Nanoparticle catalyzed reaction (NPCR): ZnO-NP catalyzed Ugi-reaction in aqueous medium

Article abstract of DOI:10.1039/c3gc41101j

The first ZnO-NP catalyzed Ugi type three-component (AB²C) reaction has been developed for the synthesis of 2-arylamino-2- phenylacetimidamide from an aldehyde, amine and isocyanide in aqueous media. This nanoparticle catalysed reaction (NPCR)

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Nano-particle Catalyzed Reaction (NPCR) : ZnO-NP catalyzed Ugi-
reaction in aqueous medium
Atul Kumar,a* Deepti Saxenaa and Maneesh Kumar Guptaa
First ZnOꢀNPs catalyzed Ugi type threeꢀcomponent (AB2C) reaction has been developed
for the synthesis of 2ꢀarylaminoꢀ2ꢀphenylacetimidamide in aqueous media. The
synthesized compound yielded 2ꢀaminoꢀ2ꢀphenylacetamide on hydrolysis with I2/SDS
/water, whereas on alkaline hydrolysis afforded Nꢀsubstituted αꢀamino acid.

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DOI: 10.1039/C3GC41101J
Cite this: DOI: 10.1039/c0xx00000x
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ARTICLE TYPE
Nanoꢀparticle Catalyzed Reaction (NPCR) : ZnOꢀNP catalyzed Ugiꢀ
reaction in aqueous medium
Atul Kumar,a* Deepti Saxenaa and Maneesh Kumar Guptaa
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
First ZnOꢀNPs catalyzed Ugi type threeꢀcomponent (AB2C) reaction has been developed for the
synthesis of 2ꢀarylaminoꢀ2ꢀphenylacetimidamide from aldehyde, amine and isocyanide in aqueous
media. This Nanoꢀparticle Catalysed Reaction (NPCR) is high yeliding and have good atom
10 economy as well as atom efficiency.The synthesized phenylacetimidamide yielded 2ꢀaminoꢀ2ꢀ
phenylacetamide on hydrolysis with I2/SDS/water, whereas on alkaline hydrolysis afforded Nꢀ
substituted αꢀamino acid.
Introduction
Fax: +91 522-26234051; Tel +91 522-2612411
50 † Electronic supplementary information (ESI) available: Experimental
The wedding of two emergent area of twenty first century,
15 nanocatalysis and green chemistry holds the answer of
sustainablity.1 Catalytst bulids the main strategy for organic
reactions via modulation of energy. Nanoꢀcatalysis (NꢀCAT) is
one of them as growing research area due to their unique
properties.2 The efficiency of Metalꢀbased heterogeneous
20 nanoparticles catalysis in organic synthesis3 can be improved by
employing nanoꢀsized catalysts because of their extremely small
and large surface to volume ratio. Recently, Nanoparticles (NPs)
as catalyst have been attracting much attention because they can
be easily recovered from the reaction mixture4 and ZnOꢀNPs5 is
25 one of them. According to the recent literature survey ZnOꢀNPs
is inexpensive, nonꢀtoxic catalyst and also has environmental
advantages i.e., disposal of the catalyst6 and utilize for waste
water treatment7 by removing chemical and biological
contaminants after reused. Many researchers have been reported
30 the potential utility of ZnOꢀNPs in biological field such as antiꢀ
cancer8, and antiꢀbacterial.9
details and NMR spectra. See supporting information.
Fig.1. (a) Ugiꢀ4CR (b) A newly reported threeꢀcomponent
reaction by List and Pan.
55 Inspired from Ugiꢀ3CR, we focused our efforts towards the
search of new efficient methodology using ZnOꢀNPs as a catalyst
for reaction and we surprised to obtain
2ꢀarylaminoꢀ2ꢀ
phenylacetimidamide 5 (AB2C) type product (scheme 1) instead
of expected 2ꢀaminoꢀ2ꢀphenylacetamide b. In odrer to establish
60 the utility of 5, we hydrolyzed it under green conditions which
yielded 2ꢀarylaminoꢀ2ꢀphenylacetamides 6 and Nꢀsubstituted αꢀ
amino acids 7 respectively.
