Unveiling the Delicate Balance of Steric and Dispersion Interactions in Organocatalysis Using High-Level Computational Methods

Emerging Conformational-Analysis Protocols from the RTCONF55-16K Reaction Thermochemistry Conformational Benchmark Set

RTCONF55-16K is a new, reactive conformational data set based on cost-efficient methods to assess different conformational analysis protocols. Our reference calculations underpinned the accuracy of the CENSO (Grimme et al. J. Phys. Chem. A, 2021, 125, 4039) procedure and resulted in alternative recipes with different cost-accuracy compromises. Our general-purpose and economical protocols (CENSO-light and zero, respectively) were found to be 10-30 times faster than the original algorithm, adding only 0.4-0.7 kcal/mol absolute error to the relative free energy estimates.

Local Energy Decomposition Analysis of London Dispersion Effects: From Simple Model Dimers to Complex Biomolecular Assemblies

ConspectusLondon dispersion (LD) forces are ubiquitous in chemistry, playing a pivotal role in a wide range of chemical processes. For example, they influence the structure of molecular crystals, the selectivity of organocatalytic transformations, and the formation of biomolecular assemblies. Harnessing these forces for chemical applications requires consistent quantification of the LD energy across a broad and diverse spectrum of chemical scenarios. Despite the great progress made in recent years in the development of experimental strategies for LD quantification, quantum chemical methods remain one of the most useful tools in the hand of chemists for the study of these weak interactions. Unfortunately, the accurate quantification of LD effects in complex systems poses many challenges for electronic structure theories. One of the problems stems from the fact that LD forces originate from long-range electronic dynamic correlation, and hence, their rigorous description requires the use of complex, highly correlated wave function-based methods. These methods typically feature a steep scaling with the system size, limiting their applicability to small model systems. Another core challenge lies in disentangling short-range from long-range dynamic correlation, which from a rigorous quantum mechanical perspective is not possible.In this Account, we describe our research endeavors in the development of broadly applicable computational methods for LD quantification in molecular chemistry as well as challenging applications of these schemes in various domains of chemical research. Our strategy lies in the use of local correlation theories to reduce the computational cost associated with complex electronic structure methods while providing at the same time a simple means of decomposition of dynamic correlation into its long-range and short-range components. In particular, the local energy decomposition (LED) scheme at the domain-based local pair natural orbital coupled cluster (DLPNO-CCSD(T)) level has emerged as a powerful tool in our research, offering a clear-cut quantitative definition of the LD energy that remains valid across a plethora of different chemical scenarios. Typical applications of this scheme are examined, encompassing protein-ligand interactions and reactivity studies involving many fragments and complex electronic structures. In addition, our research also involves the development of novel cost-effective methodologies, which exploit the LED definition of the LD energy, for LD energy quantification that are, in principle, applicable to systems with thousands of atoms. The Hartree-Fock plus London Dispersion (HFLD) scheme, correcting the HF interaction energy using an approximate CCSD(T)-based LD energy, is a useful, parameter-free electronic structure method for the study of LD effects in systems with hundreds of molecular fragments. However, the usefulness of the LED scheme reaches beyond providing an interpretation of the calculated DLPNO-CCSD(T) or DLPNO-MP2 interaction energies. For example, the dispersion energies obtained from the LED can be fruitfully used in order to parametrize semiempirical dispersion models. We will demonstrate this in the context of an emerging semiempirical method, namely, the Natural Orbital Tied Constructed Hamiltonian (NOTCH) method. NOTCH incorporates LED-derived LD energies and shows promising accuracy at a minimum amount of empiricism. Thus, it holds substantial promise for large and complex systems.

Advances and Prospects in Understanding London Dispersion Interactions in Molecular Chemistry

London dispersion (LD) interactions are the main contribution of the attractive part of the van der Waals potential. Even though LD effects are the driving force for molecular aggregation and recognition, the role of these omnipresent interactions in structure and reactivity had been largely underappreciated over decades. However, in the recent years considerable efforts have been made to thoroughly study LD interactions and their potential as a chemical design element for structures and catalysis. This was made possible through a fruitful interplay of theory and experiment. This review highlights recent results and advances in utilizing LD interactions as a structural motif to understand and utilize intra- and intermolecularly LD-stabilized systems. Additionally, we focus on the quantification of LD interactions and their fundamental role in chemical reactions.

Unraveling Atomic Contributions to the London Dispersion Energy: Insights into Molecular Recognition and Reactivity

We present a general framework that enables quantification with atomic resolution of the overall London dispersion energy, which can be readily integrated with currently available energy decomposition schemes. This approach can be used to determine the contribution of individual atoms and functional groups to molecular recognition, conformational preferences, molecular stability, and reactivity. Its efficacy across diverse realms of molecular chemistry and biology is demonstrated with application to molecular balances in solution, asymmetric organocatalytic transformations, and a subcomplex of the F1FO ATP synthase.

The Role of Aromatic Alcohol Additives on Asymmetric Organocatalysis Reactions: Insights from Theory

The presence of an aromatic additive has been seen to enhance, often significantly, the enantioselectivity and yield in asymmetric organocatalysis. Considering their success across a dizzying range of organocatalysts and organic transformations, it would seem unlikely that a common principle exists for their functioning. However, the current investigations with DFT suggest a general principle: the phenolic additive sandwiches itself, through hydrogen bonding and π⋅⋅⋅π stacking, between the organocatalyst coordinated electrophile and nucleophile. This is seen for a wide range of experimentally reported systems. That such complex formation leads to enhanced stereoselectivity is then demonstrated for two cases: the cinchona alkaloid complex (BzCPD), catalysing thiocyanation (2-naphthol additive employed), as well as for L-pipecolicacid catalysing the asymmetric nitroaldol reaction with a range of nitro-substituted phenol additives. These findings, indicating that dual catalysis takes place when phenolic additives are employed, are likely to have a significant impact on the field of asymmetric organocatalysis.

