Polythiophene-Encapsulated Bimetallic Au-Fe3O4 Nano-Hybrid Materials: A Potential Tandem Photocatalytic System for Nondirected C(sp2)-H Activation for the Synthesis of Quinoline Carboxylates

Article abstract of DOI:10.1021/acscatal.6b02681

Hetero-oligophenylene derivative 3 appended with thiophene moieties has been designed and synthesized which undergoes aggregation to form J-type fluorescent aggregates in in H₂O/THF (7/3) media. These aggregates served as reactors for the preparation of bimetallic Au-Fe₃O₄ NPs. During the reduction process, aggregates of derivative 3 were oxidized to the polythiophene species 4. Interestingly, the polythiophene species 4, having a fibrous morphology, served as a shape- and morphology-directed template for assembly of bimetallic Au-Fe₃O₄ NPs in a flower-like arrangement. Furthermore, polythiophene-encapsulated bimetallic 4:Au-Fe₃O₄ nanohybrid materials served as an efficient and recyclable catalytic system for C(sp²)-H bond activation of unprotected electron-rich anilines for the construction of synthetically versatile quinoline carboxylates via C-H activation, carbonylation, and subsequent annulation under mild and eco-friendly conditions (aqueous media, room temperature, visible-light irradiation, and aerial conditions).

Full text of DOI:10.1021/acscatal.6b02681

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Article
Polythiophene Encapsulated Bimetallic Au-Fe3O4 Nano-Hybrid
Materials: A Potential Tandem Photocatalytic System for Non-Directed
C(sp2)-H Activation for the Synthesis of Quinoline Carboxylates
Mandeep Kaur, Subhamay Pramanik, Manoj Kumar, and Vandana Bhalla
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02681 • Publication Date (Web): 31 Jan 2017
Downloaded from http://pubs.acs.org on February 4, 2017
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Polythiophene Encapsulated Bimetallic AuꢀFe O
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NanoꢀHybrid Materials: A Potential Tandem
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Photocatalytic System for NonꢀDirected C(sp )ꢀH
Activation for the Synthesis of Quinoline
Carboxylates
Mandeep Kaur, Subhamay Pramanik, Manoj Kumar, Vandana Bhalla*
Department of Chemistry, UGC Sponsored Centre for Advanced StudiesꢀII
Guru Nanak Dev University, Amritsar 143005, Punjab, India
ABSTRACT: Heteroꢀoligophenylene derivative 3 appended with thiophene moieties has been
designed and synthesized which undergoes aggregation to form Jꢀtype fluorescent aggregates in
in H O/THF (7:3) media. These aggregates served as reactors for the preparation of bimetallic
2
AuꢀFe O NPs. During the reduction process, aggregates of derivative 3 were oxidized to
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4
polythiophene species, 4. Interestingly, the polythiophene species, 4 having fibrous morphology,
served as shape and morphology directed template for assembly of bimetallic AuꢀFe O NPs in
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flower like arrangement. Furthermore, polythiophene encapsulated bimetallic 4:AuꢀFe O nanoꢀ
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hybrid materials served as an efficient and recyclable catalytic system for C(sp )ꢀH bond
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activation of unprotected electronꢀrich anilines for the construction of synthetically versatile
quinoline carboxylates via CꢀH activation, carbonylation and subsequent annulation under mild
and ecoꢀfriendly conditions (aqueous media, room temperature, visible light irradiation and
aerial conditions).
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KEYWORDS: Heteroꢀoligophenylene, Bimetallic AuꢀFe O Nanoflower, Polythiophene, C(sp )ꢀ
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H Activation, Photocatalyst, Quinoline carboxylates
1.
INTRODUCTION
Quinoline carboxylates are valuable synthons for the preparation of various derivatives having
1
applications in supramolecular and material chemistry. These derivatives are also known for
their antiꢀmalarial, antiꢀmicrobial, antiꢀHIV, antiꢀinflammatory, antiꢀTB, anticancer, antibiotic,
2
and antihypertensive activities. The versatile applications of quinoline carboxylates provide
sufficient impetus for the development of new approaches for convenient synthesis of these
3
derivatives. Among various approaches, transition metals catalyzed CꢀH functionalization has
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emerged as a powerful synthetic strategy for the preparation of quinoline derivatives. This
strategy is still growing and in recent past, various catalytic systems have been developed for
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,6,7
indirect synthesis of quinoline derivatives from protected anilines and alkynes.
Unfortunately, the literature related to the direct synthesis of quinoline 3ꢀcarboxylates from
8
readily available unprotected aniline is scarce.
Our research interest focuses on the development of catalytic/photocatalytic supramolecular
ensembles of fluorescent materials and metal nanoparticles for carrying out various carbonꢀ
carbon and carbonꢀheteroatom bond formation reactions under mild and ecoꢀfriendly conditions.
In continuation of our efforts in this direction, we planned to develop an efficient, recyclable
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catalytic system for the preparation of quinoline carboxylates from readily available substrates
i.e. unprotected anilines. Very recently, [Rh(cod)Cl] ꢀcatalyzed reaction between unprotected
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anilines and electronꢀdeficient alkynes in presence of CO source has been reported for the
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development of quinoline derivatives. However, this approach suffers from limitations such as
utilization of costly and nonꢀrecyclable Rh catalyst, prolonged heating at high temperature and
requirement of inert atmosphere which essentially decreased the environmental and economic
advantages of the strategy. Above all, the recyclability of the system was not explored. To
enhance environmental advantages of CꢀH functionalization reaction, we planned to develop a
recyclable photocatalytic system which could harvest solar radiations to synthesize quinoline
derivatives from unsubstituted aniline under mild conditions. Recently, we reported aggregates
of the perylene bisimide (PBI) derivative stabilized Fe O NPs as an efficient, recyclable
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catalytic system for preparation of quinoline derivatives through one pot multicomponent
reaction between benzaldehyde, aniline derivative and phenylacetylene via CꢀH activation under
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thermal conditions. Amazingly, this catalytic system could be reused up to five cycles without
significant loss in the yield of final product. We examined the efficiency of this catalytic system
in the reaction between unprotected anilines, benzaldehyde and alkynes under photocatalytic
conditions (vide infra). Unfortunately, the reaction did not proceed and the target compound was
not obtained. We believe that limited energy transfer between aggregates of PBI derivative and
Fe O NPs and inability of Fe O NPs to activate the electron rich orthoꢀhydrogen of aniline are
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the possible reasons for the failure of this reaction. To overcome these limitations, we then
planned to develop a photoꢀcatalytically active bimetallic catalytic system by mixing plasmonic
metal species and iron oxide nanoparticles. We envisioned that in the proposed bimetallic
system, plasmonic metal species in combination with supramolecular aggregates will harvest the
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solar energy to carry out the reaction and the Fe O NPs will ensure the recyclability of system.