More recently, numerous functionalized NPs have been
employed in a variety of organic transformations, showing
excellent catalytic properties in CꢀC bond formation,10
35 hydrogenation,11 oxidation12 amination,13 and Ritter reactions.14
However, it is an innovative stratagems, if multiꢀcomponent
reactions could be carried out via green protocol with
nanoparticle, it would be most proficient synthetic method for
organic synthesis.
40 The most widely studied MCR is Ugiꢀ4CR15 (Figure.1, path a)
have generated much interest because of their wide synthetic and
medicinal application.16 Recently, List and Pan interestingly
modified Ugiꢀ4CR into Ugiꢀ3CR (ABC type) for the synthesis of
65 Scheme 1. ZnOꢀNPs catalyzed Ugi threeꢀcomponent reaction for
the synthesis of 2ꢀarylaminoꢀ2ꢀphenylacetimidamide 5.
2ꢀaminoꢀ2ꢀphenylacetamide
b from aldehyde, amine and
45 isocyanide catalyzed by phenyl phosphinic acid.17 (Figure 1, path
b).
Nitrogenꢀcontaining heterocyclic molecules constitute the largest
portion of chemical entities, which are part of many natural
products, fine chemicals, and biologically active pharmaceuticals
aMedicinal and Process Chemistry Division, CSIR-Central Drug
Research Institute, Lucknow, India, E-mail: dratulsax@gmail.com,
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

Page 3 of 6
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DOI: 10.1039/C3GC41101J
vital for enhancing the quality of life.18 Phenylacetimidamide,19 35 catalysts for the modified Ugi threeꢀcomponent reaction. The
phenylacetamide20 and αꢀamino21 acid scaffold display a wide
range of biological activities (Fig 2).
model reaction of benzaldehyde 1, aniline 2 and tert-butyl
isocyanide 3 was performed without use of any catalyst in
different solvents at room temperature. Only a trace amount of
the product was formed (Table 1, entries 1ꢀ3). The obtained
40 results prompted us to catalyze the reaction with different
catalysts. First we choose BF3 (nonꢀmetal Lewis acid) to
catalyzed the reaction mixture, resulted to the formation of
desired product in 32% yield (Table 1, entry 4). We did not found
satisfactory results when we used metal Lewis acids such as TiCl4
45 and CuI (Table 1, entries 5 and 6). After screening of Lewis acid
catalysts we turned our attention towards the use of nanoꢀ
particles like TiO2ꢀNPs as a heterogeneous catalyst but no
improvement was observed in the yield of desired product 5a
(Table 1 entry7). Fortunately, we acquired pleasant result when
50 we carried out the reaction with ZnOꢀNPs (Table 1, entry 11).
Among the catalysts evaluated, ZnOꢀNPs was superior to others
and provided the best yield of the compound 2ꢀarylaminoꢀ2ꢀ
phenylacetimidamide 5a.
It was found that catalyst loading also played important role in
55 this reaction and 15 mol % ZnOꢀNPs was found to be sufficient
to obtained the desired product 5 with excillent yield. (Table 1,
entry 11). The yield remained unaffected when the catalyst
loading was increased 15 to 20 mol % (Table 1, entry 12).
The reaction was also screened in different solvents as well. We
60 used a variety of polar and nonpolar solvents like DMSO, ethanol
methanol toluene, THF and CH3CN (Table 2). We observed that
polar protic solvent afforded moderate yield than other solvents.
By keeping the vital aspect of water in mind we tried the same
reaction in aqueous medium and found the best results (Table 2
65 entry 7).
5
Fig. 2 Biologically active compounds containing amidine, amide
and amino acid skeleton.
Previously, Khan et al22 reported the synthesis of 2ꢀarylaminoꢀ2ꢀ
phenylacetimidamide derivative using bromodimethylsulfonium
10 bromide (BDMS) with acetonitrile. This methodology suffer from
severe drawback such as longer reaction time, harsh reaction
conditions, use of nonꢀbiodegradable solvent and expensive,
harmful, unstable catalyst.
