Performance-enhancing asymmetric catalysis unlocks tuning without rebuilding

eTOC blurb

Performance-Enhancing Asymmetric Catalysis (PEAC) is a new strategy for catalyst optimization. Rather than relying on the slow, resource- and time-intensive covalent modification of chiral ligands, this approach "tunes" a single ligand using a library of simple, achiral acids. The counterions produced upon ion pair formation perturbate the catalyst's chiral pocket through non-covalent interactions to significantly improve enantioselectivity. This method creates an adaptable chiral ligand system that is well-suited for rapid, high-throughput screening and catalyst discovery.

Introduction

Catalysis is a fundamental tool for chemical synthesis, enabling the bottom-up preparation of complex molecules, including new therapeutics. Catalyst discovery and optimization efforts fall into two broad categories: bespoke catalysts for a specific target-driven campaign, and the development of catalysts to serve a reaction type with promising, often speculative, generality of application. The discovery stage is often described metaphorically as a search for a needle in a haystack. Finding the needle is only the first step. The next step is typically a resource-intensive marathon of iterative catalyst modification through multistep synthesis where chemists act as molecular sculptors, meticulously reshaping the chiral ligand's framework to craft the perfect fit. While this hypothesis-driven approach has produced countless catalysts of exquisite selectivity, it remains a formidable undertaking. What if we could instead begin from a limited number of chiral ligands, and design instead a "personal trainer" to coach the ligand to high selectivity for a reaction of interest. Performance-Enhancing Asymmetric Catalysis (PEAC) is a strategy realized recently in ion pair catalysis, wherein the design of achiral counterions was exploited to unlock the latent potential of a single chiral ligand.² This commentary explores this concept and its integration into catalyst design, one that promotes a paradigm shift from viewing chiral ligands as static scaffolds to unlocking their hidden potential as dynamic, tunable systems.

The Pursuit of the 'Perfect Pocket': From Static Scaffolds to Dynamic Systems

One view of the evolution of asymmetric catalysis based on noncovalent substrate-catalyst design charts a steady progression away from rigid design principles. Early inspiration from enzymology gave rise to the "lock-and-key" model, where a substrate fits into a pre-organized, static active site.³ This thinking guided the development of foundational synthetic catalysts, where stereocontrol is encoded in a covalently-defined, often rigid,⁴ chiral scaffold.⁵ Small ring bidentate chelates and other conformationally-limiting features are pervasive among highly selective catalysts. In spite of these design principles, optimizing new enantioselective reactions systems required painstaking efforts to modify a chiral catalyst for each hypothesis-driven cycle. While effective, this process is inherently slow and synthetically demanding.

The creation of focused non-covalent interactions within a catalyst-substrate transition state has become an increasingly effective design element in asymmetric catalysis. Ion-pairing catalysis, for instance, demonstrated that components not covalently bound to the chiral unit could exert profound stereochemical control. A premier example is Asymmetric Counteranion-Directed Catalysis (ACDC), where a chiral counterion itself becomes a dominant source of stereoselectivity, capable of inducing or even overriding the influence of a chiral cation.⁶ This approach extended catalyst design, but still relies on pairing one complex chiral entity with another.⁷ A more recent study explores the effect of achiral additives as co-catalytic activating agents in halogenation reactions, termed cross-assembled bifunctional catalysis.⁸ The achiral, Lewis basic co-catalyst is hypothesized to activate the electrophilic reagent while pairing with the chiral catalyst. Alternatively, it may affect the transition states through non-covalent interactions. Regardless, the additive is not expected to affect the otherwise rigid chiral phosphoric acid catalyst.