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For plasmonic material, gold is metallic species of our choice due to its benign nature and its
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known catalytic efficiency in C(sp )ꢀH functionalization reactions. Thus, we designed and
synthesized thiophene appended heteroꢀoligophenylene derivative 3. We expected derivative 3 to
form supramolecular aggregates in mixed aqueous media due to presence of multiple rotors.
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Furthermore, presence of thiophene moieties could enhance its affinity toward Au and Fe
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ions. During the binding, supramolecular aggregates of derivative 3 acted as reducing agents
for preparation of bimetallic AuꢀFe O NPs and themselves got oxidized to polythiophene
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species which gratifyingly served as shape directing nanoreactors for the assembly of bimetallic
AuꢀFe O NPs in flower like arrangement. Recent studies shows that nanoflower like
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arrangement of bimetallic nanoꢀarchitecure is appropriate for offering good activity. In this
context, confinement of metallic species in nanoreactors and their subsequent reduction to form
alloy nanoparticles has emerged as a facile strategy for arranging multimetallic nanoparticles in a
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desired manner. However, this method of generation of confined alloy nanoparticles suffer
from limitations such as requirement of long preparation time and annealing at high
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temperature. In comparison, the aggregates of oxidized species 4 served as convenient shape
directing template and arranged the bimetallic NPs in flower shape arrangement at room
temperature in a simple manner in 60 minutes. The utilization of supramolecular aggregates of
heteroꢀoligophenylene derivative as nanoreactors in aqueous media for the preparation bimetallic
nanohybrid material is unprecedented. This wet chemical method for the preparation of polymer
encapsulated bimetallic AuꢀFe O nanohybrid material in the manuscript is a fast and convenient
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approach in comparison to other reported methods (Table S1 in the Supporting Information).
Further, the supramolecular nanoreactors based on oxidized polythiophene species in
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combination with AuꢀFe O NPs exhibited remarkable photocatalytic efficiency in reaction
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between unprotected anilines and electron deficient alkynes for tandem cabonylation and
annulation reaction to furnish the synthetically versatile quinoline carboxylates. As per our
information, this is the first report which demonstrates the ability of bimetallic AuꢀFe O NPs in
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nonꢀdirected C(sp )ꢀH bond activation of unprotected anilines for the efficient synthesis of
quinolines under photocatalytic and aerial condition (Table S2 in the Supporting Information).
2.
RESULTS AND DISCUSSION
2.1.1. Synthesis and characterization of heteroꢀoligophenylene derivative 3. DielsꢀAlder
reaction of phenylacetylene 1a with 2,5ꢀdiphenylꢀ3,4ꢀdi(thiophenꢀ2ꢀyl)cyclopentaꢀ2,4ꢀdienone
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a in diphenylether at 240°C furnished the desired heteroꢀoligophenylene derivative 3 in 70%
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yield (Scheme 1). The H NMR spectrum of compound 3 exhibited a singlet at 7.55 ppm (1H),
six double doublets at 7.22, 7.14, 7.07, 6.95, 6.54, 6.48 ppm (4H, 6H, 1H, 2H, 1H, 1H), two
triplets at 6.88, 6.63 ppm (1H, 1H) and a multiplet at 7.02ꢀ6.99 ppm (4H). The ESIꢀMS mass
+
spectrum of compound 3 exhibited a base peak at 471.1448 corresponding to [M+H]
.
1a
2a
3
Scheme 1. Synthesis of heteroꢀoligophenylene derivative 3.
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.1.2. Aggregation and photophyical behavior of derivative 3: The photophysical and
aggregation behavior of derivative 3 was studied using UVꢀvis and fluorescence spectroscopy.
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The absorption spectrum of compound 3 in THF showed the appearance of an absorption band at
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70 nm due to πꢀπ* transition. On increasing the water fractions up to 70% (volume fraction) in
the THF solution, the absorption band red shifted from 270 to 290 nm and its intensity decreased,
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however, an increase in intensity of level off tail in the visible region was observed (Figure 1A).
The UVꢀvis studies of derivative 3 in H O/THF (7:3) showed blue shifting of absorption band
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from 290 to 280 nm with increasing temperature up to 75°C which suggest the formation of Jꢀ
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aggregates (Figure S1 in the Supporting Information).
(A)
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Water Fraction (%)
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(C)
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Figure 1. (A) UVꢀvis spectra showing the change in absorbance of compound 3 (5.0 µM) in
H O/THF mixture (0 to 70% volume fraction of water in THF); The Scanning electron
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micrographs showing the spherical shaped aggregates in H O/THF (7:3) mixture and the
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scale bar of (B) 500 nm and (C) 100 nm.
The fluorescence spectrum of derivative 3 in H O/THF (7:3) solvent mixture exhibited a broad
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emission at 350 nm (quantum yield, Ф = 0.08) when excited at 290 nm. Further addition of water
fraction up to 70% to the THF solution, resulted in red shifting of emission band from 350 to 360
nm and its intensity decreased (Figure S2 in the Supporting Information). The scanning electron
microscopy (SEM) image of compound 3 in H O/THF (7:3) solution mixture showed the
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formation of spherical aggregates (Figure 1BꢀC). The dynamic light scattering (DLS) analysis
showed the presence of particles having average diameter around 200 nm (Figure S3 in the
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Supporting Information). Further, we performed H NMR studies by varying concentrations of
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derivative 3 in CDCl and an average upfield shift of 0.04 ppm was observed in the signals
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corresponding to aromatic protons (Figure S4 in the Supporting Information). Thus, the Hꢀ
NMR, and absorption studies suggest the formation of Jꢀtype assemblies of derivative 3 in
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aqueous media.
