Dynamic Stereochemistry of Hexaarylbenzenes

Rotational isomerism in hexaarylbenzenes has been analyzed mathematically and studied experimentally. The mathematical analysis demonstrates that 13 rearrangement modes are possible for such molecules. The isomerization pathway actually followed is revealed by experimental studies of highly substituted hexa- and pentaarylbenzenes, one of which gives rise to 16 rotational stereoisomers. Kinetic studies using both NMR and classical methods strongly support a mechanism wherein one peripheral aryl ring at a time rotates by~π radians. Measured free energies of activation for isomerization vary from about 33 kcal/mol for rotations of rings bearing an ortho methoxy group to about 17 kcal/mol for rotations of rings bearing meta substituents.