Synthesis, characterization and density functional theory investigations of monoacyl aniline derivatives

Article abstract of DOI:10.4314/bcse.v28i1.13

We synthesized a number of aniline derivatives containing acyl groups to compare their barriers of rotation around the N-CO groups. Geometry optimization for all the rotamers have been performed using density functional theory (DFT) at the B3LYP/6-31G** level of theory. For each stationary point we carried out vibrational frequency calculations at the same level to characterize their nature as minima or transition states. The methoxy group substituent on the benzene ring causes some changes in the C-C bond distances. We carried out potential energy surface scanning to find out all possible low-energy conformations, which would be used as the initial structure for further molecular calculations. There were only three kinds of conformations with local minimum potential energy and two transition states. The optimized bond lengths and bond angles are in better agreement with the experimental values.

Full text of DOI:10.4314/bcse.v28i1.13

Bull. Chem. Soc. Ethiop. 2014, 28(1), 111-120.
Printed in Ethiopia
ISSN 1011-3924
 2014 Chemical Society of Ethiopia
DOI: http://dx.doi.org/10.4314/bcse.v28i1.13
SYNTHESIS, CHARACTERIZATION AND DENSITY FUNCTIONAL THEORY
INVESTIGATIONS OF MONOACYL ANILINE DERIVATIVES
Abdullah G. Al-Sehemi1,2*, Reem Saied A. A. Al-Amri1 and Ahmad Irfan1*
1Department of Chemistry, Faculty of Science, King Khalid University, Abha 61413, P.O. Box
9004, Saudi Arabia
2Unit of Science and technology, Faculty of Science, King Khalid University, Abha 61413, P.O.
Box 9004, Saudi Arabia
(Received December 27, 2012; revised June 8, 2013)
ABSTRACT. We synthesized a number of aniline derivatives containing acyl groups to compare their barriers
of rotation around the N-CO groups. Geometry optimization for all the rotamers have been performed using
density functional theory (DFT) at the B3LYP/6-31G** level of theory. For each stationary point we carried out
vibrational frequency calculations at the same level to characterize their nature as minima or transition states. The
methoxy group substituent on the benzene ring causes some changes in the C–C bond distances. We carried out
potential energy surface scanning to find out all possible low-energy conformations, which would be used as the
initial structure for further molecular calculations. There were only three kinds of conformations with local
minimum potential energy and two transition states. The optimized bond lengths and bond angles are in better
agreement with the experimental values.
KEY WORDS: Monoacyl aniline, Synthesis, Density functional theory, Rotation barrier
INTRODUCTION
Developments in the field of stereochemistry began in the nineteenth century. The concept of
chirality was first introduced in 1815 by the French chemist Jean Baptiste Biot when he
discovered optical activity in nature [1]. One of his students, Louis Pasteur achieved the first
separation of enantiomers in 1948, he manually resolved a racemic mixture of a tartaric acid salt
based on differently shaped crystals [2].
Since then, chirality has become of tremendous importance in our daily life. The chiral
element is most usually a chiral center or, as it is sometimes called, a sterogenic center in the
molecule which is an atom (usually carbon) bearing four different substituent [3]. Electron-
accepting groups are supposed to increase the rotation energy barrier in contrast to electron-
donating groups. The influence of the resonance and the field effect of amino and nitro groups
on the electron density in the phenyl ring and the respective accelerating and decelerating effects
on the rate of enantiomeriazation of atropisomerc biphenyls has been investigated [4]. Amide
barriers to rotation have been studied extensively [5]. Clayden and co-workers [6] determined
the barriers to rotation about the Ar-CO and N-CO bonds of naphthoyl amides. The amide is a
particularly important functional group in biological and medicinal chemistry and is the means
by which polypeptides and hence proteins are assembled. Recently, a number of studies have
been carried out to shed light on the properties of interest [7].
In this work we will deal the molecules in which the element of chirality is an axis. An axis
of chirality results from the non-planar arrangement of four groups about an axis. We have
synthesized the monoacyl aniline derivatives and then investigated the rotation barrier by means
of density functional theory.
