Transition-state charge stabilization through multiple non-covalent interactions in the guanidinium-catalyzed enantioselective Claisen rearrangement

Catalysis of an SN2 pathway by geometric preorganization

Bimolecular nucleophilic substitution (SN2) mechanisms occupy a central place in the historical development and teaching of the field of organic chemistry1. Despite the importance of SN2 pathways in synthesis, catalytic control of ionic SN2 pathways is rare and notably uncommon even in biocatalysis2,3, reflecting the fact that any electrostatic interaction between a catalyst and the reacting ion pair necessarily stabilizes its charge and, by extension, reduces polar reactivity. Nucleophilic halogenase enzymes navigate this tradeoff by desolvating and positioning the halide nucleophile precisely on the SN2 trajectory, using geometric preorganization to compensate for the attenuation of nucleophilicity4. Here we show that a small-molecule (646 Da) hydrogen-bond-donor catalyst accelerates the SN2 step of an enantioselective Michaelis-Arbuzov reaction by recapitulating the geometric preorganization principle used by enzymes. Mechanistic and computational investigations show that the hydrogen-bond donor diminishes the reactivity of the chloride nucleophile yet accelerates the rate-determining dealkylation step by reorganizing both the phosphonium cation and the chloride anion into a geometry that is primed to enter the SN2 transition state. This new enantioselective Arbuzov reaction affords highly enantioselective access to an array of H-phosphinates, which are in turn versatile P-stereogenic building blocks amenable to myriad derivatizations. This work constitutes, to our knowledge, the first demonstration of catalytic enantiocontrol of the phosphonium dealkylation step, establishing a new platform for the synthesis of P-stereogenic compounds.

Mechanism of cationic ring-opening polymerisation of ε-caprolactone using metallocene/borate catalytic systems: a DFT and NCI study on chain initiation, propagation and termination

We present a comprehensive DFT investigation on the cationic ring-opening polymerisation (CROP) of ε-caprolactone (CL) using zirconocene/borate catalyst systems. All possible pathways of the interaction between cationic species [Cp2ZrMe+] and counteranions, [A-] = [MeB(C6F5)3]- and [B(C6F5)4]-, were examined during chain initiation, propagation, and termination steps. The calculations reveal an active chain-end mechanism with O-alkyl bond cleavage of the polymerisation. The catalytic performance of the two counteranions is found to be identical, and they influence the initial process through stabilisation of the cationic species via non-covalent interactions (NCI), with the [MeB(C6F5)3]- anion stabilising the catalyst-monomer complex more effectively than the [B(C6F5)4]- anion by 24.3 kJ mol-1. The first two propagations are likely the rate-determining step, with calculated free-energy barriers of 61.4-71.2 and 73.9-80.6 kJ mol-1 with and without the anions (A-), respectively. The presence of the counteranion significantly affects the third propagation rate, lowering the barriers up to 20 kJ mol-1. Comparison of the first termination and the third propagation shows that they are not competitive, with the termination being less facile. We also studied the initiation and propagation steps for the hafnocene catalyst and found that the Hf catalyst slightly favours the CL CROP in comparison to the Zr catalyst. Analysis of solvent and dispersion interaction demonstrates that both factors play an important role in the process. NCI analysis reveals weak (van der Waals) interactions at the contacts between the cationic species and the counteranions during the reaction course. Overall, our results offer insights into the structures and interactions involved in the polymerisation.

Cobalt(II)-Catalyzed Enantioselective Propargyl Claisen Rearrangement: Access to Allenyl-Substituted Quaternary β-Ketoesters

Highly enantioselective propargyl Claisen rearrangement of O-propargyl β-ketoesters was achieved under 2.5 mol % of the chiral cobalt complex as the catalyst under mild reaction conditions. With Co(OTf)2 as the Lewis acid and C1-symmetric imidazoline-pyrroloimidazolone pyridine as the ligand, diverse chiral allenyl-substituted all-carbon quaternary β-ketoesters were obtained in good yields (up to 97% yield) and high enantioselectivities (up to 98% ee).

