Synthesis of indol-3-yl aryl ketones through visible-light-mediated carbonylation

Article abstract of DOI:10.1016/j.cclet.2015.10.012

A visible-light-catalyzed synthesis of indol-3-yl aryl ketones from aryldiazonium salts, CO and indoles at room temperature was developed. This process provides a useful method for the preparation of diverse indol-3-yl aryl ketones from readily accessible reactants under base-free, acid-free and transition-metal-free conditions.

Full text of DOI:10.1016/j.cclet.2015.10.012

Accepted Manuscript
Title: Synthesis of indol-3-yl aryl ketones through
visible-light-mediated carbonylation
Author: Hong-Tao Zhang Li-Jun Gu Xiang-Zhong Huang Rui
Wang Cheng Jin Gan-Peng Li
PII:
S1001-8417(15)00403-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.cclet.2015.10.012
CCLET 3459
To appear in:
Chinese Chemical Letters
Received date:
Revised date:
Accepted date:
20-7-2015
18-9-2015
24-9-2015
Please cite this article as: H.-T. Zhang, L.-J. Gu, X.-Z. Huang, R. Wang, C. Jin, G.-P.
Li, Synthesis of indol-3-yl aryl ketones through visible-light-mediated carbonylation,
Chinese Chemical Letters (2015), http://dx.doi.org/10.1016/j.cclet.2015.10.012
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Graphical Abstract
Synthesis of indol-3-yl aryl ketones through visible-light-mediated carbonylation
Hong-Tao Zhang a, Li-Jun Gu a,b,*, Xiang-Zhong Huang a, Rui Wang a, Cheng Jin c, Gan-Peng Li a,*
a Key Laboratory of Chemistry in Ethnic Medicinal Resources, State Ethnic Affairs Commission &
Ministry of Education, Yunnan Minzu University, Kunming 650500, China
b Engineering Research Center of Biopolymer Functional Materials of Yunnan, Yunnan Minzu University,
Kunming 650500, China
c New United Group Company Limited, Changzhou 213166, China
O
Ar
Eosin Y
BF4
+ CO
ArN2
+
N
R
visible-light
N
R
transition-metal-free
base-free
acid-free
R = Me, Et, H
A visible-light-catalyzed synthesis of indol-3-yl aryl ketones from aryldiazonium salts, CO and indoles at
room temperature under acid-free, base-free and metal-free conditions was described.
Page 1 of 10

Original article
Synthesis of indol-3-yl aryl ketones through visible-light-mediated carbonylation
Hong-Tao Zhang a, Li-Jun Gu a,b, , Xiang-Zhong Huang a, Rui Wang a, Cheng Jin c, Gan-Peng Li a,*
a Key Laboratory of Chemistry in Ethnic Medicinal Resources, State Ethnic Affairs Commission & Ministry
of Education, Yunnan Minzu University, Kunming 650500, China
b Engineering Research Center of Biopolymer Functional Materials of Yunnan, Yunnan Minzu University,
Kunming, Yunnan 650500, China
c New United Group Company Limited, Changzhou 213166, China
ARTICLE INFO
ABSTRACT
Article history:
A visible-light-catalyzed synthesis of indol-3-yl aryl ketones from
aryldiazonium salts, CO and indoles at room temperature was
developed. This process provides a useful method for the
preparation of diverse indol-3-yl aryl ketones from readily
accessible reactants under base-free, acid-free and transition-metal-
free conditions.
Received 20 July 2015
Received in revised form 18
September 2015
Accepted 24 September 2015
Available online
Keywords:
Indol-3-yl aryl ketones
Carbonylation
Visible-light
Aryldiazonium salts
Indoles
1. Introduction
The synthesis and transformation of indoles have been and continue to be a focus of research efforts for
synthetic organic chemists because the indole nucleus is found in countless biologically active molecules
and medicinally relevant structures [1]. Indol-3-yl aryl ketones represent an important class of nitrogen-
containing heterocycles, which are the basic constituents of numerous natural products, biologically active
alkaloids, functional materials and pharmaceuticals [2]. Consequently, the development of synthetic
methods for the preparation of indol-3-yl aryl ketones has received considerable attention [3]. Among the
numerous methods that have been developed, the most commonly used methods are Friedel-Crafts
reactions [4], Vilsmeier-Haack type reactions [5], and Grignard reactions [6]. Recently, transition-metal-
catalyzed acylation of C-H bonds has been developed as promising protocols for the construction of indol-
3-yl aryl ketones [7]. However, these reactions employ acids, bases or transition metals as the catalysts.