2.1.3. Preparation of bimetallic AuꢀFe O NPs: The presence of thiophene moieties in
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derivative 3 prompted us to investigate its molecular recognition behavior toward different metal
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ions such as Ag , Au , Fe , Fe , Hg , Zn , Cu , Co , Ni , Pd , Cd , Ba , and Ca as
their perchlorate/chloride salts using absorption and emission studies. The absorption and
3+
3+
emission studies suggested the affinity of aggregates of compound 3 toward Au and Fe ions
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among various metal ions tested. Upon addition of Au ions (0ꢀ2 equiv.) to the H O/THF (7:3,
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v/v) solution of derivative 3, a new band at 330 nm appeared and an isosbestic point was
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observed at 310 nm which suggested the formation of 3ꢀAu complex. Further addition of Au
ions (2ꢀ20 equiv.) to the same solution led to redꢀshifting of band from 330 to 335 nm and an
appearance of plasmon resonance band at 540 nm which suggested the formation of Au NPs
(Figure S5 in the Supporting Information). These spectral changes were accompanied by solution
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color change from colorless to reddish. While upon addition of Fe ions (0ꢀ20 equiv.) to
derivative 3 solution in H O/THF (7:3, v/v) resulted in formation of an absorption band at 335
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nm and spectrum got broadened in the 500ꢀ700 nm range (Figure S6 in the Supporting
Information). Further, the color of the solution was changed from colorless to black. All these
changes suggested the formation of Fe O . The above results suggest that the supramolecular
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aggregates of derivative 3 served as reactors for the preparation of Au NPs as well as Fe O NPs.
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The UVꢀvis spectra of aggregates of derivative 3 in the presence of other metal ions such as Fe ,
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Cu , Co , Zn , Ni , Pd , Ag , Hg , Cd , Ca and Ba ions (perchlorate/chloride salts)
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Supporting Information).
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In the next part of the investigation, we planned to prepare bimetallic AuꢀFe O NPs using
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aggregates of derivative 3 as a common platform. The solution of Au ions (0ꢀ5 equiv.) and Fe
ions (0ꢀ5 equiv.) was added simultaneously to the solution of derivative 3 in H O/THF (7:3, v/v)
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as shown in Figure 2. The absorption spectrum of this solution showed a gradual enhancement in
absorbance at 290 nm and appearance of a new band spectrum at 330 nm. An isosbestic point
3
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was also observed at 316 nm which suggested the generation of 3ꢀAu and 3ꢀFe complexes, at
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low concentration (Figure 2A and inset). Upon further addition of Au and Fe ions (5ꢀ20
equiv.) resulted in red shifting of absorption bands from 290 to 295 and 330 to 335 nm
respectively. A new band was also observed at 545 nm which indicated the generation of Au NPs
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(
Figure 2B and 2C). Further, the absorption bands at 335 and 295 nm got broadened which
indicated polymerization of thiophene appended heteroꢀoligophenylene derivative 3 (vide
12, 22
infra).
In the absorption spectrum of same solution after keeping it for 60 minutes the
intensity of absorbance band at 335 nm gradually increased, and the band at 545 nm red shifted
to 570 nm. The red shifting of LSPR band of gold NPs suggested an increase in the optical index
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of the medium surrounding the NPs. On the other hand, the intensity of levelꢀoff tail increased
which suggested the decoration of the Au NPs by Fe O NPs (Figure 2C). Interestingly, the color
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of solution changed from colorless to yellow to red and finally reddish brown. (Figure 3). All
these results demonstrate that the aggregates of derivative 3 interacted with metal ions and
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served as reactors for reducing the metal ions into their nano forms and finally bimetallic Auꢀ
Fe O nanohybrid materials were generated.
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330 nm
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Figure 2. (A) Absorption spectra of derivative 3 (5 µM) in presence of Au and Fe ions (0ꢀ20
3
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equiv.) in water/THF (7:3, v/v) media; inset showing 3ꢀM (M= Au and Fe ) complexation at
lower conc. (0ꢀ4 equiv.); (B) Shifting of bands at 290 and 330 nm to 295 and 335 nm; (C)
Expanded Absorption spectra show the appearance of band at 545 nm correspond to Au NPs and
red shifting of the band to 570 nm correspond to bimetallic AuꢀFe O NPs.
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spectroelctrochemical studies of derivative 3 in presence of Au ions and Fe . The cyclic
voltammogram of derivative 3 in H O:CH CN (1:1) (15 µL THF to dissolve) showed reversible
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3
redox wave with a reduction potential of ꢀ0.30 eV (Figure S8A in the Supporting Information).
3
+
3+
In the spectroelctrochemical studies, upon addition of 20 equiv. of Au and Fe ions to
derivative 3, an absorption band was observed at 370 nm. When the reduction potential (ꢀ0.30
eV) was applied, the band at 370 nm completely disappeared, thus, indicating the reduction of
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Au to Au(0) and Fe ions to Fe ions. Further, within 60 min a new band suggesting the
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formation of Au NPs appeared at 540 nm. Gradually, the band at 540 nm was broadened and was
red shifted to 560 nm, hence, indicating the formation of Au NPs along with Fe O NPs (Figure
3
4
2
1
S8B in the Supporting Information). These spectroelctrochemical studies clearly suggest that
0
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2
3
4
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8
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1
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3
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9
0
aggregates of derivative 3 served as reducing agents for the generation of bimetallic AuꢀFe O4
3
24
NPs.
Time: 2 min
30 min
60 min
(B)
(C)
(D)
(A)
AuCl3
FeCl
3
Bimetallic Auꢀ
Fe O NPs
Aggregates of
derivative 3
3
4
Figure 3. Showing the naked eye colour change of the solution of aggregates of derivative 3
3+
3+
upon addition of Au and Fe ions (20 equiv.) under stirring; (A) colourless in absence of
3
+
3+
3+
3+
Au and Fe ions and (BꢀD) in presence of Au and Fe ions: yellow to reddish and finally
to brown after different time interval.
Next, we examined the emission behavior of aggregates of derivative 3 towards various metal
3+
3+
2+
2+
2+
2+
2+
2+
+
2+
+
2+
2+
2+
ions such as Au , Fe , Fe , Cu , Co , Zn , Ni , Pd , Ag , Hg , K , Cd , Ba and Mg as
3
+
3+
their perchlorate/chloride salts. Upon addition of Au and Fe ions (0ꢀ20 equiv.) to the solution
of derivative 3 in H O/THF (7:3, v/v) mixture, the emission band at 360 nm gradually red shifted
2
to 390 nm and its emission intensity decreased by tenꢀfolds (Figure 4). Under the same
conditions, no considerable change in emission spectrum was observed in presence of other
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metal ions (Figure S9 in the Supporting Information). The detection limit of aggregates of
3
+
derivative 3 for Au ions was found to be 53 nM (Figure S10 in the Supporting Information).
3
60
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13
14
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16
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0
3+
3+
Au + Fe
equiv.)
