First use of p-tert-butylcalix[4]arene-tetra-O-acetate as a nanoreactor having tunable selectivity towards cross azo-compounds by trapping silver ions

Article abstract of DOI:10.1039/c5gc01859e

p-tert-Butylcalix[4]arene-tetra-O-acetate was established for the first time as a member of the nanoreactor series, even without having any -OH group. The nano range distribution of this nanoreactor was ascertained by DLS, SEM and TEM studies. The capability of this cavitand towards hosting amines in a competitive manner generates a new green pathway for cross coupling of aromatic amines to give the corresponding azo-compounds. In this context, using p-tert-butylcalix[4]arene-tetra-O-acetate as a nanoreactor and silver nitrate as a catalyst, we got the cross azo-compound in good to excellent yields in the eco-friendly solvent water. This green methodology is also applicable for the synthesis of respective homo-compounds.

Full text of DOI:10.1039/c5gc01859e

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First use of p-tert-butylcalix[4]arene-tetra-O-acetate as nanoreactor
having tunable selectivity towards cross azo-compounds by trapping
silver ion
Piyali Sarkara and Chhanda Mukhopadhyay*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
5
45 synthesis that relies not only on the virtues of amphiphilicity,6 but
p-tert-Butylcalix[4]arene-tetra-O-acetate was established for
also that further adheres to the selectivity of the substrate in water
should enhance the appeal from the environmental perspective.
Presence of polar lower rim and nonpolar cavity within one stand
of a single molecule, interested us in monitoring selectivity of an
50 organic compound in aqueous medium. Noncovalently formed
surfactant capsules generally cannot show any type of selectivity
towards organic compounds (depending upon substitutions) in
the first time as a member of nanoreactor series, even without
having any –OH group. The nano range distribution of this
10 nanoreactor was ascertained by the DLS, SEM and TEM
studies. The capability of this cavitand towards hosting
amines in a competitive manner generates a new green
pathway of cross coupling of the aromatic amines to give the
corresponding azo-compounds. In this context, using p-tert-
15 butylcalix[4]arene-tetra-O-acetate as nanoreactor and silver
nitrate as catalyst, we got the cross azo-compound in good to
excellent yields in eco-friendly solvent water. This green
methodology is also applicable for the synthesis of respective
homo-compounds.
water which is
a critical drawback for surfactants. The
disadvantage of covalent capsules is their tedious multistep
55 synthesis. With this concern we attempted to find out the hosting
capability of easily synthesised pꢀtertꢀbutylcalix[4]arene tetraꢀOꢀ
acetate (1). Interestingly a single step acetate group incorporation
(Scheme 1) at the lower rim of pꢀtertꢀbutylcalix[4]arene is
sufficient to produce compound 1 at 98% yield with 100%
60 conversion. Our continuous effort reached us to the success of
binding aromatic amines to the host in water medium. Actually
this is an achievement to the way to arrange a nanoreactor from a
neutral calixarene derivative even without a single –OH group.
The interaction of amine with the cavitand 1 was confirmed by
65 1HꢀNMR, HRMS.
20 1. Introduction:
Solvent is an essential part of most of the organic reactions.
Currently, in light of the environmental E factor,1,2 toxic organic
solvents replace ecoꢀfriendly solvents like water thereby
becoming a vital target to the academic and scientific research. In
25 our body, a magnificent array of organic reactions take place in
water and this spell bounded fact motivates recent chemists to
develop clean and green protocols for organic transformations
involving water as solvent.
