Visible-Light-Induced [4+2] Annulation of Thiophenes and Alkynes to Construct Benzene Rings

Article abstract of DOI:10.1002/anie.201905971

The [4+2] annulation represents an elegant and versatile synthetic protocol for the construction of benzene rings. Herein, a strategy for visible-light induced [4+2] annulation of thiophenes and alkynes, to afford benzene rings, is presented. Under simple and mild reaction conditions, the ready availability and structural diversity of thiophenes and alkynes permit the facile synthesis of several substituted aromatic rings. Valuable drugs and amino acids are also well tolerated. Moreover, DFT calculations explain the high regioselectivity of the reaction.

Full text of DOI:10.1002/anie.201905971

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Accepted Article
Title: Visible-light Induced [4+2] Annulation of Thiophenes and Alkynes
to Construct Benzene Rings
Authors: Chunlan Song, Xin Dong, Zhongjie Wang, Kun Liu, Chien-Wei
Chiang, and Aiwen Lei
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201905971
Angew. Chem. 10.1002/ange.201905971
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Visible-light Induced [4+2] Annulation of Thiophenes and Alkynes
to Construct Benzene Rings
Chunlan Song,[a] Xin Dong,[a] Zhongjie Wang,[a] Kun Liu,[a] Chien-Wei Chiang,*[a] and Aiwen Lei*[a,b]
Abstract: [4+2] Annulation represents an elegant and versatile
synthetic protocol for the construction of benzene rings. Herein, a
strategy for visible-light induced [4+2] annulation of thiophenes and
alkynes to afford benzene rings is presented. Under simple and mild
conditions, the easy availability and structural diversity of thiophenes
and alkynes permit the facile synthesis of multifarious substituted
serve as dienes to react with maleic anhydride in Diels-Alder
reactions, but this reaction required high temperatures
(~225°C).[11] Thiophene could be activated by UV-light irradiation
to participate in cyclization reactions.[12] However, these
transformations were limited to a few substrates or involved harsh
conditions. Visible-light induced photocatalysis can serve as a
aromatic rings. Valuable drugs and amino acids are also well tolerated. powerful tool in bond formation reactions under mild, green and
Moreover, DFT calculations explain the high regioselectivity of the
reaction.
easy-to-handle conditions. [13] [14] Photocatalysis also provides an
opportunity to generate highly reactive radical ion intermediates
with unconventional reactivities. Herein, we report a visible-light
induced [4+2] annulation between thiophenes and alkynes driven
by in-situ generated thiophene radical cation. As four-carbon
synthons, readily available thiophenes allowed facile the
synthesis of aromatic rings with a diverse array of substituted
alkynes under oxidant- and metal- free conditions (Scheme 1).
Benzene ring systems possess privileged structural cores in
industrial chemicals, functional materials and biological
compounds.[1] Highly efficient methods for the construction of
benzene rings are one of the most important subjects in organic
chemistry. A well-known route to these molecules is through
stepwise electrophilic aromatic substitution between benzene
derivatives and electrophiles.[2] However, since the introduced
substituents would affect the reactivity and selectivity in follow-up
steps. Reaction conditions and synthetic pathways need to be
thoughtfully designed for avoiding side-reactions.
As a representative method, [4+2] annulation is an effective
cycloaromatization strategy for constructing benzene rings. In
1996, Pd-catalyzed [4+2] benzannulation of conjugated enynes
with enynophiles was reported.[3] Since then, the scope of this
useful transformation has been systematically investigated by
several groups.[4] Butadienes,[5] biaryls,[6] enaldiazo compounds,[7]
cyclobutenones[8], etc.[9] have been successfully employed as
four-carbon synthons to participate in the [4+2] benzannulation
reactions. These reactions allowed the rapid construction of
highly functionalized benzene frameworks in one step. Despite
these advancements, these reactions still suffer from some
problems. Since starting materials were not available, additional
pre-functionalization procedures were required for the preparation
of complicated substrates, which would consume extra materials
and energy. Moreover, stringent conditions such as high
temperature or noble metal catalyst were inevitable. Therefore,
the development of cycloaromatization with commercially
available four-carbon synthons under sustainable and mild
conditions is of great importance.
Scheme 1. Visible-light induced [4+2] annulation between thiophenes and
alkynes.