Catalytic asymmetric cationic shifts of aliphatic hydrocarbons

Asymmetric catalysis is an advanced area of chemical synthesis, but the handling of abundantly available, purely aliphatic hydrocarbons has proven to be challenging. Typically, heteroatoms or aromatic substructures are required in the substrates and reagents to facilitate an efficient interaction with the chiral catalyst. Confined acids have recently been introduced as tools for homogenous asymmetric catalysis, specifically to enable the processing of small unbiased substrates1. However, asymmetric reactions in which both substrate and product are purely aliphatic hydrocarbons have not previously been catalysed by such super strong and confined acids. We describe here an imidodiphosphorimidate-catalysed asymmetric Wagner-Meerwein shift of aliphatic alkenyl cycloalkanes to cycloalkenes with excellent regio- and enantioselectivity. Despite their long history and high relevance for chemical synthesis and biosynthesis, Wagner-Meerwein reactions utilizing purely aliphatic hydrocarbons, such as those originally reported by Wagner and Meerwein, had previously eluded asymmetric catalysis.

Accelerating σ-Bond Metathesis at Sn(II) Centers

Molecular main-group hydride catalysts are attractive as cheap and Earth-abundant alternatives to transition-metal analogues. In the case of the latter, specific steric and electronic tuning of the metal center through ligand choice has enabled the iterative and rational development of superior catalysts. Analogously, a deeper understanding of electronic structure-activity relationships for molecular main-group hydrides should facilitate the development of superior main-group hydride catalysts. Herein, we report a modular Sn-Ni bimetallic system in which we systematically vary the ancillary ligand on Ni, which, in turn, tunes the Sn center. This tuning is probed using Sn L1 XAS as a measure of electron density at the Sn center. We demonstrate that increased electron density at Sn centers accelerates the rate of σ-bond metathesis, and we employ this understanding to develop a highly active Sn-based catalyst for the hydroboration of CO2 using pinacolborane. Additionally, we demonstrate that engineering London dispersion interactions within the secondary coordination sphere of Sn allows for further rate acceleration. These results show that the electronics of main-group catalysts can be controlled without the competing effects of geometry perturbations and that this manifests in substantial reactivity differences.

An NMR Spectroscopy View on London Dispersion in Catalysis: Detection, Quantification, and Application in Ion Pair and Transition Metal Catalysis

ConspectusThe energetic contribution of London dispersion (LD) can cover a broad range from very few to hundreds of kJ mol-1 for extended interaction interfaces due to its pairwise additivity. However, for a designed and successful application of LD in chemical catalysis, there are still many obstacles and questions that remain. In principle, LD can be regarded as the attractive part of the van der Waals potential. Thus, considering the whole van der Waals potential, including the repulsive part (steric repulsion), the ideal solution to the problem in catalysis would be to design compatible interaction interfaces at exactly the correct distance. In the case of a self-assembled, flexible structure arrangement, entropic contributions and solvent interactions might be detrimental. In the case of a rigid catalyst pocket, steric hindrance might not allow for large substituents that are usually applied as dispersion energy donors (DEDs). For a working catalytic system, the following question arises: how is it possible to dissect the complex interaction interfaces in terms of energetic contributions? Usually, the energetic contribution of LD to catalysis is addressed by using calculations. However, adequately computing the correct energetic contributions can be extremely challenging for a vast conformational space with all kinds of intermolecular interactions. Thus, experimental data are essential for comparison or benchmarking.Therefore, in this Account, we describe our quest for detailed experimental data obtained via NMR spectroscopy to experimentally dissect and quantify LD in catalytic systems. In addition, we address the question of whether bulky substituents used as DEDs can be used in confined catalytic pockets. With the example of Pd phosphoramidite complexes, we show how it is possible to experimentally dissect and quantify the contribution of individual interaction areas in complicated transition metal complexes. Furthermore, a correlation between conformational rigidity and heterodimer preference clearly reveals that LD can only unfold its full potential in cases where entropic contributions are minimized. This finding can also explain the small contribution of LD in flexible and solvent-exposed molecular balances. In the field of Brønsted acid catalysis, we demonstrated that LD has a strong influence on the structures, stability, and populations of confined catalytic intermediates. LD is key for populating higher aggregates such as dimers. In addition, offsets between the experimental and computational results were observed and attributed to solvent-solute dispersion interactions. We studied the delicate interplay of attractive and repulsive interactions by adding bulky DED substituents onto a substrate, which can function as a molecular balance system. Intriguingly, the effect of LD on the free substrate was straightforwardly transferred onto the highly confined intermediates. Furthermore, this effect could even be read out in the enantioselectivities of the underlying reaction. This conceptualized a general approach regarding how LD can be used beneficially in catalysis to convert from moderate/good to excellent stereoselectivities. It showcased that bulky groups such as tert-butyl must not only be regarded as occupied volumes.