O
O
H3C
O
CH3
Nanoreactors provide a strong tie between organic compounds
30 and water that have pushed chemical technologies to the forefront
of greenness. Initially nanoreactors were made by selfꢀ
aggregation of metal nanoparticles, metal organic frameworks,
surfactants or dendrimer like molecules which belong to nonꢀ
covalent molecular capsules.1,3 Again, generation of covalent
35 organic frameworks with nanospace like hemicarcerands,
cyclotriveratrylene based capsules became a key object, but the
activity of these compounds is outshined by a parameter that
involves flexibility to be switchable at entrance and exit of the
guest molecules at least at moderate temperature.4 The behaviour
40 of calixarene or resorcenerene as nanocontainer, have
magnetically pulled them in plethora of nanochemistry.5,6
Calixarenes may play the bridging role between organic and
aqueous layer due to their amphiphilic 3D covalent framework.6,7
Now, the capability to be efficient nanoreactor for organic
reflux, 3 h
O
O
O
O
O
O
H
O
O
O
H
O
H
O
H
O
p-Tert-butylcalix[4]arene
Calix[4]acetate
Scheme 1: Synthesis of cavitand 1
In order to verify the effect of this interaction on selectivity of
amines in water, the formation of asymmetric azobenzenes were
70 investigated. Oxidation of aryl amines to azobenzenes is among
the most valuable processes in photochemistry as they behave as
photochromic ligands for optochemical genetics and
photoswitches in biological systems,8 due to their cisꢀtrans
isomerization in a photosensitive manner. In the last decade,
75 azobenzenes were applied as photoresponsive soft materials such
as smart polymers,9 liquid crystals.10 So, several methods were
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reported to exhibit easy formation of these azocompounds but the
impact of the methodology should resolute from the point of
symmetricꢀasymmetric coupling. Having secured a facile route
entire part of the amine protons alter them in the new position
without remaining any trace of the old peaks. A direct,
unambiguous evidence of 1:1 complexation ensues from the
for asymmetric or cross coupling of the amines along with 60 HRMS data. To our delight, a combined mass peak appears at
5
Principles of Green Chemistry1 in mind, we focussed at simple
and easily available metal salts like Cu(I) or Ag(I) for catalysis
involving air as the stoichiometric oxidant. Some procedures
have been reported in recent years but most of them have
followed rough conditions like Cu/pyridine,11 Ag NP/KOH
924.5407 (Fig. 2), stands for 1:1 complex pꢀtoluidine
calix[4]acetate along with the absence of any peak for 1:2 or 2:1
or onwards complex. So, the only possible way is 1:1 binding in
aquous medium as we did this NMR and HRMS experiments by
c
NH2
10 (equivalent amount),12 tꢀBuOCl/NaI/THF or Et2O etc.13 leaving a
real need of a much greener protocol. Interestingly, the cavitand
(1) having Oꢀdonor site of acetate group at the lower rim,
favoured silver ion better than the copper. On the other hand, the
hydrophobic cavity is tailored to stabilize nonpolar group
15 substituted aniline more than the polar one. These two factors
resulted in a high domination of cross coupling product over the
homo product. So, a high throughput yield of azoꢀcoupled
products were obtained using silver nitrate catalyst in presence of
calix[4]acetate nanoreactor in water at 60 ºC just in 1 h under
20 dark environment. As a consequence, we report here, an
oxidation protocol of aromatic amines to cross azocompounds (i)
using catalytic amount of a benign metal salt, (ii) involving air as
the stoichiometric oxidant, (iii) depending on an inoffensive and
recyclable nanoreactor, (iv) utilising most ecofriendly reaction
25 medium i.e., water, which offer the conditions leading to a very
low E factor1,2 based on the Principles of Green Chemistry
signifying the height of greenness allied with this new protocol
(Scheme 1).
a
65
a
H
H
H
b
b
H
d
CH3
Fig. 1 Stacked 1H NMR spectra (CDCl3, room temperature; 303 K) of the
guest p-toluidine, the host calix[4]acetate, and silver ion with p-toluidine
calix[4]acetate.
70 fishing up the complex from the water, containing equimolar (0.1
mmol of each) mixture of calix[4]acetate and pꢀtoluidine, after 10
min of stirring.