(
2
0
3
90
3
+
3+
Figure 4. Fluorescence spectra of derivative 3 (5 µM) in presence of Au and Fe ions (0ꢀ20
equiv.) in water/THF (7:3, v/v) media, λ = 300 nm.
ex
2.1.4. Preparation and characterization of polythiophene species: To get insight into the
oxidation of aggregates of derivative 3, we performed the reaction between derivative 3 in
3
+
3+
H O/THF (7:3, v/v) with Au + Fe ions at room temperature (Figure 5). After the complete
2
formation of AuꢀFe O NPs (as indicated by color change in solution) the solution of aggregates
3
4
of derivative 3 containing nanoparticles was slowly evaporated. After two days, brown
precipitates appeared which were separated by filtration and then washed with CHCl ꢀTHF to
3
obtain bimetallic AuꢀFe O NPs. The filtrate was evaporated to acquire the solid residue and its
3
4
1
1
H NMR was recoded. In the H NMR spectrum, the peak corresponding to CꢀH proton at 6.95
a
ppm was not observed. On the other hand, upfield shifting of broad signals of all the aromatic
2
5
protons was observed
(Figure S11 and Table S3 in the Supporting Information). The ESIꢀMS
spectrum showed peaks at m/z 939.2240 and 1430.3114 which corroborated very well with the
+
+
theoretical exact mass of dimeric [C H S Na] and trimeric species [C H S H] , respectively
6
4
42
4
96 62 6
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3
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3
3
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4
4
4
4
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(
Figure S12 in the Supporting Information). The FTꢀIR spectrum of oxidized species showed the
ꢀ1
ꢀ1
absence of peaks at 3092 cm and 890 cm corresponding to stretching and out of plane bending
of CꢀH bond adjacent to ‘S’ atom of thiophene moiety. All the above spectroscopic data clearly
a
0
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9
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9
0
indicates the formation of polythiophene species 4 through orthoꢀH ꢀH coupling of thiophene
a
a
2
6
rings (Figure S13 in the Supporting Information). Thus, we conclude that the aggregates of
3
+
3+
2+
derivative 3 reduced Au ions to Au NPs and Fe ions to Fe ions which were further oxidized
to Fe O NPs, however, during the reduction process, the aggregates of derivative 3 were
3
4
oxidized to polythiophene species, 4 (Figure 5). The UVꢀvis spectra of oxidised polythiophene
species exhibited an absorption band at 295 and 335 nm and the absorption spectrum of oxidized
3
+
3+
species was similar to the absorption spectrum of derivative 3 in the presence of Au and Fe
12
ions. These results support our assumption that the absorbance at 335 nm originated from the
oxidized polythiophene species 4 (vide supra) Figure S14 in the Supporting Information).
(
=
=
Au NPs
Fe O NPs
3
+
3+
Au and Fe
Selfꢀ
asse
mbly
3
4
ions (20 equiv).
in water
AuꢀFe O
3
4
=
NPs
Polythioph
ene coated
=
Polymer coated Auꢀ
Fe nanoflower
3
O
4
Polythioph
ene, 4
=
Polythiophene species, 4
Figure 5. Probable Schematic representation of formation of polythiophene species 4 from
3
+
3+
derivative 3 during reduction of Au ꢀFe ions to and further preparation of polythiopheneꢀ
protected bimetallic AuꢀFe O NPs.
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4
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Interestingly, the oxidised polythiophene species 4 exhibited broad emission band in the range of
380ꢀ500 nm (Figure S15 in the Supporting Information). The fluorescence spectrum of the
solution of oxidized species 4 (5 ꢁM) in H O/THF (7:3) showed an emission band at 405 nm.
2
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Upon addition of bare AuꢀFe O NPs (5 mol%) to this solution, emission intensity
3
4
decreased by 61% (Figure S16A in the Supporting Information). Further, a strong overlap
between fluorescence spectrum of polythiophene species 4 and absorption spectrum of AuꢀFe O4
3
NPs was observed which suggest the possibility of energy transfer from oxidized species 4 to
AuꢀFe O NPs (Figure S16B in the Supporting Information).
3
4
2
.1.5. Characterization of polythiophene capsulated AuꢀFe O nanohybrid materials: The
3 4
3
+
3+
transmission electron microscope (TEM) images of derivative 3 in the presence of Au and Fe
ions (20 equiv.) revealed the presence of flower like AuꢀFe O NPs encapsulated into the
3
4
network of fibers of polythiophene species 4 (Figure 6aꢀb). The TEM and HRꢀTEM micrographs
indicated the presence of nanofibers of polythiophene 4 coated bimetallic AuꢀFe O nanoflower
3
4
(NFs) (Figure 6bꢀc). The average size of nanoflower (Figure 6d) was found to be in the range of
7
0ꢀ80 nm (Figure 6e) (gold nanoparticles: 8ꢀ12 nm, Fe O nanoparticles: 5ꢀ10 nm). The highꢀ
3
4
resolution TEM images (Figure 6f) showed dꢀspacing of 0.23 nm corresponding to the (111)
plane of face centered cubic (fcc) gold and the dꢀspacing of 0.25 and 0.48 nm corresponding
21
to the (311) and (111) plane bearing fcc crystal lattice fringes of Fe O NPs. The
3
4
selected area electron diffraction (SAED) pattern showed three distinguishable planes of
(
311), (511), and (731) indices which suggest the fcc phase of Fe O and (111), (200),
3 4
(
220), (311) and (422) planes corresponding to Au NPs (Figure S17A in the Supporting
27
Information)
.
The SAED pattern suggested a superposition of Au and Fe O lattices
3 4
corresponding to bimetallic AuꢀFe O NPs. The energy dispersive Xꢀray spectroscopy (EDX)
3
4
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6
showed the presence of Fe, Au, O, S and C elements in the nanohybrid materials which confirm
the formation of polythiophene encapsulated bimetallic AuꢀFe O NPs (Figure S17B in the
3
4
2
3
3+
3+
Supporting Information). Further, we examined the effect of concentration of Au and Fe
0
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2
3
4
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9
0
1
2
3
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ions on the generation of AuꢀFe O NPs. The TEM images of the sample prepared by mixing
3
4
3
+
3+
Au and Fe ions in 1:2 ratio showed bigger size core shell AuꢀFe O NPs where the Au NPs
3
4
are fully encapsulated by Fe O NPs (Figure S18A in the Supporting Information).
