Synthesis of Quaternary 3,3-Disubstituted 2-Oxindoles from 2-Substituted Indole Using Selectfluor

Article abstract of DOI:10.1021/acs.orglett.6b01367

A facile method to construct quaternary 3,3-disubstituted 2-oxindole from 2-substituted indole without a catalyst in mild conditions is developed. A mechanistic study suggests that an iminium-intermediate-triggered 1,2-rearrangement is involved, and a tra

Full text of DOI:10.1021/acs.orglett.6b01367

Letter
pubs.acs.org/OrgLett
Synthesis of Quaternary 3,3-Disubstituted 2‑Oxindoles from
2‑Substituted Indole Using Selectfluor
Xiaojian Jiang,* Junjie Yang, Feng Zhang, Pei Yu, Peng Yi, Yewei Sun,* and Yuqiang Wang
Institute of New Drug Research, College of Pharmacy, Jinan University, Guangzhou 510632, China
S
* Supporting Information
ABSTRACT: A facile method to construct quaternary 3,3-disubstituted 2-
oxindole from 2-substituted indole without a catalyst in mild conditions is
developed. A mechanistic study suggests that an iminium-intermediate-
triggered 1,2-rearrangement is involved, and a trace amount of water is
required for subsequent oxidation.
xindoles, especially 3,3-disubstituted 2-oxindoles, are very
important compounds in organic synthesis due to their
oxidizable functional groups. It is possible for selectfluor to
oxidize indole to produce 2-oxindole. It also serves as a mediator
for stereoselective rearrangement processes.9 As such, combined
with the oxidation and rearrangement properties of selectfluor,
desired product 3-hydroxymethyl-2-oxindole 2a might be
formed from the corresponding 2-hydroxymethylindole 1a in a
one-pot reaction (Scheme 1, eq 2) through a 1,2-rearrangement
process.
Herein, we report a facile method to synthesize diverse
quaternary 3,3-disubstituted 2-oxindole compounds from the
corresponding 2-substituted indole using selectfluor without a
catalyst in mild conditions.
O
wide applications in natural, bioactive, and pharmaceutically
useful products.1,2 A number of methods are developed to
achieve this scaffold with transition-metal-catalyzed3 and
organocatalytic4 strategies as the main processes. Other
procedures, including photochemical,5 Ullmann-type,6 and
radical-promoted cyclizations of saturated 2-haloanilide com-
pounds,7 have been reported. Although a transition-metal-
catalyzed cyclization strategy is useful to generate diverse 3,3-
disubstituted 2-oxindoles and is important to construct chiral
ones,3n,q the high cost and air-sensitive nature of transition
metals make other methods worth developing.
We evaluated several fluorine sources for the in situ 1,2-
rearrangement oxidation process, and the results are shown in
Table 1. Selectfluor (F1) (Table 1, entry 1) was the best
mediator to convert 2-hydroxymethylindole 1a into the desired
quaternary 3-hydroxymethyl-2-oxindole 2a. Up to 76% isolated
yield of 2a could be achieved using 1.2 equiv of selectfluor when
acetonitrile was used as solvent at room temperature (Table 1,
entry 1). If we increased the amount of selectfluor, the yield
decreased (Table 1, entry 2) considerably, suggesting that 2a
could be decomposed by selectfluor. Comparable yield could be
obtained when the temperature was decreased to 0 °C (Table 1,
entry 3), albeit with a longer reaction time. Addition of a base
such as K2CO3 failed to increase the yield (Table 1, entry 4).
Replacement of selectfluor with N-fluorobenzenesulfonimide
(F2) (Table 1) failed to return the desired product with
acceptable yield (Table 1, entries 5−7). Only very low yield of 2a
was obtained when 1-fluoro-2,4,6-trimethylpyridinium triflate
(F3) (Table 1, entries 8 and 9, <5%) was applied. Alteration to
other fluorine sources such as 4-tert-butyl-2,6-dimethylphenyl-
sulfur (F4) (Table 1, entry 10, <5%) and diethylaminosulfur
trifluoride (F5) (Table 1, entry 11, <5%) could not produce the
desired product. We also tried to optimize the reaction
conditions with different solvent systems, such as acetone,
For instance, previous establishments of quaternary 3-
hydroxymethyl-2-oxindole compounds were lengthy and
required high cost and harsh conditions.3−8 As illustrated in
Scheme 1, an expensive and air-sensitive Pd catalyst was used
Scheme 1. Formation of 2a
when starting with 2-iodoanilide A, with subsequent multistep
reactions resulting in the expected 3-hydroxymethyl-2-oxindole
2a (Scheme 1, eq 1).3o Thus, it is evident that there remains a
further need to develop alternative synthetic approaches to
produce quaternary 3-hydroxymethyl-2-oxindole compounds.
