Synthesis of new Schiff bases; Investigation of their in situ catalytic activity for Suzuki CC coupling reactions and antioxidant activities

Article abstract of DOI:10.1002/jccs.202000433

New series of Schiff bases have been synthesized from the reaction between cyclohepta-2,4,6-trien-1-ylmethanamine and different aldehydes, and characterized via using ¹H NMR, ¹³C NMR, FTIR spectroscopy, and GC–MS. After the successful synthesis, the in situ catalytic activity of all Schiff bases have been examined for the Suzuki CC cross-coupling reactions using phenylboronic acid, aryl bromides, and PdCl₂ as a catalyst. Before starting these investigations, reaction conditions were optimized using different bases and solvents. At the end of these reactions, the best efficiency was obtained in Et₃N and EtOH. In addition to catalytic investigations, antioxidant activities of all synthesized Schiff bases were examined using DPPH and Iron (Fe²⁺) chelation methods, and IC₅₀ values were calculated. While many molecules show various amounts of antioxidant activity, especially molecules 8e and 8g showed the best activity compared to butylated hydroxytoluene (BHT) and butylated hydroxyanisole (BHA), which were used as positive controls, in DPPH and Iron chelating methods, respectively.

Full text of DOI:10.1002/jccs.202000433

Received: 29 September 2020
DOI: 10.1002/jccs.202000433
Accepted: 14 December 2020
A R T I C L E
Synthesis of new Schiff bases; Investigation of their in situ
catalytic activity for Suzuki C C coupling reactions and
antioxidant activities
Özgür Yılmaz
Department of Chemistry, Faculty of Arts
Abstract
and Sciences, Mersin University, Mersin,
Turkey
New series of Schiff bases have been synthesized from the reaction between
cyclohepta-2,4,6-trien-1-ylmethanamine and different aldehydes, and character-
Correspondence
1
13
ized via using H NMR, C NMR, FTIR spectroscopy, and GC–MS. After the suc-
cessful synthesis, the in situ catalytic activity of all Schiff bases have been
examined for the Suzuki C C cross-coupling reactions using phenylboronic acid,
Özgür Yılmaz, Department of Chemistry,
Faculty of Arts and Sciences, Mersin
University, 33343 Mersin, Turkey.
Email: yilmazozgur@mersin.edu.tr
aryl bromides, and PdCl as a catalyst. Before starting these investigations, reaction
2
Funding information
Mersin University, Grant/Award Number:
BAP 2017-1-AP1-2188
conditions were optimized using different bases and solvents. At the end of these
reactions, the best efficiency was obtained in Et N and EtOH. In addition to cata-
3
lytic investigations, antioxidant activities of all synthesized Schiff bases were exam-
2+
ined using DPPH and Iron (Fe ) chelation methods, and IC values were
50
calculated. While many molecules show various amounts of antioxidant activity,
especially molecules 8e and 8g showed the best activity compared to butylated
hydroxytoluene (BHT) and butylated hydroxyanisole (BHA), which were used as
positive controls, in DPPH and Iron chelating methods, respectively.
K E Y W O R D S
antioxidant activities, catalytic activity, Schiff base, Suzuki coupling
1
| INTRODUCTION
π-acceptor Schiff bases as ligands, have wide catalytic
26–30
applications in many reactions.
For example, Yilmaz
Schiff bases containing imine ( C N ) or azomethine
CH N ) groups are one of the most important class
functional groups in organic chemistry due to their exten-
and coworkers synthesized new palladium and ruthenium
complexes of Schiff bases and these complexes showed
important catalytic activity for Suzuki, Heck C C cross-
coupling reactions together with catalytic hydrogenation
(
1–5
sive biological and chemical properties. The Schiff bases
are also used as intermediates for the synthesis of impor-
tant drugs, pesticides, and other natural products because
of the conversion possibility of C N group to the desir-
able functional groups through reduction, addition, cycli-
31–33
and transfer hydrogenation reactions.
In addition,
Alimirzaei and coworkers examined copper complexes of
Schiff-bases for antimicrobial activities against gram-
positive and gram-negative bacteria, and obtained rela-
6–11
34
zation, and aziridination reactions.
Furthermore, anti-
tively good results. On the other hand, there are many
bacterial, anti-fungal, anti-oxidant, anti-tumor, anti-viral,
anti-HIV, anti-inflammatory, and anti-proliferative proper-
reported studies, which showed effective antioxidant prop-
erties of Schiff bases in the literature.
In this paper, new Schiff base derivatives have been
synthesized and all structures have been identified by
using NMR, FTIR, and GC–MS techniques. In addition,
35–37
12–25
ties of Schiff bases have been reported in the literature.
12–25
Besides the biological importance, the transition
metal complexes, which can be obtained from σ-donor and
J Chin Chem Soc. 2021;1–12.
http://www.jccs.wiley-vch.de
© 2021 The Chemical Society Located in Taipei & Wiley-VCH GmbH
1

2
YILMAZ
in situ catalytic activities for Suzuki cross-coupling reac-
tions and the antioxidant activities via DPPH
After determining the optimal conditions as shown in
Table 1/Entry 6, we tested different substrates through the
reactions of amine 4 and different aldehydes 7a–h, and
obtained new Schiff base series with high yield 8a–h
(Table 2).
2+
(2,2-diphenyl-1-picrylhydrazyl) and Ferrous ions (Fe )
chelating methods have been examined for all synthe-
sized molecules.
1
All structures have been identified by using H-NMR,
13
C-NMR, FTIR spectroscopy, and GC–MS methods. When
1
2
| RESULTS AND DISCUSSION
the H-NMR spectra of synthesized molecules were exam-
ined, the signals of the N CH protons appear as singlet
in the region between δ 8.85 and 7.77 ppm, which was also
First, cyclohepta-2,4,6-trienecarbonitrile (3) was synthe-
sized from cycloheptatriene 1 with bromination, elimina-
32
confirmed by the literature. While the aromatic protons
3
8,39
32
tion, and cyanation reactions, respectively.
