Ruthenium-Catalyzed Oxidative Dearomatization of Indoles for the Construction of C2-Quaternary Indolin-3-ones

Article abstract of DOI:10.1055/s-0036-1591876

A ruthenium-catalyzed oxidative dearomatization of 2-alkyl- or 2-aryl-substituted indoles has been developed. When coupled with a cascade transformation, it provides a new system for the construction of indolin-3-ones bearing a C2-quaternary functionality

Full text of DOI:10.1055/s-0036-1591876

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©
Georg Thieme Verlag Stuttgart · New York
2018, 28, A–E
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A
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Synlett
X.-Y. Zhou et al.
Letter
Ruthenium-Catalyzed Oxidative Dearomatization of Indoles for
the Construction of C2-Quaternary Indolin-3-ones
Xiao-Yu Zhou*a
Xia Chen*a
O
R1
Liang-Guang Wangb
Dan Yanga
Jin-Hui Lia
R1
R1
RuCl3·3H2O (5.0 mol%)
R3
R3
N
R2
N
R2
N
R2
NaIO4 (1.0 equiv)
CH CN, 70 °C
3
a
R3
School of Chemistry and Materials Engineering, Liupanshui
Normal University, Liupanshui 553004, P. R. of China
College of Chemistry and Chemical Engineering, Anshun
University, Anshun 561000, P. R. of China
zhouxiaoyu20062006@126.com
1
♣ New Catalysis System
R
R
R
= H, alkyl or Ar
= H or alkyl
= H, Me, Cl, etc
♦ Milder Reaction Conditions
♥ High Regioselectivity
2
3
b
11 examples
67–97% yield
♠
Good Yield
Received: 20.10.2017
Accepted after revision: 27.11.2017
Published online: 15.01.2018
the oxidant (Figure 1). Recently, a palladium-catalyzed oxi-
dative dearomatization of indoles has been reported by Gu-
chhait et al.8a for the synthesis of C2-quaternary indolin-3-
ones. However, tert-butyl hydroperoxide (TBHP, 2.2 equiv)
DOI: 10.1055/s-0036-1591876; Art ID: st-2017-w0774-l
Abstract A ruthenium-catalyzed oxidative dearomatization of 2-alkyl-
or 2-aryl-substituted indoles has been developed. When coupled with a
cascade transformation, it provides a new system for the construction
of indolin-3-ones bearing a C2-quaternary functionality. The reaction
occurs readily with RuCl ·3H O as a catalyst in acetonitrile. 2-(3-Indolyl)-
and MnO (2.0 equiv) were employed in excess as oxidants
2
(Figure 1). At the same time, we achieved a palladium-cata-
lyzed Wacker-type oxidation, oxidation–hydroxylation, and
oxidation–methoxylation of N-Boc-protected indoles for
3
2
substituted indolin-3-ones were obtained in medium to high yields. A
mechanism for the reaction is also proposed.
9
the preparation of 3-oxoindolines. Although many meth-
ods have been developed for the synthesis of 3-indolin-3-
ones skeletons,8 transition metal catalyzed systems to re-
alize the direct oxidation of indoles deserve more attention.
b–e
Key words ruthenium catalysis, oxidation, dearomatization, indoles,
indolinones
[
Mo], R3 = OH or OMe
Previous Work
The oxidation process has been widely recognized as a
powerful strategy for the synthesis of cyclic or heterocyclic
oxo compounds from the corresponding substrates. Howev-
er, little effort has been devoted to the oxidation of aromat-
ic or heteroaromatic compounds. Amongst the heteroaro-
matic systems, indoles are by far the most widely exploit-
mCPBA or dimethyldioxirane, R3 = OH
O
R3
R2
R2
[Ru]/2,6-Cl2PyNO, R3 = H
N
R1
N
R1
[Pd]/MnO /TBHP, R3 = indolyl
2
1
[Pd]/H O , R3 = H, OH or OMe
ed. They are easily derivatized by nucleophilic addition or
2
2
Friedel–Crafts reactions, so they have a great value in the
preparation of natural products, pharmaceuticals, and oth-
This Work
O
R1
2
R1
er fine chemicals.
