Conformational studies of 9,10-ditolylphenanthrenes: high rotational barriers for the syn–anti interconversion

Abstract

Both 9, 10-di-o- and 9,10-di-m-tolylphenanthrene have been synthesized. The existence of the syn and anti isomers of the former is apparent from ¹H n.m.r. data and one of the isomers was obtained pure by fractional recrystallization. The chemical shifts for the methyl protons of isomers of the latter are almost identical in most solvents but they are clearly resolved in nitrobenzene. Conformational studies have revealed exceptionally high barriers to rotation (ΔG^‡ > 155 kJ mol^–1) for the aryl rings in 9,10-di-o-tolyphenanthrene. At room temperature, restricted free rotation of the aryl rings was also observed in 9,10-di-m-tolylphenanthrene (ΔG_č^‡ 85 kJ mol^–1).