Then,
resonate at δ 6.84–8.08 ppm, the double bond protons res-
40
42
cyclohepta-2,4,6-trien-1-ylmethanamine 4 was obtained
in high yield from nitrile 3 with reduction via LiAlH4
onate at δ 5.23–6.73 ppm. The CH and NCH protons
2
appear at δ 2.06–2.27 and δ 3.57–4.01 ppm, respectively. In
addition, the OCH protons in the molecule 8d, resonate
(Scheme 1).
3
43
1
13
Upon successful synthesis of amine 4, a series of reac-
at δ 3.92 ppm. In addition to the H-NMR results, the C-
NMR, FTIR, and GC–MS spectra confirmed the structures
of the synthesized molecules. Structures containing OH
group in ortho position, the OH band was not observed in
the FTIR spectra. This is presumably due to the hydrogen
bonding between the CH N and the OH groups. When
the studies in the literature with similar structures are
tions have been set up between amine 4 and benzalde-
hyde 5 for the optimization of reaction conditions (Table 1).
The yields of reactions were determined with GC using
the internal standard. Results showed that polar protic
41
solvents better than polar aprotic ones. In addition, the
temperature increase improved the yield.
SCHEME 1 Synthesis of
amines 4
TABLE 1 Optimizations of condition
O
NH2
N
H
solvent, heat
4h
+
2
4
5
6
a
Entry
Condition of reaction
Yield %
64
ꢀ
1
2
3
4
5
6
Toluene, 25 C
ꢀ
Dichloromethane, 25 C
79
ꢀ
Methanol, 25 C
83
ꢀ
Methanol, AcOH, 25 C
89
ꢀ
Methanol, Na
2
SO
4
, 25 C
87
ꢀ
Methanol, 40 C
96
a
Yield was determined by GC using n-decane as internal standard.

YILMAZ
3
TABLE 2 Synthesis of new substituted Schiff bases
Entry
Amine
4
Aldehydes
Product
Isolated yield %
1
89
2
4
92
3
4
4
4
84
87
5
4
82
6
7
4
4
91
95
(
Continues)

4
YILMAZ
TABLE 2 (Continued)
Entry
Amine
4
Aldehydes
Product
Isolated yield %
92
8
TABLE 3 Optimizations for Suzuki cross-coupling reaction
a
a
Entry
Base
Solvent
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
EtOH
Conversion%
Yield%
Selectivity%
1
2
3
4
5
6
7
8
9
NaOH
KOH
56
56
29
36
76
71
61
84
11
31
10
20
89
100
48
48
23
31
68
63
56.5
79
2
85.7
85.7
79.3
86.1
89.5
88.7
92.6
94.1
18.2
75.8
10
2 3
K CO
3
Et N
NaOH
KOH
EtOH
K
2
CO
3
EtOH
Et
3
N
EtOH
NaOH
KOH
Toluene
Toluene
Toluene
Toluene
EtOH
10
11
12
13
14
23.5
1
2 3
K CO
Et
Et
3
N
N
15
83
97.0
75
b
3
93.3
97.0
c
Et
3
N
EtOH
Note: Reaction conditions: 4-bromoacetophenone (0.5 mmol), phenylboronic acid (0.6 mmol), Schiff base 8e (1 mol%), PdCl
2
(0.5 mol%), base (0.6 mmol),
ꢀ
solvent (2.0 ml), H
2
O (1.0 ml), 2 hr, 80 C.
a
The yields was calculated with GC using n-decane as internal standard.
b
ꢀ
1
4
00 C.
c
hr.
analyzed, it is also seen that OH band is not reported in
the FTIR spectrum.
been tested using different base, solvent, and temperature
to optimize the catalytic conditions. For this purpose, KOH,
NaOH, K CO , and Et N were chosen as bases to be tested.
44
2
3
3
In addition, EtOH, 1,4-dioxane, and toluene were examined
as solvent variables. The best yield was obtained while
2
.1 | In situ catalytic activity for Suzuki
C C coupling reactions
using Et N as a base in EtOH (Table 3/Entry 8). The other
3
reactions in EtOH with different bases showed better con-
version and selectivity compared to the reactions in toluene
or 1,4-dioxane. In addition, the worst results for conversion
and selectivity have been obtained in toluene especially
with K CO (Entry 11). In summary, polar protic solvents
After the synthesis of Schiff bases, the in situ catalytic activ-
ity in Suzuki C C coupling reactions of phenylboronic acid
1
0 and substituted aryl bromides were investigated for all
Schiff bases. Series of reactions between phenylboronic acid
0 and 4-bromoacetophenone 9 in the presence of 0.5 mol%
molecule 8e as ligand and 0.2 mol% PdCl as a catalyst has
2
3
1
improve the catalytic activity better compared to nonpolar
solvents. In addition, organic bases appear to be more
2

YILMAZ
5
effective than inorganic bases when ethanol is used as the
solvent. Small but various amounts of biphenyl by-product
were obtained from phenylboronic acid in all reactions. In
order not to affect the yield calculation of this product origi-
nating from phenylboronic acid 10, 4-bromoacetophenone
in the literature, besides the steric effect, substituted groups
in aryl bromide significantly affect their reaction yields. For
example, electron-donating groups and steric barriers
reduce reaction efficiency, while electron-withdrawing
groups increase reaction efficiency. In order to demonstrate
these effects, a sterically hindered aryl bromide molecule
was selected as well as aryl bromides substituted with
electron-donating or electron-withdrawing groups. 100%
conversions were observed with all Schiff bases in the reac-
tions with 4-bromoacetophenone 9 (Table 4/Entry 1) and
2-bromobenzaldehyde 12a (Entry 2), which are selected as
aryl bromide structures containing electron-withdrawing
9
was used instead of bromobenzene in optimization reac-
tions. After determining the best experimental conditions,
reactions were performed at different temperatures and
times in order to increase the conversion yield. Looking at
the results of these reactions, it was seen that the conver-
ꢀ
sion efficiency increased at 100 C but did not reach the
maximum (Entry 13). 100% conversion was obtained in the
ꢀ
0
reaction, which was continued for 4 hr at 80 C (Entry 14).
