Enzyme-Inspired Lysine-Modified Carbon Quantum Dots Performing Carbonylation Using Urea and a Cascade Reaction for Synthesizing 2-Benzoxazolinone

Article abstract of DOI:10.1021/acscatal.1c01276

Catalysts as the dynamo of chemical reactions along with solvents play paramount roles in organic transformations in long-lasting modes. Thus, developing effective and biobased catalysts in nontoxic solvents is highly in demand. In this report, carbon quantum dots (CQDs) functionalized with-lysine (Lys-CQDs) were generated from entirely nature-derived materials; they were demonstrated to be a promising catalyst for C-N bond formation in choline chloride urea (ChCl/U), a natural deep eutectic solvent (NADES). Among a number of synthesized CQDs, Lys-CQD turned out to be a powerful catalyst in the model reaction with aniline to afford phenyl urea. This type of transformation is important because aniline as a nucleophile has low activity, and urea is a very weak electrophile but an abundant natural source of the carbonyl moiety at the same time. The optimized reaction was performed under a highly desirable condition without using tedious and toxic workup processes at a low temperature (37 °C for aliphatic amines and 60 °C for aniline derivatives), as well as by embracing the broad scope of products in good to high yields even with weak nucleophiles such as aniline. A proposed acid-activated mechanism was suggested for the model reaction that was further confirmed by detecting ammonia as the leaving group. To show further multifunctionality of the catalyst, a cascade catalysis approach was developed for synthesizing 2-benzoxazolinone, which was furnished in a two-step transformation, starting from 2-aminophenol. Using X-ray crystallography, the structure of the final product in the cascade reaction was also determined. The catalyst was characterized using various analytical techniques including SEM, TEM, AFM, XRD, IR spectroscopy, UV-vis spectroscopy, DLS, and fluorescence spectroscopy. Measuring the acid/base sites by back titration, the catalyst was shown to be highly functionalized by the lysine functional group. The size of the catalyst was determined to be in the range of 1-8 nm, having a well-dispersed surface. In all, Lys-modified CQD, as a metal-free catalyst, was synthesized, characterized, and optimized for carbonylation, as well as a cascade reaction, under mild conditions. The whole process including catalyst synthesis and organic transformations is economically competitive and fulfills all requirements toward viability.

Full text of DOI:10.1021/acscatal.1c01276

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Research Article
Enzyme-Inspired Lysine-Modified Carbon Quantum Dots Performing
Carbonylation Using Urea and a Cascade Reaction for Synthesizing
2‑Benzoxazolinone
Morteza Hasani and Hamid R. Kalhor*
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ABSTRACT: Catalysts as the dynamo of chemical reactions along
with solvents play paramount roles in organic transformations in
long-lasting modes. Thus, developing effective and biobased
catalysts in nontoxic solvents is highly in demand. In this report,
carbon quantum dots (CQDs) functionalized with L-lysine (Lys-
CQDs) were generated from entirely nature-derived materials; they
were demonstrated to be a promising catalyst for C−N bond
formation in choline chloride urea (ChCl/U), a natural deep
eutectic solvent (NADES). Among a number of synthesized
CQDs, Lys-CQD turned out to be a powerful catalyst in the model
reaction with aniline to afford phenyl urea. This type of
transformation is important because aniline as a nucleophile has
low activity, and urea is a very weak electrophile but an abundant
natural source of the carbonyl moiety at the same time. The optimized reaction was performed under a highly desirable condition
without using tedious and toxic workup processes at a low temperature (37 °C for aliphatic amines and 60 °C for aniline
derivatives), as well as by embracing the broad scope of products in good to high yields even with weak nucleophiles such as aniline.
A proposed acid-activated mechanism was suggested for the model reaction that was further confirmed by detecting ammonia as the
leaving group. To show further multifunctionality of the catalyst, a cascade catalysis approach was developed for synthesizing 2-
benzoxazolinone, which was furnished in a two-step transformation, starting from 2-aminophenol. Using X-ray crystallography, the
structure of the final product in the cascade reaction was also determined. The catalyst was characterized using various analytical
techniques including SEM, TEM, AFM, XRD, IR spectroscopy, UV−vis spectroscopy, DLS, and fluorescence spectroscopy.
Measuring the acid/base sites by back titration, the catalyst was shown to be highly functionalized by the lysine functional group.
