Novel 1,3,4-thiadiazole conjugates derived from protocatechuic acid: Synthesis, antioxidant activity, and computational and electrochemical studies

Article abstract of DOI:10.1016/j.crci.2019.06.001

A series of 15 novel 1,3,4-thiadiazole amide derivatives containing a protocatechuic acid moiety were synthesized and structurally characterized. In addition, the corresponding imino (4) and amino (5) analogues of a phenyl-substituted 1,3,4-thiadiazole am

Full text of DOI:10.1016/j.crci.2019.06.001

C. R. Chimie xxx (xxxx) xxx
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ꢀ
Full paper / Memoire
Novel 1,3,4-thiadiazole conjugates derived from
protocatechuic acid: Synthesis, antioxidant activity, and
computational and electrochemical studies
b
ꢀ a
ꢀ a
ꢀ c
Katarina Jakovljevic , Milan D. Joksovic , Bruno Botta , Ljiljana S. Jovanovic ,
ꢀ a
ꢀ d
ꢀ a
ꢀ a
Edina Avdovic , Zoran Markovic , Vladimir Mihailovic , Marijana Andric ,
,
*
ꢁ
ꢀ e
ꢀ a
Snezana Trifunovic , Violeta Markovic
Faculty of Science, Department of Chemistry, University of Kragujevac, R. Domanovica 12, 34000 Kragujevac, Serbia
Dipartimento di Chimica e Tecnologie del Farmaco, Sapienza Universita di Roma, P.le Aldo Moro 5, 00185 Roma, Italy
Faculty of Sciences, University of Novi Sad, Trg D. Obradovica 3, 21000 Novi Sad, Serbia
a
ꢀ
b
c
ꢂ
ꢀ
d
ꢁ ꢀ
Department of ChemicaleTechnological Sciences, State University of Novi Pazar, Vuka Karadzica bb, 36300 Novi Pazar, Serbia
e Faculty of Chemistry, University of Belgrade, Studentski trg 16, 11000 Belgrade, Serbia
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of 15 novel 1,3,4-thiadiazole amide derivatives containing a protocatechuic acid
moiety were synthesized and structurally characterized. In addition, the corresponding imino
(4) and amino (5) analogues of a phenyl-substituted 1,3,4-thiadiazole amide derivative 3a
were prepared to compare the effects of the structural changes on the radical-scavenging
activity. The obtained compounds were examined for their antioxidative potential by 2,2-
diphenyl-1-picrylhydrazyl (DPPH) and 2,20-azino-bis(3-ethylbenzothiazoline-6-sulfonic
acid) (ABTS) assays. In addition, selected compounds were studied by density functional
theory (DFT) and cyclic voltammetry experiments. The tested compounds showed high po-
tential to scavenging DPPH radical and ABTS radical cation compared with the referent an-
tioxidants ascorbic acid and nordihydroguaiaretic acid (NDGA). On the basis of the calculated
thermodynamic parameters, it can be concluded that the sequential proton loss electron
transfer (SPLET) mechanism represents the most probable reaction path in a polar solvent for
DPPH radicalescavenging activity. On the other hand, the single electron transfer followed by
proton transfer (SET-PT) can be a likely mechanistic pathway in the case of an ABTS radical
cation.
Received 25 April 2019
Accepted 4 June 2019
Available online xxxx
Keywords:
1,3,4-thiadiazole
Protocatechuic acid
Antioxidant
DFT
Electrochemistry
© 2019 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1. Introduction
diseases [4], and neurodegenerative disorders [5]. Antioxi-
dants are substances that, in low concentration, can prevent
Reactive oxygen species (ROS), as products of cellular
metabolism, can have both beneficial and harmful roles [1].
Oxidative stress occurs as a consequence of overproduction
of reactive species and may cause significant damage to
molecules of high biological importance, thus contributing to
various health problems such as cancer [2], aging [3], heart
or slow down damage to cells caused by free radicals and
inhibit oxidative stress, DNA mutations, malignant changes,
and other forms of cell damage [6]. Among them, phenolic
compounds (including phenolic acids) have attracted much
attention because of their ubiquitous occurrence in nature
and pronounced free radicalescavenging activity attributed
to the radical-scavenging function of their phenolic hydroxyl
groups [7]. o-Dihydroxyl configuration (i.e., catecholic group)
is commonly considered to be a substantial characteristic
* Corresponding author.
ꢀ
E-mail address: markovicvioleta@kg.ac.rs (V. Markovic).
https://doi.org/10.1016/j.crci.2019.06.001
1631-0748/© 2019 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.
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Please cite this article as: K. Jakovljevic et al., Novel 1,3,4-thiadiazole conjugates derived from protocatechuic acid: Synthesis,
antioxidant activity, and computational and electrochemical studies, Comptes Rendus Chimie, https://doi.org/10.1016/
j.crci.2019.06.001

ꢀ
K. Jakovljevic et al. / C. R. Chimie xxx (xxxx) xxx
2
which governs the antiradical activity of phenolic antioxi-
dants [8]. Protocatechuic acid (3,4-dihydroxybenzoic acid) is
a naturally occurring catechol-type phenolic acid found in
many fruits and vegetables and is reported to have antibac-
terial [9], antimutagenic [10], antiinflammatory [11], and
antihyperglycemic [12] activities. It is also identified as a
potent antioxidant agent acting as a good protector against
oxidative stress, thus demonstrating the preventive effect on
different malignant diseases [13,14].
(ABTS) methods, supported by density functional theory
(DFT) study and electrochemistry.
2. Results and discussion
2.1. Chemistry
The synthesis of a series of novel 1,3,4-thiadiazole am-
ides (3aeo) is performed in two steps presented in Scheme
1. Initially, heterocyclic 1,3,4-thiadiazole precursors (2aeo)
were prepared by the reaction of corresponding carboxylic
acid (1aeo) with thiosemicarbazide using phosphoryl
chloride as the reagent and solvent [19]. In the next step,
the obtained 5-substituted-1,3,4-thiadiazole-2-amines
(2aeo) were coupled with 3,4-dihydroxybenzoyl chloride
(protocatechuic acid chloride) in dry dioxane, resulting in
the formation of the final amide derivatives 3aeo in
moderate to good yields (51e75%). The coupling reaction
was performed in the presence of sodium hydrogen car-
bonate to neutralize liberated hydrogen chloride. In
accordance with previously published synthesis of 1,3,4-
thiadiazole derivatives [20], the preparation of an amide
requires long time and high temperature because of poor
nucleophilicity of the amino group of 1,3,4-thiadiazole. In
addition, an excess of protocatechuic acid chloride was
necessary to complete the reaction. The final compounds
were prepared with satisfactory purity, but to obtain
compounds with very high purity, they can be further pu-
rified by recrystallization from a hot aqueous solution of
EtOH. All compounds were prepared for the first time, and
their structural elucidation was performed by means of 1H
and 13C nuclear magnetic resonance (NMR) spectroscopy,
infrared (IR) spectroscopy, and elemental analysis (see
Supplementary Material). Spectral data obtained from 1H
NMR spectra showed that aromatic protons corresponding
to the phenolic group appeared in the expected range of
1,3,4-Thiadiazole ring, as
a nitrogen- and sulfur-
containing heterocyclic scaffold, is well known for its syn-
thetic application as a useful synthon for the construction
of bioactive molecules with a wide range of biological ac-
tivities [15,16].
A
series of 2,5-disubstituted-1,3,4-
thiadiazoles have been synthesized by Khan et al. [17]
and evaluated for their antioxidant activities, pointing out
one compound with higher superoxide anionescavenging
activity than the standard drug n-propyl gallate. The car-
boxamide and carbothioamide derivatives of 1,3,4-
thiadiazole have been found to exhibit promising antioxi-
dant effects, which are determined by their high efficacy in
trapping the reactive species generated during the oxida-
tive damage [18].
In our previous report, we have carried out the synthesis
of novel conjugates combining 1,3,4-thiadiazole ring and a
phenolic acid moiety [19]. The obtained derivatives exerted
good antioxidant and antiproliferative activity, and these
results prompted us to proceed with the synthesis and
examination of a series of 1,3,4-thiadiazoleechalcone hy-
brids containing a phenolic moiety [20]. This series of
compounds also exhibited a strong antioxidant potential. In
continuation of this research, herein we present the syn-
thesis of new 1,3,4-thiadiazole amides obtained from pro-
tocatechuic acid and evaluation of their antioxidant
potential by 2,2-diphenyl-1-picrylhydrazyl (DPPH) and
2,20-azino-bis(3-ethylbenzothiazoline-6-sulfonic
acid)
Scheme 1. Reagents and conditions: (a) POCl3, rt, H2NHNC(¼S)NH2, 1 h, reflux; (b) 3,4-dihydroxybenzoyl chloride, dioxane, 24 h, reflux.
ꢀ
Please cite this article as: K. Jakovljevic et al., Novel 1,3,4-thiadiazole conjugates derived from protocatechuic acid: Synthesis,
antioxidant activity, and computational and electrochemical studies, Comptes Rendus Chimie, https://doi.org/10.1016/
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3
ppm scale (6.82e7.97 ppm), giving a singlet for H-2 and
two doublets corresponding to H-5 and H-6, for most of the
compounds. The proton belonging to the amide functional
antioxidants ascorbic acid and nordihydroguaiaretic acid
(NDGA). Owing to the very poor solubility of most de-
rivatives from the 3 series in methanol, determination of
radical-scavenging activity of amides was performed in
diluted dimethyl sulfoxide (DMSO). However, the solubility
of compounds 3a, 4, and 5 was sufficient to allow the
execution of DPPH assay in methanol. Because these three
compounds were selected to examine and compare the
influence of the amido, imino, and amino fragments of the
molecule on the antioxidative activity, we wanted to avoid
the possible solvent effects on the rate of H-abstraction
from phenolic compounds [21].