2. Result and Discussion:
30 2.1 Interaction Studies:
A lot of examples have been enrolled in the literature in
support of trapping capability of the calixarenes.14 In this report,
we have recognized for the first time, that calix[4]acetate may
behave as a host of electron rich amines like pꢀtoluidine. 1H
35 NMR spectroscopy is a very constructive tool for studying the
hostꢀguest interaction. The complexes can be observed very
2.2. Properties of the Nanoreactor:
The above observations inspired us to find the role of this small
75 molecule calix[4]acetate on monitoring the reactions of amines in
1
(a)
(b)
easily by following the change in HNMR spectrum of the hostꢀ
guest molecule. Quantitative complexation along with the
stoichiometry of the complex, (i.e., number of guest present in a
40 single host’s cavity) can be detected from integration of the
1
signals in the H NMR spectrum. Comparing the spectra of the
complex pꢀtoluidine calix[4]acetate with both the calixꢀ
cavitand and the free amine (Fig. 1), it is seen that there is a
downfield shift in the aromatic signals of the amine and it may be
45 due to the inclusion induced deshielding effect. Both the peaks
‘a’ (peak for mꢀprotons of the –NH2 group) and ‘b’ (peak for oꢀ
protons of the –NH2 group) show approximately same amount of
downfield shift, 0.1 ppm for ‘a’ and 0.12 ppm for ‘b’, that
implies the four aromatic protons appear in same type of
50 deshielding zone, probably inside the cavity. Due to the
formation of Hꢀbond with the acetate moieties of the lower rim of
calix[4]acetate, the peak ‘c’ for –NH2 group reflects the same
effect, 0.152 ppm. The resonance due to the methyl group of pꢀ
toluidine (peak ‘d’) supports its occurrence in the cavity via its
55 downfield shift of 0.129 ppm. The stoichiometric 1:1
complexation was confirmed from the integration study, as the
(d)
(c)
Fig. 2: (a) the mass peak for 1:1 p-toluidine calix[4]acetate complex,
(b) DLS data, (c) SEM image and (d)TEM image of calix[4]acetate.
aqueous medium to enrich the chemistry from both sides of
2
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selectivity and greenness. Interestingly, this molecule stepped up 35 just 1 h (Table 1, entry 15). Further increase in temperature did
to the nanochemistry as we found its diameter of 70ꢀ100 nm
range in water which is a general characteristic of nanoreactor.15
The size of this particle, dispersed in water, was determined from
SEM, TEM and the DLS data.
not show any further increase in the yield of the product.
2.3.2. Study of different conditions in absence of cavitand:
5
We also tried to find out the necessity of the cavitand (1) in due
course of this reaction. We screened up to 20 mol% of AgNO3 in
40 water in absence of calix[4]acetate at variable temperatures and
the results are included in Table 2. Neither cross (4a) nor the
homo products (5a & 5b) were found at room temperature. Only
a trace amount of product was observed at 40 °C. Even maximum
critical conditions i.e., high temperature ( temperature of boiling
45 water, 100 °C), high amount of catalyst, 20 mol% of AgNO3 and
prolong reaction time, 48 h, are inferior with respect to both
selectivity and yield. So, without the aid of this nanoreactor the
success of this selective oxidation methodology is almost
impossible.
2.3. Optimization of the reaction conditions:
2.3.1. Study of different conditions in presence of cavitand:
Next, we turned our attention to develop the optimized condition
of the catalytic oxidation of the amines. Azocompound derived
10 from pꢀtoluidine and pꢀbromoaniline was selected to identify a
suitable benign salt of either copper or silver to assist the
development of N=N bond. pꢀToluidine (1 mmol) and pꢀ
bromoaniline (1 mmol) were stirred with calix[4]acetate (2
mol%) for five minutes in 2 ml water and then treated with
15 various copper and silver salts (5 mol %) at 60 ºC, and the results
are tabulated in Table 1.