The size of the catalyst was determined to be in the range of 1−8 nm, having a well-dispersed surface. In all, Lys-modified CQD, as a
metal-free catalyst, was synthesized, characterized, and optimized for carbonylation, as well as a cascade reaction, under mild
conditions. The whole process including catalyst synthesis and organic transformations is economically competitive and fulfills all
requirements toward viability.
KEYWORDS: carbon quantum dots, bioinspiration, cascade catalysis, DES, carbonylation, lysine-modified
mainly metal-based, have emerged from the inspiration
provided by enzymes.19−21
INTRODUCTION
■
As the demand for eco-friendly synthetic chemicals in
sustainable and straightforward production processes grows
at a remarkable rate, tremendous scientific endeavors have
been made to develop novel, efficient, and clean catalytic
methods for generating daily chemicals.1−6 Metal-based
catalysts have accounted for most of the manufacturing output
for decades;7−12 yet, for environmental and cost-effective
reasons, they should be superseded by cleaner and practical
alternatives.13−15 Enzymes (biocatalysts), as the major catalysts
in biosystems, have been a source of inspiration and imitation
for chemists to try to mimic the actions of biocatalysts in the
laboratory and industrial organic transformations.16−18 So far, a
few promising alternatives such as nanozymes (nanomaterials
that can mimic the native action(s) of an enzyme), which are
Carbon quantum dots (CQDs) are a novel type of zero-
dimensional nanomaterials (nanomaterials with all of their
dimensions in nanoscale), and since their introduction,22 they
have quickly attracted the attention of researchers. Due to their
unique properties, such as tunable fluorescence, biocompati-
Received: March 22, 2021
Revised: July 19, 2021
https://doi.org/10.1021/acscatal.1c01276
© XXXX American Chemical Society
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a
Scheme 1. Synthesis of Lys-CQDs
a
In brief, 10 mmol of each starting material was dissolved in 50 mL of distilled water and the mixture heated at 150 °C for 5 h. The resulting brown
solid was again dissolved in water and dialyzed against water for 6 h. Finally, by removing water, fluorescent Lys-CQDs were formed.
bility, a size range of around ≤20 nm, and nontoxicity to
human health and the environment, CQDs have been
exploited in various research areas, mostly in bioimaging and
biodetecting.23−28 Our group has previously reported the role
of amino acid-modified CQDs as promising antiamyloid
carbon-based nanoparticles.29 CQDs possess a straightforward,
controllable, and clean synthetic route, embracing a broad
range of starting materials, mostly nature-derived compounds.
CQDs are facilely functionalized by various types of chemical
functional groups, such as −NH2, −CO, −OH, and −SH
groups, from a diverse range of natural compounds such as
amino acids.30−34 Despite these outstanding features, CQDs
are yet to find extensive use in catalysis and due to the
aforementioned aspects, they can be eminently suitable and
maneuverable skeletons for bioinspired and biomimetic
catalysis.35−39 The mild condition achieved in acid-catalyzed
organic synthesis has been well-established by CQDs.40,41
Wang et al. studied the phase transfer of CQDs between
aqueous−organic layers by inducing CO2 bubbles.41 Using this
phenomenon in Knoevenagel condensation as a model
reaction, it was found that amine-functionalized CQDs by
means of acid−base transformations were able to catalyze the
reaction and control their phase transfer behavior in separation
of the CQDs from the reaction media. In a recent report,35 the
catalytic properties of CQDs were exploited in C−C bond
formation through the well-known aldol condensation
reaction. In another report as an acid catalyst, it was shown
that CQDs could generate protons under irradiation with
visible light, and a number of acid-catalyzing organic reactions
were successfully inspected using CQDs as the catalyst.39
During our initial studies on the catalytic properties of
nitrogen-doped CQDs in C−C and C−N bond formations
through harsh known organic reactions such as Ullmann and
Buchwald−Hartwig reactions,42−44 it was noted that aniline
could be converted into phenyl urea in choline chloride urea
deep eutectic solvent (ChCl/U DES) with Lys-CQD as the
catalyst of the reaction in a smooth, meaningful manner.
