Anion-π Catalysis Enabled by the Mechanical Bond

Facial Selectivity in Mechanical Bond Formation: Axially Chiral Enantiomers and Geometric Isomers from a Simple Prochiral Macrocycle

In 1971, Schill recognized that a prochiral macrocycle encircling an oriented axle led to geometric isomerism in rotaxanes. More recently, we identified an overlooked chiral stereogenic unit in rotaxanes that arises when a prochiral macrocycle encircles a prochiral axle. Here, we show that both stereogenic units can be accessed using equivalent strategies, with a single weak stereodifferentiating interaction sufficient for moderate to excellent stereoselectivity. Using this understanding, we demonstrated the first direct enantioselective (70% ee) synthesis of a mechanically axially chiral rotaxane.

Mutual induced fit transition structure stabilization of corannulene's bowl-to-bowl inversion in a perylene bisimide cyclophane

Corannulene is known to undergo a fast bowl-to-bowl inversion at r.t. via a planar transition structure (TS). Herein we present the catalysis of this process within a perylene bisimide (PBI) cyclophane composed of chirally twisted, non-planar chromophores, linked by para-xylylene spacers. Variable temperature NMR studies reveal that the bowl-to-bowl inversion is significantly accelerated within the cyclophane template despite the structural non-complementarity between the binding site of the host and the TS of the guest. The observed acceleration corresponds to a decrease in the bowl-to-bowl inversion barrier of 11.6 kJ mol-1 compared to the uncatalyzed process. Comparative binding studies for corannulene (20 π-electrons) and other planar polycyclic aromatic hydrocarbons (PAHs) with 14 to 24 π-electrons were applied to rationalize this barrier reduction. They revealed high binding constants that reach, in tetrachloromethane as a solvent, the picomolar range for the largest guest coronene. Computational models corroborate these experimental results and suggest that both TS stabilization and ground state destabilization contribute to the observed catalytic effect. Hereby, we find a "mutual induced fit" between host and guest in the TS complex, such that mutual geometric adaptation of the energetically favored planar TS and curved π-systems of the host results in an unprecedented non-planar TS of corannulene. Concomitant partial planarization of the PBI units optimizes noncovalent TS stabilization by π-π stacking interactions. This observation of a "mutual induced fit" in the TS of a host-guest complex was further validated experimentally by single crystal X-ray analysis of a host-guest complex with coronene as a qualitative transition state analogue.

Anion-π catalysis on carbon allotropes

Anion-π catalysis, introduced in 2013, stands for the stabilization of anionic transition states on π-acidic aromatic surfaces. Anion-π catalysis on carbon allotropes is particularly attractive because high polarizability promises access to really strong anion-π interactions. With these expectations, anion-π catalysis on fullerenes has been introduced in 2017, followed by carbon nanotubes in 2019. Consistent with expectations from theory, anion-π catalysis on carbon allotropes generally increases with polarizability. Realized examples reach from enolate addition chemistry to asymmetric Diels-Alder reactions and autocatalytic ether cyclizations. Currently, anion-π catalysis on carbon allotropes gains momentum because the combination with electric-field-assisted catalysis promises transformative impact on organic synthesis.

Tunable Fluorescence and Morphology of Aggregates Built from a Mechanically Bonded Amphiphilic Bistable [2]Rotaxane

Advanced Organic Chemical Materials Co-constructed Mechanically bonded amphiphiles (MBAs), also known as mechanically interlocked molecules (MIMs), have emerged as an important kind of functional building block for the construction of artificial molecular machines and soft materials. Herein, a novel MBA, i. e., bistable [2]rotaxane H2 was designed and synthesized. In the solution state, H2 demonstrated pH and metal ion-responsive emissions due to the presence of a distance-dependent photoinduced electron transfer (PET) process and the fluorescence resonance energy transfer (FRET) process, respectively. Importantly, the amphiphilic feature of H2 has endowed it with unique self-assembly capability, and nanospheres were obtained in a mixed H2 O/CH3 CN solvent. Moreover, the morphology of H2 aggregates can be tuned from nanospheres to vesicles due to the pH-controlled shuttling motion-induced alternation of H2 amphiphilicity. Interestingly, larger spheres with novel pearl-chain-like structures from H2 were observed after adding stoichiometric Zn2+ . In particular, H2 shows pH-responsive emissions in its aggregation state, allowing the visualization of the shuttling movement by just naked eyes. It is assumed that the well-designed [2]rotaxane, and particularly the proposed concept of MBA shown here, will further enrich the families of MIMs, offering prospects for synthesizing more MIMs with novel assembly capabilities and bottom-up building dynamic smart materials with unprecedented functions.

Electric field-assisted anion-π catalysis on carbon nanotubes in electrochemical microfluidic devices

The vision to control the charges migrating during reactions with external electric fields is attractive because of the promise of general catalysis, emergent properties, and programmable devices. Here, we explore this idea with anion-π catalysis, that is the stabilization of anionic transition states on aromatic surfaces. Catalyst activation by polarization of the aromatic system is most effective. This polarization is induced by electric fields. The use of electrochemical microfluidic reactors to polarize multiwalled carbon nanotubes as anion-π catalysts emerges as essential. These reactors provide access to high fields at low enough voltage to prevent electron transfer, afford meaningful effective catalyst/substrate ratios, and avoid interference from additional electrolytes. Under these conditions, the rate of pyrene-interfaced epoxide-opening ether cyclizations is linearly voltage-dependent at positive voltages and negligible at negative voltages. While electromicrofluidics have been conceived for redox chemistry, our results indicate that their use for supramolecular organocatalysis has the potential to noncovalently electrify organic synthesis in the broadest sense.

