Application of a reusable Co-based nanocatalyst in alcohol dehydrogenative coupling strategy: Synthesis of quinoxaline and imine scaffolds

Article abstract of DOI:10.1016/j.catcom.2020.105927

A nitrogen doped carbon supported cobalt catalyzed efficient synthesis of imines and quinoxaline motifs is reported. Co(OAc)₂-Phen/Carbon-800 (Co-phen/C-800) showed the superior reactivity compared to other materials prepared at different temperature, in the synthesis of quinoxalines by the coupling between diamines and diols. Moreover, applying the transfer hydrogenation and acceptorless dehydrogenative coupling strategy, imines and quinoxaline derivatives were synthesized from the nitro compounds. The practical applicability of this protocol was demonstrated by the gram-scale synthesis and the reusability of the catalyst upto 8th cycle. Furthermore, several kinetic experiments were carried out to realize the probable mechanism.

Full text of DOI:10.1016/j.catcom.2020.105927

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Application of a reusable co-based nanocatalyst in alcohol
dehydrogenative coupling strategy: Synthesis of quinoxaline and
imine scaffolds
Dibyajyoti
Panja,
Bhaskar
Paul,
Bhuvaneshwari
Balasubramaniam, Raju K. Gupta, Sabuj Kundu
PII:
S1566-7367(20)30002-9
DOI:
https://doi.org/10.1016/j.catcom.2020.105927
CATCOM 105927
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
18 September 2019
3 January 2020
6 January 2020
Please cite this article as: D. Panja, B. Paul, B. Balasubramaniam, et al., Application of
a reusable co-based nanocatalyst in alcohol dehydrogenative coupling strategy: Synthesis
of quinoxaline and imine scaffolds, Catalysis Communications (2020), https://doi.org/
10.1016/j.catcom.2020.105927
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Application of a Reusable Co-Based Nanocatalyst in Alcohol
Dehydrogenative Coupling Strategy: Synthesis of Quinoxaline and Imine
Scaffolds
Dibyajyoti Panjaa, Bhaskar Paula, Bhuvaneshwari Balasubramaniamb, Raju K. Guptab and
Sabuj Kundua,* sabuj@iitk.ac.in
aDepartment of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, Uttar-
Pradesh, India.
bDepartment of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur-
208016, Uttar-Pradesh, India.
*Corresponding author.
Abstract:
A nitrogen doped carbon supported cobalt catalyzed efficient synthesis of imines and
quinoxaline motifs is reported. Co(OAc)2-Phen/Carbon-800 (Co-phen/C-800) showed the
superior reactivity compared to other materials prepared at different temperature, in the
synthesis of quinoxalines by the coupling between diamines and diols. Moreover, applying
the transfer hydrogenation and acceptorless dehydrogenative coupling strategy, imines and
quinoxaline derivatives were synthesized from the nitro compounds. The practical
applicability of this protocol was demonstrated by the gram-scale synthesis and the
reusability of the catalyst up to 8th cycle. Furthermore, several kinetic experiments were
carried out to realize the probable mechanism.
Keywords: cobalt; heterogeneous catalysis; quinoxalines; imines; mechanistic investigations.
1. Introduction

