A copper(ii)-catalyzed annulative formylation of o-alkynylanilines with DMF: a single-step strategy for 3-formyl indoles

Article abstract of DOI:10.1039/C8RA09214A

In this paper, a copper(ii)-catalyzed reaction of o-alkynylanilines with dimethylformamide (DMF) in the presence of oxygen has been developed for synthesizing multisubstituted 3-formyl indole scaffolds. This one-pot reaction proceeds through a cascade 5-endo-dig cyclization followed by formylation to construct 1,2-disubstituted 3-formyl indoles. The key aspects of this synthesis method are the broad substrate scope (with 38 examples), and well tolerating various functional groups. In addition, a detailed mechanism has been proposed, where DMF may serve as a carbon source for the in situ C3 formylation of the obtained indole derivatives.

Full text of DOI:10.1039/C8RA09214A

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A copper(II)-catalyzed annulative formylation of o-
alkynylanilines with DMF: a single-step strategy for
3-formyl indoles†
Cite this: RSC Adv., 2018, 8, 40968
a
ab
Balaji Ganesan,
and Wei-Yu Lin
Gopal Chandru Senadi,
Bing-Chun Guo,a Min-Yuan Hungc
a
*
In this paper, a copper(II)-catalyzed reaction of o-alkynylanilines with dimethylformamide (DMF) in the
presence of oxygen has been developed for synthesizing multisubstituted 3-formyl indole scaffolds. This
one-pot reaction proceeds through a cascade 5-endo-dig cyclization followed by formylation to
construct 1,2-disubstituted 3-formyl indoles. The key aspects of this synthesis method are the broad
substrate scope (with 38 examples), and well tolerating various functional groups. In addition, a detailed
mechanism has been proposed, where DMF may serve as a carbon source for the in situ C3 formylation
of the obtained indole derivatives.
Received 7th November 2018
Accepted 30th November 2018
DOI: 10.1039/c8ra09214a
rsc.li/rsc-advances
reaction,12 or other methods13 (Scheme 2, eqn (1)). Tsui and
coworkers14 recently reported an elegant method for synthe-
Introduction
sizing
2-(triuoromethyl)indoles
from
N-mesylated-2-
2,3-Disubstituted and 1,2,3-multi-substituted indoles are widely
available in various alkaloids and natural products possessing
potential biological activities. Several essential examples
include indometacin, pyrazolybisindoles and substituted
tryptamine derivatives, which exhibit signicant anticancer
activities (Scheme 1).1 Among these crucial molecules, inter-
mediates with a formyl functional group at the C3 position can
serve as a primary building block to construct substituted
indoles through their broad and specic reactivity for various
chemical transformations. Therefore, 1,2,3-tri-substituted
indole preparation has received considerable interest in
previous decades.
According to a literature survey, several pioneering methods
have been reported for the construction of 1,2 di-substituted 3-
formyl indoles through two-step reactions. The rst step
involves the use of the easily accessible starting materials of o-
alknylaniline derivatives and is mediated by transition metals
(Pt,2 Pd,3 Ag,4 Au,5 Ir,6 In,7 Fe,8 and Cu9), the molecular halogen10
catalyzed annulation of o-alknylaniline derivatives, or free
radical assisted indole construction through either C–C or C–N
bond-forming reactions.11 The second step involves direct for-
mylation by an existing indole using the Vilsmeier–Haack
alkynylanilines through domino cyclization and formylation
by using TMEDA as a carbon synthon (Scheme 2, eqn (2)).14
Although these methods are potentially useful, some have
certain limitations, such as desulfonylation, post-
functionalization is necessary for the N-substitution of the
indole derivatives, expensive metal-catalysts2–5 and strong acidic
conditions.12 Herein, we wish to report a complementary
approach by employing o-phenylethynyl N-substituted aniline
(1, 3, and 5) as a starting material for the one-pot domino
synthesis of multisubstituted-3-formyl indole (2, 4, and 6) using
DMF in the presence of oxygen (Scheme 2, eqn (3)). Moreover,
detailed mechanistic studies of the Cu(II) and O2-mediated
formylation were conducted.
Results and discussion
To optimize reaction parameters, preliminary screening was
conducted using o-phenylethynyl aniline 1a as a model
substrate (Table 1). The reaction was initially carried out using
aDepartment of Medicinal and Applied Chemistry, Kaohsiung Medical University, No.
