Visible-Light-Induced Carbonylation of Indoles with Phenols under Metal-Free Conditions: Synthesis of Indole-3-carboxylates

Article abstract of DOI:10.1021/acs.orglett.1c01494

A visible-light-induced carbonylation of indoles with phenols for the synthesis of indole-3-carboxylates has been developed. The reaction proceeded via a radical carbonylation process in which elementary I2 was used as an effective photosensitive initiator and, thus, avoided the use of transition metal catalysts. A series of different aryl indole-3-carboxylates were prepared in moderate to good yields. The broad applicability of this methodology was further highlighted by the late-stage functionalization of several phenol-containing natural products and pharmaceuticals.

Full text of DOI:10.1021/acs.orglett.1c01494

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Letter
Visible-Light-Induced Carbonylation of Indoles with Phenols under
Metal-Free Conditions: Synthesis of Indole-3-carboxylates
Zhuang Qi, Lin Li, Ying-Kang Liang, Ai-Jun Ma, Xiang-Zhi Zhang, and Jin-Bao Peng*
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ABSTRACT: A visible-light-induced carbonylation of indoles with
phenols for the synthesis of indole-3-carboxylates has been developed.
The reaction proceeded via a radical carbonylation process in which
elementary I2 was used as an effective photosensitive initiator and, thus,
avoided the use of transition metal catalysts. A series of different aryl
indole-3-carboxylates were prepared in moderate to good yields. The
broad applicability of this methodology was further highlighted by the
late-stage functionalization of several phenol-containing natural products and pharmaceuticals.
of indole with alcohol and phenol to synthesize indole-3-
ndole derivatives are ubiquitous scaffolds in pharmaceut-
Iicals, functional materials, and natural products. Numerous carboxylates.6 This reaction proceeds via an oxidative
1
iodination followed by a Pd0-catalyzed carbonylation process.
approaches have been established for the construction of
diversely functionalized indole derivatives.2 Among them, the
In addition, aryl formates were also used as efficient CO
surrogates. For example, Lan, You, and their colleagues
reported a Pd-catalyzed C−H carbonylation of (hetero)arenes
with aryl formates to prepare (hetero)aryl carboxylic esters
(Scheme 1b).7
direct C−H functionalization of indole plays an important part.
The traditional method for the synthesis of indole-3-
carboxylate derivatives involves multistep synthesis via the
intermediacy of the corresponding carboxylic acid or acid
chloride. An alternative strategy for indole-3-carboxylate
synthesis is the transition metal-catalyzed oxidative carbon-
ylation of indoles with alcohol or phenol as the nucleophile
(Scheme 1a).3 For example, the Lei group developed a Pd-
catalyzed aerobic oxidative carbonylation of N-substituted and
unsubstituted indoles to synthesize indole-3-carboxylates.4 Xia
and Li reported a Rh-catalyzed regioselective procedure for
indole-3-carboxylates via carbonylation of indoles.5 Later, the
same group developed a Pd-catalyzed oxidative carbonylation
In the past few decades, photocatalytic radical carbon-
ylations8 have attracted an increasing amount of attention from
the synthetic community owing to their distinctive features. In
this respect, visible-light photoredox catalysis has attracted
considerable attention because of its mild reaction conditions
as well as the abundance of visible light in nature.9 Several
groups, including Lu and Xiao, Wangelin, Gu, Polyzos, Li, and
others, have demonstrated that visible-light photoredox
catalysis could generate a variety of previously inaccessible
radicals, which could undergo radical carbonylation in the
presence of CO or its surrogates to form diversely function-
alized carbonyl compounds under mild conditions.10 Usually,
aryl diazonium salts, Katritzky salts, carboxylic acids, and alkyl
iodides were used as radical precursors. Ru- and Ir-based
complexes or organic dyes such as eosin Y and fluorescein were
used as photocatalysts.
