Pseudorotaxane orientational stereoisomerism driven by π-electron density

Ether Naphthotube Host-Guest Complexes and [2]Rotaxanes with Dications

The interactions between ether naphthotube and a series of dication guests in organic solution were investigated. It was found that ether naphthotube formed stable host-guest complexes selectively with these guests in a 1 : 1 stoichiometric ratio with association constants ranging from 102 to 106 M-1, which were confirmed by 1H-NMR spectra and ITC experiments. The host-guest interactions are driven by enthalpy change as the entropic factors are unfavorable. Positive correlations between ΔH and ΔS have been observed in the host-guest complexes. Furthermore, the para-substitution of the guests can significantly affect the binding affinities through a combination of field/inductive and resonance effects by following a linear free energy relationship. Based on the host-guest complexes composed of ether naphthotube and organic cations, two interlocked [2]rotaxanes were prepared by cationization reaction and Huisgen cycloaddition between the cations and the stopper components. The ether naphthotube-based host-guest complexes are useful for creating sophisticated interlocked molecules.

"Rim-Differentiated" Pillar[6]arenes

A "rim-differentiated" pillar[6]arene (RD-P[6]) was obtained successfully, with the assistance of a dimeric silver trifluoroacetate template, among eight different constitutional isomers in a direct and regioselective manner. The solid-state conformation of this macrocycle could switch from the 1,3,5-alternate to a truly rim-differentiated one upon guest inclusion. This highly symmetric RD-P[6] not only hosts metal-containing molecules inside its cavity, but also can form a pillar[6]arene-C₆₀ adduct through co-crystallization on account of donor-acceptor interactions. The development of synthetic strategies to desymmetrize pillararenes offers new opportunities for engineering complex molecular architectures and organic electronic materials.

Molecular Recognition in an Aqueous Medium Using Water-Soluble Prismarene Hosts

The synthesis of water-soluble prism[n]arenes (n = 5 and 6) bearing anionic carboxylato groups on the rims is described. The binding properties of this novel class of water-soluble hosts are studied by nuclear magnetic resonance and calorimetry. The complexation of singly and doubly charged ammonium guests with the more rigid pentamer is enthalpically driven by secondary interactions, while the thermodynamic fingerprint for the larger hexamer reveals driving forces that strongly depend on the guest charge and/or size.

Chirality Transfer in a Calixarene-Based Directional Pseudorotaxane Complex

Hexamethoxycalix[6]arene 3 forms a directional pseudorotaxane complex with the chiral axle (S)-(α-methyl-benzyl)benzylammonium 2+. Between the two (endo-chiral)-2+@3 and (exo-chiral)-2+@3 pseudorotaxane stereoisomers, the former is preferentially formed. This result confirms the validity of the “endo-α-methyl-benzyl rule”, previously reported by us. DFT calculations suggest that C-H … π interactions between the methyl group of 2+ and the calixarene aromatic rings, determine the stereoselectivity of the threading process toward the “endo-α-methyl-benzyl preference”. An amplification of optical rotation is observed upon formation of the pseudorotaxane complex (endo-chiral)-2+@3 with respect to free axle 2+. Thus, the specifical rotation of the 1:1 mixture of chiral 2+·B(ArF)4− salt and achiral 3 was augmented upon formation of the pseudorotaxane and DFT calculations were used to rationalize this result.

Chromogenic Properties of p-Pyridinium- and p-Viologen-Calixarenes and Their Cation-Sensing Abilities

The synthesis of calix[4]- and -[6]arene derivatives P6(H)₂²⁺·(Cl^–)₂, V4(H)₂⁴⁺·(Cl^–)₂·(I^–)₂, and V6(H)₂⁴⁺·(Cl^–)₂·(I^–)₂ bearing N-linked pyridinium (P) and viologen (V) units at the upper rim is described here. A rare example of an anionic conformational template is reported for p-pyridiniumcalix[6]arene P6(H)₂²⁺, which adopts a 1,3,5-alternate conformation in the presence of chloride anions. Derivatives P6(H)₂²⁺·(Cl^–)₂, V6(H)₂⁴⁺·(Cl^–)₂·(I^–)₂, and V4(H)₂⁴⁺·(Cl^–)₂·(I^–)₂ show a negative solvatochromism, while their UV–vis acid–base titration evidenced that upon addition of a base, new bands appear at 487, 583, and 686 nm, respectively, due to the formation of betainic monodeprotonated species P6(H)₁⁺, V6(H)₁³⁺, and V4(H)₁³⁺. These new bands were attributable to the intramolecular charge-transfer (CT) transition from the phenoxide to the pyridinium or viologen moiety and were responsive to the presence of cations. In fact, the band at 487 nm of P6(H)₁⁺ was quenched in the presence of a hard Li⁺ cation, and the color of its acetonitrile solution was changed from pink to colorless upon addition of LiI. Consequently, this derivative can be considered as a useful host for the recognition and sensing of lithium cations.

