A supramolecular ensemble of a PBI derivative and Cu2O NPs: Potential photocatalysts for the Suzuki and Suzuki type coupling reactions

Article abstract of DOI:10.1039/c5gc03012a

The aggregates of the PBI derivative 1 in combination with Benedict's reagent have been used for the first time as reactors for facile preparation of Cu₂O NPs at room temperature. During this process the aldehyde groups of the aggregates of derivative 1 are oxidized to carboxylate groups which act as stabilizers for Cu₂O NPs to generate the supramolecular ensemble 2:Cu₂O. Interestingly, the in situ generated supramolecular ensemble (2:Cu₂O) of Cu₂O NPs and aggregates of oxidized species 2 exhibited excellent photocatalytic efficiency in the Suzuki-Miyaura and Suzuki type cross-coupling reactions under mild and eco-friendly conditions.

Full text of DOI:10.1039/c5gc03012a

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Supramolecular ensemble of PBI derivative and Cu2O NPs:
Potential photo catalysts for Suzuki and Suzuki type coupling
reactions
Received 00th January 20xx,
Accepted 00th January 20xx
Gurpreet Singh, Manoj Kumar, Kamaldeep Sharma and Vandana Bhalla∗
DOI: 10.1039/x0xx00000x
Aggregates of PBI derivative 1 in combination with Benedict’s reagent have been used for the first time as reactors for
facile preparation of Cu2O NPs at room temperature. During this process aldehyde groups of aggregates of derivative 1 are
oxidized to carboxylate groups which act as stabilizers for Cu2O NPs to generate supramolecular ensemble 2:Cu2O.
Interestingly, the in situ generated supramolecular ensemble (2:Cu2O) of Cu2O NPs and aggregates of oxidized species 2
exhibited excellent photocatalytic efficiency in Suzuki-Miyaura and Suzuki type cross-coupling reactions under mild and
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eco-friendly
conditions.
Introduction
development of
a
supramolecular photocatalytic system
having dye stabilized semiconductor nanomaterial for carrying
out Suzuki and Suzuki type coupling reactions. In this type of
system, the semiconductor material and molecules of dye
serve as antenna and absorb the visible radiations to get
excited. Eventually, the excited species activate the substrate
molecules to facilitate the organic reactions.7 Cu2O is our
choice as semiconductor nanomaterial as it is economical,
abundant and stable p-type semiconductor which has been
used as an efficient catalyst for C-C, C-N and C-O bond
formation reactions.8 Perylene bisimide (PBI) is a scaffold of
our choice as dye stuff due to its high fluorescence quantum
yield, high photostability. In addition, PBI derivatives are also
reported for their role as light harvesting antenna.9 Recently,
Suzuki-Miyaura coupling between organohalides and
organoboronic esters/acids is one of the most powerful
synthetic methods for the preparation of synthetic building
blocks of various natural products and biologically important
compounds.1 Under conventional conditions, palladium based
catalytic systems due to their high stability and remarkable
efficiency are most preferred choice to carry out Suzuki
coupling reactions.2 Interestingly, pursuit of organic chemists
towards making the Suzuki reaction ‘green’ is a continuous
process and as a result of this a variety of new approaches
have been developed to carry out the Suzuki reaction under
eco-friendly conditions.3 In this direction, a variety of catalytic
systems have been developed which harvest clean, abundant
and economical solar energy to carry out Suzuki coupling.4
However, the use of costly and toxic palladium as major
component of these catalytic systems actually decreased the
environmental and economical advantage of the strategy.5
Very recently, we developed an in situ generated Ag@Cu2O
core-shell nanomaterials which harvested the visible light to
catalyze Suzuki-Miyaura and Suzuki type coupling reactions
under mild conditions.6 The coating of plasmonic silver
nanoparticles (AgNPs) with semiconductor Cu2O NPs served
dual purpose, first, the photocatalytic activity of plasmonic
system was enhanced. Second, by coating expensive metal NPs
with inexpensive material, we were successful in reducing the
cost of the photocatalytic system. In continuation of our
efforts in this direction, we were then interested in the
we developed aggregates of PBI derivative
aldehyde groups and utilized these aggregates for preparation
of AgNPs.10 Derivative
was found to be very stable and did
1 (Fig. 2) having
1
not exhibit any decomposition in solid as well as solution
phase even upon month long exposure to room light and air.
