Preparation of Benzothiophenes and Benzoselenophenes from Arylamines and Alkynes via Radical Cascade Reactions

Article abstract of DOI:10.1002/adsc.201501102

An intermolecular radical cascade reaction between readily prepared o-methylthio-arylamines or o-methylselanyl-arylamines and alkynes for the preparation of valuable benzothiophenes or benzoselenophenes is reported. These transformations occur efficiently with complete regioselectivity and the products are obtained in moderate to good yields. The current protocol is successfully applied to the synthesis of the key intermediates of the drug raloxifene and an AT₁receptor antagonist.

Full text of DOI:10.1002/adsc.201501102

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DOI: 10.1002/adsc.201501102
Preparation of Benzothiophenes and Benzoselenophenes from
Arylamines and Alkynes via Radical Cascade Reactions
Hao Zang,a,b Jian-Guo Sun,a Xin Dong,a Ping Li,a,* and Bo Zhanga,*
a
State Key Laboratory of Natural Medicines, China Pharmaceutical University, 24 Tongjia Xiang, Nanjing 210009, Peopleꢀs
Republic of China
Fax: (+86)-25-8327-1382; phone: (+86)-25-8327-1382; e-mail: liping2004@126.com or zb3981444@cpu.edu.cn
College of Pharmaceutical and Food Science, Tonghua Normal University, 950 Yucai Lu, Tonghua 134002, Peopleꢀs
b
Republic of China
Received: December 2, 2015; Revised: March 12, 2016; Published online: April 25, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201501102.
Abstract: An intermolecular radical cascade reac-
tion between readily prepared o-methylthio-aryla-
mines or o-methylselanyl-arylamines and alkynes
for the preparation of valuable benzothiophenes or
benzoselenophenes is reported. These transforma-
tions occur efficiently with complete regioselectivity
and the products are obtained in moderate to good
yields. The current protocol is successfully applied
to the synthesis of the key intermediates of the
drug raloxifene and an AT1 receptor antagonist.
diꢀs group[9a] and McDonaldꢀs group[9b] independently
reported the synthesis of benzothiophenes from o-
methylthio-arenediazonium salts and alkynes via radi-
cal cascade reactions. Moreover, Schiesser and co-
workers described a similar sequence for the prepara-
tion of benzoselenophenes from diazonium salts
[Scheme 1A, Eq. (1)].[9c] However, all of these meth-
ods require the introduction of stoichiometric
amounts of transition metals. To overcome this draw-
back, Kçnig and co-workers recently demonstrated
a metal-free synthesis of benzothiophenes by reacting
diazonium salts with alkynes using visible-light photo-
redox catalysis [Scheme 1A, Eq. (2)].[9d] Despite ad-
vances in synthetic methodology, the use of diazoni-
Keywords: arylamines; benzoselenophenes; benzo-
thiophenes; one-pot processes; radical reactions
Benzothiophene and its derivatives represent a highly
important and valuable class of heterocyclic com-
pounds widely present in many medicinally relevant
molecules. For example, raloxifene,[1] arzoxifene,[2]
and zileuton[3] are all active drugs on the market. On
the other hand, the structural motif of benzothio-
phene can be found in various bioactive molecules,
which exhibit prominent bioactivities (Figure 1).[4] In
addition, benzothiophenes have found wide applica-
tion in material science.[5] Accordingly, a variety of
synthetic methods for the preparation of substituted
benzothiophenes have been developed.[6] Most of
these approaches mainly focus on electrophilic cycli-
zation and coupling cyclization reactions to construct
the benzothiophene core.[7,8] However, the develop-
ment of simple and novel synthetic methods for
a facile access to benzothiophenes is still highly desir-
able.
In recent years, the preparation of benzothiophenes
via radical cascade reactions has attracted considera-
ble attention since it offers a simple and efficient ap-
Figure 1. Representative drugs and biologically active mole-
proach to construct the benzothiophene ring.[9] Zanar- cules containing benzothiophene motif.