Synthesis of Functionalized Five-Membered Heterocycles from Epoxides: A Hydrogen-Bond Donor Catalytic Approach

The synthesis of highly functionalized five-membered oxa- and aza-heterocycles has been reported utilizing hydrogen-bond donor (HBD) catalysis. In this method, an epoxide was taken as a substrate and reacted with functionalized arylidene/alkylidene malononitrile derivatives in the presence of a newly designed HBD catalyst. In all the cases, the products 2,5-disubstituted tetrahydrofurans (2,5-THFs) were obtained in good to excellent yields (up to 86%) with high diastereoselectivity (dr up to 99:1) as a single regioisomer. The stereochemistry at the 2- and 5-positions of the five-membered ring has been confirmed by single-crystal X-ray analysis, and cis is found to be the major product. The same strategy has been further utilized to obtain substituted oxazolidines whenever the epoxide has been reacted with isocyanate as an electrophile. In order to induce enantioselectivity, a chiral epoxide has been reacted with both the electrophiles in the presence of the same catalyst system to afford the single stereoisomer of the final products. This synthetic methodology involves a low catalyst loading and ambient reaction condition and has been generalized with various substituents present in the starting electrophiles to produce the resultant products in acceptable yields and stereoselectivity.

Molecular Understanding and Practical In Silico Catalyst Design in Computational Organocatalysis and Phase Transfer Catalysis-Challenges and Opportunities

Through the lens of organocatalysis and phase transfer catalysis, we will examine the key components to calculate or predict catalysis-performance metrics, such as turnover frequency and measurement of stereoselectivity, via computational chemistry. The state-of-the-art tools available to calculate potential energy and, consequently, free energy, together with their caveats, will be discussed via examples from the literature. Through various examples from organocatalysis and phase transfer catalysis, we will highlight the challenges related to the mechanism, transition state theory, and solvation involved in translating calculated barriers to the turnover frequency or a metric of stereoselectivity. Examples in the literature that validated their theoretical models will be showcased. Lastly, the relevance and opportunity afforded by machine learning will be discussed.

Exploring Structure-Function Relationships of Aryl Pyrrolidine-Based Hydrogen-Bond Donors in Asymmetric Catalysis Using Data-Driven Techniques

Hydrogen bond-based organocatalysts rely on networks of attractive noncovalent interactions (NCIs) to impart enantioselectivity. As a specific example, aryl pyrrolidine substituted urea, thiourea, and squaramide organocatalysts function cooperatively through hydrogen bonding and difficult-to-predict NCIs as a function of the reaction partners. To uncover the synergistic effect of the structural components of this catalyst class, we applied data science tools to study various model reactions using a derivatized, aryl pyrrolidine-based, hydrogen-bond donor (HBD) catalyst library. Through a combination of experimentally collected data and data mined from previous reports, statistical models were constructed, illuminating the general features necessary for high enantioselectivity. A distinct dependence on the identity of the electrophilic reaction partner and HBD catalyst is observed, suggesting that a general interaction is conserved throughout the reactions analyzed. The resulting models also demonstrate predictive capability by the successful improvement of a previously reported reaction using out-of-sample reaction components. Overall, this study highlights the power of data science in exploring mechanistic hypotheses in asymmetric HBD catalysis and provides a prediction platform applicable in future reaction optimization.

How Robust Is the Reversible Steric Shielding Strategy for Photoswitchable Organocatalysts?

A highly appealing strategy to modulate a catalyst's activity and/or selectivity in a dynamic and noninvasive way is to incorporate a photoresponsive unit into a catalytically competent molecule. However, the description of the photoinduced conformational or structural changes that alter the catalyst's intrinsic reactivity is often reduced to a handful of intuitive static representations, which can struggle to capture the complexity of flexible organocatalysts. Here, we show how a comprehensive exploration of the free energy landscape of N-alkylated azobenzene-tethered piperidine catalysts is essential to unravel the conformational characteristics of each configurational state and explain the experimentally observed reactivity trends. Mapping the catalysts' conformational space highlights the existence of false ON or OFF states that lower their switching ability. Our findings expose the challenges associated with the realization of a reversible steric shielding for the photocontrol of Brønsted basicity of piperidine photoswitchable organocatalysts.

