a versatile sulfur-based aromatic
tricyclic compound. It constitutes the main skeleton of valuable
organic compounds such as biologically active substances and
pharmaceutical drugs in organic and medicinal chemistry, and it
is a primary constituent of functional materials such as organic
semiconductors in materials chemistry.[1] Therefore, a facile and
efficient synthetic approach to various derivatives of this
valuable compound is in great demand. There are several
reliable procedures for the synthesis of dibenzothiophenes, but
most of them require the use of sulfur sources that have an
unpleasant odor, such as P4S10, and the substrates used in the
transformation are limited to a peculiar activated substrate that
undertakes either aromatic nucleophilic substitution (SNAr) or
aromatic electrophilic substitution (SEAr) (Scheme 1a, 1b).[2]
S
S
I
This work
(h)
OTf
SRX
Sulfur-Iodine
SNAr type (a)
exchange (g)
C-H/C-S
SEAr type (b)
coupling (f)
S
S
Me
C-H/C-H,
C-H/S-H
SR
C-H/C-H
O
coupling (e)
coupling (c)
C-H/C-X
coupling (d)
O
S
Over the past decade, work by many researchers has led to
significant improvements that involve the transition-metal-
catalyzed construction of dibenzothiophene via C–H bond
functionalization. For instance, Antonchick group reported the
preparation of dibenzothiophene via palladium-catalyzed double
C–H activation directed by sulfoxide (Scheme 1c).[3] Also,
palladium-catalyzed intramolecular C–H/C–X coupling using 2-
bromodiaryl sulfides lead to dibenzothiophenes were disclosed
(Scheme 1d).[4] Moreover, a dehydrogenative cyclization of
diaryl sulfides or diaryl sulfoxides with either palladium or
rhodium catalysts has been used to directly produce
dibenzothiophene frameworks (Scheme 1e).[5,6] Chatani, Tobisu
and co-workers reported the preparation of dibenzothiophene
via the unique cyclization of biphenyl sulfide, which involves C–
H/C–S coupling using a palladium catalyst (Scheme 1f).[7]
However, these procedures generally require the use of
expensive and precious metal-catalysts. Therefore, the
development of a facile construction of dibenzothiophene by
X
X
X = S, SO
S
Scheme 1. Various approach to the preparation of dibenzothiophene.
Unlike thiols, disulfides have an air-stable and odorless
sulfur source and have been utilized as important and
convenient precursors for the synthesis of sulfur-containing
molecules.[10] Cleavage of the S–S bond of a disulfide is
regarded as a key step in beginning a common reaction using a
disulfide. Although metal catalyst initiators that undertake S–S
bond cleavage have been reported,[11,12] an oxidative cleavage
of the S–S bond by molecular iodine (I2) has attracted much
attention because I2 is a reagent that is economical, easy to
handle, and environmentally friendly.[13,14] Herein, we report the
use of 2-biphenylyl disulfides with I2 as an oxidant (Scheme 1h)
in an unprecedented preparation of dibenzothiophenes
possessing various functional groups. We also disclose a novel
preparation of dibenzoselenophene.