Furthermore, the absorption spectrum of derivative
H2O/THF (1:1) covers the whole visible region. Thus, we
decided to use aggregates of derivative having aldehyde
1 in
1
groups at the periphery as dye molecule in our designed
photocatalytic system. Further, we envisioned that aldehyde
groups of derivative
1 in combination with Benedict’s reagent
could reduce cupric ion to copper NPs which under aerial
conditions will be oxidised to cuprous oxide NPs. We envisaged
that during this process aldehyde groups of derivative
oxidized to carboxylate groups which will stabilize the in situ
generated Cu2O NPs to generate supramolecular
photocatalytic ensemble. As expected, aggregates of derivative
in the presence of Benedict’s reagent served as reactors for
the generation of Cu2O NPs. Interestingly, the in situ generated
supramolecular ensemble :Cu2O) of Cu2O NPs and
aggregates of oxidized species of derivative served as
1 will be
a
a.Department of Chemistry, UGC Sponsored Centre for Advanced Studies-1, Guru
Nanak Dev University, Amritsar 143005, Punjab, India
E-mail: vanmanan@yahoo.co.in † Footnotes relaꢀng to the ꢀtle and/or authors
should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
1
(
2
1
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8h
efficient photocatalytic system for carrying out Suzuki-Miyaura
cross coupling and Suzuki type coupling reactions under mild
(visible light radiation, EtOH/H2O solvent system and room
temperature) and ecofriendly conditions. The work being
presented here has several advantages; 1) for the first time
6h
5h
4h
3h
2h
1h
aggregates of PBI derivative
1 in combination with Benedict’s
Benedict’s solution+ 1
Benedict’s solution
reagent have been used for facile preparation of Cu2O NPs at
room temperature. Various methods using wet chemistry
techniques have been used for the development of Cu2O NPs.
However, these methods suffer from the limitations of
requiring longer reaction time, high temperature, reducing
agents and additives.11 In comparison, the method being
reported in the present manuscript is a convenient method for
the preparation of stable Cu2O NPs under mild conditions
(Table S1, ESI†). 2) for the first time supramolecular ensemble
of semiconductor Cu2O NPs and PBI derivative have been used
as efficient and recyclable photocatalysts for Suzuki-Miyaura
cross-coupling reactions of aryl halides with aryl boronic acid
esters in environmentally benign solvent mixture under aerial
conditions at room temperature. In comparison, the
photocatalytic performance of supramolecular photocatalytic
Wavelength (nm)
Fig. 2: UV-vis spectra of aqueous solution of supramolecular ensemble 2:Cu2O,
Benedict’s solution in the presence and absence of derivative 1; UV-vis band at 690
nm correspond to supramolecular ensemble 2:Cu2O.
The fluorescence spectrum of derivative
1 in H2O/THF (1:1)
shows an emission band at 590 nm. Upon addition of
Benedict’s reagent (350 µL, 0.04 M), 76% quenching of the
emission band is observed (Fig. S2, ESI†). High-resolution TEM
(HR-TEM) images of aqueous Benedict’s solution in the
presence of aggregates of derivative 1 showed the presence of
spherical NPs (Fig. 3). The particle size distribution of these
NPs is shown in Fig. 3C. The lattice fringes showed d-spacing of
247.89 pm corresponding to the (111) planes of Cu2O. The
average size of Cu2O NPs was found to be 8-10 nm. The
powder XRD diffraction pattern of these NPs showed the
reflection peaks of Cu2O NPs (Fig. S3, ESI†).13 We also
ensemble
systems for Suzuki-Miyaura cross-coupling reaction (Table S2,
ESI†). 3) the supramolecular ensemble :Cu2O also exhibited
2:Cu2O is better than the reported photocatalytic
examined the effect of amount of aggregates of derivative
on shape and size of generated NPs. When ratio of aggregates
of derivative to Benedict’s reagent was changed from 1:1 to
1
2
high catalytic efficiency in Suzuki-type cross-coupling reactions
of N-tosylaziridine with different types of organoborons under
aerial conditions.
1
2:1, spherical Cu2O NPs of almost same size (8-10 nm) were
observed while on switching this ratio to 1:2, an increase in the
size of Cu2O NPs (10-15 nm) was observed (Fig. S4, ESI†).
Results and Discussion
(C)
(B)
(A)
(B)
Perylene bisimide derivative
method.10 It formed aggregates in mixed aqueous media and
TEM image of the derivative (1 mM) in H2O/THF (1:1)
1 was synthesized by reported
1
solution shows the presence of irregular shaped aggregates
(Fig. S1, ESI†). The addition of incremental amounts of
derivative
1 (1 mM) dissolved in H2O/THF (1:1) solvent mixture
Size (nm)
into the solution of Benedict’s reagent (0.04 M) generated the
Cu2O NPs. The whole process was accompanied by apparent
color change from blue to yellowish green (Fig. 1).
Fig. 3: (A) TEM image of spherical supramolecular ensemble 2:Cu2O; (B) HRTEM
image of generated supramolecular ensemble 2:Cu2O Scale bar (A) 100 nm and (B) 5
nm (C) Particle size distribution of TEM image (A).