In DPPH assay, all examined compounds showed
significantly stronger antioxidant potential than referent
ascorbic acid and parent protocatechuic acid, whereas
NDGA showed higher or similar DPPH-scavenging activity
compared with the synthesized compounds. The nature of
the R-substituents within the amide series was selected in
such a manner to cover electron-donating, electron-with-
drawing, and steric properties. Electron-donating groups
are found to increase the electron density on the radical
center formed after abstraction of a hydrogen atom from
oxygen or nitrogen and consequently stabilize free radical
intermediates. In accordance with this statement, the
highest DPPH-scavenging activity was found for derivatives
containing electron-donating alkyl groups in their struc-
ture (3c, 3d, 3l, and 3n). Among them, compound 3c, with
group, as anticipated, appeared as a singlet at the highest d-
values (12.12e12.88 ppm) for all derivatives, except for 3e,
where a broad singlet was formed combining signals from
two OH and one NH proton.
To determine the contribution of the amide functional
group to the antioxidative activity of the synthesized de-
rivatives, we have performed the synthesis of the corre-
sponding imino (4) and amino (5) analogues of the 1,3,4-
thiadiazole amide derivative 3a. The synthesis of these
two derivatives is presented in Scheme 2. In the first step,
the desired 1,3,4-thiadiazole derivative containing an imine
fragment (4) was prepared by condensation of 3,4-
dihydroxybenzaldehyde (protocatechualdehyde) and 5-
phenyl-1,3,4-thiadiazol-2-amine 2a, in the presence of
the catalytic amount of glacial acetic acid. Owing to the
decreased reactivity of the amino group of the 1,3,4-
thiadiazole derivative 2a, an excess of aldehyde and pro-
longed reaction time were necessary for completing this
reaction too. Finally, amine analogue 5 was obtained via
simple reduction of the 1,3,4-thiadiazole imine derivative
4, using NaBH4 as a reduction agent in methanol at room
temperature. Final compounds, 4 and 5, were also prepared
for the first time, and their structure was determined by
previously mentioned methods (1H and 13C NMR, IR, and
elemental analysis, see Supplementary Material). The
presence of a singlet at 8.79 ppm, corresponding to the
proton from the CH¼N group, in 1H NMR spectrum of de-
rivative 4 confirmed the proposed structure. On the other
hand, 1H NMR spectrum of compound 5 showed a doublet
at 4.35 ppm corresponding to the methylene group pro-
tons. These protons are coupled with the adjacent NH
proton, which appeared as a triplet at 8.34 ppm.
the IC50 value of 3.53
mM, possessed slightly better activity
than standard antioxidant NDGA (IC50 3.57
mM). Based on
the results obtained from the DPPH assay, derivatives
containing an electron-withdrawing chloro-substituent
Table 1
DPPH- and ABTS-scavenging activities of the 1,3,4-thiadiazoles 3aeo, 4,
and 5.a
Compounds
IC50
(
m
M)
2.2. Antioxidant activity
DPPHꢀ
ABTSꢀþ
To determine the antioxidant potential of the synthe-
sized compounds, as well as to compare them with previ-
ously reported results [19,20], we have used two of the
most widely used antioxidant methodsdDPPH and ABTS
assays. The results of DPPH radicale and ABTS radical
cationescavenging activities of the tested compounds,
expressed as a concentration of tested compound that re-
duces 50% (IC50) of the DPPH and ABTS species, are pre-
sented in Table 1. As can be seen, all examined derivatives
showed high potential to scavenging DPPH radical and
ABTS radical cation compared with the well-known
3a
4.34 0.33
4.48 0.06b
4.31 0.24
3.53 0.25
3.82 0.11
5.44 0.22
4.93 0.01
5.08 0.35
4.57 0.21
3.97 0.13
4.54 0.16
4.50 0.47
3.73 0.04
4.05 0.03
3.85 0.04
4.15 0.08
5.20 0.17b
6.08 0.07b
7.81 0.29
14.26 0.36
3.57 0.04
31.22 0.72
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
4
5
27.67 0.05
30.31 0.27
27.21 2.08
n.d.
n.d.
n.d.
27.73 0.03
27.13 0.46
33.00 0.56
n.d.
27.86 0.02
n.d.
n.d.
n.d.
25.90 0.11
21.69 0.20
99.72 0.88
125.44 1.86
45.78 0.11
PCA
Ascorbic acid
NDGA
n.d., not determined; ABTS, 2,20-azino-bis(3-ethylbenzothiazoline-6-
sulfonic acid); DPPH, 2,2-diphenyl-1-picrylhydrazyl; NDGA, nordihy-
droguaiaretic acid; PCA, protocatechuic acid (3,4-dihydroxybenzoic acid);
SD, standard deviation.
a
Scheme 2. Reagents and conditions: (a) 3,4-dihydroxybenzaldehyde, AcOH,
EtOH, 18 h, reflux; (b) NaBH4, MeOH, 1 h, rt.
Results are mean values SD from three measurements.
Tested compound dissolved in methanol.
b
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4
(3hej) showed a lower antioxidant activity. The com-
pounds 3eeg containing a methoxy group, which acts as an
electron-donating group in ortho and para positions but
shows an electron-withdrawing effect in the meta position,
also exerted weaker activity because of the negative
inductive effect of oxygen. Furthermore, it can be
concluded that the substituent position at the phenyl ring
also influenced the measured activity of the molecule, with
m-substituted derivatives having the highest DPPH-
scavenging potential. To examine the influence of the
amido group on the antioxidative potential of the tested
series, the compounds 4 and 5 were prepared and tested.
Both of them possessed a low DPPH-scavenging activity,
which is still higher than that of protocatechuic acid and
ascorbic acid.
For further examination of the antioxidative potential,
10 compounds were selected (3aed, 3hej, 3l, 4, and 5)
based on their prominent DPPH radicalescavenging activ-
ity. ABTS-scavenging assay showed that all tested com-
pounds had better ABTS radical cationescavenging ability
than ascorbic acid, NDGA, and 3,4-dihydroxybenzoic acid.
In this method, in contrast to the results obtained from the
DPPH assay, the highest activity was observed for com-
pounds 4 and 5, with IC50 values 25.90 and 21.69
mM,
respectively. This is approximately 2e5 times stronger than
the activities of referent antioxidant compounds. The
compounds of the amide series were found to be less
effective against ABTS, with IC50 values between 27 and
Fig. 1. Percentage of inhibition of DPPH (A) and ABTS (B) measured during
90 min of reaction time for compounds 3c, 3i, protocatechuic acid (PCA), and
NDGA. ABTS, 2,20-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid); DPPH,
2,2-diphenyl-1-picrylhydrazyl; NDGA, nordihydroguaiaretic acid.
33
mM, but still more active than the most powerful
examined referent antioxidant NDGA (IC50 45.78
mM). Ob-
tained IC50 values show that compounds 4 and 5 with the
best ABTS-scavenging property possessed the lowest ac-
tivities in the scavenging of DPPH radicals among synthe-
sized compounds. These differences may be attributed to
the different steric accessibility of antioxidants to the
radical site of DPPH and ABTS [22].
DPPH- and ABTS-scavenging activities of compounds 3c
and 3i, which showed high antioxidant potential in both
applied methods, were measured during 90 min and
compared with the activities of protocatechuic acid and
NDGA. The changes in the percentage of inhibition of DPPH
and ABTS for these compounds applied at a concentration
demonstrated an almost linear increase in the percentage
of inhibition of these radicals from 0 min to 90 min of re-
action. This suggests that protocatechuic acid reacts much
slower with the tested radicals than referent antioxidant
NDGA and synthesized compounds 3c and 3i, indicating
considerably better antioxidant properties of the synthe-
sized compounds in comparison with protocatechuic acid.
2.3. Conformational analysis
of 100 mM are presented in Fig. 1. In both applied methods,
referent antioxidant NDGA showed the highest percentage
of inhibition (about 60%) at the beginning of the reaction.
The percentage of inhibition of ABTS for 3c and 3i was
approximately 60% after 5 min of incubation, whereas in
the DPPH method, percentages of inhibition for these
compounds at the same time were 76.45 and 80.36,
respectively. NDGA demonstrated the maximum activity in
the neutralization of DPPH radicals after 15 min of reaction,
whereas 3c and 3i displayed maximum activity after
25 min of reaction. Regarding ABTS radical
cationescavenging activity, the percentage of inhibition for
3c and 3i greatly increased up to 20 min with a slight in-
crease of inhibition until the end of incubation. The per-
centage of inhibition for NDGA in the ABTS method was
almost the same from 0 min to 90 min. Protocatechuic acid
showed different behavior in inhibitions of DPPH and ABTS
compared with compounds 3c, 3i, and NDGA. In reactions
with DPPH and ABTS, 3,4-dihydroxybenzoic acid
The most stable rotamers of the compound 3a are
examined, with different orientations of the protocatechuic
acid part of the molecule. The most stable conformers ob-
tained by the conformational analysis (3a and 3a1) are
presented in Fig. 2. The energy difference between the two
most stable structures in methanol is not negligible
(22.19 kJ/mol). The interconversion barrier between the
two minima is 25.9 kJ/mol, which indicates that confor-
mational interconversion takes place easily at room tem-
perature. In the following text, we will focus only on the
most stable conformer 3a, as well as on the structures of
other compounds derived from this rotamer. The structures
of 3a are nonplanar and have one internal hydrogen bond
(IHB) between two hydroxyl groups bonded to aromatic
ring A (O3eH3$$$O4). The natural bond orbital (NBO)
analysis of investigated compounds reveals that the lone
paireantibonding orbital interactions between the oxygens
and adjacent OeH bond are responsible for the IHB
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Please cite this article as: K. Jakovljevic et al., Novel 1,3,4-thiadiazole conjugates derived from protocatechuic acid: Synthesis,
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5
Fig. 2. Two most stable structures of the compound 3a.
formation. Namely, it is found that the charge transfer from
the lone pair on O4 (2p orbital) to the * antibonding
O3eH3 orbital is a favorable donoreacceptor interaction.
