Regulation of Macrocycle Shuttling Rates in [2]Rotaxanes by Amino‐Acid Speed Bumps in Organic–Aqueous Solvent Mixtures

Abstract

A homologous series of two-station [2]rotaxanes incorporating amino-acid units in the molecular thread has been developed. The degenerate [2]rotaxanes exhibit amino-acid specific shuttling rates between two fumaric stations related to the steric factor associated to the amino-acid side chain, as [a

Macrocyclic Hamilton receptor-shuttling dynamics in [2]rotaxanes

“…[1] This allowed determining the key role of ring movement in molecular machines and obtaining detailed kinetic analyses providing key structure-property information in a number of different molecular machines, [1,[5][6][7] including non-degenerate molecular shuttles used as switches in response to external stimulus, [2,8,9] and degenerate rotaxanes exhibiting spontaneous ring motion. [5,[10][11][12][13][14][15][16] Relevant works include Stoddart molecular shuttles [1,[17][18][19], hydrogen-bonded molecular machines by Leigh,[3,20,21] Loeb's ring-through ring shuttling rotaxanes and related systems, [2,22] Hirose shuttling molecular machine studies on ring size and axle length, [5,23], Coutrot's pioneering studies on shuttling effects in rotaxanes, [24] and Brouwer's shuttling dynamics studies showing the complexity of that macrocycle shuttling events. [6] Among the different rotaxane formation methods, the Huisgen 1,3-dipolar cycloaddition involving reaction of alkyne and azide groups, also known as CuAAC "click" chemistry, has been extensively used as an efficient tool for the synthesis of interlocked molecules.…”

“…( 1 H,13 C)-HMBC NMR of rotaxane (600 MHz in CD 2 Cl 2 ).S8Figure S6. 1 H-NMR of thread 2 (600 MHz in TCE-d 2 ).…”

Controlling the Molecular Shuttling of pH‐Responsive [2]Rotaxanes with Two Different Stations

“…In this system, the macrocycle can move back and forth between the recognition sites along the axle [1,4–9] . Starting with the pioneering work of Stoddart, [10] numerous studies have been conducted on degenerate molecular shuttles, i. e., shuttles that contain identical recognition sites [11–21] . Furthermore, the molecular motion of rotaxanes, which are mechanically interlocked and kinetically stable, can be controlled using multiple binding sites (stations) with different affinity toward the macrocycle [22–35] .…”

[2]Rotaxanes featuring minimal helical axles

“…By far the most widely employed axles are aliphatic, typically hydrocarbon or ethylene glycol oligomers, rendering the resultant structures highly flexible. 12,16,17,18 A few reports, consider more structurally-elaborate rigid axles, offering a more well-defined ring trajectory along the axle. 19,20,21 The nature of the rigid linker is typically aromatic 20,22 or acetylenic, 21 which can necessitate multi-step synthesis and/or introduce π-conjugated systems which may be redox active and introduce low-lying excited states which may, for instance, result in unwanted rotaxane luminescence quenching.…”

“…7 Controlling the movement and dynamics of a macrocycle along a thread are the key feature on the development of interlocked-based molecular machines. During the last years, examples of directional threading, 8,9 energy ratchets 10,11 and speed bumps 12 are different approaches to control the movement of a macrocycle within an interlocked structure. Previous studies show a dependence on the number of stations and macrocycles, 13 in the case of [2]rotaxanes with more than one station, the macrocycle is preferentially positioned at the highest affinity station, 14 if the affinity between both stations is the same, the macrocycle will be in constant movement between the stations.…”