R3
R1
RuCl3·3H2O (5.0 mol%) R3
3
In the past, molybdenum pentoxide, m-chloroperoxy-
N
R2
R2
N
R2
N
NaIO4 (1.0 equiv)
MeCN, 70 °C
4
5
6
benzoic acid (MCPBA), DDQ, and dimethyldioxirane have
been used in rapid oxidations of indoles to oxoindolines as
the primary methods for synthesizing these compounds
R3
(Figure 1). However, these processes have many limitations,
Figure 1 Systems for the oxidation of indoles to give 3-oxoindolines
such as toxic metal pollution, incompatibility with many
functional groups, low yields, and poor selectivity. Fortu-
nately, a few methods for transition metal catalyzed oxida-
tions of indoles have been reported. An efficient and
highly selective ruthenium-porphyrin catalyzed oxidation
Ruthenium catalysts10 have played an important role in
the recent development of catalytic oxidation reactions.
11
12
Ruthenium-catalyzed oxidations of alkanes, alcohols,
7
13
of N-tosylindoles to give 3-oxoindolines was achieved by
Che and co-workers using 2,6-dichloropyridine N-oxide as
and ethers are typical methods for the synthesis of car-
bonyl compounds. Other oxidative transformations, such as
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 28, A–E

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Synlett
X.-Y. Zhou et al.
Letter
1
4
ruthenium-catalyzed asymmetric epoxidations of alkenes,
O
15
R1
cleavage of double bonds, and biomimetic oxidations of
R1
R3
R1
RuCl3·3H2O (5.0 mol%)
R3
16
17
amines and phenols under mild conditions also play an
important role in transition metal catalyzed oxidations of
organic compounds. Therefore, ruthenium-catalyzed oxida-
tions of heteroaryl compounds, especially indoles, should
clearly provide a variety of practical and useful methods for
the preparation of heterocyclic oxo compounds. In addition,
sodium periodate (NaIO ) has been extensively used in
ruthenium-catalyzed oxidations of alkenes, amides, and
alcohols, representing, therefore, a potentially interesting
oxidant agent in the oxidative dearomatization of indoles.
Here, we report a ruthenium-catalyzed oxidative dearoma-
N
R2
N
R2
N
R2
NaIO4 (1.0 equiv)
MeCN, 70 °C
1
2
3
R
R
R
= H, alkyl or Ar
= H or alkyl
= H, Me, Cl, etc
R3
Scheme 1 Ruthenium-catalyzed oxidative dearomatization of indoles
4
18
19
20
In the course of our research on ruthenium-catalyzed
oxidative dearomatization of indoles, we obtained 2-indolyl-
3-oxoindoline as a main product. Our initial experiment be-
gan with 2-phenylindole (1a) as a model starting material,
tization of indoles in the presence of NaIO (Scheme 1).
4
NaIO (1.5 equiv) as an oxidant, and ruthenium trichloride
4
Table 1 Optimization of the Conditions for Ruthenium-Catalyzed Oxidative Dearomatization of 2-Phenylindole (1a)a
O
Ph
Ph
Catalyst (x mol%)
Ph
N
NH
N
Oxidant (y equiv)
Solvent, T °C
H
H
1a
2a
b
Entry
Catalyst (mol%)
Oxidant (equiv)
Solvent
Temp (°C)
Yield (%)
1
2
3
4
5
6
7
8
9
RuCl
RuCl
RuCl
RuCl
RuCl
RuCl
RuCl
RuCl
RuCl
RuCl
RuCl
RuCl
RuCl
RuCl
3
3
3
3
3
3
3
3
3
3
3
3
3
2
·3H
·3H
·3H
·3H
·3H
·3H
·3H
·3H
·3H
·3H
·3H
·3H
·3H
2
2
2
2
2
2
2
2
2
2
2
2
2
O (5)
O (5)
O (5)
O (5)
O (5)
O (5)
O (5)
O (5)
O (5)
O (5)
O (5)
O (5)
O (5)
NaIO
70 wt% aq TBHP
(tBuO)
(3.0)
30 wt% aq H
AcOOH (3.0)
(1 atm)
4
(1.5)
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
THF
40
40
40
40
40
40
40
40
40
60
60
70
80
70
70
70
70
70
70
70
70
70
70
36
15
2
NR
2
O
2
(12.0)
trace
trace
NR
O
2
NaIO
NaIO
NaIO
NaIO
NaIO
NaIO
NaIO
NaIO
NaIO
NaIO
NaIO
NaIO
NaIO
NaIO
NaIO
NaIO
NaIO
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
(1.5)
(1.5)
(1.5)
(1.5)
(1.5)
(1.5)
(1.5)
(1.5)
(1.5)
(1.5)
(1.5)
(1.5)
(1.5)
(1.5)
(1.5)
(1.2)
(1.0)
trace
57
acetone
MeCN
acetone
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
55
10
11
12
13
14
15
16
17
18
19
20
21
22
23
63
75
88
86
(PPh
Ru(acac) (5)
[RuCl (p-cymene)]
FeCl (10)
Fe(acac) (10)
Cu(OAc) ·H O (10)
3
)
3
(5)
55
3
trace
59
2
2
(5)
3
41
3
trace
trace
20
2
2
CuCl
–
2
·2H
2
O (10)
trace
92
RuCl
3
3
·3H
2
O (5)
O (5)
RuCl
·3H
2
97
a
Reaction conditions: 1a (97 mg, 0.50 mmol), catalyst, oxidant, solvent (3.0 mL), 24 h.