An intramolecularly self-templated synthesis of macrocycles: self-filling effects on the formation of prismarenes

Ethyl- and propyl-prism[6]arenes are obtained in high yields and in short reaction times, independent of the nature and size of the solvent, in the cyclization of 2,6-dialkoxynaphthalene with paraformaldehyde. PrS[6]Et or PrS[6]nPr adopt, both in solution and in the solid state, a folded cuboid-shaped conformation, in which four inward oriented alkyl chains fill the cavity of the macrocycle. On these bases, we proposed that the cyclization of PrS[6]Et or PrS[6]nPr occurs through an intramolecular thermodynamic self-templating effect. In other words, the self-filling of the internal cavity of PrS[6]Et or PrS[6]nPr stabilizes their cuboid structure, driving the equilibrium toward their formation. Molecular recognition studies, both in solution and in the solid state, show that the introduction of guests into the macrocycle cavity forces the cuboid scaffold to open, through an induced-fit mechanism. An analogous conformational change from a closed to an open state occurs during the endo-cavity complexation process of the pentamer, PrS[5]. These results represent a rare example of a thermodynamically controlled cyclization process driven through an intramolecular self-template effect, which could be exploited in the synthesis of novel macrocycles.

Expanding Coefficient: A Parameter To Assess the Stability of Induced-Fit Complexes

Here we propose a new parameter, the Expanding Coefficient (EC), that can be correlated with the thermodynamic stability of supramolecular complexes governed by weak secondary interactions and obeying the induced-fit model. The EC values show a good linear relationship with the log Kapp of the respective pseudorotaxane complexes investigated. According to Cram's Principle of Preorganization, the EC can be considered an approximate mechanical measure of the host's reorganization energy cost upon adopting the final bound geometry.

Heteroditopic Calix[6]arene Based Intervowen and Interlocked Molecular Devices

Since the dawn of supramolecular chemistry, calixarenes have been employed as platforms onto which functional groups and binding sites can be loaded in a regio- and stereocontrolled manner for the recognition of charged and neutral species. Despite their wider annulus, potentially suitable to bind larger guests, the larger members of the calixarene series have been relatively less employed, mainly because of the synthetic difficulties to control their conformational flexibility and their regioselective functionalization. In this account, we will present the achievements gained during the last two decades on the use of the calix[6]arene as a platform to build-up structures in which the macrocycle acts as a wheel for the synthesis of oriented (pseudo)rotaxanes. We also account on how these calix[6]arene hosts affect the reactivity or spectroscopic properties of their bound guests.

Orientation of the α-CD component of [2]rotaxanes affects their specific molecular recognition behaviour

An α-CD component enhanced the anion recognition ability of the urea moiety and the deprotonation of the phenol moiety in the axle component in orientationally isomeric [2]rotaxanes with the OH groups on the wide rim of the α-CD, respectively.

Kinetic and Thermodynamic Behaviors of Pseudorotaxane Formation with C3v Macrocyclic BODIPY Trimers and the Remarkable Substituent Effect on Ring-Face Selectivity

The thermodynamic and kinetic behaviors of the pseudorotaxane formation between the C_3v macrocyclic BODIPY trimers and unsymmetrical secondary ammonium guests are investigated. We find a remarkable substituent effect of the BODIPY trimer on the ring-face selectivity during the threading. The difference in the small substituents (H or CH₃) in the macrocyclic host molecules significantly modulated the thermodynamic and kinetic selectivity of the threading direction of the unsymmetrical ammonium ions.

Influence of exo-Adamantyl Groups and endo-OH Functions on the Threading of Calix[6]arene Macrocycle

Calix[6]arenes bearing adamantyl groups at the exo-rim form pseudorotaxanes with dialkylammonium axles paired to the weakly coordinating [B(Ar^F)₄]⁻ anion. The exo-adamantyl groups give rise to a more efficient threading with respect to the exo-tert-butyl ones, leading to apparent association constants more than one order of magnitude higher. This improved stability has been ascribed to the more favorable van der Waals interactions of exo-adamantyls versus exo-tert-butyls with the cationic axle. Calix[6]arenes bearing endo-OH functions give rise to a less efficient threading with respect to the endo-OR ones, in line with what was known from the complexation of alkali metal cations.

Ion-Pair Selective Conformational Rearrangement of Sulfonamide Calix[6]arene-Based Pseudorotaxanes

We describe the synthesis of a new class of trisulfonamide calix[6]arene-based wheels that can bind dialkylviologen salts, in apolar media. The threading process occurs through a selective ion-pair recognition, established by the sulfonamide groups with the counterions of the bipyridinium salts, that dictates a conformational rearrangement of the corresponding pseudorotaxanes.

