Mechanochemical Enhancement of the Structural Stability of Pseudorotaxane Intermediates in the Synthesis of Rotaxanes

Diastereoselective Polypseudorotaxane Formation with Planar Chiral Pillar[5]arenes via Co-crystallization Processes

As the number of chiral ring molecules in chiral polyrotaxane increases, the number of possible stereoisomers exponentially increases. Consequently, the selective synthesis of a specific stereoisomer becomes much more challenging. To address this problem, we co-crystallized poly(ethylene glycol) and a diastereomeric ring molecule, pillar[5]arene, in the solid state. The co-crystallization formed polypseudorotaxanes with a high diastereomeric excess (ca. 88 % de), meaning that polypseudorotaxanes containing (S, pS) stereoisomer pillar[5]arene rings were synthesized selectively. By contrast, in solution and evaporation systems, the selectivity remained low (ca. 10 % de). The results suggested that the packing effect by the co-crystallization contributed to the denser assembly of ring molecules on the polymeric chain, resulting in the diastereoselective formation. High diastereoselectivity was also observed even in higher-molecular-weight poly(ethylene glycol)s. These selectivities arose from the cooperative effects of the ring molecules on the polymeric chain, which were supported by calculating the stabilization energy.

Cucurbit[8]uril-Mediated Supramolecular Heterodimerisation and Photoinduced [2+2] Heterocycloaddition to Generate Unexpected [2]Rotaxanes

In contrast to the self-assembly of homosupramolecules, the self-assembly of heterosupramolecules is more challenging and significant in various fields. Herein, we design and investigate a cucurbit[8]uril-mediated heterodimerisation based on an arene-fluoroarene strategy. Furthermore, the heteroternary complex is found to be able to undergo a photoinduced [2+2] heterocycloaddition, resulting in the formation of an unexpected [2]rotaxane. This work demonstrates a novel supramolecular heterodimerisation system that not only contributes to the development of photoisomerisation systems, but also enriches synthetic methods for mechanically interlocked molecules.

Mechanocatalytic Synthesis of Ammonia by Titanium Dioxide with Bridge-Oxygen Vacancies: Investigating Mechanism from the Experimental and First-Principle Approach

Mechanochemical ammonia (NH3) synthesis is an emerging mild approach derived from nitrogen (N2) gas and hydrogen (H) source. The gas-liquid phase mechanochemical process utilizes water (H2O), rather than conventional hydrogen (H2) gas, as H sources, thus avoiding carbon dioxide (CO2) emission during H2 production. However, ammonia yield is relatively low to meet practical demand due to huge energy barriers of N2 activation and H2O dissociation. Here, six transition metal oxides (TMO) such as titanium dioxide (TiO2), iron(III) oxide (Fe2O3), copper(II) oxide (CuO), niobium(V) oxide(Nb2O5), zinc oxide (ZnO), and copper(I) oxide (Cu2O) are investigated as catalysts in mechanochemical N2 fixation. Among them, TiO2 shows the best mechanocatalytic effect and the optimum reaction rate constant is 3.6-fold higher than the TMO-free process. The theoretical calculations show that N2 molecules prefer to side-on chemisorb on the mechano-induced bridge-oxygen vacancies in the (101) crystal plane of TiO2 catalyst, while H2O molecules can dissociate on the same sites more easily to provide free H atoms, enabling an alternative-way hydrogeneration process of activated N2 molecules to release NH3 eventually. This work highlights the cost-effective TiO2 mechanocatalyst for ammonia synthesis under mild conditions and proposes a defect-engineering-induced mechanocatalytic mechanism to promote N2 activation and H2O dissociation.

Adaptisorption of Nonporous Polymer Crystals

Although our knowledge and understanding of adsorptions in natural and artificial systems has increased dramatically during the past century, adsorption associated with nonporous polymers remains something of a mystery, hampering applications. Here we demonstrate a model system for adaptisorption of nonporous polymers, wherein dative B-N bonds and host-guest binding units act as the kinetic and thermodynamic components, respectively. The coupling of these two components enables nonporous polymer crystals to adsorb molecules from solution and undergo recrystallization as thermodynamically favored crystals. Adaptisorption of nonporous polymer crystals not only extends the types of adsorption in which the sorbate molecules are integrated in a precise and orderly manner in the sorbent systems, but also provides a facile and accurate approach to the construction of polymeric materials with precise architectures and integrated functions.

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.

Diastereoselective Rotaxane Synthesis with Pillar[5]arenes via Co-crystallization and Solid-State Mechanochemical Processes

Chiral rotaxanes have attracted much attention in recent decades for their unique chirality based on their interlocked structures. Thus, selective synthesis methods of chiral rotaxanes have been developed. The introduction of substituents with chiral centers to produce diastereomers is a powerful strategy for the construction of chiral rotaxanes. However, in case of a small energy difference between the diastereomers, diastereoselective synthesis is extremely difficult. Herein, we report a new diastereoselective rotaxane synthesis method using solid-phase diastereoselective [3]pseudorotaxane formation and mechanochemical solid-phase end-capping reactions of the [3]pseudorotaxanes. By co-crystallization of stereodynamic planar chiral pillar[5]arene with stereogenic carbons at both rims and axles with suitable end groups and lengths, the [3]pseudorotaxane with a high diastereomeric excess (ca. 92% de) was generated in the solid state because of higher effective molarity with aid by packing effects and significant energy differences between [3]pseudorotaxane diastereomers. In contrast, the de of the pillar[5]arene was low in solution (ca. 10% de) because of a small energy difference between diastereomers. Subsequent end-capping reactions of the polycrystalline [3]pseudorotaxane with high de in solvent-free conditions successfully yielded rotaxanes while maintaining the high de generated by the co-crystallization.

Pillar[5]arene based water-soluble [3]pseudorotaxane with enhanced fluorescence emission for cell imaging and both type I and II photodynamic cancer therapy

Water-soluble [3]pseudorotaxane with enhanced fluorescence emission was successfully constructed and applied in cell imaging and photodynamic cancer therapy.

Comparison of the Conventional and Mechanochemical Syntheses of Cyclodextrin Derivatives

Many scientists are working hard to find green alternatives to classical synthetic methods. Today, state-of-the-art ultrasonic and grinding techniques already drive the production of organic compounds on an industrial scale. The physicochemical and chemical behavior of cyclodextrins often differs from the typical properties of classic organic compounds and carbohydrates. The usually poor solubility and complexing properties of cyclodextrins can require special techniques. By eliminating or reducing the amount of solvent needed, green alternatives can reform classical synthetic methods, making them attractive for environmentally friendly production and the circular economy. The lack of energy-intensive synthetic and purification steps could transform currently inefficient processes into feasible methods. Mechanochemical reaction mechanisms are generally different from normal solution-chemistry mechanisms. The absence of a solvent and the presence of very high local temperatures for microseconds facilitate the synthesis of cyclodextrin derivatives that are impossible or difficult to produce under classical solution-chemistry conditions. Although mechanochemistry does not provide a general solution to all problems, several good examples show that this new technology can open up efficient synthetic pathways.