Selectfluor (1-chloromethyl-4-fluoro-1,4-diazoniabicyclo-
[2.2.2]octane bis(tetrafluoroborate) as a versatile mediator or
catalyst in organic synthesis is well-documented.9 It serves as a
strong oxidant for diverse “fluorine-free” functionalizations.10
Selectfluor is widely used as a mediator in transformations of
Received: May 11, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01367
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
protected and only 65% isolated yield was obtained (2b)
(Scheme 2). We suspected that the indolic hydrogen probably
triggered other side reactions, leading to lower yield. Bulky and
electron-poor substituents of the indolic Ar ring had a positive
effect on this reaction and delivered excellent yields (2m−2o)
(Scheme 2). However, we failed to detect any desired product
when an electron-rich substituent, such as a methoxyl group, was
introduced into the indolic Ar ring (data not shown). The
structure of these 1,2-rearrangement oxidation compounds was
determined by X-ray analysis of 2e.11
Subsequent study focused on constructing the quaternary 3-
hydroxymethyl-2-oxindole compounds with secondary alcohol.
Although the current method has a minimal effect on
constructing the desired secondary alcohol product 4a (Scheme
3), we observed that ketone compound 5a existed in a certain
Table 1. Optimization of Substrate 1a
a
b
c
entry
solvent
base
fluorine source temp (°C) yield (%)
1
2
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
Me2CO
MeCN
MeCN
MeCN
MeCN
Me2CO
THF
F1 (1.2 equiv)
F1 (2.5 equiv)
F1 (1.2 equiv)
F1 (1.2 equiv)
F2 (1.2 equiv)
F2 (2.5 equiv)
F2 (2.5 equiv)
F3 (1.2 equiv)
F3 (1.2 equiv)
F4 (1.2 equiv)
F5 (1.2 equiv)
F1 (1.2 equiv)
F1 (1.2 equiv)
F1 (1.2 equiv)
rt
rt
0
76
65
3
75
4
K2CO3
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
72
5
<5
<10
<10
35
Scheme 3. Formation of 4a and 5a
6
K2CO3
K2CO3
K2CO3
7
8
9
36
10
11
12
13
14
K2CO3
K2CO3
<5
<5
69
57
dioxane
70
amount when 1.2 equiv of selectfluor was used. We assumed that
the secondary alcohol moiety was highly sensitive to the
oxidizing reagent (i.e., selectfluor) and eventually formed the
carbonyl group instead. Indeed, 4a could be quantitatively
transformed into 5a in the presence of 1.2 equiv of selectfluor in
MeCN. As such, we took advantage of selectfluor as the strong
oxidant and synthesized diverse 3,3-disubstituted 2-oxindole
compounds with carbonyl groups, and the results are illustrated
in Scheme 4.
a
All reactions were carried out at 0.2 mmol scale in solvent (8 mL).
2.5 equiv. Isolated yield.
b
c
THF, and 1,4-dioxane. However, these solvent systems failed to
deliver higher yield (Table 1, entries 12−14).
With the optimized conditions in hand, we proceeded with the
substrate scope of the reaction, and the results are demonstrated
in Scheme 2. High yields could be achieved when an alkyl (i.e.,
a
a
Scheme 2. Substrate Scope of 2
Scheme 4. Substrate Scope of 3
a
All reactions were carried out at 0.2 mmol scale of 3 in MeCN (8
a
b
All reactions were carried out at 0.2 mmol scale of 1 in MeCN (8
mL) with 2.4 equiv of selectfluor. Isolated yield.
b
mL) with 1.2 equiv of selectfluor. Isolated yield.
