NaI-mediated divergent synthesis of isatins and isoindigoes: A new protocol enabled by an oxidation relay strategy

Article abstract of DOI:10.1039/c8cc04471f

A new approach for the synthesis of isatins and isoindigoes by an inexpensive and environmentally friendly NaI-mediated transformation is disclosed. The selectivity could be switched by simply varying the solvent, and isatins (using THF) and isoindigoes (using DMSO) could be obtained in moderate to excellent yields.

Full text of DOI:10.1039/c8cc04471f

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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NaI-mediated Divergent Synthesis of Isatins and Isoindigoes: A
New Protocol Enabled by Oxidation Relay Strategy
Hong-hua Zhang, a, c Yong-qiang Wang, a, c Long-tao Huang, a Long-qing Zhu, a Yi-yue Feng, a Ying-
mei Lu, a Quan-yi Zhao, a Xue-qiang Wang *b Zhen Wang *a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A new approach for the synthesis of isatins and isoindigoes by an ring contraction of 3-diazoquinoline-2-4-diones12
inexpensive and environmental-friendly
NaI-mediated activation chemistry13. However, these processes have
; 6) C-H
transforma-tion is disclosed. The selectivity could be switched by limitations with respect to selectivity, scope, yield, and
simply varying the solvent, isatins (using THF) and isoindigoes functional group compatibility.
(using DMSO) could be obtained in moderate to excellent yields.
Isoindigoes, as dipolymer of the indoline-2-ones, are
important bis-indole heterocyclic molecules and demonstrate
special biological activities14. For example, Meisoindigo has
been subjected to clinical trials and is used as an indirubin
substitute in China for the treatment of chronic myelocytic
leukemia (CML)15. Moreover, isoindigoes can be used to
synthesize organic solar cells such as DAD16, and used in the
design of low-band gap semiconductor materials17. To date,
several strategies for the preparation of isoindigoes were
developed such as the homocoupling of isatins and indolin-2-
ones18, metal-catalyzed intramolecular cyclization19, and
polymerization of 3-bromooxindoles20 or diazooxindoles21.
However, these protocols suffer from using metal-catalyst,
having limited diversity, and harsh reaction conditions.
Therefore, how to address issues in the isatins and isoindigoes
syntheses becomes a synthetically important question. Herein
we describe an intriguing method using NaI as an efficient
mediator to prepare the targeted isatins and isoindigoes. This
protocol is highlighted by its simple operation, metal-free
conditions, and highly functional group compatibility as shown
in Scheme 1.
Oxidation reaction is a widespread and significant strategy
in organic synthesis. Traditional methods of oxidation usually
involve the direct use of oxidants, which might cause over
oxidation issue. “Oxidation Relay Chemistry”, as
a new
oxidation model1 was developed by our group recently. This
strategy can effectively suppress the over-oxidation of the
substrate due to the in-situ generation of a small number of
halogens (X2) which serve as oxidant in this transformation. On
the other hand, it uses inexpensive and innocuous sodium-
iodide (NaI) as catalyst, and air as oxidant. Based on this
concept, we thus have developed several examples for the
construction of N-S and C-S bonds2. To further demonstrate
the universality of this strategy, we carried out the synthesis of
the isatins (Indoline-2,3-diones) and isoindigoes (1H,1'H-
bis(indolyl-3-indeno)-2,2'-diones) from readily available indole-
2-ones.
Isatins are ubiquitous and can be found in a range of
biologically active natural products3, synthetic intermediates4,
and pharmaceuticals chemistry5. Currently, a number of
protocols for the construction of isatins have been reported6:
1) strong acid or base-mediated condensation of aniline with
chloral hydrate, oxalyl chloride, and diethyl ketomalonate7; 2)
oxidation of indoles and indoline-2-ones8, 9; 3) intramolecular
oxidative cyclization using pre-existing ortho-substituents10; 4)
metal-catalyzed double carbonylation of 2-haloaniline11; 5)
Scheme 1. Divergent synthesis of isatins and isoindigoes.