The synthesis of indol-3-yl aryl ketones under acid-free, base-free and transition-metal-free conditions has
not yet been realized.
Recently visible light photoredox catalysis attracted much attention due to the rich resource of visible
light, the fact that it is environmentally friendly, and high efficiency [8]. After having garnered little
interest for about one century since being sponsored by Ciamician, reports on visible light photoredox
catalysis exploded since 2008, which exhibited powerful ability in organic synthesis and material
chemistry [9]. Many visible-light photoredox catalysts such as metal or none-metal complexes have been
used to solve the problem of visible light absorption efficiency [10]. In these methods, readily available,
reactive aryl diazonium salts have been widely utilized as a convenient aryl radical source [11]. As a result,
many unusual transformations of these reagents have been carried out by photocatalysis. Recently,
Wangelin and Xiao independently reported a visible-light-mediated redox reaction that affords alkyl
benzoates from arenediazonium salts, carbon monoxide, and alcohols [11b, 11c]. They found that in the
presence of a certain amount of CO, aryl radicals can be easily and efficiently converted into acyl radicals
that can be further oxidized to benzylidyneoxonium salts by the oxidized dye radical cation. We
———
Corresponding author.
E-mail address: gulijun2005@126.com (L.-J. Gu); ganpeng_li@sina.com (G.-P. Li)
Page 2 of 10

envisioned that indoles might serve as a platform to trap benzylidyneoxonium ions. To explore the
application of photocatalysts and to overcome the drawbacks of the synthesis of indol-3-yl aryl ketones,
herein, we disclose our preliminary results on visible light-promoted transformation of aryldiazonium salts
and indoles in the synthesis of indol-3-yl aryl ketones under base-free, acid-free and transition-metal-free
conditions in carbon monoxide atmosphere. The method involves a redox reaction driven by visible light
and catalyzed by Eosin Y (Scheme 1).
Previous work
different reactants, e.g. nitriles,
amines, carboxylic acid
O
derivatives, arylboric acid.
Ar
employing acids, bases or
transition metals as the catalysts
N
R
N
R
This work
base-free
acid-free
+
+
ArN2 BF4
CO
N
R
transition-metal-free
Scheme 1 Synthetic route to indol-3-yl aryl ketones.
2. Experimental
Reagents were obtained commercially and used as received. Solvents were purified and dried by the
standard methods. The melting points were determined on an XT-4 micro melting point apparatus and
uncorrected. IR spectra were recorded on an EQUINOX-55 spectrometer in a KBr matrix. NMR spectra
were recorded on an INOVA-400 NMR instrument at room temperature using TMS as an internal
standard. Coupling constants (J) were measured in Hz. Chemical shift values (δ) are given in ppm. High
Resolution mass spectrometer (HRMS) spectra were recorded on a Bruker micrOTOF-Q II analyzer. A
200-300 mesh silica gel was used for column chromatography.
General procedure: To an 8 mL vial equipped with a magnetic stir bar was charged with 1 (0.3 mmol), 2
(0.4 mmol), Eosin Y (1 mol%), dry MeCN (3.0 mL). The vial was purged with N2 in the dark and
transferred into an autoclave with a Quartz window bottom. The autoclave was flushed three times and
slowly filled with 70 atm of CO. The reaction was irradiated with external LEDs at room temperature for
16 h. After the reaction was finished, the gas was carefully released and the vial retrieved, the reaction
mixture was diluted with 5 mL of H2O, extracted with ethyl acetate (10 mL×3). The organic portion was
washed with a saturated solution of brine, dried (Na2SO4) and concentrated in vacuum, and the resulting
residue was purified by silica gel column chromatography (hexane/ethyl acetate) to provide the desired
products 3. The characterization data of products 3 were provided in Supporting information.