50 Table 2: oxidative coupling of amines in absence of cavitand (1)a
Table 1: optimization of metal salt in oxidative cross coupling of amines a
Br
NH2
NH2
catalyst
N
NH2
NH2
catalyst (5 mol%)
N
Br
air (1 atm), H2O, stirring
calix[4]acetate (2 mol%)
H3C
N
4a
CH3
2a
N
Br
3a
air (1 atm), H2O, stirring
H3C
CH3
2a
Br
3a
Br
CH3
4a
N
N
N
Yieldb (%)
56
N
Entry
1
2
3
4
5
6
7
8
Catalyst
Temp. (ºC)
100
100
100
100
100
100
100
100
100
100
100
r.t.c
40
Time, h
12
12
12
12
12
12
12
12
24
12
24
24
24
24
1
Br
CuI
CuBr
CuBr2
H3C
5f
5a
42
23
48
39
50
46
27
56
45
54
21
64
78
90
90
90
Entry
mol% of
Catalyst
Temp.
(ºC)
Time,
h
Yieldb
(%)
CuSO4
CuCO3
Cu(NO3)2
CuOAc
Ag2SO4
Ag2CO3
AgBr
Cross
4a
Homo
5a
Homo
5b
NFd
trace
16
1
2
3
4
5
20
20
20
10
20
r.t.c
40
80
100
100
48
48
48
24
48
NFd
NFd
12
NFd
trace
20
9
15
18
20
24
18
18
10
11
12
13
14
15
16
17
18
AgCl
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
a Reaction conditions: All reactions were carried out in H2O (2 mL) using
the substrates 2a and 3a in equivalent amount (1 mmol) in absence of
calix[4]acetate and catalyst according to the indicated ratio in mol%.
55 bIsolated yields. cHere r.t. corresponds to 30 °C. dNF=not found.
50
60
60
80
2
1
1
2.3.3. Study of the effects of different surfactants with
respected to the cavitand 1:
100
89
a Reaction conditions: All reactions were carried out in H2O (2 mL) using
20 the substrates 2a and 3a in equivalent amount (1 mmol) in presence of
calix[4]acetate and catalyst according to the indicated ratio in mol%.
bIsolated yields. cHere r.t. corresponds to 30 °C.
100
80
60
Attempt to use the copper salts did not show a glorious result in
25 producing the cross product in high yields. However, the silver
salts were more capable as oxidative catalyst under the mentioned
conditions. The probable reason is higher binding interaction than
copper with the oxygen centre of the acetate groups of
calix[4]acetate. Moreover, AgNO3 shows the maximum catalytic
30 activity, probably because of its high solubility in water. With
AgNO3 as catalyst, the reaction did not proceed in fascinated way
at all at room temperature. However, at elevated temperatures up
to 60 °C, it showed a significant improvement in the yield and
magnificently it yielded the cross product (4a) in 90% yield in
5f (homo)
40
20
0
5a (homo)
4a (cross)
60 Fig. 3: Survey of effect of different surfactant on reaction of 2a and 3a.
Surfactants like (CTAB, TTAB, SDS, SBDS etc.) can form
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micelle in water medium and their behaviour as nanoreactor is 35 selection towards pꢀtoluidene over the pꢀbromoaniline by the
reported in literature.15 We also attempted to find the capacity of
micelles formed from different type of surfactants in water
compared to the calix[4]acetate for the reaction between 2a and
3a (Fig. 3). Using the cationic surfactant, not a single trace of any
cavitand is also reflected from this data as the yield of 5b is much
lower than 5c.
The cross selection decreases as the size of the subtituent
increases from methyl to tertꢀbutyl group. So, the cavity of the
5
coupling product was found. Whereas for the anionic surfactants, 40 calix[4]acetate provides lower stability as the substituent size
SDS and SBDS showed 34 and 38% yield of the total coupled
product respectively.
increases.
2.5. Study of the effect of increasing substituent size:
2.4. Study of % Selectivity:
This study reveals the fact that as the size of the alkyl substituent
increases from methyl to the pꢀtertꢀbutyl group, cross selection
45 decreases. Interestingly the pꢀtertꢀbutyl part of the calixarene
derivative does not provide the maximum stability towards the pꢀ
tertꢀbutyl aniline which should occur if only the organic
environment, provide by the cavitand, is importent in this
reaction. So, this fact reflects the dependence of the reaction on
50 the cavity size rather than only on the organic environment
provide by the cavitand.