substituted groups, and obtained 1-([1,1 -biphenyl]-4-yl)
0
After optimizing the reaction conditions for the Suzuki
cross-coupling reaction, the catalytic activity of all synthe-
sized Schiff bases was investigated by reactions between var-
ious aryl bromides and phenylboronic acid 10. As is known
ethanone (11) and [1,1 -biphenyl]-2-carbaldehyde (13a) with
high yields (up to 93%), respectively. Other aryl bromide
structures selected to study the effect position of electron-
withdrawing groups are para- 12b and ortho-trifluoromethyl
TABLE 4 In situ catalytic activities of synthesized Schiff bases for the Suzuki C C coupling reactions between phenylboronic acid and
different aryl bromides
b
b
Conversion , (yield) , [selectivity] %
a
Entry
1
ArBr
Product
6
8a
8b
8c
8d
8e
8f
8 g
8 h
100
100
100
100
96.4
100
100
100
100
(
[
96.7)
(97.4)
[97.4]
(96.5)
[96.5]
(97.2)
[97.2]
(94.2)
[97.7]
(97.0)
[97.0]
(93.2)
[93.2]
(97.6)
[97.6]
(96.3)
[96.3]
96.7]
c
c
c
c
c
c
c
c
c
N.R.
N.R.
N.R.
N.R.
N.R.
N.R.
N.R.
N.R.
N.R.
N.R.
d
d
d
d
d
d
d
d
d
N.R.
N.R.
N.R.
N.R.
N.R.
N.R.
N.R.
N.R.
2
100
100
100
100
100
100
100
100
100
(
[
98.3)
98.3]
(98.4)
[98.4]
(96.8)
[96.8]
(97.9)
[97.9]
(97.5)
[97.5]
(97.4)
[97.4]
(97.7)
[97.7]
(98.3)
[98.3]
(97.6)
[97.6]
3
4
94.1
92.6
(89.2)
[94.9]
86.9
(81.8)
[94.1]
97.3
(93.5)
[96.1]
93.3
(89.9)
[96.4]
95.1
(93.0)
[97.8]
94.8
(89.3)
[94.2]
97.4
(93.9)
[96.4]
85.5
(82.9)
[97.0]
(
[
89.3)
97.6]
90.3
84.1
82.6
96.1
97.3
87.6
83.5
96.8
78.2
(
[
87.1)
96.5]
(80.1)
[95.2]
(78.3)
[94.8]
(93.3)
[97.1]
(91.8)
[94.3]
(84.9)
[96.9]
(78.9)
[94.5]
(95.2)
[98.4]
(75.7)
[96.8]
5
21.6
18.9
(2.7)
[10.7]
16.9
(1.8)
[10.9]
17.4
(1.9)
[10.9]
7.9
(1.1)
[13.9]
7.1
(1.2)
[16.9]
54.6
(4.3)
[7.9]
32.1
(1.7)
[5.3]
17.9
(2.4)
[13.4]
(
2.8)
[13.0]
6
7
80.0
74.8
(68.5)
[91.6]
70.2
(66.0)
[94.0]
84.1
(78.5)
[93.3]
84.4
(79.9)
[94.7]
77.5
(70.7)
[91.2]
73.1
(67.2)
[91.9]
87.0
(82.1)
[94.4]
70.7
(64.4)
[91.1]
(
[
74.5)
93.1]
62.6
63.9
52.6
69.3
78.3
68.6
69.0
78.8
42.3
(
[
57.1)
91.2]
(60.1)
[94.1]
(46.3)
[88.0]
(63.6)
[91.8]
(73.3)
[93.6]
(63.4)
[92.4]
(62.9)
[91.2]
(74.6)
[94.7]
(36.4)
[86.1]
Note: Reaction conditions: aryl bromide (0.5 mmol), phenylboronic acid (0.6 mmol), Schiff base (1 mol%), PdCl
2
(0.5 mol%), NEt
3
(0.6 mmol), EtOH (2.0 ml),
ꢀ
H
2
O (1.0 ml), 4 hr, 80 C. N.R., no reaction.
a
All biaryl products determined with NMR.
b
The yields were calculated with GC using n-decane as internal standart.
c
In addition to the reaction conditions, 1 mmol Hg was added and calculated the conversion after 1 hr.
d
Conversion after 4 hr for Hg reactions.

6
YILMAZ
bromobenzene 12c (Entry 3 and Entry 4). The obtained
results showed that the substituent attached in the ortho
position reduces the yield of the product slightly due to the
steric barrier. In reactions with sterically hindered aryl bro-
mide 12d, the best conversion was achieved with ligand 8f,
but exceptionally low selectivity was observed in all ligands
TABLE 5 The inhibition ratio of synthesized molecules at the
concentration of 800 μg/ml
DPPH inhibition
ratio (%)
Metal chelating
ratio (%)
Compound
6
74.5
83.7
57.9
89.9
81.3
97.1
56.3
68.1
67.1
86.1
31.1
93.6
35.4
32.4
38.7
95.6
98.2
93.3
8
8
a
b
(
Entry 5). In addition, the poorest results are obtained for
activated substrates 4-bromoanisole 12e and
-bromotoluene 12f when Schiff bases 8b and 8h are used
but, good conversions are obtained with ligand 8g (Entry
and Entry 7). While it can be seen that all Schiff bases
4
8c
8d
6
8
8
8
8
e
f
show moderate and good catalytic activity, it can be said
that the best catalytic activity in general is achieved with 8g
and the worst catalytic activity with 8h Schiff base. Consid-
ering the results obtained in all Suzuki C C coupling reac-
tions with synthesized Schiff bases, it is seen that Schiff
bases substituted with halogen groups that are deactivating
show less catalytic activity than Schiff bases substituted with
activating OH groups.
g
h
with high antioxidant activity at this concentration are
shown in bold type in Table 5 and lower concentrations of
these molecules were prepared to calculate IC50 values by
relevant methods.