Scheme 1. Oxidative Carbonylation of Indole Derivatives
On the contrary, elementary I2-promoted C−H functional-
ization under visible-light conditions has showed excellent
activity.11 Recent works from the groups of Muniz,11a−d
̃
Nagib,11e Shibata,11f and Jiang11h demonstrated that elemen-
Received: May 1, 2021
Published: June 1, 2021
https://doi.org/10.1021/acs.orglett.1c01494
© 2021 American Chemical Society
Org. Lett. 2021, 23, 4769−4773
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tary I2 could serve as an efficient initiator for radical catalysis in
combination with visible-light irradiation. With a continuous
interest in developing efficient carbonylation under mild
conditions, we herein report a visible-light-induced carbon-
ylation of indoles with phenols under metal-free conditions, in
which elementary I2 was used as a photosensitive initiator
(Scheme 1c).
yield when a 35 W blue LED instead of white light was used
(entry 10). Moreover, implementation of a less powerful 18 W
blue LED resulted in a slightly decreased yield (entry 13).
Only a trace of 3aa was detected without light irradiation
(entry 14). Unfortunately, the carbonylation attempts with a
catalytic amount of I2 in the presence of external oxidants
failed (see details in the Supporting Information).
With the optimal reaction conditions in hand, we then
examined the generality of this visible-light-induced carbon-
ylation reaction. As shown in Scheme 2, the scope of phenol 2
Initially, we examined this carbonylation reaction using N-
methyl indole 1a and phenol 2a as substrates (Table 1). To
a
Table 1. Optimization of the Reaction Conditions
a
Scheme 2. Substrate Scope of Phenols
b
entry
light source
solvent
base
temp (°C) yield (%)
1
2
3
4
5
6
7
8
white (35 W)
white (35 W)
white (35 W)
white (35 W)
white (35 W)
white (35 W)
white (35 W)
white (35 W)
white (35 W)
blue (35 W)
green (35 W)
red (35 W)
DMF
NMP
DEF
THF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
NEt3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
130
130
130
130
130
130
130
90
130
130
130
130
130
130
56
63
61
trace
71
66
NR
63
39
92
89
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
c
9
10
11
12
13
14
81
88
trace
blue (18 W)
without light
a
Reaction conditions: 1a (0.5 mmol), 2a (1.5 mmol), Mo(CO)6 (0.5
mmol), I2 (1.0 mmol), base (1.5 mmol), solvent (2 mL), LED
irradiation, N2 atmosphere, 12 h. Abbreviations: DMF, N,N-
dimethylformamide; NMP, N-methyl pyrrolidone; DEF, N,N-
b
c
diethylformamide. Isolated yields. 2a (2.0 equiv) was added.
our delight, the anticipated reaction proceeded smoothly when
the reaction mixture was treated with I2 in the presence of
Mo(CO)6 and K2CO3 under irradiation with a 35 W white
light-emitting diode (LED). Desired ester product 3aa was
separated in 56% yield when DMF was used as the solvent at
130 °C (entry 1). Inspired by this exciting result, we
investigated different reaction parameters for this trans-
formation. First, different solvents were examined. Amide
solvents such as N-methyl pyrrolidone (NMP) and N,N-
diethylformamide (DEF) afforded desired product 3aa in
moderate yields (entries 2 and 3, respectively). Other solvents,
including THF, MeCN, dioxane, and toluene, were ineffective
for this reaction [entry 4 (see details in the Supporting
Information)]. DMSO was found to be an optimal solvent and
produced 3aa in 71% yield (entry 5). Different bases were then
tested. When Cs2CO3 was used as the base, the yield of 3aa
decreased to 66% (entry 6). Other common bases such as t-
BuOK and NEt3 failed to produce the expected product [entry
7 (see details in the Supporting Information)]. The reaction
temperature and the ratio of 1a to 2a affected the yield of this
reaction, as well. A lower temperature and a smaller amount of
phenol led to decreased reaction efficiencies (entries 8 and 9,
respectively). Further sceening of additives did not improve
the reaction yield (see details in the Supporting Information).