[*]
Kota Nishino, Dr. Y. Ogiwara, Prof. Dr. N. Sakai
Department of Pure and Applied Chemistry, Faculty of Science and
Technology, Tokyo University of Science (RIKADAI), Noda, Chiba
278-8510 (Japan)
We selected 2-biphenylyl disulfide (1a: 0.5 mmol) as a model
substrate to achieve our purpose (Table 1). Substrate 1a was
prepared by S-arylation of 2-iodobiphenyl and a sulfur source
with CuI. Our initial attempt was performed using Na2CO3 (1
equiv) and I2 (2 equiv) in toluene at 100 °C for 12 h. When
disulfide 1a was treated with 5 mol % of PdCl2 in the hope of an
S–S bond cleavage and a subsequent C–S bond formation, the
E-mail: sakachem@rs.noda.tus.ac.jp
http://www.rs.tus.ac.jp/sakaigroup/
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejoc.201701155
European Journal of Organic Chemistry
COMMUNICATION
Table 1. Examinations of the reaction conditions.[a]
Table 2. Scope and limitations of dibenzothiophene derivatives.[a]
R
Metal cat. (5 mol %)
Na2CO3 (1 equiv)
oxidant (2 equiv)
1
I2 (2 equiv)
2
R
Na2CO3 (1 equiv)
S
S
S
S
3
toluene, 100 °C, 12 h
toluene, 100 °C, 12 h
S
4
S
R
1a
2a
1
2
Entry
Metal
PdCl2
oxidant
Conversion [%][b]
Yield [%][b]
78
Entry
1
product
Yield [%]
70
1
I2
87
>99
>99
19
>99
54
45
6
1-Me (2b
)
2
CuCl
I2
94
2
1-MeO (2c
3-Me (2d
3-MeO (2e
)
)
82
-
I2
92 (88)
3
3
)
86
4[c]
-
I2
0
4
87
5[d]
6
-
I2
71
0
5
2-MeO (2f)/4-MeO (2f’
3-tBu (2g
3-Ph (2h
)
70 (3.5:1)[b]
85
-
TBHP
6
)
7
-
DDQ
27
0
7
)
43
8
-
PhI(OAc)2
8
3-OPh (2i
)
)
23 (72)[c,d]
27 (50)[c,d]
7[c] (25)[c,d]
8[c] (24)[c,d]
0 [c]
9
-
Cu(OAc)2
8
0
9
3-OBn (2j
10
11
12[e]
13[f]
14[g]
-
-
4
0
10
11
12
13
3-F (2k
)
CuCl
-
5
0
3-Cl (2l
)
-
-
-
I2
I2
I2
68
36
37
52
27
28
3-CF3 (2m
)
75[d,e,f]
(
2n)
[a] Reaction conditions: 2-biphenylyl disulfide 1 (0.5 mmol), Metal (0.025
mmol), Na2CO3 (0.5 mmol), oxidant (1.0 mmol) in toluene (0.5 mL) at 100
˚C for 12 h. [b] GC (Isolated) yield. [c] Et3N was used instead of Na2CO3.
[d] Without Na2CO3. [e] In DMF. [f] Temperature at 60 ˚C. [g] Use of 1
equiv. of I2.
S
[a] Reaction conditions: 1 (0.2 mmol), I2 (0.4 mmol) and Na2CO3 (0.2 mmol)
in toluene (0.2 mL) at 100 ˚C for 12 h. [b] The ratio (2f/2f’ = 3.5/1) of the
regioselectivity was determined by 1H NMR. [c] The reaction was carried out
for 24 h. [d] 5 mol % of CuCl was added. [e] toluene (0.4 mL). [f] NMR yield.
desired heterocycle dibenzothiophene (2a) was obtained in a
78% yield (entry 1). Also, when CuCl, which is often used in
typical C–S coupling reactions, was used, the reaction
proceeded quantitatively (entry 2). Interestingly, product 2a was
given quantitatively without metal catalysts (entry 3). Therefore,
the use of a metal catalyst is not essential to undertake the
present reaction. Another base, Et3N, and conditions without
bases were then screened, but those treatments were ineffective
for this reaction (entries 4 and 5). Also, with the exception of I2,
oxidants such as TBHP, DDQ, PhI(OAc)2, and Cu(OAc)2, did not
improve the yield of 2a (entries 6-9). Under oxidant-free
conditions, the formation of 2a was not observed (entries 10 and
11). Consequently, I2 is the essential reagent for oxidative
cyclisation. Either the use of DMF, instead of toluene, or a
lowering of the reaction temperature to 60 °C led to a decrease
in the yield (entries 12 and 13). Moreover, a decrease in I2 to 1
equiv per 1a resulted in a lower yield (entry 14). On the other
hand, when the standard reaction was treated with molecular
bromine (2 equiv), the product yield decreased to 36% (GC
yield).
With the optimal conditions in hand, we examined the
substrate scope of dibenzothiophene derivatives (Table 2). 2-
Biphenylyl disulfides bearing an electron-donating group, such
as a methyl, a methoxy and a tert-butyl substituent, produced
the corresponding products 2b-2e and 2g in excellent yields.
