Pentacenequinone-stabilized silver nanoparticles: A reusable catalyst for the diels-alder [4 + 2] cycloaddition reactions

Article abstract of DOI:10.1021/acs.joc.5b02495

Silver nanoparticles (AgNPs) stabilized by aggregates of derivative 4 have been used as catalyst for the construction of synthetically and biologically important [4 + 2] cycloadducts at room temperature.

Full text of DOI:10.1021/acs.joc.5b02495

Article
pubs.acs.org/joc
Pentacenequinone-Stabilized Silver Nanoparticles: A Reusable
Catalyst for the Diels−Alder [4 + 2] Cycloaddition Reactions
Radhika Chopra, Kamaldeep Sharma, Manoj Kumar, and Vandana Bhalla*
Department of Chemistry, UGC Sponsored Centre for Advanced Studies-1, Guru Nanak Dev University, Amritsar, Punjab, India
S
* Supporting Information
ABSTRACT: Silver nanoparticles (AgNPs) stabilized by aggregates of derivative 4 have been used as catalyst for the
construction of synthetically and biologically important [4 + 2] cycloadducts at room temperature.
Thus, development of an efficient catalytic system for carrying
out DA cycloaddition reactions under mild and environ-
mentally benign conditions is still a challenge.
INTRODUCTION
■
Among various organic transformations, Diels−Alder (DA)
cycloaddition reactions for carbon−carbon bond formation are
ideal because of their 100% theoretical atom economy.1 Diels−
Alder cycloaddition reactions of electron rich dienes with
electron deficient carbonyl compounds proceed well, but for
electronically mismatched partners forcing conditions are
required.2−4 Under conventional conditions, DA reactions
require prolonged reaction time and heating at high temper-
ature which makes these reactions eco-unfriendly. The quest for
environmentally benign reaction conditions has stimulated
considerable research interest to develop new catalytic systems
to carry out DA reactions under environmentally mild
conditions.5−7 Consequently, over the past few years, various
aspects of DA reactions such as factors affecting rate of reaction
and catalysts have been extensively studied, and a variety of
catalytic systems have been developed. However, most of these
catalytic systems suffer from many limitations such as high cost
of catalyst, formation of side products, use of organic solvent,
and prolonged reaction time using high loading of catalyst
which actually decrease the economic and environmental
advantages of the methodology.8−11 Recently, ruthenium
complexes have been reported which harvest visible light to
carry out DA cycloaddition reactions.12−14 Though this
photocatalytic system contributes toward saving energy and
minimizing the undesirable side products, utilization of costly
and toxic ruthenium metal as a major component of
heterogeneous hybrid catalytic system and use of toxic
dichloromethane as solvent remain as big hurdles toward the
development of greener and sustainable chemical industry.
Recently, silica-supported AgNPs have been reported as
heterogeneous catalysts for the preparation of various DA
cycloadducts under thermal conditions using dichloromethane
(DCM) as solvent15 which, however, decreased the eco-friendly
advantage of the catalyst. Silver is known for its plasmonic
nature and size dependent catalytic properties.16 In this context,
we were interested in the development of a silver-based
photocatalytic system for DA reaction.
Our research work involves the development of supra-
molecular assemblies and their utilization as reactors and
stabilizers for the preparation of different types of metal NPs
which were then used as catalysts for carrying out various types
of organic transformations.17−21 Very recently, we developed
and utilized supramolecular aggregates of pentacenequinone
derivative 1 for the preparation of AgNPs having size in the
range of 20−25 nm (Figure 1). These AgNPs exhibited
Figure 1. Structure of pentacenequinone derivative 1.
Received: November 10, 2015
© XXXX American Chemical Society
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a
Scheme 1. Synthesis of Pentacenequinone-Based Derivative 4
a
Conditions: Pd(0), K2CO3(aq), 1,4-dioxane.
moderate photocatalytic efficiency in degradation of rhodamine
B under visible light illumination in the absence of any reducing
agent or additive.22 In continuation of this work, we were then
interested in the development of an efficient AgNPs based
photocatalytic system for carrying out DA reaction. Since small
sized NPs are known to exhibit better catalytic efficiency,23 in
the present investigation, we designed and synthesized
pentacenequinone derivative 4 having 4-formylphenyl groups
at 2,9 or 2,10 positions. We envisioned that the presence of
aldehyde groups at pentacenequinone derivative 4 will assist the
molecule to form a uniform layer around the silver ions during
the reduction process; hence, growth of NPs will be restricted
and thus will result in formation of AgNPs of smaller size.