3. Results and discussion
At the outset of our investigation, phenyl diazonium tetrafluoroborate 1a, N-methylindole 2a were
chosen as the model substrates to survey the reaction conditions. Gratifyingly, when a mixture of 1a (0.3
mmol), 2a (0.4 mmol), Eosin Y (1 mol%) in MeCN was irradiated with 5 W white LED light under a CO
pressure of 70 atm at room temperature for 16 h, the desired product 3aa formed in 66 % yield (Table 1,
entry 1). Interestingly, the yield of 3aa was dramatically enhanced to 82% when the reaction was
performed with 5 W green LED light (Table 1, entry 2). A brief survey of solvents such as MeCN, DMSO,
THF, EtOAc and PhCF3 led to the observation that MeCN gave the highest yield of 3aa (Table 1, entries
2-6). Subsequently, various organic dyes were further investigated and the results showed that Eosin Y was
more effective in producing the desired product (Table 1, entries 2, 7-9). Lower pressure of CO resulted in
low conversion (Table 1, entry 10). Both visible-light photoredox catalysts and visible light are essential
for the reaction to take place (Table 1, entries 11-12).
Table 1
Optimization of the Reaction Conditions.a
Page 3 of 10

O
Ph
solvent, catalysis
CO (70 atm), 16 h
PhN2 BF4
+
N
N
2a
3aa
1a
Entry Catalyst
Solvent
Yield
(%) b
66
82
52
1
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
Eosin Y
CH3CN
CH3CN
DMSO
THF
EtOAc
PhCF3
2 c
3
4
5
6
17
trace
trace
62
7
Fluorescei CH3CN
n
8
Rhodamin CH3CN
e B
40
9
RB
Eosin Y
none
CH3CN
CH3CN
CH3CN
CH3CN
27
64
0
10 d
11
12 e
Eosin Y
0
a Reaction conditions: 1a (0.3 mmol), 2a (0.4 mmol), catalyst (1 mol%), solvent (3.0 mL), room
temperature, CO (70 atm), 5 W white LED light for 16 h.
b Isolated yield.
c 5 W green LED light.
d CO (60 atm).
e In the dark.
With this preliminary result in hand, the generality of the method was explored under the optimized
conditions. The scope of aryldiazonium salts 1 was initially explored in the presence of N-methylindole 2a,
Eosin Y (1 mol%) under irradiation with 5 W green LED light and a CO pressure of 70 atm in MeCN at
room temperature, and the results are summarized in Table 2. In general, a variety of functional groups on
the phenyl ring of aryldiazonium salts were compatible in this procedure, affording the desired products in
good to excellent yields. The electron-donating groups such as methoxy and methyl groups on the aryl ring
allowed the aryldiazonium salts to react with 2a efficiently and gave the desired products 3ab-3ac in good
yields. Aryldiazonium salts bearing electron-withdrawing halogen, CF3 and NO2 groups reacted smoothly
to give the corresponding products in good yields (3ad-3af). Furthermore, the substituent at the meta
position on the arene group did not affect the reaction efficiency (Table 2, entry 6). However, a substituent
at the ortho position of the phenyl ring gave the corresponding product 3ah in a lower yield (Table 2, entry
7). These results indicated that the steric effects affected the efficiency of the reactions. Interestingly, a
polysubstituted aryldiazonium salt gave the desired product 3ai in moderate yield (Table 1, entry 8).
Notably, indol-3-yl naphthyl ketones are high affinity binding to the cannabinoid CB1 and CB2 receptors.
The visible-light-induced acylation process allowed the synthesis of indol-3-yl naphthyl ketones 3aj in
54% yield directly.
Table 2
Scope of aryldiazonium salts 1.a
O
5 W green LED
Ar
Eosin Y (1 mol%)
ArN2 BF4
+
N
CO (70 atm), 16 h
MeCN, r.t.