Searching the literature carefully, it was determined that there
are only a few reports on the catalytic approaches for the
aforementioned reaction, and for the most part, such a reaction
was performed under a very harsh condition mostly at high
temperatures using metal-based catalysts.45−47 For example, it
has been reported45 that carbamates and monosubstituted urea
derivatives could be synthesized under a 10 mol % catalyst
loading of superparamagnetic Fe3O4 in ZnCl2−choline
chloride DES at 130 °C. More recently, a polymer-supported
iron catalyst [Fe(II) (Anthra-Merf)], Merrifield anchored
iron(II)-anthra catalyst, was developed for this type of
transformation using urea as a carbonyl source in dioxane at
100−130 °C.46 Despite the efficacy of the introduced
approaches, considering the harsh conditions of the reactions,
employing metal-based catalysts and in some instances toxic
solvents, and considering the utility of the products in various
applications, introducing a milder condition under a metal-free
catalyst and an eco-friendly solvent is highly desirable.
Therefore, in the present work, conversion of aniline into
phenyl urea using Lys-CQD as a catalyst was chosen as a
model reaction since aniline is not considered as a strong
nucleophile and urea, although a natural scaffold, is not an
active carbonyl source. It is also worth mentioning that in the
case of aliphatic amines, the conversion was carried out at 37
°C. Encouraged by these accomplishments, subsequently, a
cascade catalysis pathway for synthesizing 2-benzoxazolinone
from 2-aminophenol using Lys-CQD as the tandem catalyst of
the reaction was successfully obtained at 60 °C. It is worth
mentioning that despite several reports on synthesizing 2-
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Figure 1. (A) UV−visible absorption spectrum of 0.2 mg/mL Lys-CQDs dispersed in water was used; (B) calculated band gap of the as-prepared
Lys-CQDs; (C) PL emission spectra of the as-synthesized Lys-CQDs excited at different excitation wavelengths (the inset is the optical image of as-
synthesized CQDs under UV light); and (D) FT-IR spectra of the as-synthesized Lys-CQDs (red line (a)), Arg-CQDs (blue line (b)), and Gly-
CQDs (yellow line (c)). An equal amount of each CQD (10 mg) was used to prepare KBr tablets.
benzoxazolinone, none of the approaches have been carried
out under mild conditions.
it is evident from the UV−vis spectrum, the presence of a tail
ranging from about 380 to 500 nm (visible region) suggests
that the synthesized Lys-CQDs may possess photocatalytic
activity under visible light irradiation; yet other important
parameters such as the amount of band gap and the
recombination rate of electron−hole pairs should also be
considered. Using data from the UV−vis spectrum, the band
gap of Lys-CQDs was calculated using the Tauc plot. As shown
in Figure 1B, the band gap of the CQDs is 3.3 eV, which is
comparable to some UV light-irradiated metal oxide catalysts,
such as TiO2 and ZnO.
As a part of our interest in developing green trends toward
organic transformations using bio- and bioinspired catalysts,
here, we report a metal-free catalytic approach to organic
transformations of monosubstituted urea and to carry out a
cascade catalysis approach for synthesizing 2-benzoxazolinone
using urea as a clean and abundant carbonyl source. To the
best of our knowledge, this is the first metal-free catalytic
approach for carbonylation using urea (a renewable feedstock
for carbonyl source) and the first tandem catalysis in organic
transformations using CQDs as the catalyst of the reaction.
To obtain further insights into the optical properties of the
CQDs, a detailed photoluminescence (PL) spectroscopy study
was performed with different excitation intervals ranging from
300 to 440 nm. Based on Figure 1C, as the excitation
wavelengths increase, the emission increases to its highest level,
followed by a decrease in the emission. Fourier-transform
infrared (FT-IR) spectra were studied to provide structural
insights into the synthesized Lys-CQDs. As shown in Figure
1D, in the red curve (a) for Lys-CQDs, a broad peak ranging
from 2500 cm−1 to about 3500 cm−1 was detected, suggesting
the presence of a plethora of hydroxyl groups on the Lys-
CQDs, and additionally, an obvious peak at 1354 cm−1
corresponding to O−H in-plane bending vibrations was
observed. The peaks at about 3453 and 3210 cm−1 can be
attributed to amino groups, suggesting the successful doping of
lysine on the CQD surface. A characteristic peak, centered at
1704 cm−1 confirmed the presence of carbonyl groups. The
peak at 2872 cm−1 can be attributed to the C−H stretching of
the sp3 aliphatic chain of lysine, indicating the modification of
the CQD surface with the alkyl amino side chains of lysine
instead of the amino groups doped on the CQD surface.