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In the atom economical and cost effective sustainable catalysis, one of the important
aspect is separation and recyclability of the catalyst.[1] However, reusability of the catalyst is
the prime challenge in the homogeneous catalysis.[2] Therefore, in the industry, mostly noble
metal-based heterogeneous catalysts were employed for the synthesis of various
pharmaceuticals, agrochemicals, bulk materials, etc.[3] Nevertheless, in the greener
prospective, replacement of expensive noble metal-based systems with the non-precious and
abundant transition metal catalysts received increasing interest in both academia and
industry.[4, 5]
Replacement of the hazardous and waste generating chemicals with the biomass
derived renewable feedstocks is one of the key target in the chemical research.[6] Although,
approximately 75% of the carbohydrate biomass produced by nature, only 3-4% are utilized
for the production of foods and chemicals. [7] Thus, for the synthesis of fine chemicals,
employment of naturally available biomass carbohydrates as a raw material is one of the
exciting areas of research.[8, 9] Importantly, catalytic conversion of carbohydrates to a
variety of glycols such as ethanediol, propanediol, etc. were well explored.[10] Therefore, in
the greener and sustainable perspective, use of these diols to synthesize the value added
products remains an attractive strategy.
Quinoxaline scaffolds containing molecules have significant biological[11] and
pharmacological properties.[12] However, in the traditional approach quinoxalines were
synthesized by the reaction between reactive di-carbonyls with the di-amine molecules.[13]
Recently, some other methods were also developed for their synthesis by employing 1,4-
addition, oxidative coupling, cyclization-oxidation reaction strategies.[14] Nevertheless, the
usage of expensive reactive reagents, poor yields, harsh reaction conditions are the major
issues for most of these processes.[15] Notably, applying alcohol dehydrogenative coupling
strategy[16, 17] there are several reports presented in the literature for the synthesis of

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quinoxaline molecules.[18, 19] Inspired by these reports, we tried to synthesize these
molecules through alcohol activation in presence of a heterogeneous cobalt catalysts.
In addition, imines are important class of nitrogen containing molecules used for the
synthesis of materials, fragrances, fungicides, pharmaceuticals, etc.[20] Commonly, they are
synthesized from expensive as well as reactive carbonyls and amines in the presence of Lewis
acids.[21] Therefore, alternative greener approach to access these molecules is highly
desirable. Following the alcohol dehydrogenative coupling strategy, several reports are
known for the synthesis of imines by the reaction between alcohols and amines.[22] As nitro
compounds are one of the surrogates for amines, hence direct synthesis of imines from nitro
compounds and alcohols is more attractive.[23] Notably, only few reports [24], are known for
the similar transformation under heterogeneous condition.
Herein, we report a practically useful and reusable cobalt catalyst for the direct
synthesis of structurally diverse and biologically important quinoxaline and imine molecules.
To the best of our knowledge, heterogeneous cobalt catalyzed the greener synthesis of
quinoxalines by the reactions between nitroanilines and diols is not reported yet.
2. Experimental
2.1. Procedure for Synthesis of Co-phen/C-T Catalysts
Several nitrogen doped carbon supported cobalt nanoparticles were prepared by
reacting cobalt precursor and phenanthroline (1:2 molar ratio) in ethanol, followed by the
addition of carbon powder (Carbon Black) to the resultant reaction mixture at room
temperature. Then, the solvent was evaporated under reduced pressure and the mixture was
pyrolyzed under inert atmosphere (Scheme S1). In literature it is well known that the
pyrolysis temperature has significant impact on the nature and activity of the materials.[25]

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Hence, the pyrolysis of these materials was carried out at different temperature under N2-
atmosphere to get the diverse types of cobalt nanoparticles.
2.2. Quinoxaline Synthesis from Diamine
To an oven dried 9 mL screw cap tube, a magnetic stir-bar, diamine (0.5 mmol),
vicinal diol (1.5 mmol), CsOH.H2O (0.375 mmol), Co-phen/C-800 (1.5 mol%) and toluene
(2.5 mL) were added under argon atmosphere. Then, the tube was sealed and placed in a
preheated oil bath at 150 °C for 24 h. After completion of the reaction, the tube was allowed
to cool at room temperature. Then, the solvent was evaporated under reduced pressure.
Finally, the product was purified by silica gel column chromatography using ethyl
acetate/hexane as eluent.
2.3. Imine Synthesis from Nitroarene
An oven dried 9 mL screw cap tube was charged with a magnetic stir-bar, nitroarene
(0.5 mmol), benzyl alcohol (2.0 mmol), Cs2CO3 (0.375 mmol), Co-phen/C-800 (1.5 mol%)
and toluene (2.5 mL) under argon atmosphere. Next, the tube was sealed and placed in a
preheated oil bath at 150 °C for 24 h. After completion of the reaction, the tube was allowed
to cool at room temperature. The solvent was evaporated under reduced pressure. Then, the
imine molecule was purified by deactivated silica gel column chromatography using ethyl
acetate/hexane as eluent.
2.4. Direct Synthesis of Quinoxaline from Nitroamine
In an oven dried 9 mL screw cap tube a magnetic stir-bar, nitroamine (0.5 mmol),
vicinal diol (2.5 mmol), CsOH.H2O (0.125 mmol), Co-phen/C-800 (1.5 mol%) and toluene
(2.5 mL) were added under argon atmosphere. Then, the tube was sealed and placed in a
preheated oil bath at 150 °C for 24 h. After completion of the reaction, the tube was allowed