N
2a
3
1
Entr Aryldiazonium
salts 1
Product 3
Yield
(%) b
y
Page 4 of 10

O
MeO
1
2
3
4
5
6
66
73
79
74
70
72
OMe
1b
3ab
3ac
N
O
1c
N
O
Br
Br
1d
3ad
3ae
3af
N
O
O2N
NO2
1e
N
O
F3C
F3C
CF3
1f
N
O
CF3
3ag
N
1g
Cl
O
7
57
Cl
N
1h
3ah
F
O
8
9
59
54
F
F
F
N
1i
3ai
O
1j
3aj
N
a Reaction conditions: 1 (0.3 mmol), 2a (0.4 mmol), Eosin Y (1 mol%), MeCN (3.0 mL), room
temperature, CO (70 atm), 5 W green LED light for 16 h.
b Isolated yield.
The substrate scope was further investigated by reacting phenyl diazonium tetrafluoroborate 1a with
different indoles. Apart from 2a, 1-ethyl-2-methyl-1H-indole 2b smoothly reacted with 1a to give the
corresponding product 3ba in 69% yield. 1,5-Dimethyl-1H-indole 2c and 6-chloro-1-methyl-1H-indole 2d
were all suitable substrates generating the corresponding products 3ca and 3da in 82% and 63% yields,
respectively. However, 4-methoxy-substituted N-methylindole 2e gave the desired indol-3-yl phenyl
ketone 3ea in moderate yield. Treatment of substrate 2f with 1a afforded the desired product 3fa in
moderate yield. In addition, 1-methyl-1H-pyrrolo[2,3-b]pyridine 2g could also provide the expected
product 3ga in 59% yield. It was found that reactions of free NH-indoles 2h-2j with 1a proceeded well and
gave the desired 3-acylindoles 2ha, 2ia and 2ja in 51%, 49% and 42% yields, respectively (Table 3, entries
7-9).
Table 3
Scope of indoles 2.a
O
5 W green LED
Eosin Y (1 mol%)
Ph
+
R
PhN2 BF4
R
N
R1
CO (70 atm), 16 h
MeCN, r.t.
N
R1
1a
3
2
Entr Indoles 2
y
Product 3
Yield
(%) b
69
O
1
N
2b
Et
3ba
N
Page 5 of 10

O
2
3
4
82
63
51
N
3ca
2c
N
O
Cl
N
Cl
2d
3da
N
OMe
OMe
O
N
N
2e
3ea
O
5
6
7
8
54
59
51
49
N
N
2f
3fa
O
N
N
N
N
2g
3ga
O
N
H
2h
2i
HN
3ha
O
N
H
3ia
N
H
O
F
9
42
F
N
H
2j
HN
3ja
a Reaction conditions: 1a (0.3 mmol), 2 (0.4 mmol), Eosin Y (1 mol%), MeCN (3.0 mL), room
temperature, CO (70 atm), 5 W green LED light for 16 h.
b Isolated yield.
To gain insight into the mechanism of the reaction, we conducted a series of control experiments [11b,
11c]. It has been reported that the reaction of aryldiazonium tetrafluoroborate with N-methylindoles can
deliver azo-coupling products. We evaluated the possibility of the formation of 3-arylsulfonyl-indoles by
the reactions in Tables 1-3, but no azo-coupling products or related intermediates were found under our
conditions. When a radical-trapping reagent, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) (1.0 equiv.),
was added to the model reaction, no aryl ketone was obtained (Scheme 2, a). These results suggest that the
photoreaction proceeds via a radical pathway. Furthermore, the product yields dropped precipitously when
no photocatalyst was present in the reaction and/or under dark conditions. Reactions in the dark and at
o
increased temperature (up to 80 C) gave no desired products (Scheme 2, b), which excludes homolytic
bond cleavage of the starting material to an aryl radical under these conditions.
O
Ph
N
a
5 W green LED
Eosin Y (1 mol%)
HRMS: 234.1847
HRMS: 262.1809
4
PhN2 BF4
1a
+
+
TEMPO (1.0 equiv)
CO (70 atm), 16 h
MeCN, r.t.