3
4
(
a)
(b)
(c)
(
f)
(
d)
(e)
Polythiophene
0.48 nm
0
.25 nm
3 4
Fe O NPs
Au NPs
0
.23 nm
Figure 6. (a) TEM images of polythiophene nanofibers, inset showing the diameter of nanofibres; (b)
AuꢀFe O4 NPs stabilized by polythiophene nanofibers; (c) highꢀresolution TEM images of
3
polythiophene coated AuꢀFe O NPs; (d) TEM image showing the formation of in situ polythiophene
3
4
3
+
encapsulated bimetallic AuꢀFe O nanoparticles prepared by mixing aggregates of derivative 3 and Au
3
4
3
+
and Fe ions, scale: 100 nm; (e) showed flower shape nanohybrid materials composed of dark Au NPs
and light Fe O NPs, the outer layer is polythiophene species, scale: 50 nm; (f) HRꢀTEM image
3
4
showing dꢀspacing of 0.23 nm corresponding to the (111) plane of Au NPs and the dꢀspacing of 0.25
and 0.48 nm corresponding to the (311) and (111) plane of Fe O NPs respectively.
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4
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3+
3+
3+
On the other hand, upon increasing the amount of Au by mixing Au and Fe ions in 2:1
ratio, larger sized irregular shaped Au and Fe O NPs were found Figure S18B in the
(
3
4
Supporting Information). The above studies suggest that the supramolecular aggregates of
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derivative 3 served as shape, size and morphology directing agent.
The DLS studies indicated the presence of two sets of particles having size in the range of 4ꢀ8
and 70ꢀ100 nm, respectively (Figure S19 in the Supporting Information). The powder Xꢀray
diffraction (XRD) measurements showed diffraction peaks at 18.24°, 30.14°, 35.10°, 43.16°,
53.56°, 57.12° and 62.58° which were assigned to the (111), (220), (311), (400), (422), (511),
and (440) planes corresponding to fcc lattice of Fe O NPs and the diffraction peaks at 37.96°,
3
4
44.18°, 64.34°, 77.32 and 81.58° corresponding to the (111), (200), (220), (311), and (422)
2
8
planes of Au NPs respectively (Figure S20 in the Supporting Information). The Raman
scattering spectrum of derivative 3 showed bands at 982.27, 1003.29, 1317.98, 1352.82, 1445.95,
ꢀ1
3+
3+
1
541.98 and 1603.26 cm . In presence of Au and Fe ions all the bands got broadened and
ꢀ1
new band appeared at 670 cm corresponding to the A vibration mode of Fe O NPs (Figure
1
g
3
4
23
S21 in the Supporting Information). The Xꢀray photoelectron spectroscopic (XPS) analysis of
the polythiophene coated AuꢀFe O NPs showed the presence of peaks at 84.0 and 87.8 eV
3
4
2
9
corresponding to Au 4f7/2 and Au 4f peaks of Au(0) state. Further, two peaks were observed
5/2
at 710.2 and 724.6 eV corresponding to Fe 2p3/2 and Fe 2p1/2 which confirmed the presence of
3
+
2+
30
both Fe and Fe species in the Fe O lattice. Additionally, peaks were observed at 162.4,
3
4
284.2 and 526.4 eV corresponding to S2p, C1s and O1s of organic residue, respectively
(
Figure
2
9, 31
S22 in the Supporting Information).
All the above results clearly suggest the existence of
polythiophene encapsulated bimetallic AuꢀFe O NPs. The atomic absorption spectrophotometry
3
4
(AAS) studies show the presence of 39.7 wt% of gold in bimetallic AuꢀFe O NPs Table S4 in
(
3
4
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2
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2
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3
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the Supporting Information. The thermogravimetric analysis (DTꢀTGA) of polythiophene
encapsulated bimetallic AuꢀFe O NPs has been done which showed a nonlinear continuous
3
4
weight loss above 250°C, however no weight loss was observed above 650°C. From the weight
0
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loss, the amount of polythiophene coating on AuꢀFe O NPs was estimated to be approximately
3
4
20.0%
(Figure S23 in the Supporting Information). The superparamagnetic behavior of
bimetallic AuꢀFe O was observed by magnetic hysteresis measurements (Figure S24, Table S5
3
4
in the supporting information).
3+
2.1.6. Synthesis of model compound 5 and study of its photophysical behavior with Au
3+
and Fe ions:
1
b
5
2b
Scheme 2. Synthesis of heteroꢀoligophenylene based model compound 5.
To examine the role of polythiophene species 4 as a template for preparation of flower shaped
AuꢀFe O nanohybrid materials, we prepared model compound 5 bearing only one thiophene
3
4
unit by DielsꢀAlder reaction of 1b and 2b at 240°C (Scheme 2). The absorption spectra of
3
+
3+
derivative 5 in waterꢀTHF (7:3) media in presence of Au ions and Fe ions led to the
appearance of band at 550 nm which suggested the generation of Au and Fe O NPs (Figure S25
3
4
in the Supporting Information). The TEM images showed the formation of well dispersed Au
and Fe O nanoparticles; however, no flower shaped morphology was observed (Figure S26A in
3
4
the Supporting Information). The DLS studies indicate the formation of larger sized
nanoparticles having average diameter around 20ꢀ30 nm (Figure S26B in the Supporting
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Information). These results clearly suggest that only the aggregates of oxidized polythiophene
3
species 4 served as shape directing template for the generation of bimetallic AuꢀFe O4
nanohybrid materials in flower like arrangement.
10
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2
.2.
Photocatalytic efficiency of bimetallic AuꢀFe O NPs polythiophene encapsulated
3 4
bimetallic AuꢀFe O nanoparticles: The possibility of energy transfer from polythiophene
3
4
species to AuꢀFe O NPs encouraged us to investigate the catalytic efficiency of polythiophene
3
4
2
encapsulated bimetallic AuꢀFe O NPs in the C(sp )ꢀH activation reaction in presence of visible
3
4
light. We planned to synthesize versatile 3ꢀsubstituted quinolines by reaction between
unprotected anilines with electron deficient alkynes through three steps i.e. CꢀH activation,
carbonylation and subsequent annulation. We chose model reaction between anilines (6a) and
9
, 33
methyl propiolate (7a) whereas paraformaldehyde (2.5 mmol) was used as carbonyl source
(Scheme 3). The reaction was carried out in water using AuꢀFe O nanohybrid materials (0.5
3
4
mol% with respect to Au) as catalyst under photocatalytic conditions. The reaction was complete
in 6 hours and the desired quinoline derivative 8a was acquired in excellent yield (80%) (Table
1
, entry 1). We also carried out the model reaction in several solvents such as CH CN, EtOH,
3
THF and DMF under photocatalytic conditions. In presence of all these solvents, the desired
product 8a was acquired in lower yield while under neat conditions; only 40% product formation
was observed (Table 1, entry 1ꢀ6). These studies show that maximum yield of desired product is
obtained in the presence of water as the reaction medium. We believe that in aqueous medium,
the solubility of paraformaldehyde is increased, hence, a slow flow of in situ generated CO gas is
maintained which is required for the carbonylation reaction. Thus, we chose water as an optimal
solvent for carrying out oneꢀpot multicomponent reaction to afford a green synthetic
methodology for the preparation of 3ꢀsubstituted quinolines. The same reaction when conducted
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5
5
6
using different CO sources such as aq. HCHO yielded the target quinoline product 8a in lower
amount (Table 1, entry 7). To investigate the role of in situ generated CO as a carbonylating
agent for the synthesis of quinoline derivatives, we carried out the reaction of anilines (6a) and
0
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2
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4
5
6
7
8
9
0
1
2
3
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5
6
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8
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0
34
methyl propiolate (7a) directly using CO instead of (HCHO)n and the desired product was
found in 72% yield (Table 1, entry 8). This study highlights the role of in situ generated CO in
the synthesis of quinoline. However, for carrying out reaction under ecofriendly conditions, we
choose paraformaldehyde as the carbonyl source. Therefore, paraformaldehyde was chosen as
the carbonyl source (instead of using poisonous CO gas) in the reaction.