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to cool at room temperature. Next, the solvent was evaporated under reduced pressure.
Finally, the quinoxalines was purified by silica gel column chromatography using ethyl
acetate/hexane as eluent.
3. Results and discussion
We prepared a number of nitrogen doped carbon supported cobalt nanoparticles from
various cobalt precursor and phenanthroline. To check the catalytic potentiality of these
materials, initially we tested their activity for the synthesis of 2-methylquinoxaline. For this
purpose, o-phenylenediamine and 1,2-propane diol were selected as model substrates.
At first, we screened several cobalt catalysts in this transformation, among them
Co(OAc)2-Phen/Carbon-800 (Co-phen/C-800) displayed the superior activity (Table 1,
entries 1-5). Few heterogeneous materials by using different support (e.g. alumina, silica) and
different metals (Mn, Ni) doping were also screened for the synthesis of quinoxaline.
However, all these materials were less efficient compare to Co-phen/C-800 (entries 6-9).
Nevertheless, the material without N-doping or cobalt content showed the poor catalytic
activity; signifies the importance of nitrogen and cobalt within the materials (entries 11-12).
Afterwards, screening of bases (entries 13-15), solvents (entries 16-18) and optimization of
other reaction parameters (entries 19-24 and SI) were performed. Finally, the yield of 2-
methylquinoxaline was reached to 96% in toluene by using 1.5 mol% of Co(OAc)2-phen/C-
800 in presence of 3 equiv. of diol and 0.75 equiv. of CsOH.H2O at 150 °C within 24 h (entry
24).
Table 1. Selected data for the optimizationa

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Entry
Catalyst
Base
Yield (%)
(mol%)
(equiv.)
1
2
Co(OAc)2-phen/C-700 (6)
tBuOK (0.5)
tBuOK (0.5)
tBuOK (0.5)
tBuOK (0.5)
tBuOK (0.5)
tBuOK (0.5)
tBuOK (0.5)
tBuOK (0.5)
63
84
73
23
10
45
67
55
30
15
8
Co(OAc)2-phen/C-800 (6)
Co(OAc)2-phen/C-900 (6)
Co(acac)2-phen/C-800 (6)
CoCl2-phen/C-800 (6)
3
4
5
6
Co(OAc)2-phen/Alumina-800 (6)
Co(OAc)2-phen/Silica-800 (6)
Ni(OAc)2-phen/C-800 (6)
MnCl2-phen/C-800 (6)
7
8
9
tBuOK (0.5)
-
10
11
12
13
14
15
16b
17c
18d
19e
20e
21e
22e,f
23e,g
Co(OAc)2-phen/C-800 (6)
Co(OAc)2-C-800 (6)
tBuOK (0.5)
tBuOK (0.5)
phen/C-800 (6)
2
Co(OAc)2-phen/C-800 (6)
Co(OAc)2-phen/C-800 (6)
Co(OAc)2-phen/C-800 (6)
Co(OAc)2-phen/C-800 (6)
Co(OAc)2-phen/C-800 (6)
Co(OAc)2-phen/C-800 (6)
Co(OAc)2-phen/C-800 (3)
Co(OAc)2-phen/C-800 (1.5)
Co(OAc)2-phen/C-800 (0.75)
Co(OAc)2-phen/C-800 (1.5)
Co(OAc)2-phen/C-800 (1.5)
K2CO3(0.5)
29
83
98
26
60
76
95
85
55
80
65
Cs2CO3(0.5)
CsOH.H2O (0.5)
CsOH.H2O (0.5)
CsOH.H2O (0.5)
CsOH.H2O (0.5)
CsOH.H2O (0.5)
CsOH.H2O (0.5)
CsOH.H2O (0.5)
CsOH.H2O (0.75)
CsOH.H2O (0.75)

