Supported Palladium Nanoparticles-Catalyzed Synthesis of 3-Substituted 2-Quinolones from 2-Iodoanilines and Alkynes Using Oxalic Acid as C1 Source

Article abstract of DOI:10.1002/adsc.201801127

3-Aryl/alkyl-2-quinolones were synthesized employing microwave assisted multicomponent reaction of 2-iodoanilines, terminal alkynes and oxalic acid dihydrate ((CO₂H)₂ ? 2H₂O) under polystyrene supported palladium (Pd@PS) nanoparticles-catalyzed conditions. The use of a heterogeneous palladium catalyst was explored first time for 2-quinolone synthesis involving carbonylation reaction employing (CO₂H)₂ ? 2H₂O as a solid and bench stable carbon monoxide (CO) source. The reaction exhibited good substrate generality for 2-iodoanilines and alkynes with wide functional group tolerance and good regio-selectivity. The ligand free operation, recyclability of heterogeneous Pd@PS catalyst and use of bench stable C1 source are the invaluable merits of the protocol. (Figure presented.).

Full text of DOI:10.1002/adsc.201801127

Title: Supported Palladium Nanoparticles-Catalyzed Synthesis of 3-
Substituted 2-Quinolones from 2-Iodoanilines and Alkynes Using
Oxalic Acid as C1 Source
Authors: Vandna Thakur, Ajay Sharma, Yamini --, Nishtha Sharma,
and Pralay Das
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201801127
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10.1002/adsc.201801127
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Supported Palladium Nanoparticles-Catalyzed Synthesis of 3-
Substituted 2-Quinolones from 2-Iodoanilines and Alkynes
Using Oxalic Acid as C1 Source
Vandna Thakur,a,b Ajay Sharma,a Yamini,a Nishtha Sharmaa and Pralay Dasa,b*
a
Natural Product Chemistry and Process Development Division, CSIR-Institute of Himalayan Bioresource Technology,
Palampur-176061, (H. P.), India. pdas@ihbt.res.in, pralaydas1976@gmail.com.
b
Academy of Scientific & Innovative Research, New Delhi, India.
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
efforts have been devoted for the synthesis of variety
of substituted 2-quinolones employing both
traditional[6] and metal-catalyzed protocols.[7]
Transition metal-catalyzed carbonylative approaches
provide an alternative access to 2-quinolone synthesis.
Foremost, the homogeneous palladium-catalyzed
multicomponent carbonylative annulation of 2-
iodoaniline and terminal/internal alkynes using CO
gas have emerged as a key research area for
quinolones synthesis.[8] Larock and co-workers
Abstract: 3-Aryl/alkyl-2-quinolones were synthesized
employing microwave assisted multicomponent reaction
of 2-iodoanilines, terminal alkynes and oxalic acid
dihydrate ((CO2H)2.2H2O) under polystyrene supported
palladium (Pd@PS) nanoparticles-catalyzed conditions.
The use of a heterogeneous palladium catalyst was
explored first time for 2-quinolone synthesis involving
carbonylation reaction employing (CO2H)2.2H2O as a
solid and bench stable carbon monoxide (CO) source.
The reaction exhibited good substrate generality for 2-
iodoanilines and alkynes with wide functional group
tolerance and good regio-selectivity. The ligand free
operation, recyclability of heterogeneous Pd@PS
catalyst and use of bench stable C1 source are the
invaluable merits of the protocol.
