Steric effects of a nitro group on the optical stabilities of 2,2′-bridged and unbridged biphenyls

Abstract

Quaternary azepinium salts have been prepared from 4- and 6-nitro-2,2′-bisbromomethylbiphenyl. No optical activity was observed among the compounds with the p-nitro group, but suitable salts derived from the o-nitro-compound showed mutarotation in solution at room temperature. Racemisation data for the azepinium picrate from a 6-nitrobiphenyl, in acetone, show that it has an unusually high Arrhenius parameter A. The energy barrier to racemisation is discussed in relation to that in similar bridged compounds, and some apparent discrepancies between polarimetric results and those obtained by n.m.r. spectroscopy are pointed out. A related unbridged bisquaternary salt has been prepared and obtained optically active. It is optically stable up to ca. 90°, but the very low specific rotation has so far prevented quantitative study of its racemisation.