N
2a
O
N
0.4 mmol
0.3 mmol
Ph
O
5
b
O
Eosin Y (1 mol%)
PhN2 BF4
+
N
3aa
CO (70 atm), 20 h
MeCN, 80 oC, dark
1a
2a
N
trace
0.4 mmol
0.3 mmol
Scheme 2. Control experiment.
On the basis of these preliminary results, and those of previous studies [11b, 12-14], we propose a
mechanism shown in Scheme 3. Initially, photoexcitation of Eosin Y by visible light generates excited
[Eosin Y*]. Then the electron-deficient phenyl diazonium tetrafluoroborate 1a accepts one electron from
the excited [Eosin Y*]. This single-electron transfer (SET) results in the generation of a phenyl radical
(Ph·) and the oxidized dye radical cation [Eosin Y+·]. The resulting phenyl radical (Ph·) is rapidly trapped
Page 6 of 10

by CO to give a benzoyl radical A. Further oxidation of A by [Eosin Y+·] results in the
benzylidyneoxonium B. Finally, electronic trapping of B by 2a gives the desired product 3aa.
-
N2
BF4
+
SET
1a
Ph
Eosin Y*
Eosin Y+
5 W green
LED
CO
3aa
2a
HBF4
Eosin Y
-
BF4
SET
O
O
B
A
Scheme 3 Plausible mechanism.
4. Conclusion
In summary, we have developed a visible-light-mediated carbonylation for the synthesis of indol-3-yl
aryl ketones, which is a ubiquitous component of many natural products, biomolecules and materials. Most
importantly, simple and readily available Eosin Y emerges as an efficient catalyst. Metal catalysts, which
are often expensive and difficult to be completely removed from products and can be a challenge in the
synthesis of pharmaceutical compounds, were avoided. Mechanistic studies support the sequential
operation of SET reduction, carbonylation, and back electron transfer to give aroylium cations. Further
investigations of the mechanism of the reaction and its applications are ongoing in our laboratory.
Acknowledgment
We are grateful for the financial support from the Educational Bureau of Yunnan Province (No.
2010Y431) and the State Ethnic Affairs Commission (No. 12YNZ05).
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[10] For recent selected examples for visible-light photoredox catalysts, see:
(a) H.J. Jiang, Y.Z. Cheng, R.Z. Wang, Y. Zhang, S.Y. Yu, Synthesis of isoquinolines via visible light-
promoted insertion of vinyl isocyanides with diaryliodonium salts, Chem. Commun. 50 (2014) 6164-
6166;
(b) Y. Xu, W. Zhang, Ag/AgBr-grafted graphite-like carbon nitride with enhanced plasmonic
photocatalytic activity under visible light, ChemCatChem. 5 (2013) 2343-2351;
(c) G.B. Deng, Z.Q. Wang, J.D. Xia, et al., Tandem cyclizations of 1,6-enynes with arylsulfonyl
chlorides by using visible-light photoredox catalysis, Angew. Chem. Int. Ed. 52 (2013) 1535-1538;
(d) J.D. Xia, G.B. Deng, M.B. Zhou, et al., Reusable visible light photoredox catalysts; catalyzed
benzylic C(sp3)–H functionalization/ carbocyclization reactions, Synlett. 18 (2012) 2707-2713;
(e) E.L. Tyson, Z.L. Niemeyer, T.P. Yoon, Redox mediators in visible light photocatalysis: photocatalytic
radical thiol–ene additions, J. Org. Chem. 79 (2014) 1427-1436;
(f) L.J. Gu, C. Jin, J.Y. Liu, H.Y. Ding, B.M. Fan, Transition-metal-free, visible-light induced cyclization
of arylsulfonyl chlorides with 2-isocyanobiphenyls to produce phenanthridines, Chem. Commun. 50
(2014) 4643-4645;
