A Study on the Condensation Reaction of 4-Amino-3,5-dimethyl-1,2,4-triazole with Benzaldehydes: Structure and Spectroscopic Properties of Some New Stable Hemiaminals

Studies on the stable hemiaminals and Schiff bases formation in the reaction of substituted benzaldehydes with primary 3,5-dimethyl-1,2,4-triazole 4-amine were carried out under neutral conditions. These products were investigated by IR, Raman, MS, 1H- and 13C-NMR spectra as well as by X-ray crystallography. The effect of reaction conditions: temperature, polarity of the solvents utilized, substrate concentration and the ortho and para benzaldehyde substituents on the yield of products was also examined.


Introduction
1,2,4-Triazoles and their derivatives have attracted significant attention in several different areas. These nitrogen-rich compounds represent one of the most biologically active classes of the chemical OPEN ACCESS species. This arises from their ability to bind to a variety of enzymes and receptors in biological system

X-ray Diffraction
Suitable crystals for X-ray diffraction were obtained for compounds 2, 5, 9, 10 and 15 ( Figure 1). The molecular structure consists of two phenyl and triazole aromatic rings connected with the C1Ph-C-N4Tr-N3Tr sequence. In the hemiaminals, the C and N4Tr atoms are tetrahedral with sp 3 hybridization, which enables formation of four stereoisomers (RS, SR, RR and SS). For imine 15, there is a C=N4Tr double bond with sp 2 hybridization. The general atom numbering and selected parameters are summarized in Table 1.   (2,5,9,10) and imine (15). The hemiaminal molecules in 2, 5, 9 and 10 form a centrosymmetric (RS-SR) dimer linked by a O-H···N1Tr hydrogen bond (see Figure 2). A strong π-π interaction involving pairs of triazole rings additionally stabilizes the dimers (Table 2).  The structures presented above differ from those obtained by us earlier [30,31] where hemiaminals derived from 3,5-unsubstituted triazoles occur in two conformers: stretched (with configuration RS or SR) and twisted (RR or SS). Furthermore, in the title compounds the centrosymmetric dimers are observed whereas hemiaminals described by us previously [30,31] form infinite polymeric chains or noncentrosymmetric dimers.

Spectral Studies
The characteristic IR and Raman spectral bands of hemiaminals are given in Table 3. Characteristic strong ν(C=O) stretching vibration at about 1700 cm −1 observed in the infrared spectra in aromatic aldehydes, as well as bands observed at 3243 cm −1 , 3152 cm −1 and 1650 cm −1 which were assigned to the νasNH2, νsNH2 and σ,ωNH2 vibrations [33] respectively for 4-amino-3,5-dimethyl-1,2,4-triazole, disappear after condensation reaction. A comparison between the NH and OH stretching bands, which were observed for hemiaminals, shows that they appear in the same spectral region. In the IR spectra, the strong OH bands sometimes mask the weaker NH absorption, but in the Raman spectra, the OH bands are very weak [34]. The -OH stretching vibration of the hydroxyl group is observed as a distinct peak at about 3200-3300 cm −1 in the IR spectrum. Their values increase with decreasing dC-O bond distance ( Table 1). The intramolecular hydrogen bonding interactions of C-OH with N2tr observed in the crystal structures are confirmed by an additional broad shallow -OH stretching peak observed at about 3100 cm −1 . The band appearing in the Raman spectra at about 3100 cm −1 is assigned to the stretching vibration of -NH. The NMR spectra were obtained in the DMSO solution. DMSO is one of the most polar and aprotic solvents with a high dielectric constant and, due to this, properties of the dissolving species do not come together to agglomerate. For that reason, the hydrogen bonds observed in the solid state are not detected in the 1 H-NMR spectra. In the 1 H-NMR spectra of the compounds 2-16, the singlet at δ 5.73 ppm, assigned to the NH2 protons of the starting compound 1, disappeared and additional resonances assigned to the C-NH-N, C-OH and CH-N for 2-14 (Table 3) and -CH=N-(δ = 9.17 and 9.16) for 15 and 16 were detected which confirmed the condensation between the amino and the carbonyl groups.

Hemiaminal Stability in Solution
The hemiaminal under investigation were stable for a long time in the crystalline form. This observation does not apply to the compounds in solution. The time dependent changes in the 1 H-NMR spectra were used to determine the decomposition of the hemiaminals in DMSO solution at room temperature (Scheme 2).
Compounds 2, 4, 5 and 9 decompose slowly mostly to substrates (Figure 3), similar to the hemiaminal obtained from the 4-nitrobenzaldehyde and 4-amino-1,2,4-triazole (4nba, see Figure 4a    These observations agree well with the theoretically examined Schiff base formation mechanism from benzaldehyde and 4-amine-4H-1,2,4-triazole [35]. The reaction takes place in two steps. In the first step, the hemiaminal is formed. The formation of Schiff base through the water molecule elimination requires an internal equilibrium between the twisted conformation of hemiaminals. The 1 H-NMR spectral data for all stable hemiaminals obtained from 4-amino-3,5-dimethyl-1,2,4-triazole showed that they are stretched conformers. The coupling of NH protons with vicinal CH protons is about 7 to 8 Hz (Table 3). The coupling constant 3 J(CH-NH) for 2-pyridinyl hemiaminal 17 is smaller (4.96 Hz) which indicates that the twisted isomer dominates in solution [36].