To understand the catalytic mechanism, mercury test
was performed. Hg (0) reacts with Pd (0) to form amalgam-
ate. Thus, the catalytic activity of Pd (0) in the reaction
In particular, when looking at the preliminary mea-
surements of molecule 6, which does not contain a
substituted group obtained with benzaldehyde, it can be
said that aromatic Schiff bases have various antioxidant
properties. In addition, according to these results, it is
seen that the antioxidant properties of aromatic Schiff
bases are significantly affected compared to the
substituted group in the aromatic ring. After obtaining
these results, it is aimed to calculate the IC50 values of
the molecules that are active in each method. For this
purpose, various concentrations of molecules and positive
controls BHT and BHA were prepared.
45
medium is quenched. If there is no conversion in reac-
tions after this poisoning test, it is understood that there is
46–48
heterogeneous catalysis.
The observation of palladium
black in catalytic activity reactions strengthened the idea
that the catalytic cycle is heterogeneous. For this purpose,
the reactions between phenylboronic acid 10 and
4-bromoacetophenone 12a were repeated with all Schiff
bases and 1 mmol mercury was added to the reaction
medium (Entry 1). When the results were examined by
GC, it was seen that there was no conversion in all reac-
tions. Thus, it was understood that Schiff bases catalyze
Suzuki coupling reactions with heterogeneous catalyst by
stabilizing Pd (0) formed in the reaction medium.
2.3 | DPPH scavenging activity
2
.2 | Antioxidant activities
DPPH method is one of the most used for antioxidant
measurement of molecules by UV. The ability of a mole-
cule to be active in this method depends on its ability to
deliver protons. If the molecule transfers protons to the
DPPH radical, the solution of purple-colored DPPH in
ethanol turns into yellow-colored DPPH-H. Thus, DPPH,
which absorbs at 517 nm, decreases the absorbance and
the antioxidant activity of the molecule is calculated
Antioxidant molecules are so important because they can
be kept under control by the unstable free radicals, which
are produced always in the human body.
can hinder some common diseases such as cancer and car-
diovascular so the synthesis of antioxidant molecules and
examine their antioxidant properties are important topic in
49,50
Thus, they
51–53
50,56,57
the literature.
In this work, the antioxidant activities of
according to this decrease.
synthesized molecules were examined as used DPPH free
According to preliminary studies, all aromatic Schiff
bases show various amounts of DPPH scavenging activ-
ity. The 74.5% activity of molecule 6 without any
substituted group in the first studies in Table 5 shows
that aromatic Schiff bases have DPPH scavenging activ-
ity. Since the DPPH method is dependent on the transfer
of the H atom, molecules 8a, 8c, 8d, and 8e, which
2+
radical scavenging activity and ferrous ions (Fe ) chelating
54–56
activity, which is known methods in the literature.
In
the beginning, the highest concentration (800 μg/ml) of all
synthesized molecules were prepared for specify of active
compounds in methods (DPPH or Iron chelating) using for
investigation of antioxidant capacity (Table 5). Molecules

YILMAZ
7
FIGURE 1 DPPH scavenging activities of each molecule for each concentration
TABLE 6 IC50 values of synthesized molecules for DPPH scavenging activity
Compound
6
8a
8c
8d
8e
8g
8h
BHT
BHA
IC50 values (μg/ml)
316.7
244.7
165.7
261.6
78.4
382.4
399.5
83.5
45
contain the OH groups in the phenyl ring showed
higher activity as expected. In addition, it can be seen
that deactivating groups such as -Br, -Cl slightly decrease
DPPH activity. Based on these results, IC50 values of 6,
According to preliminary study (Table 5), unlike the
DPPH results, while the Br, Cl, or C C substitu-
tion increase the Ferrous ions chelating activity, the
OH group decrease the activity dramatically. Actually,
according to the literature, the structures, which con-
taining OH, O , C O, C N, COOH groups have
8
a, 8c, 8d, 8e, 8g, and 8h molecules were calculated.
According to the results obtained, the DPPH scavenging
activities of the molecules change in the order of
BHA > 8e > BHT > 8c > 8a > 8d > 6 > 8g > 8h (Figure 1;
Table 6).
56
high potential for show iron chelating activity. However,
unlike BHT and BHA, which contain OH and show high
efficiency, molecules 8a, 8c, 8d, and 8e, which containing
OH group in ortho position showed low iron chelating
activity. This may have been caused by the hydrogen bond
between the OH group and the adjacent C N group.
Looking at the FTIR spectra of these molecules, the
absence of the OH band proves this interaction.
2+
2
.4 | Ferrous ions (Fe ) chelating
activity
Besides the radical scavenging effects of molecules, it is
important in terms of antioxidant properties that they
can form chelates with metals. Because these molecules
prevent the metal catalyzed oxidation by keeping the
metals in the body like iron, which is more active than
For this reason, the IC50 values were only calculated
for compound 6, 8b, 8f, 8g, and 8h, which showed good
activity in this method (Figure 2). In addition, BHT and
BHA used as positive controls. The IC values for ferrous
ions (Fe ) chelating activity increase in order of
8g > BHT > 8f > BHA > 8b > 8h > 6 (Table 7).
50
2+
54–56
other metals.
In order to examine the antioxidant
2+
activities of molecules in this respect, Ferrous ions (Fe )
2
method is commonly used based on the reduction of Fe
+
-
Ferrozine complex, which gives absorbance in
3 | EXPERIMENTAL
54–56
5
62 nm.
Considering that Schiff bases are used as
ligand in obtaining metal complexes, this study was con-
ducted since it was expected that the iron ions chelating
activities of the synthesized molecules would be high.