Unravelling dispersion forces in liquid-phase enantioseparation. Part I: Impact of ferrocenyl versus phenyl groups

BACKGROUND

Highly ordered chiral secondary structures as well as multiple (tunable) recognition sites are the keys to success of polysaccharide carbamate-based chiral selectors in enantioseparation science. Hydrogen bonds (HBs), dipole-dipole, and π-π interactions are classically considered the most frequent noncovalent interactions underlying enantioselective recognition with these chiral selectors. Very recently, halogen, chalcogen and π-hole bonds were also identified as interactions working in polysaccharide carbamate-based selectors to promote enantiomer distinction. On the contrary, the function of dispersion interactions in this field was not explored so far.

RESULTS

The enantioseparation of chiral ferrocenes featuring chiral axis or chiral plane as stereogenic elements was performed by comparing five polysaccharide carbamate-based chiral columns, with the aim to identify enantioseparation outcomes that could be reasonably determined by dispersion forces, making available a reliable experimental data set for future theoretical studies to confirm the heuristic hypothesis. The effects of mobile phase polarity and temperature on the enantioseparation were considered, and potential recognition sites on analytes and selectors were evaluated by electrostatic potential (V) analysis and molecular dynamics (MD). In this first part, the enantioseparation of 3,3'-dibromo-5,5'-bis-ferrocenylethynyl-4,4'-bipyridine bearing two ferrocenylethynyl units linked to an axially chiral core was performed and compared to that of the analyte featuring the same structural motif with two phenyl groups in place of the ferrocenyl moieties. The results of this study showed the superiority of the ferrocenyl compared to the phenyl group, as a structural element favouring enantiodifferentiation.

SIGNIFICANCE AND NOVELTY

Even if dispersion (London) forces have been envisaged acting in liquid-phase enantioseparations, focused studies to explore possible contributions of dispersion forces with polysaccharide carbamate-based selectors are practically missing. This study allowed us to collect experimental information that support the involvement of dispersion forces as contributors to liquid-phase enantioseparation, paving the way to a new picture in this field.

Dimerization of confined Brønsted acids in enantioselective organocatalytic reactions

The formation of Brønsted acid aggregates in the course of asymmetric organocatalytic reactions is often overlooked in mechanistic studies, even though it might have a deep impact on the stereo-controlling factors of the transformations. In this work, we shed light on the influence of the catalyst structure and reaction conditions on the spontaneity of the aggregation process for popular chiral organocatalysts derived from phosphoric acids using high-level quantum mechanical calculations. Our study encompasses small and sterically unhindered chiral phosphoric acids as well as large and "confined" imidodiphosphates and imidodiphosphorimidates. These systems have recently proven particularly effective in promoting a large number of highly relevant asymmetric transformations. While cooperative catalytic effects of sterically less hindered chiral phosphoric acid catalysts are well appreciated in literature, it is found that the formation of catalyst dimers in solution is possible for both standard and confined catalysts. The spontaneity of the aggregation process depends on reaction conditions like solvent polarity, polarizability, temperature, the nature of the interaction with the substrate, as well as the catalyst architecture. Finally, it is shown that, at low temperatures (153 K), the aggregation process can profoundly influence the reaction kinetics and selectivity.

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.

Catalytic asymmetric C-H insertion reactions of vinyl carbocations

From the preparation of pharmaceuticals to enzymatic construction of natural products, carbocations are central to molecular synthesis. Although these reactive intermediates are engaged in stereoselective processes in nature, exerting enantiocontrol over carbocations with synthetic catalysts remains challenging. Many resonance-stabilized tricoordinated carbocations, such as iminium and oxocarbenium ions, have been applied in catalytic enantioselective reactions. However, their dicoordinated counterparts (aryl and vinyl carbocations) have not, despite their emerging utility in chemical synthesis. We report the discovery of a highly enantioselective vinyl carbocation carbon-hydrogen (C-H) insertion reaction enabled by imidodiphosphorimidate organocatalysts. Active site confinement featured in this catalyst class not only enables effective enantiocontrol but also expands the scope of vinyl cation C-H insertion chemistry, which broadens the utility of this transition metal-free C(sp³)-H functionalization platform.

Tilting the Balance: London Dispersion Systematically Enhances Enantioselectivities in Brønsted Acid Catalyzed Transfer Hydrogenation of Imines

London dispersion (LD) is attracting more and more attention in catalysis since LD is ubiquitously present and cumulative. Since dispersion is hard to grasp, recent research has concentrated mainly on the effect of LD in individual catalytic complexes or on the impact of dispersion energy donors (DEDs) on balance systems. The systematic transfer of LD effects onto confined and more complex systems in catalysis is still in its infancy, and no general approach for using DED residues in catalysis has emerged so far. Thus, on the example of asymmetric Brønsted acid catalyzed transfer hydrogenation of imines, we translated the findings of previously isolated balance systems onto confined catalytic intermediates, resulting in a systematic enhancement of stereoselectivity when employing DED-substituted substrates. As the imine substrate is present as Z- and E-isomers, which can, respectively, be converted to R- and S-product enantiomers, implementing tert-butyl groups as DED residues led to an additional stabilization of the Z-imine by up to 4.5 kJ/mol. NMR studies revealed that this effect is transferred onto catalyst/imine and catalyst/imine/nucleophile intermediates and that the underlying reaction mechanism is not affected. A clear correlation between ee and LD stabilization was demonstrated for 3 substrates and 10 catalysts, allowing to convert moderate-good to good-excellent enantioselectivities. Our findings conceptualize a general approach on how to beneficially employ DED residues in catalysis: they clearly showcase that bulky alkyl residues such as tert-butyl groups must be considered regarding not only their repulsive steric bulk but also their attractive properties even in catalytic complexes.