Subsequently, visible-light sources were investigated (entries
10−13). To our delight, product 3aa was obtained in 92%
a
Reaction conditions: 1a (0.5 mmol), 2 (1.5 mmol), Mo(CO)6 (0.5
mmol), I2 (1.0 mmol), K2CO3 (1.5 mmol), DMSO (2 mL), N2
atmosphere, 130 °C, 12 h, isolated yields. At 90 °C. N-Methylindole
1a (3.0 equiv) was added.
b
c
was first investigated. A series of phenol derivatives with
electron-donating groups (3ab−3ah) and electron-withdraw-
ing groups (3ai−3aq) smoothly gave the corrsponding desired
indole-3-carboxylates in moderate to good yields. Compared
with phenols with electron-donating groups, electron-with-
drawing substituents gave slightly higher yields (3ab−3aq).
Notably, the steric hindrance of the substituent had little effect
on the efficiency of this reaction (3ab and 3ad vs 3ac). In
addtion, a series of functional groups, including ether (3ae),
acetyl (3ai), cyano (3aj), and halide (3ak−3aq) groups, were
compatible with this carbonylation reaction. It should be
mentioned that the reaction temperature could be decreased to
90 °C with slight decreases in the reaction yields (3ab and
3ac). However, for a substrate bearing a sensitive functional
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group such as cyano, an even higher yield could be obtained at
a lower temperature (3aj). Moreover, the synthetic utility of
this process is further highlighted by the late-stage
diversification of several phenol-containing pharmaceuticals
and natural products. For example, when estrone (an estrogen
receptor agonist) and eugenol (a naturally occurring guaiacol)
were treated with 3.0 equiv of N-methylindole 1a, products 3ar
and 3at were separated in 69% and 65% yields, respectively. Of
particular note was the fact that β-estradiol was esterified
selectively at the phenolic hydroxy group with the other
aliphatic one remaining untouched, thus producing 3as in 67%
yield. The essential vitamin α-tocopherol, with a hydroxy
group encumbered by the two ortho methyl groups, also
successfully engaged in this reaction and delivered product 3au
in 75% yield.
To gather further insight into this visible-light-induced
carbonylation reaction, the following control experiments were
conducted. First, when N-methylindole 1a was treated with 3.0
equiv of phenyl formate 4a under the standard conditions,
product 3aa was obtained in 81% yield (Scheme 4a). This
Scheme 4. Control Experiments
Subsequently, a series of indole derivatives as the other
reaction partner for this carbonylation were reacted with
phenol 2a to explore the generality of the reaction. As shown
in Scheme 3, a variety of N-methylindole derivatives could
a
Scheme 3. Substrate Scope of (Hetero)arenes
observation indicates that phenyl formate could be converted
to the possible intermediate under the standard conditions. On
the contrary, no desired product 3aa was detected when 3-
iodo-1-methyl-1H-indole 5a was subjected to the standard
reaction conditions without the addition of I2, which excluded
the possibility of 3-iodo-indole as an intermediate in this
reaction (Scheme 4b). Then, two commen radical scavengers,
2,2,6,6-tetramethylpiperidinooxy (TEMPO) and butylated
hydroxytoluene (BHT), were employed in the reaction of N-
methylindole 1a and phenol 2a (Scheme 4c). As a result, the
yield decreased severely to 15% when 2.0 equiv of TEMPO
was added. The reactions were completely shut down when an
excess of TEMPO or BHT was used. These phenomena
indicate that this transformation proceeds by a radical
mechanism.
Then, we analyzed the UV−vis absorbance spectra of 1a, 2a,
4a, I2, and different reaction mixtures in DMSO (see details in
the Supporting Information). The solutions of 1a, 2a, and 4a
have no absorption in the visible region. The solutions of
molecular I2 (λmax = 366 nm), 1a and I2 (λmax = 367 nm), and
2a and I2 (λmax = 365 nm) have similar visible range
absorption. These results indicated that I2 acted as a
photosensitive initiator in this reaction.
a
Reaction conditions: 1 (0.5 mmol), 2a (1.5 mmol), Mo(CO)6 (0.5
mmol), I2 (1.0 mmol), K2CO3 (1.5 mmol), DMSO (2 mL), N2
atmosphere, 130 °C, 12 h, isolated yields. Indole (3.0 equiv) was
added.
