Synthesis and quantum-chemical study of liquid-crystal derivatives of thiazole

Article abstract of DOI:10.1023/A:1013857829119

New liquid-crystall derivatives of thiazole with smectic mesomorphism over the range 5-85°C have been synthesized. Data on the spatial structure of the compounds synthesized have been obtained by quantum-chemical calculations. The effect of geometric anisotropy of the molecules of substituted thiazoles on their mesomorphic properties has been investigated.

Full text of DOI:10.1023/A:1013857829119

Chemistry of Heterocyclic Compounds, Vol. 37, No. 10, 2001
SYNTHESIS AND QUANTUM-CHEMICAL
STUDY OF LIQUID-CRYSTAL
DERIVATIVES OF THIAZOLE
M. M. Murza1, T. R. Prosochina2, M. G. Safarov1, and E. A. Kantor2
New liquid-crystall derivatives of thiazole with smectic mesomorphism over the range 5-85°C have been
synthesized. Data on the spatial structure of the compounds synthesized have been obtained by
quantum-chemical calculations. The effect of geometric anisotropy of the molecules of substituted
thiazoles on their mesomorphic properties has been investigated.
Keywords: liquid crystals, Schiff's bases, thiazoles, quantum-chemical calculations, mesomorphism.
We have previously prepared thiazole derivatives with liquid-crystal properties [1, 2]. In a continuation
of the investigation of the structures of substituted thiazoles on their mesomorphic properties we have
synthesized the following compounds:
N
N
H
C10H21O
C
O
N
CH
R
S
1–10
1 R = p-CH3OC6H4; 2 R = p-NO2C6H4; 3 R = p-(CH3)2NC6H4; 4 R = p-HOC6H4;
5 R = p-BrC6H4; 6 R = o-HOC6H4; 7 R = o-Br, o'-HOC6H3; 8 R = m-NO2C6H4;
9 R = o-CH3OC6H4; 10 R = o,p-(CH3O)2C6H3
Compounds 1-10 were obtained by esterification of 4-aminobenzoic acid with decyl alcohol in the
presence of H2SO4. (4-Decyloxycarbonyl)aniline (11) was chloroacetylated in diethyl ether with subsequent
treatment of the formed N-chloroacetyl(4-decylooxycarbonyl)aniline (12) with thiourea in absolute ethanol. The
synthesized 2-amino-4-[(4-decyloxycarbonyl)anilino]thiazole was condensed with aromatic aldehydes in
absolute THF in the presence of a trace of piperidine.
The composition and structures of the products obtained were confirmed by elemental analysis and
1H NMR spectroscopy.
The following proton signals were present in the 1H NMR spectra of compounds 1-10: 0.8-1.6 (t, CH3),
1.9-3.2 (m, CH2), 2.6-4.3 (m, CH2O), 4.9-6.0 (s, NH), 6.4-7.8 (m, C6H4), 8.2-8.6 ppm (s, N=CH).
Compounds 1-10 show monotropic mesomorphism of smectic type in the region of 5-85°C. Compounds
containing m'-NO2 and p-(CH3)2N groups showed the greatest mesophase temperature range.
__________________________________________________________________________________________
1
2
Bashkir State University, Ufa 450074, Russia. Ufa State Oil Engineering University, Ufa 450062,
Russia. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, October, 2001. Original article
submitted November 22, 1999.