reported
the
homogenous
Pd-catalyzed
multicomponent carbonylative annulation of N-
protected 2-iodoanilines with internal or terminal
alkynes using CO gas for 3/4-substituted 2-
quinolones, wherein, the reaction and product yields
depended on the nature of protecting group.[8b]
Recently, the similar approach was extended for the
synthesis of 2-quinolone based heterocycles.[8c] More
recently, benign and abundantly available CO2 gas
has also been used as CO surrogate for quinolone
synthesis employing transition metals like Pd,[9a]
Ag[9b] and Cu.[9c] Whereas, Mo(CO)6 is the only bench
stable CO surrogate applied in combination with
homogeneous Pd catalyst for 4-quinolones synthesis
under multicomponent carbonylative annulation of 2-
iodoanilines and terminal alkynes.[10]
Last few years we have been working in the area of
exploration of (CO2H)2.2H2O as in situ CO, CO2 and
H2 source and their applications in different
challenging area of organic synthesis under our
supported catalytic conditions.[11] The best
compatibility of the support (PS) was also
investigated to hold the (CO2H)2.2H2O over surface
making the combination successful for better
interaction with closely associated catalyst for fruitful
reactions.[12] In our continuous efforts to further
expand the role of bench stable (CO2H)2.2H2O as CO
source, we have targeted the synthesis of 3-aryl/alkyl-
2-quinolones using multicomponent reaction of 2-
iodoanilines, terminal alkynes and (CO2H)2.2H2O in
Keywords: Carbonylation; Quinolone ; Heterogeneous
catalyst ; CO surrogate; Microwave irradiation
Palladium-catalyzed carbonylation, a versatile and
indispensable tool for the synthesis of diverse
carbonyl compounds, is an emerging area of research
for industry and academia as well to synthesize fine
chemicals, natural products, heterocycles and other
valuable structural motifs.[1] Typically, carbonylation
reactions utilize CO gas as the major carbonyl
source,[2] but is less attractive due to its toxicity and
difficulties in handling as special equipments are
required to withstand the use of highly pressurized
gas in the reaction. Henceforth, the use of solid and
sustainable CO surrogates has garnered huge interest
in recent decades.[3] Recently, several different CO
surrogates
such
as
formic
acid,[4a,4b]
paraformaldehyde, N-formylsaccharin,[4c] 9-methyl-
9H-fluorene-9-carbonyl chloride[4d] have been
described to synthesize carbonyl compounds but their
application for quinolones still remain undervalued.
2-Quinolones are the versatile scaffolds present in
alkaloids and synthetic molecules exhibiting broad
spectrum of biological activities[5] and so far, ample
1
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10.1002/adsc.201801127
Advanced Synthesis & Catalysis
the presence of heterogeneous Pd@PS nanoparticles
(NPs) as catalyst under microwave irradiation
conditions. However, to the best of our knowledge,
this is the first such report using heterogeneous
palladium metal catalyst and (CO2H)2.2H2O as CO
source for the synthesis of 2-quinolones in a regio-
selective manner.
The Pd@PS catalyst was prepared using Amberlite
IRA 900 Cl- form resin as polystyrene support (PS)
and Pd(OAc)2 as metal precursor following reduction
deposition approach (supplementary information,
ESI) and well characterized by using powder X-ray
diffraction (pXRD), Scanning electron microscopy
(SEM), transmission electron microscopy (TEM), and
improve the reaction yield (Table 1, entry 22). But,
TBAI was preferred over Et3N as broader substrate
generality and synthesis of 3-substituted-2-
quinolones as major product with negligible quantity
of 4-quinolones detected in LC-MS along with
minimum by-products. Therefore, the solubility of
bases is major responsible factor for the reaction as
most of the screened organic bases participated well
in the reaction and gave significant yields. Other
palladium metal catalysts such as Pd(OAc)2, Pd/C
and Pd/Al2O3were also screened in the reaction and
gave the desired product 4a in 41,26 and 19% yields
Table 1. Optimization studies of reaction conditions for 3-
selected
area
electron diffraction (SAED)
aryl/alkyl-2-quinolones synthesisa
studies.[11,12] Under this study, the developed Pd@PS
catalyst was further applied in the carbonylative
annulation reaction for the synthesis of 3-substituted
2-quinolones.
To assess the optimum reaction conditions, 2-
iodoaniline 1a and phenylacetylene 2a were chosen
as model substrates. Initially, we targeted the reaction
of 1a, 2a and (CO2H)2.2H2O 3 using DBU as base in
the presence of Pd@PS in PEG-400:1,4-dioxane (1:1)
o
under MW irradiation for 1 h at 130 C, which
afforded the desired product 4a in 29% yield along
with the formation of formanilide as side product in
almost equal proportion, while 1a remains
unconsumed (Table 1, entry 1). The reaction in DMF
gave improved product yield, but was again restricted
by the formation of formanilide (Table 1, entry 2).
