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.