Microwave Assisted Solvent-Free Synthesis of Some Imine Derivatives

Compounds containing the –C=N– (azomethine group) structure are known as Schiff bases, usually synthesized from the condensation of primary amines and active carbonyl groups [1]. The reaction is acid-catalyzed and is generally carried out by refluxing the carbonyl compound and amine, with an azeotroping agent if necessary, and separating the water as formed [2]. Schiff bases are well known for their biological applications as antibacterial, antifungal, anticancer, and antiviral agents; furthermore, they have been used as intermediates inmedical substrates and as ligands in complex formationwith somemetal ions [1, 3].The synthesis of imines was firstly reported by Hugo Schiff in 1864 and they have been known since then [4]. The imine compounds have been prepared using molecular sieves [5, 6], infrared irradiation [7], Mg(ClO 4 ) 2 [8], P 2 O 5 /SiO 2 [9], ZnCl 2 [10], CaO under microwave power [11], ethyl lactate as a tunable solvent [12], K10 clay [13], TiCl 4 [14], alumina [15], CeCl 3 ⋅7H 2 O [16], ultrasound irradiation [17], polymer-supported [18], nanotube TiO 2 (in sunlight) [19], and Ti(OEt) 4 [20, 21]. The present work reveals the comparative aspects of condensation of some aromatic amines with aldehyde derivatives using microwave and conventional methods. The amine and aldehyde compounds as starting materials, β-ethoxyethanol (β-EE) as wetting reagent and microwave power as an effective source of heating are used in this study. The corresponding imine compounds were prepared in high yields and short reaction times using this effective and environment friendly method.

The present work reveals the comparative aspects of condensation of some aromatic amines with aldehyde derivatives using microwave and conventional methods.The amine and aldehyde compounds as starting materials, -ethoxyethanol (-EE) as wetting reagent and microwave power as an effective source of heating are used in this study.The corresponding imine compounds were prepared in high yields and short reaction times using this effective and environment friendly method.

Results and Discussion
In this work, we synthesized quickly and efficiently a series imine derivatives (1a-7d) by condensation of some aromatic amines (1-7) and aldehyde derivatives such as salicylaldehyde (a), p-chlorobenzaldehyde (b), p-methoxybenzaldehyde (c) and cinnamaldehyde (d) under microwave-assisted solventfree conditions using -EE as wetting reagent.-EE that is a polar molecule quickly absorbs microwaves and therefore heats up and heats around effectively.As a result, -EE, which increases the polarity of the reaction medium, has an active role in the heating of the reaction medium by microwaves.
The general reaction was summarized in Scheme 1.In addition, we tested the effect of different microwave power such as 180, 360, 600, 900 W and detected the microwave power of 180 W and 360 W are more appropriate choices for the reaction.Hence, the optimum microwave reaction conditions were determined using 180 and 360 W microwave power and neat and wetting with -EE for compound 1a.The highest reaction yield (94%) and shorter reaction time 2 Organic Chemistry International Procedure: 1 mmol aldehyde (10 mL of ethanol) and 1 mmol amine (10 mL of ethanol) are mixed a balloon and then boiled under reflux on a hot plate.
(1.5 min.)were obtained at 360 W microwave power with -EE.In the absence of wetting reagent, the reaction yields are 81% for 1.5 minutes and 88% for 5 minutes.It is understood that the reaction yield was increased by wetting reagent that increases the polarity of the reaction medium.The optimization of microwave-assisted conditions was given in Table 1.
In addition, we have synthesized some imine derivatives ( 1a, 4a, 1b, 4b, 1c, 4c, 1d, 4d) under classical conditions using a solvent (azeotropic) and traditional heating source (hot plate) for comparison with microwave conditions.Firstly, synthesis of compound 1a under the classical reaction conditions was tested at various periods of time (30, 60, 120, 240, 1300 min) to determine the optimum reaction time, which was determined as 120 minutes.Information on the optimization of the reaction time of compound 1a for classic conditions was presented in Table 2.

Conclusion
In conclusion, we have developed a simple microwave assisted solvent-free method for the synthesis of imines using a wetting reagent (-ethoxyethanol).The method works well for the reaction of this type amines and aldehydes.Because, we have synthesized imine compounds for good yields and fast reaction times in our method.Our method has some advances such as higher reaction yields, shorter reaction times, and green reaction conditions than classical requirements used of a solvent and conventional heat source (like hot plate) and other some microwave methods used of catalyst and solid supports.In addition, some imines were synthesized for the first time by us with this study.

Experimental
All of the chemicals were obtained from commercial sources or prepared according to standard methods.Melting points a Under solvent-free reaction using microwave.b Procedure: 1 mmol aldehyde (10 mL of ethanol) and 1 mmol amine (10 mL of ethanol) are mixed a balloon and then boiled under reflux on a hot plate.
were determined with an Electrothermal 9100 apparatus.
The infrared absorption spectrum of the compounds has been recorded in the region at 4000 and 400 cm −1 range using a Bruker Vertex 80/80 v spectrophotometer using KBr pressed pellet technique at room temperature.The 1 H NMR and 13 C NMR spectra were recorded using a Bruker AC 200 NMR 200 MHz spectrometer in CDCl 3 - 1 and DMSO- 6 using TMS as the internal standard.Elemental analyses were conducted at the METU Central Laboratory using a LECO, CHNS-932.All syntheses were carried out in A Bosh HMT 812 C modified microwave oven.
General Procedure for Preparation of 1a-7d.Aldehyde (1 mmoL), amine (1 mmoL) and two drops of -ethoxyethanol as a wetting reagent were mixed in a beaker.Then the beaker was placed in the microwave oven and was exposed to microwave irradiation (360 W).The product obtained by reaction was washed with cold ethanol.Then, it was recrystallized in ethanol.The physical and spectra data of the compounds 3a, 2b, 3b, 2c, 3c, 2d, and 3d are as follows.

Figure 1 :
Figure 1: Comparison of yields under the microwave and classical conditions.

Table 1 :
The optimization of microwave conditions for compound 1a.

Table 2 :
The optimization of classical conditions for compound 1a.

Table 3 :
Synthesis of imine derivatives under microwave irradiation.