3-Acylindoles Synthesis: Ruthenium-Catalyzed Carbonylative Coupling of Indoles and Aryl Iodides

Article abstract of DOI:10.1021/acs.orglett.7b02320

A novel and convenient procedure for the synthesis of 3-acylindoles from simple indoles and aryl iodides has been established. Through ruthenium-catalyzed carbonylative C-H functionalization of indoles, with Mo(CO)₆ as the solid CO source, the desired indol-3-yl aryl ketones were isolated in moderate to good yields. Not only N-alkylindoles but also N-H indoles can be applied here.

Full text of DOI:10.1021/acs.orglett.7b02320

Letter
pubs.acs.org/OrgLett
3‑Acylindoles Synthesis: Ruthenium-Catalyzed Carbonylative
Coupling of Indoles and Aryl Iodides
Zechao Wang, Zhiping Yin, and Xiao-Feng Wu*
Leibniz-Institut fur Katalyse e.V. an der Universitat Rostock, Albert-Einstein-Straße 29a, 18059 Rostock, Germany
̈
̈
S
* Supporting Information
ABSTRACT: A novel and convenient procedure for the
synthesis of 3-acylindoles from simple indoles and aryl iodides
has been established. Through ruthenium-catalyzed carbon-
ylative C−H functionalization of indoles, with Mo(CO)6 as the
solid CO source, the desired indol-3-yl aryl ketones were
isolated in moderate to good yields. Not only N-alkylindoles but also N-H indoles can be applied here.
uring the past years, transition-metal catalysts have been
extensively explored as a topic of new C−C bond
desired. Carbonylative 3-acylation of indoles is one of the most
D
straightforward procedures. In 2015, Arndtsen’s group
demonstrated a palladium-catalyzed carbonylative coupling of
heterocycles with aryl iodides via C−H functionalization.16 3-
Acylindoles can be effectively produced under CO pressure.
Meanwhile, Guan and co-workers described a novel palladium-
catalyzed carbonylative coupling of indoles with aromatic
boronic acids.17 With the addition of I2 and KOH, via in situ
generation of 3-iodoindole intermediates,18 good yields of 3-
acylindoles can be prepared with a wide range of functional
groups tolerance. More recently, the application of visible light
in carbonylative synthesis of 3-acylindoles has been realized by
the groups of Gu19 and Li and Liang20 as well. With the
assistance of photoredox catalysts under high CO pressure
(70−80 bar), 3-acylindoles were formed at room temperature.
In this paper, we report here a new procedure for the synthesis
of 3-acylindoles. With ruthenium as the catalyst using simple
indoles and readily available aryl iodides as the substrates, the
desired 3-acylindoles can be produced even from N-H indoles.
Notably, Mo(CO)6 has been applied as the solid and safe CO
source here.21
Our initial investigation began with 1,2-dimethyl-1H-indole
(1a, 1 equiv) and iodobenzene (2a, 2 equiv), [RuCl2(p-
cymene)]2 (5 mol %), and [Mo(CO)6] (1 equiv) in HFIP
(hexafluoroisopropanol). To our delight, 20% yield of the
desired carbonylation product 3a was formed after 30 h at 100
°C (Table 1, entry 1). Subsequently, various ruthenium
catalysts such as CpRuCl(PPh3)2, [RuCl2(cod)]n, and
RuCl2(PPh3)3 were tested (Table 1, entries 2−4). Unfortu-
nately, none of them could give improved results. Then the
effects of bases and acids were investigated (Table 1, entries 5−
14). TFA was found to be the best in this reaction, whereas
K2HPO4, Li2CO3, LiBr, LiCl, KH2PO4, and MesCO2H (2, 4, 6-
trimethylbenzoic acid) were all found to be inferior. Et3N and
DBU inhibited the reaction completely, and no conversion of
1a was observed (Table 1, entries 5 and 6). Only a trace of
formation through direct C−H activation.1 Among the various
noble metal catalysts, ruthenium catalysts are attractive due to
their relative low cost and high reaction selectivity.2,3 Several
challenging transformations have been realized with ruthenium
complex as the catalyst, such as C−H alkenylation,4 arylation,5
and alkyne annulations.6 However, the reports on ruthenium-
catalyzed carbonylation reactions are still very limited. In 1992,
Moore’s group reported a Ru3(CO)12-catalyzed sp2 C−H
carbonylation of aromatic heterocycles with olefins.7 This
reaction exhibited high regioselectivity and high catalyst
turnover frequencies. Subsequently, Murai’s group studied the
Ru3(CO)12-catalyzed carbonylative coupling reaction of
imidazoles with olefins, which has good yields, impressive
catalytic efficiency, and wide functional group compatibility.8
Then Chatani and co-workers demonstrated Ru3(CO)12-
catalyzed carbonylative transformation of sp2 C−H and sp3
C−H with pyridine as the directing group.9 The desired
carbonylation products were formed in moderate to good
yields. More recently, Beller’s group reported a [Ru(cod)Cl2]n-
catalyzed directing group assisted carbonylative C−H activation
of arenes.10 The reactions were carried out in water and aryl
iodides and styrenes were used as the coupling partners.
