Integration of Pd and Cu on polymer: a powerful bimetallic heterogeneous catalyst for sequential synthesis of quinoxalines

Article abstract of DOI:10.1007/s11164-019-03918-w

Pd²⁺/Cu²⁺ bimetallic catalysts immobilized on ethylenediamine-functionalized poly(vinyl chloride) (Pd^II/Cu^II@EDA-PVC) were developed by combining two metallic catalysts that complement rather than inhibit one another. The synthesized Pd^II/Cu^II@EDA-PVC were screened for their catalytic activity and found to be excellent in the one-pot sequential synthesis of quinoxalines from 1,2-diphenylacetylenes and o-phenylenediamines. Both Pd²⁺ and Cu²⁺ are indispensable and play crucial roles in this transformation. This one-pot sequential reaction proceeds well and tolerates various available substrates to form the desired product in excellent yield. A plausible mechanism was proposed for this conversion.

Full text of DOI:10.1007/s11164-019-03918-w

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-019-03918-w
Integration of Pd and Cu on polymer: a powerful
bimetallic heterogeneous catalyst for sequential synthesis
of quinoxalines
Wei Jiang1 · Wei Sun1 · Yuemin Zhou1 · Yiqun Li1
Received: 24 April 2019 / Accepted: 20 June 2019
© Springer Nature B.V. 2019
Abstract
Pd2+/Cu2+ bimetallic catalysts immobilized on ethylenediamine-functionalized
poly(vinyl chloride) (PdII/CuII@EDA-PVC) were developed by combining two
metallic catalysts that complement rather than inhibit one another. The synthe-
sized PdII/CuII@EDA-PVC were screened for their catalytic activity and found to
be excellent in the one-pot sequential synthesis of quinoxalines from 1,2-diphenyla-
cetylenes and o-phenylenediamines. Both Pd2+ and Cu2+ are indispensable and play
crucial roles in this transformation. This one-pot sequential reaction proceeds well
and tolerates various available substrates to form the desired product in excellent
yield. A plausible mechanism was proposed for this conversion.
Keywords Alkynes · o-Phenylenediamines · Quinoxaline · Pd/Cu bimetallic
catalysts · Sequential reaction
Introduction
Quinoxalines are one important class of nitrogen-containing heterocyclic com-
pounds with interesting applications in organic chemistry, pharmaceuticals, agri-
culture, and advanced materials [1–5]. In light of their usefulness, the develop-
ment of efcient and practical synthetic approaches for quinoxalines has attracted
signifcant attention, and a number of catalytic synthetic methods have been
documented [6]. Among these, the common method is the direct condensa-
tion of 1,2-dicarbonyl compounds with o-phenylenediamine in the presence of
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
4-019-03918-w) contains supplementary material, which is available to authorized users.
*
Yiqun Li
tlyq@jnu.edu.cn
1
Department of Chemistry, Jinan University, Guangzhou 510632, Guangdong,
People’s Republic of China
Vol.:(0123456789)
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W. Jiang et al.
