Structure and properties of aromatic and aliphatic-aromatic
azomethins
© Volkova Tatyana Gennadyevna, Klyuev Mikhail Vasilyevich
Department of Organic and Biological Chemistry. Ivanovo State University,
St. Ermak, 39. Ivanovo, 153025. Russia. Tel.: (4923) 37-37-03. E-mail: [email protected]
Key words.
Azomethins, structure, mobile-proton tautomerism, conformational analysis, reactivity, solvent effect
Abstract
The review of works on studying the reaction of Schiff bases, as well as peculiarities of
their structure and properties which were carried out at the Department of Organic and
Biological Chemistry of Ivanovo State University is presented. With use of quantum chemical
methods it is shown that there are two stages in azomethins formation, semiaminal being an
intermediate product. Accounting the solvent effect in the simulated system leads to lowering the
energy profile of the reaction. Peculiarities of structure of the Schiff bases with substituents of
different nature and position in the benzene ring are discussed. 1H NMR spectra proved the
existence of imine-enamine tautomerism in a solution with the predominance of enamine form,
both tautomers being stable. Conformation analysis for the molecules of aromatic Schiff bases at
temperatures of existence of different phases and phase transitions showed that the phase
transitions are accompanied by changes of conformation and length of the molecule, which is
determined by the possibility of rotation of the benzene rings and alkyl groups mobility. It is
shown that the catalysts on the base on carbon nanomaterials are considerably more active in
hydrogenation of the Schiff bases in comparison with analogs on activated carbon. A number of
computer models are developed with the use of special algorithms of analysis of multi-parameter
data and the selected molecular descriptors. Biochemical activity of the Schiff bases and
corresponding secondary amines is predicted.
Introduction
Since first synthesis of Schiff bases (azomethins) in 1864 a lot of works on their
obtaining, properties and application were published. Systematic studies of the compounds are
going on, different approaches to studying the molecule structure and properties of azomethins
are being used, quest of optimal conditions of their synthesis and new spheres of their use are
being elaborated.
Schiff bases are widely used in organic synthesis for obtaining secondary amines and
heterocyclic compounds [1], as dyes, vulcanization accelerators, stabilizers and adhesion
promoters in plastic production, fuel and oil stabilizers and inhibitors of metal corrosion [2-5]. A
number of azomethins provide liquid crystal properties [6] and are used as temperature sensors,
indicators and light modulators [7]. Polydentate ligands with azomethin fragments play an
important role in chemistry of coordination compounds [8-10].
Azomethins are high biologically active and are used in production of antimicrobial and
antiparasite medicines (furacilin, ftivazid, saluzid), antituberculosis preparations (thioacetazon)
and roentgen contrast compounds (bilimin)[11].
1. Peculiarities of reactions of Schiff bases synthesis
Condensation of carbonyl compounds with primary amines catalyzed by acids (scheme 1)
is one of the most common and simple in practical realization methods of azomethins synthesis.
Scheme 1
NH2
R1
O
H
C R2
-H2O
N=CH R2
R1
где: R1- 2-NH2, 3-NH2, 4-NH2, 2-OH, 3-OH, 4-OH, 2-Cl, 3-Cl, 4-Cl, 2-Br,
3-Br, 4-Br, 2-COOH, 3-COOH, 4-COOH, 2-SO3H, 3-SO3H, 4-SO3H;
R2- C2H5, -C6H13, -C7H15, -CH(C2H5)2, -CH2CH(CH3)2, -CH(CH3)C3H7.
Theoretical study of the reaction showed that the condensation proceeds with
participation of HOMO of the amine molecule and LUMO of the aldehyde and is of orbital
control [12-15]. This is confirmed by calculations of charge and orbital components of energy of
interaction with use of Clopman equation [16]. Orbital component in changing the energy of
interaction of the molecules is on an order higher than the value of input of charge energy and
that’s why was used for predicting the amines reactivity in azomethins formation. Aniline has the
higher interaction energy than its substituted analogs and is the most reactive. This results are
confirmed with data of [17-19] on study of charge transfer from HOMO of amine to LUMO of
carbonyl group which plays the key role when C=N bond forming.
Further studies of the reaction of azomethins formation on an example of interaction of
propanal with aniline, m- and p-aminobenzoic acids [20] using quantum chemistry methods
(HF/6-31G) showed that the reaction goes trough two transition states, the corresponding
intermediate product (semiaminal) and leads to formation of azomethin and water. It is important
that the forming semiminal has higher stability than molecular complex of reagents. This fact is
characteristic for all three studied reactions and is in agreement with data on modeling the
interaction of furfurol with ammonia and dimethylamine [18,19].
