N-alkyl imidazole-based homonuclear coordination complex as a neutral organocatalyst for the faster and efficient construction of 3,4-dihydro-2H-1,3-oxazine scaffold

Article abstract of DOI:10.1002/aoc.6425

In the present work, homonuclear coordination complex including Zn (II) and N-hexadecylimidazole ligand was used for the first time as a highly efficient homogeneous neutral organocatalyst for the synthesis of 3,4-dihydro-2H-1,3-benzoxazine monomers. Ther

Full text of DOI:10.1002/aoc.6425

Received: 8 July 2021
DOI: 10.1002/aoc.6425
Revised: 24 July 2021
Accepted: 11 August 2021
R E S E A R C H A R T I C L E
N-alkyl imidazole-based homonuclear coordination
complex as a neutral organocatalyst for the faster and
efficient construction of 3,4-dihydro-2H-1,3-oxazine scaffold
Ayhan Yıldırım
| Mustafa Göker
Department of Chemistry, Faculty of Arts
Abstract
ꢀ
and Sciences, Bursa Uludag University,
Bursa, Turkey
In the present work, homonuclear coordination complex including Zn (II) and
N-hexadecylimidazole ligand was used for the first time as a highly efficient
homogeneous neutral organocatalyst for the synthesis of 3,4-dihydro-2H-1,
3-benzoxazine monomers. Therefore, N-alkyl or N-aryl substituted, both
mono-benzoxazine and bis-benzoxazine, were successfully synthesized
(21 examples) via Mannich-type condensation reactions. Effortlessly obtaining
the pure product with high yields and the short reaction time makes this
method more useful and advantageous than the common existing benzoxazine
synthesis methods.
Correspondence
Ayhan Yıldırım, Department of
Chemistry, Faculty of Arts and Sciences,
ꢀ
Bursa Uludag University, P. O. Box 16059,
Bursa, Turkey.
Email: yildirim@uludag.edu.tr
K E Y W O R D S
1,3-benzoxazines, metal complex catalyst, multicomponent synthesis, N-alkylimidazole,
two-component synthesis
1 | INTRODUCTION
have very different molecular structures.[10,14] Therefore,
the two most general approaches for the preparation of
Compounds bearing the dihydro-1,3-benzoxazine ring
system exhibit a wide range of biological activity.[1–5] The
recently synthesized nonionic, cationic, and anionic poly-
benzoxazine surfactants draw attention with their alter-
native more advanced properties in comparison with
conventional surfactants.[6–8] On the other hand,
polybenzoxazines exhibit superior properties than previ-
ously well-known phenol-based resins.[9–11] Recently,
Yildirim et al. reported thermally curable benzoxazine-
modified renewable bio-resource based on triglyceride
oils[12] and Raicopol et al. also reported triglyceride based
polybenzoxazine derivatives as coatings on Zn–Mg–Al
alloy coated steel which can act as a corrosion protection
layer.[13] It can be concluded that benzoxazine-based
monomeric compounds can play a key role in the devel-
opment of advanced multifunctional polymeric materials
with unique advantages. Since these compounds can be
synthesized from large varieties of phenolic compounds
and amines containing different substituents, they can
compounds having this fused ring system are syntheses
based on three-component and two-component
Mannich-type condensation reactions (Figure 1). In order
to achieve the synthesis via the three-component and the
two-component condensation reactions, the most com-
monly used reagents are shown in Figure 1. As seen in
the figure, naphthols or phenols, alkyl or arylamines are
reacting with formaldehyde (Route I), and secondary
alkyl or arylamines bearing naphthol or phenol moiety
are reacting with formaldehyde (Route II). Noncatalyzed
reactions, including both of these routes, usually require
quite long reaction times, and in some cases no product
was obtained.[15]
Many approaches in the literature, revealing the syn-
thesis methods of this class of compounds, often have
some disadvantages, such as long condensation time,
high temperatures, and consequent formation of by-
products through polymerization and low product
yields.[5,8,9,16–21] On the other hand, a comprehensive
Appl Organomet Chem. 2021;e6425.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.6425