While using Et3N as base, improved the product yield
to 47% and traces of formanilide was observed in the
reaction (Table 1, entry 3). Further, increasing the
equivalency of Et3N and reaction time to 90 min. did
not affect the reaction yield. So, increasing the
catalyst loading to 5 mol% of Pd gave improved yield
(Table 1, entry 4), whereas further increase in catalyst
loading had marginal effect on the reaction. Then, the
reaction was screened at different temperatures where
o
reaction at 135 C gave 4a in 75% yield (Table 1,
o
entry 7), while raising the temperature to 140 C led
to the low product yield due to side product formation
(Table 1, entry 6). The different solvents were
screened to get optimal results (Table 1, entry 8-11)
and as evident, DMF was found to be the most
suitable solvent for the said reaction. Oxalic acid and
DMF was found to be the most suitable combination
for the reaction because of the highly efficient
dissociation of oxalic acid in DMF was noticed.
Besides assisting the dissociation of oxalic acid,
DMF also has high boiling point than the
decomposition temperature of oxalic acid dihydrate
[a]Reaction conditions: 1a (50 mg, 0.228 mmol), 2a (62.5
µL, 0.57 mmol), oxalic acid dihydrate (114.91 mg, 0.912
mmol), Pd catalyst (as mentioned above), base (as
mentioned above), solvent (1 mL), under MW irradiations
for 1 h; bIsolated yields.
o
(>= 130 C), rendering the reaction feasible and
easier.
For
further
optimization,
both
organic/inorganic bases were screened, whereas,
inorganic bases were not found efficient to carry out
the reaction while among all, the organic bases such
as Et3N and tetrabutylammonium iodide (TBAI) were
found equally efficient for the said reaction (Table 1,
entry 12-23). The use of higher equivalency i.e. 3
equiv. of TBAI and longer reaction time did not
respectively but reaction proceeded with poor
selectivity for 4a (Table 1, entry 24, 25,26). To
establish the role of Pd@PS catalyst, the reaction in
absence of Pd@PS catalyst was carried out wherein
2
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10.1002/adsc.201801127
Advanced Synthesis & Catalysis
4a was not formed (Table 1, entry 27). Also, the
The effect of functional groups present on aromatic
reaction was not feasible in absence of TBAI (Table 1, alkynes for the synthesis of 2-quinolones was also
entry
28).
The
optimal
equivalency
of
studied in detail. The reaction of 2-iodoaniline with
3-ethynyl- and 2-ethynyltoluene gave the
phenylacetylene and oxalic acid dihydrate to obtain
maximum product yield was also screened (Table S1,
ESI). Finally, the reaction stated in Table 1, entry 21
was found to be the optimal condition wherein 2-
iodoaniline (1 equiv.), phenylacetylene (2.5 equiv.),
TBAI (2 equiv.), Pd@PS (5 mol%) and
corresponding products 4k-l in 63 and 47% yields.
The relatively lower yield of 4l can be attributed to
the steric hindrance at the ortho position of alkyne.
The reaction of 2-iodoaniline with 4-tert-
butylphenylacetylene gave the corresponding product
4m in 64% yield. The 4-OCH3/C2H5 substituted
alkynes also participated in the reaction to give 4n-o
in 58 and 56% yield respectively. The halogen
substituted aromatic alkynes were found reactive to
procure corresponding 2-quinolones 4p-r in good
yields. The aromatic alkynes comprising electron
withdrawing functional groups such as -CF3 and -CN
were also screened to give products 4s-t in moderate
yields. The 3-alkyl-2-quinolones 4u-v were prepared
in moderate yields using corresponding alicyclic
alkynes that is 1-ethynylcyclohexene and 1-
ethynylcyclohexane. The aliphatic alkynes also
participated well in the reaction to afford 4w-x in 54-
57% yields (Table 2). The formanilide 5a was
obtained up to 10% yield as side product in case of
aliphatic alkynes. Moreover, the formation of 4-
substituted regio-isomer for 4b-x was not detected.
o
(CO2H)2.2H2O (4 equiv.) in DMF at 135 C gave the
highest yield (73%) of regio-selective product 4a.