All reagents were purchased from Sigma-Aldrich or
Merck. The H, C NMR spectra, the antioxidant activi-
ties, and FT-IR spectra were examined with Bruker
1
13

8
YILMAZ
FIGURE 2 Ferrous ions chelating activities of each molecule for each concentration
TABLE 7 IC50 values of
Compound
6
8b
8f
8g
8h
BHT
BHA
synthesized molecules for ferrous ions
chelating activity
IC50 values (μg/ml)
214.3
113.4
53.7
19.78
119.8
42.96
85.94
+
Ultrashield Plus Biospin Avance III 400 MHz NaNoBay
FT-NMR, Chebios Optimum-One UV–Vis spectropho-
tometer and Perkin Elmer Spectrum-100, respectively.
(M , CH NH ), 65.0, 30.1; Elemental analysis: Anal.
2 2
Cal. for C H N; C, 79.29%; H, 9.15%; N, 11.56%.
Found: C, 79.02%; H, 9.08%; N, 11.41%.
8
11
3
1
.1 | Synthesis of cyclohepta-2,4,6-trien-
3.2 | General method for the synthesis of
substituted Schiff bases
-ylmethanamine (4) (C H N)
8
11
In the beginning the nitrile 3 was synthesized according to
Synthetic method carried out by the following procedure:
0.5 g (4.1 mmol) amine 4 was dissolved with MeOH
(8 ml) and then aldehyde (4.0 mmol) was added. The
resulting solution was stirred at room temperature until
starting material was consumed by TLC analysis. The sol-
vent was evaporated, and the products were purified by
chromatographic techniques.
38,39
the literature.
Then, 10 g (85.3 mmol) of nitrile 3 was
ꢀ
dissolved in 100 mL ether and added dropwise at 0 C to
slurry of 6.48 g (170.7 mmol) LiAlH in 150 ml of ether.
42
4
The mixture was stirred 2 hr. After the starting compound
finished according to TLC analysis, 30 ml of water and
10 ml of 20% NaOH solution were added slowly in the
reaction mixture and stirred for 10 more minutes. Then,
the residue filtered, and ether phase was separated. The
water phase was washed three times with 25 ml of ether.
All ether phases were combined and dried over Na SO .
N-benzylidene-1-(cyclohepta-2,4,6-trien-1-yl)
methanamine (6) (C H N).
15
15
1
H NMR (400 MHz, CDCl ) δ 8.42 (s, 1H, N CH),
3
7.79 (dd, J = 6.6, 3.1 Hz, 2H, Ph-H), 7.45 (dd, J = 5.1,
1.8 Hz, 3H, Ph-H), 6.69 (dd, J = 2.8, 1.2 Hz, 2H,
C CH), 6.26 (dddd, J = 8.9, 3.8, 2.6, 1.3 Hz, 2H,
C CH), 5.38 (dd, J = 9.0, 5.6 Hz, 2H, C CH), 3.97
2
4
The ether phase was evaporated and the product 4 was
4
0,58
obtained in 95% yield (81.0 mmol, 9.82 g).
1
H NMR (400 MHz, CDCl ) δ 6.66–6.64 (m, 2H,
C CH), 6.24 (dddd, J = 8.9, 3.8, 2.6, 1.3 Hz, 2H,
3
(dd, J = 6.8, 1.3 Hz, 2H, NCH ), 2.25–2.17 (m, 1H,
2
13
C CH), 5.25 (dd, J = 9.0, 5.6 Hz, 2H, C CH), 3.00 (d,
CH); C NMR (101 MHz, CDCl ) δ 161.9 (C N),
3
1
3
J = 7.4 Hz, 2H, NCH ), 1.86–1.78 (m, 1H, CH);
C
136.2 (ArC), 130.9 (C C), 130.6 (ArC), 128.5 (ArC), 128.2
2
NMR (101 MHz, CDCl ) δ 130.9, 125.7, 124.4, 44.4,
2.7 ppm; IR (KBr): V = 3,351, 3,294, 3,009, 2,919, 2,849,
,566, 1,453, 742, 696 cm ; GC–MS: 121.0 (M ), 91.0
(ArC), 125.2 (C C), 124.7 (C C), 63.4 (NCH ), 40.0
3
2
ꢀ
ꢀ
4
(CH) ppm; IR (KBr): V = 3,012, 2,833, 1,643, 1,448,
−
1
+
−1
+
+
1
1,309, 1,024, 688 cm ; GC–MS: 210.1 (M ), 118.0 (M ,

YILMAZ
9
CH Ph), 91.0, 77.0, 65.0, 51.0; Elemental analysis:
2-(([cyclohepta-2,4,6-trien-1-ylmethyl]imino)
2
Anal. Cal. for C H N; C, 86.08%; H, 7.22%; N, 6.69%.
methyl)-6-methoxyphenol (8d) (C H NO ).
15
15
16 17
2
1
Found: C, 85.89%; H, 7.18%; N, 6.59%.
-(([cyclohepta-2,4,6-trien-1-ylmethyl]imino)
methyl)phenol (8a) (C H NO).