Indoles have been studied as well, but the presence of the
directing group was essential.
On the other hand, 3-acylindoles are common structural
motifs in many biologically active compounds, natural
products,11 and pharmaceutical compounds,12 such as indiacen
A, indiacen B, and MK-0533. Additionally, 3-acylindoles have
been applied as key intermediates for synthesis of some other
value-added compounds as well.13g Because of the versatile
values and applications, their preparation attracts much interest.
Traditional procedures include Friedel−Crafts acylations,13
Vilsmeier−Haack-type reactions,14 and indole Grignard reac-
tions.15 The most frequently used Friedel−Crafts reaction
requires troublesome N-protection, especially for indoles
bearing an electron-donating group with a stoichiometric
Lewis acid promoter and strict exclusion of moisture. Hence,
alternative methods for 3-acylindole preparation are highly
Received: July 27, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02320
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
the desired product could be detected when the reaction was
performed under pure O2 (1 bar) pressure or argon
atmosphere. These results suggest that the oxygen in air joined
in the reaction, but pure oxygen destroyed the catalyst activity.
With the optimized conditions in hand (Table 1, entry 21),
we began to investigate the scope of iodoarenes subsequently.
As shown in Scheme 1, carbonylation of 1,2-dimethyl-1H-
Table 1. Optimization of Reaction Conditions
b
entry
catalyst
base/acid
additive
yield (%)
a
1
[RuCl2(p-cymene)]2
CpRuCl(PPh3)2
22 (20)
Scheme 1. 3-Acylindole Synthesis: Variation of Aryl Iodides
2
0
3
[RuCl2(cod)]n
<5
0
4
RuCl2(PPh3)3
5
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p−cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
Et3N
0
6
DBU
0
7
K2HPO4
Li2CO3
LiBr
15
8
32
9
40
10
11
12
13
14
LiCl
35
KH2PO4
CH3SO3H
MesCO2H
TFA
42 (40)
<5
28
45
c
15
TFA
48
c
16
TFA
FeCl3
MgCl2
ZnCl2
ZnBr2
ZnBr2
ZnBr2
ZnBr2
44
c
17
TFA
46
c
18
TFA
50
c
19
TFA
55
cd
,
20
TFA
35
ce
,
21
TFA
62 (61)
57
cf
,
22
TFA
a
Conditions: 1a (0.2 mmol, 1 equiv), 2a (0.4 mmol, 2 equiv), catalyst
(0.01 mmol, 5 mol %), Mo(CO)6 (0.2 mmol, 1 equiv), base/acid (0.2
mmol, 1 equiv), additive (0.08 mmol, 40 mol %), HFIP (1 mL),
stirring at 100 °C for 30 h under air. Yields were determined by GC
a
b
Reaction conditions: 1,2-dimethyl-1H-indole 1a (0.2 mmol, 1 equiv),
iodoarenes 2 (0.4 mmol, 2 equiv), [RuCl2(p-cymene)]2 (0.01 mmol, 5
mol %), Mo(CO)6 (0.2 mmol, 1 equiv), TFA (0.06 mmol, 30 mol %),
ZnBr2 (0.08 mmol, 40 mol %), HFIP (1 mL), 100 °C, 30 h, air,
isolated yield.
using n-hexadecane as the internal standard. Isolated yield is in
c
d
parentheses. TFA (0.06 mmol, 30 mol %). Mo(CO)6 (0.1 mmol, 0.5
e
f
equiv). Mo(CO)6 (0.3 mmol, 1.5 equiv). Mo(CO)6 (0.4 mmol, 2
equiv).
indole with iodoarenes proceeded smoothly under our standard
reaction conditions. Using iodoarenes with either electron-
donating or electron-withdrawing groups led to the formation
of the corresponding carbonylation products in moderate to
good yields. Substrates can tolerate various functional groups
such as Bn, OCH3, CF3, Cl, F, and COOMe. However,
iodoarenes substituted with OH or NO2 could not give the
desired carbonylation product. No carbonylation products
could be obtained with 3-iodopyridine as the substrate.
Importantly, the position of substituent R1 had a critical effect
on this carbonylation reaction. Iodoarenes substituted with
functional groups in the meta position could give higher yields
than that in the para position (3e vs 3f, 3i vs 3j).