__________
*Corresponding author. E-mail: irfaahmad@gmail.com

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EXPERIMENTAL
General experimental procedure
We prepared a number of aniline derivatives containing acyl groups to compare their barriers of
rotation around the N-CO groups. The general route for the synthesis of mono-aniline derivative
is shown in Scheme 1. In general monoacylation of the aniline with acid chloride and pyridine
proceeded in high yields and most of the products monoacylaniline were crystalline; with short
reaction times and absence of excess acid chloride, formation of diacylation products was not
competitive.
1H and 13C nuclear magnetic resonance (NMR) spectra were recorded on a Bruker (ARX-
500 spectrometer at 500 MHz and 125 MHz, respectively) at room temperature in
deuterochloroform (CDCl3) (CH3Cl δ 7.26 singlet) at 500 MHz and deuterated dimethyl
sulfoxide (DMSO-d6) (CH3SOCH3 δ 2.52 singlet) at 500 MHz unless other wise stated. 13C
NMR spectra were recorded on DRX-500 (125 MHz). Infrared (IR) spectra of crystalline
compounds were determined using Nujol mulls and of liquids either in dichloromethane or
chloroform solutions or neat on a JASCO FTIR-460 Plus spectrophotometer. Melting points
(mps) were determined with a Bi/Bornsted electrothermal. Mass spectra were determined using
a Kratos Concept mass spectrometer using electron impact (EI), chemical ionization (CI) or fast
atom bombardment (FAB). Dichloromethane was distilled from calcium hydride and stored over
4 Å molecular sieves.
Physical data
For nuclear magnetic resonance (NMR) spectra, chemical shifts and expressed in ppm on the δ-
scale relative to the internal standard (TMS). The following abbreviations are used s-singlet, t-
triplet, q-quartet, d-doublet, h-heptet, m-multiplet, dd-doublet of doublets, ddd-doublet of
doublet of doublet, dq-doublet of quartets, br-broad; J is the coupling constant (Hz).
Assignment of 13C signals was assisted by using DEPT. For intensity of infra-red (IR) spectra
peaks, abbreviations used are: s-strong, m-medium, w-weak, br-broad. Mass spectra were
determined in units of mass relative to change (m/z).
General procedure for mono-acylation of aniline derivatives MAA (2)
To a stirred solution of the aniline derivatives (2 g) in dichloromethane was added pyridine (1.2
eq.) followed by drop wise addition of the acid chloride (1.2 eq.). After 24 h, further
dichloromethane (10 mL) was added, the solution then washed successively with saturated
aqueous sodium hydrogen carbonate, hydrochloric acid (2 M), water, then dried and the solvent
removed under reduced pressure to give the monoacyl aniline MAA (2), see Scheme 1.
N-(2-methoxyphenyl)acetamide MAA (2a)
The general procedure for the N-acylation of aniline derivative was followed using 2-methoxy
aniline (2 g, 16.2 mmol), pyridine (1.54 g, 19.5 mmol) and acetyl chloride (1.53 g, 19.5 mmol).
After work-up, N-(2-methoxyphenyl) acetamide was obtained as a white needle crystals from
ethanol, yield 1.9 g, 71%, m.p. 83-86 oC, 1H NMR (CDCl3) δ 2.19 (3H, s, CH3CO), 3.86 (3H, s,
OCH3), 6.86, 6.94, and 7.05 [3H, m, CH (Ar)], 7.85 [1H, br s, 6-CH (Ar)], 8.34 (1H, s, NH);
13C NMR (CDCl3, 125 MHz) δ 24.82 (CH3CO), 55.63 (OCH3), 109.94 [3-CH (Ar)], 119.87 [6-
CH (Ar)], 120.98 [5-CH (Ar)], 127.69 [4-CH (Ar)], 147.80 (C=C-O) and 168.9 (CO).