AuꢀFe O NPs,
3
4
hν
CO source
6
a
8a
7a
2
Scheme 3. C(sp )ꢀH activation reaction between anilines (6a, 1 mmol), methyl propiolate (7a,1m
mol) and paraformaldehyde (2.5 mmol) catalyzed by polythiophene encapsulated AuꢀFe O NPs
3
4
under photocatalytic conditions (irradiation of 60 W tungsten filament bulb).
2
Table 1. C(sp )ꢀH activation reaction between anilines (6a, 1 mmol), methyl propiolate
(
7a,1mmol) catalyzed by polythiophene encapsulated AuꢀFe O NPs under different conditions.
3 4
Entry Solvent
CO source
(HCHO)n
(HCHO)n
(HCHO)n
(HCHO)n
(HCHO)n
(HCHO)n
aq. HCHO
Yield (%)
1
2
3
4
5
6
7
8
H O
80
30
25
15
20
40
55
72
2
ACN
EtOH
THF
DMF
a
Neat
H O
2
3
4
H O
2
In situ generated CO
a
= 300 µL of 4:AuꢀFe O in Water:THF (7:3) was used only for catalyst
3
4
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Further, the model reaction between 6a and 7a in presence of polythiophene capped
supramolecular ensemble 4:AuꢀFe O nanohybrid materials under dark conditions furnished the
3
4
desired product in 32% yield while under photocatalytic conditions the target product was found
in 80% yield (Table 2, entry 1ꢀ2). These results suggest that the high catalytic efficiency of the
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2
system for the C(sp )ꢀH activation reaction resulted from the visible light irradiation. No reaction
product was obtained using only polythiophene species 4 as the photocatalyst (Table 2, entry 3).
In presence of AuCl and FeCl as the catalytic system, the target product was found in 15%
3
3
yield (Table 2, entry 4). On the other hand, when AuCl and FeCl in combination with
3
3
polythiophene species 4 were used as catalyst under irradiation, the target product was found in
2
5% yield (Table 2, entry 5). All these studies highlight the utility of polythiophene capped
2
supramolecular ensemble 4:AuꢀFe O nanohybrid materials in the C(SP )ꢀH activation reaction.
3
4
Further, we examined the model reaction in presence of bare Au NPs, bare Fe O NPs and bare
3
4
35
AuꢀFe O NPs, respectively, as the catalytic systems. The desired product 7a was obtained in
3
4
4
5% yield in presence of bare Au NPs, <10% yield in presence of bare Fe O NPs and 48% yield
3 4
in presence of bare AuꢀFe O NPs (Table 2, entry 6ꢀ8). These results clearly indicate that Au
3
4
2
NPs play an important role in C(sp )ꢀH activation reaction. Interestingly, bare AuꢀFe O NPs in
3
4
presence of oxidized species 4 showed better catalytic efficiency than the aggregates of
derivative 3 in the above mentioned reaction to acquire the target product 8a in 60% yield (Table
2, entry 9ꢀ10). Further, we also studied the same reaction using 0.5 mol % of supramolecular
3+
3+
ensemble prepared by mixing aggregates of derivative 5 (5 µM) with Au and Fe ions (20
equiv.). The target compound 8a was acquired in 45 % yield in 10 h (Table 2, entry 11). This
study suggest that the polythiophene capped supramolecular ensemble 4:AuꢀFe O NPs has
3
4
2
higher catalytic efficiency rather than the derivative 4 stabilized AuꢀFe O NPs in C(sp )ꢀH
3
4
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5
6
activation reaction. We also believe that polythiophene species
4 played an importanat role
in increasing the light harvesting capacity of AuꢀFe O hybrid nanomaterials.
3
4
2
Table 2: C(sp )ꢀH activation reaction between anilines (6a, 1 mmol) methyl propiolate
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(7a, 1 mmol) and and paraformaldehyde (2.5 mmol) under different catalytic conditions.
S.No. Control experiment
Time
h)
Yield
(%)
(
1
2
3
polythiophene 4:Auꢀ
36
32
a
Fe O
3
4
polythiophene 4:Auꢀ
6
80
b
Fe O
3
4
b
polythiophene 4
24
No
reaction
4
5
AuCl + FeCl
24
24
15
25
3
3
AuCl + FeCl +
3 3
polythiophene 4
b
b
6
7
8
9
Bare Au NPs
24
24
24
6
45
< 10
48
b
b
Bare Fe O NPs
3
4
Bare Auꢀ Fe O
3
4
Bare Auꢀ Fe
O
4
+
60
3
b
polythiophene 4
1
0
Bare Auꢀ Fe
3
O
4
+
6
30
45
aggregates of
derivative 3
b
11
Derivative 5:Auꢀ
10
b
Fe O
3
4
a
b
:
in dark, : under light irradiation
Afterwards, we planned to expand the scope of photocatalytic system with regard to anilines. For
this, we carried out a variety of reactions between methyl propiolate (7a) and substituted anilines
(6bꢀm) under the optimized conditions (Scheme 4, Table 3). The electron rich aryl amines (6bꢀg)
and deficient anilines underwent CꢀH activation reactions to furnish the desired products (8bꢀg)
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9
in excellent yield (Table 3, entry 1ꢀ6). This is attributed to the electronic nature of the
substituents (+R effect) which might have facilitated the electrophilic metallation step. Further,
the amines bearing electron withdrawing substituents (halogens) require extended time to
produce the desired products (8hꢀj) in good yields (Table 3, entry 7ꢀ9).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
AuꢀFe O NPs,
3
4
hν
(HCHO)n,
8
bꢀm
H O
2
6
bꢀm
7
a
2
Scheme 4. C(sp )ꢀH activation reaction between anilines (6bꢀm, 1 mmol), methyl propiolate
7a, 1 mmol) and paraformaldehyde (2.5 mmol) catalyzed by polythiophene encapsulated Auꢀ
Fe O NPs (0.5 mol% i.e., 300µl 4:AuꢀFe O in water:THF, 7:3) under photocatalytic
(
3
4
3
4
condition (irradiation of 60 W tungsten filament bulb).