100 Shi-chuan, 1st Road, Kaohsiung 807, Taiwan. E-mail: wylin@kmu.edu.tw
bDepartment of Chemistry, SRM Institute of Science and Technology, Kattankulathur,
Chennai–603203, India
cCenter for Research Resources and Development, Kaohsiung Medical University, No.
100 Shi-chuan, 1st Road, Kaohsiung 807, Taiwan
† Electronic supplementary information (ESI) available. CCDC 1876301 (2a). For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c8ra09214a
Scheme 1 Biological importance of indole derivatives.
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Scheme 2 Previous and this work on 3-formyl indoles.
1a in DMF (0.25 M) with 20 mol% of Cu(OAc)2$H2O as a catalyst, TfOH and TsOH$H2O additives (Table 1, entries 7–8). 2a was not
and TFA as an additive (2.0 equiv.) under an O2 atmosphere at obtained, when only Cu was added as a catalyst (Table 1, entry
ꢀ
120 C for 24 h. The desired product 2a was obtained in 31% 9). To our delighted, Cu(TFA)2$xH2O was used as a catalyst
yield (Table 1, entry 1). To optimize the catalyst, TFA was added instead of Cu/TFA, and the reaction yield increased to 52%
as an additive with various copper catalysts such as anhyd. (Table 1, entry 10). The reaction yield was reduced to 32% when
Cu(OAc)2, Cu(OTf)2, CuSO4$H2O, Cu(I), and Cu. The desired the reaction was performed in air (Table 1, entry 11). Further-
product 2a was obtained in 45% yield when Cu powder was used more, no product was formed under a N2 atmosphere (Table 1,
as a catalyst (Table 1, entries 2–6). The reaction yield did not entry 12). Other oxidants such as DDQ, and AgOAc under the N2
improve when different Brønsted acids were used, including atmosphere, resulted in low yield (Table 1, entries 13–14). These
Table 1 Optimization of the reaction conditionsa
Entry
Catalyst (20 mol%)
Additive (2.0 equiv.)
Oxidant
Solvent/time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
Cu(OAc)2$H2O
Cu(OAc)2
Cu(OTf)2
CuSO4$H2O
CuI
Cu
Cu
Cu
Cu
Cu(TFA)2$xH2O
Cu(TFA)2$xH2O
Cu(TFA)2$xH2O
Cu(TFA)2$xH2O
Cu(TFA)2$xH2O
Cu(TFA)2$xH2O
Cu(TFA)2$xH2O
Cu(TFA)2$xH2O
Cu(TFA)2$xH2O
Cu(TFA)2$xH2O
Cu(TFA)2$xH2O
TFA
TFA
TFA
TFA
TFA
TFA
TfOH
TsOH$H2O
—
—
—
—
—
—
—
—
—
—
—
—
O2
O2
O2
O2
O2
O2
O2
O2
DMF/20
DMF/14
DMF/28
DMF/25
DMF/22
DMF/20
DMF/24
DMF/24
DMF/20
DMF/24
DMF/24
DMF/24
DMF/24
DMF/24
Dioxane/24
DMSO/24
THF/DMF (1/1)/24
DMF/24
DMF/24
DMF/24
31
37
20
27
27
45
15
18
N.R
52
32
N.R
16
11
Trace
Trace
Trace
12
30
54
9
O2
O2
10
11c
12d
13
14
15
16
17
18e
19f
20g
Open air
—
DDQ
AgOAc
O2
O2
O2
O2
O2
O2
a
Reaction conditions 1a (1.0 equiv.), catalyst (20 mol%), solvent (3.0 mL), O2 balloon, 120 ꢀC, 24 h at indicated time unless otherwise noted.
b
c
d
e
Isolated yield. Reaction was performed under open air. Reaction was performed under nitrogen atmosphere. 5 mol% of Cu(TFA)2$xH2O
was used. 10 mol% of Cu(TFA)2$xH2O was used. g Another 20 mol% of Cu(TFA)2$xH2O was added aer 12 h.
f
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analysis, and the data was presented in ESI.†18 The scope of the
N-phenyl-2-phenylethynyl aniline derivatives (5a–5h) were
explored and the resulting data are shown in Scheme 3.