Interestingly, derivative 4 formed fluorescent aggregates in
aqueous media due to its AIEE characteristics, and these
aggregates exhibited strong affinity toward Ag+ ions and served
as reactors and stabilizers for the preparation of AgNPs of
average size of 7 nm. Furthermore, in situ generated AgNPs
served as efficient photocatalyst for DA reactions between
diene and alkynes in the absence of any additive or Lewis acid
in aqueous media. Further, the DA cycloaddition reactions of
activated systems such as quinone derivatives in the presence of
derivative 4 stabilized AgNPs were complete in much shorter
time to furnish the target molecules in excellent yields at room
temperature. The photocatalytic/catalytic performance of in
situ generated AgNPs is found to be better than the other
systems reported in the literature (Table S1−S2, Supporting
Information).
The UV−vis spectrum of derivative 4 in THF exhibits two
absorption bands at 318 and 422 nm, respectively (Figure S1,
Supporting Information). Upon addition of water (60%, v/v)
to the THF solution of derivative 4, the absorbance of the
entire spectrum gradually increased with the appearance of a
level-off tail in the visible region (Figure S2, Supporting
Information), which suggests the formation of aggregates. The
solution of derivative 4 in THF is weakly emissive when excited
at 318 nm (Φf = 0.01). Upon addition of water fraction up to
60%, an increase in emission intensity is observed at 493 nm
(Φf = 0.34). Upon increase in water fraction up to 90%, a
decrease in emission intensity was observed (Figure S3,
Supporting Information). Usually compounds with AIEE
characteristics show this phenomenon as after the aggregation
only the molecules on the surface of the aggregates emit light
and contribute to the fluorescence intensity upon excita-
tion.25−28 The viscosity dependent fluorescence studies of
derivative 4 in DMSO/glycerol were carried out to verify the
AIEE phenomena (Figure S4, Supporting Information). As
expected, the solution of derivative 4 becomes more emissive
with increase in fraction of glycerol. Further, time-resolved
fluorescence studies of derivative 4 in H2O−THF (6:4) mixture
show a large decrease in the nonradiative decay constant (Knr)
from 6.61 × 109 s−1 to 1.42 × 109 s−1 (Figure S5 and Table S3,
Supporting Information). On the basis of viscosity dependent
studies and time-resolved fluorescence studies, we believe that
restriction to intramolecular motion is the main reason for
AIEE phenomenon. The transmission electron microscopy
(TEM) images of derivative 4 in H2O−THF (6:4) solvent
mixture show the presence of irregular-shaped aggregates
(Figure S6, Supporting Information). The dynamic light
scattering (DLS) analysis of derivative 4 in H2O−THF (6:4)
mixture indicates average size of aggregates in the range of 91−
122 nm (Figure S7, Supporting Information). The presence of
aldehyde groups prompted us to evaluate the binding behavior
of aggregates of derivative 4 toward different metal ions such as
Ag+, Al3+, Ba2+, Ca2+, Pd2+, Cd2+, Co2+, Cu2+, Fe3+, Fe2+, Hg2+,
K+, Li+, Mg2+, Na+, Ni2+, Pb2+, and Zn2+ as their perchlorate/
chloride/nitrate salt using UV−vis and fluorescence spectros-
copy (Figures S8−S11, Supporting Information). However, no
significant change was observed with these metal ions except
Ag+ ions. Upon gradual addition of Ag+ ions (100 equiv) to the
solution of derivative 4 in mixed aqueous media (H2O−THF,
6:4), a characteristic surface plasmon resonance (SPR) band
was observed at 454 nm in the UV−vis spectrum which
suggests the formation of spherical AgNPs (Figure S12,
Supporting Information). The intensity of this band increased
with time, and the rate constant for the formation of AgNPs
was found to be 1.08 × 10−4 s−1 (Figure S13 and PS13,
Supporting Information).29,30 On the other hand, complete
quenching of fluorescence emission was observed upon
RESULTS AND DISCUSSION
■
Suzuki−Miyaura coupling of dibromopentacenequinone 224
[Br2PQ (2,9 or 2,10)] with 4-formylphenylboronic acid 3
afforded derivative 4 in 61% yield (Scheme 1). The structure of
derivative 4 was confirmed from its spectroscopic and analytical
data (PS52-S54, Supporting Information).
The 1H NMR spectrum of derivative 4 showed four doublets
(4H, 2H, 4H, and 2H) at 7.95, 8.01, 8.07, and 8.27 ppm
corresponding to aromatic protons and four singlets (2H each)
at 8.39, 9.02, 9.06, and 10.13 ppm corresponding to aromatic
and aldehydic protons. The mass spectrum of derivative 4
showed a parent ion peak at m/z 517.1299 (M + H)+. The FT-
IR spectrum of derivative 4 showed absorption peaks at 1701
and 1674 cm−1 corresponding to carbonyl groups of aldehydic
and ketonic moieties, respectively. The distinct peaks in the
region of 1455−1616 cm−1 correspond to the stretching
vibrations of CC double bonds of aromatic rings of derivative
4. Further, the absorbance peaks at 2840 and 3051 cm−1
correspond to −CH2 and CH stretching vibrations,
respectively. These spectroscopic data corroborate the structure
4 for this compound.
