Stereodynamics of E/Z isomerization in rotaxanes through mechanical shuttling and covalent bond rotation

The mechanical bond has opened a new world for structural and dynamic stereochemistry, which is still largely underexplored and whose significance for various applications is becoming increasingly evident. We demonstrate that designed rearrangements involving both covalent and mechanical bonds can be integrated in [2]rotaxanes, leading to interesting consequences in terms of E/Z isomerization mechanisms. Two entirely distinct and concomitant stereomutations, pertaining to the same stereogenic element but involving different kinds of linkages within the molecule, are observed and are thoroughly characterized. The rate of the two processes is affected in opposite ways upon changing solvent polarity; such a phenomenon can be used to selectively modify the rate of each motion and adjust the relative contribution of the two mechanisms to the isomerization. Although the movements are not synchronized, an analysis of the intriguing fundamental implications for transition state theory, reaction pathway bifurcation, and microscopic reversibility was triggered by our experimental observations.