10 Table 3: Comparison of % selectivity of the surfactants with the cavitand
1 for reaction between 2a and 3a:
Compound
Mol%
SDS
20
SBDS
20
Calix[4]arene
2
1
Time (h)
24
24
Temp. (ºC)
% Selectivity
100
29.41
100
60
28.95
90.91
a
Reaction conditions: equivalent amount (1 mmol) of substrates 2a and
3a were stirred in 2 ml H2O with compounds according to the indicated
ratio in mol% and AgNO3 (5 mol%) at indicated temperature and time.
Table 4: comparison of higher alkyl homologue amines a
15
Br
NH2
NH2
To compare the selectivity, the % selectivity was calculated using
the following formula and was included in
N
catalyst
N
air (1 atm), H2O, stirring
R
R= -CH3, 4a
R= -CH(CH3)2, 4a'
R= -C(CH3)3, 4b'
R
Br
3a
(Yield)4a
X 100
% Selectivity =
R= -CH3, 2a
R= -CH(CH3)2, 2b
R= -C(CH3)3, 2c
(Yield)4a
+ (Yield)5a + (Yield)5f
20
Br
R
Table 3. The anionic surfactants showed very low (28ꢀ30%)
selectivity even at high temperature and time, in expense of high
amount of those compound, 20 mol%. Delightfully the cavitand 1
induced the selectivity more than 90%, only at expense of 2
N
N
N
N
Br
R
5f
5
Entry
Substitution (R)
Yield (%)
25 mol%, at a moderate temperature with in just 1 h.
CH3
CH3
1
2
88
76
The % selectivity of the cavitand 1 was confirmed directly by
taking the HPLC data (Fig. 4) for the crude product of the
CH3
CH3
CH3
CH3
3
62
55 a Reaction conditions: equivalent amount (1 mmol) of substrates 2 and 3a
were stirred with calix[4]acetate (2 mol%) and AgNO3 (5 mol%) at 60 °C
for 1 h in 2 ml H2O.
2.6. Study of interaction of Ag+ with cavitand 1 by UV
60 Spectroscopy:
To demonstrate the interaction of Ag+ ion with calix[4]acetate
(1), change into UV absorption was studied (Fig. 5). First the
1.0
1 + Ag (1:2)
a
0.8
1 + Ag (1:1)
1
0.6
b
Fig. 4: HPLC data of the crude product of reaction of 2a and 3a.
0.4
30 reaction of compound 2a and 3a. In this HPLC spectrum the peak
1 and peak 3 appear for the homo products 5b and 5c
respectively, whereas peak 2 represents the cross product 4a. This
data clearly reveals that the cross product highly predominates
over the homo products in presence of the cavitand. Higher
0.2
0.0
250
300
350
Wavelength (nm)
4
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Fig. 5: UV-specta of calix[4]acetate (1) and the effect of silver ion
addition.
To determine the appropriate amount of catalyst needed, the
15 reaction was carried out occupying various amount of catalyst
(Fig. 6, a). A significant increase in yield had been seen up to 5
mol% of catalyst and that amount pointed the maximum yield.
Further increase was not observed with increasing the amount of
catalyst.
20 Next employing same strategy with the cavitand, we got the
critical amount (2 mol%) of the cavitand 1 to reach the maximum
yield (Fig. 6, b).
absorbance of water solution of calix[4]acetate was measured
followed by the effect in spectra after addition of the silver ion.
The absorbing intensity of peak ‘a’ increases with increasing
addition of silver ion, that implies, Ag+ interacts with the
compound 1 and favours the electronic transition responsible for
the absorption at peak ‘a’.
5
2.7. Catalyst and Cavitand loading:
2.8. Mechanistic study:
10
Involvement of the cavitand 1 as the host of the amine as well as
25 the silver ion, prompted us to outline a probable mechanistic
pathway of oxidation of aryl amines to azobenzenes (Scheme 2).
