Molecular meccano, 48 - Probing co-conformational changes in chiral [2]rotaxanes by H-1-NMR spectroscopy

The template-directed syntheses of three [2]rotaxanes possessing either chi ral centers or elements of planar chirality, in one of their two mechanical ly interlocked components, have been realized and their solid-state structu res have been analyzed by X-ray crystallography. In one instance, an enanti omerically pure dumbbell-shaped component incorporating a 1,5-dioxynaphthal ene recognition site and two (1R,2S,5R)-menthyl stoppers was employed to te mplate the formation of the achiral tetracationic cyclophane, cyclobis(para quat-p-phenylene). The resulting enantiomerically pure [2]rotaxane was isol ated in a yield of 55%. In the other two instances, an achiral 1,5-dioxynap hthalene-based dumbbell-shaped component was employed to template the forma tion of bipyridinium-based cyclophanes possessing either one or two element s of planar chirality. The resulting [2]rotaxane, possessing one element of planar chirality, was isolated as a racemate in a yield of 24%. The relate d [2]rotaxane, possessing two elements of planar chirality, was isolated as a mixture of a meso form and an enantiomeric pair in an overall yield of 2 8%. The H-1-NMR-spectroscopic analysis of this mixture revealed a diastereo isomeric ratio of 4:1. A degenerate co-conformational change was identified by variable-temperature H-1-NMR spectroscopy in all [2]rotaxanes. The symm etry loss arising from the introduction of one or two elements of planar ch irality enabled the elucidation of the mechanism of this dynamic process in two instances.