When the cyclization of m-methoxy-substituted biphenylyl
disulfide was examined, 2-methoxydibenzothiophene (2f) and
the regioisomer 4-methoxydibenzothiophene (2f’) were obtained
in high yields. Also, cyclization of a disulfide with a phenyl group
produced dibenzothiophene derivative 2h in moderate yield. In
contrast, each cyclization of disulfide derivatives bearing a
phenoxy, a benzyloxy or an electron-withdrawing group, such as
a fluoride or a chloride, proceeded in a rather low conversion
(entries 8-11). Gratifyingly, for these substrates, when an
addition of 5 mol % of CuCl and an extension of the reaction
time (24 h) were simultaneously examined, the yields of the
corresponding dibenzothiophenes 2i-2l were substantially
improved. In contrast, when the reaction with a 2-biaryl disulfide
bearing
a
trifluoromethyl group was carried out, the
corresponding heterocycle, 3-trifluoromethyl dibenzothiophene
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10.1002/ejoc.201701155
European Journal of Organic Chemistry
COMMUNICATION
(2m), was not observed. Due to the fact that cyclizations of
substrates with an electron-withdrawing group resulted in rather
low yields, we surmised that a series of cyclizations would
proceed via a SEAr-type pathway. When the biaryl disulfide with
a naphthyl moiety was reacted with I2, a tetracyclic heterocycle
benzo[b]naphtho[1,2-d]thiophene (2n) was afforded in a 75%
yield.
I2 (2 equiv)
Na2CO3 (1 equiv)
Se Se
toluene, 100 °C, 12 h
Se
4: 84%
Me
3: (0.2 mmol)
Me
Me
I2 (2 equiv)
Na2CO3 (1 equiv)
Scheme 4. Application to the preparation of dibenzoselenophene
S
S
Me
toluene, 100 °C, 12 h
S
Me
Further application of the present method of preparing a
selenium-containing heterocycle was attempted (Scheme 4).
When 2-biphenylyl diselenide (3) was treated with the I2-Na2CO3
system, a similar cyclization reaction proceeded smoothly to
provide dibenzoselenophene (4) in an 84% yield.[15] It was found
that I2 promoted the cleavage of a Se–Se bond and the
subsequent intramolecular formation of an Ar–Se bond.
2o: 68%
Me
1o: (0.2 mmol)
Cl
Cl
I2 (2 equiv)
Na2CO3 (1 equiv)
S
S
toluene, 100 °C, 12 h
S
2p: 77%
I2 (2 equiv)
Na2CO3 (1 equiv)
Cl
1p: (0.2 mmol)
toluene, 100 °C, 12 h
S
SR
2a
R = H (5: 0.5 mmol)
R = H: 94%
Scheme 2. Scope of unsymmetrical substituted dibenzothiophene derivatives
Me (6: 0.2 mmol)
Me: 0%
Scheme 5. The examination of the synthesis of dibenzothiophene from 2-
phenylbenzenethiol (5) or 2-biphenylyl methyl sulfide (6)
In addition, 2,7-dimethyldibenzothiophene (2o), bonding two
unsymmetrical methyl groups, was also given from dimethyl
substituted disulfide 1o in a good yield (Scheme 2). It is
noteworthy that although the yield of 3-chlorodibenzothiophene
(2l) was rather low, dibenzothiophene 2p bearing a chloride
group at the 2-position was obtained in a high (77%) yield.
These results prove that the electronic effect of a substituent on
the benzene ring bonded directly to the disulfide moiety had little
effect on a subsequent intramolecular aromatic electrophilic
substitution (SEAr).
Based on recent reports involving C–H/C–S coupling using
a transition metal catalyst, we attempted the preparation of
dibenzothiophene from either biphenyl thiol or biphenyl methyl
sulfide using the I2–Na2CO3 system (Scheme 5). When the
cyclisation of 2-phenylbenzenethiol (5) was treated with I2,
dibenzothiophene (2a) was obtained in a 94% yield. In contrast,
the use of 2-biphenylyl methyl sulfide (6) did not afford product
2a. These results imply that I2 has an effect on the activation of
both a S–H and a S–S bond, but not on the activation of a C(sp3)
–S bond.