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itiator and t-BuONO as a nitrosating agent in DMSO
at room temperature under N2 for 12 h. The targeted
product 3aa was isolated in 41% yield (Table 1,
entry 1). Encouraged by this promising result, we next
screened a range of organic initiators under the same
conditions. The results showed that KI, (n-Bu)4NBr,
BPO, and ascorbic acid gave similar yields (Table 1,
entries 2–5). Notably, when this reaction was run
without an initiator, a similar result was obtained
(Table 1, entry 6). It is shown that an initiator is not
essential for this transformation. Alternative nitrosat-
ing agents, such as TABNO2, amylONO, and NaNO2
provided worse results (Table 1, entries 7–9). Differ-
ent solvents were subsequently surveyed, and we
found that CH3NO2 is the best solvent for running
this reaction (Table 1, entry 12). The yield of 3aa was
significantly improved to 57% by using 2.0 equiv. of t-
BuONO (Table 1, entry 13). Further investigation
showed that a higher reaction temperature can pro-
mote this transformation, and 3aa was obtained in
70% yield at 808C (Table 1, entry 15). It is notewor-
thy that the amount of 2a was further lowered to
3.0 equiv. without affecting the yield, but a lower
yield was achieved with 2.0 equiv. of 2a (Table 1, en-
tries 16 and 17). When the reaction was carried out in
a non-dry solvent, 3aa was obtained in 63% yield
(Table 1, entry 18).
Scheme 1. Strategies for the preparation of 2-substituted
benzothiophenes or benzoselenophenes via radical cascade
reactions.
With the optimized reaction conditions in hand, we
next explored the scope of the benzothiophene syn-
thesis with regard to o-methylthio-arylamines
um salts as aryl radical precursors may limit the fur- (Scheme 2). o-Methylthio-arylamines bearing elec-
ther application of such a method, because not every tron-donating substituents (methyl, methoxy) under-
arylamine can be easily converted to the correspond- went this transformation smoothly to afford the corre-
ing aryldiazonium salt. In addition, isolation or stor- sponding products in moderate yields (41–47%).
age of diazonium salts is problematic because of their Moreover, o-methylthio-arylamines carrying halogen
limited stability.
substituents were compatible with the reaction condi-
To avoid the preparation of diazonium salts and to tions, providing the corresponding products in moder-
enhance overall reaction efficiency, we envisioned ate to good yields (48–55%). Notably, the halogen
that the preparation of benzothiophenes might be re- substituents such as Cl and Br are useful for further
alized by combining the diazotization and cyclization functionalization. O-Methylthio-aniline worked well
steps in a one-pot protocol. The advantage for the use and gave the desired product 3ja in 48% yield.
of arylamines as aryl radical precursors is that aryl-
We next examined the scope of the reaction with
amines are easy to obtain and are stable. Herein, we respect to the alkynes (Scheme 2). Phenylacetylene
describe a simple and efficient method for the prepa- derivatives bearing electron-donating substituents at
ration of valuable benzothiophenes starting with read- the para-position of the arene provided the corre-
ily available arylamines and alkynes via radical cas- sponding products 3ab (4-Me) and 3ac (4-OMe) in ex-
cade reactions. It is important to note that the current cellent yields (3ab: 68%, 3ac: 71%). The ortho- and
protocol can also be applied to the preparation of meta-substituted congeners gave similar results (3ah:
benzoselenophenes (Scheme 1B).
65%, 3ai: 65%). Phenylacetylene derivatives bearing
Recently, several organic molecules such as BPO,[10] halogen substituents such as F, Cl, and Br performed
iodide,[11] ascorbic acid,[12] and porphyrin[13] have been well with excellent yields (3ad: 70%, 3ae: 68%, 3af:
identified as efficient initiators to generate aryl radi- 71%). The substrate containing an electron-withdraw-
cals from the corresponding diazonium salts. Inspired ing substituent such CF3 was successfully converted
by these successful examples, our investigation com- into the corresponding product 3ag in 75% yield. A
menced with the radical cascade reactions of commer- heteroaryl-substituted alkyne, 3-ethynylthiophene af-
cially available o-methylthio-arylamine 1a with phe- forded the product 3aj in 60% yield. In addition,
nylacetylene 2a in the presence of (n-Bu)4NI as an in- alkyl, ester, and TMS-substituted alkynes were
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Table 1. Optimization of the reaction conditions.[a]
Entry
Initiator
Nitrosating agent (equiv.)
