A simple and sustainable tetrabutylammonium fluoride (TBAF)-catalyzed synthesis of azaarene-substituted 3-hydroxy-2-oxindoles through sp3 C-H functionalization

Article abstract of DOI:10.1039/c3ra47332e

A green, practical, and metal-free protocol for direct addition of α-and γ-alkylazaarenes to isatins has been developed via sp ³ C-H functionalization in water under controlled microwave radiation. This methodology provides a mild and fast rout

Full text of DOI:10.1039/c3ra47332e

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A simple and sustainable tetrabutylammonium
fluoride (TBAF)-catalyzed synthesis of azaarene-
substituted 3-hydroxy-2-oxindoles through sp3
C–H functionalization†
Cite this: RSC Adv., 2014, 4, 19789
*
Kumkum Kumari, Bharat Kumar Allam and Krishna Nand Singh
Received 5th December 2013
Accepted 8th April 2014
A green, practical, and metal-free protocol for direct addition of a-and g-alkylazaarenes to isatins has been
developed via sp3 C–H functionalization in water under controlled microwave radiation. This methodology
provides a mild and fast route to biologically important azaarene-substituted 3-hydroxy-2-oxindoles in
good to excellent yields.
DOI: 10.1039/c3ra47332e
www.rsc.org/advances
Water is the nature's most amazing gi.10 The use of water
as a solvent in organic synthesis is environmentally benign
and safe.11 Organocatalysis has emerged and applied rapidly
Introduction
The concept of atom economy has driven chemists to develop
more efficient and sustainable methodologies for new bond
forming reactions. In this perspective, the direct functionali-
zation of C–H bonds in organic compounds has emerged as a
powerful and ideal method for the construction of carbon–
carbon and carbon-heteroatom bonds.1 The C–H functionali-
zation logic provides step, atom, and redox economy to advance
the organic synthesis.2 The Lewis acid catalyzed activation of
the sp3 C–H bond of a-alkylazaarenes has become a focal point
of the current research.3 It is well-established that the benzylic
C–H bonds of a-alkylpyridines react with electrophiles to form
carbon–carbon bonds in nucleophilic fashion.4 However, the
substrate scope reported so far is limited to highly reactive polar
electrophiles.5 Despite its synthetic utility, only sporadic
examples are reported and the functionalization of the sp3 C–H
bond in a-alkylquinolines remains less investigated.6 Isatin as a
core structure has inspired the development of useful catalytic
strategies to give access to interesting molecular architectures
with wide biological activities.7 Interest in 3-substituted-3-
hydroxy-2-oxindoles has increased rapidly as this core structure
is present in a number of potential drug candidates for the
treatment of proliferative diseases.8 The direct addition of
benzylic C–H bonds of a-alkylazaarenes to isatins for the
synthesis of azaarene-substituted 3-hydroxy-2-oxindoles repre-
sents the most simple and straightforward method to construct
such motifs.9
because of its unique advantages.12 The exploitation of
organocatalysts that are compatible in aqueous media will
provide attractive practical applications.13 However, the use
of water as a solvent is still challenging because of the highly
insoluble nature of organic molecules in water and the
possible reaction of their functional groups with water.
Additionally water may weaken the catalytic activity and
stereocontrol through interference of the hydrogen bonds
and other polar interactions involving catalysts and
substrates. Microwave (MW)-assisted synthesis has enriched
the rapidly evolving landscape of C–H bond functionaliza-
tion,14 and results in dramatic rate accelerations, enhanced
yields, and cleaner reactions. Therefore, from the point of
sustainability and green chemistry, the development of new
organocatalytic approaches for reactions in water using
controlled MW is highly welcome.
Quaternary ammonium salts are readily available phase
transfer catalysts.15,16 Among them, tetraalkylammonium
uorides have been used as a source of naked uoride ion.17
The nucleophilic affinity of uoride ion enables the genera-
tion of nucleophiles through a deprotonation process.18 Tet-
rabutylammonium uoride (TBAF) has been recently
explored as
a
readily available, efficient and water
compatible organocatalyst and additive for various organic
transformations.19
In light of the above specics and as a part of our ongoing
research interest,20 we report herein the TBAF-catalyzed sp3 C–H
functionalization of a/g-alkylpyridines and a-alkylquinolines in
water under controlled MW (Scheme 1). To the best of our
knowledge, the ndings involve the rst time sp3 C–H bond
functionalization of a-alkylquinolines for the construction of
quinoline-substituted 3-hydroxy-2-oxindoles.
Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi
221005, India. E-mail: knsinghbhu@yahoo.co.in
† Electronic supplementary information (ESI) available: Detailed experimental
procedure and full characterisation data for all the products along with the
copies of 1H and 13C spectra. CCDC 980602. For ESI and crystallographic data
in CIF or other electronic format See DOI: 10.1039/c3ra47332e
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with much lower yields (entries 13–14). When potassium uo-
ride (20 mol%) was applied as a catalyst under the same set of
reaction conditions, it gave rise to 85% product yield (entry 15).
The use of neutral ionic liquid [Bmim]BF4 proved rather less
effective to deliver the product (entry 16). A control experiment
without using TBAF$3H2O, however, delivered the product in
20% yield aer 20 min (entry 17).
The scope of the optimized procedure was subsequently
studied for the reactions of various a/g-alkylazaarenes with
isatins. A number of a/g-alkylazaarenes viz. a-methylpyridine
(1a), a-methylquinoline (1b), g-methylpyridine (1c), and a,a0-
lutidine (1d) were successfully activated and made to react with
different isatins such as N-methylisatin (2a), isatin (2b), N-
propargylisatin (2c), N-allylisatin (2d), N-benzylisatin (2e), N-
ethylisatin (2f), 5-bromoisatin (2g), 5,7-dibromoisatin (2h), and
5-nitroisatin (2i) to provide a diverse range of products 3a–3x in
good to excellent yields (Table 2). All the reactions underwent
easily with specic product selectivity. Isatins with different
substitution patterns participated well in the reaction to deliver
the corresponding products. An increase in the yield was
noticed for N-substituted isatins in comparison to unsub-
stituted one. Isatins containing halogen in the aromatic ring also
participated well in the reaction. The present catalytic system
was also found to be effective in activating the sp3 C–H bond
of a-methylquinoline, affording 3-hydroxy-3-(quinolin-2-yl-
methyl)indolin-2-ones (3c–3k), although the formation of rather
bis(quinolin-2-ylmethyl)indolin-2-one is described in the liter-
ature.9a No side products were observed under the present
conditions. Interestingly in the case of a,a0-lutidine, only one a-
methyl group participated in the functionalization and the cor-
responding products were isolated exclusively. To conrm the
effectiveness of catalytic activity of TBAF$3H2O and to nd out
the origin of high reaction rates, a model reaction was carried out
by using a-methylquinoline (1b) and N-methylisatin (2a) under
conventional heating conditions instead of microwave irradia-
Scheme 1 Functionalization of a-and g-alkyl azaarenes.
Table 1 Evaluation of conditions for the model reactiona
Entry
Catalyst (mol %)
Solvent
T (ꢀC)
Yieldb (%)
1.
2.
3.
4.
5.
6.
7.
8.
TBAF$3H2O (5)
TBAF$3H2O (10)
TBAF$3H2O (10)
TBAF$3H2O (15)
TBAF$3H2O (10)
TBAF$3H2O (10)
TBAF$3H2O (10)
TBAF$3H2O (10)
TBAF$3H2O (10)
TBAF$3H2O (10)
TBAF$3H2O (10)
TBAF$3H2O (10)
TBAB (10)
H2O
H2O
H2O
H2O
H2O
—
EtOH
1,2-Dichloroethane
1,4-Dioxane
THF
DMSO
Toluene
H2O
H2O
H2O
H2O
H2O
80
80
55
78
90
89
85
80
47
39
60
80
29
20
69
70
85
30
20c
100
100
120
100
100
100
100
100
100
100
100
100
100
100
100
9.
10.
11.
12.
13.
14.
15.
16.
17.
