Correlated Rotation in Complex Triarylmethanes. I. The 32-Isomer System and Residual Diastereoisomerism

A triarylmethane has been prepared with three different aryl groups lacking local C₂ axes. As predicted by an earlier analysis, this compound exists in two diastereomeric forms even when all interconversions by the usual two-ring flip mechanism are rapid (residual diastereoisomerism). The two diastereomers have been separated; the barrier to isomerization is ca. 30.5 kcal/mol. The interconversion between the diastereomers is ascribed to the one-ring flip mechanism. A noteworthy feature of this novel type of stereoisomerism, and a consequence of the occurrence of correlated rotation, is that although two separable conformers exist, there are no restrictions upon the torsional angles of the individual aryl rings. Examples of the same phenomenon in molecules of the type Ar₃Z, Ar₂ZX, and Ar₂ZXY are discussed.