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YILDIRIM AND GÖKER
FIGURE 1 Routes of
multicomponent and two-
component construction of
dihydro-1,3-oxazine ring system
literature review reveals that many different compounds
have been investigated to catalyze these multicomponent
condensation reactions.[2,15,22,23]
continuation of our focus on the N-alkylated imidazole-
based organocatalyst systems and to the best of our
knowledge, metal complexes of higher N-alkylated imid-
azoles have not yet been used as catalysts for the produc-
ing of 3,4-dihydro-2H-1,3-benzoxazines. Therefore, in the
present study, symmetric metal complex compound
was prepared by reacting Zn (II) chloride with
N-hexadecylimidazole ligand, and thus, synthesized neu-
tral compound was used as a catalyst for the preparation
of a series of 3,4-dihydro-2H-1,3-benzoxazines via two-
component or multicomponent Mannich-type condensa-
tion reaction.
Many green methods have also been developed for
the synthesis of 1,3-oxazines.[1,20,23–29] Unfortunately, in
some of these studies, acidic catalysts are used, while
in some cases it is necessary to interact with strong bases
during product isolation and laborious techniques such
as extraction and column chromatography that are
applied. According to Tang et al., no product was formed
although a secondary amine and formaldehyde were
refluxed in chloroform for 5 h in the presence of Et3N or
in the absence of any catalyst.[15] In addition, very low
yields of benzoxazines were obtained in the presence of
different acidic catalysts. In the case of metal salts, over-
loading of the catalyst and long reaction times is required
for higher product yields. The use of acids or bases, either
as catalysts or in product isolation stages, may cause
structural changes in starting compounds and products
which have functional groups with high tendency toward
effects of strong acids and bases. Therefore, for the prepa-
ration of benzoxazines, the development of mild and neu-
tral catalysts, without such undesirable side-effects, is
gaining importance. In addition, some different synthesis
strategies for the preparation of these compounds
continue to be developed. Multicomponent synthesis of
bio-based benzoxazines from cardanol by using a deep
eutectic solvent[30] and a modified method based on
arylboronic acids has also been described.[31] Vengatesan
et al. reported ultrasound-assisted synthesis of
benzoxazine monomers.[32]
2 | RESULTS AND DISCUSSION
2.1 | Synthesis of catalyst ([C16Im]2MCl2)
The neutral organocatalyst used in this study was synthe-
sized easily under the reaction conditions[42,43] shown in
Scheme 1, and for this purpose, the required starting
ligand, 1-hexadecyl-1H-imidazole was prepared in accor-
dance to previously published procedure.[44] As seen in
the figure, the metal complex compound was prepared by
the reaction of two equivalents of 1-hexadecyl-1H-
imidazole with one equivalent of anhydrous zinc chloride
in EtOH and the complex product precipitates, whose
solubility decreases in EtOH due to the long alkyl side
chains. Complexation between zinc metal and
1-hexadecyl-1H-imidazole ligand caused changes in the
1H NMR chemical shift of ligand (Scheme 1). The imidaz-
ole ring protons which correspond to the protons in car-
bon atoms of the heterocyclic system appeared as singlet,
and each singlet corresponds to a single proton (three
protons in total). On the other hand, a triplet peak and a
quintet peak belonging to two protons located on the –
NCH2CH2– and –NCH2CH2– carbon atoms of the
N-alkyl and/or N, N-dialkylated imidazoles and their
modified derivatives have been used as catalysts in the
synthesis of 1,3-oxazines and variety of organic
compounds.[23,33–37] Some transition metal complexes of
N-alkylated imidazoles have been used as catalyst in sev-
eral different organic synthesis reactions.[38–41] In