Bronsted acid and Lewis acid catalysts (Scheme 1, path a). Examples of Bron-
sted acids include sulfamic acid [7], citric acid [8], H6P2W18O62 [9], protic ionic
liquids [10], polyaniline-sulfate salt [11], 2-iodoxybenzoic acid (IBX) [12],
Montmorillonite K-10 [13], etc. The employed Lewis acids comprise I2 [14–16],
InCl3 [17], MnCl2 [18], CuSO4 [19], Zn[(L)proline] [20], CAN [21], Ga(OTf)3
[22], sulfonated nanoClay [23], and so on. Another variant to develop quinoxa-
lines involved the condensation of α-hydroxy ketone with o-phenylenediamine
via a one-pot tandem oxidation and annulation process (Scheme 1, path b). A
variety of catalysts and oxidants such as MnO2 [24, 25], HgI2 [26], Pd(OAc)2
[27], RuCl2(PPh3)3 in diglyme [28], FeCl3/morpholine [29], silica gel coupled
with MWI [30], and N-Heterocyclic carbene (NHC) [31] have been employed
for this transformation. These documented protocols have their own merits and
disadvantages, however most employed substrates are confned to 1,2-diketones
or α-hydroxy ketones so they lead to quinoxalines with poor functional group
diversity. Recently, several synthetic methods have been reported for the one-pot
sequential synthesis of quinoxalines by use of commercially available alkynes
and o-phenylenediamines as starting materials should be one of the most ef-
cient and economic strategies toward the formation of quinoxalines (Scheme 1,
(a)
(b)
(c)
Scheme 1 Synthetic strategies for the synthesis of substituted quninoxalines
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Integration of Pd and Cu on polymer: a powerful bimetallic…
path c). This type of one-pot sequential reaction combines a two-step reaction in
a single vessel that has the advantages of reducing the synthesis route, simpli-
fying the operation, promoting the product outcome, and decreasing the waste
output as well. Various oxidants and catalytic systems, including NBS [32],
Cu2+/Al3+ [33], PhI(OCOCF3)2/I2 [34], PdCl2/CuCl2/PEG [35], PdI2/DMSO [36],
[AuCl(IPr)]/AgNTf [37], [Ru(cymene)Cl2]2/TEMPO [38], Au(I) [39], I2/DMSO
[14–16], PhI(OAc)2 [40], NH4I/EtOCS2K [41], and others have been reported for
the preparation of this type of compound. However, some of these procedures
still have several drawbacks such as the use of precious metal catalysts, need of
strong oxidants, generation of toxic waste, and conduction under harsh reaction
conditions. Moreover, the yields are usually unsatisfactory in some cases. Con-
sequently, the development of a simple, convenient, and general method for the
one-pot sequential synthesis of quninoxalines is still a challenge from commer-
cially available alkynes and o-phenylenediamines.
The above documented methodologies were brought to our attention and
inspired us to develop an afordable alternative catalytic system for this conver-
sion. We envisioned that a novel heterogeneous PdII/CuII dual catalyst merged
the preponderance of both Pd2+ and Cu2+ catalytical properties that would def-
nitely be of potential interest to promote one-pot two-step sequential reactions of
alkynes with o-phenylenediamines for the synthesis of quninoxalines (Scheme 1,
path c). In this integrated heterogeneous bimetallic catalyst, the combined Pd2+/
Cu2+ catalytic species can greatly activate the C≡C bond of the alkyne [42] to
facilitate the conversion of diphenylacetylene into dikentones via Wacker-type
oxidation [43], as well as it can efciently mediate the subsequently condensa-
tion of dikentones formed in situ and o-phenylenediamines. Poly(vinyl chloride)
(PVC) is one of the cheapest and most commercially available synthetic poly-
mers which is used extensively in the academic and industrial communities. It
is well known that PVC which possesses chlorine on its molecular backbone is
easily functionalized by replacement of chlorine with nucleophilic groups [44].
Palladium and copper can form stable complexes with a wide variety of organic
ligands with P, N, O, and S atoms because of its pronounced coordination proper-
ties. PVC-EDA bearing –NH2 groups on its molecular skeleton, PVC-EDA pos-
sesses the potential to act as an excellent ligand to chelate Pd2+ and Cu2+, which
makes it a suitable polymeric carrier for immobilization of bimetallic catalyst.
Our group has been interested in utilizing efcient, heterogeneous catalysts based
on the functionalized PVC for many catalysis applications and reported PVC-
EA-Pd0 [45], PVC-EDA-SA-Pd0 [46], PVC-EDA-CuII [47], PVC-EDA-CeIII [48]
and their applications in Suzuki couplings, Heck reactions, CuAAC reaction, and
multicomponent reactions.
On the basis of the described properties and our previous works, we herein
wish to advocate the successful attempt on a PdII/CuII@EDA-PVC in which PVC
not only is serving as supporting material but also acting as a N-ligand to coor-
dinate to PdII/CuII bimetallic cations for one-pot consequential construction of
quinoxalines (Scheme 2). To the best of our knowledge, a PdII/CuII@EDA-PVC
bimetallic catalyst has not yet been used for this purpose.
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W. Jiang et al.