The conformations without IHBs are less stable with
respect to the absolute minimum.
Unlike examined compounds of the 3 series, compounds
4 and 5 are not amides; the amide group which links ring A
with the rest of the molecule is replaced by imino (4) and
amino (5) groups. These two compounds are also not planar,
having one IHB between two adjacent hydroxyl groups on
the ring A. It means that similar antioxidant properties can
be expected for compounds of the 3 series. The most stable
conformers of all the investigated compounds are presented
in Fig. S35 in the Supplementary Material.
of three free radicals (Fig. 3). The exception is compound 4,
which can form only two free radicals, because of the lack
of NeH bond. The corresponding bond dissociation
enthalpy (BDE) values are presented in Table 2. The BDE
values for both of the OH groups are mutually similar and
significantly lower than those for the NH groups. Both ox-
ygen radicals (Fig. 3) are stabilized with one hydrogen
bond, unlike the nitrogen radical. Because no experimental
values are available for investigated compounds, the ob-
tained BDE values are compared with theoretical values for
protocatechuic acid in water. The BDE values for all the
investigated compounds given in this article are similar to
those obtained for protocatechuic acid (339 and 341 kJ/mol,
for 3-OH and 4-OH groups in water) [23]. It should also be
noted that calculated BDE values of these compounds are
significantly lower than BDE values calculated in benzene
and water for chrysin (392.5, 390.8 kJ/mol), pinocembrin
(396.1, 391.4 kJ/mol), apigenin (372.2, 368.1 kJ/mol), and
naringenin (368.3361.8 kJ/mol) [24]. As can be seen from
s
2.4. Radicals, radical cations, and anions
Homolytic cleavage of the OeH and NeH bonds, for
each of the investigated compounds, leads to the formation
Fig. 3. Spin density distribution in all radicals obtained from 3, 4, and 5 in methanol.
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6
Scheme 3 and Fig. 3, additional delocalization of the spin
density for compound 4 is carried out via the nitrogen of
imino group. On the other hand, no additional delocaliza-
tion of spin density over nitrogen was noticed for all other
investigated compounds. In the case of the compound 5,
there is no additional delocalization of the spin density.
When a single electron is removed from a molecule, a
radical cation is formed. Ionization potential (IP) is a
measure for ease of the formation of a radical cation. The
IP values for the investigated compounds in methanol as a
solvent are presented in Table 2. A careful analysis of the
obtained results shows that the IP values are significantly
higher than the other thermodynamic parameters. Even
more, the values of the IP of these compounds are
significantly higher than the IP value for the proto-
catechuic acid [23]. For these reasons, the single electron
transfer followed by proton transfer (SET-PT) mechanism
can be excluded in case of results obtained from the DPPH
test. On the other hand, if we compare the IP values for
compounds 3a, 4, and 5 with experimental IC50 values for
the ABTS-scavenging assay, it is clear that these values are
in good correlation with IP values. This means that SET-PT
can be a likely mechanistic pathway in the case of an ABTS
radical cation. The fact that the IC50 values for ABTS
radical cation are significantly higher than those for the
DPPH radical, i.e., that the reactions free radical neutral-
ization with ABTS radical cation are taking place much
more slower than the corresponding ones with the DPPH
radical, is supporting this assumption. The confirmation
of this assumption will be the subject of our future in-
vestigations that will address at examining the kinetics of
these reactions.
The calculated proton affinity (PA) values for both OH
groups and NH groups of all compounds are given in Table
2. The PA value is always the lowest for the 4-OH group,
indicating that proton transfer from the 4-OH group is
easier than from the NH and another OH group. Signifi-
cantly lower PA values in methanol are the consequence of
the interactions of OH and NH groups and anionic oxygen
with the solvent molecules. Distribution of the negative
charge for all anions is presented in Fig. S36. As already
mentioned, the most stable anions are obtained by depro-
tonation of the 4-OH group. The negative charge in these
anions is delocalized over O4, C5, and C6 atoms of the ar-
omatic ring A and oxygen atom of the amide group, which
contributes to the additional stability of the anion. The
neighboring of OH group additionally contributes to the
stability of the anion by formation of hydrogen bond with
the adjacent hydroxyl group.
Owing to lack of experimental PA values, the obtained
values are compared with the calculated ones for proto-
catechuic acid in water. The PA values for all the investi-
gated compounds are slightly lower than those obtained
for protocatechuic acid (130 and 119 kJ/mol, for 3-OH and
4-OH groups in water) [23]. It should also be noted that
calculated PA values of these compounds are significantly
lower than PA values calculated in water for ortho-, meta-,
and para-hydroxybenzoic acids (191, 158, 150 kJ/mol,
respectively) [25]. On the other hand, they are almost the
same as those calculated for gallic acid (129 and 109 kJ/
mol) [26].
The exception is compound 4. Only in the case of this
compound, additional delocalization of the negative charge
is noticed via the imino group, whereas for other com-
pounds, this effect was not observed. The consequence of
charge delocalization is also reflected on PA values (Table
2). It is obvious that the obtained theoretical results are
in good agreement with the experimental values for DPPH
(Table 1).
Table 2
Calculated thermodynamic parameters (kJ/mol) for the antioxidative ac-
tivity of compounds 3aed, 3hej, 3l, 4, and 5.
Compound
3a
BDE
IP
PDE
PA
ETE
554.1
3-OH
4-OH
NH
337.1
339.4
388.2
ꢁ48.0
ꢁ45.7
3.1
122.3
111.3
116.3
383.8
397.2
440.9
3b
3c
3d
3h
3i
550.2
549.4
541.5
549.9
550.4
555.0
550.1
3-OH
4-OH
NH
337.1
338.8
389.1
ꢁ44.0
ꢁ42.4
7.9
122.2
110.9
117.5
383.9
396.9
440.6
3-OH
4-OH
NH
337.2
338.8
387.2
ꢁ43.3
ꢁ41.6
6.7
122.1
111.0
116.2
384.1
396.9
440.0
3-OH
4-OH
NH
336.8
338.8
384.4
ꢁ35.7
ꢁ33.7
11.9
122.5
111.6
117.2
383.4
396.1
436.3
3-OH
4-OH
NH
340.5
339.1
394.4
ꢁ40.3
ꢁ41.8
13.6
122.3
111.1
115.1
387.2
397.0
448.3
3-OH
4-OH
NH
328.3
330.0
383.0
ꢁ44.3
ꢁ42.6
10.4
121.7
109.9
112.4
384.4
397.8
448.4
3j
3-OH
4-OH
NH
337.0
338.9
388.6
ꢁ49.0
ꢁ47.1
2.6
122.0
110.5
113.2
384.0
397.4
444.4
3l
2.5. DPPH radicalescavenging mechanisms of investigated
compounds
3-OH
4-OH
NH
337.3
338.6
400.0
ꢁ43.8
ꢁ42.4
18.9
122.9
111.5
119.5
383.4
396.2
449.5
4
5
540.4
517.9
As mentioned previously, at least three mechanisms of
the antioxidant activity are operative. To evaluate which
mechanism is dominant under given reaction conditions, it
is necessary to calculate the enthalpies associated with
these three mechanisms [hydrogen atom transfer (HAT),
SET-PT, and sequential proton loss electron transfer
(SPLET)]. It is possible to evaluate the preferred reaction
pathway by simple comparison of BDE, IP, and PA values
3-OH
4-OH
339.2
333.0
ꢁ32.2
ꢁ38.5
121.5
103.9
386.7
398.1
3-OH
4-OH
NH
328.6
325.8
369.3
ꢁ20.3
ꢁ23.1
20.4
136.2
135.5
172.1
361.4
359.3
366.2
BDE, bond dissociation enthalpy; IP, ionization potential; PDE, proton
dissociation enthalpy; PA, proton affinity; ETE, electron transfer enthalpy.
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Scheme 3. Resonance stabilization of radicals formed from amides (3aeo) after heterolytic (A) OeH and (B) NeH cleavage and radicals formed from imine 4 after
heterolytic (C) OeH cleavage and loss of one electron according to the SPLET mechanism. SPLET, sequential proton loss electron transfer.
[27e29]. The lowest value of these thermodynamic pa-
rameters indicates a more probable mechanism.
It is clear, based on the values presented in Table 2, that
IP values are significantly greater than other thermody-
namic parameters. This means that SET-PT is a nonoper-
ating mechanism for these compounds.
On the other hand, PA values for 3-OH, 4-OH, and NH
groups are significantly lower than BDE and IP values, for all
investigated compounds. It means that the SPLET mecha-
nism represents the most probable reaction path in polar
solvents such as methanol. Despite the fact that the 4-OH
group is the most active reaction center, it should be
noted that all three reaction centers have mutually similar
corresponding PA values. Moreover, unlike the HAT mech-
anism, the NH group plays a more important role in the
SPLET mechanism. The PA values for all NH groups are very
close to that of the 4-OH group, especially in the case of
compounds 3i and 3j.