In continuation of our quest for developing green protocols23
15 herein, we wish to report a first (AB2C) Ugi type 3ꢀCR for the
synthesis of 2ꢀarylaminoꢀ2ꢀphenylacetimidamide derivatives 5
from aldehyde 1, aniline 2 and isocyanide 3 by employing the
catalytic amount of ZnOꢀnanoparticle as an efficient,
heterogeneous and reusable catalyst (Scheme 1). To the best of
20 our knowledge, none methodology has been reported till now of
the use of ZnOꢀNPs as a reusable catalyst for the synthesis of 2ꢀ
arylaminoꢀ2ꢀphenylacetimidamide derivatives.
Table 1. Optimization of reaction conditions for the synthesis of
25 2ꢀarylaminoꢀ2ꢀphenylacetimidamide 5a.a
Table 2. Effect of solvents for the synthesis of 2ꢀarylaminoꢀ2ꢀ
phenylacetimidamide cactalyzed by ZnOꢀNPs.a
Entry
Solvent
DMSO
Toluene
THF
CH3CN
ethanol
Methanol
H2O
Yield (%)
1
2
3
4
5
6
7
30
34
40
45
48
50
90
e
Entry
1
Catalyst
ꢀ
Time (h)
Yield (%)
15
5
2
3
4
5
6
7
8
9
ꢀ
ꢀ
4
5
8
6
6
7
8
25
28
32
40
42
43
48
50
70 aReaction conditions : aniline (1 mmol), benzaldehyde (1 mmol), and
tertꢀbutylꢀisocyanide (1 mmol) with 15 mol % ZnOꢀNPs at r.t. for a given
time. bIsolated yield.
BF3
TiCl4
CuI
TiO2
ZnOb
ZnOb
ZnOb
ZnOc
ZnOd
Therefore, the survey of solvents with ZnOꢀNPs revealed that
75 water was the best choice (Table 2, entry 7) whereas other
organic solvents diminshed the reactivity (Table 2, entries 1ꢀ6).
6
10
11
12
15 min
5 min
6 min
60
90
90
The obtained results demonstrated the viability of ZnOꢀNPs
catalyzed strategy, therefore we explored the scope and generality
80 of the reaction. The desired product 5 obtained in good to high
aReaction Conditions
butyl isocyanide, (1 mmol), and ZnOꢀNPs (15 mol %) in water (3 ml) at
rt. 10 mol% of ZnO(NPs). 15 mol% ZnO(NPs). 20 mol% ZnO(NPs).
30 eIsolated yield.
: benzaldehyde (1 mmol), aniline (2 mmol), tertꢀ
b
c
d
yields by using
a wide range of aldehydes including
aromatic/heteroꢀaromatic, substituted aniline, and different
isocyanides. In contrast to the aniline with electronꢀwithdrawing
substitutents, the formation of desired product took slightly
85 longer reaction time than aniline with electronꢀdonating groups
Result and Discussion
Our initial efforts were focused on the actual effectiveness of
2
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(Table 3, entries 1ꢀ15).
Table 3. Synthesis of 2ꢀarylaminoꢀ2ꢀphenylacetimidamide 5 via
modified Ugi threeꢀcomponent reaction.a
Time
(h)
Yieldb
(%)
Entry
R1
R2
R3
Prod.
1
2
3
4
5
6
7
H
4ꢀCl
4ꢀOMe
3ꢀOMe
H
H
tBuNC
cHexNC
cHexNC
tBuNC
tBuNC
pTsCH2
tBuNC
5a
5b
5c
5d
5e
5f
5
8
5
6
7
5
5
90
85
80
82
88
84
79
4ꢀOMe
4ꢀCl
3ꢀCl
3ꢀOMe
4ꢀF
4ꢀCl
4ꢀCl
Fig. 3 Representation of possible tautomeric structure.