In this purpose the reaction of pꢀtoluidine (2a) and pꢀ
bromoaniline (3a) was taken as the model reaction. At the onset,
we focussed on the primary important part, the transformation of
30 the Nꢀcentre of the amine 2a from nucleophilic to the
electrophilic one. Our single molecular nanoreactor having a
beautifully organized structural arrangement, bring selectively the
amine 2a and the silver ion closer to form a probable intermediate
species B. The existence of this species was identified by HRMS
35 experiment. After addition of the catalytic amount (5 mol%)
100
90
80
70
60
50
40
30
100
90
80
70
60
50
40
30
20
10
(a)
(b)
2
3
4
5
6
7
8
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Catalyst loading (mol%)
Cavitand loading (mol%)
Fig. 6: survey of (a) yield vs catalyst loading, (b) yields vs cavitand
loading.
CH3
CH3
N
N
H
O
H
O
O
H
O
H
O
O
O
O
Ag
O
O
O
O
O
O
O
O
CH3
A
B
Ag
H
N
H
H
O
O
O
H
Ag
N
O
NPS
Ag
Br
O
O
Br
O
O
O
C
O
O
O
N
Ag
O
O
N
O
O
NPS
H3C
4a
CH3
CH3
N
N
H
O
O
H
O
O
N
N
H
O
Br
O
H
O
CH3
O
O
O
O
O
Br
D
Ag
O
O
O
O
Ag
F
N
N
H
O
O
H
O
O
Ag
Br
O2
NPS
Ag
O
O
O
O
E
Scheme 2: probable mechanism of synthesis of cross product using cavitand 1
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30 of the reaction for the synthesis of 4a. The work up of the catalyst
was carried out in a dark place to preserve the silver catalyst
either in form of Ag+ or Ag NPS in water. Pleasingly, after
filtration of the organic parts, the filtrate was reused as the source
of catalyst and it showed excellent catalytic activity up to four
AgNO3 and (2 mol%) cavitand 1, to the 2aꢀ3a mixture in water at
room temperature, we got a peak at m/e 1029.4367 in mass
spectra of the crude starting material mixture (Fig. 7) that
indicated the exclusive generation of 1:1:1 complex formation of
5
2a and the Ag+ ion with the cavitand 1 supporting the 35 cycles only by the expense of a little extra time (Table 5).
intermediate B. Now in this platform the silver ion may induce a
2.9.1 Cavitand:
positive charge on the Nꢀcentre of 2a representing a new
10 electrophilic centre. The acetate group of the calix[4]acetate may
also stabilize the amine group of the incoming nucleophile 3a
(intermediate C), which may attack the initially formed
electrophilic Nꢀcentre to produce the oxidised compound
hydrazobenzene (intermediate D) and remove the reduced form
15 of Ag+ as silver nanoperticle (Ag NPS). These Ag NPS further
oxidised to the Ag+ ion in presence of air. Again the exposure of
intermediate D to the Ag+ ion oxidised the hydrazobenzene in a
similar fashion to produce the desired oxidised product 4a.
The next important obligation is the recovery of the cavitand 1.
The reusability of this single molecular nanoreactor was
scrutinized for the formation of 4a in water by using the recycled
40 cavitand (Fig. 8). After the filtration, mentioned before, the
cavitand and the crude product was obtained in the residue part.
We got the pure compound 1 by column chromatography using
5% ethyl acetate in petroleum ether as eluent. This compound
became ready for further use just after ultraꢀsonication for five
45 minutes in 1 ml water. It was able to produce the product upto
five cycles without any significant decrease in the yield. The
1
stability of the compound 1 was examined using HꢀNMR, 13Cꢀ
NMR and TEM experiment.
