β-Cyclodextrin catalysed C-C bond formation via C(sp3)-H functionalization of 2-methyl azaarenes with diones in aqueous medium

Article abstract of DOI:10.1039/c4gc02287d

The first β-cyclodextrin catalysed C(sp³)-H functionalization of 2-alkyl-azaarenes with homocyclic as well as heterocyclic diones in water has been developed. This biomimetic catalyst oriented methodology provides a sustainable and green protocol for C-H functionalization, an area mainly dominated by transition metals. This journal is

Full text of DOI:10.1039/c4gc02287d

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Cite this: DOI: 10.1039/x0xx00000x
β-Cyclodextrin Catalysed C-C Bond Formation via
C(sp3)-H Functionalization of 2-Methyl azaarenes
with Diones in aqueous medium
Received 00th January 20xx,
Accepted 00th January 20xx
Atul Kumar* and Ratnakar Dutt Shukla
DOI: 10.1039/x0xx00000x
www.rsc.org/
ABSTRACT: First βꢀcyclodextrin catalysed C(sp3)ꢀH 1,4ꢀglycosidic bonds possessing a hydrophilic outer
functionalization of 2ꢀalkylꢀazaarenes with homocyclic as well surface and a hydrophobic central cavity which have
as heterocyclic diones in water has been developed. This bioꢀ attracted much attention as aqueousꢀbased hosts for
mimetic catalyst oriented methodology provides a sustainable inclusion complex phenomena with a wide variety of
and green protocol for CꢀH functionalization, the area mainly guests.20 They have substrates selective binding ability,
dominated by transition metals.
and catalyse a wide range of chemical reactions by
supramolecular catalysis, involving reversible host–guest
complexes.21ꢀ22 To the best of our knowledge, the use of
βꢀcyclodextrin in C(sp3)ꢀH bond functionalization of 2ꢀ
alkylazaarens in aqueous medium is not reported.
In twenty first century the development of new methods
for C–H functionalization and their applications has been
one of the most blossomed areas of research.1ꢀ2 CꢀH
bond functionalization is considered as powerful and
shortest approach3 for construction of carbon–carbon and
Here, first time we wish to report
a biomimetic,
biodegradable
functionalization
catalyst
of
βꢀcyclodextrin
2ꢀalkylazaarens
for
CꢀH
with
carbonꢀheteroatom
bonds, which have potential
application in the synthesis of biologically relevant
molecules. 2ꢀAlkyl azaarenes such as 2ꢀmethyl pyridine
and 2ꢀmethyl quinoline are ubiquitous structural motifs in
biologically compatible molecules and act as valuable
precursors for a wide range of 2ꢀalkyl heterocycles.4
Forasmuch, digits of attempts have been presumed to the
homo/heterocyclic diones (Scheme 1).
Scheme 1 Reaction of 2ꢀMethylazaarenes with Diones
O
β
-CD
X
N
O
OH
functionalization of
2ꢀmethylꢀazaarenes.
Recently,
O
Water, 80°C
N
X
numerous catalysts and procedures have been reported
for direct C(sp3)–H bond functionalization of 2ꢀalkylꢀ
substituted azaarenes.5ꢀ9 Keeping in account the utility of
C–H bond functionalization, in the scenario of green
X=C: Homocyclic diones
X=NR, S :Heterocyclic diones
To look into the prospects of new CꢀC bond formation, a
model reaction using easily available and reasonably
priced 2ꢀmethylquinoline and 1ꢀmethylisatin was
investigated in detail by varying catalysts to develop the
appropriate conditions (Table 1).
The screening was initiated by using types of cyclodextrin
for example αꢀCD, βꢀCD, ϒꢀCD, βꢀCDꢀOTs and βꢀCDꢀ
BIMOTs to determine their catalytic efficacy. Out of all the
trials, an exceedingly high product yield (91%) was
obtained in the presence of βꢀCD (Table 1, entry 4)
whereas αꢀCD, ϒꢀCD, βꢀCDꢀBIMOTs gave poor yields.
When the reaction was carried out without the use of
catalyst, it did not provide the desired product (Table 1,
entry 1). After that when the same reaction was carried
chemistry,
the
search
of
more
economical,
environmentally benign catalyst and medium is yet
desirable.