Without CuCl, 12 h:
(57%, 881 mg, 4.2 mmol scale)
Without CuCl, 36 h:
(85%, 2162 mg, 6.8 mmol scale)
I2 (2 equiv)
Na2CO3 (1 equiv)
1a
2a
toluene, 100 °C, time
I2
With CuCl (5 mol %), 12 h:
(93%, 1721 mg, 5.0 mmol scale)
2
S
S
S
I
Scheme 3. Gram-scale preparation of a dibenzothiophene
A
1
The gram-scale synthesis of dibenzothiophene (2a) was
then examined (Scheme 3). When 2-biphenylyl disulfide (1a:
2520 mg, 6.8 mmol) was treated with I2 and Na2CO3 for 12 h,
dibenzothiophene (2a) was obtained in a 57% yield (881 mg).
When prolonging the reaction time to 36 h, the product yield was
further improved to an 85% yield (2162 mg). Also, the addition of
CuCl (5 mol %) to the reaction system dramatically improved the
yield to 93% (1721 mg). We anticipate that in a series of reaction
process the capper(I) catalyst would promote a cleavage step of
the S-S bond of the starting disulfide and a subsequent
cyclization step.
S
S
base
base-H+I-
H
2
B
I
Scheme 6. Plausible reaction pathway
On the bases of the above results, a plausible mechanism
for the cyclization is shown in Scheme 6. Initially, the S–S bond
of a 2-biphenylyl disulfide is oxidatively cleaved by I2 to form
intermediate A with a S–I bond that is highly reactive due to its
high level of polarity.[16] Then, a tricyclic intermediate B is formed
by intramolecular SEAr to the S–I bond. Finally, the
deprotonation of B by a base produced the corresponding
dibenzothiophene.
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10.1002/ejoc.201701155
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COMMUNICATION
[10]
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We have demonstrated an iodine-mediated cyclization of 2-
biaryl disulfides possessing various substituted groups, which
leads to the direct preparation of dibenzothiophenes. This
synthetic process is a new protocol for the preparation of
dibenzothiophenes and dibenzoselenophene. By comparison
with the conventional approaches to the synthesis of
dibenzothiophene, this novel approach features (i) transition
metal-free conditions, and (ii) the use of inexpensive and
environmentally friendly I2 as an oxidant. The reaction pathway
proceeded via an oxidative cleavage of the S–S bond of 2-
biphenylyl disulfides by I2 and a subsequent C–S bond formation
via SEAr.
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Experimental Section
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To a freshly distilled toluene solution (0.2 mL) in a screw-capped test
tube under a nitrogen atmosphere were successively added a magnetic
stirrer bar, 2-biphenylyl disulfide 1 or 2-biphenylyl diselenide (3) (0.20
mmol), Na2CO3 (0.20 mmol, 21 mg) and molecular iodine (0.400 mmol,
102 mg). The test tube was sealed with a cap that contained a PTFE
septum and was heated to 100 °C for 12 h. After the reaction, the
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reaction resultant mixture was quenched by
a saturated Na2S2O3
aqueous solution (3 mL). The aqueous layer was extracted with ethyl
acetate (3 mL x 3). The combined organic phase was dried over
anhydrous Na2SO4, filtered, and then evaporated under reduced
pressure. The crude material was purified by column chromatography
(hexane as an eluent) to give dibenzothiophene derivative
dibenzoselenophene (4).
2 or
Keywords: dibenzothiophene • iodine • disulfide •
dibenzoselenophene • green preparation
[15]
[16]
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10.1002/ejoc.201701155
European Journal of Organic Chemistry
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
Cyclization
K. Nishino, Y. Ogiwara, and N. Sakai*
R
R
R
R
R
I2
Page No. – Page No.
S
S
S
15 examples
up to 88% yield
Green Preparation of
R
dibenzothiophene Derivatives Using
2-Biphenylyl Disulfides in the
Presence of Molecular Iodine and Its
Application to Dibenzoselenophene
Synthesis
One-pot preparation of dibenzothiophene derivatives by a green method:
Preparation of dibenzothiophenes from 2-biphenylyl disulfides using molecular
iodine as an oxidant. This protocol could lead to a direct preparation of
dibenzoselenophene.
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