YILDIRIM AND GÖKER
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SCHEME 1 Synthesis of
catalyst [C16Im]2ZnCl2
SCHEME 2 Two-component synthesis of benzoxazine monomers
hexadecyl alkyl side chain attached to the N atom in the
imidazole ring were observed at 3.96 and 1.78 ppm,
respectively. Because of decrease in the electron, density
of the imidazole ring yields downfield shifts at all ring
positions. Thus, the downfield shifts of imidazole
ring protons in the complex, compared with free
1-hexadecyl-1H-imidazole ligand, confirm the successful
synthesis of the desired complex compound.
2.2 | Synthesis of benzoxazine
monomers
A series of 3,4-dihydro-2H-1,3-benzoxazines (21 examples)
were synthesized based on 2-((alkylamino)methyl)phe-
nols or 2-((arylamino)methyl)phenols as secondary
amines with the assistance of [C16Im]2ZnCl2 as presented
in Scheme 2. The starting secondary amines (Mannich

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YILDIRIM AND GÖKER
bases) were prepared readily and with good yields, via
common two step reactions from salicylaldehyde and
different primary amines. For this, the corresponding
Schiff bases were first obtained and then reduced with
NaBH4 to give secondary amines. The 1,3-benzoxazine
ring formation was achieved by reacting the
reactions of all benzoxazine compounds were carried out
with the catalyst containing zinc as metal.
When the reaction temperature is increased by 10ꢀC,
the yields of benzoxazines decrease slightly, most likely
due to the formation of polymeric by-products during the
condensation
between
paraformaldehyde
and
corresponding
2-((arylamino)methyl)phenols
or
2-((arylamino)methyl)phenols or 2-(alkylamino)methyl)
phenols (Table 1, Entry 3, and Table 2, Entry 5). In addi-
tion, it was observed that the increase in the amount of
2-((alkylamino)methyl)phenols with 2 equivalent of para-
formaldehyde in the presence of appropriate amount of
the catalyst, [C16Im]2ZnCl2 in toluene for 10 to 15 min
(Tables 1 and 2). After the reaction is complete, the sol-
vent was removed and the remaining residue was crystal-
lized from the appropriate solvent mixture to obtain the
corresponding benzoxazines with good to excellent
yields. As can be seen from Table 1, when using the imid-
azole complex containing the cobalt transition metal as
the catalyst, the corresponding benzoxazine 1a was
obtained with lower yield than the reaction carried out
with the catalyst containing the zinc metal. Therefore, in
the present paper, optimization studies and the synthesis
catalyst
in
the
condensation
reaction
with
2-((phenylamino)methyl)phenol significantly reduced the
yield of the corresponding benzoxazine (Table 1, Entry
4). Thus, it is concluded that the optimum reaction condi-
tions for condensation paraformaldehyde with
2-((arylamino)methyl)phenols or 2-((alkylamino)methyl)
phenols are as indicated in Table 1, Entry 1, and Table 2,
Entry 4, respectively.
In the case of preparation of benzoxazines in aqueous
medium, the oxazine ring is usually opened and
unwanted oligomeric side products are formed in the
TABLE 1 Optimization of reaction conditions for the synthesis of benzoxazines from 2-((arylamino)methyl)phenols
Entry
Catalyst
ImZn
ImZn
ImZn
ImZn
ImCo
Catalyst loading (mmol%)
Time (min)
Temperature (ꢀC)
Yield (%)a
1
2
3
4
5
2
10
15
10
10
10
90
90
96
75
43
59
41
2
2
100
90
2.5
2
90
aYields after crystallization from MeOH/H2O.
TABLE 2 Optimization of reaction
conditions for the synthesis of
benzoxazines from 2-((alkylamino)
methyl)phenols
Entry Catalyst loading (mmol%) Time (min) Temperature (ꢀC) Yield (%)a
1
2
3
4
5
2
10
15
20
15
15
90
90
92
90
87
97
83
2
2
90
2.5
2.5
90
100
aYields after crystallization from MeOH/H2O.