Theoretical Perspectives in Organocatalysis

It is clear that the field of organocatalysis is continuously expanding during the last decades. With increasing computational capacity and new techniques, computational methods have provided a more economic approach to explore different chemical systems. This review offers a broad yet concise overview of current state-of-the-art studies that have employed novel strategies for catalyst design. The evolution of the all different theoretical approaches most commonly used within organocatalysis is discussed, from the traditional approach, manual-driven, to the most recent one, machine-driven.

A Combined DFT, Energy Decomposition, and Data Analysis Approach to Investigate the Relationship Between Noncovalent Interactions and Selectivity in a Flexible DABCOnium/Chiral Anion Catalyst System

Developing strategies to study reactivity and selectivity in flexible catalyst systems has become an important topic of research. Herein, we report a combined experimental and computational study aimed at understanding the mechanistic role of an achiral DABCOnium cofactor in a regio- and enantiodivergent bromocyclization reaction. It was found that electron-deficient aryl substituents enable rigidified transition states via an anion-π interaction with the catalyst, which drives the selectivity of the reaction. In contrast, electron-rich aryl groups on the DABCOnium result in significantly more flexible transition states, where interactions between the catalyst and substrate are more important. An analysis of not only the lowest-energy transition state structures but also an ensemble of low-energy transition state conformers via energy decomposition analysis and machine learning was crucial to revealing the dominant noncovalent interactions responsible for observed changes in selectivity in this flexible system.

Recognition in the Domain of Molecular Chirality: From Noncovalent Interactions to Separation of Enantiomers

It is not a coincidence that both chirality and noncovalent interactions are ubiquitous in nature and synthetic molecular systems. Noncovalent interactivity between chiral molecules underlies enantioselective recognition as a fundamental phenomenon regulating life and human activities. Thus, noncovalent interactions represent the narrative thread of a fascinating story which goes across several disciplines of medical, chemical, physical, biological, and other natural sciences. This review has been conceived with the awareness that a modern attitude toward molecular chirality and its consequences needs to be founded on multidisciplinary approaches to disclose the molecular basis of essential enantioselective phenomena in the domain of chemical, physical, and life sciences. With the primary aim of discussing this topic in an integrated way, a comprehensive pool of rational and systematic multidisciplinary information is provided, which concerns the fundamentals of chirality, a description of noncovalent interactions, and their implications in enantioselective processes occurring in different contexts. A specific focus is devoted to enantioselection in chromatography and electromigration techniques because of their unique feature as "multistep" processes. A second motivation for writing this review is to make a clear statement about the state of the art, the tools we have at our disposal, and what is still missing to fully understand the mechanisms underlying enantioselective recognition.

Nonadiabatic Molecular Dynamics with Extended Density Functional Tight-Binding: Application to Nanocrystals and Periodic Solids

In this work, we report a new methodology for nonadiabatic molecular dynamics calculations within the extended tight-binding (xTB) framework. We demonstrate the applicability of the developed approach to finite and periodic systems with thousands of atoms by modeling "hot" electron relaxation dynamics in silicon nanocrystals and electron-hole recombination in both a graphitic carbon nitride monolayer and a titanium-based metal-organic framework (MOF). This work reports the nonadiabatic dynamic simulations in the largest Si nanocrystals studied so far by the xTB framework, with diameters up to 3.5 nm. For silicon nanocrystals, we find a non-monotonic dependence of "hot" electron relaxation rates on the nanocrystal size, in agreement with available experimental reports. We rationalize this relationship by a combination of decreasing nonadiabatic couplings related to system size and the increase of available coherent transfer pathways in systems with higher densities of states. We emphasize the importance of proper treatment of coherences for obtaining such non-monotonic dependences. We characterize the electron-hole recombination dynamics in the graphitic carbon nitride monolayer and the Ti-containing MOF. We demonstrate the importance of spin-adaptation and proper sampling of surface hopping trajectories in modeling such processes. We also assess several trajectory surface hopping schemes and highlight their distinct qualitative behavior in modeling the excited-state dynamics in superexchange-like models depending on how they handle coherences between nearly parallel states.

Hydrogen Atom Transfer Driven Enantioselective Minisci Reaction of Alcohols

Catalytic enantioselective Minisci reactions have recently been developed but all instances so far utilize α-amino radical coupling partners. We report a substantial evolution of the enantioselective Minisci reaction that enables α-hydroxy radicals to be used, providing valuable enantioenriched secondary alcohol products. This is achieved through the direct oxidative coupling of two C-H bonds on simple alcohol and pyridine partners through a hydrogen atom transfer (HAT)-driven approach: a challenging process to achieve due to the numerous side reactions that can occur. Our approach is highly regioselective as well as highly enantioselective. Dicumyl peroxide, upon irradiation with 390 nm light, serves as both HAT reagent and oxidant whilst selectivity is controlled by use of a chiral phosphoric acid catalyst. Computational and experimental evidence provide mechanistic insight as to the origin of selectivity, revealing a stereodetermining deprotonation step distinct from the analogous reaction of amide-containing substrates.

Revisiting the Bonding Model for Gold(I) Species: The Importance of Pauli Repulsion Revealed in a Gold(I)-Cyclobutadiene Complex

Understanding the bonding of gold(I) species has been central to the development of gold(I) catalysis. Herein, we present the synthesis and characterization of the first gold(I)-cyclobutadiene complex, accompanied with bonding analysis by state-of-the-art energy decomposition analysis methods. Analysis of possible coordination modes for the new species not only confirms established characteristics of gold(I) bonding, but also suggests that Pauli repulsion is a key yet hitherto overlooked element. Additionally, we obtain a new perspective on gold(I)-bonding by comparison of the gold(I)-cyclobutadiene to congeners stabilized by p-, d-, and f-block metals. Consequently, we refine the gold(I) bonding model, with a delicate interplay of Pauli repulsion and charge transfer as the key driving force for various coordination motifs. Pauli repulsion is similarly determined as a significant interaction in AuI -alkyne species, corroborating this revised understanding of AuI bonding.