We believe that in the presence of high amount of Benedict’s
reagent, the number of reduced copper ions is increased, thus,
Cu2O nanoparticles of slightly bigger size were obtained. Thus,
by changing the amount of aggregates of derivative 1, size of
Cu2O NPs could be controlled; however, shape of generated
NPs remained same. We then slowly evaporated the solution
Aggregates of
derivative 1
After 8 h
of aggregates of derivative
1 containing Cu2O NPs. The
Benedict, s
reagent
Benedict, s reagent +
derivative 1
Supramolecular
ensemble 2:Cu2O
precipitates were observed after 2 days which were filtered
1
and washed with CHCl3 and THF. The H NMR spectrum of the
Fig. 1: Schematic diagram for the formation of supramolecular ensemble 2:Cu2O
from Benedict‘s solution on addition of derivative 1.
residue so obtained after evaporation of the solvent showed
upfield shift of the aromatic protons and no peak
corresponding to aldehyde protons. (Fig. S5, ESI†). The FTIR
studies of the residue showed the absence of the band
corresponding to aldehyde groups and presence of two new
bands at 1600 and 1400 cm-1 corresponding to carboxylate
groups.14 Further, the absorption band corresponding to Cu(I)-
The UV-vis absorption spectra of derivative
1 in H2O/THF (1:1)
in presence of aqueous solution of Benedict’s reagent
exhibited a broad absorption band at 690 nm corresponding to
Cu2O NPs after 8 h.12 Furthermore, from UV-vis studies, it is
clear that in absence of derivative
NPs is formed which highlights the importance of aggregates
of derivative in generation of supramolecular ensemble
:Cu2O (Fig. 2).
1, no band corresponding to
1
O vibration appeared at 625 cm-1 (Fig. S6, ESI†). The H NMR
1
2
2 | J. Name., 2012, 00, 1-3
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and IR studies suggest the oxidation of aldehyde groups to the yield (Table 1, entry 3). The above studies show the catalytic
carboxylate groups during the formation of Cu2O NPs.
efficiency of supramolecular ensemble 2:Cu2O in the Suzuki
coupling reaction. Further, we planned to carry out the model
reaction under photocatalytic condition. For this, we carried
out the reaction of bromobenzene and phenyl boronic acid in
the presence of supramolecular ensemble
2:Cu2O in a
H2O/EtOH (1:1) solvent mixture under visible light irradiation.
A 100 W tungsten filament bulb was used as the irradiation
source and to avoid the photo-heating effect the reaction vial
was submerged in a water bath. The reaction was complete in
8 h and the desired product was obtained in 73 % yield
(Table1, entry 4). Interestingly, when same reaction was
carried out in THF, the desired product was obtained in 40%
yield (Table 1, entry 5). We believe that electron transfer is
Fig. 4: Schematic diagram showed the formation of supramolecular ensemble 2:Cu2O.
The fluorescence spectrum of oxidized species of derivative
1
(10 µM) in H2O/THF (1:1) solvent mixture exhibited an suppressed in aprotic solvent which decreases the yield of the
emission band at 600 nm. Upon addition of aqueous desired product.18 The same reaction when carried out in the
dispersion of Cu2O NPs (300 µL, 0.04 M) to this solution, 83% absence of K2CO3 furnished no product (Table 1, entry 6),
quenching of the emission was observed due to the energy hence, basic medium is a prerequisite for carrying out Suzuki
transfer from oxidized species of PBI derivative
(Fig. S7 in ESI†). We also carried out time resolved conditions; however, the reaction did not proceed to furnish
fluorescence studies of oxidized species of derivative in the the desired product (Table 1, entry 7).
1 to Cu2O NPs reactions. We also carried out the model reactions under dark
1
absence and presence of aqueous dispersion of Cu2O NPs. The
solution of oxidized species in H2O/THF (1:1) showed average
decay time of 7.12 ns, however, in presence of Cu2O NPs (300
µL, 0.04 M) the average decay time decreased to 2.87 ns (Fig.
Table 1: Screening of the photocatalytic Suzuki coupling reaction of bromobenzene
and phenyl boronic acid using supramolecular ensemble 2:Cu2O as the
photocatalyst.
Supramolecular ensemble 2:Cu2
(0.02 mmol)
O
OH
OH
Br
B
H2O/EtOH (1;1) K2CO3 (1.5 eq)
S8, ESI†). Furthermore,
a strong overlap was observed
5
4
3
between emission spectrum of oxidized species of derivative
1
Entry
Catalyst
Reaction
conditions
THF, heating
(80–90 OC), N2
atm
Base
Time
14 h
Yield
and absorption spectrum of Cu2O NPs (Fig. S9 in ESI†). All the
above studies support the energy transfer from oxidized
1
Supramolecular
ensemble
2:Cu2ONPs
K2CO3
75%
species of PBI derivative
1
to Cu2O NPs.9,15 On the basis of
in
these studies, we believe that aggregates of derivative
1
2
3
4
5
6
7
Supramolecular
ensemble
2:Cu2ONPs
H2O/EtOH,
heating
K2CO3
K2CO3
K2CO3
K2CO3
-
14 h
14 h
8 h
8 h
77%
68%
73%
40%
-
combination with Benedict’s reagent reduce the copper ions to
Cu2O NPs and during this process the aldehyde groups of
(80-90 0C), N2 atm
H2O/EtOH,
heating
Supramolecular
ensemble
2:Cu2ONPs
derivative
1 get oxidized to carboxyl groups. Further,
aggregates of oxidized species of derivative
stabilizers to generate supramolecular ensemble
4).16
1
2
acted as
(80–90 0C), Air
H2O/EtOH, visible
light
:Cu2O (Fig.
