Selective reductive annulation reaction for direct synthesis of functionalized quinolines by a cobalt nanocatalyst

Article abstract of DOI:10.1016/j.jcat.2020.01.034

Due to the extensive applications of quinolines, the search for selective construction of such products has long been an attractive subject in scientific community. Herein, by developing a new N-doped ZrO₂@C supported cobalt nanomaterial, it has been successfully applied as an efficient catalyst for the reductive annulation of 2-nitroaryl carbonyls with alkynoates and alkynones. The catalytic transformation allows synthesizing a wide array of funcitonalized quinolines with the merits of broad substrate scope, good functional group tolerance, excellent hydrogen transfer selectivity, reusable earth-abundant metal catalyst, and operational simplicity. The developed chemistry paves the ways for further design of hydrogen transfer-mediated coupling reactions by developing heterogeneous catalysts with suitable supports.

Full text of DOI:10.1016/j.jcat.2020.01.034

Journal of Catalysis 383 (2020) 239–243
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Selective reductive annulation reaction for direct synthesis of
functionalized quinolines by a cobalt nanocatalyst
⇑
Rong Xie, Guang-Peng Lu, Huan-Feng Jiang, Min Zhang
Key Lab of Functional Molecular Engineering of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou
510641, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Due to the extensive applications of quinolines, the search for selective construction of such products has
long been an attractive subject in scientific community. Herein, by developing a new N-doped ZrO2@C
supported cobalt nanomaterial, it has been successfully applied as an efficient catalyst for the reductive
annulation of 2-nitroaryl carbonyls with alkynoates and alkynones. The catalytic transformation allows
synthesizing a wide array of funcitonalized quinolines with the merits of broad substrate scope, good
functional group tolerance, excellent hydrogen transfer selectivity, reusable earth-abundant metal cata-
lyst, and operational simplicity. The developed chemistry paves the ways for further design of hydrogen
transfer-mediated coupling reactions by developing heterogeneous catalysts with suitable supports.
Ó 2020 Elsevier Inc. All rights reserved.
Received 10 December 2019
Revised 29 January 2020
Accepted 29 January 2020
Keywords:
Heterogeneous catalysis
Transfer hydrogenation
Reductive annulation
Functionalized quinolines
Cobalt
1. Introduction
to access ester substituted quinolines. As shown in Scheme 1, the
catalytic transfer hydrogenation (TH) of the nitro group of 2-
Quinolines constitute a class of significantly important N-
heterocycles, and they are extensively distributed in functional
materials, marketing drugs [1–5], biologically active agents (e.g.,
anti-HIV, antimalarial, and anti-inflammation,), etc [6–8]. In gen-
eral, quinolines are prepared by the well-established names reac-
tions (i.e. Doebner-von Miller [9], Skraup [10,11], Doebner [12],
Combes [13], Friedländer [14–16], etc.). In recent years, various
approaches have also been nicely developed to achieve the related
end [17–21]. Among them, the cyclization of 2-aminoaryl car-
bonyls and alkynoates offers a valuable way to access ester substi-
tuted quinolines that are used frequently for the discovery of
bioactive and therapeutical products [22,23]. However, the related
transformations generally suffer from difficult catalyst reusability,
low chemoselectivity due to the occurrence of side reactions
induced by the active 2-aminoaryl carbonyls [24,25]. In this con-
text, the search for direct and selective synthesis of ester-linked
quinolines with easily available catalysts, preferably reusable ones,
still remains a higly demanding goal.
nitroaryl carbonyl 1 initially generates the 2-aminoaryl carbonyl
10. Then, the hydroamination of 10 with alkynoate 2 affords enam-
ine A. Finally, the quinoline product 3 is afforded via intramolecu-
lar nucleophilic addition (B) followed by proton-promoted
dehydration.
However, it is important to note that, to achieve the above syn-
thetic purpose, at least two challenging issues have to be
addressed: (1) There should be a compatible catalyst system to
ensure that, among various reducible functionalities (nitro, alky-
nyl, carbonyl groups) in the electron-deficient substrates (1 and
2), the transfer hydrogenation selectively occurs on the nitro
group. (2) The in situ generated active 2-aminoaryl carbonyls 10
should be timely consumed up by its hydroamination (A) with
alkynoates 2, thus suppressing the occurrence of undesired side
reactions such as homo-condensation.
Enlightened by the capability of cobalt in reducion of unsatu-
rated chemical bonds [29–32], we believed that the design of a
suitable supported heterogeneous cobalt catalyst might offer a
solution to achieve the above synthetic goal. On one hand, the
cobalt metal has preferential interaction with electron-rich het-
eroatoms, which would result in unique transfer hydrogenation
selectivity toward the nitro group among different reducible func-
tionalities of substrates 1 and 2. On the other hand, the supporting
materials or dopants introduced drastically affect the morphology
and surface chemistry of the catalytic materials, thus resulting
As our sustained effort toward hydrogen transfer-mediated
synthesis of N-heterocycles [26–28], we believe that, in the pres-
ence of a suitable hydrogen donor (HD) and catalyst, the replace-
ment of 2-aminoaryl carbonyls with more cost-effective and
stable 2-nitroaryl carbonyls would offer an appealing approach
⇑
Corresponding author.
https://doi.org/10.1016/j.jcat.2020.01.034
0021-9517/Ó 2020 Elsevier Inc. All rights reserved.