TBAI (10)
KF (20)
[Bmim]BF4 (10)
—
a
Reaction conditions: 1b (2 mmol), 2a (2 mmol), 80 W (MW), 5 min.
b
Isolated yield. c Reaction conducted for 20 min.
Results and discussion
The optimization of the reaction conditions was carried out by tion using the same set of optimized reaction conditions (10 mol
ꢀ
using a-methylquinoline (1b) and N-methylisatin (2a) as model % TBAF$3H2O, 100 C, H2O). To our delight, the reaction went
substrates in the presence of TBAF$3H2O as a catalyst under smoothly to deliver the product 3c in 83% yield within 3 h, which
various conditions (Table 1). The screening was initiated using clearly demonstrates the efficient catalytic role of TBAF$3H2O in
5 mol% of TBAF$3H2O as a catalyst in pure water at 80 ꢀC, 80 W this methodology. However, we may not completely rule out the
for 5 min. Gratifyingly, the reaction proceeded to afford selec- effect of MW irradiation to help further accelerate the reaction.
tively the target 3-hydroxy-1-methyl-3-(quinolin-2-ylmethyl) The product structure of a representative product 1-allyl-3-
indolin-2-one (3c) in 55% isolated yield (entry 1). Increase of the hydroxy-3-(pyridin-4-ylmethyl)-indolin-2-one (3n) was conclu-
catalyst concentration to 10 mol% delivered the product in 78% sively conrmed by its single crystal X-ray determination (Fig. 1).
yield under the same conditions (entry 2).
In order to further advance the yield, reaction temperature
was raised to 100 C, which provided the optimum yield (90%,
entry 3). Further increase in the catalyst concentration, and
Experimental
General experimental procedure
ꢀ
temperature did not enhance the yield again (entries 4, 5). In a sealed pressure regulation 10 mL pressurized vial were
Various organic solvents like EtOH, 1,2-dichloroethane, 1,4- placed a/g-alkyl azaarene (2 mmol), TBAF$3H2O (10 mol%, 0.2
dioxane, THF, DMSO, and toluene were also probed for their mmol, 62 mg), isatin (2 mmol), H2O (2 mL), and a teon coated
effect on the reaction yield but did not help (entries 7–12). When magnetic stir bar. The vial was closed with a snap on cap, stirred
the reaction was conducted under solvent-free conditions, a at room temperature for 1 min and then placed into the MW
decrease in the product yield was noticed probably due to cavity. Microwave irradiation of 80 W at a set temperature of
ꢀ
decomposition of the product (entry 6). To ascertain the role of 100 C was used and the reaction was held under these condi-
uoride ion, tetrabutylammonium salts with different counter tions for 5 min. Aer completion of the reaction (monitored
ions such as bromide and iodide were also checked, but ended through TLC), the mixture was cooled to room temperature,
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Table 2 Substrate scope for the construction of azaarene-substituted 3-hydroxy-2-oxindolesab
a
b
c
Reaction conditions: a/g-alkylazaarene (2 mmol), isatin (2 mmol), TBAF$3H2O (10 mol%), MW (80 W) 100 ꢀC, 5 min. Isolated yield. Yields
without catalyst.
poured to a vessel containing distilled water and then extracted
with ethyl acetate (2 ꢁ 10 mL). The combined organic phase was
dried over anhydrous Na2SO4, ltered and concentrated under
rotary vacuum evaporator. The resulting crude product was
puried using preparative TLC.
Conclusions
In conclusion, the work demonstrates a highly efficient, prac-
tical, and environmentally benign approach for the sp3 C–H
functionalization of a-and g-alkyl azaarenes catalyzed by a
simple water compatible organocatalyst in aqueous media
under controlled MW. This study will open a new organo-
catalytic way for the functionalization of sp3 C–H bonds. The
Fig. 1 ORTEP diagram of Product “3n” showing atomic numbering
scheme with ellipsoid of 50% probability.
application of this protocol for the functionalization of
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Acknowledgements
We acknowledge the nancial support received from the
Department of Biotechnology (DBT), New Delhi. K. K. and B. K.
Allam thank Council of Scientic Industrial Research (CSIR),
New Delhi for the award of Senior Research Fellowships.
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