Table 1
Energetic parameters of stationary points and transition states of reaction of condensation of
benzaldehyde and aniline
TS2
TS1
Еа1
C6H5HC=NC6H5
C6H5CHO*NH2C6H5
nb
Еа2
C6H5COH2 - NHC6H5
Molecular system
[C6H5CHO*NH2C6H5]
TS1
[C6H5COH2-NHC6H5]
TS2
[C6H5HC=NC6H5*H2O]
Еtot, kcal/mole
Gas phase model
- 394826,74
- 394737.33
- 394828,78
-394723,07
- 394823,84
Continual modal
-394671,25
-394588,03
-394673,24
-394574,85
-394670,83
Еа#, kcal/mole
Gas phase model
Continual modal
89.41
83,22
105.71
98,41
In synthesis of secondary amines by hydroamination reaction the stages of synthesis and
reduction of azomethins are usually combined (in situ), alcohols (ethanol, propanol-2) are being
most often used as solvents. Route and transition states of benzylideneaniline formation in gas
phase was studied by method HF/6-31G**(Hyper Chem7.05). Then solvent molecules influence
was taken into account by use of continual approach (method PCM HF/6-31G (GAMESS) [21].
Comparison of the results of the gas phase and continual models allows pointing out that
accounting the solvation leads to decreasing the height of activation barriers of the condensation
reaction (Table 1).
2. Molecule structure and mobile-proton tautomerism of mixed aliphatic-aromatic
amines
In works [22-25] with the use of HF/AM1 method 64 azomethins molecules with
substituents of different nature and positions in benzene ring, including alkyl substituents of
various length and structure which are forming according with scheme 1 are studied.
Results of quantum chemical calculations show that the molecules of studied
alkylidenanilines are non-planar, angle of rotation of the benzene ring plane in relation to
nitrogen-carbon bond is about 350.
Introducing the bulky substituents (brom-, carboxy- sulfo-groups) to ortho-position leads
to noticeable increasing (to 70 - 800) the angle of azomethin group formation regarding benzene
ring plane. Length of >C=N- bond in the studied azomethins molecules does not change. Total
charge on propylideneaniline reaction center (azomethin group >C=N-) is equal -0.203 atomic
charge units.
Presence of substituent in propylideneaniline benzene ring leads to re-spreading the
charge on imino-group atoms. In comparison with propylideneaniline q>C=N- in its substituted
analogs rises essentially to -0.216 and -0.207 atomic charge units in propylidene-ortho-,paraphenylendiamines molecules correspondingly that is explained first of all by electron donor
properties of NH2-group.Variations of alkyl fragment structure in azomethins molecules does not
essentially change energetic, electron and geometric characteristics of the molecules and
apparently has small influence onto their reactivity in hydrogenation.
10
10
8
6
4
2
0
, м.д.
8
Fig. 1. 1H NMR spectrum of tautomer equilibrium
propylidene-para-aminobenzoic acid  N- propylene(para-aminobenzoic acid).
Solvent ethanol.
Azomethin in a solution can participate in mobile-proton triad tautomerism [26].
4-(propylideneamino)benzoic acid was synthesized by condensation of propanal with paraaminobenzoic acid. 1H NMR spectra study in DMSO-d6-C2D5OD (1:0.2) showed that tautomer
equilibrium azomethin (A)  enamine (B) (scheme 2) exist [27]. Two separate signals of
protons of azomethin group –N=CH- (s, 7.66 m.d.) and enamine group >NCH=CH- (d, 6.62
m.d.) are observed in the spectrum (Fig. 1). The hydrogen atoms are specific for every tautomer
and the ratio of intensity of their signals shows ratio of imine and enamine forms in a solution.
Integral intensities of the mentioned lines are in regard as 1:2.8, ethanol as polar proton solvent
[28,29] promoting the enamine form stabilization.
Quantum-chemical study of the equilibrium for five imine-enamine pairs (scheme 2) in
gas phase (method HF/6-31G) and with accounting the solvent (ethanol) effects by using
continual approach (method PCM (PCGAMESS7.1)) has been realized. It is shown that the
solvation promotes polarization of azomethines and enamines and increasing the electron density
on the reaction centers of the molecules (>C=N- and >C=C< bonds). Thus, presence in the
solution relatively stable tautomer forms evidences that the final product – secondary amine can
be formed as a result of reduction of azomethin and enamine as well.
