Synthesis of carbonylated heteroaromatic compounds: Via visible-light-driven intramolecular decarboxylative cyclization of o -alkynylated carboxylic acids

Article abstract of DOI:10.1039/c7cc04813k

An efficient strategy for the easy access to carbonylated heteroaromatic compounds has been developed via a visible-light-promoted intramolecular decarboxylative cyclization reaction of o-alkynylated carboxylic acids. This method is characterized by its b

Full text of DOI:10.1039/c7cc04813k

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Synthesis of Carbonylated Heteroaromatic Compounds via Visible-
Light-Driven Intramolecular Decarboxylative Cyclization of o-
Alkynylated Carboxylic Acids
Received 00th January 20xx,
Accepted 00th January 20xx
Fei Gao, a Jiu-Tao Wang, a Lin-Lin Liu, a Na Ma, a Chao Yang, a Yuan Gaob and Wujiong Xia* a
DOI: 10.1039/x0xx00000x
www.rsc.org/
An efficient strategy for the easy access to carbonylated typical methods include Friedel-Crafts reaction5, Vilsmeier-
heteroaromatic compounds has been developed via a visible-light- Haack reaction6, and transition-metal-catalyzed C3-H
promoted intramolecular decarboxylative cyclization reaction of activation reaction7, which almost rely on preformed indoles,
o-alkynylated carboxylic acids. This method is characterized by its benzofurans or benzothiophenes to achieve carbonylated
benign conditions and the tolerance of
functionalities.
a
wide range of products. However, most of traditional strategies suffer from
limited functional group tolerability and harsh conditions.
Compared with the prosperous achievements, photoredox
Heteroaromatic compounds, including 3-acylindoles, 3-
acylbenzofurans and 3-acylbenzothiophenes, have been widely
employed as pervasive structural motifs in numerous natural
catalysis may provide an efficient method for the assembly of
3-acylindoles, 3-acylbenzofurans and 3-acylbenzothiophenes
under milder conditions.
products and biologically active pharmaceutical molecules1-4
.
Carboxylic acids are readily available and inexpensive basic
chemicals, and their radical decarboxylative functionalization
has recently received considerable attention as a powerful
synthetic method that allows to rapidly construct C-C and/or
C-X bonds9. In recent years, visible-light-promoted radical
decarboxylative functionalization of carboxylic acids and their
derivatives has arguably gained considerable momentum10.
With this regard, the applications on the visible-light-driven
decarboxylation process for the construction of C-C and/or C-X
bonds have been reported11. In contrast to the previous
achievements in intermolecular C−C bond and/or C-X bonds
formation via decarboxylative functionalization, research on
visible-light-induced intramolecular decarboxylative cyclization
to construct C-C bond and C-O bond remains scarce. Herein,
For example, pravadoline, which has been synthesized by
Sterling Drug (Sterling Research Group, Rensselaer, NY),
exhibits significant analgesic activity in humans2. Daphnodorin
A and B show a broad range of biological activities such as
antitumor properties, antiviral activities, and anti-
inflammatory properties3. Raloxifene has been found as an
oral selective estrogen receptor modulator (SERM) to prevent
the osteoporosis in postmenopausal women4. Therefore
considerable efforts have been made towards the
development of efficient protocols for the synthesis of
carbonylated heteroaromatic compounds5-8. Among them the
we reported
a novel visible-light-driven intramolecular
decarboxylative cyclization of o-alkynylated carboxylic acids in
the presence of air, base and photocatalyst for the formation
of
3-acylindoles,
3-acylbenzofurans
and
3-
acylbenzothiophenes, in which alkyl radical generated from
decarboxylation could undergo intramolecular radical addition
to the C-C triple bond, followed by a C-O bond formation, to
synthesize the corresponding carbonylated compounds.
Optimization of the reaction conditions was investigated by
irradiating
a
mixture of carboxylic acid, base, and
Figure 1. Examples of bioactive carbonylated heteroaromatic compounds
.
photocatalyst under air atmosphere using blue LEDs as the
light source (Table 1). On the basis of reported successes of
radical decarboxylative reactions10-11, our initial investigation
was carried out using o-alkynylated α-amino acid 1a as the
substrate, K2HPO4 as base and [Ir{dF(CF3ppy)}2(dtbbpy)]PF6 as
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Table 1. Optimization of the Reaction Conditionsa
withdrawing or electron-donating substituents on the
aromatic ring at the terminal alkyne were all competent
partners in this reaction (2f-2l), especially substrate 1j with a
strong electron-donating group was successfully converted to
2j in 78% yield. Note that the yield of product 2j remained
positive when the reaction was carried out even at gram scale.