Supramolecular
ensemble
2:Cu2ONPs
To understand the role of oxidized species of derivative
1 in
Supramolecular
ensemble
2:Cu2ONPs
THF, visible light
the preparation of Cu2O NPs. We carried out UV-vis studies of
Benedict’s solution in the presence of oxidized species of
Supramolecular
ensemble
2:Cu2ONPs
H2O/EtOH,visible
light
8 h
derivative 1. Upon addition of oxidized species to Benedict’s
solution, no band corresponding to Cu2O NPs was observed
(Fig. S10 in ESI†). On the basis of these studies, we believe that
oxidized species play no role in generation of Cu2O NPs.
Supramolecular
ensemble
H2O/EtOH, dark
K2CO3
8 h
-
2:Cu2ONPs
Since copper containing compounds are known as co-
catalyst in C-C bond formation reactions,17 we planned to
investigate the catalytic efficiency of in situ generated
8
Aggregates
of derivative 1
Oxidized species 2
H2O/EtOH,visible
light
K2CO3
K2CO3
K2CO3
K2CO3
8 h
8 h
-
9
H2O/EtOH,visible
light
-
supramolecular ensemble
2:Cu2O in the Suzuki-Miyaura
10
11
Bare Cu2ONPs
(Ref. 12a)
H2O/EtOH,visible
light
12 h
8 h
35%
36%
coupling reactions. We began with Suzuki-Miyaura cross
coupling reaction between bromobenzene and phenyl boronic
acid as the model reaction under thermal conditions. The
reaction went smoothly in the presence of supramolecular
Bare Cu2ONPs (Ref.
12a) + aggregates
of derivative 1
Bare Cu2ONPs (Ref.
12a) + oxidized
species 2
H2O/EtOH,visible
light
12
H2O/EtOH,visible
light
K2CO3
8 h
70%
ensemble 2:Cu2O (0.02 mmol) in THF as well as in H2O/EtOH
(1:1) solvent mixture (Table 1, entries 1 and 2). The reaction
between bromobenzene and phenyl boronic acid in presence
of supramolecular ensemble 2:Cu2O under aerial conditions Furthermore, we carried out the model reaction in presence of
also moved smoothly to furnish the desired product in 68% aggregates of derivative
1 and oxidized species of derivative 1
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without using Cu2O NPs (Table 1, entries 8 and 9). The oxidized to furnish 4-aryl-1-bromobenzene (Table 3). This study shows
species of derivative (compound ) was prepared by the the high selectivity of in situ generated supramolecular
oxidation reaction of derivative :Cu2O in Suzuki coupling.
with 2 equiv of 70% tBuOOH ensemble
1
2
1
2
in acetonitrile (pS5, ESI†). As expected none of these reactions
furnished the desired product. To understand the role of
Table 3: supramolecular ensemble 2:Cu2O catalyzed photocatalytic Suzuki coupling
reactions between various aryl halides and phenyl boronic acid.
aggregates of derivative 1/oxidized species of derivative 1 in
the photocatalytic efficiency of NPs, we prepared bare Cu2O
NPs by reported method12a and carried out model reaction
using these NPs. The reaction was complete in 12 h and the
desired product was obtained in 35% yield (Table 1, entry 10).
In the next part of our work, we carried out the model reaction
in the presence of bare Cu2O NPs and aggregates of derivative
Entry
Reactant
Product
Time
Yield (%)
1
8a;X=Cl,R=H
5 = 9a;R1=H
24 h
48%
2
3
4
8b;X=Br,R=H
8c;X=I,R=H
8d;X=I,R=Br
5 = 9b;R1=H
5 = 9c;R1=H
9d;R1=Br
8 h
4.3 h
5 h
73%
81%
80%
1
and the desired product was obtained in 36% yield (Table 1,
entry 11). We also carried out model reaction in presence of
bare Cu2O NPs and oxidized species of derivative (residue
1
obtained after evaporating the solution of in situ generated We further checked the scope of the coupling by carrying out
NPs). The reaction was complete in 8 h and the desired the reaction of bromobenzene with different types of boronic
product was obtained in 70% yield (Table 1, entry 12). We acids having electron rich and withdrawing groups as
believe that aggregates of oxidized species of derivative
1
substituents using supramolecular ensemble 2:Cu2O as
assisted the Cu2O NPs in harvesting visible radiations more photocatalyst and results are given in Table 4. It is clear from
efficiently. On the basis of above studies, we conclude that these results that higher yields are obtained in the case of
aggregates of oxidized species of derivative
photocatalytic efficiency of the system and are important for entries 1 and 2). Further, photocatalytic Suzuki coupling of
stabilization of generated NPs. phenyl boronic ester with bromobenzene furnished desired
To check the scope of substrates, we utilized the in situ product in 72% yield indicating no effect of boronic acid or
generated supramolecular ensemble :Cu2O as a photocatalyst ester groups on this reaction.