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addition of Ag+ ions (0−100 equiv) to the solution of derivative
4 (H2O−THF, 6:4) (Figure S14, Supporting Information).
This quenching in fluorescence emission is attributed to the
binding of Ag+ ions with nanoaggregates of derivative 4. The
TEM image of aggregates of derivative 4 in the presence of Ag+
ions shows the presence of spherical AgNPs having average size
of 7 nm (Figure 2 and Figure S15, Supporting Information).
Table 1. Optimization of Reaction Conditions for AgNPs
Catalyzed DA Reaction of Phenyl Acetylene, 5a, with 2,3-
Dimethyl-1,3-butadiene, 6
S. no.
reaction conditions
DCM, RT
time (h)
yield
1
2
3
4
5
6
7
4
70%
−
30%
toluene, RT
24
24
24
20
12
1.5
toluene, 100 °C
EtOH, RT
−
45%
−
EtOH, 80 °C
EtOH: H2O (1:1), RT
EtOH: H2O (1:1), visible light
70%
Figure 2. HRTEM image of spherical AgNPs of derivative 4; scale bar
5 nm.
we planned to examine the catalytic efficiency of in situ
generated AgNPs as photocatalysts in DA reaction. For this, we
carried out the reaction of 5a and 6 in the presence of in situ
generated AgNPs in EtOH−H2O (1:1) solvent mixture under
visible light irradiation. A 60 W tungsten filament bulb was used
as the irradiation source, and the reaction vial was submerged in
a water bath to prevent the photoheating effect. Interestingly,
the reaction was complete in 1.5 h, and the desired product was
obtained in 70% yield (Table 1, entry 7). This result suggests
the possible application of derivative 4 stabilized AgNPs as a
photocatalyst in DA reaction. To understand the influence of
stabilizing agent on photocatalytic efficiency of AgNPs in the
DA reaction, we carried out model reaction using pentacene-
quinone derivative 1 stabilized AgNPs. In presence of 0.07
mmol of pentacenequinone derivative 1 stabilized AgNPs, the
reaction was complete in 4 h, and the desired product was
obtained in 65% yield (Table 2, entry 1).
The powder X-ray diffraction (XRD) studies of crystalline
residue obtained after evaporation of solution of 4 and Ag+ ions
show the presence of diffraction peaks located at 2θ values of
29.46, 43.34, 70.36, 75.06, and 78.04, which suggests the
formation of AgNPs (Figure S16, Supporting Information).31
To get insight into the mechanism of formation of AgNPs,
the solution of aggregates of derivative 4 containing AgNPs was
slowly evaporated. After several days, precipitates were
observed which were filtered and washed with THF. After
1
evaporation of THF, a residue was obtained whose H NMR
spectrum showed the upfield shift of aromatic signals, and a
new peak was observed at 11.45 ppm corresponding to proton
of carboxylic acid group (Figure S17, Supporting Information).
The FTIR studies of the residue showed the presence of a new
absorption band at 1776 cm−1 corresponding to carbonyl group
of carboxylic acid (Figure S18, Supporting Information). We
also carried out UV−vis studies of derivative 4 in the presence
of Ag+ ions in THF, but absorption spectra did not indicate any
absorption band corresponding to AgNPs (Figure S19,
Supporting Information). These studies highlight the role of
aggregates of derivative 4 in preparation of AgNPs. On the
basis of these results, we believe that upon addition of Ag+ ions
to the solution of aggregates of derivative 4, the Ag+ ions
interact with the aldehyde groups and get reduced to AgNPs.
On the other hand, the aldehyde groups of aggregates of
derivative 4 are oxidized to carboxylic acids which then stabilize
AgNPs (Figure S20, Supporting Information).32 Thus,
aggregates of derivative 4 having aldehyde groups serve as
reactors for the generation of AgNPs, and the oxidized species
of pentacenequinone derivative 4 having acidic groups serve as
stabilizers for AgNPs.
In the next part of our work, we planned to examine the
catalytic activity of in situ generated AgNPs in Diels−Alder
(DA) cycloaddition reaction. To evaluate the effect of solvents,
first, DA reaction of phenyl acetylene, 5a, with 2,3-dimethyl-
1,3-butadiene, 6, was carried out in different solvents such as
toluene, EtOH, DCM, and EtOH−H2O (1:1) solvents at room
temperature. The reaction did not proceed at room temper-
ature in all the solvents except DCM (Table 1, entry 1). The
desired product was obtained in lower yield upon heating the
reaction mixture at high temperature for a longer period (24 h)
in the presence of toluene and EtOH (Table 1, entries 3 and 5).