This stands in contrast to many established chiral proton catalysts, where the chiral information resides exclusively on the cation.⁹ In these systems, the counterion is typically a simple, dissociated species like ^-OTf or ^-NTf₂, which is not readily modified structurally to tune the catalyst's stereochemical environment.¹⁰

PEAC: Performance Enhancement Through Achiral Counterion Matching

The central hypothesis of PEAC is that an achiral counterion, when engaged in a sufficiently intimate ion pair, can perturb the topology of a chiral catalyst to create a chiral environment that is similar yet unique as the achiral counterion is modified. This conformational fine-tuning is strategically distinct from the traditional approach that recasts the chiral ligand with each iteration. This strategy was first demonstrated in the enantioselective addition of azide to nitroalkenes, serving as a compelling proof-of-principle (Figure 1A).² A chiral bis(amidine) ligand, when protonated by the strong Brønsted acid triflimidic acid (HNTf₂), showed minimal selectivity (15% ee). The dissociated triflimide anion (⁻NTf₂) is a quintessential "spectator," exerting little discriminating influence on the competing diastereomeric catalyst-substrate transition states.

Figure 1. The Achiral Counterion as a Molecular 'Coach'.

(A) Reactivity/selectivity-tuning by achiral acid co-catalyst. (B) DFT-calculated transition state models illustrating the PEAC concept. (B-Left) The chiral ligand paired with a weakly-coordinating, dissociated anion (Tf₂N⁻) results in an open and conformationally flexible binding pocket, leading to low enantioselectivity. (B-Right) The same chiral ligand paired with an optimal achiral bistriflamide counterion (I8). The tight, two-point ion pairing interaction constricts the ligand's diamine backbone, creating a more compact and well-defined chiral pocket that enhances enantioselectivity. (C) Traditional catalyst development, where a single chiral ligand is iteratively and covalently modified to optimize selectivity. This is a synthetically intensive process. (D) The Performance-Enhancing Asymmetric Catalysis (PEAC) approach, where a single chiral ligand is paired with a library of simple, achiral counterions, which tune its conformation to achieve high selectivity. This shifts the optimization effort from complex ligand synthesis to rapid counterion screening.

The selectivity outcome changed dramatically when a library of N-aryl triflamides (ArNHTf) was applied to the reaction. These achiral acids are designed to form proton-coupled ion pairs with the chiral ligand. They are sufficiently acidic (pKa (DMSO) = 2-3) to activate the catalyst but designed with an intimate ion pair in mind, thereby providing tunable stereo-modulators. By systematically screening these achiral "coaches," a 1,3-bistriflamide bearing a 5-trifluoromethylphenyl group (I8) was discovered that increased enantioselectivity to 85% ee. This transformation from a lackluster catalyst (L·HNTf₂) to a highly effective one was achieved without covalent modification of the chiral ligand.

DFT calculations support this hypothesis, revealing that the achiral counterion reshapes the catalytic pocket (Figure 1B). The bistriflamide engages in a two-point hydrogen-bonding interaction with the protonated bis(amidine) ligand, constricting the chiral scaffold and altering its conformation. This induced-fit mechanism pre-organizes the substrate and nucleophile more effectively in the transition state. In the case of the weakly coordinating triflimide, the catalyst remains in a more open conformation that correlates with lower selectivity. The achiral counterion, therefore, acts as a tuning agent for the chiral ligand, increasing its performance by creating a more selective catalyst.

The Selectivity-Generality Paradox

The use of PEAC to accelerate discovery and lead optimization of a new catalyst and reaction could also address the need for more general catalysts. The field of reaction development stands at the precipice of questioning the absolute generality of a specific catalyst type, both old and new. The need to alleviate substrate restrictions on catalysis has long been recognized (e.g. alkene epoxidation), and recently developed tools enable the quantification of relative generality. At the center of general reactions is the selectivity-generality paradox which recognizes the limitations of a rigid pocket design if generality is a goal. The use of noncovalent interactions in PEAC to achieve a binary array of catalyst modules may provide for a more flexible pocket that imparts greater generality without loss of the noncovalent interactions that contribute to high selectivity.