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24e
25e
Co(OAc)2-phen/C-800 (1.5)
CsOH.H2O (0.75)
CsOH.H2O (0.75)
96
6
-
aReaction conditions: o-phenylenediamine (0.3 mmol), 1,2-propane-diol (0.9 mmol), toluene (1.5 mL)
b
c
d
e
f
at 150 °C for 15 h; yields were determined by GC; dioxan; THF; p-xylene; 24 h; 0.6 mmol diol;
g0.45 mmol diol.
All the cobalt catalysts were characterized through various techniques such as X-Ray
Diffraction Spectroscopy (XRD), X-Ray Photoelectron Spectroscopy (XPS), Scanning
Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) etc. The powder
pattern of the materials at different temperatures was shown in Figure S4. The characteristic
2θ values of 43.44°, 49.30° for these materials advocated the presence of metallic cobalt and
31.72°, 37.01°, 42.41°, 59.88° 62.84° indicated the existence of the oxide of cobalt.[26] To
get insight into the surface of the materials, XPS measurement was carried out. The XPS
analysis of the materials, formed at 700, 800 and 900 °C disclosed the nitrogen content of
6.2, 2.7 and 2.3 wt%, respectively. Notably, the N1s plots for these materials exposed mainly
three characteristic peaks at i) 396.9 eV for pyridinic nitrogen, ii) 398.3 eV for pyrrolic
nitrogen or nitrogen coordinated to cobalt, and iii) 402 eV for graphitic nitrogen (Figure
S6a).[25] In addition, characteristic peaks of cobalt appear with binding energy of 778.5 eV
and 793.8 eV, corresponds to Co2p3/2 and Co2p1/2, respectively. Importantly, the satellite
peaks at 786 eV and 802.4 eV also represent the oxidic nature of cobalt present in the surface
(Figure 1a). The TEM images of these materials disclosed the existence of CoO and Co3O4
which were uniformly dispersed over the N-doped carbon support. The average particle size
of 17.53 nm and 18.2 nm was observed for the material at pyrolyzed at 700 °C and 800 °C,
respectively. However, at 900 °C aggregation of the material lead to the larger particle size of
31.44 nm which indicate that growth of cobalt nanoparticle happened with increasing
pyrolyzing temperature.[27] Importantly, presence of Co3O4 with well resolved lattice

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spacing of 0.24 nm for 311(Co3O4) plane was realized from the high resolution TEM images
of the Co-phen/C-800 material (Figure 1b).[25] Furthermore, the homogeneous distribution
of the several elements such as Co, N, O and C in the material Co-phen/C-800 was
recognized by the elemental mapping analysis using high-angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM) as shown in the Figure S9. Additionally,
BET analysis of the Co-phen/C-800 material was carried out which displayed the BET
surface area of 59 m2/g and the average pore size of 1.4 nm.
Figure 1. a) XPS Spectra of Co2p for Co-phen/C-800 material, b) TEM Images of Co-phen/C-800
material (scale bar 20 nm).
After achieving the optimum conditions (Table 1, Entry 24) for 2-methylquinoxaline
synthesis, next, we moved to explore the scope of the present catalytic process by employing
variety of diamines and diols as mentioned in Table 2. The reaction of 1,2-diaminobenzene
with different diol molecules afforded the corresponding quinoxaline moieties up to 96%
yield (2a-g). Additionally, quinoxaline scaffolds were synthesized efficiently, by altering
both of diamine and diol molecules (2h-q). However, with diol having diphenyl or tert-butyl
substitution lower yield of the desired product was observed (Table 2). In addition to that,
1
two regio-isomers (2t1 and 2t2) were obtained with an isomeric ratio of 1:1 (analyzed by H-
NMR spectroscopy) when unsymmetrical diamine and diol was used. 2-Methylpyrazine (2u)
also synthesized using this protocol from ethylenediamine with good yield.