2
Table 3: C(sp )ꢀH activation reaction between anilines (6bꢀm) (1 mmol), methyl propiolate (7a)
(
1 mmol) and paraformaldehyde (2.5 mmol) as carbonyl source and polymer encapsulated 4:Auꢀ
Fe O nanohybrid materials (0.5 mol% i.e., 300µl 4:AuꢀFe O in Water:THF, 7:3) as catalyst,
3
4
3
4
under photocatalytic condition (irradiation of 60 W tungsten filament bulb).
Entry
6bꢀm
Product (8bꢀm)
Time Yield
(h)
(%)
1
6
78
6
b
8b
2
3
6
8
82
75
8c
6
c
6d
8d
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2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
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5
5
5
5
5
5
5
5
5
6
4
5
6
7
8
9
6
6
8
8
8
85
82
75
72
70
8
e
6
f
e
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
8f
6
8
g
6g
8h
6
h
8i
6
i
6
6
68
75
8j
6
j
10
6
k
8
k
11
MeO
OMe
COOMe
N
9
45
MeO
MeO
H N
NH2
2
6l
N
COOMe
8
l
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9
1
2
COOMe
N
10
40
10
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54
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58
59
60
H N
NH2
2
N
6m
COOMe
8m
To check the practical applications of polythiophene encapsulated 4:AuꢀFe O nanohybrid
3
4
materials as photocatalytic system, we carried out the CꢀH activation reaction in case of pyrene
(6k), terphenyl (6l) and hexaphenylbenzene (6m) derivatives. Under optimized conditions, all
these derivatives reacted smoothly with methyl propiolate (7a) to furnish the desired products 8k
in 75%, 8l in 45% and 8m in 40% yield, respectively (Table 3, entry 10ꢀ12).
In the next section, we selected the model reaction between aniline (6a) and methyl propiolate
(
7a) to examine the reusability and recyclability of the photocatalytic system. The in situ
generated polythiophene capped supramolecular ensemble 4:AuꢀFe O nanohybrid materials
3
4
could be easily separated from the reaction mixture by simple magnetic separation upto 11 times.
th
After 11 cycle the yield of final product was reduced to 54% (Figure S27 in the Supporting
Information). The TEM image of recycled 4:AuꢀFe O obtained after recycling revealed small
3
4
variation in size and morphology of the nanoparticles (Figure S28 in the Supporting
Information). To measure the amount of gold leached into the residual solution, we carried out
th
the atomic absorption spectroscopic (AAS) studies of the residual solution remained after 11
th
cycle and it was found that 3.849 ppm of gold (19.2 wt%) leached through the solution after 11
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
catalytic cycle (Table S6 in the Supporting Information). We believe that higher amount of
leaching of gold is the main reason behind the decrease in the catalytic efficiency of the reaction
th
after 11 catalytic cycle. Further, we investigated the catalytic efficiency of supramolecular
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ensemble 4:AuꢀFe O nanohybrid materials for the reaction between 6a with 7a at different
3
4
amounts of catalyst (Table S7 in the Supporting Information). When 5000 ppm (0.5 mol%)
catalyst was used, then the target compound 8a was found in 80% yield (Table S7, entry 1)
within 6 h and no product was formed in absence of the catalyst. On decreasing the amount of
catalyst (1 ppm), target product was acquired in 40% yield within 48 h (Table S7, entry 8). Such
small amount of catalyst has rarely been employed for tandem cyclization for the preparation of
2
quinoline via C(sp )ꢀH activation reactions under photocatalytic conditions before the present
study. Further, to determine the role of polythiophene species 4 in reaction between 6a with 7a,
we prepared various photocatalytic systems of supramolecular ensemble 4:AuꢀFe O nanohybrid
3
4
materials by mixing polythiophene species 4 and AuꢀFe O NPs in different ratios and
3
4
investigated the progress of the reaction (Table S8 in the Supporting Information). The above
results clearly suggest that upon decreasing the amount of polythiophene species 4 in the
catalytic system by varying the ratio from 1:1 to 1:5 (4:AuꢀFe O ), the product yield (59%)
3
4
considerably decreased, however, upon increasing the amount of polythiophene species 4 in the
catalytic system by varying the ratio from 1:1 to 5:1, the yield of product was not affected
significantly (Table S8, entry 8ꢀ9 in the Supporting Information).
2
.2.1 Substrate scope for the CꢀH activation catalyzed by 4:AuꢀFe O nanohybrid
3 4
2
materials: To examine the influence of the electronic nature of alkynes in ortho C(sp )ꢀH
activation, we carried out different reactions between ethyl propiolate (7b) and substituted
anilines (6a/6e/6g/6h) under the optimized conditions and the desired products were obtained in
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good to excellent yield (Scheme 5, Table 4, entry 1ꢀ4). The switching of the alkynes brought
very small changes in reaction rate or yield of the desired products.
AuꢀFe O NPs, hν
3
4
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(HCHO)n, H O
2
6
a/6e/6g/6h
2
7b
9aꢀd
Scheme 5. C(sp )ꢀH activation reaction between substituted anilines (1 mmol) and ethyl propiolate
(
7b, 1 mmol) and paraformaldehyde (2.5 mmol) catalyzed by polythiophene encapsulated AuꢀFe O4
3
NPs (0.5 mol% i.e., 300µl 4:AuꢀFe O in Water:THF, 7:3) in presence of visible light irradiation
3
4
(irradiation of 60 W tungsten filament bulb).
2
Table 4. C(sp )ꢀH activation reaction between substituted anilines (6a/6e/6g/6h, 1 mmol), ethyl
propiolate (7b, 1 mmol) and paraformaldehyde (2.5 mmol) catalyzed by polythiophene encapsulated
AuꢀFe O NPs (0.5 mol% i.e., 300µl 4:AuꢀFe O in Water:THF, 7:3) under photocatalytic condition
3
4
3
4
(
irradiation of 60 W tungsten filament bulb).
Entry Reactant
Product
Time (h)
Yield (%)
1
2
.
.
6
75
6
6
a
g
9a
8
8
6
72
70
82
9b
3
.