1 influence the boronic acids/esters having electron rich substituents (Table 4,
2
in coupling of different types of aryl halides (having electron
donating and withdrawing groups) with phenyl boronic acid to
afford the desired products in moderate to excellent yields
(Table 2). Further, the present set of reaction conditions are
tolerant to diverse functional groups such as aldehyde,
hydroxyl and nitrile (Table 2).
Table 4: supramolecular ensemble 2:Cu2O catalyzed photocatalytic Suzuki coupling
reactions between bromobenzene and various boronic acids/esters.
Entry
Bornic
ester/acid used
Substrate
Product
Yield
75%
1
Table 2: Supramolecular ensemble 2:Cu2O catalyzed photocatalytic Suzuki type
coupling reactions between various bromo derivatives and phenyl boronic acid
2
77%
Entr
y
Aryl bromo
derivatives
Product
Yield
(%)
76
3
4
72%
1
6a
7a
7b
7c
73 %
2
65
CN
6b
3
4
5
6
67
79
70
78
5
65 %
6c
7c
7d
6d
Reaction conditions: (a) Phenyl boronic acid (1 equiv.); (b) Supramolecular
ensemble 2:Cu2O NPs (0.02mmol); (c) K2CO3 (1.5 equiv.); RT.
All the products were isolated and characterized by 1H NMR
spectroscopy (PS14-PS22, ESI†). We carried out the Suzuki
coupling of bromobenzene with phenyl boronic acid in a
H2O/EtOH (1:1) solvent mixture to examine the catalytic
efficiency of catalyst. When the quantity of supramolecular
6e
6f
7e
7f
Reaction conditions: (a) Phenyl boronic acid (1 equiv.); (b) Supramolecular
ensemble 2:Cu2O NPs (0.02mmol); (c) K2CO3 (1.5 equiv.); RT.
To check the scope of substrates, we carried out ensemble
photocatalytic Suzuki coupling of aryl halides with phenyl However, the yield of desired product remains quantitative (68
boronic acid in the presence of in-situ generated %) upon decrease of supramolecular ensemble :Cu2O loading
supramolecular ensemble :Cu2O which afforded desired from 0.02 to 0.005 mmol (Table S4, ESI†) and no product was
products in moderate to excellent yields (Table 3). In the obtained when same reaction is carried out without catalyst.
presence of in situ generated supramolecular ensemble :Cu2O The recyclability of catalyst is a key factor from environment
NPs, 1-bromo-4-iodobenzene reacted with phenyl boronic acid and synthetic point of view. The reaction between
2:Cu2O is 0.02 mmol the yield is 75% in 8 h.
2
2
2
4 | J. Name., 2012, 00, 1-3
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bromobenzene and phenyl bornic acid was chosen as the out reaction between bromobenzene and phenyl boronic acid
model reaction to study the recyclability of the catalyst.
in the presence of a hole scavenger (10%, triethanolamine
(TEOA) and the desired product was obtained in 34% yield. We
also carried out the same reaction in the presence of
benzoquinone as a radical scavenger and the desired product
was obtained in 35% yield. However, when both the
scavengers were used together the target compound was
obtained in 12% yield. These studies suggest that
photogenerated hole and electron pair are essential for
carrying out the Suzuki coupling reactions. However, when the
photo-generated electron-hole pair is blocked, the
100
80
60
40
20
0
1
2
3
4
supramolecular ensemble
2:Cu2O electron pair still remain
No. of cycles
active to catalyse the coupling reaction with one activated aryl
species but in lower yield. We believe that electron-hole pair is
Fig. 5: Recyclability of supramolecular ensemble 2:Cu2O catalyst for Suzuki coupling
reaction.