H NMR (400 MHz, CDCl ) δ 8.42 (s, J = 1.1 Hz,
3
2
1H,
N CH), 6.96–6.90 (m, 2H, Ph-H), 6.84 (d,
J = 7.8 Hz, 1H, Ph-H), 6.69–6.65 (m, 2H, C CH), 6.25
(dddd, J = 9.0, 3.8, 2.6, 1.2 Hz, 2H, C CH), 5.34 (dd,
1
5
15
1
H NMR (400 MHz, CDCl ) δ 8.45 (s, 1H, N CH),
3
7.36–7.31 (m, 2H, Ph-H), 6.99 (d, J = 7.4 Hz, 1H, Ph-H),
6.93–6.90 (m, 1H, Ph-H), 6.73–6.67 (m, 2H, C CH),
6.28 (dddd, J = 8.9, 3.8, 2.6, 1.2 Hz, 2H, C CH), 5.35
J = 9.0, 5.7 Hz, 2H, C CH), 3.92 (s, 3H, OCH ), 3.89
3
(dd, J = 7.1, 1.1 Hz, 2H, NCH ), 2.23–2.16 (m, 1H,
2
13
CH) ppm; C NMR (101 MHz, CDCl ) δ 165.6
3
(
1
dd, J = 9.0, 5.7 Hz, 2H, C CH), 3.91 (dd, J = 7.0,
(C N), 152.0 (ArC), 148.5 (ArC), 130.9 (C C), 125.7
(C C), 123.7 (C C), 122.9 (ArC), 118.5 (ArC), 117.8
(ArC), 114.1 (ArC), 60.5 (NCH ), 56.1 (OCH ), 39.9
13
.1 Hz, 2H, NCH ), 2.27–2.12 (m, 1H, CH) ppm;
C
2
NMR (101 MHz, CDCl ) δ 165.7 (ArC), 161.2 (C N),
1
1
(
3
2
3
ꢀ
32.3 (ArC), 131.3 (ArC), 131.0 (C C), 125.8 (C C),
23.8 (C C), 118.8 (ArC), 118.6 (ArC), 117.1 (ArC), 61.2
(CH) ppm; IR (KBr): V = 3,011, 2,835, 1,629, 1,461,
−
1
+
1,249, 1,078, 907, 725, 645 cm ; GC–MS: 255.1 (M ),
+
ꢀ
NCH ), 39.9 (CH) ppm; IR (KBr): V = 3,011, 2,846,
207.0 (M , OH,-OCH ), 168.9, 105.0, 91.0, 77.0, 66.9,
2
3
−1
1
,629, 1,579, 1,494, 1,276, 1,150, 753, 731, 702, 647 cm ;
54.9, 43.9, 32.0; Elemental analysis: Anal. Cal. for
C H NO ; C, 75.27%; H, 6.71%; N, 5.49%. Found: C,
+
+
GC–MS: 225.1 (M ), 207.9 (M , OH), 134.0, 104.0, 91.0,
7.0, 65.0, 51.0, 39.0, 32.0; Elemental analysis: Anal.
Cal. for C H NO; C, 79.97%; H, 6.71%; N, 6.22%.
16
17
2
7
75.19%; H, 6.67%; N, 5.48%.
4-(([cyclohepta-2,4,6-trien-1-ylmethyl]imino)
methyl)benzene-1,3-diol (8e) (C H NO ).
15
15
Found: C, 79.68%; H, 6.69%; N, 6.14%.
1
5
15
2
1
N-(4-chlorobenzylidene)-1-(cyclohepta-2,-
H NMR (400 MHz, CDCl ) δ 7.77 (s, 1H, N CH),
3
4
,6-trien-1-yl)methanamine (8b) (C H ClN).
6.90 (d, J = 8.8 Hz, 1H, Ph-H), 6.62–6.59 (m, 2H, C CH),
6.34 (s, 1H, Ph-H), 6.26–6.20 (m, 3H, C CH and Ph-H),
5.23 (dd, J = 9.2, 6.2 Hz, 2H, C CH), 3.57 (d, J = 7.2 Hz,
15
14
1
H NMR (400 MHz, CDCl ) δ 8.36 (s, 1H, N CH),
3
7
.72 (d, J = 8.5 Hz, 2H, Ph-H), 7.41 (d, J = 8.5 Hz, 2H, Ph-
13
H), 6.70–6.67 (m, 2H, C CH), 6.28 (dddd, J = 8.9, 3.7, 2.6,
.1 Hz, 2H, C CH), 5.36 (dd, J = 9.1, 5.6 Hz, 2H,
C CH), 3.95 (dd, J = 6.8, 1.3 Hz, 2H, NCH2),
2H, NCH ), 2.32–2.23 (m, 1H, CH) ppm; C NMR
2
1
(101 MHz, CDCl ) δ 164.1 (ArC), 164.0 (ArC), 162.9
3
(C N), 131.0 (ArC), 131.0 (C C), 126.9 (ArC), 126.1
(C C), 123.1 (C C), 110.9 (ArC), 107.9 (ArC), 104.3
1
3
2
.23–2.15 (m, 1H, CH) ppm; C NMR (101 MHz,
ꢀ
CDCl ) δ 160.5 (C N), 136.6 (ArC), 134.7 (ArC), 130.9
(ArC), 50.6 (NCH ), 39.7 (CH) ppm; IR (KBr): V = 3,314,
3
2
(
(
=
5
1
C C), 129.4 (ArC), 128.8 (ArC), 125.2 (C C), 124.5
3,011, 2,922, 1,632, 1,600, 1,537, 1,446, 1,221, 904, 790,
−
1
+
+
ꢀ
C C), 63.3 (NCH ), 39.9 (CH) ppm; IR (KBr): V
700 cm ; GC–MS: 224.1 (M , OH), 207 (M , -2OH),
132.0, 118.0, 104.0, 91.0, 77.0, 65.0, 39.0, 32.0; Elemental
analysis: Anal. Cal. for C H NO ; C, 74.67%; H,
2
3,011, 2,840, 1,643, 1,593, 1,487, 1,085, 1,012, 819, 700,
−
1
+
+
04 cm ; GC–MS: 243.0 (M ), 208.1 (M , Cl), 178.0,
51.9, 124.9, 104.0, 91.0 77.0, 65.0, 51.0, 32.0; Elemental
15
15
2
6.27%; N, 5.81%. Found: C, 74.61%; H, 6.24%; N, 5.73%.
N-(2-bromobenzylidene)-1-(cyclohepta-2,-
4,6-trien-1-yl)methanamine (8f) (C H BrN).
analysis: Anal. Cal. for C H ClN; C, 73.92%; H,
15
14
5
.79%; N, 5.75%. Found: C, 73.84%; H, 5.74%; N, 5.71%.