In comparison to the Gly-CQDs (Figure 1D, yellow curve
(c)), the characteristic peak due to sp3 C−H stretching was
not observed, likely due to the lack of a long amino side chain.
RESULTS AND DISCUSSION
■
The Lys-CQDs were synthesized by a mild and clean
solvothermal method (Scheme 1), using citric acid as a natural
carbon source and lysine as the nitrogen source (Supporting
Information, the Experimental Section). Doping CQD with
lysine was inspired due to the presence of Lys in a number of
enzymes’ active sites and the ubiquity of the long amino arm of
its side chain in various biocatalytic processes. Envisioning that
through modifying synthesized CQDs with lysine, it is possible
to make the amino functionality more available for catalytic
operations. The optical image of synthesized Lys-CQDs
dispersed in water under normal and UV light is shown in
Figure S1. To investigate the importance of doping CQDs with
lysine, glycine-doped CQDs (Gly-CQDs) and arginine-doped
CQDs (Arg-CQDs) were also synthesized (Supporting
Information, the Experimental Section), and they were tested
in parallel with Lys-CQDs in the model reaction. A typical
ultraviolet and visible (UV−vis) absorption spectrum is
depicted in Figure 1A. The peaks at around 270 and 345 nm
are attributed to the absorption of hydroxyl, amino, and
carboxyl groups on the surface of Lys-CQDs, resulting from
n−π* transitions of the corresponding groups. Furthermore, as
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Figure 2. (a) Back titration of Lys-CQD (4.5 mg/mL Lys-CQD was titrated with HCl 0.1 M after adding 1 mL of NaOH 0.5 M). (b) Gran plot of
Lys-CQD based on back titration of Lys-CQD.
Figure 3. (a) SEM image of the as-synthesized Lys-CQDs, (b) EDX analysis of the as-synthesized Lys-CQDs, (c) TEM image of Lys-CQDs, (d)
particle size distribution histogram, and (e) the XRD pattern of Lys-CQD.
In addition, there was no sign of the broad peak, ranging from
2500 to 3500 cm−1, that could be attributed to the hydroxyl
groups, which most likely was due to insufficient functionaliza-
tion with hydroxyl groups on the CQD surface. In the case of
Arg-CQDs (Figure 1D, blue curve (b)), the characteristic peak
of C−H stretching of the sp3 aliphatic chain was observed,
demonstrating the insertion of a lengthy alkyl amino side chain
in the CQDs using amino acid with a long side chain.
After a qualitative analysis of lysine-modified CQDs, a Gran
plot analysis of back titration (Supporting Information, the
Experimental Section) of Lys-CQDs was carried out to obtain
the total number of acid/base functionalities on the surface of
the CQDs (Figure 2).48,49 The number of acid/base sites
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Figure 4. (a) AFM image of the as-synthesized Lys-CQDs and (b) a 3D view of the as-synthesized Lys-CQDs.
Table 1. Control Experiments on the Efficiency of (a) Lys-CQD and (b) Choline Chloride Urea Deep Eutectic Solvent (ChCl/
U DES)
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calculated on the surface of the CQDs was found to be 22100
μmol/g, which reveals a high rate of functionalization
compared with values reported for modified CQDs in the
literature.48 Furthermore, by comparison with citric acid and
lysine as starting materials used in the synthetic process of Lys-
CQD, an estimation of the acid/base sites retained was
calculated to be 61%, suggesting that many carboxyl and amino
groups could retain themselves during the synthetic process of
Lys-CQD. Furthermore, the NH2 loading of Lys-CQD was
calculated using the quantitative Kaiser test protocol48,50
(Supporting Information, the Experimental Section), and 5500
μmol/g was calculated for Lys-CQD. The obtained amount of
NH2 loading reveals that the total number of acid/base sites is
about four times higher than the amount of NH2 loading.
Field-emission scanning electron microscopy (FE-SEM) was
performed to investigate the surface properties of Lys-CQDs.
As shown in Figure 3a, the SEM image revealed a uniform
surface for Lys-CQDs. Furthermore, EDX analysis indicated
the successful doping of nitrogen on the CQDs (Figure 3b).