100
80
60
40
20
0
1
2
3
4
5
6
No. of cycles
50
Fig.8: the reusability of the cavitand 1 for the synthesis of 4a
20
Fig.7: the mass peak for 1:1:1 p-toluidine and Ag+
2.10. Substrate scope:
calix[4]acetate complex
2.10.1 Syntheses of cross coupled products:
2.9. Recyclability:
2.9.1 Catalyst:
Now, a wide range of polar and nonpolar group substituted
55 aromatic amines were inspected to assess the scope of the
reaction (Scheme 3). First the influence of methyl substitution at
different positions was investigated using more polar group
attached aniline systems as the counter amine part (Scheme 1, 4aꢀ
4k), and we got excellent yields of the cross product. Probably
60 due to nonpolarꢀnonpolar hydrocarbon interaction the
calix[4]acetate binds more favourably the toluidines than the
counter parts. Unsubstituted aniline also showed the repetition of
yields (Scheme 1, 4lꢀ4n). A slight large substitution –OCH3 at the
phenyl ring reflected a slight decrease in yield (Scheme 3, 4oꢀ4q).
65 Reaction proceeded with relatively lower ease when ꢀCOOEt or ꢀ
COCH3 were employed as substitutions (Scheme 1, 4rꢀ4w), but
the yields did not leave the arena of excellency. Probably the
reason of minor lowering is their lower capability of compatible
25 Now, having scared the recyclability of catalyst, the bridging
point of the catalytic chemistry with the green chemistry, we tried
Table 5: Recyclability of the catalyst (silver) for the synthesis of 4a
No of cycles a
yield b (%)
Time (h)
1
1
1
2
3
4
5
6
90
89
88
86
83
80
1.1
1.3
1.5
2
a Reaction was carried out with recovered catalyst. b Isolated yield.
to reuse the catalyst several times after completion of one cycle
6
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Green Chemistry
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DOI: 10.1039/C5GC01859E
complex formation with the cavitand, taking size and polarity
factor of the substitution of phenyl ring in consideration. Also the
same problem (or competition) appeared when two different
anilines with same type, small sized electron donating
significantly not bad, we got (Scheme 1, 4wꢀ4y). So, in all cases
we got a good to excellent yields using an easy green
methodology i.e., using a recyclable catalyst, a reusable cavitand
along with mentionable green solvent, water.
5
substitution were used and
a
relatively lower yield, but
10
Library of compounds:
R2
NH2
NH2
AgNO3 (5 mol%)
calix[4]acetate (2 mol%)
N
N
air (1 atm), H2O, 60°C,
1h stirring
R1
R2
R1
4
3
2
NO2
Cl
Br
N
N
N
N
N
NO2
N
N
N
H3C
CH3
H3C
H3C
H3C
4c(89%)
4d(88%)
H3C
4b(87%)
4a(88%)
CO2Et
COCH3
COOEt
N
N
N
N
N
N
Cl
N
N
H3C
H3C
4f(85%)
4g(84%)
4e(86%)
4h(83%)
COOEt
Br
Cl
COOEt
N
N
N
N
N
N
N
N
4i(84%)
4k(83%)
4j(84%)
4l(89%)
N
CH3
CH3
CH3
Br
Cl
Br
Cl
N
N
N
N
N
N
N
H3CO
H3CO
4o(85%)
4m(83%)
NO2
4n(82%)
4p(85%)
Cl
Br
NO2
N
N
N
N
N
N
N
N
H3COC
H3CO
H3COC
H3COC
4q(86%)
4t(73%)
4s(75%)
4r(76%)
NO2
Br
OCH3
N
N
N
N
N
N
N
N
EtOOC
H3COC
H3C
EtOOC
H3CO
4v(73%)
4w(87%)
4u(71%)
4x(70%)
Br
Br
N
N
N
N
N
N
N
N
H3C
CH3
H3C
H3C
4y(68%)
4z(66%)
4a'(76%)
4b'(62%)
H3C
CH3
15 Scheme 3: Cross products synthesized. Reaction conditions: equivalent amount (1 mmol) of substrates 2 and 3 were stirred with
calix[4]acetate (2 mol%) and AgNO3 (5 mol%) at 60 °C for 1 h in 2 ml H2O.
