Transamidation-Driven Molecular Pumps

Abstract

We report a new class of synthetic molecular pumps that use a stepwise information ratchet mechanism to achieve the kinetic gating required to sequester their macrocyclic substrates from bulk solution. Threading occurs as a result of active template reactions between the pump terminus amine and an acyl electrophile, whereby the bond-forming reaction is accelerated through the cavity of a crown ether. Carboxylation of the resulting amide results in displacement of the ring to the collection region of the thread. Conversion of the carbamate to a phenolic ester provides an intermediate rotaxane suitable for further pumping cycles. In this way rings can be ratcheted onto a thread from one or both ends of appropriately designed molecular pumps. Each pumping cycle results in one additional ring being added to the thread per terminus acyl group. The absence of pseudorotaxane states ensures that no dethreading of intermediates occurs during the pump operation. This facilitates the loading of different macrocycles in any chosen sequence, illustrated by the pump-mediated synthesis of a [4]rotaxane containing three different macrocycles as a single sequence isomer. A [5]rotaxane synthesized using a dual-opening transamidation pump was structurally characterized by single-crystal X-ray diffraction, revealing a series of stabilizing CH···O interactions between the crown ethers and the polyethylene glycol catchment region of the thread.

The Role of Dethreading Process in Pseudorotaxanes and Rotaxanes towards Advanced Applications: Recent Examples

“…Rotaxane-based molecular pumps are artificial molecular machines which operate through the transport of macrocycles from bulk solution to a linear component upon the application of external stimuli and the other way around, thus pumping macrocycles in an out-of-equilibrium manner. [69][70][71][72][73] An early example of a pseudorotaxane operating as a molecular pump was reported by Credi and colleagues in 2014, working through the application of light stimuli. [74] In the initial state, dinaphthocrown ether macrocycle 11 threaded a linear component having an azobenzene unit with E-configuration and an ammonium binding site (E-12) in a directional manner, affording pseudorotaxane 13 (Figure 5).…”

“…Rotaxane‐based molecular pumps are artificial molecular machines which operate through the transport of macrocycles from bulk solution to a linear component upon the application of external stimuli and the other way around, thus pumping macrocycles in an out‐of‐equilibrium manner [69–73] …”

Sequence‐controlled synthesis of rotaxanes

“…With the rise of artificial molecular pumping technology, [130][131][132][133][135][136][137] the range of accessible polyrotaxanes structures has already been expanded extensively. A quantitative ring-threading process affords polyrotaxanes with high concentrations of rings on the collecting chains.…”

Out of Equilibrium Chemical Systems Fueled by Trichloroacetic Acid

“…The complexity of the different functions obtained through such technologies enables chemists to design chemical systems close to Nature’s biomachinery or alternatives possessing purely artificial properties. These processes have been applied, for example, to trigger the motion in different molecular machines, to design switchable catalysts, or for temporary self-assemblies in smart materials. − …”