1H n.m.r. spectroscopic studies of rotational isomers of several 1,2-diarylacenaphthylenes: conformational barriers and buttressing effects

Abstract

The conformational behaviour of 1,2-di-o-tolylacenaphthylene and seven of its 3′,3″-disubstituted derivatives has been investigated. The existence of the syn and anti isomers is evident from the resolution of the two respective pairs of methyl groups in their ¹H n.m.r. spectra at room temperature. Dynamic ¹H n.m.r. studies indicate that the rotational barriers, in the range 76–85 kJ mol^–1, are dependent on the nature of the 3′,3″-substituents. The buttressing effect of these substituents is found to follow the order: H < CN < CH₃ < Cl < CH₂OH < CHO ≈ CH₂Br. Interestingly, one of the precursors. (a pinacol) to 1,2-di-o-tolylacenaphthylene shows both a propelling process and free rotation of the aryl rings. The corresponding pinacolone also exhibits a possible propelling interconversion of the two tolyl rings.