Macrocycle Unidirectional Transport Along a Linear Molecule by a Two‐Step Chemical Reaction Sequence.

Chemical systems displaying directional motions are relevant to the operation of artificial molecular machines. Herein we present the functioning of a molecule capable of transporting a cyclic species in a preferential direction. Our system is based on a linear, non‐symmetric, positively charged molecule. This cation integrates into its structure two different reactive regions. On one side features a bulky ester group that can be exchanged by a smaller substituent; the other extreme contains an acid/base responsive moiety that plays a dual role, as part of the recognition motif and as a terminal group. In the acidic state, a dibenzo‐24‐crown‐8 ether slides into the linear component attracted by the positively charged recognition site. It does this selectively through the extreme that contains the <italic>azepanium</italic> group, since the other side is sterically hindered. After base addition, intermolecular interactions are lost; however, the macrocycle is unable to escape from the linear component since the energy barrier to slide over the neutral <italic>azepane</italic> is too large. Therefore, a metastable mechanically interlocked molecule is formed. A second reaction, now on the ester functionality, exchanges the bulky mesityl for a methyl group, small enough to allow macrocycle dissociation, completing the directional transit of the ring along the track. [ABSTRACT FROM AUTHOR]