Isolated yield.
b
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 28, A–E

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Synlett
X.-Y. Zhou et al.
Letter
(
RuCl ·3H O) as a catalyst in ethyl acetate (EtOAc). The re-
we also tested N-methyl and N-ethyl indoles 1i–k under the
optimal reaction conditions and we obtained the corre-
sponding products 2i–k in 81–87% yield (entries 9–11).
However, in the case of the 2-ethoxycarbonyl indole 1l (en-
try 12), no product was detected and the starting material
was recovered. Indole (1m; entry 13) and N-methylindole
(1n; entry 14), without any substituents on the 2-position,
also failed to give the target products, and the reaction
systems were complex.
3
2
quired product, 2-phenyl-2-(2-phenyl-1H-indol-3-yl)indolin-
-one (2a), was isolated, but only in 36% yield (Table 1,
3
entry 1). This result encouraged us to adhere to our initial
hypothesis that a ruthenium catalyst might be a suitable
catalyst in the oxidative dearomatization of indoles. We
therefore evaluated other oxidants, solvents, and catalysts
with the aim of improving the yield of indolinone 2a.
First, we tested tert-butyl hydroperoxide (TBHP; Table 1,
t
entry 2), di-tert-butyl peroxide [( BuO) , entry 3], H O (en-
2
2
2
try 4), peracetic acid (AcOOH, entry 5), and oxygen (O , en-
try 6) as oxidants in an attempt to improve the yield of 2a.
Table 2 Ruthenium-Catalyzed Oxidative Dearomatization Reactions of
2
Indolesa
The results showed that NaIO is the best oxidant for the
4
O
ruthenium-catalyzed oxidative dearomatization of indoles,
although it gave relatively low yields. A 15% yield of 2a was
obtained when TBHP was chosen as the oxidant, and the
other oxidant showed no oxidative activity in the rutheni-
R1
R1
R3
R3
R1
RuCl ·3H O (5.0 mol%)
3
2
N
R2
N
R2
N
R2
NaIO4 (1.0 equiv)
MeCN, 70 °C
1
2
um-catalyzed oxidative dearomatization of 1a. When H O2
2
R3
or AcOOH was chosen as the oxidant, many bubbles es-
caped from the solvent surface, possibly as a result of the
decomposition of these oxidants in the presence of the
ruthenium salt. Tetrahydrofuran (THF, entry 7), acetone
R1
Ph
R2
R3
Yield (%)
b
Entry Substrate
Product
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
H
H
2a
2b
2c
2d
2e
2f
97
85
75
85
85
82
67
79
87
82
81
NA
NA
NA
4-Tol
H
H
(entry 8), and acetonitrile (entry 9) were then evaluated as
solvents in the presence of NaIO (1.5 equiv) at 40 °C in an
4-EtC
6
H
4
H
H
4
attempt improve the yield of the product 2a. The reaction
was completely inhibited when THF was used as a solvent
in the presence of RuCl ·3H O, and the yield improved to
4-FC H
H
H
6
4
Me
H
H
3
2
Me
Me
Me
Ph
H
Me
OMe
Cl
H
57% and 55% when acetone or acetonitrile, respectively, was
1g
1h
1i
H
2g
2h
2i
used as the solvent. Next, the temperature was increased in
an attempt to improve the yield of 2a in acetone and aceto-
nitrile. An 88% yield was obtained when the reaction was
carried out at 70 °C in acetonitrile (entry 12). Higher tem-
peratures did not improve this result (entry 13; 80 °C, 86%).