240
R. Xie et al. / Journal of Catalysis 383 (2020) 239–243
corresponding pore size distribution is mainly concentrated at
~10 nm (Fig. S2 in the SI). The results also confirm that the pre-
pared material contains the mesoporous structure.
Further, TEM, HRTEM, and EDS have been adopted to analyze
the morphology of the Co/N-ZrO2@C composites. The TEM and
HRTEM images (Fig. 2a,b) revealed the as-prepared Co/N-ZrO2@C
structure remained the initial octahedral shape of UiO-66 [46].
The TEM image (Fig. 2c) and high-angle annular dark-field scan-
ning TEM (HAADF-STEM) image (Fig. 2d) clearly showed that the
cobalt species are homogenously dispersed, which is in line with
the XRD test results. Meanwhile, the corresponding element map-
ping images (Fig. 2e, Fig. S4) also indicate that the elements of Co,
Zr, N, C and O are uniformly distributed, and the Co nanoparticles
are embedded in the matrix consisting of zirconia and carbon
derived from the UiO-66 MOF template, which prevents cobalt
nanoparticles from agglomerating during the pyrolysis process.
The surface chemistry of the as-prepared nanomaterial was
subsequently analyzed by the X-ray photoelectron spectroscopy
(XPS). A range of elements corresponding to Co, N, C, O, and Zr
on the samples surface were detected as 2.26%, 2.58%, 59.97%,
29.52%, and 5.67%, respectively, which are in accordance with the
elemental mapping results (Fig. S4 in the SI). The N 1s XPS spec-
trum (Fig. S5b in the SI) revealed three different signals, corre-
sponding to Co-N (399.01 eV), graphitic N (400.8 eV), and N
oxide species (403.5 eV), respectively [47,48]. Both graphitic-N
and oxidized-N emanate from the calcination of uncoordinated
1,10-Phen ligand, whereas the Co-N moieties are from the calcina-
tion of Co-1,10-Phen complex [49]. Similarly, the spectra of Co 2p
(Fig. 3) could be resolved into four constituents, the two character-
istic peaks located at 781.9 eV and 787.1 eV are allotted to Co-N
structures, while the binding energies of 797.9 eV and 803.5 eV
correspond to the cobalt oxides [50–53]. The appearance of cobalt
oxides are derived from partial oxidation of cobalt atoms on the
surface of Co-N species, as cobalt is susceptible to air [54,55].
The XPS results manifest that the cobalt nitrides (Co-Nx) and
cobalt oxides (Co-Ox) constitute the catalytically active species in
the prepared materials.
Scheme 1. Envisioned new synthetic protocol.
tunable catalytic activity and selectiviy [33,34]. In recent years, dif-
ferent heteroatoms (i.e. B, N, P, S, etc.) have been applied for the
design of functional materials [35,36]. Especially, the silica is used
to adjust the pore size [37–41], and the N-element could change
electronic properties of supporting materials [42,43]. Based on
the information, we wish herein to report the preparation of a
cobalt nanocatalyst supported on nitrogen-doped ZrO2@C by a
template method, and describe, for the first time, its application
in selective reductive annulation of 2-nitroaryl carbonyls with
alkynoates, which offers an efficient new platform for the synthesis
of functional quinolines.
2. Experimental and result analysis
2.1. Synthesis of Co/N-ZrO2@C
The new nanomaterial was prepared by the method shown in
Fig. 1. Firstly, the metal organic framework UiO-66 composing of
inorganic zirconium clusters and benzenedicarboxylate (H2BDC)
bridged linkers was synthesized according to the solvothermal
method [44]. Subsequently, the mixture of Co(OAc)2ꢀ4H2O and
1,10-phenanthroline (1,10-phen) (1:2 M ratio) in EtOH was stirred
at 80 °C for 1 h to generate the cobalt complex. The silica was then
introduced into the above solution by in situ hydrolysis of the
added Si(OC2H5)4 (TEOS) with aqueous ammonia. After that, the
dried UiO-66 was added to the above mixture and refluxed for
4 h at 80 °C. At last, the obtained composites were pyrolyzed under
argon flow at 800 °C for 3 h, followed by treatment with NaOH
solution to remove silica and form the cavity [45], affording the
cobalt material (Co/N-ZrO2@C; see details in the Supporting Infor-
mation (SI)).
To evaluate the catalytic performance of the prepared nanoma-
terial, it was applied to test a benchmark reaction, the reductive
annulation of 2-nitrobenzaldehyde 1a with diethyl acetylenedicar-
boxylate 2a. Pleasingly, the initial test resulted in the quinoline
product 3aa in 51% yield (entry 1, table 1). Then, we evaluated var-
ious reaction parameters, including hydrogen donors, solvents,
2.2. Characterization
To understand the constituent of the prepared new material
accurately, X-ray diffraction (XRD) detection was carried out.
Except for the characteristic peaks of zirconium dioxide (PDF No.
49-1642), no other significant peaks belonging to cobalt species
are presented in the XRD spectrum of the Co/N-ZrO2@C (Fig. S1
in the SI), which implies that the Co species are evenly dispersed
or amorphous phase. The N2 adsorption-desorption tests of
Co/N-ZrO2@C clearly exhibit a typical IV isotherm, indicating the
inclusion of mesoporous structure (Fig. S2 in the SI). Moreover,
the specific BET surface area is shown as 199.2 m2/g, and the
Fig. 2. (a, b, c) TEM images of Co/N-ZrO2@C, respectively. (d) magnified HAADF-
STEM image of Co/N-ZrO2@C. (e) the corresponding elemental mapping of Co, Zr, N
and C, respectively.
Fig. 1. Preparation of cobalt nanocatalyst.