YILDIRIM AND GÖKER
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TABLE 3 Structures of benzoxazines synthesized in the presence of [C16Im]2ZnCl2 catalyst
Entry
Compound
1a
Structure
Yield (%)a
1
85
2
1b
1c
1d
1e
1f
89
3
89
76
64
77
76
69
4
5
6
7
1g
1h
87
(Continues)

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YILDIRIM AND GÖKER
TABLE 3 (Continued)
Entry
Compound
1i
Structure
Yield (%)a
9
89
10
1j
70
11
12
13
14
15
1k
1l
89
76
89
90
75
1m
1n
2a
16
17
2b
2c
92
95

YILDIRIM AND GÖKER
7 of 12
TABLE 3 (Continued)
Entry
Compound
2d
Structure
Yield (%)a
18
95
19
20
21
3a
3b
3c
96
96
97
aYields after crystallization from MeOH/H2O.
reaction
medium.[45]
Therefore,
to
increase
The molecular structures of all the synthesized
1,3-benzoxazines were established on the basis of their
spectroscopic data (1H NMR, 13C NMR, and Fourier
Transform Infrared (FTIR)) and elemental analysis. For
the benzoxazine 1a, representative 1H NMR and 13C
the benzoxazine yield, paraformaldehyde is preferred
instead of formalin, and nonpolar solvents such as tolu-
ene are generally preferred as solvents. Therefore, we pre-
ferred to perform condensation reactions between
secondary amines and paraformaldehyde in toluene as a
most reliable method for the synthesis of 1,3-benzoxazine
monomers. In order to evaluate the substrate scope of the
condensation procedure developed in the present work,
we used a variety of secondary amines for the synthesis of
1,3-benzoxazine monomers. All of these attempts showed
that either aryl or alkyl substituted secondary amine as
starting compounds can provide the corresponding
1,3-benzoxazines with good to excellent yields in the pres-
ence of [C16Im]2ZnCl2 as an efficient organocatalyst. The
structures of all the benzoxazines synthesized within the
scope of this work are given in Table 3. As can be seen
from the table, the (2-((alkylamino)methyl)phenols)
formed the related benzoxazines with higher yields than
(2-((arylamino)methyl)phenols) (Entries 15–20). On the
other hand, generally, the (2-((arylamino)methyl)phe-
nols) with electron-withdrawing groups on the amine-
based aryl ring gave slightly lower yields than those with
electron-donating groups. The reason for these higher
benzoxazine yields can be attributed to the fact that the
nucleophilic characters of the amino groups in
(2-((alkylamino)methyl)phenols) are higher than that of
the N-aryl substituted amines.
1
NMR spectra are given in Figure 2. The H NMR spec-
trum of 1a showed characteristic two clear singlet signals
at 4.62 and 5.35 ppm, corresponding to the two protons
each in the 1,3-oxazine ring (ArCH2N– and –NCH2O–,
respectively). On the other hand, when the 13C NMR
spectrum of this compound is examined, it is seen that
the characteristic signals of the C atoms in the ArCH2N–
and –NCH2O– groups appear at 50.4 and 79.5 ppm,
respectively.
The fact that the [C16Im]2ZnCl2 catalyst facilitates the
synthesis of 1,3-benzoxazines via the two-component
condensation reaction has encouraged us to investigate
the catalytic activity of this complex compound for the
synthesis of abovementioned compounds through
the three-component condensation reaction. For this pur-
pose, hexadecyl amine, phenol, and paraformaldehyde
were reacted under the reaction conditions shown in
Table 4. As can be seen from the table, with the assis-
tance of this catalyst, benzoxazines can also be success-
fully synthesized following a three-component reaction
strategy. For instance, the benzoxazine 3e was obtained
in good yield by using this synthesis strategy. (Table 4,
Entry 6).

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YILDIRIM AND GÖKER
FIGURE 2 1H NMR and 13C NMR
spectra of benzoxazine 1a
2.3 | Reusing of the reaction medium
2.4 | Plausible mechanism of
[C16Im]2ZnCl2 catalyzed two-component
condensation reaction
In order to investigate reusability of the catalyst,
2-((phenylamino)methyl)phenol and paraformaldehyde
were treated under the optimum condensation reaction
conditions given in Table 1, Entry 1. After the
reaction was complete, the solvent was removed and the
residue was crystallized from the MeOH/H2O system.
After isolation of the desired product, the resulting fil-
trate was evaporated to dryness and the remaining crude
catalyst was used directly in the next similar runs without
further purification. The results obtained are given in
Table 5. According to the yields obtained, it can be con-
cluded that the synthesized catalyst is convenient to use
two times without a significant loss in its activity.
The
reaction
catalytic
between
two-component
condensation
and
paraformaldehyde
(2-((hexadecylamino)methyl)phenol) is considered to
proceed through the mechanism shown in Figure 3.
Formaldehyde molecule interacts via its unpaired elec-
trons on the carbonyl oxygen atom with the zinc atom of
catalyst [C16Im]2ZnCl2 which act as an electron pair
acceptor. Thus, the amino group on the Mannich base
readily makes a nucleophilic attack on the carbonyl
group of the aldehyde molecule to form a tetrahedral
intermediate. Following a proton migration, the related