The AgNPs are known to absorb the visible light,33 and thus,
Table 2. Optimization of Reaction Conditions for AgNPs
Catalyzed DA Reaction of Phenylacetylene, 5a, with 2,3-
Dimethyl-1,3-butadiene, 6
S. no.
catalyst
time (h)
yield
1
2
derivative 1 stabilized AgNPs
4
7
65%
63%
citrate-stabilized AgNPs
Furthermore, we also carried out a model reaction in the
presence of citrate-stabilized AgNPs prepared by the reported
method.34 The reaction was complete in 7 h, and the desired
product was obtained in 63% yield (Table 2, entry 2). The
above studies highlight the importance of catalytic efficiency of
AgNPs stabilized by aggregates of pentacenequinone derivative
4.
We believe that higher catalytic activity of pentacenequinone
derivative 4 stabilized AgNPs is due to their small size, large
surface area, and spherical shape. In addition, aggregates of
derivative 4 also play an important role in bringing the reactant
molecules closer to catalytic sites, thus providing a facile
synthetic route to prepare DA cycloadducts.
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Thus, for carrying out further reaction, we choose 0.07 mmol
of pentacenequinone derivative 4 stabilized AgNPs and EtOH−
H2O (1:1) as solvent mixture. To check the scope of substrates,
we utilized the in situ generated AgNPs as catalyst in DA
reactions of various dienophiles having electron-withdrawing or
electron-donating groups (Table 3). All the reactions
proceeded well, and desired products were obtained in good
to excellent yields (PS26-S32, Supporting Information).
groups. Further, the efficiency of the in situ generated AgNPs
was tested using various amounts of catalyst loading for the
reaction between 5a and 6. With decrease in catalyst loading
from 0.07 to 0.033 mmol, the product was formed in almost the
same yield, but it took a longer time for completion (12 h).
Such an extremely low quantity of catalyst has never been used
successfully for DA reactions at room temperature before the
present study. The same reactions when carried out in the
absence of AgNPs furnished no product even after 24 h. To test
the reusability of these NPs, the DA cyclization between 5a and
6 was chosen as a model reaction. After completion of reaction,
the reaction mixture containing catalyst was subjected to the
next catalytic sequence by adding 5a and 6 to the reaction
mixture. The complete conversion of 5a to DA cycloadduct
occurs up to three cycles in the presence of derivative 4
stabilized AgNPs. Further, recyclability of AgNPs generated by
aggregates of derivative 4 was checked for the same reaction.
The oily product was extracted with pentane, and the aqueous
layer containing AgNPs was reused at least three times using
this recycling procedure for identical reactions (Figure S21,
Supporting Information). The yield of desired product
remained almost the same after three recycling reactions
(respectively 70%, 67%, 65%). To get insight into the
mechanism of photocatalytic DA reaction, we carried out DA
reaction between 5a and 6 in the presence of triethanolamine
(TEOA) as hole scavenger, and the reaction was complete in
1.5 h to afford desired cycloadduct in 62% yield. We also
Table 3. AgNPs Catalyzed Photocatalytic DA Reaction of
Various Alkynes with 2,3-Dimethyl-1,3-butadiene, 6
S. no.
X
yield
1
2
3
4
5
5a/7a = −Ph
5b/7b = −COOH
5c/7c = −COOC2H5
5d/7d = −CH2OH
5e/7e = −CH2Br
70%
83%
80%
64%
45%
The substrates bearing electron-withdrawing groups fur-
nished the desired products in excellent yields, and the reaction
conditions tolerated the presence of hydroxyl and carboxylic
Table 4. DA Cyclization Strategy Involving Quinones as Dienophiles and 2,3-Dimethyl-1,3-butadiene, 6, Utilizing Derivative 4
Stabilized AgNPs
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carried out the same reaction in the presence of CuSO4 as an
electron scavenger, and the desired product was obtained in
traces. These studies suggest that photogenerated electron pair
is essential to carry out the DA reactions.
Scheme 2. Proposed Reaction Mechanism for DA Reaction
Involving p-Benzoquinone as Dienophile
Further, we employed DA cyclization strategy using highly
activated dienophiles such as pyran and maleic anhydride.
Interestingly, in situ generated AgNPs exhibited good catalytic
efficiency in these reactions. The reactions were complete in 3
h at room temperature to furnish pyran and maleic anhydride
derivatives in good yield (PS33-S36 and Table S4, Supporting
Information). Encouraged by excellent catalytic activity of
derivative 4 stabilized AgNPs in DA reactions, we planned to
utilize in situ generated AgNPs as catalyst for carrying out
synthesis of various quinone derivatives. Quinone derivatives
are oxidative intermediates of natural polyphenols and widely
exist in medicinal plants or food. The conventional conditions
require prolonged heating of the reaction mixture to get the
desired products in good yields. However, in the presence of
0.07 mmol of AgNPs, the reaction between 2,3-dimethyl-1,3-
butadiene, 6, and p-benzoquinone, 8a, was complete in 4 h.