To further explore the CQDs, transmission electron micros-
copy (TEM) was used (Figure 3c). It became apparent that
the CQDs were well-dispersed with a diameter of about 1−8
nm. These results along with the fluorescence properties of
CQDs (Figures 1C and S1) are entirely consistent with
previous reports on the characterization of CQDs,51 revealing
blue emission for CQDs with a size range below 10 nm. The
particle size distribution histogram clearly demonstrates that
the particles have uniform dispersion with the maximum
frequency of size distribution at about 3 nm (Figure 3d). The
formation of CQDs is also additionally represented in the XRD
pattern of Lys-CQD (Figure 3e). The broad diffraction peak at
2θ = 20.19° (d = 0.56 nm) demonstrated the formation of
amorphous CQDs, and the noticeable peak broadening at
around 20° could be attributed to the reduction of the
crystalline domain size below 10 nm, as observed in the TEM
image (Figure 3c).52 Moreover, the considerably large amount
of interlayer spacing could be attributed to the existence of
abundant functional groups as observed in quantification of
acid/base sites by the back titration of Lys-CQD.
Dynamic light scattering (DLS) analysis was performed to
obtain the hydrodynamic size distribution of CQDs dispersed
in water. A size distribution from about 28 to 58 nm, with the
maximum frequency at around 30 nm, was computed (Figure
S2). Due to hydration, the hydrodynamic diameter of particles
in water should be larger than the diameter of particles in the
vacuum condition. The observed size difference between the
results obtained from TEM and DLS indicates that the
synthesized Lys-CQDs are highly hydrophilic, suggesting the
presence of many functional groups on the CQDs as confirmed
by FT-IR and UV−vis spectroscopies, PL spectra (Figure 1A−
D), and the XRD pattern (Figure 3e). To further verify the
formation of CQDs, atomic force microscopy (AFM) was
carried out. As shown in Figure 4a,b, the AFM images confirm
the presence of well-dispersed CQDs, and the 3D image of
synthesized Lys-CQDs demonstrates a highly functionalized
surface of Lys-CQDs.
toward positive charge yet with less intensity than CQDCit,
suggesting the presence of both negatively and positively
charged functional groups on the surface of synthesized Lys-
CQDs.
The catalytic properties of Lys-CQDs were evaluated in the
carbonylation reaction using urea as a carbonyl source. The
Lys-CQD-catalyzing reaction of aniline in ChCl/U DES
(Supporting Information, the Experimental Section) was
chosen as the model reaction. To our surprise, the reaction
of aniline as a weak nucleophile resulted in a 54% reaction
yield. To explore the influence of the catalyst and ChCl/U
DES in the reaction, the selected model reaction was inspected
under different circumstances (Table 1). As shown in Table 1a,
entry 2, the control reaction in the absence of the catalyst
resulted in no product, indicating the catalytic effect of Lys-
CQDs on the reaction. To further explore the efficiency of the
catalyst, three reactions with lysine, citric acid, and a 1:1
mixture of lysine and citric acid as catalysts were examined, and
in all cases, only a trace amount of the product was observed,
confirming the efficacy of the Lys-CQD as the catalyst for the
reaction (Table 1a, entries 8, 9, and 10, respectively). To test
whether other types of nitrogen-doped CQDs using amino
acids as the nitrogen source can effectively catalyze the
reaction, N-doped CQDs were synthesized using citric acid as
the carbon source and L-glycine or L-arginine as the nitrogen
sources (Supporting Information, the Experimental Section).
As depicted in Table 1a, entries 3 and 4, Gly-CQD catalyzed
the reaction to a much lesser extent than Lys-CQD, while Arg-
CQD was very ineffective as a catalyst for the reaction. Based
on the control experiments, it can be inferred that Lys-CQDs
possess an alkyl amino arm on their surface instead of a mere
amino group, allowing for amino groups in Lys-CQDs to move
freely and be available in catalyzing the reaction more
effectively. As shown in Figure 1D, FT-IR spectra of Lys-
CQDs showed a characteristic peak at 2872 cm−1, which
corresponded to a C−H stretch of the sp3 aliphatic chain of
lysine. However, Gly-CQD lacked such sp3 C−H stretching,
and in the case of Arg-CQD, a likely explanation would be that
its side chain ends with a guanidino group instead of a primary
amino group. Furthermore, lysine and citric acid were
separately used as starting materials to synthesize CQDs,
CQDLys, and CQDCit, respectively (Supporting Information,
the Experimental Section), and they were used in the place of
Lys-CQD as the catalyst in the model reaction. Observing no
product in both cases (CQDLys and CQDCit) was further
considered evidence of the suitability of the Lys-CQD catalyst
(Table 1a, entries 5 and 6). Moreover, when a 1:1 weight ratio
of CQDCit and CQDLys was tested, only a 13% yield of the
product was achieved, demonstrating the importance of
merging citric acid with lysine in the fabrication step of Lys-
CQD (Table 1a, entry 7). To test the catalytic influence of
ammonium ion in the reaction, perhaps by activation of the
carbonyl group of the urea moiety, ammonium acetate was
used as the catalyst of the model reaction instead of Lys-CQD
(Table 1a, entry 11). A lower yield compared with Gly-CQD
and Lys-CQD underlined our initial hypothesis about much
better accessibility of the amino group for catalytic function in
Lys-CQD.
