Synthesis of ( 1-ethyl-2-phenyl-1 , 4-dihydroquinolin-4-yl )-( 2 , 4 , 6-trimethyl-phenyl )-amine by Electrochemical Methods in Aprotic Media

The formation of (1-ethyl-2-phenyl-1,4-dihydroquinolin-4-yl)-(2,4,6-trimethylphenyl)-amine by the electro reduction of (1-ethyl-2-phenyl-1H-quinolin-4-ylidene)-(2,4,6-trimethylphenyl)-amine occurs through acceptance of two electrons accompanied by two successive peaks, each electron peak followed by a chemical reaction. The electrochemical reduction of (1-ethyl-2-phenyl-1H-quinolin-4-ylidene)(2,4,6-trimethyl-phenyl)-amine was investigated in 0.1 M tetrabutylammoniumbromide in N,N-dimethylformamide at glassy carbon electrode using the technique of cyclic voltammetry at the room temperature (290K). In this medium the first peak was observed at –0.831 V (vs. Ag |Ag) at the glassy carbon electrode (GCE) surface, which is more stable and well defined as compared to the second peak. The diffusion coefficient (D) of investigated imine in the investigated solvent media has been calculated using the modified Randles-Sevcik equation. The electron transfer coefficient (α) of the reactant species has also been calculated. (doi: 10.5562/cca1855)


INTRODUCTION
Imines are the class of most important and fundamental unsaturated organic compound with a >C=N double bond as their characteristic chemical bond and are extensively present in natural products and many drugs.Since imines have many interesting biological activities and roles and can be converted into various very useful amines, they have received a much intense attention of chemists for a long time. 1,2mines are reduced by electrolysis without difficulty and some of the articles have indicated that the amines are the product of the reduction of imines in either protic or aprotic solvent.][5][6] In a solvent having good proton donor capability, the initial reduction is assumed to occur via an immonium cation deriving from the addition of a proton to the nitrogen atom and is therefore more likely to be related to the reduction of the alkyl immonium salts rather than to the imine group itself.Studies in dimethylformamide give a more direct insight into the electrochemical properties of the >C=N− group, for under the usual conditions, a pre-protonation step can be excluded in this solvent.Another advantage of nonaqueous studies over that in aqueous media is the stability of the imines. 7he electrochemical-electrochemical-chemical (EEC) mechanism is known to be applicable to the reduction of the aromatic imines in the low proton donor capability solvents, where two successive one electron wave are usually observed by the cyclic voltammetry experiments, and the first-step being reversible. 8,9CHNAr' + 1e - However, in a solvent with good proton donor capability and with unsaturated imine, a single two electron wave electrochemical-chemical-electrochemical (ECE) mechanism has been reported. 10,11The formed radical anion in the first step is rapidly protonated, resulting into a radical that will be easily reduced and finally protonated.
][14] The electrochemical reduction mechanism of numerous aromatic imines at a glassy carbon electrode has been reported in aprotic media. 15On the other hand, relatively few studies have focused on the electrochemical behavior of heteroaromatic imines.In the present study, electro-reduction of (1-ethyl-2-phenyl-1,4-dihydroquinolin-4-yl)-(2,4,6-trimethylphenyl)-amine was studied at the GCE in N,N-dimethylformamide (DMF).Adsorption properties of imine on a GCE surface, the mechanism and the kinetics of the reduction were also investigated using cyclic voltammetry.

Reagents
All the reagents used were of analytical grade.2-phenylquinolin-1-ethyl-4-one was prepared according to reported method. 16Stock solution of imine (Q=NAr) were prepared at a concentration of 1×10 -3 mol dm -3 in DMF.The supporting electrolyte of tetrabutylammonium bromide (TBAB) was purchased from SISCO Research laboratories Pvt. Ltd., and used without further purification.

Apparatus
The voltammetric measurements were carried out on an The following observations were made:  (a) (b) 1) A single clear spot on silica gel-G plate was obtained in iodine chamber, confirming that the product was a single compound and not a mixture.
2) The percentage of Carbon, Hydrogen and Nitrogen in the product was determined by Perkin Elmer elemental analysis.3) IR spectra were recorded in KBr on a Shimadzu 400-50 infrared spectrophotometer (ν max in cm -1 ).4) 1 H NMR spectra were recorded on JEOL AL 300 1 H NMR spectrophotometer using CDCl 3 as solvent and TMS as an internal standard (chemical shift in δ /ppm).A study of effect of scan rate is made in order to evaluate the mechanism and the feasibility of electrochemical reactions involved at GCE in this medium.The relationship between the peak current (i p ) and the voltage scan rate (ν) is described by the modified Randles-Sevcik equation: 18  