Further, to widen the scope of reaction, various quinones
(8a−8e) were subjected to the DA reaction with the diene 6 in
the presence of in situ generated AgNPs at room temperature
to furnish the desired products (9a−9e) in good yields (Table
4 and PS43-S51, Supporting Information). Interestingly, the
target products were purified by simple recrystallization with
EtOH without using any other purification technique.
To get insight into the mechanism of the reaction, we
selected the reaction between 2,3-dimethyl-1,3-butadiene, 6,
and p-benzoquinone, 8a, as the model reaction. The UV−vis
studies of reaction mixture of 6 and 8a in EtOH−H2O (1:1) in
the absence of catalyst showed two absorption bands at 221
and at 440 nm corresponding to diene 6 and dienophile 8a,
respectively. Upon addition of AgNPs, the band at 440 nm is
blue-shifted to 370 nm which suggests the interaction between
the substrate 8a and AgNPs. The absorption band at 370 nm
may be assigned to the adduct A formed between AgNPs and
substrate 8a. After 4 h, a new band corresponding to product 9a
appeared at 305 nm (Figure S22, Supporting Information).
Furthermore, we investigated the kinetic studies of the
reaction between 6 and 8a in the presence of in situ generated
AgNPs by UV−vis spectroscopy (Figures S23−S34, Supporting
Information). These studies suggest that rate of reaction
follows first-order kinetics with respect to diene 6. In case of
dienophile 8a, rate of reaction follows first-order kinetics at low
concentration (1 μM); however, saturation kinetics is observed
at high concentration. We also carried out kinetic measure-
ments of model reaction by using different concentrations of in
situ generated AgNPs, and a linear relationship was observed
between initial rate of reaction and concentration of in situ
generated AgNPs. On the basis of these studies, a plausible
reaction mechanism has been proposed for DA reaction
between diene 6 and dienophile 8a (Scheme 2). We propose
that AgNPs interact with p-benzoquinone and get activated by
acquiring a partial positive charge on their surface.35 Further,
the activated AgNPs bring diene and dienophile closer to each
other which results in the formation of cycloadducts via
concerted mechanism as shown in Scheme 2.
forms fluorescent aggregates in aqueous media. The aggregates
of derivative 4 serve as reactors and stabilizers for the
preparation of spherical AgNPs having size in the range of 7
nm which serve as an efficient and reusable photocatalysts/
catalysts in the DA cycloaddition reactions.
EXPERIMENTAL SECTION
■
General Experimental Methods. Instruments. General
experimental methods used in the present work were the same
as reported earlier by us.36 To record TEM images,
transmission electron microscopy was used. IR spectrum
(400−4000 cm−1) was taken on an infrared (IR) spectrometer
by using KBr pellets. Vibrational bands in IR spectra were
reported as follows: frequency in cm−1, intensity (s = sharp, w =
weak). To perform elemental analysis (C, H, N), a CHNS-O
analyzer was used. For carrying out photocatalytic experiments,
a tungsten filament bulb (60 W) was used as irradiation source.
FT-NMR (300 MHz) and FT-NMR (500 MHz) spectropho-
1
tometers were used to record H NMR spectra. Coupling
constants (J) and chemical shifts (δ) were expressed in Hz and
ppm, respectively. Further, multiplicities of signals such as
broad singlet, singlet, doublet, and multiplet were quoted as br
s, s, d, and m, respectively.
Quantum yield calculations. To determine fluorescence
quantum yield of derivative 4, optically matching solution of
diphenylanthracene (Φfr = 0.90 in cyclohexane) was used as
reference at an excitation wavelength (λex) of 318 nm. In order
to calculate quantum yield, the following equation was used:
Ns2
Nr2
D
s
1 − 10−ArLr
Φ = Φ ×
×
×
fs
fr
1 − 10−AsLs
D
r
Φfs and Φfr are the fluorescence quantum yields of sample
and reference, respectively. As, Ls, Ns, and Ds are the
absorbance, length of the absorption cells, refractive index,
and respective areas of emission of sample, respectively. Ar, Lr,
Nr, and Dr are the absorbance, length of the absorption cells,
refractive index, and respective areas of emission of reference,
respectively.