Unveiling a key catalytic pocket for the ruthenium NHC-catalysed asymmetric heteroarene hydrogenation

The chiral ruthenium(ii)bis-SINpEt complex is a versatile and powerful catalyst for the hydrogenation of a broad range of heteroarenes. This study aims to provide understanding of the active form of this privileged catalyst as well as the reaction mechanism, and to identify the factors which control enantioselectivity. To this end we used computational methods and in situ NMR spectroscopy to study the hydrogenation of 2-methylbenzofuran promoted by this system. The high flexibility and conformational freedom of the carbene ligands in this complex lead to the formation of a chiral pocket interacting with the substrate in a "lock-and-key" fashion. The non-covalent stabilization of the substrate in this particular pocket is an exclusive feature of the major enantiomeric pathway and is preserved throughout the mechanism. Substrate coordination leading to the minor enantiomer inside this pocket is inhibited by steric repulsion. Rather, the catalyst exhibits a "flat" interaction surface with the substrate in the minor enantiomer pathway. We probe this concept by computing transition states of the rate determining step of this reaction for a series of different substrates. Our findings open up a new approach for the rational design of chiral catalysts.

Concise enantioselective synthesis of non-proteinogenic α-aminoacids via an organocatalytic Mannich-type reaction

Scalable asymmetric synthesis of non-proteinogenic (S)-aminoacids and their derivatives based on an organocatalyzed Mannich-type reaction involving allomaltol applicable to subsequent RuIII-catalyzed oxidative cleavage has been developed.

An induced-fit model for asymmetric organocatalytic reactions: a case study of the activation of olefins via chiral Brønsted acid catalysts

We elucidate the stereo-controlling factors of the asymmetric intramolecular hydroalkoxylation of terminal olefins catalyzed by bulky Brønsted acids [Science2018, 359 (6383), 1501–1505] using high-level electronic structure methods.

Stereodivergent atom-transfer radical cyclization by engineered cytochromes P450

Naturally occurring enzymes can be a source of unnatural reactivity that can be molded by directed evolution to generate efficient biocatalysts with valuable activities. Owing to the lack of exploitable stereocontrol elements in synthetic systems, steering the absolute and relative stereochemistry of free-radical processes is notoriously difficult in asymmetric catalysis. Inspired by the innate redox properties of first-row transition-metal cofactors, we repurposed cytochromes P450 to catalyze stereoselective atom-transfer radical cyclization. A set of metalloenzymes was engineered to impose substantial stereocontrol over the radical addition step and the halogen rebound step in these unnatural processes, allowing enantio- and diastereodivergent radical catalysis. This evolvable metalloenzyme platform represents a promising solution to tame fleeting radical intermediates for asymmetric catalysis.

London Dispersion Helps Refine Steric A-Values: Dispersion Energy Donor Scales

We suggest a scale of dispersion energy donors (DEDs) that allows for direct comparisons with steric effects. This scale is based on the classic A-values and allows groups to reorient to minimize strain, thereby providing an advantage over raw group polarizabilities. The A-value can no longer be considered purely a steric factor. Even for groups that do not participate in charge transfer or electrostatic interactions, the A-value includes Pauli repulsion (steric hindrance) and attractive London dispersion (LD) interactions. Although the common assumption is that, at the distances found in monosubstituted cyclohexanes, steric demands are the key factors influencing conformer preferences, we show in this computational study that there is a non-negligible LD part. We use this system to build a DED scale and a complementary steric scale. These scales are quantitatively comparable, as they are based on the same system, and allow for comparison of the two competing interactions in experimentally relevant settings. In addition, we show that LD interactions can be used to explain puzzling data regarding relative group sizes.

Ternary complexes of chiral disulfonimides in transfer-hydrogenation of imines: the relevance of late intermediates in ion pair catalysis

In ion pairing catalysis, the structures of late intermediates and transition states are key to understanding and further development of the field. Typically, a plethora of transition states is explored computationally. However, especially for ion pairs the access to energetics via computational chemistry is difficult and experimental data is rare. Here, we present for the first time extensive NMR spectroscopic insights about the ternary complex of a catalyst, substrate, and reagent in ion pair catalysis exemplified by chiral Brønsted acid-catalyzed transfer hydrogenation. Quantum chemistry calculations were validated by a large amount of NMR data for the structural and energetic assessment of binary and ternary complexes. In the ternary complexes, the expected catalyst/imine H-bond switches to an unexpected O-H-N structure, not yet observed in the multiple hydrogen-bond donor-acceptor situation such as disulfonimides (DSIs). This arrangement facilitates the hydride transfer from the Hantzsch ester in the transition states. In these reactions with very high isomerization barriers preventing fast pre-equilibration, the reaction barriers from the ternary complex to the transition states determine the enantioselectivity, which deviates from the relative transition state energies. Overall, the weak hydrogen bonding, the hydrogen bond switching and the special geometrical adaptation of substrates in disulfonimide catalyst complexes explain the robustness towards more challenging substrates and show that DSIs have the potential to combine high flexibility and high stereoselectivity.