To test the activity of other catalysts, several metal salts
were evaluated. RuCl (PPh ) (entry 14), Ru(acac)3 (entry
H
Me
Et
Me
H
1
0
1
2
3
4
1j
Ph
H
2j
1
1
1
1k
1l
Me
H
2k
2l
CO
H
2
Et
H
2
3 3
1m
1n
H
H
2m
2n
1
5), [RuCl (p-cymene)] (entry 16), FeCl3 (entry 17),
2 2
1
H
Me
H
Fe(acac)3 (entry 18), Cu(OAc) ·H O (entry 19), and
2
2
a
CuCl ·2H O (entry 20), when tested in the presence of NaIO
3 2
Reaction conditions: 1 (0.50 mmol), RuCl ·3H O (6.5 mg, 0.025 mmol, 5.0
2
2
4
mol%), NaIO
Isolated yield.
4
(107 mg, 0.50 mmol, 1.0 equiv), MeCN (3.0 mL), 70 °C, 24 h.
(1.5 equiv) in MeCN at 70 °C failed to give better results,
b
suggesting that RuCl ·3H O might be the most effective cat-
3
2
alyst. Finally, the amount of NaIO was examined, and a 97%
4
yield of 2a was obtained when NaIO (1.0 equiv) was used
These results indicated that substrates with 2-aryl or
2-methyl groups react under the optimal conditions, but no
target product was detected when a 2-H indole was chosen
as the substrate. Furthermore, the optimized conditions are
also appropriate for the oxidation of N-methyl and N-ethyl
indoles. Note that the catalytic system is not appropriate
for a cross-reaction between two different indoles, such
as 2-methylindole and 2-phenylindole or 2-methylindole
and indole, because of the low selectivity of the C–C bond-
formation reaction. This is attributed to the relatively high
reactivity of the starting materials and the relatively high
oxidative activity of the catalyst system, which lead to low
selectivity of the C–C bond-formation process.
4
as the oxidant (entry 23). By the optimization of oxidant,
solvent and catalyst, the best reaction conditions were
identified as RuCl ·3H O (5.0 mol%) as catalyst with NaIO
3
2
4
(1.0 equiv) as the oxidant in MeCN at 70 °C.
Having identified the optimal conditions, we examined
the substrate scope by subjecting a variety of indoles 1a–n
to the ruthenium-catalyzed oxidative dearomatization
(
Table 2). Indoles 1a–d with 2-aryl groups gave moderate to
high isolated yields (75–97%) of products 2a–d, respectively
Table 2, entries 1–4). Products 2e–h were obtained in 67–
5% yield from the ruthenium-catalyzed oxidative dearo-
matization reactions of the 2-methyl substituted indoles
e–h (entries 5–8). To extend the scope of the substrates,
(
8
1
©
Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 28, A–E

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Synlett
X.-Y. Zhou et al.
Letter
To investigate the mechanism of the ruthenium-cata-
lyzed oxidative dearomatization of indole 1a, we intro-
duced TEMPO (1.05 equiv) into the reaction system, and we
obtained only a trace amount of the target product 2a
In summary, by using NaIO as an oxidant, the ruthenium-
4
catalyzed oxidative dearomatization of indoles was success-
fully achieved, with up to 97% yield.21 The reactions afford-
ed 2-indolyl-3-oxoindolines with high regioselectivity un-
der the optimal conditions. The principle of these reactions
will provide a new strategy for developing new types of ru-
thenium-catalyzed oxidative dearomatizations of aromatic
and heteroaromatic compounds.
(Scheme 2). Most of the starting material 1a was recovered.
This result indicated that the reaction might proceed by a
radical process.
O
Ph
Ph
RuCl3·3H2O (5.0 mol%)
Ph
Funding Information
N
H
NH
N
NaIO4 (1.0 equiv)
TEMPO (1.05 equiv)
MeCN, 70 °C
H
The work was supported by the Youth Science and Technology Talent
Development Project in the Education Department of Guizhou
Province (Grant No. qianjiaohe KY zi [2016] number 263) and by the
Innovation Team of Liupanshui Normal University (Grant No. LPSSYK-
1a
2a, trace
Scheme 2 A radical-quenching experiment for the ruthenium-
catalyzed oxidative dearomatization of indoles
JTD201601).