The application of various other CO surrogates has
also been screened so as to compare the efficiency of
all C1 sources under the optimized reaction
conditions (Table S2, ESI). The study established
(CO2H)2.2H2O as most efficient C1 source in
presence of heterogeneous Pd@PS catalyst for the
synthesis of 3-substituted-2-quinolones (Table S2).
Further, the versatility of the reaction was
determined with variety of substrates (Table 2). The
reaction was feasible with various substituted 2-
iodoaniline derivatives. All 4-Cl, 5-Cl, 4-Br, 4-F, 4-
CH3, 4-COOCH3, 4-CN substituted 2-iodoanilines
participated well in the reaction with phenylacetylene
or 4-ethynyltoluene to give corresponding 2-
quinolones derivatives 4b-j in good to moderate
yields. We have also subjected 2-bromoaniline and 2-
chloroaniline under the same catalytic conditions.
However, in case of 2-bromoaniline we ended up
with complex reaction mixture while no reaction
occurred with 2-chloroaniline.
The molecular structure of 4a was also confirmed
using X-ray crystallography (Figure 1).
Table 2. Substrate scope for the regio-selective synthesis
of 3-aryl/alkyl-2-quinolonesa
Figure 1. The molecular structure of 4a determined by X-
ray crystallography.[13]
A set of control experiments was performed to
elucidate the reaction pathway. Wherein, the reaction
of 6 under the standard conditions didn’t give the
desired product 4a which effectively ruled out the
possibility of Sonogashira coupling and subsequent
carbonylative annulation reaction (Scheme 1, entry
‘a’). As per literature reports, (CO2H)2.2H2O is
known to decompose into CO and CO2, hence, to
discover the fate of CO2 molecule, we performed the
reaction of 1a and 2a with CO2 (dry ice) under
standard conditions and obtained the final product 4a
in 34% yield which led to the conclusion that CO and
CO2 both can participate in the carbonylation reaction
under this condition (Scheme 1, entry ‘b’). Similarly,
reaction of 1a was also performed with 1-octyne (2x)
using CO2 (dry ice) to procure 4x in 31% yields
(Scheme 1, entry ‘c’). The reaction of 7 with 2a
under optimized conditions gave the product 4a in
54%yield but the formation of 8 was not detected
(Scheme 1, entry ‘d’). Likewise, when 9 was treated
with 2a under standard reaction conditions gave 4a in
[a]Reaction conditions: 2-iodoaniline derivatives (1 equiv.),
terminal alkyne (2.5 equiv.), oxalic acid dihydrate (4
equiv.), TBAI (2 equiv.) and Pd@PS (5 mol% of Pd) in 1
mL of DMF under MWI for 1 h at 135 oC; Isolated yields.
3
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10.1002/adsc.201801127
Advanced Synthesis & Catalysis
traces but the formation of 10 was not observed
may follow a minor pathway, wherein, the amine
group of intermediate (III) may be formylated and
which in turn interacts with Pd metal to generate
intermediate (VI). The final annulation step leads to
the formation of desired product (4) and regeneration
of the catalyst (Scheme 2).
