Remote Control of Anion–π Catalysis on Fullerene‐Centered Catalytic Triads

The design, synthesis and evaluation of catalytic triads composed of a central C₆₀ fullerene with an amine base on one side and polarizability enhancers on the other side are reported. According to an enolate addition benchmark reaction, fullerene–fullerene–amine triads display the highest selectivity in anion–π catalysis observed so far, whereas NDI–fullerene–amine triads are not much better than fullerene–amine controls (NDI=naphthalenediimide). These large differences in activity are in conflict with the small differences in intrinsic π acidity, that is, LUMO energy levels and π holes on the central fullerene. However, they are in agreement with the high polarizability of fullerene–fullerene–amine triads. Activation and deactivation of the fullerene‐centered triads by intercalators and computational data on anion binding further indicate that for functional relevance, intrinsic π acidity is less important than induced π acidity, that is, the size of the oriented macrodipole of polarizable π systems that emerges only in response to the interaction with anions and anionic transition states. The resulting transformation is thus self‐induced, the anionic intermediates and transition states create their own anion–π catalyst.     

Asymmetric Anion–π Catalysis on Perylenediimides

Anion–π catalysis, that is the stabilization of anionic transition states on π‐acidic aromatic surfaces, has so far been developed with naphthalenediimides (NDIs). This report introduces perylenediimides (PDIs) to anion–π catalysis. The quadrupole moment of PDIs (+23.2 B) is found to exceed that of NDIs and reach new records with acceptors in the core (+70.9 B), and their larger surface provides space to better accommodate chemical transformations. Unlike NDIs, the activity of PDI catalysts for enolate and enamine addition is determined by the twist of their π surface rather than their reducibility. These results, further strengthened by nitrate inhibition and circular dichroism spectroscopy, support an understanding of anion–π interactions centered around quadrupole moments, i.e., electrostatic contributions, rather than redox potentials and charge transfer. The large PDI surfaces provide access to the highest enantioselectivities observed so far in anion–π catalysis (96 % ee).     

Anion–π Catalysis of Enolate Chemistry: Rigidified Leonard Turns as a General Motif to Run Reactions on Aromatic Surfaces

To integrate anion–π, cation–π, and ion pair–π interactions in catalysis, the fundamental challenge is to run reactions reliably on aromatic surfaces. Addressing a specific question concerning enolate addition to nitroolefins, this study elaborates on Leonard turns to tackle this problem in a general manner. Increasingly refined turns are constructed to position malonate half thioesters as close as possible on π‐acidic surfaces. The resulting preorganization of reactive intermediates is shown to support the disfavored addition to enolate acceptors to an absolutely unexpected extent. This decisive impact on anion–π catalysis increases with the rigidity of the turns. The new, rigidified Leonard turns are most effective with weak anion–π interactions, whereas stronger interactions do not require such ideal substrate positioning to operate well. The stunning simplicity of the motif and its surprisingly strong relevance for function should render the introduced approach generally useful.     

Anion‐π Catalysis: Focus on Nonadjacent Stereocenters

Anion‐π interactions have been recently introduced to catalysis with the idea to stabilize anionic intermediates on π‐acidic surfaces. Realized examples include enolate, enamine and iminium chemistry, domino processes and Diels–Alder reactions. Moving on from the formation of contiguous stereogenic centers on π‐acidic surfaces, herein we report the first asymmetric anion‐π catalysis of cascade reactions that afford nonadjacent stereocenters. Conjugate addition‐protonation of achiral disubstituted enolate donors to 2‐chloroacrylonitrile generates 1,3‐nonadjacent stereocenters with moderate enantioselectivity and diastereoselectivity. The explored catalysts operate with complementary naphthalenediimide and fullerene surfaces with highly positive quadrupole moments and high polarizability, respectively, and proximal amine bases. We find that anion‐π catalysts can increase the diastereoselectivity of the reaction beyond the maximal 1:4.0 dr with conventional catalysts to maximal 5.3:1 dr on the large fullerene surfaces. The enantioselectivity of anion‐π catalysts, best on the confined naphthalenediimide surfaces with strong quadrupole moment, exceed the performance of conventional catalysts except for comparable results with a new, most compact, surprisingly powerful bifunctional control catalyst. Simultaneously increased rates and stereoselectivities compared to control catalysts without π‐acidic surface support that contributions of anion‐π interactions to the catalytic cascade process are significant.     

Anion–π Catalysis on Carbon Nanotubes

Induced π acidity from polarizability is emerging as the most effective way to stabilize anionic transition states on aromatic π surfaces, that is, anion–π catalysis. To access extreme polarizability, we propose a shift from homogeneous toward heterogeneous anion–π catalysis on higher carbon allotropes. According to benchmark enolate addition chemistry, multi‐walled carbon nanotubes equipped with tertiary amine bases outperform single‐walled carbon nanotubes. This is consistent with the polarizability of the former not only along but also between the tubes. Inactivation by π‐basic aromatics and saturation with increasing catalyst concentration support that catalysis occurs on the π surface of the tubes. Increasing rate and selectivity of existing anion–π catalysts on the surface of unmodified nanotubes is consistent with transition‐state stabilization by electron sharing into the tubes, i.e., induced anion–π interactions. On pristine tubes, anion–π catalysis is realized by non‐covalent interfacing with π‐basic pyrenes.     