)(
On the basis of the above results and the known mecha-
Supporting Information
8,9
nism of the Pd-catalyzed oxidation of indoles and the ru-
17
Supporting information for this article is available online at
https://doi.org/10.1055/s-0036-1591876.
thenium-catalyzed oxidation of phenols, a possible mech-
anism for the ruthenium-catalyzed oxidative dearomatiza-
tion reaction of indoles is proposed (Figure 2). The activated
S
u
p
p
o
nrtogI
i
f
rm oaitn
S
u
p
p
ortioIgnfmr oaitn
ruthenium catalyst RuO is formed from ruthenium precur-
4
References and Notes
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3
2
4
(
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Letter
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Komiya, N.; Oda, Y.; Kuwabara, T. J. Org. Chem. 2000, 65, 9186.
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Bäckvall, J.-E. J. Chem. Soc., Chem. Commun. 1994, 1037.
(20) Carlsen, P. H. J.; Katsuki, T.; Martin, V. S.; Sharpless, K. B. J. Org.
Chem. 1981, 46, 3936.
(21) Ruthenium-Catalyzed Oxidative Dearomatization of Indoles
1; General Procedure
(
A mixture of indole 1 (0.50 mmol), NaIO (107 mg, 0.50 mmol,
4
1.0 equiv), and RuCl ·3H O (6.5 mg, 0.025 mmol, 5.0 mol%) in
3
2
(
c) Dijksman, A.; Arends, I. W. C. E.; Sheldon, R. A. Chem.
MeCN (3 mL) was added to a 25 mL Schlenk flask at r.t. and the
mixture was then stirred at 70 °C until the reaction was com-
plete. Then the solvent was evaporated under reduced pressure,
and the residue was purified by column chromatography [silica
gel, PE–EtOAc (10:1 to 5:1)].
Commun. 1999, 1591. (d) Dijksman, A.; Marino-González, A.;
Mairata i Payaras, A.; Arends, I. W. C. E.; Sheldon, R. A. J. Am.
Chem. Soc. 2001, 123, 6826. (e) Csjernyik, G.; Éll, A.; Fadini, L.;
Pugin, B.; Bäckvall, J.-E. J. Org. Chem. 2002, 67, 1657.
(
13) Gonsalvi, L.; Arends, I. W. C. E.; Sheldon, R. A. Chem. Commun.
2,2′-Diphenyl-1,2-dihydro-1′H,3H-2,3′-biindol-3-one (2a)
Yellow solid; yield: 97.0 mg (97%); mp 220–223 °C. IR (neat):
3058, 3025, 2910, 1696, 1618, 1470, 1439, 1074, 1026, 919, 742
2002, 202.
(14) (a) Stoop, R. M.; Mezzetti, A. Green Chem. 1999, 1, 39. (b) Stoop,
R. M.; Bachmann, S.; Valentini, M.; Mezzetti, A. Organometallics
–1 1
cm . H NMR (500 MHz, DMSO-d ): δ = 11.34 (s, 1 H), 8.33 (s, 1
6
2
000, 19, 4117. (c) Yamada, T.; Hashimoto, K.; Kitaichi, Y.;
H), 7.51 (dd, J = 8.3, 7.1 Hz, 1 H), 7.39–7.33 (m, 3 H), 7.25 (d, J =
7.7 Hz, 1 H), 7.16–7.11 (m, 3 H), 7.06–7.01 (m, 6 H), 6.98 (d, J =
8.3 Hz, 1 H), 6.78–6.69 (m, 2 H), 6.61 (d, J = 8.0 Hz, 1 H). 13C NMR
Suzuki, K.; Ikeno, T. Chem. Lett. 2001, 268.
(
(
15) Yang, D.; Zhang, C. J. Org. Chem. 2001, 66, 4814.
16) Murahashi, S.-I.; Naota, T.; Taki, H. J. Chem. Soc., Chem. Commun.
(126 MHz, DMSO-d ): δ = 200.5, 160.1, 139.8, 138.0, 137.5,
6
1985, 613.
135.8, 133.2, 129.5, 127.6, 127.4, 127.0, 124.4, 121.2, 120.3,
(
17) (a) Murahashi, S.-I.; Naota, T.; Miyaguchi, N.; Noda, S. J. Am.
Chem. Soc. 1996, 118, 2509. (b) Ratnikov, M. O.; Farkas, L. E.;
McLaughlin, E. C.; Chiou, G.; Choi, H.; El-Khalafy, S. H.; Doyle, M.
118.7, 118.5, 117.5, 111.9, 111.3, 111.0, 71.2. HRMS (ESI): m/z
+
[M + Na] Calcd for C28H20N NaO: 423.1473; found: 423.1477.
2
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Georg Thieme Verlag Stuttgart · New York — Synlett 2018, 28, A–E