SPLET mechanism, the phenoxyl radical formed from
amide derivatives can be stabilized through resonance
which includes a phenyl ring and neighboring carbonyl
group (Scheme 3A). Herein, the heterocyclic scaffold does
not participate in the stabilization of the radical formed
after homolytic cleavage of the OeH bond. In the case of
homolytic cleavage of the NeH bond, the resonance stabi-
lization includes only 1,3,4-thiadiazole ring (Scheme 3B).
The imino analogue (4) was found to have the lowest
calculated PA value for the 4-OH bond (Table 2), whereas
amine 5 had generally the highest calculated PA values for
3-OH, 4-OH, and NH bonds. This is in agreement with the
high resonance stabilization of the phenoxyl radical formed
after homolytic cleavage of the 4-OH bond of imino
In our previous research, we have determined that
better radical-scavenging activity of phenolic acid de-
rivatives containing 1,2,4-triazole [30] or 1,3,4-oxadiazole
[31] ring in comparison with parent acids can be attrib-
uted to the participation of the heterocyclic scaffold in the
delocalization of the unpaired electron after homolytic
cleavage of the OeH and NeH bonds by the DPPH radical.
This was also supported by DFT calculations for 1,2,4-
triazole derivatives [30]. Similarly, in case of previously
reported conjugates combining 1,3,4-thiadiazole ring and a
phenolic acid moiety [19], as well as 1,3,4-
thiadiazoleechalcone hybrids containing a phenolic moi-
ety [20], the resonance stabilization of the resulting phe-
noxyl or nitrogen radical was achieved by the participation
of the 1,3,4-thiadiazole ring. In this study, as we can see
from Table 1, the presence of 1,3,4-thiadiazole moiety
significantly increases the radical-scavenging activity of the
compounds 3aeo in comparison with parent proto-
catechuic acid and referent antioxidants. As a result of the
Fig. 4. Cyclic voltammograms for 3a (c ¼ 1.14 mM) and PCA (c ¼ 1.15 mM);
GC electrode, v ¼ 0.10 Vsꢁ1. PCA, protocatechuic acid (3,4-dihydroxybenzoic
acid); GC, glassy carbon; SCE, saturated calomel electrode.
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K. Jakovljevic et al. / C. R. Chimie xxx (xxxx) xxx
8
derivative, where the unpaired electron can be delocalized
across the whole molecule (Scheme 3C).
presented in Fig. 4, suggesting reversible chemical steps
after the electron release.
The second oxidation process for all 3-type compounds
(a, d, and j) is shifted to the positive limit of potential and
has the characteristics of irreversible electron transfer. In
these voltammograms, the current function of the peak II is
somewhat higher than for the peak I, which suggests a
possible two-electron process (Fig. 4). Assessment of these
peaks is not easy partly because the aforementioned
compounds display some adsorptive features at these po-
tentials. This oxidation process, however, is much better
defined for compound 5 where the second process appears
at the least positive potentials of all compounds (Fig. 5).
These peaks have apparently similar characteristics as the
peaks I/I'. Because by cycling the potential in the amplitude
ꢁ0.2 to þ1.5 V both oxidation peaks are fully recovered, it
means that in the cited potential range, no irreversible
chemical reaction is present in either step. The second
oxidation process may be ascribed to oxidation of NH; that
is why no peak was observed at 4. On the other hand, the
process in 3l voltammograms is shifted behind the limit of
positive potentials; the reason for that might lie in the
highest BDE value for the NeH bond, which makes the
oxidation more difficult (vide infra, Table 2).
The potentials of the characteristic peaks on the voltam-
mograms of the selected compounds are given in Table 3.
It is common that the compounds containing amino,
imino, or amido nitrogen display reduction processes
with a variety of chemical reactions coupled with electron
transfer [32]. The chemical steps usually involve either
radical dimerization or addition of a proton to stabilize
the obtained anion from the medium or through self-
protonation if there is such a possibility [33e36]. As
mentioned previously, in our cases, three nitrogen forms
are present: amido in compounds denoted by 3, imino in
4, and amino in 5. Among all these, only 4 displayed well-
defined peaks, whereas the voltammograms of other
compounds have the ill-shaped irreversible peaks shifted
to potentials around ꢁ2.0 V (Fig. 6). These processes are
situated near the adsorptive multielectron peaks at the
negative potential limit and therefore appear to be un-
suitable for a detailed characterization. However, judging
by the current functions, all the peaks correspond to a
one-electron reduction. In contrast to that, reduction of
the imine group in 4 is carried out in a one-electron
2.6. Electrochemistry
Selected compounds (3a, 3d, 3j, 3l, 4, and 5) were
investigated in N,N-dimethylformamide (DMF) solutions
containing tetrabutylammonium perchlorate in the po-
tential amplitude of þ1.6 V to ꢁ2.1 V.
In the applicable positive potential range, all com-
pounds exhibit one (3l and 4) or two (3a, 3d, 3j, and 5)
oxidation peaks. For the sake of comparison, the oxidation
process for 3a is presented together with a voltammogram
of PCA, the parent phenolic acid, protocatechuic acid
(Fig. 4). In this range, as reported previously [30], this acid
displays a two-step one-electron oxidation process at peaks
located at close potentials. Here, the shape and the current
function of the first oxidation peak of 3a are quite similar to
those of PCA. Because this is observed at all new com-
pounds, it seems reasonable to attribute the first oxidation
process to the already described oxidation located at hy-
droxyl groups [30]. Shifting of the peak potential toward
less-positive values and better “reversibility” of its counter
peak must be understood as a result of an easier oxidation
process at a conjugated molecule which is in accordance
with the aforementioned discussion.
The electrochemical characteristics of the first process
are given as follows:
D
Eap/c for peaks I/I' for all the com-
pounds at moderate scan rates lie between 300 and
500 mV, while cathodic parts significantly deform at higher
scan rates showing irreversibility of the electron transfer
process. In addition, kinetic parameters of the peak I such
as half-peak width which amounts from 80 mV (at
0.02 Vsꢁ1) up to about 150 mV for higher scan rates
together with
D
Ep/
D
log v ranging from 80 mV (v ¼
0.02e0.20 Vsꢁ1) to 130 mV (0.2e2 Vsꢁ1) are evidence of an
overall complex process. However, if the amplitude of po-
tential is limited behind the peak I, excellent repeatability
of the peaks’ I/I' shapes and potentials was recorded just as
quasireversible pair of peaks:
DEp(Ir'/Ir) amounts from
65 mV (0.02 Vsꢁ1) to 130 mV (2 Vsꢁ1). At scan rates higher
than 0.02 Vsꢁ1, a new small oxidation peak at ~þ0.10 V
appears which in voltammetric time scale apparently
Table 3
Potentials of characteristic peaks; GC electrode (v ¼ 0.10 V sꢁ1).
Compounds
Ep(I)a
Ep(I')
Ep(II)
Ep(Ir)
3a
3d
3j
3l
4
þ0.81
þ0.84
þ0.83
þ0.86
þ0.75
þ0.76
þ0.39
þ0.41
þ0.44
þ0.34
þ0.41
þ0.46
þ1.44
þ1.44
þ1.47
>1.5
ꢁ1.82
ꢁ1.86
ꢁ1.75
<ꢁ2.1
ꢁ1.80
ꢁ2.06
e
5
þ1.20
Fig. 5. Cyclic voltammograms for 5 (c ¼ 0.74 mM); GC electrode, v ¼
GC, glassy carbon; SCE, saturated calomel electrode.
a
0.10 V sꢁ1. GC, glassy carbon; SCE, saturated calomel electrode.
In V versus SCE.
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K. Jakovljevic et al. / C. R. Chimie xxx (xxxx) xxx
9
lowest unoccupied molecular orbital (LUMO) energies
(Table 4), according to the procedure described in the study
ꢀ
by Filipovic et al. [37]. The obtained HOMO values are
compared with that for protocatechuic acid, which appears
to be the lowest of all. Because the acid does not show any
reduction process in the applied potential range, its LUMO
cannot be calculated. The highest value of HOMO in 5 is
related with the lowest IP (Table 2), whereas its LUMO
energy correlates with the lowest electron affinity, i.e.,
value of electron transfer enthalpy (ETE) (vide infra).
3. Conclusion
In conclusion, 15 novel 1,3,4-thiadiazole amide de-
rivatives containing a protocatechuic acid moiety (3aeo),
as well as the corresponding imino (4) and amino (5) an-
alogues of 3a, were prepared and examined for their anti-
oxidative potential. All tested compounds exerted better
DPPH radicalescavenging activity than parent 3,4-
dihydroxybenzoic acid and referent ascorbic acid, with
the highest activity found for derivatives containing
electron-donating alkyl groups (3c, 3d, 3l, and 3n). The
ABTS assay showed that all examined derivatives had bet-
ter radical cationescavenging ability than referent ascorbic
acid, NDGA, and 3,4-dihydroxybenzoic acid. In this method,
in contrast to the results obtained from the DPPH assay, the
highest activity was observed for compounds 4 and 5. To
evaluate which radical-scavenging mechanism is dominant
under given reaction conditions, the values of thermody-
namic parameters associated with these mechanisms were
calculated and compared. The obtained theoretical results
are in good agreement with the experimental values for the
DPPH assay. The PA values are significantly lower than BDE
and IP values, for all investigated compounds, which means
that the SPLET mechanism represents the most probable
reaction path in polar solvents. Furthermore, the IP values
for compounds 3a, 4, and 5 are in good correlation with
experimental IC50 values for the ABTS-scavenging assay.