(Scheme 1, entry ‘e’). This implies that
Scheme 1. Set of control experiments to understand the
reaction mechanism
Scheme 2. The plausible mechanism of Pd@PS-catalyzed
multicomponent carbonylative annulation for 2-quinolone
synthesis
Pd@PS (5 mol%)
X
a)
+
(COOH)2.2H2O
TBAI, DMF
N
O
NH2
135 oC, 1 h, MW
H
6
4a
(not detected)
I
Pd@PS (5 mol%)
+
b)
c)
CO2
CO2
+
TBAI, DMF
135 oC, 1 h, MW
NH2
N
H
O
O
1a
1a
2a
2x
4a (34%)
5
I
Pd@PS (5 mol%)
+
5
+
TBAI, DMF
135 oC, 1 h, MW
N
H
NH2
4x (31%)
(COOH)2.2H2O
I
Pd@PS (5 mol%)
d)
+
+
TBAI, DMF
135 oC, 1 h, MW
N
O
NHCHO
N
O
H
CHO
7
2a
4a (54%)
8 (not detected)
(COOH)2.2H2O
I
Pd@PS (5 mol%)
+
+
e)
TBAI, DMF
135 oC, 1 h, MW
N
O
N
H
O
NHTs
Ts
9
2a
4a (traces)
10 (not detected)
I
Pd@PS (5 mol%)
f)
+
TBAI, DMF
135 oC, 1 h, MW
N
O
NHCHO
H
7
2a
4a (5-7%)
Moreover, in pathway ‘b’, the CO2 generated from
(CO2H)2.2H2O interacts with amine moiety of
vinylpalladium intermediate (III) to afford (VII)
which in presence of base gave intermediate (VIII)
which is also supported by control experiments
(Scheme 1, ‘b and c’). The intermediate (VIII)
afforded the final product (4) following the
nucleophilic attack of vinyl group at the carbon of
isocyanate group and regenerate the Pd@PS.
deprotection of compound 7 and 9 might have
occurred before it participated in the reaction.
Whereas, the reaction of 7 with 2a in absence of
(CO2H)2.2H2O and under standard reaction
conditions gave 4a in traces (5-7% yield).
Based on the control experiments and literature
reports, we propose the possible reaction mechanism,
wherein, both CO and CO2 (decomposition products
of (CO2H)2.2H2O) participates in the reaction as
carbonyl source (Scheme 2). Also, the insertion of
alkyne bond to Pd-C bond is preferred over CO
insertion and is a decisive point in the course of
reaction for 3-substituted 2-quinolones synthesis. The
Pd@PS undergoes oxidative addition with 2-
iodoaniline (I) to procure arylpalladium intermediate
(II). The insertion of alkyne to intermediate (II)
results in the formation of vinylpalladium
intermediate (III). However, the intermediate (III)
might follow two pathways, ‘a’ or ‘b’. In pathway ‘a’,
the interaction of CO with palladium metal afforded
intermediate (IV) which follows insertion reaction to
give acylpalladium intermediate (V).Further, the
nucleophilic attack of amine moiety on the carbonyl
of acylpalladium intermediate afforded the desired
product 4 (3-substituted 2-quinolones). On the other
hand, based on the experiment (Scheme 1, ‘f’),
However, the intermediate (III) might follow two
pathways, ‘a’ or ‘b’. In pathway ‘a’, the interaction of
CO with palladium metal afforded intermediate (IV)
which follows insertion reaction to give
acylpalladium intermediate (V). Further, the reaction
Moreover, the Pd@PS was recycled up to four
times with no substantial loss of activity (ESI). The
ICP-AES analysis of reaction mixture was performed
for 2nd and 4th cycle but no significant Pd metal
leaching (<0.01 ppm) was detected (ESI). The TEM
studies of Pd@PS after 4th cycle showed notable
increase in Pd NPs size which might be the potential
cause of loss of catalytic activity (ESI). Besides, the
heterogeneity of Pd@PS in the reaction was
established using the mercury poisoning test which
gave 4a in traces only (ESI). To see that the solution
phase Pd is not responsible for the reaction, we have
removed Pd@PS through hot filtration from reaction
mixture after 0.5 h and the reaction mixture further
continue for another 0.5 h, but no progress in
reactivity was noticed (ESI).
In
conclusion,
we
have
developed
a
multicomponent approach involving 2-iodoaniline
derivatives, terminal alkynes, and (CO2H)2.2H2O as
bench stable CO source under microwave irradiation
for the synthesis of regio-selective 3-substituted 2-
quinolones. The detailed mechanistic evaluation
established the role of CO itself and CO2 as CO
4
This article is protected by copyright. All rights reserved.

10.1002/adsc.201801127
Advanced Synthesis & Catalysis
source in the reaction. The use of recyclable
heterogeneous catalyst, bench stable CO surrogate,
microwave irradiations, easy handling of reaction and
regio-selectivity of products are the major highpoints
of the protocol.
