Na⁺ Ions Induce the Pirouetting Motion and Catalytic Activity of [2]Rotaxanes

Abstract

We have prepared [2]rotaxanes, the behavior of which as switchable catalysts depends on their pirouetting motion, which can be controlled through the addition and removal of Na ions. At least three sequential on/off cycles of a Michael reaction can be performed in situ when using the NaTFPB/[2.2.2]cryptand reagent pair to switch "on" and "off" the catalytic ability of the [2]rotaxanes.

Alkali metal cations can inhibit non-covalent catalysis

“…This is well seen on the 1 H NMR spectra of hydrolysis that signals positions of product PhOH@CB7 depend on the concentration of cations (ESI Fig. To summarize, we scrutinized the effect of ionic strength on supramolecular catalysis by cucurbit [7]uril macrocycle. The studied reaction was the acid hydrolysis of cationic phenyl acetate derivative.…”

“…Moreover, the irreversibility of the ester hydrolysis in an aqueous solution facilitates data interpretation. As a catalyst, we chose a water-soluble cucurbit [7]uril (CB7). The effectiveness of CB7 depends on the concentration of a substrate-catalyst complex.…”

“…To date, attention has been rather paid to the positive effect of inorganic cations. For example, sodium ions by the complexation with crown ether-based enzyme mimics were shown to promote condensation reaction 7 and facilitate the release of a rhodium catalyst to enable asymmetric dehydrogenation 8 . Similarly, potassium ions combined with crown ether pockets were used to accelerate ester hydrolysis 9 .…”

Interlocking the Catalyst: Thread versus Rotaxane-Mediated Enantiodivergent Michael Addition of Ketones to β-Nitrostyrene

“…Owing to the particular properties that the mechanical bond endows to such systems, they have become ideal candidates to be applied in different fields, remarking their use as catalysts or ligands in metal-catalyzed processes . Switchable rotaxane-based catalysts, able to change their catalytic activity (rate, chemoselective control, activation mode), are available . Also specially selective, rigid and confined interlocked systems have been designed, by which the chemo- or the enantioselectivity of a selected process is controlled and even enhanced due to the presence of the mechanical bond …”

Synthesis of Oxygen-Free [2]Rotaxanes: Recognition of Diarylguanidinium Ions by Tetraazacyclophanes

“…Interlocked molecules, particularly rotaxanes and catenanes, can be useful materials in various applications (e.g., gelation, catalysis, material transport, molecular memory). Although many different types of interlocked molecules have been synthesized over the past two decades, the number of recognition systems upon which they are based has remained limited .…”