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Synthesis, characterization and DFT investigations of monoacyl aniline derivatives
113
N-(2-Methoxy-phenyl)-4-nitro-benzamide MAA (2b)
The general procedure for the N-acylation of aniline derivative was followed using 2-methoxy
aniline (2 g, 16.2 mmol), pyridine (1.54 g, 19.5 mmol) and 4-nitrobenzoyl chloride (3.62 g, 19.5
mmol). After work-up, N-(2-methoxy-phenyl)-4-nitro-benzamide was obtained as a yellow
o
1
crystals shiny, yield 4 g, 91%, m.p. 135-139 C, H NMR (CDCl3) δ 3.96 (3H, s, OCH3), 6.97,
7.13, 7.15 and 8.07 [4H, m, CH (Ar)], 8.07, 8.53, 8.49 and 8.50 [4H, m, CH(Ar) ] and 8.59 (1H,
s, NH); 13C NMR (CDCl3, 125 MHz) δ 55.89 (OCH3), 110.1, 120.03, 121.24, 123.98, 123.98,
124.73 and 127.1 [8×CH (Ar)], 128.24 [2×CN (Ar)], 140.73 [C=C-O (Ar)], 148.26 [COC (Ar)],
149.76 [CNO2 (Ar)] and 163.07 (CO); IR ν/cm-1: 3321 (N-H), 1647(C=O), 1341(C-O-C).
N-(2-Methoxy-phenyl)-4-bromo-benzamide MAA (2c)
The general procedure for the N-acylation of of aniline derivative was followed using 2-
methoxy aniline (2 g, 16.2 mmol), pyridine (1.54 g, 19.5 mmol) and 4-bromobenzoyl chloride
(4.28 g, 19.5 mmol). After work-up, N-(2-methoxy-phenyl)-4-bromo-benzamide was obtained
1
as a white crystals shiny, yield 4.4 g, 89%, m.p. 127-131oC, H NMR (CDCl3) δ 3.39 (3H, s,
OCH3), 6.94 [1H, dd, J 8.2, 2.0, CH (Ar)], 7.04 [1H, ddd, J 12.2, 8.2, 2.0, CH (Ar)], 7.11 [1H,
ddd, J 12.2, 8.2, 2.0, CH (Ar)], 7.63 [2H, d, J 8.2, CH (Ar)], 7.78 [2H, d, J 8.2, CH (Ar)], 8.51
[1H, br s, CH (Ar)] and 8.53(1H, s, NH); 13C NMR (CDCl3, 125 MHz) δ 55.85 (OCH3), 110.01,
119.93, 121.20, 24.16, 126.41, 127.54, 127.54 and 131.98 [8×CH (Ar)], 131.98, 134.13 and
134.13 [3× C=C (Ar)], 148.19 (COCH3) and 164.22 (CO); IR ν/cm-1: 3409 (N-H), 1667 (C=O),
1341 (C-O-C); MS m/z (%): 309 (M+2, 11), 307 (M+, 12), 183 (100), 155 (38), 104 (21), 76
(37), 50 (24).
4-Nitro-N-o-tolyl-benzamide MAA (2d)
The general procedure for the N-acylation of of aniline derivative was followed using 2-methyl
aniline (2 g, 18.7 mmol), pyridine (1.77 g, 22.4 mmol) and 4-nitro benzoyl chloride (4.16 g,
22.4 mmol). After work-up, 4-nitro-N-o-tolyl-benzamide was obtained as a pale yellow crystals
o
1
shiny, yield 4.3 g, 90%, m.p. 157-160 C, H NMR (CDCl3) δ 2.34 (3H, br s, CH3), 7.32-8.03
[8H, br m, CH(Ar)] and 8.30 (1H, s, NH); 13C NMR (CDCl3, 125 MHz) δ 17.86 (CH3), (CH-Ar-
NO2, C-3.5), 123.9, 126.45, 126.93, 128.39 and 130 [8×CH (Ar)], 130.42 [C=C (Ar)], 135.0
(C=C-NH), 140.33 (C=C- CO), 149.72 (C=C-NO2) and 163.99 (CO).
O
R1
NH
NH2
O
R
N
+
R1
Cl
CH2Cl2
R
(1)
a R = OCH3 , R1 = CH3
(2)
b R = OCH3 , R1 = P-Nitrobenzene
c R = OCH3 , R1 = P-Bromobenzene
d R = CH3 R1 = P-Nitrobenzene
,
Scheme 1. The schematic diagram of synthesized compounds 2a-2d.
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Abdullah G. Al-Sehemi et al.