To provide an insight into the charge of the functional
groups on the surface of Lys-CQDs, agarose gel electrophoresis
was conducted (Supporting Information, the Experimental
Section). CQDLys, as expected, migrated toward the negative-
charge electrode while CQDCit moved toward the positive-
charge electrode, verifying the positively charged CQDLys and
negatively charged CQDCit, respectively. Lys-CQDs migrated
With no yield or a trace amount of the product in the
reaction even with an excess amount of urea, it was inferred
that ChCl/U DES as the solvent is essential for the reaction
and may indicate its co-catalyzing role in the carbonylation
reaction using urea (Table 1b). The reaction also failed to
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occur when ChCl/glycerol DES (Supporting Information, the
Experimental Section) was used in place of ChCl/U DES as
the solvent of the reaction.
be 0.1 g in the presence of 2 mmol of the substrate. Moreover,
the effect of catalyst loading was investigated by following
UV−vis absorption and tracing the absorption of aniline at 280
nm (Supporting Information, the Experimental Section). To
this, the modal reactions with 0.05, 0.1, and 0.15 g of the
catalyst loadings were carried out for 30 and 120 min, and the
consumption of aniline compared to its initial absorption was
studied using UV−vis spectra. The results revealed that the
absorption of aniline considerably decreased from 0.05 g of the
catalyst loading to 0.1 g. Yet, a fairly low decrease in the
absorption of aniline was observed by increasing the catalyst
loading from 0.1 to 0.15 g (Figure S3), revealing that a further
increase in the catalyst loading could not furnish a remarkable
increase in the yield of the reaction.
The effect of the stoichiometric relationship between citric
acid and lysine in synthesizing the catalyst was studied by
setting up the model reaction with Lys-CQDs composed of
lysine and citric acid moieties in 1:1, 2:1, and 3:1 molar ratios,
respectively. The yield of the reaction decreased dramatically
with an increase in the lysine concentration, indicating that the
negative effect of the excess amount of lysine likely resulted in
good dispersion of the amino functionalities on CQDs (Table
2). Furthermore, the model reaction was investigated to
Table 2. Evaluation of the Effect of the Molar Ratio of Citric
Acid and Lysine in as-Synthesized Lys-CQD on the Yield of
the Reaction
After attaining the optimal condition of the reaction, the
scope of the reaction was investigated. Several organic
compounds were utilized as nucleophiles in carbonylation
with urea by forming C−N bonds in monosubstituted urea
scaffolds. All nucleophiles reacted with urea in good to
excellent yields; in the case of some nucleophiles, such as
aniline, despite their weak nucleophilicity, a surprisingly good
yield was achieved (near 60%) (Table 4). In the case of 2,6-
dimethylaniline and 2,4,6-trimethylaniline in comparison with
aniline, a significantly enhanced yield was achieved; this result
may suggest that not only did the steric effect of two methyl
groups not have an undue influence on the yield but also, due
to the imposing steric effect of two methyl groups on the
amino group, the compound may have deviated from the
planar sp2 geometry, thereby being more available to react with
urea. Further evaluation of the scope of the reaction was
achieved by investigating various aniline derivatives with
electron-donating and electron-withdrawing groups (entries
4−10). As expected, the electron-donating groups (entries 4−
6) slightly increased the yield of the model reaction, and in the
case of the 1-(2-methoxyphenyl) urea derivative (entry 4), the
yield reached around 70%. But conversely, aniline derivatives
with electron-withdrawing groups (entries 7−10) reduced the
yield, and a fairly sharp decrease in the yield was observed in
the case of the 1-(3-acetylphenyl) urea derivative (entry 10).