Electron-donating groups at the alkynyl position, exhibiting the
desired product (6a–6c) in 33–55% yields. 4-Me (5d), and 4-F
(5e) groups on the N-phenyl were formed smoothly, with 10–
18% recovery of starting materials. A cyclohexyl group (5f)
present on a nitrogen atom gave the desired product (6f).
Surprisingly, when N-tosylate-2-phenylethynyl aniline deriva-
tives (5g and 5h) used as substrates, only led to the formation of
indole14 (Scheme 5, 6g, 6h).
With the successful outcome of 2,3 di-substituted indole
derivatives, this method was extended to synthesize of 1,2,3 tri-
substituted indole derivatives by using the N-benzylated 2-
phenylethynyl aniline derivatives (3a–3u) under the optimized
conditions (Scheme 4). When 3a was tested, the desired product
4a was obtained in 34% yield. Substituents at the R2 position
with a phenyl ring having electron-donating groups, such as 2-
Me (3b), 4-Me (3c), 2-OMe (3d), and 4-OMe (3f) had appropriate
reactivity. By contrast, the electron-withdrawing groups,
including 3-Cl (3g), 3-NO2 (3h), and 3-CF3 (3i), produced the
corresponding products in moderate yields (32–44%). Thio-
phene group (3j) smoothly proceeded in 60% yield. Aliphatic
groups, such as hexyl (3k) and cyclopropane (3l), were exam-
ined, the desired products were obtained in 30% and 33%
yields, respectively. The substitution of a phenyl ring bearing an
Scheme 3 Scope of 2-phenylethynyl aniline derivativesa,b aReaction
.
conditions: 1a (0.3 mmol), Cu(TFA)2$xH2O (20 mol%), DMF (3 mL),
120 ꢀC, 24 h, O2. bIsolated yield. c3.0 mmol scale. dRecovered yield of
starting material.
results indicate that the oxygen supply is crucial for the reac-
tion. The use of alternative solvents such as, dioxane, DMSO,
and a binary solvent system (THF/DMF 1 : 1) generated trace
yield (Table 1, entries 15–17). When the catalyst was reduced to
5 mol% or 10 mol%, the reaction yield was reduced (Table 1,
entries 18–19). The addition of another 20 mol% of Cu(TFA)2-
$xH2O catalyst aer 12 h, did not substantially improve the
reaction yield (Table 1, entry 20).
With the optimized conditions (Table 1, entry 10), the scope
of this reaction was studied using o-phenylethynyl aniline
derivatives [(1a–1i) and (5a–5h)], and the reaction results are
summarized in Scheme 3. To evaluate the reaction scope and
probable substituent effects of R2, various functional groups
including substituted phenyls, and the heterocyclic group were
studied (Scheme 3, 1a–1g). Compared with 1a, substrates with
electron-donating groups on the phenyl ring, such as 2-Me and
2 or 4-OMe were suitable for this reaction, generating the cor-
responding products 2b, 2c and 2e in moderate yield. A
substrate with electron-withdrawing groups, such as 2-F (1f)
gave 2f in lower yield. When the substituent group was thio-
phene (1g), the desired was obtained in a low yield of 25% (2g).
The substituent on the aryl ring (R) was subsequently studied
(Scheme 3, 1h–1i). Electron-withdrawing groups, including 4-F
(1l), and 4-Cl (1m) generated the corresponding products 2l and
2m in 38% and 50% yield, respectively. A scale up experiment
could be performed in a 3 mmol scale of 1a, to obtain 2a in 55%
yield, indicating the efficacy and scalability of the reaction. The
structure of 2a was validated through X-ray crystallography
Scheme
4 Scope N-benzylated-2-phenylethynyl aniline deriva-
tivesa,b,c aReaction conditions: 3a–3v (0.3 mmol), Cu(TFA)2$xH2O
.
(20 mol%), DMF (3 mL), 120 ꢀC, 24 h, O2. bIsolated yield. cPlease see
Table S1 in ESI† for isolated yield of 4a0–4u0.