From the standpoint of green chemistry water is
known to be a potential replacement for organic
solvents10ꢀ14 which is readily available, cost effective,
environmentally compatible and have been used several
times as solvent in organic synthesis.15ꢀ18 In continuation
to our work on development of green synthesis for natural
products and medicinal agents19 we wish to report here
use of biomimetic catalyst cyclodextrin (CD) for CꢀH
functionalization in water. Cyclodextrins are cyclic
oligosaccharides of D(+)ꢀglucopyranosyl units linked by αꢀ
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Table 1 Optimization of reaction conditionsa
with isatin and βꢀCD with isatin and 2ꢀmethylquinoline
after two hours were undertaken (Fig. 2). It is evident
from Fig. 2 that there is an up field shift of protons of
CD when complexed with 2ꢀmethylquinoline as compared
to that of neat ꢀCD indicating the formation of an
inclusion complex of 2ꢀmethylquinoline with ꢀCD. The up
field character of protons retained in spectra of reaction
mixture (after 2h) reveals the retention of complex during
reaction (Fig. 2).
Intrigued by these observations and with the stipulated
reaction conditions in hand, the scope and versatility of
present method was extended to the reaction of 2ꢀ
methylpyridine as well as 2ꢀ methylquinolines with isatins
to afford a diverse range of 3ꢀsubstitutedꢀ3ꢀhydroxyꢀ
oxindoles (Scheme 2). Isatins and 2ꢀmethylquinoline with
different substitution patterns participated well in the
reaction to deliver the corresponding products. An
increase in the yield was noticed for Nꢀsubstituted isatins
in comparison to unsubstituted one.
βꢀ
O
Catalyst
O
N
O
OH
β
Water
N
N
β
N
Entry
Solvent
Time(h)
Temp
Catalyst
Yield
(%)
ꢀ
ꢀ
(°C)
(20 mol %)
1
2
3
Water
Water
Water
12
12
9
80
100
80
ꢀ
ꢀ
22
α-CD
β-CD
-CD
4
Water
4
80
91
5
6
7
Water
Water
Water
8
12
14
80
80
80
21
30
17
ϒ
β CDꢀBIMOTs
β CDꢀTs
aReaction conditions: a mixture of 1a(1.0 mmol ), 2a (1.2
mmol ) and the catalyst (20 mol%) in the water was stirred at
80°. CD –cyclodextrin, β ꢀCDꢀBIMOTs: Monoꢀ6ꢀdeoxyꢀ6ꢀ(3ꢀ
benzylimidazolium)ꢀβ ꢀcyclodextrin
.
out without adding catalyst at higher temperature such as
100°C, no product was obtained (Table 1, entry 2) which
indicate the essential role of CD in activation of C (sp3)ꢀH
bond of the 2ꢀmethylquinoline.
Scheme 2 Synthesis of azaareneꢀsubstituted 3ꢀhydroxyꢀ2ꢀ
oxindoles
R1
O
R2
N
O
β
-CD
OH
R1
O
H2O
N
N
R3
N
R2
R3
3a-3o
N
O
N
O
N
O
N
OH
OH
OH
OH
O
NO2
HN
3b, 85%
N
HN
N
3c, 86%
Fig. 1 Schematic representation of α -, β -, ϒ -CD and derivatives of β-CD with
their equivalent truncated cone structures
3d, 85%
3a, 82%
N
N
O
N
O
N
O
OH
OH
OH
OH
Besides this we have also explored the evidence for
O
β
ꢀCD using 1H NMR
NO2
Cl
association of 2ꢀmethylquinoline with
spectroscopy. A comparison of 1H NMR spectra (D2O
solutions) of ꢀCD, ꢀCD with 2ꢀmethylquinoline, ꢀCD
HN
HN
HN
HN
3h, 85%
3f, 84%
3g, 84%
3e, 82%
β
β
β
N
N
N
N
OH
OH
OH
F
OH
O
O
O
O
Br
I
HN
HN
HN
N
3j, 84%
3i, 82%
3k, 85%
3l, 91%
F
Cl
Br
N
O
N
N
O
N
N
O
N
N
O
N
OH
OH
OH
OH
Cl
3o, 84%
3m, 89%
3n, 86%
3p, 83%
Reaction conditions: Reactions were performed with 2ꢀ
methylazaarene (1.2 mmol), isatin (1.0 mmol), and βꢀ
cyclodextrin (20 mol %) in aqueous medium at 80°C.
Fig. 2 1H NMR spectra of (A) β-CD (B) β-CD with 2-methylquinoline (C) β-CD with
isatin (D) β-CD with isatin and 2-methylquinoline after 2 h
To examine the efficacy of present greener protocol, next,
it was imperative to explore the benzo [b] thiopheneꢀ 2, 3ꢀ
2 | Green Chem, 2014, 00, 1-3
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dione with 2ꢀmethylazarenes like 2ꢀmethylpyridine and 2ꢀ
methylquinolines. To our delight, expected products (4a-
4d) were obtained in good yields and the outcomes are
given in Scheme 3.