COMPUTATIONAL DETAILS
Density functional theory (DFT) calculations of the rotational barrier of six 3-(o-aryl)-5-methyl-
rhodanines were carried out by Yeliz Aydeniz et al. They found good agreement with the
available experimental results [8]. Geometry optimization for all the rotamers have been
performed using density functional theory (DFT) at the B3LYP functional and 6-31G** basis
set which has been proved to be an excellent method [9-12]. Moreover, B3LYP/6-31G(d,p)
level is adequate which has been applied to computed the different properties and successfully
reproduce the experimental data previously [13]. Recently, rotational energy barrier for
quinazolinone derivatives has been computed at different levels of theory B3LYP/6-31G**,
PM3, AM1, MNDO, HF/6-31G* and found that AM1 is rational and reasonable approach
which reproduce the experimental data (details can be found in reference 14). In the present
study, we have optimized the geometries at B3LYP/6-31G** and B3LYP/6-31+G** level of
theories. We have also applied AM1 method. We found that basis set has no significant effect
thus we focused on B3LYP/6-31G** and B3LYP/6-31G** level of theory.
RESULTS AND DISCUSSION
Geometrical parameters
The optimized structures of the aniline derivatives (2a-d) for the most stable conformer with
labelling are presented in Figure 1. The experimental and optimized geometric parameters (bond
lengths, bond angles and dihedral angles) have been listed in Table 1. The theoretical results are
comparable with the reported structural parameters of related molecules [15-19].
The calculated values are in good agreement with the experimental values. The experimental
results were obtained at solid phase and the theoretical calculations at gaseous phase. The
experimental crystallographic parameters and the predicted bond lengths and bond angles of the
four compounds MAA (2a-d) of minimum energy were compared by linear regressional
analysis. The statistical results of the linear dependence between the experimental X-ray versus
the predicted bond lengths and of the experimental X-ray versus predicted bond angles and
dihedral angle are presented in Figures 2.
(2a)
(2b)
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Synthesis, characterization and DFT investigations of monoacyl aniline derivatives
115
(2c)
(2d)
Figure 1. Optimized geometries at B3LYP/6–31G** level of theory of the studied aniline
derivatives (2a-d) for the most stable conformers, showing intermolecular H-bond (in
Å).
Table 1. Optimized geometrical parameters (bond lengths (Å), bond angles and dihedral angles (degrees) at
B3LYP/6-31G** level of theory.
Parameters
2a
2b
2c
2d
X-ray
Bond lengths
C1-C2
C1-C6
C1-H1
C2-C3
C2-H2
C3-C4
C3-H3
C4-C5
C4-H4
C5-C6
C5-O1
C5-C10
C6-N1
N1-C7
N1-H5
C7-C8
C7-O2
O1.....H5
1.398
1.397
1.081
1.391
1.086
1.400
1.086
1.392
1.084
1.416
1.376
1.398
1.397
1.081
1.391
1.086
1.400
1.086
1.392
1.084
1.416
1.374
1.398
1.397
1.081
1.391
1.086
1.400
1.086
1.392
1.084
1.416
1.375
1.392
1.400
1.088
1.395
1.086
1.394
1.086
1.402
1.087
1.408
1.401
1.399
1.393
1.399
1.399
1.404
1.371
1.507
1.428
1.379
1.012
1.508
1.224
1.407
1.375
1.011
1.523
1.224
2.111
1.408
1.371
1.011
1.509
1.227
2.094
1.407
1.374
1.011
1.505
1.228
1.424
1.343
1.508
1.244
2.099
Bond angles
120.6
119.1
C1-C2-C3
C1-C2-H2
120.6
119.1
120.6
119.1
119.4
119.9
119.6
120.2
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C1-C6-C5
Abdullah G. Al-Sehemi et al.