The Cation–π Interaction in Small‐Molecule Catalysis

Catalysis by small molecules (≤1000 Da, 10^?9 m) that are capable of binding and activating substrates through attractive, noncovalent interactions has emerged as an important approach in organic and organometallic chemistry. While the canonical noncovalent interactions, including hydrogen bonding, ion pairing, and π stacking, have become mainstays of catalyst design, the cation–π interaction has been comparatively underutilized in this context since its discovery in the 1980s. However, like a hydrogen bond, the cation–π interaction exhibits a typical binding affinity of several kcal mol^?1 with substantial directionality. These properties render it attractive as a design element for the development of small‐molecule catalysts, and in recent years, the catalysis community has begun to take advantage of these features, drawing inspiration from pioneering research in molecular recognition and structural biology. This Review surveys the burgeoning application of the cation–π interaction in catalysis.     

Shifting from Hydrogen Bond Network to π–π Stacking: A Key Mechanism for Reversible Thermochromic Sulfonated Poly(Ether Ether Ketone)

Sulfonated poly(ether ether ketone) (SPEEK) thin film performs reversible thermochromic property by developing the color to be yellowish at the temperature above 190 °C. The detailed analyses based on temperature‐dependent techniques suggest the thermal treatment inducing the shifting of the hydrogen bond network between the sulfonated group and the hydrated water molecules to the π–π stacking among aromatic rings in SPEEK chains. Although it is general that the polymer chain packing is unfavorable at high temperature, the present work shows a good example that when the polymer chains can form specific molecular interaction, such as π–π stacking, even in harsh thermal treatment, a rearrangement will effectively occur, which leads to an external stimuli‐responsive property.

     

Anion–π and Cation–π Interactions on the Same Surface

Herein, we address the question whether anion–π and cation–π interactions can take place simultaneously on the same aromatic surface. Covalently positioned carboxylate–guanidinium pairs on the surface of 4‐amino‐1,8‐naphthalimides are used as an example to explore push–pull chromophores as privileged platforms for such “ion pair–π” interactions. In antiparallel orientation with respect to the push–pull dipole, a bathochromic effect is observed. A red shift of 41 nm found in the least polar solvent is in good agreement with the 70 nm expected from theoretical calculations of ground and excited states. Decreasing shifts with solvent polarity, protonation, aggregation, and parallel carboxylate–guanidinium pairs imply that the intramolecular Stark effect from antiparallel ion pair–π interactions exceeds solvatochromic effects by far. Theoretical studies indicate that carboxylate–guanidinium pairs can also interact with the surfaces of π‐acidic naphthalenediimides and π‐basic pyrenes.     

σ–π Continuum in Indole–Palladium(II) Complexes

The intrinsic features of (hetero‐arene)–metal interactions have been elusive mainly because the systematic structure analysis of non‐anchored hetero‐arene–metal complexes has been hampered by their labile nature. We report successful isolation and systematic structure analysis of a series of non‐anchored indole–palladium(II) complexes. It was revealed that there is a σ–π continuum for the indole–metal interaction, while it has been thought that the dominant coordination mode of indole to a metal center is the Wheland‐intermediate‐type σ‐mode in light of the seemingly strong electron‐donating ability of indole. Several factors which affect the σ‐ or π‐character of indole–metal interactions are discussed.     

Photophysical properties of σ–π‐conjugated alternating oligothienylene–oligosilylene polymers

UV‐visible absorption and fluorescence properties of three series of σ–π‐conjugated polymers (copolymers of alternative oligothienylene and oligosilylene units) have been studied in dioxane solution. The energies of the absorption maximum, fluorescence maximum, and the 0–0 transition are found to be linearly dependent on the reciprocal of the number of thiophene rings in the repeating unit of the polymer chain, but almost independent of the silicon atom number. The σ–π‐conjugation in the polymers results in red shift in the absorption and fluorescence maxima, higher fluorescence quantum yields, and longer fluorescence lifetimes of the polymers, with respect to their corresponding analogous α‐oligothiophenes. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1873–1880, 1999     

Enolate Stabilization by Anion–π Interactions: Deuterium Exchange in Malonate Dilactones on π‐Acidic Surfaces

