A study of the racemisation rates of 2,2'-disubstituted biphenyl derivatives

Abstract

Experimental data have been accumulated which appear to reveal a relationship between optical stability and electron density in the region of the 1,1'-bond of sterically hindered biphenyls: an attempt has been made to isolate this factor by working with a series of 2,2'-di-iodobiphenyls in which 4,4' -substituents of varying electron donating power are introduced. 2,2'-di-iodobiphenyl and four of its 4,4'-derivatives (dicarboxy, dicarbomethoxy, diacetamido and diamino) have been obtained in their optically active states; for the first time, except for the 4,4'-dicarboxylic acid. Their rate coefficients of racemisation have been determined at several temperatures. The solvent used in all the five cases was N,N'-dimethylformamide, but in addition, racemisation has been followed in absolute ethanol and in 0.1N aqueous sodium hydroxide for 2,2'-di-iodobiphenyl-4,4'-dicarboxylic acid, and in 0.5N aqueous alcoholic hydrochloric acid for 2,2'-di-iodobenzidine. From the experimental results, the Arrhenius parameters E and log10A, and the transition state theory functions [delta]H[dagger], [delta]S[dagger] and [delta]F[dagger] were evaluated. In the discussion, a suggested geometry of the ground state, and of the transition state during racemisation, of 2,2'-di-iodobiphenyl are described and whenever possible, are correlated with the experimental values of the entropy and energy of activation, which determine the optical stability. The experimentally determined activation energies for the series of 2,2'-di-iodobiphenyl derivatives have been compared with theoretically calculated values, and discussed with respect to the geometry of the ground and transition states. The experimental results show a decrease in energy of activation with increasing electron donating power of the 4,4' -substituents. This observation is discussed in terms of resonance involving the 1,1'-bond, and in terms of out-of-plane bond distortion of the 1 and 1-carbon atoms. The preparations of this series of 2,2'-di-iodo-biphenyl derivatives are described in the experimental sections, which also include descriptions of the syntheses of three 4,4'-derivatives of 2,2'-di-bromobiphenyl, and of the attempts to obtain these in the optically active state.