1258
0009-3122/01/3710-1258$25.00©2001 Plenum Publishing Corporation

TABLE 1. Yields and Characteristics of the Compounds Synthesized
Found, %
K-I*2,
°C
——————
Tc*,
°C
T
Com-
pound
Empirical
formula
T*,
Yield,
%
∆
°C
Calculated, %
C
H
N
C28H35N3O3S
C27H32N4O4S
C29H38N4O2S
C27H33N3O3S
C27H32BrN3O2S
C27H33N3O3S
C27H32BrN3O3S
C27H32N4O4S
C28H35N3O3S
C29H37N3O4S
C17H27NO2
67.93
68.15
7.04
7.10
8.36
8.52
8
5
34
36
54
83
31
35
85
47
26
72
42
59
38
26
31
44
8
41
54
44
43
41
45
46
36
50
52
60
78
60
1
65.56
6.29
10.79
2
65.85
6.50
11.38
67.04
66.67
7.58
7.28
11.06
10.73
10
75
9
3
67.42
6.75
8.52
4
67.64
6.89
8.77
59.64
59.67
5.72
6.08
7.56
7.73
22
29
11
39
19
11
—
—
—
5
67.46
6.82
8.54
6
6
67.64
6.89
8.77
57.66
58.06
5.62
5.73
7.44
7.53
74
8
7
63.48
5.96
10.84
8
63.78
6.30
11.02
68.02
68.15
6.94
7.10
8.40
8.52
7
9
66.42
6.84
7.95
61
—
—
—
10
11
12
13
66.54
7.07
8.03
73.24
73.60
9.84
9.75
5.21
5.05
C19H28ClNO3
C20H29N3O2S
64.34
7.75
3.82
64.50
7.92
3.96
63.84
64.00
7.58
7.73
10.88
11.20
_______
* Temperature at which the smectic phase is formed.
*2 Transition temperature into an isotropic liquid.
We carried out quantum-chemical analyses on the thiazoles 1-10 using the semiempirical AM1 method
in order to investigate the effect of substituents on the molecular spatial structure and to estimate the
relationship between the molecular structure and the properties which give rise to substances capable of forming
the liquid crystal mesophase. In calculations of the molecules of compounds 6-10 we took into account the
possibility that two rotational isomers exist with the following disposition of the substituents: 6a R1 = OH; 6b
R5 = OH; 7a R1 = OH, R5 = Br; 7b R1 = Br, R5 = OH; 8a R2 = NO2; 8b R4 = NO2; 9a R1 = OCH3; 9b R5 = OCH3;
10a R1 = R3 = OCH3; 10b R3 = R5 = OCH3 (Fig. 1).
Fig. 1. Numbering of atoms and substituents in molecules of the thiazoles studied.
1259

From the results of the quantum-chemical calculation of the heats of formation (Table 2), the isomers
can be placed in the following order of increasing thermodynamic stability relative to the simple substance:
6b (-61.8 kcal/mol) < 6a (-63.6 kcal/mol)
7b (-54.6 kcal/mol) < 7a (-55.2 kcal/mol)
8b (-16.0 kcal/mol) ~ 8a (-16.2 kcal/mol)
9b (-55.4 kcal/mol) < 9a (-56.5 kcal/mol)
10b (-93.9 kcal/mol) < 10a (-96.9 kcal/mol)
Thus among the o-isomers conformers 6a and 9a are somewhat more stable. The same is true for the
1,6-disubstituted compound 7 and the 2,4-disubstituted compound 10. There is practically no difference in
thermodynamic stability in the conformers of the m-isomer 8. Since the differences in thermodynamic stability
of the isomers a and b for compounds 6-10 do not exceed 3 kcal/mol, then, considering the accuracy of the
calculation, it is not possible to conclude that one or other of the isomers predominates to any extent in the
Schiff base formation reaction.
It may be noted from analysis of the geometric structures of the substituted thiazoles that the bond
lengths and valence angles in molecules with different substituents change very little. Nevertheless some
regularities can be noticed. Changes in bond lengths with different substituents are observed only in the
benzylidene component: the lengths of the bonds C(18)–C(19) and C(19)–C(20) increase with increasing electron
donor properties of the substituents in the p-position. For conformer a, the presence of an o-substituent in the
benzylidene component increases the length of the C(16)–C(17) and C(17)–C(18) bonds, whereas there is no change
in conformer b. On the other hand, the length of the N(14)–C(15) bond is decreased in conformer b but is
unchanged in conformer a. In the presence of a m-substituent, the C(18)–C(19) and N(14)–C(15) bond lengths are
decreased and the C(20)–C(21) bond length is increased in conformer b, while in conformer a the C(17)–C(18) bond
is lengthened and the C(19)–C(20) bond is shortened. Among the molecules calculated the angle N(10)–C(11)–S(12) is
decreased only for compounds 9a and 10a with a CH3O substituent at position 2. In comparison with the
p-substituted, the angle C(15)–C(16)–C(17) is larger in the o-isomers of form a and smaller in form b, whereas the
reverse is true for the m-isomers.