-bromo-2-(([cyclohepta-2,4,6-trien-1-ylmethyl]
imino)methyl)phenol (8c) (C H BrNO).
1
5
14
1
3
H NMR (400 MHz, CDCl ) δ 8.77 (s, 1H, N CH),
3
8.05 (dd, J = 7.7, 1.9 Hz, 1H, Ph-H), 7.60 (dd, J = 7.9, 1.1 Hz,
1H, Ph-H), 7.38–7.33 (m, 1H, Ph-H), 7.29 (ddd, J = 9.7, 6.9,
1.9 Hz, 1H, Ph-H), 6.69 (dd, J = 3.5, 2.8 Hz, 2H, C CH),
6.27 (dddd, J = 8.9, 3.8, 2.6, 1.3 Hz, 2H, C CH), 5.38 (dd,
J = 9.0, 5.6 Hz, 2H, C CH), 4.01 (dd, J = 6.8, 1.4 Hz, 2H,
1
5
14
1
H NMR (400 MHz, CDCl ) δ 8.34 (s, 1H, N CH),
3
7
.40 (dd, J = 7.3, 2.3 Hz, 2H, Ph-H), 6.90–6.85 (m, 1H,
Ph-H), 6.69 (dd, J = 3.5, 2.9 Hz, 2H, C CH), 6.28
dddd, J = 9.0, 3.7, 2.6, 1.2 Hz, 2H, C CH), 5.32 (dd,
J = 9.0, 5.7 Hz, 2H, C CH), 3.89 (dd, J = 7.0, 1.1 Hz,
(
1
3
NCH ), 2.25–2.18 (m, 1H, CH) ppm; C NMR (101
2
1
3
2
(
H, NCH ), 2.25–2.18 (m, 1H, CH) ppm; C NMR
MHz, CDCl ) δ 161.0 (C N), 134.6 (ArC), 132.9 (ArC), 131.7
2
3
101 MHz, CDCl ) δ 164.4 (ArC), 160.3 (C N), 134.9
(ArC), 130.9 (C C), 129.0 (ArC), 127.6 (ArC), 125.3 (ArC),
3
(
(
(
1
ArC), 133.4 (ArC), 131.0 (C C), 125.9 (C C), 123.5
C C), 120.1 (ArC), 119.1 (ArC), 109.9 (ArC), 61.0
125.0 (C C), 124.5 (C C), 63.3 (NCH ), 39.9 (CH) ppm; IR
2
ꢀ
(KBr): V = 3,011, 2,886, 1,633, 1,435, 1,269, 1,020, 752,
−
1
+
+
ꢀ
NCH ), 39.8 (CH) ppm; IR (KBr): V = 3,012, 2,842,
702, 591 cm ; GC–MS: 287.9 (M ), 206.0 (M , -Br),
168.9, 104.0, 91.0, 77.0, 65.0, 51.0, 39.0, 32.0; Elemental
analysis: Anal. Cal. for C H BrN; C, 62.52%; H,
2
−
1
,631, 1,474, 1,274, 1,178, 816, 730, 695, 623 cm ; GC–
+ +
MS: 304.9 (M ), 287.9 (M , OH), 211.9, 184.9, 104.0,
1.0, 77.0, 51.0, 39.0, 32.0; Elemental analysis: Anal.
Cal. for C H BrNO; C, 59.23%; H, 4.63%; N, 4.60%.
15
14
9
4.90%; N, 4.86%. Found: C, 62.44%; H, 4.88%; N, 4.82%.
1-(cyclohepta-2,4,6-trien-1-yl)-N-(4-([E]-styryl)
benzylidene)methanamine (8 g) (C H N).
15
14
Found: C, 59.11%; H, 4.61%; N, 4.55%.
23
21

10
YILMAZ
1
H NMR (400 MHz, CDCl ) δ 8.26 (s, 1H, N CH),
concentrations (6,75–400 μg/ml) of all synthesized com-
pounds and reference test materials (Butylated hydro-
xytoluene (BHT) and butylated hydroxyanisole (BHA))
were prepared in EtOH. Then 1 ml of DPPH solution was
added into different test tubes and 1 ml of solutions in
different concentrations was added for each sample. After
incubation for half an hour in the dark, UV measure-
ments at 517 nm were taken. According to the decrease
in absorbance value, antioxidant activity was calculated
3
7
7
7
.65 (d, J = 8.3 Hz, 2H, Ph-H), 7.49–7.40 (m, 4H, Ph-H),
.27 (t, J = 7.5 Hz, 2H, Ph-H), 7.21–7.15 (m, 1H, Ph-H),
.05 (d, J = 9.8 Hz, 2H, Ph-CH CH-), 6.60–6.55 (m, 2H,
C CH), 6.17 (dddd, J = 8.9, 3.8, 2.6, 1.3 Hz, 2H,
C CH), 5.25 (dd, J = 9.0, 5.6 Hz, 2H, C CH), 3.84
dd, J = 6.8, 1.0 Hz, 2H, NCH ), 2.13–2.06 (m, 1H,
CH) ppm; C NMR (101 MHz, CDCl ) δ 161.6
C N), 139.7 (ArC), 137.0 (ArC), 135.3 (ArC), 130.9
C C), 130.0 (ArC), 128.7 (ArC), 128.6 (ArC), 128.0
ArC), 127.9 (C C), 126.7 (ArC), 125.2 (C C), 124.7
C C), 63.5 (NCH ), 40.0 (CH) ppm; IR (KBr): V
3,014, 2,835, 1,639, 1,602, 1,447, 960, 906, 815, 727, 687,
(
2
13
3
(
(
(
(
=
5
53,57
using the formula below (Equation 1).
ꢀ
%DPPH Scavenging = ðAo −As=AoÞ × 100
ð1Þ
2
−
1
+
37 cm ; GC–MS: 208.0 (M , C C-Ph), 179.0, 165.0,
52.0, 89.0, 76.0, 51.0, 39.0, 32.0; Elemental analysis:
where A is the absorbance of the control; A is the
o
s
1
absorbance of the sample at 517 nm.