Noncovalent CH-π and π-π Interactions in Phosphoramidite Palladium(II) Complexes with Strong Conformational Preference

The weak noncovalent interactions and flexibility of ligands play a key role in enantioselective metal‐catalyzed reactions. In transition metal complexes and their catalytic applications, the experimental assessment and the design of key interactions is as difficult as the prediction of the enantioselectivities, especially for flexible, privileged ligands such as chiral phosphoramidites. Therefore, the interligand interactions in cis‐Pd^IIL₂Cl₂ phosphoramidite complexes were investigated by NMR spectroscopy and computations. We were able to induce a strong conformational preference by breaking the symmetry of the C₂‐symmetric side chain of one of the ligands, and shift the equilibrium between hetero‐ and homocomplexes towards heterocomplexes because of interligand interactions in the cis‐complexes. The modulation of aryl substituents was exploited, along with the solvent effect. The combined CH–π and π–π interactions reveal design patterns for binding and folding of chiral ligands and catalysts.

Nazarov Cyclizations Catalyzed by BINOL Phosphoric Acid Derivatives: Quantum Chemistry Struggles To Predict the Enantioselectivity

Quantum chemical calculations have successfully predicted the stereoselectivities of many BINOL phosphoric acid catalyzed reactions over the past 10-15 years. Herein we report a contrasting example: a reaction for which standard quantum chemistry techniques have proven unexpectedly ineffective at explaining the stereoselectivity. The Nazarov cyclizations of a divinyl ketone catalyzed by a BINOL phosphoric acid or H₈-BINOL dithiophosphoric acid were studied with a conventional contemporary quantum chemical approach, consisting of transition state optimizations with B3LYP-D3(BJ) and single-point calculations with several functionals in implicit solvent. Unexpectedly, different functionals gave widely different predictions of the level of enantioselectivity and were unable even to agree on which enantiomer of the product would predominate. Molecular dynamics simulations with the OPLS-AA force field provided evidence that the transition state geometries optimized with DFT in the gas phase or in implicit solvent are not good representations of the true transition states of these reactions in solution. One possible reason for this, which may also explain the failure of quantum chemical techniques to reliably predict the enantioselectivity, is the fact that the transition states contain ion pairs which are not highly organized and do not contain any strongly directional noncovalent interactions between the substrate and the catalyst.

Kinetic Analysis of a Cysteine-Derived Thiyl-Catalyzed Asymmetric Vinylcyclopropane Cycloaddition Reflects Numerous Attractive Noncovalent Interactions

Kinetic studies of a vinylcyclopropane (VCP) cycloaddition, catalyzed by peptide-based thiyl radicals, are described. Reactions were analyzed by using reaction progress kinetic analysis, revealing that ring-opening of the VCP is both rate- and enantio-determining. These conclusions are further corroborated by studies involving racemic and enantiopure VCP starting material. Noncovalent interactions play key roles throughout: both the peptide catalyst and VCP exhibit unproductive self-aggregation, which appears to be disrupted by binding between the catalyst and VCP. This in turn explains the requirement for the key catalyst feature, a substituent at the 4-position of the proline residue, which is required for both turnover/rate and selectivity.

Computational Investigation on the Origin of Atroposelectivity for the Cinchona Alkaloid Primary Amine-Catalyzed Vinylogous Desymmetrization of N-(2-t-Butylphenyl)maleimides

Mechanistic studies clarifying how chiral primary amines control the stereochemistry of vinylogous processes are rare. We report a density functional theory (DFT) computational study for the comprehension of the reaction mechanism of the vinylogous atroposelective desymmetrization of N-(2-t-butylaryl)maleimide catalyzed by 9-amino(9-deoxy)epi-quinine. Our results illustrate how the origin of the atroposelectivity was realized by the catalyst through steric and dispersion interactions. The role of N-Boc-l-Ph-glycine was crucial for the formation of a closed transition-state geometry and the activation of both reaction partners.

Could London Dispersion Force Control Regioselective (2 + 2) Cyclodimerizations of Benzynes? YES: Application to the Synthesis of Helical Biphenylenes

In recent years, London dispersion interactions, which are the attractive component of the van der Waals potential, have been found to play an important role in controlling the regio- and/or stereoselectivity of various reactions. Particularly, the dispersion interactions between substrates and catalysts (or ligands) are dominant in various selective catalyzes. In contrast, repulsive steric interactions, rather than the attractive dispersion interactions, between bulky substituents are predominant in most of the noncatalytic reactions. Herein, we demonstrate the first example of London dispersion-controlled noncatalytic (2 + 2) cyclodimerization of substituted benzynes to selectively afford proximal biphenylenes in high yields and regioselectivities, depending on the extent of dispersion interactions in the substituents. This method can be applied for the synthesis of novel helical biphenylenes, which would be fascinating for chemists as these compounds are potential skeletons for ligands, catalysts, and medicines.