Synthesis of 2,9 or 2,10-Bis(4-formylphenyl)-6,13-penta-
cenequinone, 4. To a mixture of compounds 2 (0.30 g, 1.56
mmol) and 3 (0.52 g, 3.44 mmol) in 20 mL of 1,4-dioxane was
added 1 mL of aqueous solution of K2CO3 (1.72 g, 12.50
mmol) followed by addition of [Pd(PPh)3]4 (0.35 g, 0.47
mmol) under N2 atmosphere. The reaction mixture was
CONCLUSIONS
■
In summary, pentacenequinone derivative 4 having 4-
formylphenyl groups at 2,9 or 2,10 positions has been designed
and synthesized. Derivative 4 exhibits AIEE characteristics and
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refluxed for 24 h. After completion of the reaction, the excess
solvent was removed under reduced pressure, and the residue
so obtained was dissolved in DCM. The organic layer was
washed with water, dried over anhydrous Na2SO4, and removed
under reduced pressure to give a crude product which was
purified by column chromatography using chloroform/hexane
(80:20) as an eluent to afford derivative 4 (0.51 g in 61%
NMR (125 MHz, CDCl3, ppm) δ = 23.8, 28.9, 29.7, 38.8,
128.8, 130.9, 132.5; ESI-MS mass spectrum of compound 7e
showed a parent ion peak, m/z = 202.1938 [M + H]+.
Elemental analysis: Calcd for C9H13Br: C 53.75; H 6.52.
Found: C 53.73; H 6.51.
General Procedure for Synthesis of DA Cycloadducts (7f/
7g) Using in Situ Generated AgNPs. A mixture of activated
dienophiles 5f/5g (0.1 g, 1.0 equiv) and 2,3-dimethyl-1,3-
butadiene, 6 (4.0 equiv), were mixed together in 10 mL of
EtOH:H2O (1:1) solvent mixture in the presence of AgNPs
(0.07 mmol) . The reaction mixture was stirred at room
temperature for 3 h. After completion of the reaction, the
ethanol was removed under reduced pressure, and the
remaining aqueous solution was extracted using pentane (2 ×
10 mL). The organic layer was dried over anhydrous Na2SO4
and distilled off to yield the desired product (7f). In case of
reaction between 5g and 6, solid product (7g) was separated
out after 3 h, filtered, and recrystallized using ethanol for 3 h.
The aqueous layer containing AgNPs was used as such in the
next catalytic reaction.
1
yield); mp: >280 °C. H NMR (500 MHz, CDCl3, ppm) δ =
7.95 (d, 4H, J = 7.5 Hz), 8.01(d, 2H, J = 8.5 Hz), 8.07 (d, 4H, J
= 7.5 Hz), 8.27 (d, 2H, J = 8 Hz), 8.39 (s, 2H), 9.02 (s, 2H),
9.06 (s, 2H), 10.13 (s, 2H); m/z = 517.1299 [M + H]+;
Elemental analysis: Calcd for C36H20O4: C 83.71; H 3.90; O
12.39. Found: C 83.70; H 3.87; O 12.38. IR: (in cm−1)
3051(w), 2840(w), 1701(s), 1674(s), 1616(s), 1603(s),
1582(s), 1455(s). The 13C NMR spectrum of compound 4
could not be recorded due to its poor solubility.
Synthesis of AgNPs. A solution of compound 4 (10 μM, 3
mL) in H2O/THF (6/4, v/v) and AgNO3 in water (0.1 M, 30
μL) was stirred for 4 h at room temperature. After 4 h, the
color of the reaction mixture changed from colorless to greyish
black which indicated the formation of AgNPs. These AgNPs
were washed 4−5 times with distilled water to remove
unreacted AgNO3 and then were used as such for further
catalytic application.
General Procedure for Synthesis of DA Cycloadducts (7a/
7b/7c/7d/7e) Using in Situ Generated AgNPs. In a 25 mL
round-bottom flask (RBF), terminal alkynes 5a/5b/5c/5d/5e
(0.1 g, 1.0 equiv) and 2,3-dimethyl-1,3-butadiene, 6 (4.0 equiv)
were mixed in 10 mL of EtOH:H2O (1:1) solvent mixture in
the presence of AgNPs (0.07 mmol). The reaction mixture was
degassed under vacuum for 2−3 min and then irradiated with a
60 W tungsten filament bulb (0.24 W/cm2) from a distance of
10 cm to provide visible light for 75 min. The reaction
temperature was maintained at room temperature by putting
RBF in a water bath. After completion of the reaction, excess
EtOH was removed, and the remaining aqueous solution was
extracted with pentane (2 × 10 mL). The organic layer was
dried over anhydrous Na2SO4 and distilled off to yield the
desired product (7a/7b/7c/7d/7e). The aqueous layer
containing AgNPs was then used as such for further catalytic
reactions.
Compound 7f. (6,7-Dimethyl-3,4,4a,5,8,8a-hexahydro-2H-
1
chromene): (Colorless liquid, 0.15 g in 78% yield). H NMR
(300 MHz, CDCl3, ppm) δ = 1.40−1.43 (m, 4H), 1.90 (s, 6H),
2.39−2.46 (m, 4H), 2.81−2.83 (m, 1H), 3.62 (t, 2H, J = 3.8
Hz), 4.19−4.24 (m, 1H). 13C NMR (125 MHz, CDCl3, ppm) δ
= 19.0, 25.2, 29.7, 30.7, 31.9, 63.8, 94.4, 112.6; ESI-MS mass
spectrum of compound 7f showed a parent ion peak, m/z =
205.0989 [M + K]+. Elemental analysis: Calcd for C11H18O: C
79.46; H 10.91; O 9.62. Found: C 79.43; H 10.89; O 9.60.