Scheme 2
R
NH C1H=C2H CH3
(B)
N=C1H C2H2 CH3
(A)
R
R = H (I), 4-OH (II), 3-NH2 (III), 3-COOH (IV), 4-COOH (V).
3. Structure and conformations of aromatic azomethins
Study of structure and electron characteristics of molecules of benzylidenaniline and
azomethines of para-n-alkyloxybenzylidene-para’-toluidine row (scheme 3) (methods PBE /sbk
(DFT) (Priroda) и HF/АМ1, РМ3, MNDO (HyperChem 7.5)) showed that all the studied
molecules are non-coplanar and torsion angle С–N–СAr–СAr value is near 34º [31,32].
Scheme 3
Studied molecules of para-n-alkyloxybenzylidene-para’-toluidine row (n=1-12)
R1
CH N
R2
R1=CnH2n+1O; R2=-CH3
The mentioned methods provide sufficiently good description of geometrical
characteristics of azomethins of para-n-alkyloxybenzylidene-para’-toluidine row. However
semi-empirical АМ1, РМ3, MNDO methods give understated values of electron characteristics,
in particular, of dipole moments values of the studied azomethins.
PBE (DFT) method, contrary, gives a little bit higher dipole moments values (excepting
benzylidenaniline), but these values are close to those obtained experimentally.
Studied in works [32,33] aromatic azomethins provide mesomorphic (liquid crystal (LC))
properties.
LC properties (state of a substance, which is characterized by fluidity from one hand and
by anisotropy of properties from the other hand) are determined by molecular structure.
Compounds with the same qualitative and quantitative composition but different chemical
structure (isomers) can have different properties. Liquid crystal state of a substance is
determined by joint interaction of all the molecules in it composition and that is why the
structure and |consequently liquid crystal properties depend on geometrical structure of the
separate molecules. Because of this it was interesting to study conformation states of the
azomethins molecules at temperatures of various phases and phase transitions. Conformation
analysis for seven azomethins molecules was carried out (scheme 4) [33-40].
Structure and conformation properties of molecule of MBBA was studied by methods of
quantum chemistry and molecular dynamics at temperatures of various phases and phase
transitions [39,40]. It is found that “non-rigidity” of MBBA molecule is due to rather small
mobility of benzene rings and butyl group.
Results of different modeling methods were compared. According to data of quantumchemical modeling in approximation of isolated molecule the conjugation of p-electrons of
oxygen atom with -electrons of benzene ring leads to planar location of methoxy-group. At the
same time influence of the surrounding molecules in volume phase leads to turning CH3O-group
to plane which is perpendicular to benzene ring plane.
Scheme 4
R1
CH N
R2
R3
R1
R2
R3
Название
соединения
CH3O-
-C4H9
-Н
C3H7O-
-C4H9
-ОН
C4H9O-
-ОСОС2Н5
-Н
C4H9O-
-CH3
-Н
C5H11O-
-CH3
-Н
C7H15O-
-CH3
-Н
CH3O-
-CH3
-Н
p-methoxybenzylidenep`-butylaniline (MBBA)
p-propyloxy-ohydroxybenzylidene-p`butylaniline
p-n-butyloxybenzylidenep`- propionyloxyamine
p-n-butyloxybenzylidenep`-toluidin (BOBT)
p-n- toluidin
pentyloxybenzylidene-p`p-nheptyloxybenzylidene-p`toluidin
p-methoxybenzylidenep`- toluidin
Conformation analysis of other molecules of aromatic azomethins at temperatures of
various phases and phase transitions showed that phase transitions are accompanied with
changing the conformation and lengths of the molecule. Conformation behavior of the studied
molecules is determined by possibility of benzene rings rotation and alkyl groups mobility. Our
data [33-38] correspond to results of radiographic studies [41] where possibility of the molecular
structure was shown. The authors of [41] point that the ending fragments of the molecules
undergo twisting, the longer the alkyl chain of alkyloxy-group the larger the shortening of the
whole molecule.
Study of LC molecular mobility [40] of structure isomers with the use of molecular
dynamics method showed that conformations of butyl and butoxy-chains in these molecules are
significantly different. In butyl chain of МBBА fully stretched trans-state is in two times more
probable than in butoxy-chain of BOBT (Table 2).
Different conformational behavior influences upon local structure of the studied
compounds. Mutual regulating the molecules will be reflected in LC properties.
Table 2
Probability of realization of conformations in aliphatic chains
Conformations
%
BOBT
МBBА
g– g–
1.8
0.1
g+ g+
1.8
0.3
g– t
22.7
3.6
g+ t
24.3
3.5
g– g+
1.4
0.1
g+ g–
1.5
0.1
t g–
2.7
6.8
t g+
2.5
6.3
tt
41.4
79.2
4. Liquid phase catalytic hydrogenation of azomethins in the presence of metal
containing carbon nanomaterials.