YILDIRIM AND GÖKER
9 of 12
TABLE 4 Optimization of reaction conditions for the three-component synthesis of benzoxazines
Entry
Catalyst loading (mmol%)
Time (h)
Temperature (ꢀC)
Yield (%)a
1
2
3
4
5
6
7
8
9
2
3
3
4
3
3
4
2
3
4
110
110
110
120
110
100
110
100
90
83
75
75
2.5
2
2
Polymerization occurs, and product isolation is difficult
1
56
85
64
83
56
2
2
2
2
aYields after crystallization from MeOH/H2O.
TABLE 5 Catalyst reuse studies
Entry
Catalyst cycle
Catalyst loading (mmol%)
Time (min)
Temperature (ꢀC)
Yield (%)a
1
2
3
4
1
2
3
4
2
2
2
2
10
10
10
10
90
90
90
90
96
94
69
57
aYields after crystallization from MeOH/H2O.
benzoxazine compound is formed via an intramolecular
cyclization of N-(2-hydroxybenzyl)-N-met-
hylenehexadecan-1-aminium intermediate.
In order to explain the catalytic effect of
[C16Im]2ZnCl2 in the condensation reaction, a different
mechanistic approach was adopted and some simple the-
oretical calculations were also made. Geometry optimiza-
Orbital (LUMO) orbitals of the 2-(hexadecylamino)
methyl)phenol and formaldehyde molecules are given,
respectively, and the electron flow from the
2-(hexadecylamino)methyl)phenol molecule to formalde-
hyde molecule is also shown. On the other hand, in
Figure 4b the energies of the LUMO and HOMO orbitals
of the catalyst and formaldehyde molecules are given,
respectively, and the electron flow from formaldehyde
molecule to the catalyst molecule is also shown. As can
be seen from Figure 4, the LUMOcatalyst - HOMO-
tions
of
the
2-(hexadecylamino)methyl)phenol,
formaldehyde and [C16Im]2ZnCl2 molecules were per-
formed using the MM2 force field option through the
Chem3D. The calculated Frontier Molecular Orbital
(FMO) energetics of the abovementioned molecules with
their optimized geometries are given in Figure 4. In
Figure 4a, the energies of the Highest Occupied Molecu-
lar Orbital (HOMO) and Lowest Unoccupied Molecular
energy
gap
is
lower
than
the
formaldehyde
HOMO2-(hexadecylamino)methyl)phenol
-
LUMOformaldehyde
(4.461 and 6.656 eV, respectively). Thus, after this favor-
able interaction with the catalyst, the formaldehyde mol-
ecule is more easily subjected to nucleophilic attack by

