Molecular Shuttles by the Protecting Group Approach

Abstract

Two new [2]rotaxane-based molecular shuttles, in which a mechanically bound dibenzo[24]crown-8 (DB24C8) macroring shunts back and forth between two dialkylammonium recognition sites situated on a chemical dumbbell, have been constructed by a novel synthetic strategy that relies upon the use of the tert-butoxycarbonyl (Boc) protecting group. During the syntheses of both molecular shuttles, this protecting group masks a dialkylammonium recognition center which is liberated only after the [2]rotaxane constitution is established. In both cases, the molecular shuttles' other dialkylammonium center is essential for the rotaxane-forming reactions and it ensures that DB24C8 is interpenetrated by threadlike precursors, as a result of noncovalent bonding interactions, to produce [2]pseudorotaxanes which are stoppered subsequently through 1,3-dipolar cycloadditions between azides and bulky acetylenedicarboxylates. The new molecular shuttles have been examined by means of dynamic 1H NMR spectroscopy, which reveals that the movements of the DB24C8 macroring are very highly dependent both on solvent properties and on the nature of the spacer unit linking the two dialkylammonium centers. Thus, DB24C8 shunts facilely between the dialkylammonium centers when the shuttles are dissolved in solvents that readily donate their nonbonding electrons into noncovalent bonds, e.g., DMF, and when spacer units that do not offer much steric resistance to shuttling, e.g., hexamethylene, are used. On the other hand, shuttling is difficult in solvents that are less inclined to donate their electrons into noncovalent bonds, e.g., (CDCl2)2, and when relatively bulky benzenoid spacer units, e.g., p-xylylene, link the two dialkylammonium centers. It has been proposed that the DB24C8 might act as a "ferry" which carries a proton between dialkylammonium and dialkylamine moieties in a singly protonated [2]rotaxane by means of ion-dipole interactions.

Remote electrochemical modulation of pK a in a rotaxane by co-conformational allostery

“…Such a phenomenon, that can be rationalized with both thermodynamic and kinetic arguments, is in line with earlier observations (22)(23)(24)(25)(26) that deprotonation of ammonium axles in crown etherbased rotaxanes is extremely difficult to achieve. We also found that the decrease of acid strength is mitigated in the presence of an alternative recognition site for the ring.…”

Shuttling Dynamics in an Acid–Base‐Switchable [2]Rotaxane

“…The activation barriers DG°for both shuttling processes are similar to those observed for related rotaxanes such as those composed [25] of DB24C8 and a dumbbell whose main station is ÀNH 2 + À, and to that determined [26] in the case of a dumbbell with two identical stations threaded into a bis(p-phenylene) [34]crown-10. In the case of dumbbell components containing asymmetrically substituted monopyrrolotetrathiafulvalene (MPTTF) and 1,5-dioxynaphthalene (DNP) recognition sites for encirclement by cyclobis(paraquat-p-phenylene) (CBPQT 4 + ), the measured activation energies are comparable to those obtained in the present case, or higher if the dumbbell contains a "speed-bump" in the form of a thiomethyl group (SMe), for example, situated between the two stations.…”

A Rational Design for the Directed Helicity Change of Polyacetylene Using Dynamic Rotaxane Mobility by Means of Through‐Space Chirality Transfer

“…The balanced helix sense around the main chains can be explained by assuming that the optically active wheel components are located at such a distance from the main chain that they do not influence the main-chain conformation, because a strong hydrogen-bonding interaction operates between the components of the rotaxane moiety to control the mobility of the wheel. [15] This is consistent with our initial assumption described in Scheme 1.…”