Solvation stabilizes intercarbonyl n→π* interactions and polyproline II helix

n→π* interactions between consecutive carbonyls stabilize the α-helix and polyproline II helix (PPII) conformations in proteins. n→π* interactions have been suggested to provide significant conformational biases to the disordered states of proteins. To understand the roles of solvation on the strength of n→π* interactions, computational investigations were conducted on a model n→π* interaction, the twisted-parallel-offset formaldehyde dimer, as a function of explicit solvation of the donor and acceptor carbonyls, using water and HF. In addition, the effects of urea, thiourea, guanidinium, and monovalent cations on n→π* interaction strength were examined. Solvation of the acceptor carbonyl significantly strengthens the n→π* interaction, while solvation of the donor carbonyl only modestly weakens the n→π* interaction. The n→π* interaction strength was maximized with two solvent molecules on the acceptor carbonyl. Urea stabilized the n→π* interaction via simultaneous engagement of both oxygen lone pairs on the acceptor carbonyl. Solvent effects were further investigated in the model peptides Ac-Pro-NMe₂, Ac-Ala-NMe₂, and Ac-Pro₂-NMe₂. Solvent effects in peptides were similar to those in the formaldehyde dimer, with solvation of the acceptor carbonyl increasing n→π* interaction strength and resulting in more compact conformations, in both the proline endo and exo ring puckers, as well as a reduction in the energy difference between these ring puckers. Carbonyl solvation leads to an energetic preference for PPII over both the α-helix and β/extended conformations, consistent with experimental data that protic solvents and protein denaturants both promote PPII. Solvation of the acceptor carbonyl weakens the intraresidue C5 hydrogen bond that stabilizes the β conformation.

Assessing the persistence of chalcogen bonds in solution with neural network potentials

Non-covalent bonding patterns are commonly harvested as a design principle in the field of catalysis, supramolecular chemistry and functional materials to name a few. Yet, their computational description generally neglects finite temperature and environment effects, which promote competing interactions and alter their static gas-phase properties. Recently, neural network potentials (NNPs) trained on Density Functional Theory (DFT) data have become increasingly popular to simulate molecular phenomena in condensed phase with an accuracy comparable to ab initio methods. To date, most applications have centered on solid-state materials or fairly simple molecules made of a limited number of elements. Herein, we focus on the persistence and strength of chalcogen bonds involving a benzotelluradiazole in condensed phase. While the tellurium-containing heteroaromatic molecules are known to exhibit pronounced interactions with anions and lone pairs of different atoms, the relevance of competing intermolecular interactions, notably with the solvent, is complicated to monitor experimentally but also challenging to model at an accurate electronic structure level. Here, we train direct and baselined NNPs to reproduce hybrid DFT energies and forces in order to identify what are the most prevalent non-covalent interactions occurring in a solute-Cl$^-$-THF mixture. The simulations in explicit solvent highlight competition with chalcogen bonds formed with the solvent and the short-range directionality of the interaction with direct consequences for the molecular properties in the solution. The comparison with other potentials (e.g., AMOEBA, direct NNP and continuum solvent model) also demonstrates that baselined NNPs offer a reliable picture of the non-covalent interaction interplay occurring in solution.

Tunable and Cooperative Catalysis for Enantioselective Pictet-Spengler Reaction with Varied Nitrogen-Containing Heterocyclic Carboxaldehydes

Herein we report an organocatalytic enantioselective functionalization of heterocyclic carboxaldehydes via the Pictet-Spengler reaction. Through careful pairing of novel squaramide and Brønsted acid catalysts, our method tolerates a breadth of heterocycles, enabling preparation of a series of heterocycle conjugated β-(tetrahydro)carbolines in good yield and enantioselectivity. Careful selection of carboxylic acid co-catalyst is essential for toleration of a variety of regioisomeric heterocycles. Utility is demonstrated via the three-step stereoselective preparation of pyridine-containing analogues of potent selective estrogen receptor downregulator and U.S. FDA approved drug Tadalafil.

Organocatalyst-controlled site-selective arene C-H functionalization

Over the past three decades, organocatalysis has emerged as a powerful catalysis platform and has gradually been incorporated into the routine synthetic toolbox to obtain chiral molecules. However, its application in the site- and enantioselective functionalization of inactive aryl C-H bonds remains in its infancy. Here, we present an organocatalyst-controlled para-selective arene C-H functionalization strategy that addresses this issue, which remains an enduring challenge in arene functionalization chemistry. By emulating enzyme catalysis, the chiral phosphoric acid catalyst offers an ideal chiral environment for stereoinduction, and the projecting substituents give control of chemo- and site-selectivity. Various types of nucleophile are compatible with this method, affording more than 100 para-selective adducts with stereodefined carbon centres or axes in viable molecular contexts. This protocol is expected to provide a general strategy for para-selective functionalization of arene C-H bonds in a controlled manner.