R. Xie et al. / Journal of Catalysis 383 (2020) 239–243
241
and other hydrogen donors, non-N-doped or Si-non-etching mate-
rials using UiO-66 as the support of cobalt resulted in either no
activity or low efficiency (entries 9, 12–13).
3. Practical applicability of the catalyst
With the optimal conditions established, we next got down to
evaluating the universality of this catalytic scheme. Initially, a ser-
ies of 2-nitrobenzaldehydes 1 in combination with alkynoate 2a
(for their structures, see Scheme S1 in the SI) were examined. As
shown in scheme 2, all the reactions underwent smooth transfer
hydrogenative annulation and delivered the desired quinoline
products in reasonable to excellent isolated yields (3aa-3ra). Grat-
ifyingly, various functional groups (i.e. –Me, –OMe, –NMe2, –OH,
–Ph, –F, –Cl, –Br, –COOMe and –NO2) on substrates 1 were well tol-
erated, and the electrical properties of these functionalities
affected the product yields to a certain extent. Especially, reactants
1 bearing electron-withdrawing groups generated the desired pro-
duct (3ka-3oa) in relatively higher yields that those of substrates
containing strong electron-donating substituents (3fa, 3ha) in rel-
atively lower yields that those of substrates containing, presum-
ably because the electron-rich 2-nitrobenzaldehydes are less
beneficial to the reduction of the nitro group, whereas the
electron-withdrawing groups could enhance the reactivity of the
carbonyl group in substrates 1, thus favoring condensation step
(Scheme 2). Notably, substrate 1o has two nitro groups, but only
one is involved in the formation of product 3oa. The unique selec-
tivity is rationalized as the interaction between the carbonyl and
ortho nitro groups with the cobalt catalyst.
Fig. 3. Co 2p XPS spectra of Co/N-ZrO2@C.
Table 1
Screening of optimal reaction conditions.a
Entry
Catalyst
solvent
3aa Yield %b
Subsequently, we turned our attention to the variation of both
coupling partners. Thus, the reactions employing various 2-
nitroaryl carbonyls and alkynoates 2 were performed under the
optimized conditions. As outlined in Scheme 3, all the reactions
1
2
3
4
5
6
7
8
Co/N-ZrO2@C
Co/N-ZrO2@C
Co/N-ZrO2@C
Co/N-ZrO2@C
Co/N-ZrO2@C
–
H₂O
51
53
41
20
EtOH
t-amyl alcohol
DMSO
HFIP
71
HFIP
NR
NRc
83d
(5, 0)e
NR
NRf
70
Co/N-ZrO2@C
Co/N-ZrO2@C
Co/N-ZrO2@C
Co(OAc)2ꢀ4H2O
Co/N-ZrO2@C
Co/ZrO2@C
HFIP
HFIP
HFIP
HFIP
HFIP
HFIP
9
10
11
12
13
Co/N-Si-ZrO2@C
HFIP
65
a
Reaction conditions: Unless otherwise specified, all reactions were performed
at 120 °C for 18 h under N2 protection by using 1a (0.25 mmol), 2a (0.3 mmol),
catalyst (6 mol %), HFIP (1.2 mL), HCOOH (3.0 eq, 0.75 mmol), Co/N-ZrO2@C pyr-
olyzed at 800 °C.
b
GC yield using hexadecane as an internal standard.
Without HCOOH.
Reaction temperature: 110 °C.
the hydrogen sources are respect to HCOONa, and hydrogen balloon,
c
d
e
respectively.
f
Non-pyrolyzed Co/N-ZrO2@C. EA: ethyl acetate, HFIP: 1,1,1,3,3,3-Hexafluoro-2-
propanol. NR = no reaction.
temperatures and catalysts (Table 1, Table S1 in the SI). It was
found that the solvents significantly affect the reaction, and
1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) exhibit the highest effi-
ciency in affording product 3aa (entries 1–5), which might be
rationalized as such alcohol could serve as an efficient proton
transferring shuttle [56–58]. The absence of cobalt catalyst or
hydrogen donor failed to give any products (entries 6 and 7). An
optimal GC yield (83%) of product 3aa was gained when the reac-
tion in HFIP was performed at 110 °C in the presence of 6 mol % of
catalyst and HCOOH as the hydrogen donor (entry 8, standard con-
ditions). However, the use of catalyst precursor Co(OAc)2∙4H2O,
non-pyrolyzed Co/N-ZrO2@C showed no activiy (entries 10, 11),
Scheme 2. Variation of 2-nitrobenzaldehydes.