RESULTS AND DISCUSSION
where, i p is the peak current, α is the charge-transfer coefficient, n a is the number of electron equivalents exchanged during the oxidation/reduction reversible process (electron stiochiometry), A/cm 2 is the active surface area of working electrode, D/cm 2 s -1 the diffusion coefficient, C/mol cm -3 is the bulk concentration of the diffusing species, ν/V s -1 is the voltage scan rate, F is the faraday constant, R is the gas constant, and T/K is the absolute temperature.
In the present studies, the plot of the cathodic peak current (i pc ) was plotted against the square root of the scan rate (ν 1/2 ) in order to apply the Nicholson-Shain criteria to elucidate the reaction mechanism, for the 1 st wave.The plot of peak current (i pc ) and square root of the scan rate (ν 1/2 ) is clearly a straight line (Figure 2a).Nicholson-Shain criteria state that the linear change of the current with the scan rate is an indication that, the process was diffusion-controlled. 19,20 But there appear a non zero intercept, this may be due to the electron transfer processs complicated by the associated adsorption.The current function (i pc /ν 1/2 ) values were also plotted against the scan rate (Figure 3a).The current function (i pc /ν 1/2 ) decrease exponentially towards higher scan rate is an indication that the electron transfer is preceded by a chemical reaction.In the absence of chemical complication, this plot would be expected to be a nearly horizontal line.So the first electron transfer may be coupled to a fast chemical reaction which is very likely a protonation reaction by the tetrabutylammonium cation via Hofmann elimination 21 or by residual proton impurities.All these fact suggest that the overall reaction in 1 st step followed by an EC mechanism.
The second reduction wave which corresponds to the addition of one electron to the radical [QNHAr] • leading to the formation of the anion [QNHAr] -, was found to be electrochemically irreversible, with no anodic reversal current associated with it.The plot of the cathodic peak current (i pc ) plotted against the square root of the scan rate (ν 1/2 ) and the current function  (i pc /ν 1/2 C) values plotted against the scan rate are given in Figures 2b and 3b respectively.Furthermore, linear change of the current with the square root of scan rate and the ratio i pc /ν 1/2 decreases on increasing the scan rate.All the above evidence suggests that the irreversibilibty of the second wave is due to a moderately fast first-order reaction involving the product of the second electron-transfer.
The adsorptive character of the (1-ethyl-2-phenyl-1H-quinolin-4-ylidene)-(2,4,6-trimethylphenyl)-amine on the GCE was identified from the peak current's (i p ) dependence on the scan rate (ν).Plots of log i p vs. log ν is a straight line and its slope is 0.57 and 0.66 for 1 st and 2 nd peak respectively, which is less than the theoretical value of 1.0 that is expected for an ideal reaction of surface species.

Estimation of αn a and Diffusion Coefficient D
The cathodic peak potential (E pc ) of the reduction peak was dependent on scan rate.The shift of peak potential was observed towards more negative values with the increase in scan rates which indicates a diffusion controlled irreversible nature of the system, 22 where the peak potential is given by, where, α is the cathodic charge transfer coefficient, n a is the number of electrons involved in the rate determining step, D the diffusion coefficient and k° is the standard rate constant of the electrochemical reaction.In the present work, the plot of E pc vs. log ν was linear having a correlation coefficient of 0.983 and 0.972 for 1 st and 2 nd peak respectively (Figures 4a and 4b) and this behavior was consistent with the EC nature of the reaction in which the electrode reaction is coupled with an irreversible follow-up chemical step. 23he value of αn a is calculated from the slope of the plot between E pc and log ν, the value of αn a is 0.678 and 0.583 for 1 st and 2 nd peak respectively.In most of irreversible case, α is the range from 0.30 to 0.70, thus the number of electrons transfer for each reduction step is most probably to be 1.
The D values for Q=NAr can be determined from the slope of i pc vs ν 1/2 plot, after careful substitution and unit analysis.The values of diffusion coefficients (D) are found to be 5.05×10 -5 cm 2 s -1 and 5.92×10 -6 cm 2 s -1 for 1 st and 2 nd peak respectively.

Controlled Potential Electrolysis
CPE experiment were carried out in DMF containing 0.1 M tetrabutylammoniumbromide (TBAB) at potential about 200 mV more negative than the peak potential of irreversible reduction wave.The number of electron was calculated from the plot of amount of charge passes vs. t 1/2 and the value was found to be two.After electrolysis, the cell was disconnected from the circuit and the solvent was evaporated in vacuum.The residue was shaken with dry ether and the supporting electrolyte was filtered off.The ethereal layer was evaporated in turn.The resulting solid was identified.

C, H, N, Estimation Value
The observed values of the carbon, hydrogen and nitro-  gen, in the product, were 85.13 %, 6.97 %, 7.39 % respectively, as compared to the their theoretical values, which are 85.24 %, 7.10 %, 7.65 % respectively, thus, confirming the product.

Electrode Reaction Pathway
The result of cyclic voltammetry and controlled potential electrolysis suggest the consumption of two electron following the proton transfer reaction for reduction of the −CH=N− centre to −CH 2 −NH−.The data in the cyclic voltammetry results indicate that the reactant examined in this study (Q=NAr), is reduced in two discrete one electron transfer steps in DMF containing Bu 4 NBr.After the first charge transfer, the protonation of the radical anion leads to a neutral radical whose charge is more favorable for the second electron transfer to occur.Therefore a second charge transfer occurred after protonation, leading to the saturation of carbon nitrogen double bond.Thus, the sequence of the electrode reaction pathway of the examined Q=NAr at the glassy carbon electrode can be represented as follows:

CONCLUSION
The investigation have demonstrated that investigated Q=NAr i.e. (2-phenyl-1H-quinolin-4-ylidene)-(2,4,6trimethylphenyl)-amine has two reduction peak at glassy carbon electrode in DMF.On the basis of results obtained, the proposal of an electrode reaction mechanism pathway for Q=NAr can conveniently be claimed as EC mechanism.The electrochemical reduction occurs through acceptance of two electrons by successive one electron peak followed by chemical reaction.In order to test the validity of the proposed mechanism controlled-potential preparative electrolysis was carried out at the potential 200 mV more than the potential of the second peak for Q=NAr and the reduction products were isolated and identified by spectroscopic methods.

Figure 2 .
Figure 2. Plot of cathodic peak current (i p ) as a function of ν 1/2 for 1 st peak (a), plot of cathodic peak current (i p ) as a function of ν 1/2 for 2 nd peak (b).

Figure 4 .
Figure 4. Plot of cathodic peak potential (E p ) as a function of log ν for 1 st peak (a), plot of cathodic peak potential (E p ) as a function of log ν for 2 nd peak (b).