Me) or an aryl (i.e., Ph) group was attached to the C3 position. A
number of amine protecting groups were compatible with the
1,2-rearrangement oxidation process. For instance, protecting
groups containing alkyl, aryl, allyl, propargyl, and chloride
tolerated the reaction conditions. Up to 91% isolated yield could
be obtained (2i) (Scheme 2) when an allyl protecting group was
used. An exception was the NH situation, where amine was not
We discovered that up to 86% isolated yield of quaternary 3-
ketone-2-oxindole compound 5a could be obtained when 2.4
equiv of selectfluor was applied. A number of modifications could
be made to the secondary alcohol indole substrate 3, resulting in
various quaternary 3-ketone-2-oxindole compounds 5. As shown
in Scheme 4, up to 95% isolated yield could be achieved when a
phenyl ring was attached to the secondary alcohol (5f). Other
B
DOI: 10.1021/acs.orglett.6b01367
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Letter
substituents were well-suited for these reaction conditions and
returned excellent yields.
The current 1,2-rearrangement oxidation process also works
for ester, aldehyde, and ketone functional groups. As shown in
Scheme 5, various quaternary 3-ester/aldehyde/ketone-2-
Note that the alkyl group might not be suitable to direct the
1,2-rearrangement process, as the 2-methylindole substrate 11
failed to form the desired 3,3-disubstituted 2-oxindole
compound. As illustrated in Scheme 7, the strong oxidative
Scheme 7. Reaction of Substrates 11, 12, and 14
a
Scheme 5. Substrate Scope of the Directing Group
effect of selectfluor preferred to turn the methyl group into an
aldehyde group and generated 2-aldehyde indole 7b in 65%
isolated yield instead (Scheme 7, eq 1). We also discovered that
hydride could direct selectfluor to perform the 1,2-hydride shift.
As indicated in Scheme 7, hydride at the C2 position migrated to
the C3 position, leading to the formation of the desired tertiary 3-
substituted 2-oxindole product 13 (Scheme 7, eq 2). Because
hydride, hydroxymethyl, ester, aldehyde, and ketone were able to
create hydrogen bonds, we assumed that a hydrogen bonding
effect was necessary to coordinate selectfluor and direct the
rearrangement process. However, we failed to detect the 1,2-
hydride shift oxidation process if hydride was placed at the C3
position. Instead, fluorination of the C3 position resulted in
formation of 3-fluoroindole compound 15 (Scheme 7, eq 3).
In summary, a facile method to construct diverse quaternary
3,3-disubstituted 2-oxindole without catalyst in mild conditions
is developed. The current chemistry can be extended to
synthesize a tertiary 3-substituted 2-oxindole compound if
hydride is placed at the C2 position of an indole skeleton.
Various functional groups, such as hydroxymethyl, ketone,
aldehyde, and ester groups, can be easily incorporated into a
quaternary carbon center. A mechanistic study implies that an
iminium-intermediate-triggered 1,2-rearrangement is involved,
and a trace amount of water is required for subsequent oxidation.
a
All reactions were carried out at 0.2 mmol scale of 8, 9, or 10 in
b
MeCN (8 mL) with 1.2 equiv of selectfluor. Isolated yield.
oxindole 9, 10, and 5 could be synthesized with 1.2 equiv of
selectfluor when MeCN was used as solvent. Indolic hydrogen,
halogen, aryl, alkene, and alkyne moieties were well-tolerated in
this transformation and delivered up to 95% (10b) (Scheme 5)
isolated yield. Up to 96% isolated yield could be achieved when
R2 was substituted by a ketone group (5f) (Scheme 5). 3-Ester/
aldehyde-2-oxindole supplemented the 3-hydroxymethyl/ke-
tone-2-oxindole by simply using the corresponding 2-ester/
aldehyde indole starting material.
For a mechanistic study, we believed that the nitrogen atom in
substrates 1 and 3 could contribute to the fluorination to give an
iminium ion intermediate I (Scheme 6). The cation intermediate
Scheme 6. Proposed Mechanism
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b01367.
Experimental details and characterization data (PDF)
X-ray crystal structure of compound 2e (CIF)
II initiated the 1,2-rearrangement process and formed
intermediate III. This 1,2-rearrangement process was further
supported by rearrangement of indolyl acetates and carbonates.12
Subsequent oxidation with a trace amount of water in the media
furnished the quaternary 3,3-disubstituted 2-oxindole product 2
or 4. Further oxidation of secondary alcohol 4 with an extra
equivalent of selectfluor furnished the final quaternary 3-ketone-
2-oxindole products 5.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: chemjxj2015@jnu.edu.cn.
*E-mail: yxy0723@163.com.
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.6b01367
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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3955.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Grant No. 21502068).
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