Influence of substituents on the rotational energy barrier of atropisomeric biphenyls — studies by polarimetry and dynamic gas chromatography

Abstract

AbstractWe have synthesized a series of 2,2′‐substituted biphenyls and particularly derivatives of 2,2′‐bis(trifluoromethyl)biphenyl (4) with either one or two fluoro, chloro, bromo, amino, nitro or methoxy substituents in the para positions for investigating the influence of these substituents on the rotational energy barrier of the biphenyls by polarimetry and dynamic gas chromatography (DGC). Axially chiral biphenyls with at least two bulky substituents in the ortho positions were separated by gas chromatography on selectively modified cyclodextrins. Due to competition between resolution and enantiomerization in the gas chromatographic column 2‐tert‐butyl‐2′‐isopropyl‐ (2), 2‐isopropyl‐2′‐(trifluoromethyl)biphenyl (5), and some para‐substituted derivatives of 4 show temperature‐dependent interconversion profiles and coalescence phenomena in analogy to dynamic NMR studies. The accelerating effect on the rate of enantiomerization of electron‐donating groups in the para positions and the decelerating effect of electron‐accepting groups has been determined. Dynamic gas chromatography is shown to be a useful supplement to studies of thermal enantiomerization kinetics of optically enriched or separated compounds by polarimetry for the investigation of energy barriers of atropisomeric biphenyls.