Future Directions: Toward Catalyst Systems and Accelerated Discovery

The PEAC approach invites us to reconsider long-standing goals in catalyst development, such as generality and the pursuit of "privileged" catalysts.¹¹ A privileged catalyst is typically a single core design/structure effective across a broad range of substrates, optimized by iterative chiral ligand modification (Figure 1C). PEAC suggests an alternative, more attainable paradigm: the "privileged ligand system." Here, a single, readily accessible chiral ligand can be evaluated with a large library of stoichiometric modifiers leading to a small, curated set of optimal achiral counterions to achieve high performance across a diverse substrate landscape (Figure 1D). We observed this in our work, where different substrates achieved maximum enantioselectivity with different achiral counterions. Generality, therefore, arises not from a single rigid scaffold but from a dynamic, adaptable catalytic system. The unbiased screening approach may benefit early-stage decisions based on sparse data, potentially leading to less general catalysts as judgment is exercised to decide what constitutes an actionable activity-selectivity profile.

This concept is perfectly synergistic with high-throughput experimentation (HTE). The synthetic barrier to creating a library of 100 simple achiral acids is vastly lower than that for preparing 100 complex chiral ligands. The discovery and optimization workflow can thus be dramatically accelerated, shifting the bottleneck from multi-step synthesis to rapid, parallel screening (Figure 2). A reaction that once might have been abandoned due to the low performance of an initial catalyst hit can now be explored more fully by screening a diverse set of achiral tuning agents.

Figure 2. Unlocking Performance via High-Throughput Screening.

A plot demonstrating the power of the PEAC strategy for rapid catalyst optimization. The enantioselectivity (% ee) for the azide addition to nitrostyrene is shown for a single chiral ligand paired with a representative subset of the 102 achiral N-aryl triflamide counterions screened. While the majority of counterions yield low-to-modest selectivity, a small number of "hits" emerge, dramatically enhancing performance and quickly identifying an optimal catalyst system without modifying the chiral ligand.

This leads to a final, more philosophical point. The traditional catalyst design process often aims to distill the source of selectivity down to a single, minimal, and elegant structural hypothesis. While intellectually satisfying, this reductionist goal can be limiting. The PEAC strategy encourages a more holistic, systems-thinking approach. It acknowledges that the active catalyst may be a complex, dynamic ensemble of ligand, counterion, and substrate. By embracing this complexity and developing tools to tune it systematically, more robust and adaptable catalyst solutions might be within reach.

Conclusion

The lion’s share of development in enantioselective catalysis has focused on the discovery and optimization of enantioselection first by covalent remodeling of a chiral ligand (Figure 1C). Must the modulating entity always be chiral? Our work suggests the answer is no. The PEAC approach demonstrates that strategically chosen achiral counterions can serve as powerful modulators (Figure 1A). If ACDC is akin to swapping one great player for another, PEAC advocates retention of the original player but with the addition of a dedicated coach. This coach—here, the achiral counterion—stands at an influential distance while providing direction to operate at a level they could not achieve alone. This conceptual shift opens the door to a more modular and efficient method of catalyst optimization, providing substantive practical benefit. Importantly, it enhances traditional hypothesis-driven catalyst development by increasing the number of parallel experiments to tune selectivity, thereby allowing structure-selectivity trends for the counterion to be more readily uncovered. The PEAC concept provides a new avenue to unlock the full potential of both existing and new chiral scaffolds, potentially turning underperforming ligands into more effective and selective agents of synthesis.

Biographies

Zihang Deng received his PhD degree from Vanderbilt University where his research interest focused on the development of asymmetric organocatalyst generality, and employing high-throughput screening in organocatalysis. He is currently a postdoctoral scholar at Harvard University (Richard Liu) developing innovative coupling reagents for bioconjugation.

Jeffrey N. Johnston is a Stevenson Professor of Chemistry at Vanderbilt University, where he leads a research program that develops new reactions and reagents for the synthesis of complex natural products and therapeutics. The integrative design of sustainable catalysts with strategic fragment-assembling schemes, and the acceleration of the discovery phase in enantioselective catalysis, are high priorities.