(g) J. Xuan, X. Xia, T. Zeng, et al., Visible-light-induced formal [3+2] cycloaddition for pyrrole
synthesis under metal-free conditions, Angew. Chem. Int. Ed. 53 (2014) 5653-5656;
(h) J. Liu, Q. Liu, H. Yi, et al., Visible-light-mediated decarboxylation/oxidative amidation of α-keto
acids with amines under mild reaction conditions using O2, Angew. Chem. Int. Ed. 53 (2014) 502-506;
(i) D.P. Hari, B. König, Synthetic applications of eosin Y in photoredox catalysis, Chem. Commun. 50
(2014) 6688-6699;
(j) A.K. Yadav, V.P. Srivastava, L.D.S. Yadav, Visible-light-mediated eosin Y catalyzed aerobic
desulfurization of thioamides into amides, New J. Chem. 37 (2013) 4119-4124;
(k) T.B. Xiao, L.Y. Li, G.L. Lin, et al., Synthesis of 6-substituted phenanthridines by metal-free, visible-
light induced aerobic oxidative cyclization of 2-isocyanobiphenyls with hydrazines, Green Chem. 16
(2014) 2418-2421;
(l) M.N. Hopkinson, B. Sahoo, J. Li, F. Glorius, Dual catalysis sees the light: combining photoredox
with organo-, acid, and transition-metal catalysis, Chem. Eur. J. 20 (2014) 3874-3886;
(m) C. Yu, N. Iqbal, S. Park, E.J. Cho, Selective difluoroalkylation of alkenes by using visible light
photoredox catalysis, Chem. Commun. 50 (2014) 12884-12887;
(n) J.C. Tellis, D.N. Primer, G.A. Molander, Single-electron transmetalation in organoboron cross-
coupling by photoredox/nickel dual catalysis, Science. 345 (2014) 433-436;
(o) Z. Zuo, D.T. Ahneman, L. Chu, et al., Merging photoredox with nickel catalysis: coupling of α-
carboxyl sp3-carbons with aryl halides, Science, 345 (2014) 437-440.
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[11] (a) D.P. Hari, P. Schroll, B. König, Metal-free, visible-light-mediated direct C–H arylation of
heteroarenes with aryl diazonium salts, J. Am. Chem. Soc. 134 (2012) 2958-2961;
(b) M. Majek, A.J. Wangelin, Metal-free carbonylations by photoredox catalysis, Angew. Chem. Int. Ed.
54 (2015) 2270-2274;
(c) W. Guo, L.Q. Lu, Y. Wang, et al., Metal-free, room-temperature, radical alkoxycarbonylation of
aryldiazonium salts through visible-light photoredox catalysis, Angew. Chem. Int. Ed.54 (2015) 2265-
2269;
(d) T. Hering, D.P. Hari, B. König, Visible-light-mediated α-arylation of enol acetates using aryl
diazonium salts, J. Org. Chem. 77 (2012) 10347-10352;
(e) F.P. Crisóstomo, T. Martin, R. Carrillo, ascorbic acid as an initiator for the direct C-H arylation of
(hetero)arenes with anilines nitrosated in situ, Angew. Chem. Int. Ed. 53 (2014) 2181-2185;
(f) L.J. Gu, C. Jin, J.Y. Yan, Metal-free, visible-light-mediated transformation of aryl diazonium salts
and (hetero)arenes: an efficient route to aryl ketones, Green Chem. 17 (2015) 3733-3736.
[12] T. Kawamoto, T. Okada, D.P. Curran, I. Ryu, Efficient hydroxymethylation reactions of iodoarenes
using CO and 1,3-dimethylimidazol-2-ylidene borane, Org. Lett. 15 (2013) 2144-2147.
[13] B. Sahoo, M.N. Hopkinson, F. Glorius, Combining gold and photoredox catalysis: visible light-
mediated oxy- and aminoarylation of alkenes, J. Am. Chem. Soc. 135 (2013) 5505-5508.
[14] D.P. Hari, B. König, The photocatalyzed meerwein arylation: classic reaction of aryl diazonium salts
in a new light, Angew. Chem. Int. Ed. 52 (2013) 4734-4743.
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