At the beginning of our investigation, the optimization of
reaction conditions was carried out by evaluating a variety of
reaction parameters, which uses 1-methylindolin-2-one (1a) as
a model substrate (Table 1). Inspired by our previous work2,
we firstly tested the reaction of 1a in the presence of 1.0
equivalent of NaI in dimethylformamide (DMF) at 100 oC. After
12 h, the desired products 2a and 3a were obtained in 33%
and 15% yield (Table 1, entry 1), respectively. At that point, a
a.School of Pharmacy, Lanzhou University, West Donggang Road. No. 199, Lanzhou
730000, China. E-mail: zhenw@lzu.edu.cn .
b.State Key Laboratory of Chemo/Biosensing and Chemometrics, College of
Chemistry and Chemical Engineering, Hunan University, 2 Lushan Nan Road,
Changsha, Hunan 410082, China. E-mail: wangxq@hnu,edu.cn
c. These authors contributed equally. E-mail: zhenw@lzu.edu.cn
†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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series of solvents were tested (Table 1, entries 2-7). To our excellent yields (80%-92%). In general, the compounds with
delight, 2a was obtained in 82% yield when THF was used electron-donating group on phenyl ring could be generated in
(Table 1, entry 4). The results of control-reactions in the better yields as compared to the ones with electron-
absence of O2 or NaI (Table 1, entries 8-9) indicated that both withdrawing group. To further demonstrate the versatility of
O2 and NaI played a vital role in this transformation. Increasing this protocol, reactions with indolin-2-ones bearing
or decreasing the quantity of NaI gave lower yields of 2a (Table substituents on the phenyl ring and nitrogen atom were then
1, entries 10-11). The effect of temperature was also examined examined (2t-2w). The reactions of 1t-1v bearing a halogen on
o
for 2a, and 60 C was found to be the best of choice (Table 1, the phenyl ring and a methyl group on the nitrogen atom
entries 4 and 12). To our surprise, 3a could be obtained in 68% provided the expected products 2t-2v in 82%, 83%, and 80%
yield while DMSO was used as solvent instead of THF (Table 1, yields respectively. With 1w bearing a bromine (Br) on the
entry7). Changing temperature or the quantity of NaI showed phenyl and a butyl group on the nitrogen atom produced 2w in
a negative influence, delivering 3a in inferior yields (Table 1, 85% yield. These reactions provide a rapid synthetic route for
entries 13-16). Subsequently, we tried to increase the yield of the synthesis of various isatin derivatives.
3a by prolonging the reaction time, and only little
improvement was observed. (Table 1, entries 17-18).
Entry
1
Solvent
DMF
Temp. (oC)
100
100
100
60
Yield 2a (%)
Yield 3a (%)
33
N.R.
38
82
5
15
N.R.
10
15
<1
<1
68
0
2
Toluene
n-PrCN
THF
3
4
5
Dioxane
NMP
100
100
100
60
6
<5
29
0
7
DMSO
THF
8b
9c
THF
60
0
0
10d
11e
12
13d
14e
15
16
17f
18g
THF
60
38
67
43
15
19
25
27
22
20
24
9
a
a
Scheme 2. Scope of the oxidation of indolin-2-ones to isatins
conditions：
.
Reaction
1
(0.1 mmol), NaI (0.1 mmol), O2 (1 atm), 60 oC, THF (1 mL), 12 h.
THF
60
THF
40
Next, we focused our attention on isoindigoes. The
reactions of a series of substituted indolin-2-ones were
examined under the optimal reaction condition (Table 1, entry
7), and the results were summarized in Scheme 3. Substrates
bearing hydrogen (3b), ethyl (3c), phenyl (3d), benzyl (3e), p-
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
100
100
80
25
50
55
66
69
70
120
100
100
tolyl (3f), 4-methoxyphenyl (3g), and 4-chlorophenyl (3h
)
20
a
Table 1: Optimization of the reaction conditions
. ：1a (0.1
a Reaction conditions
groups on nitrogen atom displayed better reactivity in this
reaction, and produced isoindigoes efficiently. However, the
substrate with tert-Butyloxy carbonyl (Boc) group (3i) gives the
detert-Butyloxy carbonyl product (3b) in 56% yield. Halogen (-
F) substituted indolin-2-ones produced the corresponding
products 3j in moderate yield. The 3k with a bromine (Br)
group on phenyl and a butyl group on the nitrogen atom, was
also obtained in good yield. Meanwhile, the 3k is used to
prepare a series of isoindigo-based low-band gap copolymers,
b
mmol), NaI (0.1 mmol), O2 (1 atm), and solvent (1 mL) for 12 h. Isolated yields.
f
The reaction was conducted under Ar atmosphere. c The absence of NaI. d NaI
was used in 0.05 mmol (0.5 eq). e NaI was used in 0.2 mmol (2.0 eq). The
reaction time was 16 h. g The reaction time was 20 h. n-PrCN = Butanenitrile.
