Synthesis and nuclear magnetic resonance spectra of benzidines and some derivatives

Abstract

Benzidine, 2,2'-disubstituted and 2,6,2',6'- tetra substituted benzidines are usually prepared through the rearrangement of their corresponding hydrazobenzenes in acid medium. However, hydrazobenzenes which would lead to benzidines heavily substituted in the 2,6,2',6' positions were found to be difficult to rearrange or to give low yields when rearranged in concentrated acids. Best yields were obtained when sulfuric acid (2:1 by weight) at 85--90°C was used. Column chromatography was found to be the most satisfactory method of separating the rearrangement products (benzidine, diphenyline and semidine).

1H N.m.r. spectra of the benzidines prepared throughout this work were done and the chemical shifts of the amino protons and of the ring protons in CDCl₃, in CDCl₃ with few drops of TFA and in neat were determined and discussed.

Many attempts were made to achieve optical resolution of 2,2'-di-t-butylbenzidine through the (+)-hydrogen tartrate, (+)-camphor-10-sulfonate salts, and the (-)-menthoxyacetyl derivative but without success. On the other hand, the optical activation of 2 ,2'-dibromo-6,6f-diethoxybenzidine gave only the (-) base through the (+)-camphor-10-sulfonate salt.

Temperature variable high resolution n.m.r. (220MHz) spectroscopy was employed to study the inversion process of the two enantiomeric forms by means of magnetic behaviour of the diastereotopic protons of the ethoxy group of 2,2'-dibromo-6,6'-diethoxybenzidine.

Schiff's bases derived from different benzidines and benzaldehydes were prepared and the substituent effect in both components on the rate of the reaction was studied. Moreover, 1H n.m.r. spectra of these bases was extensively studied and the chemical shifts of the azomethine proton3and of the ring protons of the aldehyde and of the benzidine components were determined and discussed. The de shielding effect of the azomethine group CH=N on the chemical shifts of the benzylidene ring protons of these bases was determined for the first time.