Table 1. Reaction conditions optimization[a]
Isolated yield
Entry
Variation from Standard Conditionsa
(%)
1
2
standard conditions
Ir(ppy)3 as the photocatalyst
Ru(bpy)3Cl2 as the photocatalyst
Eosin Y as the photocatalyst
3 mmol% Mes-Acr+ClO4- was used
7 mmol% Mes-Acr+ClO4- was used
DCE as the solvent
84
N.D.[b]
N.D.
N.D.
80
3
4
5
Thiophene derivatives are widely present in industrial raw
materials and natural products.[10] It’s known that thiophenes can
6
84
7
11
8
DMF as the solvent
N.D.
N.D.
N.D.
-
9
no Mes-Acr+ClO4
[a]
[b]
C. Song, X. Dong, Z. Wang, K. Liu, Prof. C.-W. Chiang, Prof. A. Lei
College of Chemistry and Molecular Sciences and The Institute for
Advanced Studies (IAS), Wuhan University, Wuhan 430072, Hubei,
P. R. China.
E-mail: aiwenlei@whu.edu.cn; cwchiang@whu.edu.cn
National Research Center for Carbohydrate Synthesis Jiangxi
Normal University, Nanchang 330022, Jiangxi, People’s Republic of
China.
10
no light
[a] Standard conditions: reactions were performed with 1 (0.3 mmol), 2 (0.6
mmol) and photocatalyst (5 mol %) in 6 mL CH3CN under N2 with irradiation of
3 W blue LEDs at room temperature for 12 h. [b] N.D. = Not Detected.
We initially investigated the reaction conditions using thiophene
(1) and 4-tert-butylphenylacetylene (2) as model starting
materials. After screening photocatalysts, the desired product 3
Supporting information and the ORCID identification numbers for
authors of this article can be found under: *******
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-
could be obtained in 84% yield with Mes-Acr+ClO4 under the
irradiation of blue LEDs (Table 1, Entry 1), along with S8 as by-
product (Figure S1). When other photocatalysts were applied, no
product could be detected (Table 1, Entry 2-4). Neither increasing
nor decreasing the amount of catalyst could improve the yield
(Table 1, Entry 5 and 6). Further attempts, such as modifying the
solvent from CH3CN to DCE or DMF, couldn’t enhance the yield
(Table 1, Entry 7 and 8). Additionally, control experiments
indicated that no product was produced in the absence of light or
photocatalyst (Table 1, Entry 9 and 10).
and aliphatic alkynes also participated in the reaction efficiently
(21–24). Besides, when diethynylbenzene was applied into this
protocol, 53% yield of dicyclization product 25 could be obtained
along with 13% yield of monocyclization product. The annulation
of internal alkynes into corresponding products could also be
achieved (26–28). Remarkably, this protocol provides direct
access to late-stage derivatization of valuable drugs. For example,
when probenecid-containing and estrone-containing alkynes
were reacted with thiophene, the desired product 29 and 30 could
be obtained in 72% and 80% yield, respectively. Additionally,
sarcosine derivative (31) and phenylalanine derivative (32) were
successfully prepared, which highlighted the potential of this
method to bioconjugation reactions.
With optimized conditions in hand, the scope of alkynes was
examined (Table 2). Phenylacetylene derivatives bearing
electron-donating substituents produced the corresponding
products in good yields (3–10). However, amino-group couldn’t
be tolerated (11), for the competitive quenching of photocatalyst
by more electron-rich 4-ethynylaniline (Figure S2). This
transformation also tolerated alkynes with halogen groups (12–
14), which could provide further flexible avenues for diversification.
Phenylacetylenes with electron-withdrawing groups (15–18) were
also amenable. Moreover, heterocyclic-ethynes were suitable for
the formation of benzene derivatives (19 and 20). Enyne, alkynol
Subsequently, the scope of thiophenes was investigated (Table
3). Thiophene derivatives substituted at C3-position with alkyl,
acetoxy, methoxy and halogen groups all afforded the products in
good yields (33–40). The boric substituted product could be
synthesized with 74% yield, providing a handle for further
synthetic manipulations (41). This method could also be efficiently
extended to an electron-deficient thiophene (42). Thiophenes
substituted at the C2-position delivered products in moderated
yields because of the steric hindrance (43 and 44). When 3,4-
dimethylthiophene was utilized as a substrate, two methyl
substituents would not disturb the formation of 45. Furthermore,
using 3-ethynylthiophene as a sole substrate, the transformation
occurred smoothly to yield 3-(4-ethynylphenyl) thiophene (46) in
55% yield.