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Scheme 5 Control experiments.
electron donating groups such as 4-Me (3m), 4-OMe (3n), and t-
Bu (3o), on R1 provided moderate yield (44–54%). The presence
of electron withdrawing groups (3p–3r) on a benzyl ring gener-
ated a similar moderate yield. An indole phenyl ring bearing
with electron withdrawing groups such as F (3s), Cl (3t), and Br
(3u) provided in good yield. During this reaction, the formation
of debenzylation products was observed (ESI†). This formation
might be a side reaction of substrate with molecular oxygen
under high temperature.14,15
To understand the formation of the 3-formyl indoles deriv-
atives, several control experiments were performed (Scheme 5).
Table 1 (vide supra) indicates that when oxygen was replaced by
air or nitrogen, the reaction yield was reduced to 32% and 0%,
respectively (Table 1, entries 11 and 12). These results revealed
that sufficient supply of oxygen is necessary for this reaction.
When a 2.5 equivalent of common radical scavengers, including
TEMPO or BHT was added, a trace amount (<5%) of 2a was
detected through GC-MS and the starting material remained.
The results suggested that this reaction may occur through
a radical process (Scheme 5, eqn (1) and (2)). The addition of
D2O aer the completion of the reaction could not afford the
deuterium-labeled aldehyde in 50% yield (Scheme 5, eqn (3)).
The addition of 18O-labeled water to the reaction mixture under
a standard condition afforded 100% of 18O-labeling product 2a0
(Scheme 5, eqn (4), and ESI†), where the oxygen atom of the
formyl group is originated from water. The reaction with the 3-
H-indole substrate 7 generated 2a in good yield (73%) under the
standard condition (Scheme 5, eqn (5)). These results indicate
that the product formation was through a pathway involving
radical intermediates that utilized molecular oxygen as an
oxidant.
Scheme 6 Proposed mechanism.
activated by Cu(II) and oxygen to generate the iminium ion
8.13e,14,16 The simultaneous electrophilic activation of alkyne by
Cu(II) and intramolecular 5-endo-dig cyclization generated the
vinyl Cu(II) intermediate A.14 The carbocupration of the inter-
mediate A and the reaction with an iminium ion led to forma-
tion of the N–Cu(II) intermediate B.13e,14,16,17 The b-hydride
elimination of the B gave the primary iminium species C along
with the catalytic regeneration of Cu(0) in the presence of
oxygen. The nucleophilic addition of 18O-labeled water to the C
generated the hemiketal intermediate D. Finally, the elimina-
tion of dimethylamine from the D generated the desired 3-
formyl indoles derivatives.
Experimental
Experimental procedure,11a crystallographic data and spectro-
scopic data of new starting materials and nal products were
given in ESI.†18
In sealed tube, o-alkynylaniline derivatives (1a–1i, 5a–5h &
3a–3u) (0.3 mmol) was taken in DMF (3 mL). To the stirred
solution, Cu(TFA)2$xH2O (20 mol%) was added and allowed to
stir under O2 source at 120 ꢀC, until the completion of starting
material (ꢁ24 h). The reaction mixture was quenched with ice
cold water and extracted with ethyl acetate. Combined organic
layer washed with brine, dried over MgSO4 and concentrated in
vacuum. Then the crude material was puried by column
chromatography using 20% ethyl acetate in hexane as eluent to
afford the desired product (2a–2i, 6a–6h & 4a–4u).
The proposed mechanism is based on the control experi-
ments and previous reports13e,14,16,17 (Scheme 6). DMF was
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In summary, in this paper, a new synthetic method was devel-
oped for the direct synthesis of 1,2-disubstituted 3-formyl
indole or 2-substituted 3-formyl indole from the common
starting materials of o-alkynylaniline through 5-endo-dig cycli-
zation followed by an iminium ion attack at the C3 position.
Compared with other methods, this one-pot reaction was using
the inexpensive Cu-catalyst and DMF as a formyl carbon source,
without any additives. This method can be used with a wide
range of functional groups and the desired products can be
obtain up to moderate yield. In the near future, this method can
be adapted as a potential route for the direct synthesis of 3-
formyl indole. Further investigation of this reaction in contin-
uous ow system are currently ongoing in our laboratories.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors gratefully acknowledge funding from the Ministry
of Science and Technology (MOST 106-2113-M-037-009-), Tai-
wan, and the Center for Research Resources and Development
(CRRD) of Kaohsiung Medical University for Mass and 400 MHz
NMR analyses.
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crystallographic data for this paper.
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