Hemiaminal Formation-Effect of Substituents.
To gain a better understanding of the substrate structure effect on the hemiaminal formation, a series of 2-and 4-substituted benzaldehydes was examined, focusing on their reaction with 1 in a 1:1 stoichiometry in CH3CN solution. The reaction mixtures were stirred at 50 °C over 9 h. After solvent evaporating, the remaining solids were investigated by 1 H-NMR in DMSO solution.
The good correlation between the imine and hemiaminal formation and electronic effects of the substituents is observed only for para derivatives (Figure 5a). From the theoretical studies [35,37] it is known that the N-H amine bond is broken first and then the hydrogen atom is transferred to the aldehyde O atom forming an O-H bond. Subsequently, the C-N bond is formed. It seems that the hemiaminal formation must be dependent on the carbonyl C atom electrophilicity. Benzaldehydes containing electron-withdrawing (-R) substituents reduce the hemiaminal formation in order: NO2 > CN > CF3 > CHO > H. Opposite to this, in the case of the substituents containing electron-donating groups (+R), the formation yield increases in order: OH < OCH3 < CH3 < F < Cl < Br < H. The next step of reaction is water molecule elimination from hemiaminal.  [38]. All reaction were performed using 0.172 mmol of substrates in CH3CN (2 mL) at 50 °C. Product yields were obtained from the CH3 1 H-NMR signals in the region of 2.00-2.50 ppm.
The C-OH bond is broken first. Then, the N-H bond is broken and finally an imine and water are formed. It seems that the stability of the C-OH bond is also dependent on the phenyl ring substituent and this relation is opposite to that described above for hemiaminal formation. The C-O bond is being broken more easily for +R than for −R substituents.
The effect of ortho substituents on the condensation product reaction is more complex (Figure 5b) than for para substituents and could not be explained by the differences in electrophilicity of the carbonyl C atom.

Hemiaminal Formation-Solvent Effect
The condensation reaction of 2-nitrobenzaldehyde with 4-amino-3,5-dimethyl-1,2,4-triazole was studied in 12 different organic solvents. The solvent effect on the reaction rate and efficiency was investigated by the 1 H-NMR spectroscopy ( Table 4). The results indicate a higher hemiaminal content in apolar aprotic solvents than in dipolar aprotic media. The hemiaminal yield increases with solvent hydrophobicity, whereas a polar solvent shifts the equilibrium towards the Schiff base formation. Although, at first sight, it is surprising that increasing solvent polarity diminishes the hemiaminal content, this is understandable in terms of changing substrates and products dipole moment. The rate of the first step of condensation decreases with increasing solvent polarity because the activated complex must be less dipolar than the reactants. It means that the dipole moment of the activated complex should be less than the sum of the reactant dipole moment [39]. From the theoretical study [35], it is known that the hemiaminal, as an intermediate of the condensation, is non-ionic. On the other side, due to the strong intermolecular hydrogen interaction, the existence of dimers is possible, which can reduce the polarity of a hemiaminal. The strong influence of the solvent on the second step of the formation of Schiff base and elimination of the water molecule was also observed. The rate is slowest in polar aprotic solvents with high dipole moment. It seems that the activated complex, which leads to the Schiff base, appears to be less dipolar and hence less strongly solvated. In the aprotic electron-pair donor solvents with small dipole moments, the rate of this step is faster. In the hydrogen bonding solvent such as water or iso-propanol, the hemiaminal formation can proceed via a zwitterionic intermediate. The calculations of zwitterion formation between methylamine and formaldehyde have been performed [40] and found that two water molecules reduce the reaction barriers of proton-transfer step [41]. As indicated in Table 4, the water role in the 2-nitro hemiaminal formation in acetonitrile solution is not restricted only to solvent effects [42], as water also acts as a reactive species. The catalytic properties of water molecules in this reaction were thought to be essential in order to facilitate the nucleophilic attack of the amine on the carbonyl group and the proton transfers from amine to water molecule and from water to aldehyde oxygen. The rate for the first step of condensation reaction of 1 with 2-nitrobenzaldehyde depends on the water content in acetonitrile and maximum rate acceleration was observed at 15% by volume water in acetonitrile (Table 4).

Hemiaminal Formation-Benzaldehyde Concentration and Temperature Effect
The 4-amino-3,5-dimethyl-1,2,4,-triazole is in dynamic equilibrium with the reactant aldehydes. In order to determine the experimental conditions that favor the shift of the equilibrium toward the hemiaminal as a product, the effects of temperature and benzaldehyde concentration were determined using 2-and 4-nitro substituted benzaldehydes. As can be seen (Figure 6), the highest hemiaminal yield was obtained in the upper range of the aldehyde to amine molar ratio.   In Table 5, the values of molar ratio K calculated for the formation of hemiaminal 2 and respective Schiff base (from the amine 1 and 2-nitrobenzaldehyde) in acetonitrile at different temperatures are presented. The results show that the temperature increase favors the imine formation. However, it must be noticed that the summary yield of products (HA + SB) at all temperatures is about 30%. This probably indicates that the first step of the reaction-the hemiaminal formation-is a reversible and exothermic process [43]. The second step of Schiff base formation is endothermic.