After complete conversion, the solvent was evaporated and generated within the supramolecular ensemble
product was extracted using dichloromethane. The residue light irradiation. The photoexcited electrons activate the C-X
containing supramolecular ensemble :Cu2O catalyst was bond of aryl halide and breaking of the C-X bond is the rate
2:Cu2O upon
2
subjected to the next run of the Suzuki reaction by charging determining step. Further, our previous investigation suggests
with the same substrate molecules. The supramolecular that the active substrates such as iodobenzene,
ensemble 2:Cu2O could be recycled as a catalyst at least 4 bromobenzene and aryl halides having electron rich
times with a decrease in the activity (Fig. 5). We believe that substituents furnished the desired products in higher yields.
due to the continuous photoexcitation of catalyst, Cu2O NPs These findings also support our corresponding results of
get deactivated due to photocorrosion and thus the catalytic cleavage of C-X bond in the rate determining step. Taking
efficiency of supramolecular ensemble decreased.19 After the account of all these observations, we propose that the holes
catalytic reactions, the TEM images of recycled supramolecular inside the phocatalysts activates the C-B bond of
ensemble
2:Cu2O revealed no change in its size and phenylboronic acid in basic medium to transform the boronic
morphology (Fig. S11, ESI†). We also carried out atomic acid into electronically negative B(OH)3 species. Afterwards,
absorption studies (AAS) of the residual liquid left after the reductive elimination and transmetalation are followed which
recycling of the catalyst and found that only 0.012 ppm of Cu lead to the formation of the desired product. A plausible
had leached into the solution (pS13 in ESI†).
To evaluate the versatility of in situ generated photocatalytic Suzuki cross-coupling reaction (Fig. 7).
supramolecular ensemble :Cu2O, we planned to evaluate its
catalytic efficiency in the reaction involving preparation of
perylenedimide (PBI) derivatives and 12 (Fig. 6). Under
photocatalytic conditions, derivatives and 12 were obtained
in 47% and 48% yields, respectively (PS23-PS25, ESI†). Earlier,
we have reported synthesis of derivative
and 1220 under
mechanism is proposed for the Cu2O NPs catalyzed
2
1
1
CB
VB
1
palladium catalyzed thermal conditions in 50% and 78%,
respectively. We also utilized the in situ generated
supramolecular ensemble
2:Cu2O as photocatalyst for
preparation of electron rich hexaphenylbenzene derivative 13
and the desired product was obtained in 44% yield (Fig. 6).
Earlier, the synthesis of HPB is reported in 55% yield by
carrying out Suzuki reaction in THF solvent in the present of
Fig. 7: Proposed photocatalytic mechanism of Suzuki cross-coupling reaction.
Pd2+ catalyst.21 These studies highlight the practical Having done all this, we planned to examine the photocatalytic
applicability of supramolecular ensemble
coupling reactions.
2
:Cu2O in Suzuki efficiency of in situ generated supramolecular ensemble
2
:Cu2O in the ring opening reactions of N-tosylaziridine to
C12H25
C12H25
obtain phenylethylamine. Derivatives having phenylethylamine
motifs are important due to their extensive use in synthesis of
drug targets for the treatment of various diseases.22
Traditionally, synthesis of these compounds via ring opening
reaction of aziridines requires strong organometallic
nucleophiles.23 In the present investigation, we carried out the
reactions between N-tosylaziridine and different type of
boronic acids/esters in the presence of in situ generated
O
N O
NH2
O
N O
OHC
H2N
CHO
NH2
O
N
O
NH2
O
N
O
C12H25
C12H25
13 (44%)
1 (47%)
12 (48%)
Fig. 6: Practical applicability of supramolecular ensemble 2:Cu2O in the synthesis of
PBI and HBC derivatives by Suzuki coupling reactions.
To get insight into the mechanism of supramolecular ensemble supramolecular ensemble
2
:Cu2O by using K2CO3 as the base in
2:Cu2O photocatalyzed Suzuki coupling reaction, we carried H2O/EtOH (1:1) under aerial conditions. All the reactions went
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smoothly and desired products were obtained in good to pentahydrate dissolved in 10 mL of water was added to the above
excellent yields. The results are summarized in Table 5 which solution and further diluted to make the total volume 100 mL.
clearly show that electron rich boronic acids/esters yielded the Then, 0.04 M of this solution was used for the preparation of
desired products in higher yields (PS26-PS29, ESI†).
supramolecular ensemble 2:Cu2O.
Preparation of supramolecular ensemble 2:Cu2O
Table 5: Supramolecular ensemble 2:Cu2ONPs catalyzed photocatalytic Suzuki type
To a 2 mL solution of aggregates of derivative
1 (1 mM) was
Coupling reactions between different boronic acid/ester and tosyl aziridine.
Entry
Bornic ester/acid
used
Product
Time
Yield
added 1 mL Benedict’s solution (0.04 M). The colour of
solution changed from blue to purple immediately. After
continuous stirring for 8 h, the colour of the mixture further
changed from purple to yellowish green indicating the
1
6 h
70%
2
3
8h
4h
60%
83%
formation of supramolecular ensemble
2:Cu2O of oxidized
species of derivative 1 (2) and Cu2O NPs. The resulting solution
was sonicated to get homogeneous mixture and was used as
such in the photocatalytic experiments.