Different substituents, including F, Cl and Me on the aromatic
ring of the aniline moiety all smoothly led to the desired 3-
entry
1
base
K2HPO4
2,6-Lutidine
Cs2CO3
DBU
solvent
DMF
yield (%)b
44
48
58
32
31
54
46
36
54
0
acylindoles (2m
the acids with
substrates for this transformation
-
2o). Moreover, we were pleased to find that
heteroaromatic ring were competent
2p 2q). Finally, when
2
DMF
a
3
DMF
(
-
4
DMF
carboxylic acid 1r with a bulky tert-butyl group at the terminal
alkyne was employed, the desired 3-acylindoles 2r could be
isolated in 29% yield.
5
no
DMF
6
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
DMSO
MeCN
1,2-DCE
DMF
7
Table 2. Scope of α-Amino Acid Derivativesa, b
8
O
9c
10d
11e
12f
R3
R3
R2
[Ir{dF(CF3ppy)}2(dtbbpy)]PF6 (5 mol %)
R2
DMF
Cs2CO3 (1.5 equiv), DMF, air
30W blue LEDs
N
R1
N
COOH
R4
R4
DMF
0
R1
2a-2r
DMF
34
1a-1r
aReaction conditions: 1a (0.1 mmol), [Ir{dF(CF3ppy)}2(dtbbpy)]PF6 (0.005
mmol), base (0.15 mmol), solvent (anhydrous, 4 ml), air, 30 W blue LEDs,
room temperature (rt), 24 h. bIsolated yield. cUnder O2 atmosphere. dUnder
N2 atmosphere. eIn the dark. fNo photocatalyst.
R
O
O
N
O
N
R1
2a, R1 = Me, 58%
2b, R1 = Et, 56%
2c, R1 = H, 0%
2d, R1 = Ac, 0%
2f
,
R = F, 60%
N
photocatalyst in DMF. Gratifyingly, the desired 3-acylindole 2a
was obtained in 44% isolated yield (entry 1), and further
screening on diverse bases showed that Cr2CO3 could provide
an obviously improvement in reaction efficiency (entries 2-5).
Subsequently, examination on the solvent revealed that DMF
was still the optimal reaction medium for this transformation
(entries 6-8). Interestingly, it was found that the yield of the
product was decreased when using pure oxygen instead of air,
and the reaction was completely suppressed under N2
atmosphere (entries 9-10). A control experiment confirmed
that visible light was essential for the desired transformation
to occur (entry 11). Notably, when the photocatalyst was
absent in the reaction mixture, the desired product 2a could
also be obtained with a decreased yield of 34% (entry 12),
which might be owing to the fact that the o-alkynylated α-
amino acid 1a could be directly oxidized by oxygen of air under
irradiation with light to form the corresponding radical
intermediate.
2g, R = Cl, 63%
2h
, R = Et, 48%
2e, 57%
2i
,
R = t-Bu, 55%
2j, R = MeO, 78%
(gram scale, 72%)
O
O
O
R4
R
N
N
N
Cl
2k, R = F, 67%
2l, R = Me, 57%
2o, 51%
2m, R4 = F, 60%
2n, R4 = Me, 56%
O
O
O
S
S
N
N
N
2q, 63%
2p, 55%
2r, 29%
aReaction conditions:
1 (0.1 mmol), [Ir{dF(CF3ppy)}2(dtbbpy)]PF6 (0.005
mmol), Cr2CO3 (0.15 mmol), DMF (anhydrous, 4 ml), air, 30 W blue LEDs, rt,
24 h. bIsolated yield.
The generality of this method was next explored for the synthesis
of various 3-acylbenzofurans. As described in Table 3, a series of o-
alkynylated phenoxyacetic acids could undergo efficient
decarboxylative cyclization/carbonylation to afford the desired 3-
acylbenzofurans successfully. Importantly, this transformation
appears to be restricted to the substituent at the α-position of the
carboxyl group. When phenoxyacetic acid 3b incorporated with α-
phenyl group was employed as a substrate, the reaction worked
With the optimized reaction conditions in hand, we then
explored the scope of α-amino acid derivatives. As shown in
Table 2, a wide range of structurally diverse o-alkynylated α-
amino acids were readily converted to the corresponding 3-
acylindoles. It is important to note that the reaction efficiency
is closely relevant to the substitution effect on the nitrogen
atom, wherein substrates with electron-donating groups, e.g.