Hemiaminal-Aldehyde Interchange Reaction
Finally, the aromatic aldehyde interchange reaction in DMSO solution at room temperature was studied by the 1 H-NMR. The spectra in Figure 7 show that upon addition of 2-nitrobenzaldehyde and 4-nitro substituted hemiaminal 4 (12.5 mM) in molar ratio 2:1, respectively, in DMSO-d6 at 25 °C, a new signal appears in the hemiaminal proton region. The above experiments also show that the metathesis reaction is occurring quite slowly and that the first step of this process is the hemiaminal disintegration to amine and aldehyde (Scheme 3).

Materials and Physical Measurements
The reagents and solvents employed were commercially available and used as received without further purification. Elemental analyses were carried out with a CHNS Vario EL III analyzer (Elementar Analysensystem GmbH, Hanau, Germany). The NMR spectra were recorded on a Bruker 300 or 500 MHz spectrometer (Bruker, Poznań, Poland) using solvent as an internal standard. The mass spectra of electrospray ionization (ESI)-MS were obtained on MicrOTOF-Q mass spectrometer (Bruker). The Fourier transform IR spectra were recorded from KBr pellets in the range of 400-4000 cm −1 on a Bruker IFS 66 FT-IR (Bruker). The Fourier-Transform Raman Nicolet Magna 860 FTIR/FT Raman spectrometer (Spectro-Lab, Warszawa, Poland) was used for the Raman spectral measurements at room temperature. 4-amino-3,5-dimethyl-1,2,4-triazole (MeATR) was synthesized in accordance with the published procedure and checked with 1 H-NMR spectra and elemental analysis [32].

Synthesis of Hemiaminals 2-14
Compounds 2-12 were synthesized according to the following general procedure. A mixture of equimolar amounts (0.54 mmol) of MeATR (1) and a suitable aldehyde ArCHO (in molar ratio 1:1) were dissolved in acetonitrile (5 mL) and refluxed for 3 h. After removing volatile components, the raw solid products was washed with cold acetonitrile and dried in air. Crystals of four hemiaminals were obtained upon slow evaporation of the solvent from the reaction mixtures.

Reaction of MeATR (1) with 2-Pyridinecarboxaldehyde
A mixture of equimolar amounts (0.17 mmol) of MeATR (1) and 2-pyridinecarboxaldehyde (in molar ratio 1:1) were dissolved in hexane (2 mL) and stirred at 50 °C for 9 h. After removing volatile components, raw solid products washed with cold hexane dried and dissolved in DMSO-d6 and analyzed by NMR spectroscopy.

Reaction Survey
The effect of substituents on the condensation reaction-aldehyde (0.172 mmol) and amine 1 (0.172 mmol) were dissolved in acetonitrile (2 mL) and stirred for 9 h at 50 °C. After removing volatile components, the solid products were dissolved in DMSO-d6 and 1 H-NMR spectra were measured. The amount of hemiaminal, Schiff base and unreacted amine were determined from integrated peak intensities.
The effect of solvent on the condensation reaction-2-nitrobenzaldehyde (0.172 mmol), amine 1 (0.172 mmol) and 2 mL of solvent were stirred for 9 h at 50 °C. After removing volatile components, the solid products were dissolved in DMSO-d6 and 1 H-NMR spectra were measured. The amount of hemiaminal (HA), Schiff base (SB) and unreacted amine (A) were determined from integrated peak intensities of the δ(C-CH3) signals (A-2.25 ppm, HA-2.27 ppm, SB-2.47 ppm).

Conclusions
In this paper, a new group of hemiaminals derived from aromatic aldehydes (benzyl, pyridyl) and 4-amine-3,5-dimethyl-1,2,4-triazole was presented. We found that most of the electron-withdrawing substituents in the aromatic aldehydes can stabilize the creation of stable hemiaminals e.g., compounds 9, 10, 12, 13 and 14 presented in this paper. The presence of two methyl substituents in the triazole ring significantly affects the crystal and molecular structure of hemiaminals, which form centrosymmetric dimers only, while predominantly polymeric structures have been reported previously. The presence of the methyl groups also affects the conformation of molecules which, in solution and in crystalline form, have the stretched geometry. This means that our hemiaminals in solution have the RS/SR configuration. The current study revealed the enormous influence of the environment on the reaction course and its efficiency. In this respect, the solvent polarity, the presence of water and its catalytic performance are important factors. A simple relationship between temperature and the product yield as well as the metathesis phenomena observed in this work led to the conclusion that the first stage of condensationthe creation of a hemiaminal-is an exothermic process, while the second-a Schiff base formation-is an endothermic process.