A Case Study in Catalyst Generality: Simultaneous, Highly-Enantioselective Brønsted- and Lewis-Acid Mechanisms in Hydrogen-Bond-Donor Catalyzed Oxetane Openings

Generality in asymmetric catalysis can be manifested in dramatic and valuable ways, such as high enantioselectivity across a wide assortment of substrates in a given reaction (broad substrate scope) or as applicability of a given chiral framework across a variety of mechanistically distinct reactions (privileged catalysts). Reactions and catalysts that display such generality hold special utility, because they can be applied broadly and sometimes even predictably in new applications. Despite the great value of such systems, the factors that underlie generality are not well understood. Here, we report a detailed investigation of an asymmetric hydrogen-bond-donor catalyzed oxetane opening with TMSBr that is shown to possess unexpected mechanistic generality. Careful analysis of the role of adventitious protic impurities revealed the participation of competing pathways involving addition of either TMSBr or HBr in the enantiodetermining, ring-opening event. The optimal catalyst induces high enantioselectivity in both pathways, thereby achieving precise stereocontrol in fundamentally different mechanisms under the same conditions and with the same chiral framework. The basis for that generality is analyzed using a combination of experimental and computational methods, which indicate that proximally localized catalyst components cooperatively stabilize and precisely orient dipolar enantiodetermining transition states in both pathways. Generality across different mechanisms is rarely considered in catalyst discovery efforts, but we suggest that it may play a role in the identification of so-called privileged catalysts.

Enantioselective, Catalytic Multicomponent Synthesis of Homoallylic Amines Enabled by Hydrogen-Bonding and Dispersive Interactions

We report a one-step catalytic, enantioselective method for the preparation of homoallylic N-Boc amines directly from acetals. Reactive iminium ion intermediates are generated in situ through the combination of an acetal, a chiral thiourea catalyst, trialkylsilyl triflate, and N-Boc carbamate and are subsequently trapped by a variety of allylsilane nucleophiles. No homoallylic ether byproducts are detected, consistent with allylation of the iminium intermediate being highly favored over allylation of the intermediate oxocarbenium ion. Acetals derived from aromatic aldehydes possessing a variety of functional groups and substitution patterns yield homoallylic amines with excellent levels of enantiomeric enrichment. Experimental and computational data are consistent with an anchoring hydrogen-bond interaction between the protioiminium ion and the amide of the catalyst in the enantiodetermining transition state, and with stereodifferentiation achieved through specific noncovalent interactions (NCIs) with the catalyst pyrenyl moiety. Evidence is provided that the key NCI in the major pathway is a π-stacking interaction, contrasting with the cation-π interactions invoked in previously studied reactions promoted by the same family of aryl-pyrrolidino-H-bond-donor catalysts.

Catalytic Enantioselective Desymmetrizing Fischer Indolization through Dynamic Kinetic Resolution

The first catalytic enantioselective Fischer indolization of prochiral diketones containing enantiotopic carbonyl groups is developed and shown to proceed through dynamic kinetic resolution (DKR). Catalyzed by the combination of a spirocyclic chiral phosphoric acid and ZnCl₂ (Lewis acid assisted Brønsted acid), this direct approach combines 2,2-disubstituted cyclopentane-1,3-diones with N-protected phenylhydrazines to furnish cyclopenta[b]indole derivatives containing an all-carbon quaternary stereocenter with good to excellent enantioselectivities.

Gold-Catalyzed Asymmetric Thioallylation of Propiolates via Charge-Induced Thio-Claisen Rearrangement

A gold(I)-catalyzed enantioselective thioallylation of propiolates with allyl sulfides is described. The key mechanistic element is a sulfonium-induced Claisen rearrangement which helps minimize the allyl dissociation and render higher enantioselectivity. This protocol features remarkable scope of the allyl moiety, allowing enantiocontrolled synthesis of all-carbon quaternary centers, and exhibits exceptional functional group compatibility with many Lewis bases and π-bonds. This intermolecular variant of Claisen rearrangement forges both C-S and C-C bonds concomitantly, providing efficient access to interesting optically active organosulfur compounds which can be transformed further through the vinyl sulfide as a functional handle. The rate of the reaction was zeroth order with respect to allyl sulfides, which suggested a reversible inhibition, providing a resting state for the catalyst. The Hammett plot displayed a correlation with σ_p values, suggesting a turnover-limiting sigmatropic rearrangement where decreased electron-density on sulfur accelerated the rearrangement.