The effects of substituents on the torsion and dihedral angles are shown in Fig.2. We note that the size
(φ1), C
(φ2), and C
(φ3) are effectively
of the torsion angles O(22)–C(1)–C(2)–C(7)
(4)–C(5)–N(8)–C(9)
(13)–C(9)–N(8)–C(5)
unchanged by the introduction of a substituent into the benzylidene residue, whereas the values of the torsion
(φ4) and N (φ5) change considerably and depend on the nature
angles N(10)–C(11)–N(14)–C(15)
(14)–C(15)–C(16)–C(21)
and position of the substituent. For example for compounds with substituents in the m- and p- positions the size
of the torsion angles φ4 and φ5 change within the ranges from -13.8° to -8.0° and from -22.8° to -11.8°
-isomers the values of the torsion angles φ4 and φ5 for the form
respectively. For o
a lie within the limits from
-5.6° to -3.2° and -25.4° to -14.9° respectively; in the form b the thiazole and benzene rings are rotated more
considerably: φ4 and φ5 are found within the limits from -2.4° to 10.2° and from -45.3 to -43.0° respectively.
Maximum rotation of the rings occurs in the compounds with two o-substituents (from 15.1° to 16.8° and from
-59.1° to -57.2° for φ4 and φ5 respectively). Introduction
of two o-substituents into the molecule leads to
rotation of these rings in opposite directions, the amplitude of the rotation being greater for torsion angle φ5
than for angle φ4.
We have observed a relation between the electronic properties of the substituents in the benzene ring of
the benzylidene unit and the values of the torsion angles φ4 and φ5: the increase in electron acceptor properties
of the substituents in the series N(CH3)2 < OCH3 < OH < Br < NO2 leads to an increase in the amplitudes of the
deviation of the size of the angles (Fig. 2).
1260

TABLE 2. Calculated Parameters Which Change in Value on Introduction of Substituent into the Molecule
Compound
Bond, d, Å
6
7
8
9
10
1
2
3
4
5
a
7
b
8
a
b
10
a
11
b
12
a
13
b
14
a
15
b
16
1
2
3
4
5
6
9
N(14)–С(15)
С(15)–С(16)
С(16)–С(17)
С(17)–С(18)
С(18)–С(19)
С(19)–С(20)
С(20)–С(21)
С(21)–С(16)
1.30
1.47
1.40
1.39
1.41
1.41
1.39
1.40
1.30
1.47
1.40
1.39
1.40
1.40
1.39
1.40
1.30
1.46
1.40
1.39
1.42
1.42
1.39
1.40
1.30
1.47
1.40
1.39
1.40
1.41
1.39
1.40
1.30
1.47
1.40
1.39
1.40
1.40
1.39
1.40
1.30
1.47
1.41
1.41
1.39
1.40
1.39
1.40
1.29
1.47
1.40
1.39
1.40
1.39
1.41
1.41
1.29
1.47
1.41
1.41
1.39
1.40
1.40
1.40
1.30
1.47
1.40
1.40
1.39
1.39
1.40
1.41
1.30
1.47
1.40
1.40
1.41
1.39
1.39
1.40
1.29
1.47
1.40
1.39
1.39
1.40
1.40
1.40
1.30
1.46
1.42
1.40
1.40
1.40
1.40
1.41
1.29
1.47
1.40
1.39
1.40
1.39
1.41
1.41
1.30
1.47
1.42
1.40
1.40
1.41
1.40
1.41
1.29
1.46
1.40
1.39
1.40
1.39
1.41
1.41
Valence angle, ω, deg.