9c
6
h
4.
9d
6e
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2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
2
.2.2 Mechanism of C(sp )ꢀH activation by polythiophene encapsulated AuꢀFe O NPs: To
3 4
understanding the mechanism of polythiophene encapsulated supramolecular ensemble 4:Auꢀ
2
Fe O nanohybrid materials catalyzed C(sp )ꢀH activation reaction, we investigated the model
3
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
reaction of 6a with 7a in presence of the radical scavenger 2,2,6,6ꢀtetramethylꢀ1ꢀpiperidinoxyl
(TEMPO, 2 equiv.). The presence of TEMPO completely inhibited the reaction, thus, confirming
the formation of free radical under the standard reaction conditions. On the basis of above
results, we purpose the plausible radical/single electron transfer (SET) mechanism as shown in
scheme 6.
A
C
TSꢀI
IMꢀI
B
IMꢀII
TSꢀII
2
Scheme 6. Probable reaction mechanism of quinoline synthesis via C(sp )ꢀH activation reaction of
substituted anilines, alkynes and paraformaldehyde (where R = substituents and R’ = Me/Et)
catalyzed by polythiophene encapsulated AuꢀFe O NPs (0.5 mol% i.e., 300µl 4:AuꢀFe O in
3
4
3
4
Water:THF, 7:3) under photocatalytic condition (irradiation of 60 W tungsten filament bulb).
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0
5
0
5
0
5
0
5
0
5
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
3
2
2
1
1
1.89 Å
TSꢀII
IMꢀII
ꢁG =
+1.975
kcal/mol
C
TSꢀI
ꢁ
+
G =
31.454
kcal/mol
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ꢁG =
ꢀ
25.590
ꢁ
G =
ꢁ
G =
kcal/mol
ꢁG =
ꢀ30.132
kcal/mol
+25.212
+30.133
kcal/mol
kcal/mol
CO
addition
ꢁ
G =
ꢁG =
ꢀ
42.111
+29.479
kcal/mol
kcal/mol
B
1.09 Å
IMꢀI
A
B
ꢀ
ꢀ
10
2
Scheme 7. Possible reaction mechanism for the synthesis of quinoline derivative via C(sp )ꢀH
activation reaction of anilines, alkynes and paraformaldehyde catalyzed by 4:AuꢀFe O NPs (AuNPs
3
4
ꢀ1
is considered as the main active site) (relative Gibbs free energy: kcal mol , distance: Å).
To gain insight into the mechanism of formation of quinoline derivatives, we carried out density
functional theory (DFT) calculation and the possible transition states were performed with
theoretical modeling in gas phase by using B3LYP functional and 6ꢀ31G(d)/LANL2DZ basis
sets in the Gaussian 09 program (Scheme 7 and Table S9ꢀS10 in the Supporting Information). In
3
6
the catalytic system being proposed in this manuscript, gold is the main reactive site. We chose
Au system (modeled as gold clusters) as a general catalytic system for monitoring the reaction
3
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2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
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4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
36
1
mechanism. The metallic gold has 6s configuration which generates a possibility of radical
catalysis. Thus, we performed DFT calculations on the transient electron transfer from the light
3
7
excited AuNPs surface to the reactant aniline molecule. As a result of interaction between
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
AuNPs and ortho C(sp )ꢀH bond of aniline (A), the bond becomes longer (from 1.09 to 1.89 Å)
in the transition stateꢀI (TSꢀI), which makes the cleavage of this bond much easier. The energy
of the photo excited electrons on AuNPs is high enough to activate the CꢀH bond which
3
8
facilitates the generation of AuNPs stabilized aniline radical intermediateꢀI (IMꢀI) (Scheme 7)
.
Metal catalyzed photocatalytic reactions are capable of achieving high activation energy as these
39
involve light as well as catalyst. The formation of radical is also experimentally supported by
complete inhibition of the reaction in the presence of TEMPO (vide supra). In the next step,
AuNPs interacted with πꢀelectron cloud of the alkyne (7a) and activate the alkyne. Subsequently,
both the intermediates IMꢀI and IMꢀII reacted to form CꢀC bonded productꢀB via transition
stateꢀII (TSꢀII) (Scheme 7). Next, the reaction involves insertion of CO into the intermediate B
to generate the intermediate C. Finally, annulation of the intermediate C furnished the desired
stable product quinoline carboxylate (8a) as shown in scheme 7.
2.2.3 Synthesis of Quinolines from aromatic aldehyde via CꢀH activation catalyzed by 4:Auꢀ
Fe O nanohybrid materials: In the next part of our work, we investigated the scope of
3
4
supramolecular ensemble 4:AuꢀFe O nanohybrid materials in one pot multicomponent reaction
3
4
for the synthesis of a quinoline derivatives. The three component reaction between aniline 6a,
methyl propiolate 7a and benzaldehyde 10a was chosen as the model reaction (Scheme 8). The
reaction was carried out in the presence of 0.5 mol% of catalyst under photocatalytic conditions.
The reaction took 5 h to provide the target product 11a in 80% yield (Table 5, entry 1). To
examine the scope of the CꢀH activation by in situ generated catalytic system with respect to
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aldehydes, we carried out the reaction between anilines (6a), methyl propiolate (7a) and a variety
of aromatic aldehydes bearing electronꢀdonating groups (10bꢀc). To our pleasure, all the
reactions proceeded very well to furnish the corresponding products (11bꢀc) in excellent yields
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(
Table 5, entry 1ꢀ3).
AuꢀFe O NPs, hν
3
4
H O
2
6
a
10aꢀc
7a
11aꢀc
2
Scheme 8. C(sp )ꢀH activation reaction between substituted aniline 6a (1 mmol), methyl
propiolate 7a (1 mmol) and substituted benzaldehyde 10aꢀc (1 mmol) catalyzed by
polythiophene encapsulated AuꢀFe O NPs (0.5 mol% i.e., 300µl 4:AuꢀFe O in Water:THF,
3
4
3
4
7
:3) under photocatalytic condition (irradiation of 60 W tungsten filament bulb).
2
Table 5. C(sp )ꢀH activation reaction between substituted anilines (6ab, 1 mmol), methyl
propiolate (7a, 1 mmol) and substituted benzaldehyde (10aꢀc, 1 mmol) catalyzed by
polythiophene encapsulated AuꢀFe O NPs (0.5 mol% with respect to AuꢀFe O ) under
3
4
3
4
irradiation of 60 W tungsten filament bulb.
Entry
Reactant
Product
Time
Yield
1
7
80
10a
11a
2
3
8
9
75
76
1
0b
11b
1
0c
11c
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