Stereoisomerism of molecular multipropellers. 2. Dynamic stereochemistry of bis- and tris-triaryl systems.

The dynamic stereochemistry of bis- and tris-triaryl systems, the most simple "molecular multipropellers", is discussed on the basis of an extension of a systematic stereochemical analysis based on a symmetry-adapted symbolic notation developed specifically for these molecules. A suitable theoretical basis for our study is provided by the classical hypotheses concerning the dynamics of simple triaryl systems as formulated by Mislow and co-workers (J. Am. Chem. Soc. 1973, 95, 1535-1547), which, once applied to molecular multipropellers, show the existence of two modes of rearrangement for each propeller. Interconversion graphs for all molecules under study, covering a wide span of structural complexity, are presented. A complete NMR study of a two- and a three-propeller molecule indicates that all experimentally observable exchange pathways are indeed predicted by theoretical analysis. Moreover, quantitative analysis of 2D-EXSY experiments affords the activation energy of the subset of pathways that give rise to observable interconversions on the NMR time scale. Assuming that two-ring flips are the threshold mechanism for individual propeller interconversion, the experimental evidence indicates a preference for the flip of the central ring and one of the outer rings over the flip of two outer rings.