Relative Rotational Motion between α-Cyclodextrin Derivatives and a Stiff Axle Molecule

Novel [2]rotaxanes bearing α-cyclodextrin (α-CD) derivatives and a diphenylacetylene axis molecule
with trinitrobenzene as a bulky stopper have been prepared to investigate the relative rotary movement
of a ring relative to an axis molecule and that of an axis molecule in a ring by NMR techniques. [2]Rotaxanes 2 and 3 were composed of α-CD derivatives (2:  6-phenyl-amide-α-CD; 3:  6-stilbene-amide-α-CD). The protons of α-CDs in rotaxanes were thoroughly assigned by the two-dimensional NMR
techniques (TOCSY, COSY, ROESY, HMQC, and HMBC). The protons of α-CD in rotaxane 1 did not
show splitting, whereas the resonance peak shifts and splitting for the corresponding protons of α-CD
derivatives in rotaxanes 2 and 3 were observed by the shielding and deshielding effects from a
diphenylacetylene axis molecule. The splitting of resonance peaks was closely related to the rotary
movements of α-CDs and an axis molecule. We supposed that α-CD in rotaxane 1 rotates freely around
a diphenylacetylene axis molecule, and vice versa, whereas the rotary movement of α-CD derivatives
and the axis molecules of rotaxanes 2 and 3 were restricted by the steric repulsion between the substituent
group of α-CD and the stopper group of an axis molecule. To estimate the relative rotary movement of
CDs and an axis molecule in rotaxanes, the rotational correlation time (τc) of rotaxanes was measured by
13C NMR. The results indicate that the corresponding rotary movement of the modified α-CD and the
axis molecules in rotaxanes 2 and 3 depends on the size of the substituent group.