3ꢀOMe
5g
Neutral hydrolysis of 5a, 5b, 5h, 5i, and 5k in the presence of I2/
SDS sodium dodecyl sulfate)/water afforded corresponding 2ꢀ
35 aminoꢀ2ꢀphenylacetamide derivatives24
(Table 4), which
8
9
4ꢀBr
4ꢀCl
4ꢀF
3ꢀOMe
4ꢀOMe
4ꢀOMe
tBuNC
tBuNC
5h
5i
5
5
5
80
83
80
(
6
10
cHexNC
5j
suggested that compound 5A was the actual isomer instead of 5B.
11
12
13
H
2,4diMe
4ꢀCl
H
cHexNC
cHexNC
tBuNC
5k
5l
6
5
5
82
75
80
Table 4. Synthesis of 2ꢀarylaminoꢀ2ꢀphenylacetamide 6 via
hydrolysis of 2ꢀarylaminoꢀ2ꢀphenylacetimidamide 5.a
4ꢀpy
4ꢀpy
5m
2ꢀmorp
ethyl
14
15
4ꢀCl
H
4ꢀCl
5n
5o
5
73
70
R2
N
O
H
4ꢀNO2
tBuNC
15
H
R3
N
R3
I2, 20 mol%
N
N
N
H
R2
aReaction conditions: bezaldehyde (1 mmol), aniline (2 mmol),
isocyanide (1 mmol), and 15 mol% ZnOꢀNPs in water (3 mL) at rt.
bIsolated Yield.
R2
H
R1
5
Water, SDS
80oC
R1
5
6
40
The Green Chemistry movement has developed a series of
metrics to support and reinforce behaviour change on both
10 industry and academia in the move towards green and more
sustainable chemistry. Two most common green metrics, Atom
economy and Atom efficiency are calculated below.
Entry
R1
R2
R3
Prod.
Time(h)
Yieldb(%)
1
2
3
H
H
tBuNC
cHexNC
tBuNC
6a
6b
6h
5
7
6
70
80
78
4ꢀCl
4ꢀBr
4ꢀOMe
3ꢀOMe
4
5
4ꢀCl
H
4ꢀOMe
tBuNC
6i
5
75
74
Atom Economy = 357.22/ (106.04+186.12+83.07)×100 = 95.2%
2,4ꢀ
diMe
cHexNC
6k
10
15
Atom Efficiency = 90% × 95.2% = 85.6%
aReaction conditions: 2ꢀArylaminoꢀ2ꢀphenylacetimidamide 5 (1 mmol)
with I2 (20 mol %), and SDS (10 mol %), in water (3 mL) at
80°C.bIsolated Yield.
From the green chemistry point of view, efficient recovery and
reuse of the catalyst are highly desirable, thus the recovery and
20 reusability of ZnOꢀNPs were investigated. After completed the
reaction, ethylacetate was added until the solid crude product was
dissolved. Then, ZnOꢀNPs as the catalysts were isolated from the
mixture of reaction by simple filtration and reused again after
washing with ethylacetate (see supporting information).
25
45 Searching for the utility of 5 we further converted it into Nꢀ
substituted αꢀamino acid via alkaline hydrolysis. To
7
demonstrate the feasibility of this reaction, initially experiments
were performed using 5a and found the expected product 7a. To
generalize this scope various derivatives of 5 were successfully
50 hydrolyzed and results have been shown in Table 4.
The structural data of compound 5a, highlighted the possibility of
tautomeric structures 5A and 5B. In order to address this issue it
was anticipated that the hydrolysis of 5a could offer either 6a
when 5A form is predominant or 8 when 5B is predominant
30 (Fig.3).
55
60
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Table 5. Synthesis of αꢀamino acid via alkaline hydrolysis of 2ꢀ
arylaminoꢀ2ꢀphenylacetimidamide 5.a
Arylaminoꢀ2ꢀphenylacetamide have been synthesized by using
green hydrolysis system I2/SDS(sodium dodecyl sulfate)/water
from 2ꢀarylaminoꢀ2ꢀphenylacetimidamide.
Nꢀsubstituted αꢀ
30 amino acids via alkaline hydrolysis. These ecofriendly procedures
can be regarded as new methods for the preparation of
pharmaceutically
relevant
phenylacetimidamide,
phenylacetamide and αꢀamino acid derivatives.