Scheme 2 Synthesis of quinoxaline derivatives catalyzed by PdII/CuII@EDA-PVC
Experimental
Chemicals and reagents
All chemicals were used directly as received. Melting points were recorded on
a X5 digital apparatus. The infrared spectrum of products is recorded using a
1
Perkin Elmer PE-1600-FTIR. The H NMR (300 MHz) and 13C NMR (75 MHz)
were run on a Bruker Avance spectrometer. Mass spectra were recorded on a Shi-
madzu GC MS-QP 1000 EX 85 apparatus. The known products were identifed
by comparison of their melting points and spectral data with those reported in the
literature. Progress of the reactions was monitored by TLC using silica gel HF
254 plates.
General procedure for one‑pot synthesis of quinoxalines
A mixture of 1,2-diphenylethyne (1 mmol), o-phenylenediamine (1 mmol),
DMSO (2 mL), and PdII/CuII@EDA-PVC (0.14 mmol) was charged into a sealed
vessel at 140 °C with vigorously stirring. The progress was tracked by TLC. Upon
the end of the reaction, the mixture was cooled to room temperature, extracted
with ethyl acetate (10 mL × 3) to separate the product. The organic fraction was
successively washed with dilute HCl, brine, and dried on anhydrous sodium sul-
fate. After evaporating the organic solvent, the solid residue was recrystallized by
a mixture of ethanol and water to give the pure product.
Result and discussion
Following the procedure of our previous work, PVC was partially converted into
aminated PVC (PVC-EDA) with the use of ethylenediamine at 80 °C. Complexation
of the as-prepared PVC-EDA with equivalent of PdCl2 and CuCl2 solution provides
the desired PdII/CuII@EDA-PVC bimetallic catalyst. The content of Pd and Cu was
determined to be 1.0 mmol/g and 0.005 mmol/g in the catalyst by ICP-AES. The
synthetic pathway is depicted in Scheme 3.
To explore the feasibility of our strategy, we began our investigation by exert-
ing 1,2-diphenylethyne and o-phenylenediamine as a model reaction in the presence
1 3

Integration of Pd and Cu on polymer: a powerful bimetallic…
Scheme 3 Schematic route for the preparation of PdII/CuII@EDA-PVC bimetallic catalysts
of diferent amounts of PdII/CuII@EDA-PVC at the range of room temperature to
140 °C in various solvents. The results are shown in Table 1. At the beginning, the
efect of the amount of PdII/CuII@EDA-PVC on the model reaction was investigated
in DMSO at 140 °C. As shown in Table 1, it was found that the yield of the desired
product was reached up to 95% when the model reaction was carried out in the pres-
ence of PdII/CuII@EDA-PVC (3 mol% Pd, 0.02 mol% Cu) (Table 1, entry 3). This
result encouraged us to optimize the dosage of PdII/CuII@EDA-PVC on the cata-
lytic performance. Increasing the amount of Pd/Cu from 3/0.02 to 4.5/0.03 mol%,
even to 6/0.04 mol% had no apparent improvement in yield at all (Table 1, entries
4–5). Lowering the amount of the Pd/Cu to 2/0.013 mol% still allowed the reac-
tion to proceed smoothly, decreasing the desired product in 91% yield respectively
(Table 1, entry 2). Further decreasing the amount of the Pd/Cu to 1/0.007 mol%,
the product yield dramatically dropped to 44% (Table 1, entry 1). Next, the efect of
reaction temperature was evaluated on the model reaction. Remarkably, the tempera-
ture had an apparent impact on the model reaction. The highest yield of the product
was obtained when the reaction was carried out DMSO at 140 °C (Table 1, entry 3),
and the yield of the product decreased from 95 to 43, 17, 11, and 0% as the tempera-
ture was decreased from 140 to 120, 100, 80, 60 °C and even to room temperature
(Table 1, entries 5–10). Finally, the used solvents were explored. Among the various
screened solvents, DMSO was identifed as most efective for the model reaction
under the catalytic system. Replacement of DMSO with other solvents such as DMF,
acetonitrile, 1,4-dioxane, THF, toluene, and dichloromethane resulted in no product
detected (Table 1, entries 11–16). Taken together, optimal conditions for this model
reaction were identifed by employing 1,2-diphenylacetylene (1 mmol), o-phenylen-
ediamine (1 mmol), and PdII/CuII@EDA-PVC (3 mol% of Pd and 0.02 mol% of Cu)
in DMSO at 140 °C, which yielded 95% of the desired product.
