Stereochemistry of biaryls

Abstract

Hydrogen atoms attached to an aromatic nucleus have been found to be particularly effective in causing hindrance to free rotation about the inter-nuclear bond of certain biaryls. In order to investigate the limits of this effect, some substituted phenylnaphthalenes and dinaphthyls have been synthesised by means of the Ullmann reaction and their stereochemical properties investigated. In some cases the compounds have been obtained in optically active forms, while others were shown to be incapable of optical activation even at low temperatures. The optically active compounds have been racemised under controlled conditions and accurate values for their optical half-lives have thus been obtained. During the course of the work, it was observed that optically active biaryls in which there are two adjacent carboxyl groups (e.g. 1-phenylnaphthalene-2':8-dicarboxylie acid; 1:1'-dinaphthyl-8:8'-dicarboxylie acid), racemise more quickly in sodium hydroxide solution than in a non-polar solvent. It has further been shown that the rate of racemisation of such compounds in chloroform is sensitive to the percentage of ethyl alcohol present. Thus it appears that the presence of two adjacent carboxylate ions in a compound decreases the optical half-life. Experimental data obtained from the literature supports this conclusion. The ultra-violet absorption spectra of the compounds under investigation have been measured and an attempt has been made to correlate the degree of conjugation between the two rings, as inferred from the spectra, with the optical half-lives of the compounds concerned.