Methane Adsorption on Heteroatom-Modified Maquettes of Porous Carbon Surfaces

Experimental and theoretical studies disagree on the energetics of methane adsorption on carbon materials. However, this information is critical for the rational design and optimization of the structure and composition of adsorbents for natural gas storage. The delicate nature of dispersion interactions, polarization of both the adsorbent and the adsorbate, interplay between H-bonding and tetrel bonding, and induced dipole/Coulomb interactions inherent to methane physisorption require computational treatment at the highest possible level of theory. In this study, we employed the smallest reasonable computational model, a maquette of porous carbon surfaces with a central site for substitution and methane binding. The most accurate predictions of methane adsorption energetics were achieved by electron-correlated molecular orbital theory CCSD(T) and hybrid density functional theory MN15 calculations employing a saturated, all-electron basis set. The characteristic geometry of methane adsorption on a carbon surface ("lander approach") arises due to bonding interactions of the adsorbent π-system with the proximal H-C bonds of methane, in addition to tetrel bonding between the antibonding orbital of the distal C-H bond and the central atom of the maquette (C, B, or N). The polarization of the electron density, structural deformations, and the comprehensive energetic analysis clearly indicate a ∼3 kJ mol^-1 preference for methane binding on the N-substituted maquette. The B-substituted maquette showed a comparable or lower binding energy than the unsubstituted, pure C model, depending on the level of theory employed. The calculated thermodynamic results indicate a strategy for incorporating electron-enriched substitutions (e.g., N) into carbon materials as a way to increase methane storage capacity over electron-deficient (e.g., B) modifications. The thermochemical analysis was revised for establishing a conceptual agreement between the experimental isosteric heat of adsorption and the binding enthalpies from statistical thermodynamics principles.

Organocatalysis emerging as a technology

During the last 20 years, organocatalysis has significantly advanced as a field. Thanks to contributions from hundreds of groups and companies around the world, the area has risen from a few mechanistically ill-defined niche reactions, to one of the most vibrant and innovative fields in chemistry, providing several well-defined generic activation modes for selective catalysis. Organocatalysis is also on the rise in industrial settings, especially for the production of enantiomers, which are of use in fine chemistry, pharma, crop-protection, and fragrance chemistry. Here we will look at some of the specific elements of organocatalysis that we think are particularly attractive and contribute to this successful development.

Dispersion forces in chirality recognition - a density functional and wave function theory study of diols

In the discussion of chirality recognition, steric considerations and strongly directed interactions such as hydrogen bonds are primarily discussed. However, given the sheer size of biomolecules, it is expected that dispersion forces could also play a determining role for aggregate formation and associated chirality recognition. With the example of diol molecules, we explore different factors in the formation of homo- and hetero-dimers as well as their relative stability. By comparing density functional results with the analysis of local correlation methods, we infer the impact of dispersion not only on the energies but also on the structures of such chiral aggregates. A local orbital based scheme is used to calculate wave function dispersion-free gradients and compare to uncorrected density functional structures.

Monitoring of the Pre-Equilibrium Step in the Alkyne Hydration Reaction Catalyzed by Au(III) Complexes: A Computational Study Based on Experimental Evidences

The coordination ability of the [(ppy)Au(IPr)]²⁺ fragment [ppy = 2-phenylpyridine, IPr = 1,3-bis(2,6-di-isopropylphenyl)-imidazol-2-ylidene] towards different anionic and neutral X ligands (X = Cl⁻, BF₄⁻, OTf⁻, H₂O, 2-butyne, 3-hexyne) commonly involved in the crucial pre-equilibrium step of the alkyne hydration reaction is computationally investigated to shed light on unexpected experimental observations on its catalytic activity. Experiment reveals that BF₄⁻ and OTf⁻ have very similar coordination ability towards [(ppy)Au(IPr)]²⁺ and slightly less than water, whereas the alkyne complex could not be observed in solution at least at the NMR sensitivity. Due to the steric hindrance/dispersion interaction balance between X and IPr, the [(ppy)Au(IPr)]²⁺ fragment is computationally found to be much less selective than a model [(ppy)Au(NHC)]²⁺ (NHC = 1,3-dimethylimidazol-2-ylidene) fragment towards the different ligands, in particular OTf⁻ and BF₄⁻, in agreement with experiment. Effect of the ancillary ligand substitution demonstrates that the coordination ability of Au(III) is quantitatively strongly affected by the nature of the ligands (even more than the net charge of the complex) and that all the investigated gold fragments coordinate to alkynes more strongly than H₂O. Remarkably, a stabilization of the water-coordinating species with respect to the alkyne-coordinating one can only be achieved within a microsolvation model, which reconciles theory with experiment. All the results reported here suggest that both the Au(III) fragment coordination ability and its proper computational modelling in the experimental conditions are fundamental issues for the design of efficient catalysts.

A New Ligand Design Based on London Dispersion Empowers Chiral Bismuth-Rhodium Paddlewheel Catalysts

Heterobimetallic bismuth-rhodium paddlewheel complexes with phenylglycine ligands carrying TIPS-groups at the meta-positions of the aromatic ring exhibit outstanding levels of selectivity in reactions of donor/acceptor and donor/donor carbenes; at the same time, the reaction rates are much faster and the substrate scope is considerably wider than those of previous generations of chiral [BiRh] catalysts. As shown by a combined experimental, crystallographic, and computational study, the new catalysts draw their excellent application profile largely from the stabilization of the chiral ligand sphere by London dispersion (LD) interactions of the peripheral silyl substituents.

Efficient Quantum Chemical Calculation of Structure Ensembles and Free Energies for Nonrigid Molecules

The application of quantum chemical, automatic multilevel modeling workflows for the determination of thermodynamic (e.g., conformation equilibria, partition coefficients, pK_a values) and spectroscopic properties of relatively large, nonrigid molecules in solution is described. Key points are the computation of rather complete structure (conformer) ensembles with extremely fast but still reasonable GFN2-xTB or GFN-FF semiempirical methods in the CREST searching approach and subsequent refinement at a recently developed, accurate r²SCAN-3c DFT composite level. Solvation effects are included in all steps by accurate continuum solvation models (ALPB, (D)COSMO-RS). Consistent inclusion of thermostatistical contributions in the framework of the modified rigid-rotor-harmonic-oscillator approximation (mRRHO) based on xTB/FF computed PES is also recommended.