This indicates that SET-PT can be a likely mechanistic
pathway in the case of an ABTS radical cation. The obtained
electrochemical data showed that the most easily oxidiz-
able compounds are 4 and 5. On the other hand, reduction,
taking place at amido (3), imino (4), and amino (5) nitrogen
atoms, is irreversible for all compounds except for 4. This
can be ascribed to the possibility of additional stabilization
in the imino derivative, which also justifies the lowest
calculated PA value for this compound.
Fig. 6. Cyclic voltammograms for 3j (c ¼ 0.87 mM); GC electrode, v ¼
0.10 V sꢁ1. GC, glassy carbon; SCE, saturated calomel electrode.
does not affect either oxidation or reduction processes.
This peak is most pronounced at 4 but only slightly visible
in voltammograms of all other compounds. Obviously, it
belongs to a reactive intermediate formed after reduction,
the composition and nature of which could be established
by application of other methods such as controlled po-
tential electrolysis. However, it was beyond the scope of
the present study.
As can be seen in Table 3, the most easily oxidizable
compounds are 4 and 5. In general, the first oxidation
process is similar for all six compounds and easier than
oxidation of the parent protocatechuic acid recorded
under the same experimental conditions (Ep
¼
þ0.96 V at v ¼ 0.10 Vsꢁ1) [30]. The second oxidation
process ascribed to oxidation of amido (3), imino (4),
and amino (5) nitrogen proceeds most easily at the
latter one. On the other hand, reduction, which takes
place also at these nitrogen atoms, is irreversible for all
compounds except for 4; this can be ascribed to the
possibility of additional stabilization of the transferred
electron by charge delocalization in the imino
derivative.
The obtained electrochemical data served for calcula-
tion of highest occupied molecular orbital (HOMO) and
Table 4
Calculated values of HOMO and LUMO orbitals.
4. Experimental
Compounds EOxonset/[V]a HOMO/[eV]b ERedonset/[V] LUMO/[eV]c
3a
3d
3j
3l
4
þ0.128
þ0.163
þ0.148
þ0.175
þ0.032
þ0.008
þ0.213
ꢁ4.93
ꢁ4.96
ꢁ4.95
ꢁ4.98
ꢁ4.83
ꢁ4.81
ꢁ5.01
ꢁ2.140
ꢁ2.093
ꢁ2.082
e
ꢁ2.66
ꢁ2.71
ꢁ2.72
e
4.1. Chemistry
4.1.1. Physical measurements and methods
ꢁ2.103
ꢁ2.377
e
ꢁ2.70
ꢁ2.42
e
Melting points were determined on a Mel-Temp capil-
lary melting points apparatus, model 1001 and are uncor-
rected. Elemental (C, H, N, S) analysis of the samples was
carried out in the Center for Instrumental Analysis, Faculty
of Chemistry, Belgrade. UV spectra were recorded using an
Agilent Technologies, Cary 300 Series UV-Vis Spectropho-
tometer. IR spectra were obtained using a PerkinElmer
Spectrum One FT-IR spectrometer with a KBr disc. 1H and
5
PCA
GC, glassy carbon; SCE, saturated calomel electrode; DMF, N,N-
dimethylformamide; TBAP, tetrabutylammonium perchlorate.
a
V versus Fc/Fcþ in DMF containing 0.1 M TBAP as the supporting
electrolyte at a scan rate of 0.10 V sꢁ1, GC electrode.
b
EHOMO ¼ ꢁ4.8 ꢁ (EOx
ꢁ EF1c/2/Fcþ) [37].
onset
c
Fc/Fcþ
1/2
ELUMO¼ ꢁ4.8 ꢁ (ERed
ꢁ EF1c/2/Fcþ); E
¼ þ0.487 V versus SCE.
onset
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10
13C NMR spectra were recorded using a Varian Gemini
200 MHz spectrometer.
Afterward, 20.0 ml of distilled water was added, as well as
2 M HCl, until pH 2 was reached, and the formed precipitate
was stirred for 30 more minutes at the room temperature.
After cooling the flask in a refrigerator for 1.5 h, the pre-
cipitate was filtrated, dried over CaCl2, and recrystallized
from hot 50% aqueous EtOH.
4.1.2. Procedure for the preparation of 3aeo, 4, and 5
4.1.2.1. Procedure for the synthesis of 2aeo. A mixture of
corresponding acid, 1 (2.00 mmol), and POCl3 (0.80 ml) was
stirred for 20 min at room temperature. Then, thio-
semicarbazide (2.0 mmol, 0.182 g) was added, and the
resulting suspension was refluxed for 1 h. After cooling the
flask in an ice bath, 2.4 ml of distilled water was added
carefully, and reflux was continued for 1 h. The mixture was
then cooled to the room temperature, saturated aqueous
solution of NaOH was added until pH 8.5 was reached, and
the suspension was stirred for 1 h at the room temperature.
The formed precipitate of the corresponding 2-amino-
1,3,4-thiadiazole derivative (2aeo) was then filtrated, dried
over CaCl2, and recrystallized from hot 50% aqueous EtOH.
4.1.2.5. 3,4-Dihydroxy-N-(5-phenyl-1,3,4-thiadiazol-2-yl)ben-
zamide ꢂ H2O (3
а). Beige powder; yield: 0.20 g (64%); mp >
250 ꢃC; 1H NMR (200 MHz, DMSO-d6): 6.86 (d, 1H, J ¼
8.2 Hz, AreH); 7.53e7.62 (m, 5H, AreH); 7.97 (s, 1H, AreH);
7.97 (d, 1H, J ¼ 8.2 Hz, AreH); 9.44 (s, 1H, OH); 9.93 (s, 1H,
OH); 12.81 (s, 1H, NH); 13C NMR (50 MHz, DMSO-d6):
115.22, 116.09, 120.94, 122.37, 126.89 (2C), 129.33 (2C),
130.39, 130.47, 145.26, 150.48, 159.56, 161.75, 164.73; IR
(KBr, cmꢁ1): 3524, 3414, 3107, 1632, 1614, 1592, 1520, 1291,
754; anal. calcd. for C15H11N3O3S ꢂ H2O (313.33 g/mol): C,
57.50; H, 3.54; N, 13.41; S, 10.23; found: C, 57.48; H, 3.55; N,
13.39; S, 10.25.
4.1.2.2. Procedure for the synthesis of 3aeo. To the mixture
of 3,4-dihydroxybenzoic acid (2.00 mmol, 0.308 g) in dry
dichloromethane (8.0 ml), SOCl2 (20.00 mmol, 1.5 mL) was
slowly added, followed by two drops of DMF. The resulting
mixture was then stirred for 2 h at room temperature. Af-
terward, the solvent was evaporated under reduced pres-
sure, and the excess of SOCl2 was removed by azeotropic
distillation with toluene. Corresponding substituted 2-
amino-1,3,4-thiadiazole, 2aeo (1.00 mmol), NaHCO3
(2.00 mmol, 0.168 g), and dry dioxane (10.0 ml) were added
to the formed acid chloride, without previous isolation, and
the mixture was then refluxed for 24 h. Afterward, the
solvent was evaporated under reduced pressure, 15.0 mL of
H2O was added to the residue, as well as solid Na2CO3, until
pH 9 was reached, and the formed suspension was stirred
for 30 min at the room temperature, filtrated, washed with
water, and dried over CaCl2. The final compounds were
obtained with satisfactory purity, but to obtain compounds
with very high purity, they can be subjected to further
purification by recrystallization from a hot aqueous solu-
tion of 50% EtOH, for 3l, 65% EtOH, for 3b and 3o, 70% EtOH,
for 3a, 3c, 3f, and 3n, 75% EtOH, for 3m, 80% EtOH, for 3k,
85% EtOH, for 3d and 3e, 90% EtOH, for 3j and 3g, and 96%
EtOH, for 3h and 3i.
4.1.2.6. 3,4-Dihydroxy-N-(5-(o-tolyl)-1,3,4-thiadiazol-2-yl)
benzamide (3b). Beige powder; yield: 0.20 g (61%); mp ¼
246e247 C (Dec.); 1H NMR (200 MHz, DMSO-d6): 2.53 (s,
ꢃ
3H, CH3); 6.86 (d, 1H, J ¼ 8.0 Hz, AreH); 7.31e7.42 (m, 3H,
AreH); 7.55e7.62 (m, 2H, AreH); 7.70 (d, 1H, J ¼ 7.0 Hz,
AreH); 9.77 (bs, 2H, OH); 12.43 (s, 1H, NH); 13C NMR
(50 MHz, DMSO-d6): 21.27, 115.28, 116.11, 121.01, 122.46,
126.47, 129.57, 129.93, 130.12, 131.52, 136.41, 145.34, 150.55,
160.06, 161.31, 164.85; IR (KBr, cmꢁ1): 3404, 3154, 2955,
1685, 1604, 1527, 1300, 1204, 753; anal. calcd. for
C
16H13N3O3S (327.36 g/mol): C, 58.70; H, 4.00; N, 12.84; S,
9.80; found: C, 58.72; H, 4.01; N, 12.82; S, 9.82.