С(1)–С(2)–С(3)
С(2)–С(3)–С(4)
С(3)–С(4)–С(5)
С(4)–С(5)–С(6)
С(7)–С(2)–С(3)
С(4)–С(5)–N(8)
N(8)–С(9)–N(10)
С(9)–N(10)–С(11)
121.7
120.6
120.5
118.4
119.5
118.0
121.0
109.0
121.8
120.5
120.5
118.4
119.5
118.0
121.1
109.0
121.8
120.6
120.5
118.3
119.4
118.1
121.1
109.1
121.7
120.5
120.5
118.3
119.5
118.0
121.0
109.0
121.7
120.5
120.6
118.3
119.5
118.0
121.0
109.0
121.7
120.5
120.5
118.4
119.5
118.0
121.0
109.1
121.7
120.5
120.6
118.3
119.4
118.0
121.0
109.0
121.5 121.6
120.5 120.5
120.4 120.5
118.5 118.4
119.5 119.4
117.8 118.0
121.0 120.7
109.0 109.0
121.9
120.5
120.5
118.4
119.5
118.0
121.2
109.0
121.7
120.6
120.5
118.4
119.4
118.0
121.2
108.9
121.7
120.5
120.5
118.4
119.5
118.0
121.1
109.3
121.6
120.5
120.5
118.3
119.4
117.9
120.8
109.1
121.8
120.5
120.5
118.4
119.5
118.2
121.5
109.3
121.8
120.5
120.5
118.4
119.5
118.1
121.1
109.1

TABLE 2 (continued)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
N(10)–С(11)–S(12)
С(11)–S(12)–С(13)
С(12)–С(13)–С(9)
С(13)–С(9)–С(10)
N(10)–С(11)–N(14)
S(12)–С(11)–N(14)
С(11)–N(14)–С(15)
N(14)–С(15)–С(16)
С(15)–С(16)–С(17)
С(16)–С(17)–С(18)
С(17)–С(18)–С(19)
С(18)–С(19)–С(20)
С(19)–С(20)–С(21)
С(20)–С(21)–С(16)
114.3
91.0
114.3
91.1
114.2
91.0
114.3
91.0
114.3
91.0
114.3
91.1
114.3
91.1
114.4 114.3
91.1 91.1
114.4
91.0
114.4
91.1
113.8
91.2
114.3
91.1
113.8
91.1
114.3
91.1
111.7
114.0
129.2
116.5
122.2
122.2
118.2
120.8
119.2
120.6
119.4
120.6
111.7
114.0
129.2
116.7
121.5
122.6
118.0
120.5
119.1
121.0
119.3
120.4
111.6
114.0
129.5
116.3
122.3
122.3
118.6
121.1
120.8
117.6
120.9
121.0
111.7
114.0
129.1
116.5
122.2
122.3
118.1
120.6
119.3
120.8
119.2
120.6
111.7
114.0
129.0
116.6
122.2
122.0
118.1
120.3
120.0
119.9
120.1
120.2
111.7
113.9
129.0
116.7
122.0
122.3
120.4
120.7
119.5
120.4
120.2
120.8
111.6
114.0
129.1
116.6
122.2
122.2
117.6
121.0
120.0
120.2
119.8
120.4
111.7 111.6
113.9 113.9
128.5 128.7
117.0 117.0
122.3 122.0
121.3 121.2
118.9 120.3
121.0 120.5
119.1 120.2
120.6 120.4
120.4 119.4
120.2 120.9
111.7
113.9
128.8
116.8
122.3
121.6
117.8
119.2
121.1
118.9
120.7
120.2
111.7
113.8
128.9
116.7
122.7
121.4
118.4
120.3
120.6
118.9
121.2
119.0
111.6
114.0
129.1
117.1
121.7
122.4
118.8
121.1