Encouraged by the optimized conditions, we then explored the
generality of this newly developed protocol. A variety of
different oxindoles were converted into the corresponding
isatins in excellent yields (Scheme 2). Substrates bearing
hydrogen (2b), ethyl (2c), allyl (2d), benzyl (2e), phenyl (2f), p-
isoindigo15.
tolyl (2g), 4-methoxyphenyl (2h), and 4-chlorophenyl (2i
)
such
as,
Thieno[3,2-b][1]benzothiophene
groups on nitrogen atom illustrated better reactivity in this
reaction comparing with the substrate with tert-Butyloxy
carbonyl (Boc) group (2j). Instead, detert-Butyloxy carbonyl
product (2b) was generated as the major product in 85% yield.
Interestingly, meisoindigo (3l) that can be used for treatment
of chronic myelocytic leukemia (CML)13 was also obtained in
45% yield. And 5-fluoro-5'-methoxy-[3,3'-biindolinylidene]-
2,2'-dione (3m) was also produced in 46% yield. This protocol
provides a practical way to synthesise isoindigo derivatives
with the advantages of readily commercially available
materials and convenient operation process.
Both electron-donating group
(2k, 2l, 2r) and electron-
withdrawing group (2m-2q, 2s) on phenyl ring worked well,
and the corresponding products were obtained in moderate to
2 | J. Name., 2012, 00, 1-3
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that 1z might be the critical intermediate. On the other hand,
when BHT was added to the system in THF, the reaction was
completely suppressed (Scheme 4, eq 5), indicating the
reaction in THF might involve radical process.
Based on the above results and the literatures, a plausible
mechanism for this reaction was depicted in Scheme 5. First, I-
can be oxidized to I2 by oxygen or DMSO. Then 1-
methylindolin-2-one
intermediate22, 23
3-dione 2a by DMSO23,
( ) reacted with I2 to give the
1a
I
，which was oxidized to 1-methylindoline-2,
24
.
Then, the enolate of 1-
methylindolin-2-one nucleophilic attacked 2a to produce
intermediate II, which was dehydrated and the final product
was generated. On the other hand, iodine radical was
generated from molecular iodine by heat or light.
Subsequently, iodine radical abstracts a hydrogen atom from
substrate 1a to generate a key intermediate radical III
.
Meanwhile, iodine radical also abstracts α-hydrogen atom
from the THF and the tetrahydrofuran-2-yl radical was
generated. Peroxy radical IV was generated from
a
.
a Reaction
25
Scheme 3. Scope of the oxidation of indolin-2-ones to isoindigoes
conditions
：1
(0.1 mmol), NaI (0.1 mmol), O2 (1 atm), 100 oC, DMSO (1 mL), 12 h.
tetrahydrofuran-2-yl radical and O2
intermediate
radical IV Then intermediate V can be oxidized to 1-
. Subsequently, the
V
was generated from the radical III and the
.
To gain insight into the mechanistic details, several control
experiments were designed and conducted. When BHT was
added to the system in DMSO, the desired products 2a and 3a
were obtained in 20% and 56% yield (Scheme 4, eq 1),
respectively. This result might allow us to exclude the
possibility of radical pathway for the reaction in DMSO. When
O2 was replaced by Ar in DMSO, the products 2a and 3a were
obtained in 15% and 8% yield (Scheme 4, eq 2), respectively,
which indicated that the DMSO participated in the oxidation of
indolin-2-one22-24. To our delight, when the reaction time was
shortened to 4 h, 3-hydroxy-1,1'-dimethyl-[3,3'-biindoline]-
2,2'-dione(II) was observed in 30% yield ( Scheme 4, eq 3),
methylindoline-2, 3-dione (2a)9.
Scheme 5. Plausible reaction mechanism.
In conclusion, we have developed an efficient, metal-free,
and practical method for the straightforward synthesis of
isatins and isoindigoes from commercially available indolin-2-
ones. The protocol featured with the use of inexpensive NaI as
mediator, simple test procedure, wide substrates scope and
high functional group tolerance. These isatins and isoindigoes
are, meanwhile, presented as the key structural motifs in
many biologically active molecules, and will find their
applications in medicinal chemistry.
Financial support was provided by the Recruitment Program
of Global Experts (1000 Talents Plan).
Scheme 4. Control experiments.
which manifested that the II may be the intermediate of the
reaction. When 3-iodo-1-methylindolin-2-one
(
1z
)
was
Conflicts of interest
subjected to the reaction under standard conditions, 2a could
be obtained in 80% yield (Scheme 4, eq 4). This result indicated
There are no conflicts to declare.
This journal is © The Royal Society of Chemistry 20xx
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