Anion-(π)n -π Catalytic Micelles

Anion-π catalysis operates by stabilizing anionic transition states on π-acidic aromatic surfaces. In anion-(π)n -π catalysis, π stacks add polarizability to strengthen interactions. In search of synthetic methods to extend π stacks beyond the limits of foldamers, the self-assembly of micelles from amphiphilic naphthalenediimides (NDIs) is introduced. To interface substrates and catalysts, charge-transfer complexes with dialkoxynaphthalenes (DANs), a classic in supramolecular chemistry, are installed. In π-stacked micelles, the rates of bioinspired ether cyclizations exceed rates on monomers in organic solvents by far. This is particularly impressive considering that anion-π catalysis in water has been elusive so far. Increasing rates with increasing π acidity of the micelles evince operational anion-(π)n -π catalysis. At maximal π acidity, autocatalytic behavior emerges. Dependence on position and order in confined micellar space promises access to emergent properties. Anion-(π)n -π catalytic micelles in water thus expand supramolecular systems catalysis accessible with anion-π interactions with an inspiring topic of general interest and great perspectives.

The Interplay of Weakly Coordinating Anions and the Mechanical Bond: A Systematic Study of the Explicit Influence of Counterions on the Properties of (Pseudo)rotaxanes

Weakly coordinating anions (WCAs) have attracted much attention in recent years due to their ability to stabilise highly reactive cations. It may well be argued, however, that a profound understanding of what truly defines a WCA is still lacking, and systematic studies to unravel counterion effects are scarce. In this work, we investigate a supramolecular pseudorotaxane formation reaction, subject to a selection of anions, ranging from strongly to weakly coordinating, which not only aids in fostering our knowledge about anion coordination properties, but also provides valuable theoretical insight into the nature of the mechanical bond. We employ state-of-the-art DFT-based methods and tools, combined with isothermal calorimetry and 1H NMR experiments, to compute anion-dependent Gibbs free association energies ΔGa, as well as to evaluate intermolecular interactions. We find correlations between ΔGa and the anions’ solvation energies, which are exploited to calculate physico-chemical reaction parameters in the context of coordinating anions. Furthermore, we show that the binding situation within the (pseudo)rotaxanes can be mostly understood by straight-forward electrostatic considerations. However, quantum-chemical effects such as dispersion and charge-transfer interactions become more and more relevant when WCAs are employed.

Chloride Binding Modulated by Anion Receptors Bearing Tetrazine and Urea

Modulation and fine-tuning of the strength of weak interactions to bind anions are described in a series of synthetic receptors. The general design of the receptors includes both a urea motif and a tetrazine motif. The synthetic sequence towards three receptors is detailed. Impacts of H-bond strength and linker length between urea and tetrazine on chloride complexation are studied. Binding properties of the chloride anion are examined in both the ground and excited states using a panel of analytical methods (NMR spectroscopy, mass spectrometry, UV/Visible spectroscopies, and fluorescence). A ranking of the receptors by complexation strength has been determined, allowing a better understanding of the structure-properties relationship on these compounds.

Benzene, an Unexpected Binding Unit in Anion–π Recognition: The Critical Role of CH/π Interactions

We report high-level ab initio calculations (CCSD(T)(full)/CBS//SCS-RI-MP2(full)/aug-cc-pwCVTZ) that demonstrate the importance of cooperativity effects when Anion–π and CH/π interactions are simultaneously established with benzene as the π-system. In fact, most of the complexes exhibit high cooperativity energies that range from 17% to 25.3% of the total interaction energy, which is indicative of the strong influence of the CH/π on the Anion–π interaction and vice versa. Moreover, the symmetry-adapted perturbation theory (SAPT) partition scheme was used to study the different energy contributions to the interaction energies and to investigate the physical nature of the interplay between both interactions. Furthermore, the Atoms in Molecules (AIM) theory and the Non-Covalent Interaction (NCI) approach were used to analyze the two interactions further. Finally, a few examples from the Protein Data Bank (PDB) are shown. All results stress that the concurrent formation of both interactions may play an important role in biological systems due to the ubiquity of CH bonds, phenyl rings, and anions in biomolecules.

Decoded fingerprints of hyperresponsive, expanding product space: polyether cascade cyclizations as tools to elucidate supramolecular catalysis

Simple enough to be understood and complex enough to be revealing, cascade cyclizations of diepoxides are introduced as new tools to characterize supramolecular catalysis. Decoded product fingerprints are provided for a consistent set of substrate stereoisomers, and shown to report on chemo-, diastereo- and enantioselectivity, mechanism and even autocatalysis. Application of the new tool to representative supramolecular systems reveals, for instance, that pnictogen-bonding catalysis is not only best in breaking the Baldwin rules but also converts substrate diastereomers into completely different products. Within supramolecular capsules, new cyclic hemiacetals from House–Meinwald rearrangements are identified, and autocatalysis on anion–π catalysts is found to be independent of substrate stereochemistry. Decoded product fingerprints further support that the involved epoxide-opening polyether cascade cyclizations are directional, racemization-free, and interconnected, at least partially. The discovery of unique characteristics for all catalysts tested would not have been possible without decoded cascade cyclization fingerprints, thus validating the existence and significance of privileged platforms to elucidate supramolecular catalysis. Once decoded, cascade cyclization fingerprints are easily and broadly applicable, ready for use in the community.

Hyperresponsive XL product space identifies polyether cascade fingerprinting as an attractive tool to elucidate supramolecular catalysis, including pnictogen-bonding, capsule and anion–π catalysts.