The synthesis of a novel pH-sensitive hetero[4]rotaxane molecular machine through a self-sorting strategy is reported. The original tetra-interlocked molecular architecture combines a [c2]daisy chain scaffold linked to two [2]rotaxane units. Actuation of the system through pH variation is possible thanks to the specific interactions of the dibenzo-24-crown-8 (DB24C8) macrocycles for ammonium, anilinium, and triazolium molecular stations. Selective deprotonation of the anilinium moieties triggers shuttling of the unsubstituted DB24C8 along the [2]rotaxane units.
Interlocked molecular machines like [2]rotaxanes are intriguing aesthetic molecules. The control of the localization of the macrocycle, which surrounds a molecular axle, along the thread leads to translational isomers of very different properties. Although many moieties have been used as sites of interactions for crown ethers, the very straightforwardly obtained amide motif has more rarely been envisaged as molecular station. In this article, we report the use of secondary and tertiary amide moieties as efficient secondary molecular station in pH-sensitive molecular shuttles. Depending on the N-substitution of the amide station, and on deprotonation or deprotonation-carbamoylation, the actuation of the molecular machinery differs accordingly to very distinct interactions between the axle and the DB24C8.
The efficient synthesis and very easy isolation of dibenzo[24]crown-8-based [2]pseudorotaxane building blocks that contain an active ester motif at the extremity of the encircled molecular axle and an ammonium moiety as a template for the dibenzo[24]crown-8 is reported. The active ester acts both as a semistopper for the [2]pseudorotaxane species and as an extensible extremity. Among the various investigated active ester moieties, those that allow for the slippage process are given particular focus because this strategy produces fewer side products. Extension of the selected N-hydroxysuccinimide ester based pseudorotaxane building block by using either a mono- or a diamino compound, both containing a triazolium moiety, is also described. These provide a pH-dependent two-station [2]rotaxane molecular machine and a palindromic [3]rotaxane molecular machine, respectively. Molecular machinery on both interlocked compounds through variation of pH was studied and characterized by means of NMR spectroscopy.
This work reports on the use of molecular translocators to capture a dibenzo-24-crown-8 (DB24C8) and then release it onto targeted molecular axles to afford, after removal of the translocator, [2]rotaxanes that do not hold any template site. Various translocators were studied and successfully aided the synthesis, with more or less efficacy, of [2]rotaxanes of different lengths. During the releasing step, the DB24C8 macrocycle shuttles along the thread, and the localization of the macrocycle might be driven by steric repulsion on the translocator part and/or electronic attraction of the targeted part of the axle to be encircled, which depends on both the nature of the translocator and the targeted thread to be encircled.
A straightforward slippage strategy has been used for the synthesis of three [2]rotaxane building blocks that all contain an ammonium template for the dibenzo-24-crown-8 macrocycle and an N-hydroxysuccinimide end. The kinetic rate of the slipping-on process proved to be highly dependent on both the length and flexibility of the thread.
Very efficient slipping-on of the dibenzo-24-crown-8 over the NHS end of an ammonium-containing molecular axle was carried out through a solvent-less procedure.
The reverse anomeric effect (RAE) was investigated in different mannosyl [2]rotaxane molecular shuttle isomers that contain dibenzo-24-crown-8 (DB24C8) as the macrocycle, and anilinium and pyridinium amide as molecular stations. The switching on or off of the RAE was possible depending on both the pyridinium amide motif and the localization of the DB24C8 along the thread. The (1) C4 mannopyranosyl chair-like conformation was observed in all the non-interlocked molecules because the anomeric carbon of the mannose is linked to the positively charged nitrogen of the pyridinium unit. In the protonated rotaxanes, the (1) C4 chair conformation of the mannose end remains because the DB24C8 resides around the best anilinium station, which is located at the other end of the axle. Upon deprotonation of the anilinium, the DB24C8 shuttles with a large-amplitude motion toward the pyridinium amide stations, where it interacts in a different fashion depending on the pyridinium motif. In one molecular shuttle, the RAE could be switched on or off with control at one end of the encircled thread upon protonation/deprotonation of the other end, through shuttling of the DB24C8.
Molecular muscles: The self‐sorting strategy between a naked dibenzo‐24‐crown‐8 (DB24C8), a DB24C8‐based hermaphrodite molecule, and a molecular axle that contains both an ammonium or an anilinium template allows for the efficient synthesis of a hetero[4]rotaxane molecular machine. Deprotonation of the anilinium stations allows shuttling of the DB24C8 units along the axle, which is linked to the daisy scaffold (see figure). More information can be found in the Full Paper by F. Coutrot et al. on page 6837 ff.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.