Correlated Rotation in Complex Triarylmethanes. II. The 16- and 8-Isomer Systems and Residual Diastereotopicity

The dynamic stereochemistry of two complex triarylmethane systems has been studied. In one of these, the molecule has three different aryl groups, two of which lack local C₂ axes coincident with the bonds to the central carbon atom. Although all conformers of this methane are rapidly interconverted at ambient temperatures by two-ring flip pathways, and in spite of the fact that there are no restrictions upon the individual torsional angles of the aryl groups, this compound still exhibits residual diastereotopicity, i.e., the groups in the ortho positions of the ring with a C₂ axis still give rise to separate resonances in the ¹H nmr spectrum. Isomerization, presumably by the one-ring flip mechanism, is necessary to cause coalescence of these signals. The barrier for this process is greater than 26 kcal/mol. Residual diastereotopicity is also observed in methanes with three different aryl groups, only one of which lacks a local C₂ axis, but in this case all conformers and all diastereomeric environments are averaged by two-ring flips.