Amount of 2a (equiv.)
Solvent
Yield [%][b]
1
2
3
4
5
6
7
8
(n-Bu)4NI
KI
(n-Bu)4NBr
BPO
ascorbic acid
none
none
none
none
none
none
none
none
none
none
none
none
none
t-BuONO (1.5)
t-BuONO (1.5)
t-BuONO (1.5)
t-BuONO (1.5)
t-BuONO (1.5)
t-BuONO (1.5)
TBANO2 (1.5)
amylONO (1.5)
NaNO2 (1.5)
t-BuONO (1.5)
tBuONO (1.5)
t-BuONO (1.5)
tBuONO (2.0)
t-BuONO (2.0)
t-BuONO (2.0)
t-BuONO (2.0)
t-BuONO (2.0)
t-BuONO (2.0)
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
3
2
3
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
CH3CN
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
CH3NO2
41
44
40
44
48
46
0
33
trace
40
40
48
57
62
70
70
62
63
9
10
11
12
13
14[c]
15[d]
16[d]
17[d]
18[e]
[a]
Reaction conditions: 1a (0.4 mmol), 2a (2.0 mmol), initiator (0.04 mmol), and nitrosating agent (0.6 mmol) in dry solvent
(1.0 mL) at room temperature under N2 for 12 h.
Isolated yields.
The reaction was conducted at 608C.
The reaction was conducted at 808C.
[b]
[c]
[d]
[e]
The commercial CH3NO2 (1.0 mL) was directly used. BOP=benzoyl peroxide, TBANO2 =tetrabutylammonium nitrite.
proved to be competent, leading to the products 3ak– easily prepared by using this metal-free route.[9b,d] We
3an in moderate to good yields (30–55%). prepared 1c from commercially available 2-amino-6-
Benzoselenophenes are also an important class of methoxybenzothiazole 1c’ and reacted it with alkyne
heterocycles, which have found wide application in or- 2c under our conditions to afford the benzothiophene
ganic synthesis, medicinal chemistry, and materials 6 in 50% yield (Scheme 4, a). In addition, this method
science.[9c,14] In light of this importance, we subse- can also be successfully applied to the synthesis of the
quently extended the present protocol to prepare ben- key intermediate 8 of AT1 receptor antagonist.[9c] The
zoselenophenes starting with readily prepared o- o-methylselanyl-aniline 4a was easily prepared in two
methylselanyl-arylamines and alkynes under metal- steps from 1-fluoro-2-nitrobenzene 4a’, which reacted
free conditions (Scheme 3). Reactions with o-methyl- with alkyne 2o to form the benzoselenophene 8 in
selanyl-arylamines carrying electron-donating and 45% (Scheme 4, b).
electron-withdrawing substituents proceeded well,
To gain mechanistic insights into the reaction path-
and the corresponding products 5aa–5da were ob- way, some preliminary studies were conducted. Radi-
tained in good yields (50–60%). Moreover, a variety cal trapping experiment was conducted by using
of alkynes bearing different substituents were tested. 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) as
We found that electron-donating (methyl, methoxy) a radical trap, and the TEMPO-trapped product 11
and also electron-withdrawing substituents (halides) could be identified by high-resolution mass spectrom-
were well tolerated, affording the corresponding etry [Scheme 5a, Eq. (1)]. This result suggested that
products 5db–5df in good yields (57–60%). With het- aryl radicals are likely involved in these reactions. In
eroaryl, alkyl, and TMS substituents on the alkynes, addition, the o-methylthio-benzenediazonium salt was
moderate yields were obtained (5dj: 43%, 5dl: 36%, prepared independently and used under our standard
5dn: 34%).
conditions, affording traces of the targeted product
The current method provides many opportunities 3ja. This result indicated that diazonium salts as the
for application to organic synthesis. For example, the key intermediates in the reaction mechanism appear
key intermediate 6 of the raloxifene synthesis can be rather unlikely [Scheme 5a, Eq. (2)]. Moreover, the
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Scheme 2. Various 2-substituted benzothiophenes prepared.