Scheme
5
Synthesis
of
azaareneꢀsubstitutedꢀ2ꢀ
hydroxyaceanthrylenꢀ1(2H)ꢀones
O
O
N
HO
O
Scheme 3 Synthesis of azaareneꢀsubstituted 3ꢀhydroxyꢀ
benzo[b]thiophenꢀ2(3H)ꢀones
β-CD
H2O
N
R2
R1
6a-6b
O
R2
N
O
β
-CD
OH
R1
O
H2O
S
N
S
N
N
O
O
4a-4d
HO
HO
Cl
F
6a, 88%
6b, 91%
N
O
N
O
N
O
N
O
OH
OH
OH
OH
Reaction conditions: Reactions were performed with 2ꢀ
methylazaarene (1.2 mmol), aceanthryleneꢀ1, 2ꢀdione (1.0
mmol), and βꢀcyclodextrin (20 mol %) in aqueous medium at
80°C.
S
S
S
S
4a, 81%
4b, 87%
4b, 88%
4b, 86%
Reaction conditions: Reactions were performed with 2ꢀ
methylazaarene (1.2 mmol), benzo[b]thiopheneꢀ2, 3ꢀdione
(1.0 mmol), and βꢀcyclodextrin (20 mol %) in aqueous
The catalyst reusability was studied five times including
fresh catalyst for the synthesis of compounds 3c, 3m, 4b,
and 5b and inevitable losses of catalytic activities were
medium at 80°C
.
observed during reusability experiment (Fig. 3).
As the heterocyclic diones gave the expected results with
established conditions, various homocyclic diones were
subjected to the reaction with 2ꢀmethyazaarens to assess
the scope and versatility of the methodology.
Scheme 4 Synthesis of azaareneꢀsubstituted 2ꢀhydroxyꢀ
acenaphthylenꢀ1(2H)ꢀones
O
O
N
HO
O
β
-CD
H2O
N
5a-5b
Fig. 3 Reusability data for β -cyclodextrin
N
HO
O
N
HO
O
A
cross catalytic reusability has been studied for
compounds 3c and 3m and no appreciable loss is
observed. The fresh catalyst used for the synthesis of 3c
is recovered and than used for synthesis of 3m for first
batch synthesis and viceꢀversa (Fig. 4).
5b, 88%
5a, 82%
Reaction conditions: Reactions were performed with 2ꢀ
methylazaarene (1.2 mmol), acenaphthyleneꢀ1, 2ꢀdione (1.0
mmol), and βꢀcyclodextrin (20 mol %) in aqueous medium at
80°C.
In
this
context,
first
we
explored
bicyclic.
Acenaphthyleneꢀ1, 2ꢀdione with 2ꢀmethylpyridine as well
as 2ꢀmethylquinoline and desired products (5a, 5b) were
obtained (Scheme 4). Then after, we tried the same
protocol on polycyclic aceanthryleneꢀ1, 2ꢀdione with 2ꢀ
methylpyridine as well as 2ꢀmethylquinoline and results
are mentioned in Scheme 5.
Fig. 4 Cross reusability data for β -cyclodextrin
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In conclusion, we report first example of βꢀcyclodextrin
catalysed CꢀH Bond functionalization of 2ꢀ
methylazaarenes with various diones in water to afford 2ꢀ
methylazaareneꢀsubstitutedꢀheterocycles.This quest
confers a greener route toward establishing a new
horizon for CꢀH functionalized reactions. Furthermore, we
are trying our best to exploit βꢀcyclodextrin in other CꢀH
functionalized reactions in an environmental way, and the
related work is underway in our laboratory.
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Medicinal and Process Chemistry Division, CSIR-Central Drug Research
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Research (AcSIR) New Delhi India. E-mail:dratulsax@gmail.com;
Fax: 91-522-26234051; Tel:91-522-2612411
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Graphical abstract
βꢀCyclodextrin Catalysed CꢀC Bond Formation via C(sp3)ꢀH
Functionalization of 2ꢀMethyl azaarenes with Diones in aqueous
medium
Atul Kumar*, Ratnakar Dutt Shukla
A βꢀcyclodextrin catalysed C(sp3)ꢀH functionalization of 2ꢀalkylꢀazaarenes with homocyclic as well
as heterocyclic diones in water has been developed. This bioꢀmimetic catalyst oriented methodology
provides a sustainable protocol for CꢀH functionalization, the area mainly dominated by transition
metals.
O
O
N
N
O
O
O
O
O
O
OH
S
N
S
HN
HO
H
N
O
O
N
N
HO
HO
O
O