Enantioselective Catalysis of an Anionic Oxy-Cope Rearrangement Enabled by Synergistic Ion Binding

Charge-accelerated rearrangements present interesting challenges to enantioselective catalysis, due in large part to the competing requirements for maximizing reactivity (ion-pair separation) and stereochemical communication. Herein, we describe application of a synergistic ion-binding strategy to catalyze the anionic oxy-Cope rearrangement of a symmetric bis-styrenyl allyl alcohol in up to 75:25 e.r. Structure-reactivity-selectivity relationship studies, including linear free-energy-relationship analyses, with bifunctional urea catalysts indicate that H-bonding and cation-binding interactions act cooperatively to promote the chemo- and enantioselective [3,3]-rearrangement. Implications for catalyst designs applicable to other transformations involving oxyanionic intermediates are discussed.

Why do thioureas and squaramides slow down the Ireland-Claisen rearrangement?

A range of chiral hydrogen-bond-donating organocatalysts was tested in the Ireland–Claisen rearrangement of silyl ketene acetals. None of these organocatalysts was able to impart any enantioselectivity on the rearrangements. Furthermore, these organocatalysts slowed down the Ireland–Claisen rearrangement in comparison to an uncatalyzed reaction. The catalyst-free reaction proceeded well in green solvents or without any solvent. DFT calculations showed that the activation barriers are higher for reactions involving hydrogen-donating organocatalysts and kinetic experiments suggest that the catalysts bind stronger to the starting silyl ketene acetals than to transition structures thus leading to inefficient rearrangement reactions.

Quaternary Phosphonium Salts as Active Brønsted Acid Catalysts for Friedel-Crafts Reactions

A readily available quaternary phosphonium salt containing a trifluoroacetonyl group and a tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BAr^F₄^-) counterion was demonstrated to be a highly active Brønsted acid catalyst for Friedel-Crafts-type reactions of an array of electron-rich heteroarenes and aniline derivatives with isatin-derived ketimines, even at 0.1 mol % catalyst loadings.

Cyclopropenium Enhanced Thiourea Catalysis

An integral part of modern organocatalysis is the development and application of thiourea catalysts. Here, as part of our program aimed at developing cyclopropenium catalysts, the synthesis of a thiourea-cyclopropenium organocatalyst with both cationic hydrogen-bond donor and electrostatic character is reported. The utility of the this thiourea organocatalyst is showcased in pyranylation reactions employing phenols, primary, secondary, and tertiary alcohols under operationally simple and mild reaction conditions for a broad substrate scope. The addition of benzoic acid as a co-catalyst facilitating cooperative Brønsted acid catalysis was found to be valuable for reactions involving phenols and higher substituted alcohols. Mechanistic investigations, including kinetic and ¹H NMR binding studies in conjunction with density function theory calculations, are described that collectively support a Brønsted acid mode of catalysis.

Toward a Predictive Understanding of Phosphine-Catalyzed [3 + 2] Annulation of Allenoates with Acrylate or Imine

Both theoretical and experimental studies were performed to explore the mechanism, regioselectivity, and enantioselectivity of phosphine-catalyzed [3 + 2] annulation between allenoates and acrylate or imine. Using density functional theory computations, we predicted that the enantioselective determining step is the nucleophilic addition of acrylate or imine to the catalyst-activated allenoate. In the key step, we proposed two hydrogen bonding interaction models (intermolecular H-bond model and intramolecular H-bond model). For acrylate substrates, the reaction proceeds via the intramolecular H-bond model and the strong noncovalent interactions between the 2-naphthyl ester moiety lead to the re-face attack pathway being more favorable. For imine substrates, the intermolecular H-bond model operates. In the annulation process, the bulky n-propyl oriented toward a crowded, sterically demanding environment plays a significant role in asymmetric induction. The theoretical calculation results agreed with experimental observations, and these results provide valuable insight into catalyst design and understanding of mechanisms of related reactions.