4.1.2.7. 3,4-Dihydroxy-N-(5-(m-tolyl)-1,3,4-thiadiazol-2-yl)
benzamide ꢂ 0.5H2O (3c). Beige powder; yield: 0,24 g (74%);
mp > 250 ꢃC; 1H NMR (200 MHz, DMSO-d6): 2.39 (s, 3H,
CH3); 6.86 (d, 1H, J ¼ 8.2 Hz, AreH); 7.32 (d, 1H, J ¼ 7.6 Hz,
AreH); 7.41 (t,1H, J ¼ 7.6 Hz, AreH); 7.56 (s,1H, AreH); 7.61
(d, 1H, J ¼ 8.2 Hz, AreH); 7.73e7.79 (m, 2H, AreH); 9.72 (bs,
2H, OH); 12.60 (s, 1H, NH); 13C NMR (50 MHz, DMSO-d6):
21.05, 115.29, 116.14, 121.02, 122.45, 124.12, 127.41, 129.26,
130.38,131.19,138.80,145.35,150.56,159.59,161.92,164.78;
IR (KBr, cmꢁ1): 3407, 3281, 3187, 1685, 1608, 1542, 1521,
1320,1298,1107, 749; anal. calcd. for C16H13N3O3S ꢂ 0.5H2O
(327.36 g/mol): C, 58.70; H, 4,00; N, 12.84; S, 9.80; found: C,
58.71; H, 3.99; N, 12.87; S, 9.79.
4.1.2.3. Procedure for the synthesis of 4. To the mixture of
3,4-dihydroxybenzaldehyde (1.87 mmol, 0.259 g) and 2-
amino-5-phenyl-1,3,4-thiadiazole (1.50 mmol, 0.266 g) in
absolute ethanol (5.0 ml), three drops of glacial acetic acid
were added, and the mixture was refluxed for 18 h. The
mixture was then cooled to the room temperature, 10.0 ml
of distilled water was added, and the suspension was then
stirred for 30 min at the room temperature. The precipitate
was then filtrated, dried over CaCl2, and recrystallized from
hot acetone.
4.1.2.8. 3,4-Dihydroxy-N-(5-(p-tolyl)-1,3,4-thiadiazol-2-yl)
benzamide ꢂ 0.5H2O (3d). White powder; yield: 0.23 g
(70%); mp > 250 ꢃC; 1H NMR (200 MHz, DMSO-d6): 2.36 (s,
3H, CH3); 6.86 (d, 1H, J ¼ 8.2 Hz, AreH); 7.34 (d, 2H, JAB
¼
8.0 Hz, AreH); 7.55 (s, 1H, AreH); 7.59 (d, 1H, J ¼ 8.2 Hz,
AreH); 7.85 (d, 2H, JBA ¼ 8.0 Hz, AreH); 9.54 (bs, 1H, OH);
9.74 (bs, 1H, OH); 12.75 (s, 1H, NH); 13C NMR (50 MHz,
DMSO-d6): 19.04, 113.17, 114.03, 118.89, 120.34, 124.76 (2C),
125.63, 127.84 (2C), 138.31, 143.23, 148.42, 157.22, 159.77,
162.61; IR (KBr, cmꢁ1): 3397, 3168, 2956, 1682, 1609, 1532,
1459, 1309, 1298, 1206, 810; anal. calcd. for
4.1.2.4. Procedure for the synthesis of 5. Solid NaBH4
(15.67 mmol, 0.593 g) was slowly added to the stirring
suspension of formed imine, 4 (1.00 mmol, 0.297 g) in
methanol (10.0 ml), and stirring of the resulting mixture
was then continued for 1 h at the room temperature.
C
16H13N3O3Sꢂ0.5H2O (327,36 g/mol): C, 58.70; H, 4.00; N,
12.84; S, 9.80; found: C, 58.74; H, 4.02; N, 12.85; S, 9.81.
ꢀ
Please cite this article as: K. Jakovljevic et al., Novel 1,3,4-thiadiazole conjugates derived from protocatechuic acid: Synthesis,
antioxidant activity, and computational and electrochemical studies, Comptes Rendus Chimie, https://doi.org/10.1016/
j.crci.2019.06.001

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11
4.1.2.9. 3,4-Dihydroxy-N-(5-(2-methoxyphenyl)-1,3,4-
thiadiazol-2-yl)benzamide 0.5H2O (3e). Beige powder;
8.2 Hz, AreH); 7.53e7.63 (m, 4H, AreH); 7.87e7.95 (m, 1H,
AreH); 8.02 (d, 1H, J ¼ 1.8 Hz, AreH); 9.42 (s, 1H, OH); 9.95
(s, 1H, OH); 12.88 (s, 1H, NH); 13C NMR (50 MHz, DMSO-d6):
115.26, 116.11, 121.01, 122.22, 125.78, 126.16, 130.24, 131.30,
132.35, 134.08, 145.32, 150.59, 160.08, 160.33, 164.74; IR
(KBr, cmꢁ1): 3421, 3365, 3114, 1659, 1617, 1601, 1518, 1300,
1256, 1115, 753; anal. calcd. for C15H10ClN3O3S (347.78 g/
mol): C, 51.80; H, 2.90; N, 12.08; S, 9.22; found: C, 51.78; H,
2.89; N, 12.09; S, 9.23.
ꢂ
yield: 0.23 g (68%); mp > 250 ꢃC; 1H NMR (200 MHz,
DMSO-d6): 4.03 (s, 3H, OCH3); 6.86 (d, 1H, J ¼ 8.0 Hz,
AreH); 7.13 (t, 1H, J ¼ 8.0 Hz, AreH); 7.27 (d, 1H, J ¼ 8.0 Hz,
AreH); 7.50 (dd, 1H, J ¼ 8.0 and 1.4 Hz, AreH); 7.55 (s, 1H,
AreH); 7.70 (d, 1H, J ¼ 8.0 Hz, AreH); 8.30 (dd, 1H, J ¼ 8.0
and 1.4 Hz, AreH); 10.41 (bs, 2H, OH and 1H, NH); 13C NMR
(50 MHz, DMSO-d6): 54.14, 110.37, 113.16, 113.99, 117.09,
118.85, 19.14, 120.46, 125.32, 129.60, 143.26, 148.41, 153.32,
154.45, 159.27, 162.60; IR (KBr, cmꢁ1): 3398, 3153, 2944,
1681, 1601, 1545, 1526, 1314, 1299, 1260, 1018, 747; anal.
calcd. for C16H13N3O4S ꢂ 0.5H2O (343.36 g/mol): C, 55.97;
H, 3.82; N, 12.24; S, 9.34; found: C, 55.99; H, 3.81; N, 12.26;
S, 9.35.
4.1.2.14. N-(5-(4-Chlorophenyl)-1,3,4-thiadiazol-2-yl)-3,4-
dihydroxybenzamide (3j). White powder; yield: 0.22
g
(64%); mp > 250 ꢃC; 1H NMR (200 MHz, DMSO-d6): 6.86 (d,
1H, J ¼ 8.2 Hz, AreH); 7.55e7.62 (m, 2H, AreH); 7.60 (d, 2H,
JAB ¼ 8.4 Hz, AreH); 7.99 (d, 2H, JBA ¼ 8.4 Hz, AreH); 9.47
(bs, 1H, OH); 9.91 (bs, 1H, OH); 12.84 (bs, 1H, NH); 13C NMR
(50 MHz, DMSO-d6): 115.19, 116.06, 120.96, 122.22, 128.50
(2C), 129.21, 129.35 (2C), 135.04, 145.25, 150.50, 159.80,
160.58, 164.66; IR (KBr, cmꢁ1): 3426, 3182, 1681, 1657, 1614,
1603, 1523, 1455, 1298, 1234, 829; anal. calcd. for
4.1.2.10. 3,4-Dihydroxy-N-(5-(3-methoxyphenyl)-1,3,4-
thiadiazol-2-yl)benzamide ꢂ 0.5H2O (3f). Light brown pow-
der; yield: 0.24 g (70%); mp > 250 ꢃC (Dec.); 1H NMR
(200 MHz, DMSO-d6): 3.85 (s, 3H, OCH3); 6.86 (d, 1H, J ¼
8.0 Hz, AreH); 7.09 (d, 1H, J ¼ 8.0 Hz, AreH); 7.40e7.62 (m,
5H, AreH); 9.68 (bs, 2H, OH); 12.72 (bs, 1H, NH); 13C NMR
(50 MHz, DMSO-d6): 55.49, 111.47, 115.27, 116.12, 116.65,
119.55, 121.00, 122.40, 130.60, 131.69, 145.33, 150.55, 159.78
(2C), 161.62, 164.79; IR (KBr, cmꢁ1): 3386, 3169, 2940, 1676,
1605, 1526, 1457, 1304, 1217, 1217, 1112, 745; anal. calcd. for
C
15H10ClN3O3S (347.78 g/mol): C, 51.80; H, 2.90; N, 12.08; S,
9.22; found: C, 51.84; H, 2.90; N, 12.06; S, 9.19.
4.1.2.15. N-(5-(4-Bromophenyl)-1,3,4-thiadiazol-2-yl)-3,4-
dihydroxybenzamide ꢂ 0.5H2O (3k). Light yellow powder;
yield: 0.21 g (54%); mp > 250 ꢃC; 1H NMR (200 MHz,
DMSO-d6): 6.85 (d, 1H, J ¼ 8.2 Hz, AreH); 7.54e7.61 (m, 2H,
C
16H13N3O4S ꢂ 0.5H2O (343,36 g/mol): C, 55.97; H, 3.82; N,
12.24; S, 9.34; found: C, 55.94; H, 3.83; N, 12.23; S, 9.31.
AreH); 7.73 (d, 2H, JAB ¼ 8.4 Hz, AreH); 7.92 (d, 2H, JBA
¼
8.4 Hz, AreH); 9.48 (bs, 1H, OH); 9.89 (bs, 1H, OH); 12.84
(bs, 1H, NH); 13C NMR (50 MHz, DMSO-d6): 115.26, 116.12,
121.01, 122.29, 123.83, 128.78 (2C), 129.63, 132.34 (2C),
145.32, 150.57, 159.90, 160.72, 164.76; IR (KBr, cmꢁ1): 3391,
3199, 1675, 1605, 1521, 1455, 1299, 1209, 1071, 749; anal.
calcd. for C15H10BrN3O3S ꢂ 0.5H2O (392.23 g/mol): C,
45.93; H, 2.57; N, 10.71; S, 8.18; found: C, 45.92; H, 2.58; N,
10.73; S, 8.19.
