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 (<i>J. Am. Chem. Soc.</i> <b>1973</b>, <i>95</i>, 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.