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Stereoisomerism of Molecular Multipropellers. 2. Dynamic Stereochemistry of Bis- and Tris-Triaryl Systems
journal contribution
posted on 2001-02-06, 00:00 authored by Josep Sedó, Nora Ventosa, Ma Antònia Molins, Miquel Pons, Concepció Rovira, Jaume VecianaThe 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.