Enantioselective synthesis of chiral tetrasubstituted allenes: harnessing electrostatic and noncovalent interactions in a bifunctional activation model for N-triflylphosphoramide catalysis

DFT calculation reveals that the oxygen activation model is preferred than the nitrogen activation model due to the preferred chiral electrostatic environment.

Bicyclic Guanidine-Catalyzed Asymmetric Cycloaddition Reaction of Anthrones-Bifunctional Binding Modes and Origin of Stereoselectivity

We report a computational analysis of the [5,5] bicyclic guanidine-catalyzed asymmetric cycloaddition reaction of anthrones. Based on extensive conformational search of key intermediates and transition states on the potential energy surface and density functional theory calculations, we studied five plausible binding modes between the guanidine catalyst and substrates for this reaction. Our results indicate that the most favorable pathway is a stepwise conjugate addition-Aldol sequence via the dual hydrogen-bond binding mode. The predicted level of enantioselectivity is in good agreement with experimental values. Trends in variation of substrates and catalysts have also been reproduced by our calculations. Decomposition analysis revealed the significance of aromatic interactions in stabilizing the key enantioselectivity-determining transition state structures.

Hydrogenative Metathesis of Enynes via Piano-Stool Ruthenium Carbene Complexes Formed by Alkyne gem-Hydrogenation

The only recently discovered gem-hydrogenation of internal alkynes is a fundamentally new transformation, in which both H atoms of dihydrogen are transferred to the same C atom of a triple bond while the other position transforms into a discrete metal carbene complex. [Cp*RuCl]₄ is presently the catalyst of choice: the resulting piano-stool ruthenium carbenes can engage a tethered alkene into either cyclopropanation or metathesis, and a prototypical example of such a reactive intermediate with an olefin ligated to the ruthenium center has been isolated and characterized by X-ray diffraction. It is the substitution pattern of the olefin that determines whether metathesis or cyclopropanation takes place: a systematic survey using alkenes of largely different character in combination with a computational study of the mechanism at the local coupled cluster level of theory allowed the preparative results to be sorted and an intuitive model with predictive power to be proposed. This model links the course of the reaction to the polarization of the double bond as well as to the stability of the secondary carbene complex formed, if metathesis were to take place. The first application of “hydrogenative metathesis” to the total synthesis of sinularones E and F concurred with this interpretation and allowed the proposed structure of these marine natural products to be confirmed. During this synthesis, it was found that gem-hydrogenation also provides opportunities for C–H functionalization. Moreover, silylated alkynes are shown to participate well in hydrogenative metathesis, which opens a new entry into valuable allylsilane building blocks. Crystallographic evidence suggests that the polarized [Ru–Cl] bond of the catalyst interacts with the neighboring R₃Si group. Since attractive interligand Cl/R₃Si contacts had already previously been invoked to explain the outcome of various ruthenium-catalyzed reactions, including trans-hydrosilylation, the experimental confirmation provided herein has implications beyond the present case.

Mechanistic Insight into the Stereoselective Cationic Polymerization of Vinyl Ethers

The control of the tacticity of synthetic polymers enables the realization of emergent physical properties from readily available starting materials. While stereodefined polymers derived from nonpolar vinyl monomers can be efficiently prepared using early transition metal catalysts, general methods for the stereoselective polymerization of polar vinyl monomers remain underdeveloped. We recently demonstrated asymmetric ion pairing catalysis as an effective approach to achieve stereoselective cationic polymerization of vinyl ethers. Herein, we provide a deeper understanding of stereoselective ion-pairing polymerization through comprehensive experimental and computational studies. These findings demonstrate the importance of ligand deceleration effects for the identification of reaction conditions that enhance stereoselectivity, which was supported by computational studies that identified the solution-state catalyst structure. An evaluation of monomer substrates with systematic variations in steric parameters and functional group identities established key structure-reactivity relationships for stereoselective homo- and copolymerization. Expansion of the monomer scope to include enantioenriched vinyl ethers enabled the preparation of an isotactic poly(vinyl ether) with the highest stereoselectivity (95.1% ± 0.1 meso diads) reported to date, which occurred when monomer and catalyst stereochemistry were fully matched under a triple diastereocontrol model. The more complete understanding of stereoselective cationic polymerization reported herein offers a foundation for the design of improved catalytic systems and for the translation of isotactic poly(vinyl ether)s to applied areas.

Protobranching as repulsion-induced attraction: a prototype for geminal stabilization

Noncovalent interactions are traditionally defined within the context of their attractive components, such as electrostatics and dispersion. Sources of molecular strain are derived through the destabilization of Coulombic and exchange repulsion. Due to this binary designation, the underlying origin of geminal stability with respect to alkanes (referred to as protobranching) has been an active subject for debate between these competing perspectives. We recast this stabilization as a complementary (Gestalt) interaction between dispersion and exchange repulsion, each impacting the other. We use triplet hydrogen and argon dimer as foundational van der Waals adducts to develop a procedure for the visualization and quantification of both exchange repulsion, Δρ_SCF, and medium-range correlation, ΔΔρ, as perturbations in electron density. We use the framework of the DFT-D3 correction to reproduce the shape of the dispersion potential at medium range and successfully model the trend in stability for the eighteen isomers of octane with a diverse series of functionals: BLYP, B3LYP, BP86, PBE, and PBE0. Collectively, our findings show that protobranching is a manifestation of steric repulsion-reduction in vibrational enthalpy and medium-range electron correlation.