Catalytic properties of metal containing (Pt, Pd) carbon nanomaterials in model reaction
of hydrogenization amination of propanal with para-aminobenzoic acid (scheme 5) were studied
[42-46].
Different metal containing carbon nanomaterials (CNM): Pd- and Pt-fullerene soot
[42,44,46], Pt-containing soot Vulcan XC 72, multi-wall carbon nanotubes (MNT), carbon
nanofibers (CNF) with diameter 20-40 nm and 100-200 nm [43,45,46] as well as Pt and Pd
nanodiamonds (ND) [46,47] were obtained and tested to catalytic properties in the reaction.
Detonation nanodiamond with specific surface 280–320 m2/g (average size of crystal
diamond nuclear of ND particles is near 4 nm) and total non-carbon impurities content not more
than 0.5 weight % was used as a carrier for platinum group metals. Synthesis of Pt or Pd
containing nanodiamonds was carried out according to method described in patent [48].
Analysis of the obtained experimental data (Table 3) showed that all the studied Pt- and
Pd-nanodiamonds provide catalytic activity in model hydroamination reaction. In Ptnanodiamonds presence the reaction rate increases in almost two times with growth of platinum
content by every 5 %, effectiveness of the catalyst growing in 1.5 times. In the case of Pdcontaining nanodiamonds when growing palladium content from 6 % to 10 % the reaction rate
grows in 1.6 times but the catalyst effectiveness does not change. The reaction rates are almost
the same when using catalysts with 10% and 15% metal content, effectiveness of 15% Pd/ND is
in 1.6 lower in comparison with that of 10% Pd/ND. In whole Pd-containing catalysts appeared
to be more effective in the model hydroamination reaction than Pt-containing catalysts.
Scheme 5
HOOC
NH2
O
H
C CH2 CH3
-H2O
HOOC
N=CH CH2 CH3
H2
Kat
HOOC
H2
Kat
HOOC
NH CH=CH CH3
NH CH2 CH2 CH3
Table 3.
Effectiveness of Pt- and Pd-containing nanodiamonds in hydroamination*
N
Catalyst
Metal content in catalyst
Wн2·106,
TN,
with activated carbon, %
mole/(l·s)
mole/(g-at.·min)
1
15% Pt/ND
2.7
4.7
1.7
2
20%Pt/ND
3.6
9.1
2.5
3
25% Pt/ND
4.5
18.0
3.9
4
6% Pd/ND
1.1
11.0
5.4
5
10% Pd/ND
1.8
18.0
5.2
6
15% Pd/ND
2.7
17.0
3.3
7
4.4% Pt/MNT
4.4
7.5
1.7
8
5% Pt/CNF
5
10.0
2.0
9
1% Pd/C
1
2.0
0.2
*
Conditions: Т=318 K, Рн2=0.1 MPa, 30 mg of catalyst, 10 mg of NaBH4, 25 ml of
ethanol, 2 mmole of propanal, 2 mmole of para-aminobenzoic axid. Error of measuring the
reaction rates 5%.
To compare catalytic activity of the obtained contacts in the mentioned model reaction
the results of study of ordinary Pd/C and the best catalysts on the base of CNM: Pt/MNT and
Pt/CNF are shown in Table 3. These catalysts appeared to be less active in hydrogenization
amination of propanal with para-aminobenzoic acid than Pd/ND and Pt/ND.
5. Predicting the target specific activity of azomethins and secondary amines
For prognostication of effectiveness of formation of non-covalent complexes between
low molecular organic compounds (azomethins and secondary amines in our case) and protein
macromolecules (biotargets) classification version of the quantitative relationship between
structure and properties (QSPR) was used [49-51]. In all the cases specific protein receptors and
ferments which are pharmaceutically significant bio targets were studied. QSPR-modeling
consists from the next principal stages: a) preparation of model experiment, b) building QSPRmodel, c) testing or validation of the built model.
Model experiment includes gathering and treating the information to a high-quality
training set. Medicine compounds from Prous Integrity Database were used as training set. The
selection was consisted from 17000 organic compounds with experimentally found and stated
target specific activity. The selection contained medicine compounds active in regard to various
groups of bio targets, for example, to tyrosine kinases, receptors connected related with Gproteins, nuclear receptors, HIV-integrase, DNA-topoisomerase, caspase and kinase receptors,
chemokine receptors, lipoxygenase, etc. (totally more than 210 unique types of bio targets were
in the training selection). High structure variety of compounds of the training selection allows
concluding that the results of QSPR modeling will not be distorted because of predominance of
several chemotypes of the compounds.