methyl and ethyl, readily delivered products (2a-2b). However,
for less-substituted or acetyl-substituted carboxylic acid, the more smoothly to deliver the desired product (4a-4c). This result
corresponding products were not observed (2c-2d). Substrate
may be rationalized by stability of the respective radical
intermediate. Moreover, for ortho-substituted carboxylic acid 3d, it
was observed that the reactivity was decreased compared with its
para- or meta- substituted counterpart (4d-4f). Subsequently, we
1e bearing a methyl group in the α-position of the carboxyl
group performed well under the optimized conditions to
furnish a 2, 3-disubstituted indole product. To our delight, a
variety of α-amino acid derivatives with either electron-
2 | J. Name., 2012, 00, 1-3
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Table 4. Scope of (Phenylthio)acetic Acid Derivatives a, b
evaluated the substitution effect of the aromatic ring at the
terminal alkyne. It was observed that both electron-withdrawing
and electron-donating aromatic substituents were well tolerated
(4g-4m). In addition, this protocol could be applied to heteroaryl-
substituted phenoxyacetic acids (4n-4o). A substrate bearing a tert-
butyl group at the terminal alkyne was also subjected to this
reaction, and 4p was obtained in 22% yield. Interestingly, this
process was extended to the acid 3q with a naphthalene ring.
Table 3. Scope of Phenoxyacetic Acid Derivativesa, b
aReaction conditions: 5 (0.1 mmol), [Ir{dF(CF3ppy)}2(dtbbpy)]PF6 (0.005
mmol), Cr2CO3 (0.15 mmol), DMSO (anhydrous, 4 ml), air, 30 W blue LEDs, rt,
48 h. bIsolated yield.
Scheme 1. Control Experiments
aReaction conditions: 3 (0.1 mmol), [Ir{dF(CF3ppy)}2(dtbbpy)]PF6 (0.005
mmol), Cr2CO3 (0.15 mmol), DMSO (anhydrous, 4 ml), air, 30 W blue LEDs, rt,
48 h. bIsolated yield.
Finally, to further explore the scope of this protocol, we turn our
attention to the decarboxylative cyclization of (phenylthio)acetic
acid derivatives (Table 4). As a result of this endeavor, this
transformation could also be applied to furnish a variety of
differentially substituted 3-acylbenzothiophenes in acceptable
yields. Fortunately, various substitutions at the α-position of the
carboxyl group were well tolerated with the reaction conditions to
obtain the expected products with moderate efficiency (6a-6c). As
expected, the acids with a substituent on the aromatic ring at the
terminal alkyne also worked smoothly (6d-6f). However, in the case
of substrates with a 3-thienyl group or a tert-butyl group at the
terminal alkyne, lower yields of the desired products were formed
(6g and 6h, 32% and 25%, respectively).
To gain some information about this transformation, several
control experiments were conducted. As illustrated in Scheme 1,
when the acid 3b or 5a was employed, the reactions did not
proceed in the absence of either photocatalyst or light, which
highlighted the essential roles of the light and photocatalyst in this
transformation (eq 1). Under the optimized conditions, 2a was not
observed in the presence of TEMPO, which revealed that the
reaction might occur through a radical process (eq 2). Meanwhile,
to further elucidate the origin of the oxygen atom of the ketonic
carbonyl group, 18O-labeling experiments were carried out. As
expected, it was observed that an 18O atom was incorporated into
the ketonic carbonyl group under 18O2 atmosphere based upon GC-
MS analysis (eq 3). However, in the presence of H218O (5 equiv) and
anhydrous DMF, an 18O-labelled product was not observed (eq 4).
On the basis of the above results, a plausible mechanism was
proposed as outlined in Scheme 212. Upon irradiation with visible
light, the photocatalyst was excited to *Ir[dF(CF3)ppy]2(dtbbpy)+ (2)
(ERed
= +1.21 V vs SCE), which was readily reduced by o-
1/2
alkynylated carboxylic acids (1a, ERed1/2 = +0.45 V vs SCE; 3b, ERed
=
1/2
+1.18 V vs SCE; 5a, ERed = +1.09 V vs SCE; see SI) to form the
1/2
corresponding carboxyl radical specie. The immediate CO2-
extrusion would provide the radical intermediate (4). It is to note
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Scheme 2. Plausible Mechanism
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8
·-
radical intermediate (5) was then captured by O2 that was derived
from the oxidation of IrII to IrIII by air to form the peroxide radical
intermediate (6). After abstraction of a proton13, intermediate (6)
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Synthesis of Carbonylated Heteroaromatic Compounds via Visible-Light-Driven Intramolecular Decarboxylative Cyclization of
o-Alkynylated Carboxylic Acids
Fei Gao, Jiu-Tao Wang, Lin-Lin Liu, Na Ma, Chao Yang, Yuan Gao and Wujiong Xia*
An efficient strategy for the easy access to carbonylated heteroaromatic compounds has been developed via a visible-light-promoted
intramolecular decarboxylative cyclization reaction of o-alkynylated carboxylic acids.
1