2.10.2 Syntheses of homo coupled products:
surroundings of the hydrophobic cavity may provide stability
selectively to the nonpolar group than the polar one. Hence using
mixture of amines we got the hetero azoꢀcompound as the major
product. But for one single amine such compitating stability
30 factor is absent. So, the organic environment provided by the
cavitand, facilitated the formation of the homo azo-compound in
good yields.Then we got outstanding yields for the amines with
electron donating group (EDG) and slight lower but also
admirable yields for the amines with electron withdrawing group
The success in the heterodimerization prompted us to bring
20 greenness in the homodimerization reaction (Scheme 4). The
hydrophobic cavity of the calix[4]acetate is tailored to stabilize
nonpolar group substituted aniline more than the polar one. So,
when we use a mixture of two different amines having different
substitutions considering polarity, the amines get stability in a
25 competitive manner. Also not only the cavity but also the
This journal is © The Royal Society of Chemistry [year]
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Green Chemistry
Page 8 of 9
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DOI: 10.1039/C5GC01859E
(EWG).
R
AgNO3 (5 mol%)
NH2
calix[4]acetate (2mol%)
N
N
air (1 atm), H2O, 60°C,
1h stirring
R
R
5
CH3
CH3
N
H3C
N
N
N
N
CH3
Br
N
N
N
CH3
5b(94%)
H3C
5d(96%)
5c(94%)
5a(96%)
Cl
OCH3
Cl
N
N
N
Cl
N
N
N
N
N
5h(88%)
Cl
Br
5g(91%)
5f(90%)
H3CO
5e(93%)
CH3
COOEt
NO2
COCH3
CH3
N
N
N
N
N
N
N
N
O2N
EtOOC
H3COC
5l(83%)
5j(86%)
5k(88%)
H3C
O2N
5i(92%)
CH3
CH3
CH3
CH3
CH3
CH3
N
NO2
N
N
N
N
N
5m(85%)
H3C
H3C
5n(86%)
5o(84%)
H3C
CH3
CH3
Scheme 4: Homo products synthesized. Reaction conditions: aniline (2 mmols) was stirred with calix[4]acetate (2 mol%) and AgNO3 (5
mol%) at 60 °C for 1 h in 2 ml H2O
5
2.11 Identification of Ag nanoparticle :
spectroscopy. We got the characteristic peak for Ag NPS at 400
15 nm (Fig. 9, a) in UV spectrum. These particles were subjected to
TEM and 10ꢀ15 nm size range was obtained (Fig. 9, b). From the
EDS analysis the presence of the Ag particle is confirmed (Fig. 9,
c).
0.4
(a)
(b)
0.3
0.2
0.1
0.0
3. Conclusions
20 In summary, we report the cross selectivity of the N=N coupling
of aromatic amines with the aid of the easily synthesized
tetraacetate derivative of simple pꢀtertꢀbutylcalix[4]arene, using
catalytic amount of AgNO3. This is the first use of this calixarene
as a new member of nanoreactor family. The cavity size and
25 nonpolar environment tuned the selectivity of the cross product
formation. Also the reaction mechanism involved attachment of
the amine moiety to the hydrophobic cavity and a simultaneous
trapping of Ag+ ion at the acetate centres. Catalytic amount of
AgNO3 is sufficient for this oxidation reaction due to reformation
30 of Ag+ form the arial oxidation of the Ag nanoparticles.
Interestingly fishing out of Ag NPS was possible using N2
atmosphere and it was characterized by UV spectrum, TEM and
EDS analysis. So, this reaction is an example of insitu formation
and utilisation of Ag NPS. So, this is a green synthetic protocol
35 for the formation of a library of cross coupled aromatic azo
compounds with high throughput yield using just catalytic
amount of calix[4]acetate at moderate temperature in green
solvent water. Also the use of this simply synthesized calixarene
300
400
500
600
700
800
Wavelength (nm)
(c)
Fig. 9: (a) UV spectrum, (b) TEM image and (c) EDS analysis of
silver nanoparticle.
10
The presence of the Ag NPS as intermediate, the reaction was
carried out in N2 atmosphere. After 1 hour we got the Ag NPS in
the water solution which was confirmed from the UV
8
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DOI: 10.1039/C5GC01859E
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5
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Kolkata, 700009,
cmukhop@yahoo.co.in
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+91
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