Next, various indoles were investigated for further extending
the substrates scope (Scheme 2). N-Substituted indoles can be
readily carbonylated with iodoarenes to provide moderate to
good yields of the corresponding carbonylation products (4a−
d). A yield of 70% can be achieved with −COOMe-decorated
aryl iodide. Remarkably, free NH-indoles can be successfully
applied as well (4e−i) and gave good yields of the desired
products with total chemoselectivity. Here, the obtained N-H
free 3-acylindole products (4e−i) are ready for further C−N
coupling reactions. However, we could not detect the products
4j or 4k when indole or 3-methyl-1H-indole was used as the
substrate. These results revealed that the methyl group on the
carbonylation product 3a could be detected when CH3SO3H
was added (Table 1, entry 12). Further study showed that a
lower amount of TFA led to higher yield (48%; Table 1, entry
15). To further improve the outcome, different additives were
used in this reaction (Table 1, entries 16−19). Surprisingly, we
found that ZnBr2 can give the carbonylation product in 55%
yield (Table 1, entry 19). Replacement of ZnBr2 with FeCl3,
MgCl2, or ZnCl2 proved detrimental to the efficiency of the
process, with product 3a being formed in diminished 44%, 46%,
and 50% yield, respectively (Table 1, entries 16−18). Notably,
the amount of Mo(CO)6 played a crucial role for the outcome
of this reaction (Table 1, entries 20 and 21). The yield could be
improved by increasing the amount of Mo(CO)6 to 1.5 equiv,
and we could obtain the carbonylation product 3a in 61% yield
(Table 1, entry 21). In addition, the model reaction was also
performed with other CO sources such as formic acid, CO,
Cr(CO)6, Co2(CO)8, and Fe2(CO)9. However, none of them
gave better results (see the Supporting Information). These
results imply that Mo(CO)6 might play several roles in this
transformation. In the case of temperature testing, we found
that less than 5% of the desired product was formed at 80 °C
and the yield decreased as well at higher temperature (120 °C;
28% yield). Importantly, no product could be detected in the
absence of ruthenium catalyst. Additionally, less than 10% of
B
DOI: 10.1021/acs.orglett.7b02320
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
molybdenum slat. The in situ generated zinc reagent then
moved to transmetalation with ruthenium catalyst to produce
the ruthenium intermediate A. Following this, the ruthenium
species A underwent oxidative addition upon formation of
intermediate B. Subsequently, intermediate C was generated
through the coordination and insertion of CO into the Ru−C
bond. Finally, the terminal product could be eliminated through
reductive elimination, and the active ruthenium catalyst for the
next catalytic cycle was regenerated under the presence of air.
In summary, we have developed an interesting procedure for
the synthesis of 3-acylindoles. Through ruthenium-catalyzed
carbonylative C−H functionalization with Mo(CO)6 as the
solid CO source, moderate to good yields of the desired
products can be prepared with good functional group tolerance.
Scheme 2. 3-Acylindole Synthesis: Variation of Indoles
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b02320.
Experimental procedures and NMR spectra for obtained
compounds (PDF)
AUTHOR INFORMATION
■
a
Reaction conditions: indoles 1a (0.2 mmol, 1 equiv), iodoarenes 2
(0.4 mmol, 2 equiv), [RuCl2(p−cymene)]2 (0.01 mmol, 5 mol %),
Mo(CO)6 (0.2 mmol, 1 equiv), TFA (0.06 mmol, 30 mol %), ZnBr2
(0.08 mmol, 40 mol %), HFIP (1 mL), 100 °C, 30 h, air, isolated yield.
Corresponding Author
*E-mail: xiao-feng.wu@catalysis.de.
ORCID
Xiao-Feng Wu: 0000-0001-6622-3328
Notes
2-position of indoles played a crucial role in the carbonylative
C−H activation reaction. Additionally, heterocycles such as
benzothiazole, benzothiophene, and pyrrole were also tested in
the reaction conditions, but none of them could give the
desired carbonylation products.
On the basis of the above results and literature, we postulated
a possible reaction mechanism for the ruthenium-catalyzed
carbonylation of indoles (Scheme 3). First, indoles were
activated and transformed into the corresponding organo-
metallic reagents in the presence of ZnBr2, TFA, and
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Chinese Scholarship Council for financial
support. We thank the analytical department of Leibniz-
Institute for Catalysis at the University of Rostock for their
excellent analytical service here. We appreciate the general
support from Professor Armin Borner and Professor Matthias
̈
Beller in LIKAT.
Scheme 3. Proposed Reaction Mechanism
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