Table 2. Substrate scope of alkynes[a]
Table 3. Substrate scope of thiophenes[a]
[a] Standard conditions as shown in Table 1. [b] 0.6 mmol thiophene and 0.3
mmol alkyne were used as substrates. [c] The ratio of the isomer was
determined by NMR. [d] 0.9 mmol 3-ethynylthiophene was used as a sole
substrate.
[a] Standard conditions as shown in Table 1. [b] 0.6 mmol Thiophene and 0.3
mmol alkyne were used as substrates. [c] 0.1 mmol Alkyne was used as
substrate. [d] GC yield was determined by using biphenyl as an internal
standard. [e] Ratio of the isomer was determined by NMR.
To further explore the utilities of this method, selenophene (47)
was utilized to react with 4-tert-butylphenylacetylene, the desired
product could be obtained with 77% yield (Scheme 2a).
Meanwhile, 0.98 g of 37 was synthesized with good selectivity and
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efficiency in a gram scale synthesis (Scheme 2b). This result
indicated that this photochemical synthesis have the potential for
practical industrial production. This reaction could also be
conducted under sunlight irradiation without stir, giving the
desired product in 55% yield (Scheme 2c).
Furthermore, kinetic studies of this reaction were performed by
detecting the initial reaction rate with different amounts of
thiophene, 4-tert-butylphenylacetylene and photocatalyst
(Scheme 4a-c). The initial reaction rates demonstrated a first
order dependence of thiophene, and was independent of the
concentration of alkyne. Concerning the kinetic profiles of the
photocatalyst, first-order dependencies at low concentration was
observed and the reaction rate was saturated at high
concentrations. This can be attributed to the limited number of
photons from the LED lamp. These kinetic behaviors suggested
that single-electron transfer from thiophene to photocatalyst is
involved in the rate-determining step.
To get a deeper understanding about this protocol, some
mechanistic studies have been performed. Firstly, intermolecular
competition experiments were carried out (Scheme 3a). In the
mixture of electron-rich and electron-deficient substrates, the
totally preferential consumption of electron-rich thiophene and
alkyne were observed, indicating that electron-rich substrates are
favored for this reaction. In addition, when two equiv. of 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO) was added, the reaction
was completely inhibited, indicating that a radical process might
be involved (Scheme 3b). Moreover, 4-chloro-1H-pyrazole was
introduced to trap the in-situ generated thiophene radical
cation.[13d, 15] The C-N bound adduct 48 could be detected by GC-
MS, demonstrating that this transformation might go through a
radical cation pathway (Scheme 3c).
Scheme 4. Kinetic studies
Scheme 2. Investigation and application of this protocol. (a) Selenophene as
substrate to construct benzene ring. (b) Gram-scale synthesis experiment. (c)
Photoredox synthesis experiment using sunlight source.
Scheme 5. DFT calculation for site selectivity
In-depth analysis of the selectivity issue, a C3 methyl group
substituted thiophene could react with 4-tert-butylphenylacetylene
to produce para-methyl biphenyl in 59% yield with only trace
amounts of meta-substituted product. In cases of applying ether
and ester group substituents, the generation of meta-substituted
biphenyl was enhanced. To determine the reason for this different
selectivity, the production of single-electron oxidative substituted
Scheme 3. Mechanistic studies. (a) Intermolecular competition experiments. (b)
Radical-inhibiting experiment. (c) Nucleophile trapping experiment.
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thiophenes were investigated by employing the spin density map
of a DFT calculation. Regarding the DFT results, radical species
were delocalized on thiophene rings and mainly distributed at the
C2 and C5 positions (Scheme 5). Interestingly, differences
between the population of C2 and C5 position would directly affect
the selectivity of para- and meta-substituted products. Since the
electricity of phenylacetylene at the terminal position would be
partially positive during the cyclization, the radical species
location of thiophene radical cation would pointedly affect the
formation of products.
(2017CFA010, 2017CFB152). Support from the Program of
Introducing Talents of Discipline to Universities of China (111
Program) is also appreciated.
Keywords: [4+2] annulation • photochemical • benzene ring •
thiophene • alkyne
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21520102003, 21701127, 21390402) and
the Hubei Province Natural Science Foundation of China
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Chunlan Song,[a] Xin Dong,[a] Zhongjie
Wang,[a] Kun Liu,[a] Chien-Wei Chiang,*[a]
and Aiwen Lei*[a,b]
Page No. – Page No.
Visible-light Induced [4+2] Annulation
of Thiophenes and Alkynes to
Construct Benzene Rings
Text for Table of Contents
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