Exploring and Exploiting the Symmetry-Breaking Effect of Cyclodextrins in Mechanomolecules

Cyclodextrins (CDs) are cone-shaped molecular rings that have been widely employed in supramolecular/host–guest chemistry because of their low cost, high biocompatibility, stability, wide availability in multiple sizes, and their promiscuity for binding a range of molecular guests in water. Consequently, CD-based host–guest complexes are often employed as templates for the synthesis of mechanically bonded molecules (mechanomolecules) such as catenanes, rotaxanes, and polyrotaxanes in particular. The conical shape and cyclodirectionality of the CD “bead” gives rise to a symmetry-breaking effect when it is threaded onto a molecular “string”; even symmetrical guests are rendered asymmetric by the presence of an encircling CD host. This review focuses on the stereochemical implications of this symmetry-breaking effect in mechanomolecules, including orientational isomerism, mechanically planar chirality, and topological chirality, as well as how they support applications in regioselective and stereoselective chemical synthesis, the design of molecular machine prototypes, and the development of advanced materials.

Threading of Conformationally Stable Calix[6]arene Wheels Substituted at the Methylene Bridges

Calix[6]arenes disubstituted at the methylene bridges, which are stable in the cone or 1,2,3-alternate conformation, form pseudorotaxanes with dialkylammonium axles. The cone wheel-based pseudorotaxanes are 10-100 times more stable than those obtained with the native conformationally mobile calix[6]arene wheel, as a consequence of their higher degree of preorganization. The threading of conformationally stable 1,2,3-alternate calix[6]arenes is unprecedented in the literature. Therefore, very peculiar NMR features are here evidenced for this threading process involving the less symmetrical 1,2,3-alternate calix[6]arene conformation, which implies a peculiar rototranslation motion of the axle.

Threading of an Inherently Directional Calixarene Wheel with Oriented Ammonium Axles

The threading of monostoppered alkylbenzylammonium axles 7⁺ and 8⁺ with the calix[6]-wheel 3 can occur by both routes of entering the macrocycle 3 in the cone conformation: passage through the upper rim and the through the lower rim. Thus, under thermodynamic conditions, with both the axles 7⁺ and 8⁺, the two possible orientations of calix[2]pseudorotaxane, namely, endo-benzyl and endo-alkyl, are formed by a stereoselectivity controlled by the endo-alkyl rule. Interestingly, by ¹H NMR monitoring of the threading process between 8⁺ and 3, we revealed two calix[2]pseudorotaxane isomers in which the calix-wheel adopts 1,2,3-alternate and cone conformations, which represent the kinetic and thermodynamic species, respectively. Finally, the synthesis of ammonium-based oriented calix[2]rotaxane is here described.

Facile and efficient formation and dissociation of a pseudo[2]rotaxane by a slippage approach using pillar[5]arene-based cyclic host liquid and solvent

A pillar[5]arene-based [2]rotaxane was synthesized by heating an axle with valine ends in a cyclic host liquid.

Guest-dependent directional complexation based on triptycene derived oxacalixarene: formation of oriented rotaxanes

Structural changes in the guest cause inversion of the dominant threading direction in triptycene derived oxacalixarene with different semi-cavities.

The manipulation of supramolecular devices to carry out sophisticated and programmed tasks is bound up with the spatial allocation of their components, especially the threading direction of the guest, which controls the host–guest orientation in the device. However, insights are needed to probe more possibilities for steering the threading direction. We have developed a new system consisting of a three-dimensional nonsymmetric oxacalixarene (H) with a fixed comformation and (bi)pyridinium salts (G1–G3), in which we found that based on the intrinsic discrepancies between the two semi-cavities of H, the electron densities of the axles greatly affect the threading direction. This was unequivocally demonstrated by NMR spectra and single crystal structures. With elaborate design, unidirectional threading was achieved, resulting in an oriented rotaxane. Therefore, we describe a new approach in which the threading direction and final orientation may be finely controlled by adjustment of the structure of the guest.

Nucleophilic Functionalization of the Calix[6]arene Para- and Meta-Position via p-Bromodienone Route

It is here demonstrated that the p-bromodienone route, previously reported for calix[4]arenes, is also effective for the functionalization of the calix[6]arene macrocycle. Thus, alcoholic O-nucleophiles can be introduced at the calix[6]arene exo rim. In addition, the reaction of a calix[6]arene p-bromodienone derivative with an actived aromatic substrate, such as resorcinol, led to the first example of a meta-functionalized, inherently chiral calix[6]arene derivative.