Upon the optimal reaction conditions in hand, we tested the scope of internal
alkynes and o-phenylenediamine. As shown in Table 2, a variety of 1,2-disubstituted
alkynes and o-phenylenediamine proceeded smoothly in this reaction to aford the
corresponding quinoxalines in excellent yields. It should be noted that both electron-
donating and electron-withdrawing substitution such as –OMe, –OEt, –Me, –Cl,
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W. Jiang et al.
Table 1 The screening of optimal reaction conditions
Entry
Solvent
Catalyst (mol%)
Tem. (°C)
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
CH3CN
Dioxane
THF
PhMe
CH2Cl2
PdII/CuII@EDA-PVC (1/0.007)
PdII/CuII@EDA-PVC (2/0.013)
PdII/CuII@EDA-PVC (3/0.02)
PdII/CuII@EDA-PVC (4.5/0.03)
PdII/CuII@EDA-PVC (6/0.04)
PdII/CuII@EDA-PVC (3/0.02)
PdII/CuII@EDA-PVC (3/0.02)
PdII/CuII@EDA-PVC (3/0.02)
PdII/CuII@EDA-PVC (3/0.02)
PdII/CuII@EDA-PVC (3/0.02)
PdII/CuII@EDA-PVC (3/0.02)
PdII/CuII@EDA-PVC (3/0.02)
PdII/CuII@EDA-PVC (3/0.02)
PdII/CuII@EDA-PVC (3/0.02)
PdII/CuII@EDA-PVC (3.2/0.015)
PdII/CuII@EDA-PVC (3.2/0.015)
140
140
140
140
140
120
100
80
60
r.t.
140
82 (refux)
100 (refux)
60 (refux)
110 (refux)
40 (refux)
10
8
8
7
6
10
10
10
10
10
10
10
10
10
10
10
44
91
95
95
94
43
17
11
0
0
0
0
0
9
10
11
12
13
14
15
16
0
0
0
Reaction conditions: 1a (1 mmol) and solvent (3 mL) were conducted in absence and presence of cata-
lytic amount of catalyst at specifc temperature in air
aIsolated yield
–NO2 on the aromatic ring of internal alkynes and o-phenylenediamine were toler-
ated in the reaction to give the corresponding products in good yields.
Recyclability of the catalyst was examined upon the model reaction. After com-
pletion of the reaction, the reaction mixture was extracted with ethyl acetate to sepa-
rate the product. The left catalyst was washed with EtOAc thoroughly, and reused
for the next run without additional treatment. The catalyst can be reused at least four
times with satisfying yield (Fig. 1).
To gain further insights into the reaction mechanism, some control experiments
have been explored (Scheme 4). Initially, the Wacker-type reaction of diphenylacety-
lene was performed in the absence and presence of CuII@EDA-PVC, PdII@EDA-
PVC, and PdII/CuII@EDA-PVC separately in DMSO under optimal conditions. As
can be seen from Scheme 4a, the Wacker-type reaction did not process under cata-
lyst-free conditions (Scheme 4a, i). Also, the CuII@EDA-PVC is null and void when
1 3

Integration of Pd and Cu on polymer: a powerful bimetallic…
Table 2 The PVC-EDA-Pd2+/Cu2+ catalyzed synthesis of quinoxalines
Time
(h)
Yield
(%)a
Entry
1
1,2-Diphenylethyne (1)
Damine (2)
Product (3)
6.5
91
80
(1a)
(2a)
(3aa)
(3ba)
2
3
8
(1b)
(2a)
8
81
97
(1c)
(1d)
(2a)
(2a)
(3ca)
(3da)
4
8
72
5
8
(1e)
(1f)
(2a)
(2a)
(3ea)
(3fa)
95
6
8
it was employed as a catalyst for this transformation (Scheme 4a, ii). When PdII@
EDA-PVC was used as a catalyst, the moderate yield (48%) of the corresponding
benzil was obtained (Scheme 4a, iii), which indicates PdII@EDA-PVC has certain
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W. Jiang et al.