Of central importance in chemistry and biology, enolate chemistry is an attractive topic to elaborate on possible contributions of anion–π interactions to catalysis. To demonstrate the existence of such contributions, experimental evidence for the stabilization of not only anions but also anionic intermediates and transition states on π‐acidic aromatic surfaces is decisive. To tackle this challenge for enolate chemistry with maximal precision and minimal uncertainty, malonate dilactones are covalently positioned on the π‐acidic surface of naphthalenediimides (NDIs). Their presence is directly visible in the upfield shifts of the α‐protons in the ¹H NMR spectra. The reactivity of these protons on π‐acidic surfaces is measured by hydrogen–deuterium (H–D) exchange for 11 different examples, excluding controls. The velocity of H–D exchange increases with π acidity (NDI core substituents: SO₂R>SOR>H>OR>OR/NR₂>SR>NR₂). The H–D exchange kinetics vary with the structure of the enolate (malonates>methylmalonates, dilactones>dithiolactones). Moreover, they depend on the distance to the π surface (bridge length: 11–13 atoms). Most importantly, H–D exchange depends strongly on the chirality of the π surface (chiral sulfoxides as core substituents; the crystal structure of the enantiopure (R,R,P)‐macrocycle is reported). For maximal π acidity, transition‐state stabilizations up to ?18.8 kJ mol^?1 are obtained for H–D exchange. The Brønsted acidity of the enols increases strongly with π acidity of the aromatic surface, the lowest measured pK_a=10.9 calculates to a ΔpK_a=?5.5. Corresponding to the deprotonation of arginine residues in neutral water, considered as “impossible” in biology, the found enolate–π interactions are very important. The strong dependence of enolate stabilization on the unprecedented seven‐component π‐acidity gradient over almost 1 eV demonstrates quantitatively that such important anion–π activities can be expected only from strong enough π acids.     

A–π–D–π–A Electron‐Donating Small Molecules for Solution‐Processed Organic Solar Cells: A Review

Organic solar cells based on semiconducting polymers and small molecules have attracted considerable attention in the last two decades. Moreover, the power conversion efficiencies for solution‐processed solar cells containing A–π–D–π–A‐type small molecules and fullerenes have reached 11%. However, the method for designing high‐performance, photovoltaic small molecules still remains unclear. In this review, recent studies on A–π–D–π–A electron‐donating small molecules for organic solar cells are introduced. Moreover, the relationships between molecular properties and device performances are summarized, from which inspiration for the future design of high performance organic solar cells may be obtained.     

Energetic N‐Nitramino/N‐Oxyl‐Functionalized Pyrazoles with Versatile π–π Stacking: Structure–Property Relationships of High‐Performance Energetic Materials

N‐Nitramino/N‐oxyl functionalization strategies were employed to investigate structure–property relationships of energetic materials. Based on single‐crystal diffraction data, π–π stacking of pyrazole backbones can be tailored effectively by energetic functionalities, thereby resulting in diversified energetic compounds. Among them, hydroxylammonium 4‐amino‐3,5‐dinitro‐1H‐pyrazol‐1‐olate and dipotassium N,N′‐(3,5‐dinitro‐1H‐pyrazol‐1,4‐diyl)dinitramidate, with unique face‐to‐face π–π stacking, can be potentially used as a high‐performance explosive and an energetic oxidizer, respectively.     

Peptide Stapling with Anion‐π Catalysts

We report design, synthesis and evaluation of a series of naphthalenediimides (NDIs) that are bridged with short peptides. Reminiscent of peptide stapling technologies, the macrocycles are conveniently accessible by a chromogenic nucleophilic aromatic substitution of two bromides in the NDI core with two thiols from cysteine sidechains. The dimension of core‐bridged NDIs matches that of one turn of an α helix. NDI‐stapled peptides exist as two, often separable atropisomers. Introduction of tertiary amine bases in amino‐acid sidechains above the π‐acidic NDI surface affords operational anion‐π catalysts. According to an enolate chemistry benchmark reaction, anion‐π catalysis next to peptides occurs with record chemoselectivity but weak enantioselectivity. Catalytic activity drops with increasing distance of the amine base to the NDI surface, looser homocysteine bridges, mismatched, shortened and elongated α‐helix turns, and acyclic peptide controls. Elongation of isolated turns into short α helices significantly increases activity. This increase is consistent with remote control of anion‐π catalysis from the α‐helix macrodipole.     

Anion–π Catalysis of Diels–Alder Reactions

Among concerted cycloadditions, the Diels–Alder reaction is the grand old classic, which is usually achieved with acid catalysis. In this report, hydroxypyrones, oxa‐, and thiazolones are explored because they provide access to anionic dienes. Their [4+2] cycloaddition with cyclic and acyclic dienophiles, such as maleimides and fumarates, affords bicyclic products with four new stereogenic centers. Bifunctional anion–π catalysts composed of amine bases next to the π surface of naphthalenediimides (NDIs) are shown to selectively stabilize the “open”, fully accessible anionic exo transition state on the π‐acidic aromatic surface. Our results also include reactivities that are hard to access with conventional organocatalysts, such as the exo ‐specific and highly enantioselective Diels–Alder reaction of thiazolones and maleimides with complete suppression of the otherwise dominant Michael addition. With increasing π acidity of the anion–π catalysts, the rates, chemo‐, diastereo‐, and enantioselectivities increase consistently.