Compound 7g.40 (5,6-Dimethyl-3a,4,7,7a-tetrahydroisoben-
zofuran-1,3-dione): (0.12 g in 65% yield) 1H NMR (300 MHz,
CDCl3, ppm) δ = 1.72 (s, 6H), 2.28 (d, 2H, J = 5.7 Hz), 2.46
(d, 2H, J = 5.7 Hz), 3.35 (t, 2H, J = 4.2 Hz).
General Procedure for Synthesis of DA Cycloadducts (9a/
9b/9c/9d9e) Using in Situ Generated AgNPs. In a 25 mL
round-bottom flask, quinones 8a/8b/8c/8d/8e (0.1 g, 1.0
equiv) and 2,3-dimethyl-1,3-butadiene, 6 (4.0 equiv), were
mixed in 10 mL of EtOH:H2O (1:1) solvent mixture in the
presence of AgNPs (0.07 mmol). After stirring at room
temperature for 4 h, the solid product (9a/9b/9c/9d/9e)
separated which was removed by filtration and recrystallized
with ethanol, and the remaining filterate containing AgNPs was
used as such in the next catalytic reaction.
Compound 7a.37 (4,5-Dimethylcyclohexa-1,4-dienyl)-
benzene): (0.056 g in 70% yield). 1H NMR (300 MHz,
CDCl3, ppm) δ = 1.64 (s, 6H), 2.52−2.68 (m, 4H), 5.75 (t, 1H,
J = 6.0 Hz), 7.43 (d, 2H, J = 6.0 Hz), 7.75−7.78 (m, 3H).
Compound 7b.38 (4,5-Dimethylcyclohexa-1,4-dienecarbox-
ylic acid): (0.180 g in 83% yield). 1H NMR (300 MHz, CDCl3,
ppm) δ = 1.67 (s, 3H), 1.70 (s, 3H), 2.87−2.88 (m, 4H), 7.08−
7.10 (m, 1H).
For the synthesis of DA cycloadducts 9d and 9e, reactants
(8d and 8e) were synthesized by reported methods41,42(PS37-
S42, Supporting Information).
Compound 8d. (6-Bromoanthracene-1,4-dione): (dark
1
green solid, 0.035 g in 45% yield). H NMR (300 MHz,
Compound 7c.39 (Ethyl 4,5-Dimethylcyclohexa-1,4-diene-
CDCl3, ppm) δ = 7.09 (s, 2H), 7.77(d, 1H, J = 8.7 Hz), 7.94 (d,
1H, J = 8.7 Hz), 8.24 (s, 1H), 8.54 (s, 1H), 8.61 (s, 1H); 13C
NMR (125 MHz, CDCl3, ppm) δ = 124.3, 127.8, 128.8, 131.6,
132.2, 133.1, 135.8, 140.1, 140.1, 184.4; ESI-MS mass spectrum
of compound 8d showed a parent ion peak, m/z = 288.9859
[M + H]+. Elemental analysis: Calcd for C14H7BrO2: C 58.57;
H 2.46; O 11.14 Found: C 58.55; H 2.45; O 11.12. M.P. =
240−242 °C.
1
carboxylate): (0.072 g in 80% yield). H NMR (300 MHz,
CDCl3, ppm) δ = 1.30 (t, 3H, J = 7.05 Hz), 1.65 (s, 3H), 1.68
(s, 3H), 2.82−2.83 (m, 4H), 4.21 (q, 2H, J = 7.05 Hz), 6.92−
6.94 (m, 1H).
Compound 7d.39 [(4,5-Dimethylcyclohexa-1,4-dienyl)-
1
methanol]: (0.079 g in 64% yield). H NMR (300 MHz,
CDCl3, ppm) δ = 0.92 (br s, 1H), 1.52−1.62 (m, 6H), 2.36−
2.42 (m, 2H), 2.52−2.58 (m, 2H), 3.76 (s, 2H), 5.50−5.56 (m,
1H).
Compound 8e. (6,7-Dibromoanthrace-1,4-dione): (yellow-
1
ish-green solid, 0.03 g in 40% yield). H NMR (300 MHz,
CDCl3, ppm) δ = 7.11 (s, 2H), 8.38 (s, 2H), 8.53 (s, 2H); 13
C
Compound 7e. (4-Bromo-1,2-dimethylcyclohexa-1,4-diene):
1
NMR (125 MHz, CDCl3, ppm) δ = 126.8, 127.7, 129.4, 134.2,
134.2, 140.1, 184.0; ESI-MS mass spectrum of compound 8e
showed a parent ion peak, m/z = 389.8685 [M + Na]+.