b
A plausible reaction pathway for this carbonylation reaction
is proposed on the basis of the control experimental results and
previous reports (Scheme 5). First, a single-electron oxidation
of phenol 2a with I2 under visible-light irradiation generates
phenol radical A. Then, trapping of a CO molecule yields
benzoyl radical B, which was further oxidized by I2 to give
acylium ion C. Finally, nucleophilic attack of N-methylindole
1a on the carbonyl center of acylium ion C gives desired
product 3aa. The use of elementary I2 as an efficient initiator
for radical catalysis under visible-light irradiation has been
elegantly demonstrated in previous reports. Importantly, the
product yields severely decreased when the reaction was
conducted under dark conditions (Table 1, entry 14).
Accordingly, it is reasonable that this carbonylation reaction
afford the corresponding phenyl esters in moderate to good
yields. Both electron-donating (3ba−3fa) and electron-with-
drawing (3ga−3ia) substituents on the indole skeleton were
tolerated. In addition, we also investigated the effect of
substituents on the nitrogen of indoles. Generally, N-alkyl-
substituted indoles reacted smoothly to give expected products
3ja and 3ka in moderate yields.12 However, indoles substituted
with N-electron-withdrawing groups (including Ac, Boc, Piv,
and Ts) failed to give the desired products under the standard
conditions. In addition, other (hetero)arenes such as N-methyl
7-azaindole and 1,3,5-trimethoxybenzene also engaged in this
reaction and afforded the corresponding phenyl esters 3la and
3ma, respectively, in good yields.
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https://pubs.acs.org/10.1021/acs.orglett.1c01494
Scheme 5. A Plausible Reaction Pathway
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the NSFC (21801225), the Science
Foundation for Young Teachers (2019td02) and High-level
Talent Research Start-up Project (2018TP018) of Wuyi
University, the Guangdong Province Universities and Colleges
Pearl River Scholar Funded Scheme (2019), and the
Department of Education of Guangdong Province
(2020KCXTD036) is gratefully acknowledged. The authors
appreciate the general support provided by Prof. Yong-Qiang
Tu at LZU and SJTU.
proceeds via a visible-light-induced radical process. In addition,
the fact that indoles substituted with N-electron-withdrawing
groups failed to give the desired products indicates that indole
acts as the nucleophile that attacks intermediate C to afford the
desired product.
In summary, an efficient protocol for the construction of
indole-3-carboxylates via visible-light-induced carbonylation of
indoles with phenols under metal-free conditions is described.
The reaction proceeds via a radical carbonylation process in
which elementary I2 is used as an effective photosensitive
initiator. A range of functionalized indole-3-carboxylates were
obatained in moderate to good yields from easily available
starting materials. Importantly, the broad applicability of this
method is further highlighted by the late-stage functionaliza-
tion of several phenol-containing natural products and
pharmaceuticals.
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ASSOCIATED CONTENT
* Supporting Information
■
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AUTHOR INFORMATION
Corresponding Author
■
Jin-Bao Peng − School of Biotechnology and Health Sciences,
Wuyi University, Jiangmen, Guangdong 529020, People’s
Republic of China; orcid.org/0000-0002-0568-7740;
Email: pengjb_05@126.com
Authors
Zhuang Qi − School of Biotechnology and Health Sciences,
Wuyi University, Jiangmen, Guangdong 529020, People’s
Republic of China
Lin Li − School of Biotechnology and Health Sciences, Wuyi
University, Jiangmen, Guangdong 529020, People’s Republic
of China
Ying-Kang Liang − School of Biotechnology and Health
Sciences, Wuyi University, Jiangmen, Guangdong 529020,
People’s Republic of China
Ai-Jun Ma − School of Biotechnology and Health Sciences,
Wuyi University, Jiangmen, Guangdong 529020, People’s
Republic of China
Xiang-Zhi Zhang − School of Biotechnology and Health
Sciences, Wuyi University, Jiangmen, Guangdong 529020,
People’s Republic of China; orcid.org/0000-0003-0631-
0758
Complete contact information is available at:
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