119.4
120.1
120.5
120.5
111.6
114.0
129.1
116.6
122.1
122.4
117.5
120.9
119.9
120.5
119.8
120.2
111.6
114.2
129.6
116.6
122.3
121.9
119.3
121.3
118.8
121.0
119.3
121.5
111.6
114.0
129.3
116.5
121.9
122.7
117.5
121.6
119.0
121.1
119.0
120.7
Enthalpy of formation,
-59.2
-15.9
-13.2
-65.5
-15.5
-63.6
-61.8
-55.2 -54.6
-16.2
-16.0
-56.5
-55.4
-96.9
-93.9
ΔΗ°f, kcal/mol

TABLE 3. Calculated Parameters Which do not Change on Introduction of
the Substituent into the Molecule
Bond
d, Å
Bond
d, Å
С(1)–С(2)
С(2)–С(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(6)–C(7)
C(7)–C(2)
C(5)–N(8)
N(8)–C(9)
C(9)–N(10)
N(10)–C(11)
C(11)–S(12)
S(12)–C(13)
1.47
1.40
1.39
1.42
1.41
1.39
1.40
1.40
1.41
1.40
1.35
1.73
1.65
C
(13)–C(9)
1.41
1.40
1.24
1.31
1.42
1.52
1.51
1.51
1.51
1.51
1.51
1.51
1.51
С(11)–N(14)
C(1)–O(22)
C(1)–O(23)
O(23)–C(24)
C(24)–C(25)
C(25)–C(26)
C(26)–C(27)
C(27)–C(28)
C(28)–C(29)
C(29)–C(30)
C(30)–C(31)
C(32)–C(33)
Some Valence Angles
ω, deg.
Angle
Angle
ω, deg.
C(5)–C(6)–С(7)
120.5
120.6
117.7
116.3
106.0
-110.1
111.0
C
(26)–C(27)–С(28)
111.2
111.2
111.3
111.3
111.3
111.5
C(6)–C(7)–С(8)
C(27)–C(28)–С(29)
C(28)–C(29)–С(30)
C(29)–C(30)–С(31)
C(30)–C(31)–С(32)
C(31)–C(32)–С(33)
O(22)–C(1)–С(23)
C(1)–O(23)–С(24)
O(23)–C(24)–С(25)
C(24)–C(25)–С(26)
C(25)–C(26)–С(27)
Some Torsion Angles
τ, deg
Angle
Angle
τ, deg
C(1)–C(2)–С(3)–C(4)
C(2)–C(3)–С(4)–C(5)
C(13)–C(9)–N(10)–C(11)
C(13)–S(12)–С(11)–N(10)
C(16)–C(17)–С(18)–C(19)
177.73
-0.25
1.02
1.00
0.87
O
(22)–C(1)–С(23)–C(24)
-2.21
5.58
-177.04
179.33
O(22)–C(1)–С(2)–C(3)
C(1)–O(23)–С(24)–C(25)
C(30)–C(31)–С(32)–C(33)
To investigate the influence of the substituents on the spatial characteristics of the molecules studied the
dihedral angles between the thiazole ring and the benzene rings of the benzylidene and aniline components (θ1
and θ2 respectively) were calculated from the results of the quantum-chemical calculations. The size of angle θ1
is practically independent of the substituents in the benzene ring and has a value of 44.2° - 47.0°, whereas the
size of the dihedral angle θ2 lies within the limits 8.2° and 44.3° and does depend on the nature and position of
the substituent (Fig. 2).