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Table 2. Synthesis of quinoxalines from diaminesa
aReaction conditions: diamine (0.5 mmol), diol (1.5 mmol), toluene (2.5 mL) at 150 °C for 24 h;
b
isolated yield; 3 mol% catalyst for 28 h; c36 h.
Next, we tested the potential of this catalytic system towards the coupling between
nitroarenes and benzyl alcohols for the synthesis of imines. However, during the optimization
process, for the coupling between nitrobenzene and benzyl alcohol, CsOH.H2O displayed

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lower activity towards the imine formation (see SI). Hence, we again screened several bases
and among them, Cs2CO3 presented superior activity in this transformation (SI, Table S5,
88% yield). Afterwards, to check the general applicability of this process, several substituted
alcohols and nitro compounds were employed (Table 3). Following this protocol, various
imine molecules were synthesized in 61-96% yields (3a-l). In addition, alcohols containing
naphthalene moiety or hetero nuclei also worked fine by employing this protocol (3f-h).
Gratifyingly, the reducible -CN group was unaffected during this transformation;
demonstrated the selectivity of this catalytic system (3l).
Table 3. Scope for the imines synthesisa
°
aReaction conditions: nitroarene (0.5 mmol), benzyl alcohol (2.0 mmol), toluene (2.5 mL) at 150 C
for 24 h; isolated yield.
After achieving the optimum conditions (Table S6), we explored the potential of this
catalytic system towards the direct synthesis of a variety of quinoxaline scaffolds from 2-
nitroanilines and diols (Table 4). Several mono- and di-substituted diol derivatives furnished
the desired products efficiently (4a-g). Additionally, a number of substituted 2-nitroanilines

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smoothly furnished the desired quinoxaline scaffolds up to 95% yield (4h-i). Notably,
substitution at the both of alcohols and nitroamines effectively transformed to the quinoxaline
molecules in 60-93% yield (4j-q).
To demonstrate the practical applicability of this protocol, we carried out gram-scale
synthesis of various quinoxalines and imines, where up to 87% yield was observed (Figure
S1). Then, we evaluated the green chemistry metrics for the preparative scale synthesis of 2,
3, 6-trimethylquinoxaline (2h) from 4-methylbenzene-1, 2-diamine. The calculated E-factor
of 4.23, 81% atom economy, 66.42% atom efficiency, 100% carbon efficiency and 66.58% of
reaction mass efficiency in this transformation clearly exposed the novelty and sustainability
of this protocol.
Table 4. Direct synthesis of quinoxalines from nitroaminesa

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aReaction conditions: nitroaniline (0.5 mmol), diol (2.5 mmol), toluene (2.5 mL) at 150 °C for 24 h;
b
isolated yield; 3 mol% catalyst.
The significant advantage of the heterogeneous system over the homogeneous
catalytic condition is the separation and recyclability of the catalyst. To test the recyclability
of this system, we carried out the synthesis of 2, 3-dimethylquinoxaline from 2-nitroaniline
and isolated the heterogeneous cobalt catalyst after the reaction. Remarkably, this
heterogeneous system did not lose its catalytic activity significantly even after 8th catalytic
cycle (Figure 2).
Figure 2. Recycling of Co-phen/C-800 catalyst.
The yield of 2-methylquinoxaline dropped considerably when the reaction was carried
out with acid treated Co-phen/C-800 catalyst (Scheme S2). This experiment suggested that
both Co-Nx and CoOx played vital role in this reaction.[28]
To understand these catalytic transformations using the heterogeneous cobalt system
few kinetic experiments were carried out. The reduction of nitro compound follows either a
direct route or condensation route depending upon the catalytic system.[29] Thus, in order to
find which pathway was operating during the reduction of nitroarenes to anilines, a time
dependent product distribution was carried out for the coupling of nitrobenzene and benzyl