At the second stage various molecular descriptors were calculated and analyzed, minimal
set of key descriptors that give the most adequate description of the training selection was
determined as well. Program SmartMining created by researchers of Institute of Physiologically
Active Substances [52] was used. It can calculate more than 100 unique molecular descriptors
including number of donors and acceptors of hydrogen bond, logarithm of solubility of the
compounds in octanol/water system, many topological and electrotopologycal indexes and some
quasi-3D-descriptors. Computer modeling was carried out on the base of leading high effective
neuronetting algorithms (self organizing Kohonen maps, classical neural networks of back
propagation of error).
Compounds of central chemotype (scheme 6) (35 structures) were taken to tested
selection.
Scheme 6
N CH CH2 CH3
NH CH2 CH2 CH3
I
II
R
R
R: H, NH2, OH, COOH, -N=CH-CH2-Me, -NH-CH2-CH2-Me3
R1
CH N
R2
III
R1=CnH2n+1O; R2=-Me, -OMe
Structures of tested compounds.
I – mixed aliphatic-aromatic azomethins;
II – mixed aliphatic-aromatic secondary amines;
III – aromatic azomethins
To estimate profile of target specific activity of compounds from the tested selection
regularities of position of various medicine substances on Kohonen map were used. The
methodology is described in details in work [50]. Tested compound was classified as belonging
to unique class (classes) of physiologically active medicine substances if it appeared to be in the
point (unit) on the map where agents with concrete target specific activity from the training
selection prevail.
Corresponding Kohonen maps reflecting spreading the structures from the tested
selection against a background of target specific activity were analyzed. The majotity of the
compounds from the tested selection have close positions on the map.
Alternating “developing” the target specific areas found from results of Kohonen
algorithm teaching revealed several the most significant activities that can be regard to
compounds from virtual library. For example mixed aliphatic-aromatic azomethins are active as
Acetylcholine inhibitors receptors (5 compounds) and beta-Adrenoceptor inhibitors (4
compounds) (Table 4, I). Secondary aliphatic-aromatic amines obtained by hydrogenation of
aliphatic-aromatic azomethins are also
Table. 4. Representative profile of target –
active as beta-Adrenoceptor inhibitors (3
compounds) (Table 4, I). At the same time specific activity of structures from virtual library
(compounds I and II)
they are not active as Acetylcholine
I II
inhibitors but can be used as Potassium Соединение
5 1
Acetylcholine
inhibitors
Blockers (3 compounds) and reverse
4 3
beta-Adrenoceptor
inhibitors
transcriptase inhibitors (3 compounds)
Monoamine oxidase inhibitors (MAOIs) 1 1
(Table 4, II).
1 1
For aromatic azomethins a series of antagonists / agonists of NMDA
specific activity was predicted (Table 5): receptors
0 3
GABA antagonists (10 compounds), Potassium Blockers
0 3
Lipoxydenase inhibitors (10 compounds), reverse transcriptase inhibitors
1 0
histamine
receptor
antagonists
Cyclooxygenase inhibitors (10 compounds),
inhibitors of ACAT inhibitors (5 Receptor antagonists serotonin reuptake 0 1
1 0
compounds), Cannabinoid antagonists (5 Antagonists of nicotine receptors
0 1
compounds), Estrogen antagonists (7 Progesterone receptor antagonists
0 1
compounds), Potassium Blockers (7 antagonists of aldose reductase
0 1
tyrosine
kinase
inhibitors
compounds). In other tested systems
0 1
activity of compounds of chemotypes I-III antagonists / agonists of serotonin
receptors
is not significant or equal zero.
Table. 5. Representative profile of target –
Thus these bio targets ought to be
specific
activity of structures from virtual library
considered among the priorities when
(compounds III)
biological testing planning.
Соединение
III
Conclusion
10
Nowadays constant interest of GABA antagonists
10
researches to azomethins is due to wide Lipoxydenase inhibitors
10
spectrum of their properties. Use of various Cyclooxygenase inhibitors
5
theoretical (quantum chemical calculations, inhibitors of ACAT inhibitors (АХАТ)
5
Cannabinoid
antagonists
molecular dynamics modeling, virtual
7
screening) and experimental methods is a Estrogen antagonists
7
Potassium
Blockers
guarantor of successful study of the
1
sigma receptor antagonists
compounds of the given class.
1
histamine receptor antagonists
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