One of the most important factors which determines the appearance of liquid-crystal properties of
azomethines is the geometrical anisotropy of the molecule, the figure of rotation of which around the long axis
is a cylinder [3]. An increase in the ratio of the height of the cylinder to its diameter widens the temperature
range of the mesophase (Δ
T). Because the acoplanar azomethines pack more loosely and have lower melting
points, it may be expected that an increase in the acoplanarity of the substituted thiazoles studied (i.e., the angle
θ2) should lead to a decrease in the melting point and Δ
T. This relation is followed for the p-substituted
compounds 1-5, with the exception of compound 4, with an OH substituent, for which an increase (not a
is observed together with a sharp decrease in the range of existence of a mesophase, Δ
decrease) in TK-1
T. This is
evidently connected with the formation of intramolecular hydrogen bonds, the appearance of which, as a rule,
1263

Fig. 2. Relation between the calculated values of the torsion and dihedral angles of the thiazoles
φ1; C
φ2;
investigated on the substituents: torsion angles O(22)–C(1)–C(2)–C(7)
(4)–C(5)–N(8)–C(9)
φ3; N
(10)–C(11)–N(14)–C(15)
φ4; N
φ5;
C(13)–C(9)–N(8)–C(5)
(14)–C(15)–C(16)–C(21)
dihedral angles between the planes of the benzene ring of the aniline component
and the thiazole ring θ1 and between the benzylidene component and the thiazole ring θ2.
leads to increased thermostability of the mesophase, but in some cases to its degeneration. For compounds 6-10
with various substituents in different positions on the benzene ring, no unambiguous correlation was
established, because introduction of secondary substituents can considerably decrease the temperature range of
the liquid crystal state as a result of decreasing the relative contribution of secondary dipole-dipole interactions
and the formation of intermolecular hydrogen bonds.
It is shown, therefore, that geometrical anisotropy is not the determining factor in guaranteeing
liquid-crystal properties in the molecules synthesized.
EXPERIMENTAL
1H NMR Spectra of CDCl3 solutions with HMDS as internal standard were recorded on a Bruker
WP-200 (200.13 MHz) instrument. Temperatures of the phase transitions were measured on an MIN-10
polarizing microscope with a heating attachment. The purity of all the compounds described were monitored by
TLC on alumina (toluene–chloroform 3:7).
The semiempirical AM1 (DFP) method, from the AMPAC package, was used for quantum-chemical
calculations. Complete optimization of the molecules was carried out without limiting any parameters.
(4-Decyloxycarbonyl)aniline (11). 92% H2SO4 (11.3 g, 0.115 mol) was added with vigorous stirring to
a mixture of 4-aminobenzoic acid (12.3 g, 0.1 mol) and decyl alcohol (94.8 g, 0.6 mol). After refluxing for 6 h
the reaction mixture was distilled with steam, the residue was cooled, (4-decyloxycarbonyl)anilinium sulphate
was isolated, dissolved in hot water, and treated with conc. NH4OH (16 ml). The precipitate was filtered off and
recrystallized from hexane.
N-Chloroacetyl-(4-decyloxycarbonyl)aniline (12). Chloroacetyl chloride (6.3 g, 4.45 ml, 0.055 mol)
was added dropwise to a solution of compound 11 (13.9 g, 0.5 mol) in diethyl ether (100 ml). The mixture was
kept for 3 h at room temperature, the precipitate was filtered off, washed several times with water, and
recrystallized from ethanol.
2-Amino-4-[(4-decyloxycarbonyl)anilino]thiazole (13). A mixture of compound 12 (10.6 g, 0.03 mol)
and thiourea (2.28 g, 0.03 mol) was dissolved in absolute ethanol (50 ml) and refluxed for 8 h. At the end of the
reaction the solvent was distilled off, the residue was neutralized with 20% sodium carbonate solution, the
precipitate was filtered off and recrystallized from ethanol.
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2-Arylidenenamino-4-[(4-decyloxycarbonyl)anilino]thiazoles (1-10). A mixture of compound 13
(1.5 g, 0.04 mol) and an aromatic aldehyde (0.04 mol) in absolute THF (40 ml) was refluxed for 4 h in the
presence of a catalytic amount of piperidine. The solvent was distilled off and the residue was recrystallized
from ethanol.
REFERENCES
1.
2.
3.
M. M. Murza, A. S. Golovanov, and M. G. Safarov, Khim. Geterotsikl. Soedin., 546 (1996).
M. M. Murza, A. S. Golovanov, and M. G. Safarov, Zh. Org. Khim., 31, 1701 (1995).
Liquid Crystals (in Russian). ed. S. I. Zhdanov, Khimiya, Moscow (1979).
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