Anal. Cal. for C H N; C, 88.71%; H, 6.80%; N, 4.50%.
23
21
Found: C, 88.60%; H, 6.77%; N, 4.49%.
2+
N-(2-chlorobenzylidene)-1-(cyclohepta-2,-
3.4 | Ferrous ions (Fe ) chelating
activity
4
,6-trien-1-yl)methanamine (8 h) (C H ClN).
15
14
1
H NMR (400 MHz, CDCl ) δ 8.85 (s, 1H, N CH),
3
8
6
.08 (d, J = 7.5 Hz, 1H, Ph-H), 7.44–7.28 (m, 3H, Ph-H),
.76–6.64 (m, 2H, C CH), 6.27 (d, J = 9.1 Hz, 2H,
C CH), 5.38 (dd, J = 9.1, 5.6 Hz, 2H, C CH), 4.01 (d,
Concentrations prepared in the DPPH method were also
used for this method. 1 ml of each solution prepared for
each synthesized molecule was taken and added to 0.4 ml
J = 6.2 Hz, 2H, NCH ), 2.25–2.17 (m, 1H, CH) ppm;
FeCl solution of 2 mM previously taken into the test
2
2
13
C NMR (101 MHz, CDCl ) δ 158.7 (C N), 135.1
tubes. 0.2 ml of 5 mM ferrozine solution was added to
these mixtures and the resulting solutions were left to
incubate in the dark for 10 min. After the incubation was
completed, the activity was calculated using the Equa-
tion (2) according to the decrease in absorbance value by
3
(ArC), 133.2 (ArC), 131.5 (ArC), 130.9 (C C), 129.7 (ArC),
1
28.5 (ArC), 126.9 (ArC), 125.2 (C C), 124.5 (C C), 63.5
ꢀ
(NCH ), 39.9 (CH) ppm; IR (KBr): V = 3,012, 2,888,
2
−
1
1
2
7
,634, 1,591, 1,436, 1,270, 1,051, 753, 701 cm ; GC–MS:
+
64
08.0 (M , Cl), 180.9, 167.0, 153.0, 144.0, 104.0, 91.0,
taking UV measurements at 562 nm.
7.0, 65.0, 51.0, 39.0, 32.0; Elemental analysis: Anal.
À
Á
2
+
Cal. for C H ClN; C, 73.92%; H, 5.79%; N, 5.75%.
%Ferrous ions Fe
chelating activity = ðAo −As=AoÞ × 100
ð2Þ
15
14
Found: C, 73.79%; H, 5.74%; N, 5.71%.
General method for the synthesis of Suzuki C C
coupling products.
Synthetic method carried out by the following pro-
cedure : Aryl bromide (0.5 mmol), phenylboronic acid
A is the absorbance of the control; A is the absorbance
of the sample at 562 nm
o
s
(
0.6 mmol), base (0.6 mmol), EtOH-H O (2:1 ml), Schiff
2
base (0.8 mol%), and PdCl2 (0.4 mol%) were added to a
ꢀ
sealed tube and stirred for 4 hours at 80 C. After the
4 | CONCLUSIONS
reaction time was completed, the mixture was washed
with CHCl and dried with Na SO . After filtering rap-
Several new Schiff bases were synthesized, and their
structures were illuminated by various spectroscopic
methods. The catalytic activity of all synthesized Schiff
bases in Suzuki coupling reactions using phenyl boronic
acid and various aryl bromides were investigated. All
Schiff bases showed good to excellent catalytic activity
according to aryl bromide used in the reactions. In addi-
tion to their catalytic activities, the antioxidant properties
of all Schiff bases were examined by DPPH and iron che-
lation methods. IC50 values of Schiff bases, which give
good results compared to BHT and BHA used as positive
control, were calculated.
3
2
4
idly through SiO , internal standard was added, and
2
GC–MS was taken. The biphenyl product was character-
1
ized by H NMR and GC, and agreement with the litera-
5
9–63
ture data.
3.3 | DPPH free radical scavenging
activity
Free radical scavenging activity was measured using the
DPPH. In order to examine the activity tests, the

YILMAZ
11
ACKNOWLEDGMENTS
[24] K. Zhang, P. Wang, L.-N. Xuan, X.-Y. Fu, F. Jing, S. Li, Y.-M.
Liu, B.-Q. Chen, Bioorg. Med. Chem. Lett. 2014, 24, 5154.
I would like to thank Prof. Dr. Nermin S¸ im s¸ ek KU S¸ and
Assoc. Prof. Dr. Mustafa Kemal YILMAZ for their sup-
port. This research was supported by Scientific Research
Foundation, Mersin Üniversitesi (BAP 2017-1-AP1-2188).
[25] M. Shebl, O. M. I. Adly, E. M. Abdelrhman, B. A. El-Shetary,
J. Mol. Struct. 2017, 1145, 329.
[
[
26] K. Buldurun, M. Özdemir, J. Mol. Struct. 2020, 1202, 127266.
27] X. Liu, C. Manzur, N. Novoa, S. Celedón, D. Carrillo, J.-R.
Hamon, Coord. Chem. Rev. 2018, 357, 144.
ORCID
[28] W.-G. Jia, S. Ling, S.-J. Fang, E.-H. Sheng, Polyhedron 2017,
138, 1.
Özgür Yılmaz https://orcid.org/0000-0001-9278-1091
[29] D. Tarinc, H. Muslu, M. Cesme, A. Golcu, M. Tumer, S. A.
Ozkan, Curr. Anal. Chem. 2013, 9, 319.
30] D. Pandiarajan, R. Ramesh, J. Organomet. Chem. 2013, 723, 26.
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How to cite this article: Yılmaz Ö. Synthesis of
new Schiff bases; Investigation of their in situ
catalytic activity for Suzuki C C coupling
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