SYNTHESIS, IR AND NMR SPECTRAL CORRELATIONS IN SOME SYMMETRICAL DIIMINES

A series of diimines have been synthesized by coupling of diamine with substituted benzaldehydes. The purities of these diimines were checked by their analytical and spectroscopic data. The spectral frequencies νCN (cm), NMR chemical shifts (δ, ppm) of C-H and C=N of these diimines have been correlated with Hammett substituent constants, 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.


INTRODUCTION
In this decade synthesis of enantiometrically pure compounds from prochiral compounds using chiral reagents have become topic of immense interest.Effective and enthusiastic efforts by synthetic organic chemists in the last two decades are converting this difficult task to a practically possible level.Many chiral molecules have been synthesized in excellent optical purities through chemical process which reveal natural processes.Aldehydes and ketones react with primary amines to give imines, in which the carbon-oxygen double bond is replaced by a carbon-nitrogen double bond was discovered in 1864 by Hugo Schiff [1] and been always the reaction of interest to chemists up to date in the fields of macrocyclic and supramolecular chemistry.The diimines or bisimines or bis-Schiff's bases also been synthesised by the reaction of 1 mole diamine with 2 moles of aldehydes or ketones.Primary amine and β-diketones also used for synthesis of cyclic diimines.Optically active amines and alkaloids, with a wide range of molecular structures are used as chiral reagents.Numerous solvent assisted [2] or solvent free greener [3] methods and reagents were available in the literature for synthesis of optically active diimines.These diimines have been used as a precursor for synthesis of optically active cyclic imines like piperazine derivatives [4], heterocycles such as imidazolines, pyrimidines, carbapenem intermediates, bis-β-lactams, bisketenes [5,6], polymers [7] and many metal complexes [8].Diimines possess important biological activities such as antioxidant [9], antiinflammatory and anti-analgesics [10].Spectroscopic data were used for prediction of ground state molecular equilibration of organic compounds such as, ketones, chalcones [11], imines [12], diimines [13], acyl chlorides [14], bromides and their esters [15].Recently, Suresh et al., [12] have studied the effects of substituent on infrared C=N, nuclear magnetic resonance chemical shifts (ppm) δCH and CN.In this work they observed good, satisfactory and fair degree of correlations for the above frequencies with Hammett substituent constants, F and R parameters.In this view, there is no report available for the synthesis and spectral correlations of diimines in literature in the past.Therefore the author has taken efforts to synthesis and studied the effect of substituents on the spectral group frequencies of synthesized diimines.

General
All the chemicals involved in the present investigation were procured from Sigma-Aldrich and E-Merck chemical companies.Melting points of all diimines were determined in open glass capillaries on SUNTEX melting point apparatus and are uncorrected.Infrared spectra (KBr, 4000-400 cm -1 ) were recorded on AVATAR-300 Fourier transform spectrophotometer.The Bruker AV400 NMR spectrometer was used for recording NMR spectra operating 400 MHz for 1 H and 100 MHz for 13 C spectra in CDCl 3 solvent using TMS as internal standard.Mass spectra of diimines were recorded on SHIMADZU spectrometer using chemical ionization technique.

Synthesis of diimines
To a solution of aryl aldehydes (100 mmol) in methanol (80 mL) was added ethylenediamine (60 mmol) at 0 °C slowly in drops.Then the reaction mixture was stirred at room temperature for 12 h (Scheme 1) [4].The crude diimines were subjected to evacuate under vacuum.The residue was treated with 100 mL of methylene chloride and dried over anhydrous potassium carbonate.The diimines were obtained after evaporated the solvent under high vacuum.The analytical, physical constants and mass fragments of these diimines are presented in Table 1.The infrared and NMR spectroscopic data of these diimines are given in Table 2.

Entry
Table 1.The analytical, physical constants and mass fragments (m/z) of diimines.

Infrared spectral study
In the present study, author has recorded the infrared spectrum of all synthesised diimines containing single substituents in phenyl moieties with KBr disc.The C=N stretches (cm -1 ) of all diimines have been assigned and listed in Table 2.These stretches (cm -1 ) were correlated with Hammett substituent constants, F and R parameters.In this correlation, the Hammett equation is employed as, where ν o is the frequency for the parent member of the series.The assigned νC=N stretching frequencies (cm -1 ) were correlated with various Hammett substituent constants and F and R parameters through single and multi-regression analyses [12,[17][18][19] including Swain-Lupton's parameters [20].The results of statistical analysis of single parameter correlation are shown in Table 3.
The correlation of νC=N (cm -1 ) frequencies of diimines with Hammett σ constants and R parameters were found to be satisfactory.All correlations gave positive ρ values.This implies that the normal substituent effect operates in all systems.The remaining constants gave poor correlations with νC=N (cm -1 ) frequencies.This is due to the absence of polar, field and inductive effects of the substituents were unable to predict the reactivity on C=N stretches.This is associated with the conjugative structure shown in (Figure 1).