J. Chil. Chem. Soc., 63, Nº 2 (2018) 3918 MICROWAVE ASSISTED SYNTHESIS, SPECTRAL CORRELATION AND ANTIMICROBIAL EVALUATION OF SOME ARYL IMINES S PAZHINIVEL SAKTHINATHAN 1 , RAMAMOORTHY SURESH 1 , DAKSHNAMOORTHY KAMALAKKANNAN 1 , VEERANDIRAN MALA 1 , KALIYAN SATHIYAMOORTHI 1 , GANESAN VANANGAMUDI 1 , AND GANESAMOORTHY THIRUNARAYANAN 2,* 1 PG & Research Department of Chemistry, Government Arts College, C-Mutlur, Chidambaram-608102 2 Department of Chemistry, Annamalai University, Annamalainagar-608002, India. ABSTRACT A series of aryl imines have been synthesized by SiO 2 -H 3 PO 4 catalyzed microwave assisted condensation of amine and aldehyde under solvent-free conditions. The yield of the imines has been found to be more than 80%. The purity of all imines has been checked using their physical constants and spectral data as published earlier in literature. The UV λ max (nm), infrared νC=N(cm -1 ), NMR δ(ppm) of C-H and C=N spectral data have been correlated with Hammett substituent constants and F and R parameters using single and multi-linear regression analysis. From the results of statistical analysis, the effect of substituents on the above spectral data has been studied. The antimicrobial activities of all imines have been studied using Bauer-Kirby method. Key words: Solvent-free synthesis, SiO 2 -H 3 PO 4 , Aryl imines, IR and NMR spectra, Spectral correlation study, Antimicrobial activities e-mail: drgtnarayanan@gmail.com INTRODUCTION The Schiff’s bases are bimolecular condensation products of primary amine with carbonyl compound. They are generally known as azomethines possess (-C=N-) to honor Hugo Schiff’s, who had synthesized such compounds earlier. Schiff’s bases are characterized by the –N=CH– (imine) group which fnds importance in elucidating the mechanism of transamination and racemization reactions in biological systems 1 . Schiff’s bases of aliphatic aldehydes are relatively unstable which readily undergo polymerization while those of aromatic aldehydes having an effective conjugation system are found to be more stable. Schiff’s bases have been reported to play very important role in many biological and chemical reactions, due to the presence of the imine linkage. Schiff’s bases are generally Bi- or tri- dentate ligands capable of forming very stable complexes with transition metals 2 . Schiff’s bases, derived from aromatic amines and aromatic aldehydes are reported to be involved in the study of asymmetric catalysis 3 , agnetic properties 4 , photochromism 5 , binding with DNA 6 , construction of supra molecular structures 7 , the study of activity against Ehrlichascites carcinoma (EAC) 8 the feld of dyes and pigments 9 , the development of corrosion inhibitors 10 , anti-HIV 11 and in the evaluation of physical properties in the crystalline state 12 . Optically active imine derivatives possess multipronged biological activities such as antimicrobial 13 , anticancer 14 anti-tubularcular 15-17 , nematicidal-insecticidal 18 , anti-infammatory and lipoxygenas 19 . The imine moieties are important intermediates and versatile starting materials for the synthesis of chiral amines 20 , pyrimidine derivatives 21 , phenylhydrazones 22 , Mannich bases 23 , indoles 24 , quinoxalines 25 , imidazoles 26 , α-ethoxycarbomates 27 , aminotriphenylmethanes 28 , Michael adducts 29 , allyl products 30 , optically active α-alkyl aldehydes 31 by hydrogenation 32 , nucleophilic addition with organometallics 33 and cycloaddition reaction 34 . Many reagents have been used for the synthesis of optically active imines such as Lewis acids 35 , molecular sieves in ionic liquids 36 , solid super acids, K-10 montmorillonite 37 , Tandam catalysts 38 , MnO 2 39 , CaO 40 , ZnCl 2 20 , MgSO 4 -PPTS 22 , alumina 41 , Ti(OR) 4 42 , CuCl 2 43 , MCM-41-SO 3 nanocatalyst 44 , P 2 O 5 -SiO 2 45 promoted by microwave irradiation 39 , Cinchona alkaloid-thiourea 46 , Infrared 47 and ultrasound radiation 48 . These catalysts have been applied for the synthesis of chiral amines by oxidative coupling of amines 49, 50 with carbonyl compounds 17, 37, 38 alcohols 30 and acid chlorides 33, 35 . The microwave assisted synthesis has become popular in academic and pharmaceutical areas since this involves a new enabling technology for developing new drugs. Chemists and scientists 17, 37, 51 prefer solvent-free microwave synthetic methods for synthesizing organic compounds, since they involve shorter duration, operational simplicity, easy workup procedure, less hazardousness to humans and environment and better yields. No report has been found in the literature regarding the synthesis of imines with SiO 2 -H 3 PO 4 catalyst under microwave assistance and spectral as well as biological activities of imines. Therefore the authors have taken efforts to synthesis of imines from amine and carbonyl compound using SiO 2 -H 3 PO 4 catalyst with microwave irradiation under solvent-free conditions. The various spectral data of these imines have been utilized for studying the quantitative structure activity relationships through Hammett correlations. The biological activities of these imine derivatives have been studied with the help of standard 52 method. EXPERIMENTAL General All the chemicals involved in the present investigation, have been procured from Sigma-Aldrich and E-Merck chemical company. Melting points of all imines have been determined in open glass capillaries on SUNTEX melting point apparatus and are uncorrected. The UV spectra of all synthesized imines have been recorded withELICO-BL222 spectrophotometer(λ max nm) in spectral grade methanol solvent. Infrared spectra (KBr, 4000-400 cm -1 ) have been recorded on AVATAR-300 Fourier transform spectrophotometer. Bruker AV400 NMR spectrometer operating at 400 MHz has been utilized for recording 1 H NMR spectra and 100 MHz for 13 C spectra in CDCl 3 solvent using TMS as internal standard. General procedure for synthesis of substituted benzylidene-2-chloro- 4-methyl aniline Appropriate quantities of aryl amine (2 mmol), substituted benzaldehydes (2 mmol) and SiO 2 -H 3 PO 4 53 (0.5 g) have been taken in borosil tube and tightly capped. The mixture has been subjected to microwave irradiation for 6-12 minutes in a microwave oven (Scheme-1) (LG Grill, Intellowave, Microwave Oven, 160-800W) and then cooled to room temperature. After separating the organic layer with dichloromethane the solid product has been obtained on evaporation. The solid, on recrystallization with benzene-hexane mixture gives glittering product. The insoluble catalyst has been recycled by washing with ethyl acetate (8 mL) followed by drying in an oven at 100°C for 1h and reused for further reactions. RESULTS AND DISCUSSION Spectral linearity Spectral linearity of synthesized imines has been studied by evaluating the substituent effects on the spectral frequencies. The spectral data observed for the imines, UV λmax (nm), infrared νC=N, the proton chemical shifts δ(ppm), of C-H and carbon chemical shifts of C=N are correlated with various substituent constants. UV-Vis spectral study The measured absorption maxima (λ max nm) of synthesized imines are presented in Table-1. These values have been correlated with Hammett substituent constants and F and R parameters using single and multi-linear regression analyses 54-56 . Hammett equation employed, for the correlation analysis, involving the absorption maxima is as shown below in equation (1). λ = ρσ + λ o --- (1)