To further expand the scope of the reaction, heterocyclic,
aliphatic, and secondary amines were studied (entries 11−14).
As expected, excellent yields were obtained with aliphatic
nucleophiles such as phenyl propyl amine and benzylamine, 93
and 85%, respectively (entries 12 and 13). It is worth
mentioning that in these two cases, the reaction was carried
out at 37 °C, demonstrating the effectiveness of the catalyst at
such a low temperature. To show the desired effect of the
catalyst on the reaction progress with aliphatic amines, a
control reaction with benzylamine was conducted under the
same conditions as the aliphatic amines (37 °C, 6 h) yet in the
absence of the catalyst. Only 20% of the product was isolated,
which was around four times lower than that of the product
yield in the presence of the catalyst. Furthermore, N-
methybenzyl amine as a secondary amine afforded 55% of
the product (entry 14), indicating the efficiency of Lys-CQD in
carbonylation of the secondary amines as well. When
tryptamine, a naturally occurring monoamine alkaloid and an
agonist for serotonin receptor, was used as substrate, the
corresponding urea derivative was obtained with a high yield,
indicating the desirability of the method for functionalization
of bioactive compounds, e.g., tryptamine-derived drugs (entry
15). Additionally, it should be noted that the reactions with
a
b
c
entry
Lys-CQD
yield (%)
1
2
3
1:1
1:2
1:3
54
40
28
a
Reaction conditions: 2 mmol of aniline, 1 mL of ChCl/U DES, 0.07
g of the catalyst, and 6 h. Molar ratio of citric acid and lysine in as-
synthesized Lys-CQDs. Isolated yield.
b
c
determine whether the yield of the reaction could exceed the
calculated yield versus time. It was observed that the reaction
yield reached 54% in 6 h and, except for subtle increases, the
yield remained constant. Subsequently, the effect of catalyst
loading was examined in the reaction; the model reaction was
performed under the same conditions with 0.03, 0.05, 0.07, 0.1,
and 0.15 g of catalyst loading with Lys-CQDs produced by
lysine and citric acid in a 1:1 molar ratio. The yield of the
reaction remained relatively constant up to 0.07 g of the
catalyst loading, and then a remarkable increase in the yield
was achieved at 0.1 g of the catalyst loading with a 60% yield of
the reaction (Table 3). A further increase in the catalyst
loading up to 0.15 g did not increase the yield of the reaction,
and therefore the optimal catalyst loading was determined to
Table 3. Evaluation of the Effect of the Catalyst Loading on
the Yield of the Reaction
a
b
c
entry
catalyst loading (g)
yield (%)
1
2
3
4
5
0.03
0.05
0.07
0.1
49
52
54
60
60
0.15
a
Reaction conditions: 2 mmol of aniline, 1 mL of ChCl/U DES, and
6 h. Lys-CQD produced by citric acid and lysine in a 1:1 molar ratio
was used. Isolated yield.
b
c
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Table 4. Scope of the Reaction with a Diverse Range of Organic Substrates
a
All reactions were performed with 2 mmol of substrate, 1 mL of ChCl/U DES, and 0.1 g of the catalyst at 60 °C for 6 h unless otherwise
b
c
mentioned. Isolated yield. At 37 °C.
Scheme 2. Catalytic Cascade Reaction for Synthesizing 2-Benzoxazolinone from 2-Aminophenol in Two Steps
phenol and benzyl alcohol failed to afford the corresponding
carbamates.
To show further the multifunctionality of the catalyst, a
cascade catalytic approach for synthesizing 2-benzoxazolinone
starting with 2-aminophenol was developed. Benzoxazolinones,
in general, are commercially important compounds mostly due
to their applications in pharmaceuticals. As shown in Scheme
2, the entire transformation requires two steps: 2-aminophenol
is converted to 1-(2-hydroxyphenyl) urea in the first step and,
subsequently, 2-benzoxazolinone is obtained in the second
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step. In the presence of the synthesized catalyst (Lys-CQD),
the reaction afforded a 73% yield. To verify the cascade
catalytic formation of 2-benzoxazolinone, 1-(2-hydroxyphenyl)
urea as the product of the first step was isolated and its
transformation into 2-benzoxazolinone was investigated under
the same conditions used for the scope reaction (60 °C, 6 h) in
the absence of the catalyst. With no detection of 2-
benzoxazolinone, the cascade catalytic formation of the final
product was experimentally confirmed. Moreover, to ensure
the structure of the final product of the cascade reaction (2-
benzoxazolinone), the crystals of the product were prepared
and the atomic structure of the compound was determined
using X-ray crystallography (Figure 5).