119.2
124.7
120.0
121.4
120.0
120.6
120.2
119.7
119.8
119.4
120.5
124.8
119.4
124.8
119.8
121.3
120.1
120.5
120.3
119.7
119.9
119.3
120.4
124.9
119.2
124.9
119.9
121.3
120.1
120.6
120.2
119.7
119.9
119.4
120.5
124.9
120.8
117.8
120.7
120.2
119.7
120.4
120.7
122.1
119.5
119.9
117.3
119.9
122.9
120.1
120.0
120.8
119.6
120.2
119.5
120.2
119.6
120.0
125.0
C1-C6-N1
C2-C1-C6
C2-C1-H1
C2-C3-C4
C2-C3-H3
C3-C2-H2
C3-C4-C5
C3-C4-H4
C4-C3-H3
C4-C5-C6
C4-C5-O1
C4-C5-C9
C5-C4-H4
C5-C6-N1
C5-O1-C9
C6-C5-O1
C6-C5-C9
C6-C1-H1
C6-N1-C7
C6-N1-H5
N1-C7-C8
N1-C7-O2
C7-N1-H5
C8-C7-O2
O1–C9-H9
O1-C9-H10
O1-C9-H11
Parameters
120.3
118.3
121.4
120.4
116.1
118.3
114.8
120.4
115.8
118.4
114.7
120.4
115.9
118.4
114.7
120.2
117.1
116.7
115.0
122.3
119.1
124.6
115.4
115.2
123.6
116.1
121.2
118.6
128.7
113.7
114.3
124.3
117.7
121.4
106.0
111.4
111.4
2a
118.9
128.5
113.3
114.8
124.5
117.9
120.7
106.0
111.3
111.3
2b
118.8
128.6
113.4
115.0
124.0
117.8
121.0
106.0
111.3
111.3
2c
120.0
126.7
107.0
115.5
124.7
126.0
119.7
109.5
109.5
109.5
X-ray
2d
Dihedral angles
C1-C2-C3-C4
C1-C2-C3-H3
C1-C6-C5-C4
C1-C6-C5-O1
C1-C6-C5-C9
C1-C6-N1-C7
C1-C6-N1-H5
C2-C1-C6-C5
C2-C1-C6-N1
C2-C3-C4-C5
C2-C3-C4-H4
C3-C2-C1-C6
C3-C2-C1-H1
C3-C4-C5-C6
C3-C4-C5-O1
C3-C4-C5-C9
C4-C3-C2-H2
C4-C5-C6-N1
C4-C5-O1-C9
C5-C4-C3-H3
C5-C6-C1-H1
C5-C6-N1-C7
C5-C6-N1-H5
0.001
-180.0
0.001
-180.0
0.022
-180.0
0.080
-179.9
0.017
-179.9
0.051
-179.9
-0.938
179.6
-1.643
-1.000
-2.000
177.9
177.2
120.7
-36.14
0.799
177.8
0.041
-178.9
0.524
-178.6
1.236
0.031
180.0
0.000
180.0
0.000
-180.0
-0.001
180.0
-0.001
180.0
-2.846
-177.5
-0.038
-179.5
0.021
179.9
-0.013
179.8
-0.072
179.9
-2.812
-177.5
0.001
-179.5
0.034
179.9
-0.034
179.7
-0.068
179.9
29.10
1.300
178.0
0.400
0.100
1.100
-178.8
-177.6
179.7
-178.5
-180.0
-180.0
0.003
-180.0
180.0
-180.0
-0.007
180.0
-179.6
1.223
180.0
-179.8
-177.6
2.969
180.0
-179.6
1.294
180.0
-179.7
-177.7
2.984
-178.8
5.600
179.5
179.9
-62.40
140.8
-154.1
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Synthesis, characterization and DFT investigations of monoacyl aniline derivatives
117
C6-C1-C2-H2
C6-C5-C4-H4
C6-C5-O1-C9
C6-N1-C7-C8
C6-N1-C7-O2
N1-C6-C1-H1
N1-C6-C5-O1
N1-C6-C5-C9
O2-C7-N1-H5
180.0
180.0
-180.0
180.0
-0.021
-0.002
0.004
-180.0
180.0
-178.8
178.8
-1.838
0.685
0.380
-180.0
180.0
-178.7
178.3
2.317
0.788
0.425
179.9
179.8
-174.3
178.3
0.800
-175.6
4.625
-3.127
1.000
-3.127
161.3
180.0
172.6
172.2
1.6
1.5
1.4
1.3
1.2
1.1
1.0
R = 0.9917
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
Experimental
(a)
130
125
120
115
110
R = 0.9748
105
110
115
120
125
130
Experimental bonds angles
(b)
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Abdullah G. Al-Sehemi et al.
200
150
100
50
R = 0.9999
0
-50
-100
-150
-200
-200
-150
-100
-50
0
50
100
150
200
Experimental DihedralAngles (o)
(c)
Figure 2. Correlations between the experimental and theoretical (a) bond lengths (Å), (b) bond
angles, and (c) dihedral angles for 2a obtained at B3LYP/6-31G** level of theory.