A. Ishii, M. Kono, F. Suzuki, J. Med. Chem. 1992, 35,
4893-4902.
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Experimental Section
Typical Experimental procedures for the synthesis of 2-
quinolones
2-Iodoaniline 1a (50 mg, 0.228 mmol), oxalic acid
dihydrate (114.66 mg, 0.91 mmol), tetrabutylammonium
iodide (168.43 mg, 0.456 mmol), Pd@PS (256 mg, 5
mol% of Pd) were taken in 1.5 mL DMF in a MW tube.
Then, phenylacetylene (62.60 µL, 0.57 mmol) was added
to the reaction mixture. The resultant reaction mixture was
[8] a) M. Genelot, V. Dufaud, L. Djakovitch, Tetrahedron
2011, 67, 976-981; b) V. Dmitry, Kadnikov, C. L.
Richard, J. Org. Chem. 2004, 69, 6772-6780; c) X.
Zhang, H. Liu, Y. Jia, Chem. Commun. 2016, 52, 7665-
7667; d) A. C. Tadd, A. Matsuno, M. R. Fielding, M. C.
Willis, Org. Lett. 2009, 11, 583-586.
o
treated under MW irradiation at 135 C, 60 W for 1 h. The
reaction was monitored by TLC. The reaction mixture was
cooled to ambient temperature and quenched with water
and then extracted using ethyl acetate (4X5 mL). The
resulting organic layer was treated with anhy. Na2SO4 and
dried under reduced pressure. The corresponding 3-
phenylquinolin-2(1H)-one 4a was obtained in 37 mg, 73%
yield as off white solid after purification by column
chromatography on silica gel (60-120 mesh) using
Hexane:EtOAc (75:25) as eluent.
[9] a) S. Sun, W. -M. Hu, N. Gu, J. Cheng, Chem. Eur. J.
2016, 22, 18729-18732; b) T. Ishida, S. Kikuchi, T.
Yamada, Org. Lett. 2013, 15, 3710-3713; c) C.-X. Guo,
W.-Z. Zhang, S. Liu, X. -B. Lu, Catal. Sci. Technol.
2014, 4, 1570-1577.
[10] a) L. Åkerbladh, P. Nordeman, M. Wejdemar, L. R.
Odell, M. Larhed, J. Org. Chem. 2015, 80, 1464-1471;
b) J. R. Chen, J. Liao, W. J. Xiao, Can. J. Chem. 2010,
88, 331-337.
Acknowledgements
Authors are grateful to Director CSIR-IHBT for providing
necessary facilities during the course of the work. The authors
thank DST Nano Mission Project (SR/NM/NS-1340/2014) for
financial support. VT, AS, Y and NS thank UGC and CSIR New
Delhi for awarding fellowships.
[11] a) N. R. Guha, V. Thakur, D. Bhattacherjee, R. Bharti,
P. Das, Adv. Synth. Catal. 2016, 358, 3743-3746; b) R.
Bharti, C. B. Reddy, P. Das, Chemistry Select, 2017, 2,
4626-4629; c) A. K Shil, S. Kumar, C. B. Reddy, S.
Dadhwal, V. Thakur, P. Das, Org. Lett. 2015, 17,
5352-5355.
#IHBT communication No.4273
[12] a) C. B. Reddy, R. Bharti, S. Kumar, P. Das, RSC
Adv. 2016, 6, 71117-71121; b) V. Thakur, S. Kumar, P.
Das, Catal. Sci. Technol. 2017, 7, 3692-3697; c) V.
Thakur, A. Kumar, N. Sharma, A. K. Shil, P. Das, Adv.
Synth. Catal. 2018, 360, 432-437.
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5
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10.1002/adsc.201801127
Advanced Synthesis & Catalysis
Supported Palladium Nanoparticles-Catalyzed
Synthesis of 3-Substituted 2-Quinolones from 2-
Iodoanilines and Alkynes Using Oxalic Acid as
C1 Source
Adv. Synth. Catal. Year, Volume, Page – Page
Vandna Thakur,a,b Ajay Sharma,a Yamini,a Nishtha
Sharmaa and Pralay Dasa,b*
6
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