Reaction conditions: see entry 16, Table 1. The yields given
are for the isolated products.
Scheme 4. Synthetic applications.
reactions of 2-(ethylthio)aniline 13 and 2-(phenyl-
thio)aniline 14 were investigated. When the reaction
was conducted on 2-(ethylthio)aniline 13, the product
3ja was obtained in 45% yield [Scheme 5a, Eq. (3)].
When the reaction was conducted on 2-(phenylthio)-
aniline 14, the product 3ja was obtained in 33% yield
[Scheme 5a, Eq. (4)]. In this reaction, the product 15
that was likely generated via intramolecular cycliza-
tion could not be observed. In addition, benzene
could be detected by GC-MS from reaction mixture.
On the basis of the above experiments and previous
reports,[9a,d,15,16] a plausible mechanism is proposed in
Scheme 5b. First, arylamine 1 or 4 is transformed with
t-BuONO to the corresponding nitrosamine I, which
Scheme 3. Various 2-substituted benzoselenophenes pre-
pared. Reaction conditions: see entry 16, Table 1. The yields
given are for the isolated products.
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undergoes self-condensation to generate diazo anhy-
dride II. It is important to note that the formation of
this type of dimeric diazo anhydride structure is well
established in the literature.[16–18] N O homolysis of II
À
provides aryl radical III along with azoxy radical IV
and nitrogen. Addition of III to alkyne 2 leads to the
vinyl radical V, which reacts via intramolecular homo-
lytic substitution at the sulfur atom or selenium atom
to form the final products 3 or 5 along with a R3 radi-
cal.[19] The R3 radical can further react via H-abstrac-
tion from the solvent. In addition, the generated
azoxy radical IV abstracts a hydrogen atom from the
solvent to afford the diazo hydroxide I’ (the species I
and I’ are in equilibrium and can thus be readily inter-
converted),[16c] which can further react to generate the
aryl radical III.
In summary, we have developed a simple, efficient,
and unified strategy for the preparation of benzothio-
phenes and benzoselenophenes from readily prepared
starting materials. Notably, these transformations
occur efficiently without the help of any transition
metal or additive. Reactions are very easy to conduct
and products are obtained in moderate to good yields
with a good functional group tolerance. The applica-
tions of the method are demonstrated by the synthesis
of the key intermediates of the drug raloxifene and
AT1 receptor antagonist. Further studies on the scope,
mechanism, and synthetic application are ongoing in
our laboratory.
Experimental Section
General Procedure for the Preparation of Benzo-
thiophenes
o-Methylthio-arylamines
1
(0.4 mmol, 1.0 equiv.) were
placed in a dry standard Schlenk tube or a dry sealed tube
under N2. Dry CH3NO2 (1.0 mL) was added, followed by
the addition of alkynes 2 (1.2 mmol, 3.0 equiv.) and t-
BuONO (0.8 mmol, 2.0 equiv.). The reaction mixture was
stirred at 808C for 12 h, and the reaction was monitored
with TLC. The crude reaction mixture was purified by flash
column chromatography on silica gel to afford the corre-
sponding product.
General Procedure for the Preparation of Benzo-
selenophenes
o-Methylselanyl-arylamines 4 (0.4 mmol, 1.0 equiv.) were
placed in a dry standard Schlenk tube or a dry sealed tube
under N2. Dry CH3NO2 (1.0 mL) was added, followed by
the addition of alkynes 2 (1.2 mmol, 3.0 equiv.) and t-
BuONO (0.8 mmol, 2.0 equiv.). The reaction mixture was
stirred at 808C for 12 h, and the reaction was monitored
with TLC. The crude reaction mixture was purified by flash
column chromatography on silica gel to afford the corre-
sponding product.
Scheme 5. a) Mechanistic studies. b) Proposed reaction
mechanism.
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Acknowledgements
This work is financially supported by the 111 Project (No.
B16046), and China Pharmaceutical University (1054070012,
3014070049). We thank Prof. A. Studer for valuable discus-
sions and suggestions. Dr. R. N. Alolga in this group is
thanked for proofreading the manuscript.
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