General experimental procedure for photocatalytic Suzuki-
Miyaura coupling reaction:
4
5
5 h
5 h
79%
77%
Phenyl boronic acid (0.318 mmol, 0.038 g), K2CO3 (0.477
mmol, 0.066 g) was added to the stirred solution of
supramolecular ensemble 2:Cu2O (0.020 mmol) in H2O/EtOH
Reaction conditions: a) Aziridine (1 equiv.); b) Boronic acid/ester (1 equiv.); c)
(1:1) mixture. Aryl halide (0.318 mmol, 0.05 g) was then added
to the reaction mixture and degassed with vacuum pump for 3
min. The reaction mixture was then irradiated with a 100 W
tungsten filament bulb (0.4 W/cm2) to provide visible light for
8 h. To prevent the heating effect the reaction vial was put in
water bath to maintain the room temperature. After
completion of the reaction, solvent was removed under
reduced pressure and the residue so obtained was extracted
with dichloromethane (3×20 mL). The combined organic layer
was then washed with water, dried over anhydrous Na2SO4
and distilled under reduced pressure to obtain a crude product
which was recrystallized from dichloromethane and hexane
solvent mixture to get pure product.
Supramolecular ensemble 2:Cu2O NPs (0.02 mmol), RT.
Conclusion
In conclusion, aggregates of derivative
Benedict’s reagent have been used for in situ generation of
supramolecular ensemble :Cu2O NPs. The supramolecular
ensemble :Cu2O NPs served as efficient photocatalysts in
1 in combination with
2
2
Suzuki-Miyaura and Suzuki type cross coupling reactions under
mild and environmental friendly conditions. In contrast to the
conventional systems, this catalytic system is economically,
easy to prepare, recyclable and ecofriendly.
Experimental Section
General Experimental Procedures
General experimental procedure for photocatalytic Suzuki
type cross-coupling reaction:
Instrumentations
To a 50 mL round bottom Flask (RBF) were added N-tosyl-
aziridine (0.253 mmol, 0.05 g), phenylboronic acid (0.253
mmol, 0.03 g), K2CO3 (0.380 mmol, 0.053 g) and 0.02 mmol of
UV-vis spectra of synthesized compounds were performed
in THF and H2O/THF mixture by using SHIMADZU UV-2450
spectrophotometer having a quartz cuvette (path length, 1
cm). High Resolution Transmission Electron Microscope (HR-
TEM) images were recorded on a TEM-JEOL 2100F. FTIR
spectrum was recorded on Varian 660-IR spectrometer.
Photocatalytic experiments were carried out by using the 100
supramolecular ensemble 2:Cu2O in H2O/EtOH (1:1) mixture.
The resulting mixture was degassed under reduced pressure
for 3 min. and then irradiated from a distance of 10 cm with a
100 W tungsten filament bulb (0.4 W/cm2) to provide visible
light for 5 h. To prevent the heating effect the reaction vial was
put in water bath to maintain the room temperature. After
completion of the reaction, solvent was evaporated under
vacuum pump and residue so obtained was extracted with
1
W tungsten filament bulb as an irradiation source. The H and
13C NMR spectra were recorded on a JEOL-FT NMR-AL 300
MHz and Bruker-FT NMR-AL 500 MHz spectrophotometers
using CDCl3 and DMSO-d6 as solvent and tetramethylsilane
(SiMe4) as internal standards. Data are reported as follows:
chemical shifts in ppm (δ), multiplicity (s = singlet, d = doublet,
dichloromethane (3×20 mL). The combined organic layer was
then washed with water, dried over anhydrous Na2SO4 and
distilled under reduced pressure to obtain a crude product
which was recrystallized from dichloromethane and hexane
solvent mixture to get pure product.
br
= broad, m = multiplet), coupling constants J (Hz),
integration and interpretation.
Characterization
Preparation of stock solution of Benedict’s reagent
(5)24 1H NMR (300 MHz, CDCl3): δ = 7.6 [d, 4H, J = 6.9 Hz,
ArH], 7.44 [t, 4H, J = 7.3 Hz, ArH], 7.36 [d, 2H, J = 7.5 Hz, ArH].
1 M stock solution of Benedict’s reagent was prepared by
dissolving 10 g of sodium carbonate and 17.3 g of sodium citrate
dihydrate in 85 mL of distilled water. Then, 1.73 g copper sulfate
6 | J. Name., 2012, 00, 1-3
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1
(7a)25: H NMR (300 MHz, CDCl3) δ 7.77-7.70 (m, 2H), 7.60 (br, 4H), 1.28 (br, 36H), 0.87 (t, J = 6 Hz, 6H). TOF MS ES+:
(d, J = 8.1 Hz, 2H), 7.50-7.46 (m, 2H), 7.40-7.43 (m, 1H), 7.32- [M+H]+ 967.0084.
7.39 (m, 2H), 2.02 (s, 3H).
(7b )26: 1H NMR (300 MHz, CDCl3) δ 7.75 (d, J = 8.4 Hz, 2H),
Acknowledgements
7.68 (d, J = 8.4 Hz, 2H), 7.60 (d, J = 7.6 Hz, 2H), 7.44 (t, J = 7.3
V. B. is thankful to SERB, New Delhi (ref. no.
EMR/2014/000149) for financial support. We are also thankful
to UGC for ‘‘University with Potential for Excellence’’ (UPE). G.