4.1.2.11. 3,4-Dihydroxy-N-(5-(4-methoxyphenyl)-1,3,4-
thiadiazol-2-yl)benzamide ꢂ H2O (3g). Beige powder; yield:
0.18 g (51%); mp > 250 ꢃC; 1H NMR (200 MHz, DMSO-d6):
3.83 (s, 3H, OCH3); 6.85 (d, 1H, J ¼ 8.4 Hz, AreH); 7.08 (d,
2H, JAB ¼ 8.8 Hz, AreH); 7.54 (s, 1H, AreH); 7.59 (d, 1H, J ¼
8.2 Hz, AreH); 7.90 (d, 2H, JBA ¼ 8.8 Hz, AreH); 9.66 (bs, 2H,
OH); 12.65 (bs, 1H, NH); 13C NMR (50 MHz, DMSO-d6):
55.56, 114.84 (2C), 115.27, 116.11, 120.96, 122.49, 123.00,
128.51 (2C), 145.33, 150.51, 159.04, 161.06, 161.61, 164.72; IR
(KBr, cmꢁ1): 3397, 3181, 2937, 1663, 1607, 1521, 1462, 1305,
1258, 1178, 829; anal. calcd. for C16H13N3O4S ꢂ H2O
(343.36 g/mol): C, 55.97; H, 3.82; N, 12.24; S, 9.34; found: C,
55.98; H, 3.80; N, 12.27; S, 9.36.
4.1.2.16. N-(5-Benzyl-1,3,4-thiadiazol-2-yl)-3,4-
dihydroxybenzamide (3l). Gray powder; yield: 0.19
g
(57%); mp > 250 ꢃC (Dec.); 1H NMR (200 MHz, DMSO-d6):
4.36 (s, 2H, CH2); 6.82 (d, 1H, J ¼ 8.2 Hz, AreH); 7.24e7.37
(m, 5H, AreH); 7.48 (s, 1H, AreH); 7.52 (d, 1H, J ¼ 8.2 Hz,
AreH); 9.38 (s, 1H, OH); 9.88 (s, 1H, OH); 12.57 (s, 1H,
NH); 13C NMR (50 MHz, DMSO-d6): 35.20, 115.23, 116.04,
120.85, 122.48, 127.06, 128.84 (2C), 128.91 (2C), 137.92,
145.27, 150.40, 159.90, 163.53, 164.67; IR (KBr, cmꢁ1):
3433, 3347, 3063, 1673, 1612, 1527, 1454, 1304, 1218, 750;
anal. calcd. for C16H13N3O3S (327.36 g/mol): C, 58.70; H,
4.00; N, 12.84; S, 9.80; found: C, 58.65; H, 3.98; N, 12.80;
S, 9.83.
4.1.2.12. N-(5-(2-Chlorophenyl)-1,3,4-thiadiazol-2-yl)-3,4-
dihydroxybenzamide ꢂ H2O (3h). Beige powder; yield: 0.21 g
(61%); mp > 250 ꢃC; 1H NMR (200 MHz, DMSO-d6): 6.86 (d,
1H, J ¼ 8.2 Hz, AreH); 7.49e7.62 (m, 4H, AreH); 7.65e7.72
(m, 1H, AreH); 8.12e8.17 (m, 1H, AreH); 9.59 (bs, 1H, OH);
9.80 (bs, 1H, OH); 12.85 (bs, 1H, NH); 13C NMR (50 MHz,
DMSO-d6): 115.28, 116.14, 121.05, 122.27, 127.90, 129.25,
130.64, 130.93, 131.16, 131.75, 145.34, 150.60, 157.86, 161.25,
164.84; IR (KBr, cmꢁ1): 3420, 3160, 2969, 1684, 1606, 1527,
1451, 1300, 1203, 753; anal. calcd. for C15H10ClN3O3S ꢂ H2O
(347.78 g/mol): C, 51.80; H, 2.90; N, 12.08; S, 9.22; found: C,
51.82; H, 2.91; N, 12.10; S, 9.20.
4.1.2.17. 3,4-Dihydroxy-N-(5-(phenoxymethyl)-1,3,4-
thiadiazol-2-yl)benzamide (3m). Beige powder; yield: 0.26 g
(75%); mp > 250 ꢃC; 1H NMR (200 MHz, DMSO-d6): 5.51 (s,
2H, CH2); 6.84 (d, 1H, J ¼ 8.2 Hz, AreH); 7.00 (t, 1H, J ¼
7.4 Hz, AreH); 7.08 (d, 2H, J ¼ 8.2 Hz, AreH); 7.33 (t, 2H, J ¼
8.2 Hz, AreH); 7.51 (s, 1H, AreH); 7.56 (d, 1H, J ¼ 8.2 Hz,
AreH); 9.43 (bs, 1H, OH); 9.89 (s, 1H, OH); 12.73 (s, 1H, NH);
13C NMR (50 MHz, DMSO-d6): 64.22, 115.08 (2C), 115.26,
4.1.2.13. N-(5-(3-Chlorophenyl)-1,3,4-thiadiazol-2-yl)-3,4-
dihydroxybenzamide (3i). White powder; yield: 0.19 g (54%);
mp > 250 ꢃC; 1H NMR (200 MHz, DMSO-d6): 6.86 (d, 1H, J ¼
ꢀ
Please cite this article as: K. Jakovljevic et al., Novel 1,3,4-thiadiazole conjugates derived from protocatechuic acid: Synthesis,
antioxidant activity, and computational and electrochemical studies, Comptes Rendus Chimie, https://doi.org/10.1016/
j.crci.2019.06.001

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K. Jakovljevic et al. / C. R. Chimie xxx (xxxx) xxx
12
116.09, 120.97, 121.65, 122.31, 129.72 (2C), 145.32, 150.54,
157.48, 160.42, 160.73, 164.82; IR (KBr, cmꢁ1): 3391, 3178,
1684, 1601, 1531, 1497, 1455, 1299, 1231, 746; anal. calcd. for
4.38; N, 14.04; S, 10.72; found: C, 60.17; H, 4.39; N, 14.05; S,
10.71.
C
16H13N3O4S (343.36 g/mol): C, 55.97; H, 3.82; N, 12.24; S,
4.2. Biology
9.34; found: C, 55.99; H, 3.83; N, 12.21; S, 9.35.
4.2.1. DPPH free radicalescavenging assay
The DPPH free radicalescavenging activity of the stud-
ied compounds was determined according to the method
described by Kontogiorgis and Hadjipavlou-Litina [38].
Briefly, 1 mL (0.05 mM) of DPPH solution in methanol was
mixed with an equivalent volume of the tested compound
4.1.2.18. N-(5-Cyclohexyl-1,3,4-thiadiazol-2-yl)-3,4-
dihydroxybenzamide (3n). Beige powder; yield: 0.20
g
(64%); mp > 250 ꢃC; 1H NMR (200 MHz, DMSO-d6):
1.17e1.55 (m, 5H, CH2cyclohexyl); 1.62e1.79 (m, 3H, CH2cy-
clohexyl); 2.01e2.07 (m, 2H, CH2cyclohexyl); 2.97e3.10 (m, 1H,
CH2cyclohexyl); 6.83 (d, 1H, J ¼ 8.2 Hz, AreH); 7.50 (s, 1H,
AreH); 7.54 (d, 1H, J ¼ 8.2 Hz, AreH); 9.59 (bs, 2H, OH);
12.52 (s, 1H, NH); 13C NMR (50 MHz, DMSO-d6): 25.49 (2C),
25.58, 33.15 (2C), 38.84, 115.20, 116.05, 120.82, 122.65,
145.25, 150.32, 158.92, 164.66, 169.01; IR (KBr, cmꢁ1): 3391,
3294, 2932, 2855, 1686, 1614, 1552, 1524, 1312, 1293, 1193,
656; anal. calcd. for C15H17N3O3S (319.38 g/mol): C, 56.41;
H, 5.37; N, 13.16; S, 10.04; found: C, 56.37; H, 5.38; N, 13.15;
S, 10.05.
[20
mL of the compound solution (in DMSO for 3aeo or in
methanol for 3a, 4, and 5), and 980
mL of methanol]. The
sample was stored in dark at room temperature. The
absorbance was acquired at 517 nm after an incubation
period of 30 min. Methanol was taken as the control. IC50
values represent the concentration necessary to obtain 50%
of a maximum scavenging activity. Ascorbic acid was used
as the positive control. The results are presented as mean
standard deviation (SD) calculated from the independent
triplicate experiments using Microsoft Excel software.
4.1.2.19. N-(5-(Adamantan-1-yl)-1,3,4-thiadiazol-2-yl)-3,4-
dihydroxybenzamide (3o). White powder; yield: 0.25
4.2.2. ABTS-scavenging assay
g
(67%); mp > 250 ꢃC; 1H NMR (200 MHz, DMSO-d6): 1.75 (s,
6H, Hadamantyl); 2.02 (s, 9H, Hadamantyl); 6.82 (d, 1H, J ¼
8.2 Hz, AreH); 7.50 (s, 1H, AreH); 7.54 (d, 1H, J ¼ 8.2 Hz,
AreH); 9.74 (bs, 2H, OH); 12.12 (s, 1H, NH); 13C NMR
(50 MHz, DMSO-d6): 28.09 (3C), 36.07 (3C), 37.37, 42.90
(3C), 115.19, 116.03, 120.80, 122.65, 145.26, 150.35, 158.84,
164.68,173.44; IR (KBr, cmꢁ1): 3381, 3220, 2906, 2851,1637,
1607, 1525, 1448, 1296, 1226, 744; anal. calcd. for
The radical-scavenging activity of synthesized com-
pounds using stable ABTS radical cation was determined
according to the method of Re et al. [39] with slight mod-
ifications. ABTSꢀþ was generated by addition of Na2S2O8
(2.45 mM) to 7 mM ABTS solution during 16 h in the dark.