Computational insight into the cooperative role of non-covalent interactions in the aza-Henry reaction catalyzed by quinine derivatives: mechanism and enantioselectivity

Density functional theory (DFT) calculations were performed to elucidate the mechanism and the origin of the high enantioselectivity of the aza-Henry reaction of isatin-derived N-Boc ketimine catalyzed by a quinine-derived catalyst (QN). The C-C bond formation step is found to be both the rate-determining and the stereo-controlled step. The results revealed the important role of the phenolic OH group in pre-organizing the complex of nitromethane and QN and stabilizing the in situ-generated nitronate and protonated QN. Three possible activation modes for C-C bond formation involving different coordination patterns of catalyst and substrates were studied, and it was found that both the ion pair-hydrogen bonding mode and the Brønsted acid-hydrogen bonding mode are viable, with the latter slightly preferred for the real catalytic system. The calculated enantiomeric excess (ee) favouring the S enantiomer is in good agreement with the experimental result. The high reactivity and enantioselectivity can be ascribed to the cooperative role of the multiple non-covalent interactions, including classical and non-classical H bonding as well as anionπ interactions. These results also highlight the importance of the inclusion of dispersion correction for achieving a reasonable agreement between theory and experiment for the current reaction.

Computing organic stereoselectivity - from concepts to quantitative calculations and predictions

Advances in theory and processing power have established computation as a valuable interpretative and predictive tool in the discovery of new asymmetric catalysts. This tutorial review outlines the theory and practice of modeling stereoselective reactions. Recent examples illustrate how an understanding of the fundamental principles and the application of state-of-the-art computational methods may be used to gain mechanistic insight into organic and organometallic reactions. We highlight the emerging potential of this computational tool-box in providing meaningful predictions for the rational design of asymmetric catalysts. We present an accessible account of the field to encourage future synergy between computation and experiment.

Chiral guanidines and their derivatives in asymmetric synthesis

This article reviews current achievements of chiral guanidines and their derivatives in organocatalysis, and updates versatile guanidine–metal salt combinations in asymmetric catalytic reactions.

Systematic Coupled Cluster Study of Noncovalent Interactions Involving Halogens, Chalcogens, and Pnicogens

The noncovalent interactions of 32 complexes involving pnicogens, chalcogens, and halogens atoms were investigated at the CCSD(T)/aug-cc-pVTZ level of theory. Two different types of complexes could be distinguished on the basis of geometric parameters, electron difference densities, and the charge transfer mechanisms associated with each type. In the type I conformation, the monomers adopt a skewed orientation allowing charge to be transfer between both monomers, whereas in the type II conformation the monomers adopt a linear arrangement, maximizing charge transfer in only one direction. Type I complexes involving the interaction between pnicogens and chalcogens cannot be unambiguously defined as chalcogen or pnicogen bonds, they are an admixture of both. The charge transfer dependence on the conformation adopted by the complexes described in this work can serve as a novel conformationally driven design concept for materials.

Exploiting non-covalent π interactions for catalyst design

Molecular recognition, binding and catalysis are often mediated by non-covalent interactions involving aromatic functional groups. Although the relative complexity of these so-called π interactions has made them challenging to study, theory and modelling have now reached the stage at which we can explain their physical origins and obtain reliable insight into their effects on molecular binding and chemical transformations. This offers opportunities for the rational manipulation of these complex non-covalent interactions and their direct incorporation into the design of small-molecule catalysts and enzymes.

A mechanistic study of the Lewis acid–Brønsted base–Brønsted acid catalysed asymmetric Michael addition of diethyl malonate to cyclohexenone

Reaction shown to exhibit biomimetic behaviour.

Claisen rearrangements of benzyl vinyl ethers: theoretical investigation of mechanism, substituent effects, and regioselectivity

Theoretical calculations are reported which examine the mechanisms of Claisen rearrangements of benzyl vinyl ethers and the ways in which substituents influence reactivity and regioselectivity.

Efficient catalytic application of a binary ionic liquid mixture in the synthesis of novel spiro[4H-pyridine-oxindoles]

Novel pyrazole-fused spiro[4H-pyridine-oxindoles] were synthesized under the catalysis of the binary ionic liquid mixture [1,1,3,3-tetramethylguanidinium chloride][1-methylimidazolium-3-sulfonate] in solvent-free conditions.

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.