Experimental Section
Entry
R1
H
R2
H
R3
Prod.
7a
Time(h)
Yieldb(%)
35 General procedure for the synthesis of 2ꢀArylaminoꢀ2ꢀ
phenylacetimidamide (5)
1
2
3
tBuNC
9
8
70
72
65
4ꢀCl
4ꢀCl
4OMe cHexNC
7b
Aldehyde (1 mmol), aniline (2 mmol), isocyanide (1 mmol) and
ZnOꢀNPs <50 nm (15 mol %) were placed into a flask. Water (3
mL) was added to the mixture and stirred for 5 min at room
40 temperature. Progress of reaction was monitored by TLC, after
completion of the reaction, the reaction mixture was diluted with
water and extracted with ethyl acetate, dried over sodium
sulphate and evaporated under vacuum to give crude product,
which was purified by silica gel (100ꢀ200 mesh) coloum
45 chromatography to afford the correponding product.
OMe
tBuNC
7b
10
2,4ꢀ
diMe
4
H
cHexNC
7k
10
69
aReaction conditions: 2ꢀArylaminoꢀ2ꢀphenylacetimidamide 5 (1 mmol),
5
NaOH (20 mol%) in H2O:EtOH (1:3, 4 mL) at 70°C. bIsolated Yield.
On the basis of the above observations and the literature survey,25
we hypothised the plausible mechanism for the synthesis of 2ꢀ
arylaminoꢀ2ꢀphenylacetimidamide 5a as shown in Scheme 2. The
General procedure for synthesis of 2ꢀarylaminoꢀ2ꢀ
phenylacetamide (6)
In a 50ꢀmL roundꢀbottom flask, iodine (0.2 mmol), 2ꢀarylaminoꢀ
2ꢀphenylacetimidamide (1 mmol), and surfactant (sodium
50 dodecyl sulfate, 10 mol %) were added in H2O (3 mL) and stirred
for 6h at 80°C and reaction was monitored by TLC. The aqueous
part was diluted and extracted with ethyl acetate, the organic
layer was washed with brine and dried over anhydrous Na2SO4.
Evaporation of solvent gave a crude product which was purified
55 by column chromatography (silica gel, ethyl acetate:hexane).
10 reaction commences with the formation of shiffꢀbase (1a) from
aldehyde and aniline catalyzed by ZnOꢀNPs. Now 1a was further
activated by ZnOꢀNPs via protonation favouring the nucleophilic
attack of isocyanide to provide the intermediate 1b.
Subsequently, another molecule of aniline react with 1b, leads to
15 the formation of αꢀamino amidines 4 which undergoes [1,3]
hydrogen shift to yield the desired product 5.
General procedure for the synthesis of αꢀamino acid (7)
In
a
50ꢀmL
roundꢀbottom
flask,
2ꢀarylaminoꢀ2ꢀ
phenylacetimidamide (1 mmol) was added in 1:3 ratio of H2O:
60 EtOH with 20 mol% NaOH and stirred for 3 h at 70°C. After
completion of reaction as evidenced by TLC, solvent was
removed in vacuum. The aqueous part was diluted and extracted
with ethyl acetate, the organic layer was washed with brine and
dried over anhydrous Na2SO4. Evoperation of solvent gave a
65 crude product which was purified by column chromatography
(silica gel, ethyl acetate:hexane).
Supporting Information see footnote on the first page of
this article): 1H and 13C NMR spectra of all compounds and other
70 details.
Note and References :
Scheme 2. Plausible mechanistic pathway for the synthesis of 2ꢀ
arylaminoꢀ2ꢀphenylacetimidamide 5.
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Conclusion
In conclusion, we have demonstrated Nanoꢀparticle Catalyzed
Reaction (NPCR) Ugi type threeꢀcomponent reaction in water. A
highly efficient and environmentally green methodology for the
25 synthesis of 2ꢀarylaminoꢀ2ꢀphenylacetimidamide derivatives
from amine, aldehyde and isocyanide has been developed. 2ꢀ
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