For experimental purposes, the obtained solution was
diluted with methanol to reach the absorbance of 0.70
0.02 at 734 nm. Tested compounds were dissolved in DMSO
to obtain a concentration of 10 mM and then diluted with
methanol to prepare a concentration range between 20 and
C
19H21N3O3S (371.45 g/mol): C, 61.44; H, 5.70; N, 11.31; S,
8.63; found: C, 61.47; H, 5.69; N, 11.33; S, 8.65.
40
mixing 100
at different concentrations with 900
m
M. The ABTSꢀþ scavenging test was carried out by
L of examined compounds solutions prepared
L of ABTSꢀþ solution
m
4.1.2.20. 4-(((5-Phenyl-1,3,4-thiadiazol-2-yl)imino)methyl)
benzene-1,2-diol (4). Yellow powder; yield: 0.24 g (53%); mp
¼ 228e229 ꢃC (Dec.); 1H NMR (200 MHz, DMSO-d6): 6.92
(d, 1H, J ¼ 8.2 Hz, AreH); 7.39 (dd, 1H, J ¼ 8.2 and 1.8 Hz,
AreH); 7.51 (d, 1H, J ¼ 1.8 Hz, AreH); 7.54e7.58 (m, 3H,
AreH); 7.93e7.98 (m, 2H, AreH); 8.79 (s, 1H, N¼CH); 9.65
(s, 1H, OH); 10.18 (s, 1H, OH); 13C NMR (50 MHz, DMSO-d6):
115.02, 115.84, 125.44, 126.23, 127.20 (2C), 129.38 (2C),
130.14, 131.11, 146.01, 151.96, 165.02, 168.56, 174.15; IR (KBr,
cmꢁ1): 3533, 2924, 1603, 1585, 1516, 1297, 1291, 1192, 762;
anal. calcd. for C15H11N3O2S (297.33 g/mol): C, 60.59; H,
3.73; N, 14.13; S, 10.78; found: C, 60.56; H, 3.74; N, 14.12; S,
10.76.
m
and incubated at ambient temperature for 30 min. Then,
the absorbance at 734 nm was measured. The percentage of
ABTSꢀþ-scavenging activity was expressed as % radical-
scavenging activity ¼ [(Ac ꢁ As)/Ac] ꢂ 100, where Ac is the
absorbance of ABTSꢀþ solution without the test sample (the
blank control) and As is the absorbance of the test sample.
The concentration of the tested compound that reduces
50% of the free-radical concentration (IC50) was calculated
using the equation obtained from the linear regression
analysis.
4.3. Computational details
4.1.2.21. 4-(((5-Phenyl-1,3,4-thiadiazol-2-yl)amino)methyl)
benzene-1,2-diol (5). Yellow powder; yield: 0.24 g (80%); mp
¼ 217e218 ꢃC (Dec.); 1H NMR (200 MHz, DMSO-d6): 4.35
(d, 2H, J ¼ 5.4 Hz, CH2); 6.64 (dd, 1H, J ¼ 8.0 and 1.6 Hz,
AreH); 6.69 (t, 1H, J ¼ 8.0 Hz, AreH); 6.79 (d, 1H, J ¼ 1.4 Hz,
AreH); 7.44e7.52 (m, 3H, AreH); 7.73e7.77 (m, 2H, AreH);
8.34 (t,1H, J ¼ 5.4 Hz, NH); 8.85 (s,1H, OH); 8.94 (s,1H, OH);
13C NMR (50 MHz, DMSO-d6): 48.21, 115.33, 115.52, 118.84,
126.38 (2C), 129.21 (2C), 129.28, 129.67, 130.99, 144.65,
145.28, 156.00, 168.56; IR (KBr, cmꢁ1): 3431, 3205, 3108,
2955, 1607, 1580, 1573, 1516, 1425, 1362, 1289, 1195, 761;
anal. calcd. for C15H12N3O2S (299.35 g/mol): C, 60.19; H,
Theoretical calculations of the investigated com-
pounds were carried out using Gaussian 09 software
program [40]. Geometry optimizations of all investigated
compounds and their corresponding reactive species
such as radicals, radical cations, and anions have been
performed using M062X hybrid meta functional [41]. To
find the most stable geometries, all possible rotamers
are calculated. The geometries of investigated com-
pounds were fully optimized by using the DFT method
using the 6e311þþg(d,p) basis set. This theoretical
model provides satisfactory results in the analysis of a
range of organic molecules [42e44]. All geometries were
ꢀ
Please cite this article as: K. Jakovljevic et al., Novel 1,3,4-thiadiazole conjugates derived from protocatechuic acid: Synthesis,
antioxidant activity, and computational and electrochemical studies, Comptes Rendus Chimie, https://doi.org/10.1016/
j.crci.2019.06.001

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K. Jakovljevic et al. / C. R. Chimie xxx (xxxx) xxx
13
3. Sequential proton loss electron transfer (SPLET)
optimized without any symmetry constraints. In addi-
tion, vibrational frequency calculations were performed
to confirm the minima on the potential surface. The
solvent effect of methanol (ε ¼ 32.67) on the calculated
structures was investigated by the solvation model based
on the quantum mechanical charge Density of a solute
molecule interacting with a continuum (SMD) [45]. To
obtain thermodynamic descriptors of the antioxidant
activity, such as BDE, IP, PDE, structures of not only the
neutral molecules but also the corresponding radical
cations, radicals, and anions were optimized.
ArOH/ArOꢁ þ Hþ
ArOꢁ/ArOꢀ þ eꢁ
PA ¼ HðArOꢁÞ þ HðHþÞeHðArOHÞ
ETE ¼ HðArOꢀÞ þ HðeꢁÞeHðArOꢁÞ
This is also a two-step reaction mechanism. In the first
step, phenoxide anion is formed by a heterolytic cleavage of
OH bond of the investigated antioxidant. In the second step,
this anion loses an electron that leads to formation of the
stable phenoxide radical. The thermodynamic descriptors
derived from this mechanism are the PA and ETE for the
first and the second step, respectively.
4.4. Antioxidant capacity
Antioxidant activity of phenols has been explained via
at least three possible mechanisms [46e55]. These mech-
anisms will be discussed here in short.
1. Hydrogen atom transfer (HAT):
4.5. Cyclic voltammetry
The cyclic voltammetric experiments were performed
on a PST050 Voltalab instrument. The three-electrode cell
consisted of a glassy carbon (GC) disk electrode (3 mm)
polished with fine alumina suspension before each curve,
with Pt wire counter electrode and a saturated calomel
electrode (SCE) as a reference one. Voltammetric mea-
surements were carried out in the potential range þ1.6 to
ꢁ2.1 V, applying scan rates from 0.01 to 2 Vsꢁ1. The solu-
tions of the compounds (0.6e1.5 mM) were prepared in
doubly distilled DMF previously dried over molecular
sieves for two days; the solutions contained tetrabuty-
lammonium perchlorate (TBAP) as the supporting electro-
lyte. The solutions were carefully deaerated by purging
nitrogen before an experiment, and a strong stream of the
gas was maintained during the measurements.
ArOH/ArOꢀ þ Hꢀ
BDE ¼ HðArOꢀÞ þ HðHꢀÞ
The HAT mechanism is presenting the ability of a
different phenolic compound to exist as stable radical
species after the abstraction of a hydrogen atom from the
OH group. The stability of the resulting radical in-
termediates (ArO$) formed during the abstraction of
hydrogen atoms is determined by the fact whether the
reaction is going to terminate or continue. It means that if
the radical is better stabilized by the resonance delocal-
ization of the unpaired electron within the aromatic ring,
its antiradical activity is better. This mechanism is governed
by the OeH bond dissociation enthalpy (BDE).
Acknowledgments
2. Single electron transfer followed by proton transfer
(SET-PT):
The authors are grateful to the Ministry of Education,
Science and Technological Development of the Republic of
Serbia for financial support (Grant Nos 172016, 172014,
172015, and III43004).
ArOH/ArOHꢀþ þ eꢁ
ArOHꢀþ/ArOꢀ þ Hþ
Appendix A. Supplementary data
IP ¼ HðArOHꢀþÞ þ HðeꢁÞeHðArOHÞ
Supplementary data to this article can be found online
at https://doi.org/10.1016/j.crci.2019.06.001.
PDE ¼ HðArOꢀÞ þ HðHþÞeHðArOHꢀþ
Þ
References
The first step of this mechanism is governed by an
electron transfer from the antioxidant to the free
radical. The second step of this two-step mechanism is
abstraction of a proton from the radical cation, which
is formed during the first step. The main product in
this case is the same as in the HAT mechanism, a new
phenoxy radical. The IP and proton dissociation
enthalpy (PDE) are thermodynamic parameters
responsible for the first and the second step of the SET-
PT mechanism.
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Please cite this article as: K. Jakovljevic et al., Novel 1,3,4-thiadiazole conjugates derived from protocatechuic acid: Synthesis,
antioxidant activity, and computational and electrochemical studies, Comptes Rendus Chimie, https://doi.org/10.1016/
j.crci.2019.06.001