S. (JRF) is thankful to CSIR for fellowship.
Hz, 2H), 7.31-7.37 (m, 1H).
1
(7c)26: H NMR (300 MHz, CDCl3) δ 9.95 (s, 1H), 7.81 (d, J =
8.1 Hz, 2H), 7.56 (d, J = 8.1 Hz, 2H), 7.45 (d, J = 7.5 Hz, 2H),
7.22-7.30 (m, 3H).
Notes and references
(7d)26: 1H NMR (300 MHz, CDCl3) δ 7.54 (t, J = 8.85 Hz, 4H),
7.44 (t, J = 7.8 Hz, 2H), 7.34 (t, J = 7.35 Hz, 1H), 6.91 (d, J = 9.0
Hz, 2H), 3.88 (s, 3H).
1
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(7e)26: H NMR (300 MHz, CDCl3) δ 7.59 (d, J = 8.1 Hz, 2H),
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(7f)26: H NMR (300 MHz, CDCl3) δ 7.50 (d, J = 7.5Hz, 2H),
7.31 (t, J = 7.5 Hz, 4H), 7.06 (t, J = 7.2 Hz, 1H), 6.53 (d, J = 8.1
Hz, 2H), 4.67 (s, 2H).
(9d)25: 1H NMR (500 MHz, CDCl3): δ = 7.35 [d, 2H, J = 8.5 Hz,
ArH], 7.34 [d, 2H, J = 8.5 Hz, ArH], 7.11 [t, 2H, J = 9.3 ArH],
7.00-7.04 [m, 3H, ArH]
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(11c)26: 1H NMR (500 MHz, CDCl3): δ 7.56 (d, J = 8.5 Hz,
2H), 7.43 (d, J = 7.5 Hz, 1H), 7.41 (d, J = 7.5 Hz, 1H), 7.31 (t, J =
7.5 Hz, 1H), 7.15 (dd, J = 7.5, 1.5 Hz, 1H), 7.07 (d, J = 8.5 Hz,
1H), 6.95 (d, J = 8.5 Hz, 1H), 3.95 (s, 3H), 3.92 (s, 3H).
(1)11: 1H NMR (300 MHz, CDCl3, ppm). δ = 10.15 (s, 2H),
8.61 (s, 2H), 8.15 (d, J = 8.1 Hz, 2H), 8.05 (d, J = 7.2 Hz, 4H),
7.74 (d, J = 7.8 Hz, 4H), 7.64 (d, J = 7.8 Hz, 2H, Ar-H), 4.18 (t, J =
7.05 Hz, 4H), 1.72 (br, 4H), 1.25 (br, 36H), 0.87 (t, J = 5.7 Hz,
6H).
(12)20: 1H NMR (300 MHz, CDCl3) δ = 8.61 (s, 2H, ArH), 8.10
(d, J = 5.1 Hz, 2H, ArH), 7.99 (d, J = 12 Hz, 2H, ArH), 7.38 (d, J =
9.3Hz, 4H, ArH), 6.83 (d, J = 8.7 Hz, 4H, ArH), 4.18 (t, J = 6.75
Hz, 4 H) 4.00 (s, 4H,), 1.72 (br, 4H), 1.25 (br, 36H), 0.87 (t, J =
5.7 Hz, 6H).
(13)21: 1H NMR (500 MHz, CDCl3, ppm) δ = 7.25-7.27 (m,
4H); 7.21-7.24 (m, 4H), 7.05 (d, J = 8.0 Hz, 4H), 6.81-6.85 (m,
16H), 6.73 (d, J = 8.0 Hz, 4H), 6.66 (d, J = 8.0 Hz, 4H), 3.65 (br,
4H).
(15a)7: 1H NMR (300 MHz, CDCl3) δ 7.74 (d, J = 8.1 Hz, 2H),
7.36 (s, 1H), 7.30 (d, J = 7.2 Hz, 2H), 7.12 (d, J = 8.1 Hz, 1H),
6.96 (d, J = 8.1 Hz, 1H), 5.00 (br s, 1H), 3.59 (t, J = 4.95 Hz, 2H),
3.04 (t, J = 4.95 Hz, 2H), 2.42 (s, 3H).
7
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Table of Contents
CB
VB
Supramolecular
ensemble 2:Cu2O
Synopsis
Aggregates of PBI derivative 1 in combination with Benedict’s reagent have been as reactors for
facile preparation of supramolecular ensemble 2:Cu2O at room temperature. Interestingly, the in
situ generated supramolecular ensemble (2:Cu2O) of Cu2O NPs and aggregates of oxidized
species 2 exhibited excellent photocatalytic efficiency in Suzuki-Miyaura and Suzuki type cross-
coupling reactions under mild and eco-friendly conditions.