(Oily liquid, 0.038 g in 45% yield). H NMR (300 MHz,
CDCl3, ppm) δ = 1.63 (s, 3H), 1.68 (s, 3H), 2.37−2.39 (m,
2H), 2.48−2.50 (m, 2H), 3.66 (s, 2H), 5.86 (br s, 1H); 13C
F
DOI: 10.1021/acs.joc.5b02495
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
Elemental analysis: Calcd for C14H6Br2O2: C 45.94; H 1.65; O
8.74. Found: C 45.92; H 1.63; O 8.73. M.P. = 245−247 °C.
Compound 9a.43 (6,7-Dimethyltetrahydro-4a,5,8,8a-1,4-
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naphtoquinone): (0.15 g in 84% yield). H NMR (300 MHz,
CDCl3, ppm) δ = 1.63 (s, 6H), 2.04−2.10 (m, 2H), 2.36−2.42
(m, 2H), 3.16−3.21 (m, 2H), 6.65 (s, 2H).
Compound 9b.44 (1,4,4a,9a-Tetrahydro-2,3-dimethylanthra-
1
quinone): (0.12 g in 81% yield). H NMR (300 MHz, CDCl3,
ppm) δ = 1.64 (s, 6H), 2.11−2.17 (m, 2H), 2.43−2.45 (m,
2H), 3.35−3.38 (m, 2H), 7.73−7.76 (m, 2H), 8.03−8.06 (m,
2H).
Compound 9c.45 2,3-Dimethyl-1,4,4a,12a-tetrahydrotetra-
cene-5,12-dione): (0.11 g in 80% yield). 1H NMR (300
MHz, CDCl3, ppm) δ = 1.66 (s, 6H), 2.15−2.21(m, 2H),
2.47−2.54 (m, 2H), 3.39−3.43 (m, 2H), 7.66−7.69 (m, 2H),
8.05−8.08 (m, 2H), 8.60 (s, 2H).
Compound 9d. (8-Bromo-2,3-dimethyl-1,4,4a,12a-tetrahy-
1
drotetracene-5,12-dione): (green solid, 0.1 g in 76% yield). H
NMR (300 MHz, CDCl3, ppm) δ = 1.66 (s, 6H), 2.14−2.21
(m, 2H), 2.46−2.53 (m, 2H), 3.39−3.43 (m, 2H), 7.74 (d, 1H,
J = 8.7 Hz), 7.93 (d, 1H, J = 8.7 Hz), 8.23 (s, 1H), 8.50 (s, 1H),
8.57 (s, 1H); 13C NMR (125 MHz, CDCl3, ppm) δ = 18.9,
30.7, 47.3, 123.6, 123.8, 125.8, 127.6, 128.7, 131.4, 131.9, 132.7,
136.3, 198.0; ESI-MS mass spectrum of compound 9d showed
a parent ion peak, m/z = 370.0563 [M + H]+. Elemental
analysis: Calcd for C20H17BrO2: C 65.05; H 4.64; O 8.67.
Found: C 65.03; H 4.63; O 8.65. M.P. = 250−253 °C.
Compound 9e. (8,9-Dibromo-2,3-dimethyl-1,4,4a,12a-tetra-
hydrotetracene-5,12-dione): (brown solid, 0.09 g in 73% yield).
1H NMR (300 MHz, CDCl3, ppm) δ = 1.66 (s, 6H), 2.16−2.22
(m, 2H), 2.46−2.53 (m, 2H), 3.39−3.42 (m, 2H), 8.36 (s, 2H),
8.48 (s, 2H). 13C NMR (125 MHz, CDCl3, ppm) δ = 18.9,
30.6, 47.3, 123.5, 126.3, 127.6, 131.4, 134.0, 134.6, 197.5; ESI-
MS mass spectrum of compound 9e showed a parent ion peak,
m/z = 487.4008 [M + K]+. Elemental analysis: Calcd for
C20H16Br2O2: C 53.60; H 3.60; O 7.14. Found: C 53.58; H
3.59; O 7.11. M.P. = 260−262 °C.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.5b02495.
■
S
Characterization data, optical properties of derivative 4,
kinetic studies of catalytic reactions, NMR data of DA
cycloadducts, and tables showing comparison of photo-
catalytic/catalytic activity of present system over other
catalytic systems reported in the literature (PDF)
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AUTHOR INFORMATION
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Corresponding Author
*E-mail: vanmanan@yahoo.co.in.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
V.B. is thankful to the Science and Engineering Research Board
(SERB), New Delhi (ref no. EMR/2014/000149) for financial
support. R.C. is thankful to UGC (New Delhi) for a Junior
Research Fellowship (JRF). We are also thankful to UGC
(New Delhi) for the “University with Potential for Excellence”
(UPE) project.
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