The methoxy group on the benzene ring causes some changes in the C–C bond distances.
Also, their ortho positions play vital role in shaping the structural and electronic properties of
the molecules. The methoxy group oxygen shares its loan pair of electrons with the p electrons
in a ring and also interacts with the nearby π systems. Therefore, the interaction of amide groups
with the ortho-substituents enhances intermolecular interaction including π-π stacking from
phenyl groups. The existence of an intermolecular hydrogen bonding between the hydrogen
atom of amine and the oxygen atom of the methoxy group (N-H---O), which was confirmed by
the short distance found (2.094-2.111 Å). The hydrogen bonding is critical in determining the
structure stability. In general the hydrogen bond strength is indicated by the length, e.g. shorter
hydrogen bonds lengths typically indicate stronger hydrogen bonds and Vice versa.
Barriers to rotation
Planarity of the amide plane and information about amide bond are the key factors for
investigation of the peptide molecules. The dihedral angle between amide moieties with respect
to a plane of aromatic ring is important for consideration. It is well known that the conformation
is restricted by partial double bond. So, we carried out a further potential energy surface
scanning to find out all possible low-energy conformations, which would be used as the initial
structure for further molecular calculation and dynamic simulations. MAAs (2a-d) was selected
for this purpose. The dihedral angle (ϕ = C5–C6–N1–C7, for MAA 2a and 2d, C1–C6–N1–C7
for MAA (2b) and C5–C6–N1–C8 of MAA (2c) were subjected to rotate from 180o to -180o
with a step of 5o. The energy values in the vertical axis were the relative, which were obtained
by all energy values minus the maximum energy value at the dihedral angle. We found that there
are four local minimum potential energies, where the torsion angles of the corresponding
conformations range 180-150o, 55o, -55o and -150 to -180o, respectively. Herein the
conformations with torsion angles of 180 to 150o and -155 to -180o were in fact the same. So
there were only three kinds of conformations with local minimum potential energy. There are
also two transitions states: TS1 where the torsion angles were found to be very close to 0o. The
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Synthesis, characterization and DFT investigations of monoacyl aniline derivatives
119
TS2 was found to be more stable than TS1 one, which may be due to the electron shell repulsion
between electrons pairs of oxygen atoms (Table 2).
Table 2. Computational energy values of TS1 and TS2 corresponding to the dihedral angels of MAAs (2a-
d).
No
R
R1
C10-N9-N14-N15
TA(o)
TA(o)
-20
-30
-20
∆G TS1
∆G TS2
2a
2b
2c
2d
OCH3
OCH3
OCH3
CH3
CH3
8.5
10.9
8.4
0
0
0
0
6.9
6.9
6.9
7.2
p-Nitrobenzene
p-Bromobenzene
p-Nitrobenzene
-25
10.5
TS1 = Transition state 1; TS2 = Transition state 2; ∆G = Enthalpy in kcal/mol; TA = Torsion angle.
CONCLUSIONS
The N-(2-methoxyphenyl)acetamide, N-(2-methoxy-phenyl)-4-nitro-benzamide, N-(2-methoxy-
phenyl)-4-bromo-benzamide, and 4-nitro-N-o-tolyl-benzamidehave been synthesized. The
theoretical results obtained are in good agreement with the reported structural parameters and
experimental values. The experimental X-ray and the predicted bond lengths and angles were
compared by linear regressional analysis. The methoxy group substituent on the benzene ring
causes some changes in the C–C bond distances. The methoxy group oxygen shares its loan pair
of electrons with the p electrons in a ring and also interacts with the nearby π systems.
Therefore, the interaction of amide groups with the ortho-substituents enhances intermolecular
interaction including π-π stacking from phenyl groups and also existence of an intermolecular
hydrogen bonding between the hydrogen atom of amine and the oxygen atom of the methoxy
group. Four local minimum potential energies and two transition states have been observed.
ACKNOWLEDGEMENTS
We thank King Abdul Aziz city of Science and technology (KACST) for their financial support
to Reem Saied Abdulaziz Muhammad Al-Yabess Al-Amri by the grant No. GSP-18-138.
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