Solution and Solvent-Free Stopper Exchange Reactions for the Preparation of Pillar[5]arene-containing [2] and [3]Rotaxanes

Diamine reagents have been used to functionalize a [2]rotaxane building block bearing an activated pentafluorophenyl ester stopper. Upon a first acylation, an intermediate host-guest complex with a terminal amine function is obtained. Dissociation of the intermediate occurs in solution and acylation of the released axle generates a [2]rotaxane with an elongated axle subunit. In contrast, the corresponding [3]rotaxane can be obtained if the reaction conditions are appropriate to stabilize the inclusion complex of the mono-amine intermediate and the pillar[5]arene. This is the case when the stopper exchange is performed under mechanochemical solvent-free conditions. Alternatively, if the newly introduced terminal amide group is large enough to prevent the dissociation, the second acylation provides exclusively a [3]rotaxane. On the other hand, detailed conformational analysis has been also carried out by variable temperature NMR investigations. A complete understanding of the shuttling motions of the pillar[5]arene subunit along the axles of the rotaxanes reported therein has been achieved with the help of density functional theory calculations.

Solution and Solvent-Free Stopper Exchange Reactions for the Preparation of Pillar[5]arene-Containing [2] and [3]Rotaxanes

Invited for the cover of this issue are the group of Iwona and Jean-François Nierengarten from the University of Strasbourg (LIMA, UMR 7042, CNRS) and collaborators from the University of Carthage and the IPHC (University of Strasbourg and CNRS, UMR 7178). The image illustrates the fast motions of a pillar[5]arene subunit along the axle of a rotaxane, reminiscent of those of a guitarist's hand along the neck allowing him to use random parts of a scale with certain sweet spots when improvising a solo. Read the full text of the article at 10.1002/chem.202304131.

Stepwise Functionalization of a Pillar[5]arene-Containing [2]Rotaxane with Pentafluorophenyl Ester Stoppers

Detailed investigations into the stepwise bis-functionalization of a pillar[5]arene-containing rotaxane building block have been carried out. Upon a first stopper exchange, the pillar[5]arene moiety of the mono-acylated product is preferentially located close to its reactive pentafluorophenyl ester stopper, thus limiting the accessibility to the reactive carbonyl group by the nucleophilic reagents. Selective mono-functionalization is thus very efficient. Introduction of a second stopper is then possible to generate dissymmetrical rotaxanes with different amide stoppers. Moreover, when dethreading is possible upon the second acylation, the pillar[5]arene plays the role of a protecting group allowing the synthesis of dissymmetrical axles that are particularly difficult to prepare under statistical conditions. Finally, detailed conformation analysis of the rotaxanes revealed that the position of the pillar[5]arene moiety on its axle subunit is mainly governed by polar interactions in nonpolar organic solvents, whereas solvophobic effects play a major role in polar solvents.

Stepwise Functionalization of a Pillar[5]arene-Containing [2]Rotaxane with Pentafluorophenyl Ester Stoppers

Invited for the cover of this issue are the group of Iwona and Jean-François Nierengarten from the University of Strasbourg (LIMA, UMR 7042, CNRS) and collaborators from the University of Carthage. The image depicts a sky of umbrellas floating in the air to illustrate the protective effect allowing the efficient mono-acylation of a symmetrical rotaxane building block with two pentafluorophenyl ester stoppers. Read the full text of the article at 10.1002/chem.2023003501.