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alcohol (Figure S2). In this reaction formation of substantial amount of azobenzene indicated
the preferential involvement of condensation route.
Afterwards, during the formation of quinoxaline from 2-nitroaniline a time dependent
product distribution was carried out (Figure S3). Initially, considerable amount of diamine
was observed and the amount of diamine decreased steadily with the progress of the reaction.
Subsequently, we performed a number of control experiments as mentioned in
Scheme S3. During the reductive imination of nitrobenzene, at the initial time (0.5 h)
formation of a significant amount of the azobenzene and azoxybenzene was detected
(Scheme S3). This study clearly specified the preferential involvement of condensation route
in this transformation. To further support this, an experiment between azobenzene and benzyl
alcohol was carried out which smoothly delivered the desired imine molecule (Scheme S3B).
The conversion of nitrobenzene was quite low with only base which exposed the necessity of
cobalt catalyst in this transformation (Scheme S3C).
Based on kinetic experiments, we proposed the schematic pathway for the formation
of quinoxaline from the 2-nitroaniline molecule (Scheme S4). The time dependent product
distribution study showed the formation of significant amount of diamine during the reaction
which suggested that path 1 was more favourable. Nevertheless, the possibility of path 2
cannot be ruled out. Initially, 2-nitroaniline was transformed to the 1, 2-diamino benzene.
After that, condensation between the o-phenylenediamine and the in situ generated
dicarbonyl lead to form the species R. Finally, intra-molecular cyclization of the species R
provided the quinoxaline product. In addition based on literature reports a interaction
mechanism was proposed for this systhem (Scheme S5).[25]
4. Conclusion

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In summary, we synthesized a number of N-doped carbon supported cobalt catalysts
by varying pyrolysis temperature. Catalytic activity of these materials were tested towards the
synthesis of 2-methylquinoxaline from o-phenelenediamine and among them, Co-phen/C-800
showed the superior results. PXRD analysis revealed the presence of metallic Co and oxidic
cobalt in these materials. XPS study disclosed that minimum nitrogen content in Co-phen/C-
900 compared to Co-phen/C-700 and Co-phen/C-800. The nanoparticle growth with higher
pyrolyzing temperature and existence of Co3O4-species was suggested by TEM analysis.
Additionally, employing transfer hydrogenation and alcohol dehydrogenative coupling
approach, quinoxalines and imines were also synthesized from nitro compounds and alcohols.
Moreover, the preferential involvement of diamine and condensation route during the
formation of quinoxalines from 2-nitroamines was revealed by the kinetic studies and control
experiments. Importantly, the recyclability of this system was tested up to 8th cycle and this
heterogeneous system did not lose its catalytic activity significantly.
Author contribution statement:
Dibyajyoti Panja: Conceptualization, Methodology, Software, Validation, Formal analysis,
Investigation, Resources, Data Curation, Writing - Original Draft, Visualization
Bhaskar Paul: Data Curation, Formal analysis, Writing - Review & Editing
Bhuvaneshwari Balasubramaniam: Software, Formal analysis, Resources, Writing - Review &
Editing
Raju K. Gupta: Resources, Writing - Review & Editing
Sabuj Kundu: Conceptualization, Validation, Writing - Review & Editing, Supervision, Project
administration, Funding acquisition
Conflicts of interest
There are no conflicts to declare
Acknowledgements
We are thankful to Science and Engineering Research Board (SERB), India; Council
for Industrial and Scientific Research (CSIR), India and Indian Institute of Technology

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Kanpur for financial support. DP thanks IITK, BP thanks UGC and BB thanks Department of
Science and Technology (DST) for fellowship.
Supplementary data
Supplementary material
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Graphical abstract:

Journal Pre-proof
Highlights
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Quinoxalines were synthesized fromnitroamines usingaheterogeneous Co-catalyst.
Various imines werealso synthesized fromnitroarenes and alcohols.
This catalyst was recycled up to 8th cycle.
Green chemistry metrics calculation showed the sustainability of this protocol.
Several kineticexperiments were performed to understand the mechanisms.