Clicked interlocked molecules

The Cu-I-catalyzed Huisgen 1,3-dipolar cycloaddition, popularized as "click chemistry," is one of the latest acquisitions to the synthetic arsenal for the making of mechanically interlocked molecular compounds. The high efficiency and functional group tolerance of "click chemistry" allows this reaction to be employed at the stoppering step in the "threading-followed-by-stoppering" sequence that produces rotaxanes, and in the macrocyclization step that affords catenanes. The use of this kind of "click chemistry" alleviates some of the drawbacks associated with previous approaches to the template-directed synthesis of mechanically interlocked molecular compounds-approaches such as "clipping; "-and opens up the way to more exotic mechanically interlocked molecules. Employing this new approach, [2]-, [3]-, and [4]rotaxanes and [2]catenanes have all been prepared in a convergent and efficient manner. Their template-directed synthesis relies, in the beginning, on the formation of [n]pseudorotaxanes, which can either (i) be stoppered, affording [n]rotaxanes, or (ii) induced to undergo an intramolecular cyclization, affording [2]catenanes. This new synthetic approach has also been employed in the template-directed synthesis of a self-complexing system as well as a two-station, degenerate [2]rotaxane (molecular shuttle) which was used to measure the energy barrier (15 kcal mol(-1)) to shuttling by a cyclobis(paraquat-p-phenylene) ring between two identical 1,5-dioxynaphthalene (DNP) stations across a 1,2,3-triazole unit. It has also been found that the Cu-I-salt, employed in the click reaction between an azide and an alkyne, can act as both a catalyst and a template in the formation of [2]rotaxanes. The "click" approach has also been used in the formation of a bistable [2]rotaxane containing a DNP station and a tetrathiafulvalene one. Electrochemical experiments performed on this compound indicate that the presence of the triazole unit does not influence the kinetics or thermodynamics of the redox-controlled switching process. Finally, the synthesis of a liquid-crystalline bistable [2]rotaxane, which shows smectic A phase behavior over a wide range of temperatures, is described.