activated mechanism is proposed (Scheme 3). The mechanism
suggests that the carbonyl moiety in urea in ChCl/U DES is
most likely activated by Lys acting as an acid, from Lys-CQDs,
and the urea in the solvent becomes capable of readily
accepting nucleophiles such as weak amines. Following the
proton transfer from the newly bonded nucleophile to the
amine group in urea, a molecule of ammonia could leave the
intermediate, and the product could be formed as mono-
substituted urea. To provide precise experimental evidence for
detecting ammonia as the leaving group, the Nessler method
was used (Supporting Information, the Experimental Section).
As shown in Figure S5, the result of the Nessler experiment,
the appearance of a reddish-brown color in the reaction
mixture (Eppendorf tube d) can be taken as evidence for
ammonia as the leaving group in the reaction while Eppendorf
tubes of the reaction mixture in the absence of the catalyst and
solely ChCl/U DES remained colorless in the presence of the
Nessler reagent (K2[HgI4]) (tubes a and b, respectively).
In conclusion, Lys-CQD was synthesized through a mild
solvothermal process from nature-derived precursors and used
as an acid-activated catalyst for carbonylation of amines using
urea as a renewable feedstock in ChCl/U DES. The control
experiments have suggested that both Lys-CQD as the catalyst
and ChCl/U DES as the reaction media and the source of urea
are indispensable parts of the reaction. The desired trans-
formation was achieved at a highly mild condition at 60 °C for
aniline and at 37 °C for aliphatic amines with a low catalyst
loading (0.1 g); in other words, the reactions met all of the
Figure 5. Molecular structure of the compound with displacement
ellipsoids at 50% probability and the atom numbering scheme.
Based on the control experiments, the characterization of the
catalyst, and the substrate scope of the reaction, an acid-
a
Scheme 3. Proposed Acid-Activated Mechanism
a
(1) Lys-CQD as a proton-donor catalyst, activating one molecule of urea in ChCl/U DES. (2) Nucleophile attacking the carbon of the acid-
activated urea. (3) Proton transfer from the nucleophile to one amine moiety of urea. (4′) Release of an ammonia molecule and the protonated
product. (4) Regeneration of ChCl/U DES. (5) Regaining the acidic proton by Lys-CQD and generating the final product.
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requirements of green chemistry. Furthermore, a cascade
approach for synthesizing 2-benzoxsazolinone from 2-amino-
phenol was developed. The catalyst was characterized using
various analytical techniques to obtain a profound insight into
the catalytic features of synthesized Lys-CQDs. TEM, SEM,
XRD, and AFM analyses of the catalyst proved a uniformly
well-dispersed surface with a particle size of around 1−8 nm.
Moreover, comparative results of IR spectra and the control
reactions with Gly-CQDs and Arg-CQDs proved the initial
hypothesis about installing a long alkyl amino arm as the active
site of the catalyst on the CQD surface using lysine as the
source of nitrogen doping, thereby allowing for the amino
functionality to be more available for catalyzing the reaction.
The total number of acid/base sites of Lys-CQD was
calculated by back titration of the CQDs with HCl 0.1 M,
and it was found that there was a high rate of acid/base sites on
the surface of CQDs. Based on the obtained data, an acid-
activated mechanism for the carbonylation reaction was
proposed.
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
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AUTHOR INFORMATION
■
Corresponding Author
Hamid R. Kalhor − Biochemistry Research Laboratory,
Chemistry Department, Sharif University of Technology,
Tehran, Iran; orcid.org/0000-0002-0654-0513;
Phone: +982166165315; Email: kalhor@sharif.edu;
Fax: +982166165315
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors would like to thank Amin Yahyazadeh for
technical assistance. They are also grateful to Dr. Reza Kia
from Sharif University of Technology and Prof. Abbasi, from
University of Tehran, for their help in X-ray crystallography
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