Synthesis and stereochemistry ofortho-methylated octaphenylnaphthalenes

Two exceptionally crowded derivatives of octaphenylnaphthalene bearing ortho-methyls on the phenyl substituents—2,3,5,6,7,8-hexaphenyl-1,4-di(o-tolyl)naphthalene (2) and 2,3,6,7-tetraphenyl-1,4,5,8-tetra(o-tolyl)naphthalene (3)—were prepared by the addition of tetraarylbenzynes to tetraarylcyclopentadienones. Both compounds exist as a mixture of conformational isomers which do not interconvert at ordinary temperatures. The yield of 2 and its distribution of conformers is strongly influenced by the choice of precursors in the final cycloaddition; one method gives a high yield of a 55:45 trans-2:cis-2 mixture, while another gives a very low yield of a 15:85 trans-2:cis-2 mixture. Kinetic epimerization of the latter indicated that the barrier to rotation about the tolyl-naphthalene bonds is 33.1 kcal/mol. Chirality 10:154–158, 1998.