10 of 12
YILDIRIM AND GÖKER
FIGURE 3 Proposed mechanistic pathway for the production of benzoxazines via the two-component condensation reaction
the 2-(hexadecylamino)methyl)phenol molecule as
shown previously in Figure 3.
4 | EXPERIMENTAL
4.1 | Reagents and analyses
3 | CONCLUSIONS
All reagents and solvents were purchased from Merck
(Merck,
Darmstadt,
Germany),
Sigma-Aldrich
As a result, the catalyst [C16Im]2ZnCl2, prepared from
N-alkylimdazole and anhydrous zinc chloride, is a
highly efficient catalyst for the two- or three-component
synthesis of many different types of 3,4-dihydro-2H-1,
3-benzoxazines. In this study, for the first time, an
imidazole-based metal-containing compound was suc-
(St. Louis, MO), or Acros Organics (Thermo Fisher Sci-
entific, Geel, Belgium) and used without further purifi-
cation. Thin layer chromatography was performed using
silica gel (60 F254, Merck, Darmstadt, Germany) plates.
Melting points were recorded by BÜCHI melting
pointB-540 apparatus (BÜCHI Labortechnik AG in
Flawil, Switzerland). A Bruker Tensor II Fourier trans-
form infrared (FT-IR) spectrometer (Billerica, MA,
USA) was used for acquisition of the FT-IR spectra.
The NMR spectra were measured using A600a Agilent
DD2 600-MHz NMR spectrometer (Santa Clara,
California, USA) and chloroform-d (CDCl3) as a solvent.
Chemical shifts (δ) are reported in ppm and J values in
Hertz. The elemental analyses were performed using an
LECO CHNS-932 elemental analyzer (Saint Joseph,
MI, USA).
cessfully used as
a catalyst in the synthesis of
1,3-benzoxazines. This compound is very stable at room
conditions and is not hygroscopic, so it can be stored
easily and its use in condensation reactions is quite
simple. The most important factors that make this
invented practical method more considerable are its
simplicity and very short reaction times (10–15 min).
The condensation reaction can proceed at fairly low cat-
alyst load (down to 2 mmol%) under mild reaction
conditions.

YILDIRIM AND GÖKER
11 of 12
FIGURE 4 FMO orbitals of 2-(hexadecylamino)methyl)phenol, formaldehyde, catalyst, and energy gap
4.2 | Preparation of catalyst
The flask was attached to a reflux condenser and heated
under atmospheric conditions in an oil bath at 90ꢀC for
15 min. Thereafter, the solvent was removed by rotary
evaporator and the resulting solid product was crystal-
lized from acetone–water solvent mixture to give pure
1,3-benzoxazine as a white crystalline solid.
Into a 50-ml flat bottom reaction flask anhydrous ZnCl2
(0.12 g, 0.88 mmol) was added and dissolved in 10 ml of
EtOH. With stirring at room temperature, 0.5 g
of 1-hexadecyl-1H-imidazole dissolved in 10 ml of etha-
nol was added slowly and heated at 50ꢀC for 1 h. The
resulting mixture was then left at room temperature over-
night, and the precipitated yellowish solid was filtered
and washed with ether. The yield of the product was
0.5 g (41%) with mp: 102.4–103.2ꢀC.
4.4 | Typical experimental procedure for
the three-component preparation of
1,3-benzoxazines
To a single-necked 50-ml reaction flask, 0.2 g, 0.83 mmol
of hexadecylamine, 0.078 g, 0.83 mmol of phenol,
0.050 g, 1.66 mmol of paraformaldehyde, 2 mmol% of the
catalyst, and 2-ml toluene were added. The flask was
attached to a reflux condenser and heated under atmo-
spheric conditions in an oil bath at 100ꢀC for 4 h. There-
after, the solvent was removed by rotary evaporator and
the resulting solid product was crystallized from acetone–
water solvent mixture to give pure 1,3-benzoxazine as a
4.3 | Typical experimental procedure for
the preparation of 1,3-benzoxazines from
2-((alkylamino)methyl)phenols
To a 50-ml round-bottomed flask containing 2 ml of tolu-
ene were added 2-((hexadecylamino)methyl)phenol
(0.20 g,
0.58 mmol),
paraformaldehyde
(0.035 g,
1.16 mmol, 2 equivalent), and 2.5 mmol% of the catalyst.

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YILDIRIM AND GÖKER
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white crystalline solid with sufficient purity for further
spectroscopic analysis. (Supporting information includes
1
characterization data and FTIR, HNMR, 13CNMR spec-
tra of all the synthesized benzoxazines).
DATA AVAILABILITY STATEMENT
Data openly available in a public repository that issues
datasets with DOIs.
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ORCID
Ayhan Yıldırım https://orcid.org/0000-0002-2328-9754
Mustafa Göker https://orcid.org/0000-0003-4667-1670
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How to cite this article: A. Yıldırım, M. Göker,
Appl Organomet Chem 2021, e6425. https://doi.org/
10.1002/aoc.6425
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