Table 2 (continued)
Time
(h)
Yield
Entry
1,2-Diphenylethyne (1)
Damine (2)
Product (3)
(%)a
7
(
8
88
1g)
(2a)
(3ga)
8
9
10
10
6.5
83
74
(1h)
(2a)
(3ha)
(3ia)
(3ab)
(1i)
(2a)
(2b)
10
92
(1a)
6.5
6.5
99
11
(1a)
(2c)
(2d)
(3ac)
(3ad)
92
12
(1a)
13
14
6.5
95
97
(2e)
(2b)
(1a)
(1b)
(3ae)
(3bb)
8
1 3

Integration of Pd and Cu on polymer: a powerful bimetallic…
Table 2 (continued)
Time
(h)
Yield
(%)a
Entry
1,2-Diphenylethyne (1)
Damine (2)
Product (3)
93
15
8
8
(1e)
(1d)
(2b)
(3eb)
(3db)
16
97
(2b)
17
97
96
99
8
(1g)
(2b)
(3gb)
18
8
(1b)
(2e)
(3be)
19
3.5
(1j)
(2b)
(3jb)
Reaction conditions: 1 (1 mmol), 2 (1 mmol), PdII/CuII@EDA-PVC (3 mol% of Pd and 0.02 mol% of
Cu) in DMSO at 140 °C in air
aIsolated yield
catalytic performance. Remarkably, PdII/CuII@EDA-PVC aforded the highest yield
(97%) of the desired benzil under the standard conditions (Scheme 4a, iv). These
results clarify that both of the Pd(II) and Cu(II) species are indispensable and played
a critical role in the Wacker-type reaction. These fndings may contribute to the syn-
ergic efect of Pd(II) and Cu(II). [33, 35, 42, 43]. Subsequently, the condensation
of benzil and o-phenylenediamine was investigated (Scheme 4b). All the catalyst of
CuII@EDA-PVC, PdII@EDA-PVC, and PdII/CuII@EDA-PVC gave the desired qui-
noxaline in excellent yields.
A plausible mechanism based on the control experimental results and previous
literatures is outlined in Scheme 5. The Pd2+ combined with Cu2+ to form a Pd2+/
Cu2+-π-complex intermediate (I) should greatly activate C≡C bond of the alkyne
through coordination to generate the acceptors of DMSO [42]. The nucleophilic
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W. Jiang et al.
Fig. 1 The recyclability of catalyst
(a)
(b)
Scheme 4 Control experiments
1 3

Integration of Pd and Cu on polymer: a powerful bimetallic…
Scheme 5 Plausible synergic catalytic mechanism for the synthesis of quinoxalines
addition of DMSO to the as-generated π-complex intermediate (I) followed by the
release of Me2S and the addition of another DMSO produces a ketoalcohol-like
intermediate (II), which proceeds to aford the corresponding 1,2- diphenyldiketone
along with the Pd2+/Cu2+ bimetallic species and Me2S. Noticeably, the Pd2+/Cu2+-
π-complex leads to a more efcient nucleophilic attack of DMSO onto the acti-
vated C≡C bond of the alkyne than Pd2+ alone, thus afording more efcient alkyne
Wacker-type oxidation [43]. Subsequently, condensation reaction of 1,2-dikentone
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W. Jiang et al.
with o-phenylenediamine catalyzed by PdII/CuII of the catalysts afords the desired
quinoxaline.
Conclusion
In conclusion, we have developed a PdII/CuII@EDA-PVC bimetallic catalyst in a
simple and efcient way by the treatment of a solution of Pd and Cu salts with PVC-
EDA which could be easily achieved by a substitution reaction of PVC and EDA.
The experimental results demonstrate that PdII/CuII@EDA-PVC bimetallic cata-
lysts possess a good catalytic activity in the synthesis of a broad variety of quinox-
aline derivatives. This approach undergoes through one-pot sequential reaction of
Wacker-type oxidation of alkyne giving 1,2-dikentones, which are then condensed
with o-phenylenediamine smoothly afording substituted quinoxaline derivatives in
excellent yields. High reactivity, wide substrate scope, short reaction time, and reus-
ability of the catalyst are the merits of this practical catalytic process.
Acknowledgements We are grateful to the National Natural Science Foundation of China (No 21372099)
for fnancial support.
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