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R. Xie et al. / Journal of Catalysis 383 (2020) 239–243
Next, several verification experiments were performed to gain
mechanistic insight into the reaction (Scheme 4). Treating 2-
nitrobenzaldehyde 1a under the standard conditions resulted in
2-aminobenzaldehyde 1a’ exclusively, and the retention of the car-
bonyl group shows that the catalyst exhibits excellent transfer
hydrogention selectivity (Eq. (1)). Moreover, the coupling of com-
pound 1a’ with diethyl acetylenedicarboxylate 2a was able to deli-
ver product 3aa in 76% yield (Eq. (2)), indicating that compound 1a’
is key a reaction intermediate. These results are in good agreement
with the assumed pathway proposed in Scheme 1.
4. Conclusions
In summary, we have developed a new N-doped ZrO2@C sup-
ported cobalt nanocatalyst with uniform dispersion, which exhi-
bits good catalytic performance toward the reductive annulation
of 2-nitroaryl carbonyls with alkynoates and alkynones. The cat-
alytic transformation allows synthesizing a wide array of funci-
tonalized quinolines with the merits of broad substrate scope,
good functional compability, excellent transfer hydrogenation
selectivity, reusable earth-abundant metal catalyst, and opera-
tional simplicity. The developed chemistry paves the ways for fur-
ther development of hydrogen transfer-mediated coupling
reactions by design of heterogeneous catalysts with suitable
supports.
Scheme 3. Variation of both coupling agents.
Declaration of Competing Interest
went smoothly and furnished the desired products in moderate to
high yields. Both symmetrical and unsymmetrical alkynoates were
able to generate the desired product in exclusive regioselectivity
(3lb, 3fb, 3ab-3ad, 3pd, 3gd, 3af). Noteworty, the reaction of 1-
(2-nitrophenyl)ethan-1-one 1q with 2a did not hamper the pro-
duct formation (3qa), and the somewhat low yield is ascribed to
the partial hydrogenation of the C-C triple bond of 2a due to the
relatively low reactivity of substrate 1q. In addition, the alkynes
without an electron-withdrawing group failed to give the quino-
line products, indicating that the use of electron-deficient alkynes
is essential for the transformation under the current reaction sys-
tem. In addition to alkynoates, alkynone 2e is also compatible with
the transformation, affording the 3-carbonylquinolines in reason-
able yields (3ae, 3ee and 3ne).
In order to check the stability and reusability of the newly
developed catalyst material, we performed the circulation experi-
ments with the model reaction. As presented in Fig. S6 (SI), the
reaction conversion has a decreasing trend during the five consec-
utive runs. After five recycling experiments, the catalyst was
detected by ICP-OES, and the cobalt content decreased from
4.21% to 2.95% compared to the fresh catalyst. Therefore, the
decreased catalytic activity of the catalyst is attributable to the loss
of active cobalt species, which arises out of mechanical wear-
induced exfoliation during the course of reaction. In addition,
XRD spectra and TEM images of Co/N-ZrO2@C catalyst after five
runs (Fig. S1 and S3a-b in the SI) were examined, the results
showed that the morphology of the reused catalyst maintained
well.
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
We thank the National Key Research and Development Program
of China (2016YFA0602900), the National Natural Science Founda-
tion of China (21971071), and Guangdong Province Science Foun-
dation (2017B090903003) for financial support.
Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2020.01.034.
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