Catalytic α-Deracemization of Ketones Enabled by Photoredox Deprotonation and Enantioselective Protonation

Visible-light-driven enantioselective protonation: a new Frontier in asymmetric catalysis

Enantioselective protonation represents a direct and effective method for constructing tertiary carbon stereocenters, which are prevalent in natural products and bioactive compounds. However, achieving catalytic asymmetric protonation has long posed challenges due to difficulties in controlling stereoselectivity. The small size of protons and their rapid transfer rates complicate the selective delivery to active intermediates, resulting in convoluted reaction pathways and pronounced background reactions. In recent years, light-driven photocatalysis has emerged as a powerful strategy in asymmetric protonation reactions, significantly broadening the range of reaction pathways and substrate types. In this review, we highlight recent advancements in this area, focusing on photoinduced single-electron transfer and energy transfer processes, where photosensitizers generate reactive intermediates for asymmetric protonation. This approach not only expands the scope of asymmetric catalysis but also presents new opportunities for green and sustainable chemistry, effectively addressing critical challenges in the construction of complex molecules.

Enantioselective Photocatalysis Using a Privileged Al-Salen Complex

ConspectusEnantioselective catalysts that exhibit broad generality are disruptive innovators in contemporary synthesis and are considered to be "privileged" on account of their expansive reactivity/selectivity profiles. Operating in the ground state, these species simultaneously regulate reactivity and orchestrate the translation of chiral information with exquisite efficiency: achieving parity in higher-energy (excited-state) scenarios remains a frontier in contemporary catalysis. Advancing this field will require new structure-activation guidelines to be delineated that reflect the energetic realities of achieving chiral induction in non-ground-state environments, thereby expediting the discovery of privileged photocatalysts. Earth-abundant aluminum-salen (Al-salen) complexes, which have a venerable history in ground-state enantioselective catalysis, show great promise in reconciling this disparity on account of their well-defined photophysical properties. In this Account, the potential of these catalysts in engaging various substrates via discrete activation modes to furnish optically enriched products with high levels of reliability is discussed. The deployment of commercial Al-salen complexes in the single electron transfer (SET)-enabled deracemization of cyclopropyl ketones is an exemplar. Irradiation of a commercial Al-salen complex augments the function of the catalyst to enable efficient deracemization (up to 98:2 e.r.), thereby eliminating the need for directing units. In stark contrast to conventional deracemization approaches that are predicated on C(sp3)-H deprotonation/reprotonation sequences, the transformation is characterized by a key C(sp3)-C(sp3) bond cleavage/cyclization process. Subsequent downstream manipulations of the enantioenriched products demonstrate the synthetic utility of the methodology. To illustrate mechanistic diversity using the same Al-salen complex, an enantioselective photocyclization under the auspices of energy transfer (EnT) catalysis is described. The photocyclization of acrylanilides under operationally simple conditions facilitates access to a diverse group of heterocyclic products (up to quantitative yield and 96:4 e.r.) using an Al-salen as the sole chiral operator. Collectively, these mechanistically distinct scenarios illustrate that light activation is a powerful strategy to augment the reactivity arsenal of a ubiquitous small molecule catalyst that is considered to be privileged in the ground state. The mechanistic foundations of reaction development are surveyed (combined experimental and computational approach), together with a perspective on the impact of this enabling technology in chiral functional molecule discovery. This Account serves to emphasize the synthetic utility of leveraging photochemical activation to mitigate intrinsic constraints of processes that might be considered to be thermochemically challenging.

Photocatalyst-Dependent Enantioselectivity in the Light-Driven Deracemization of Cyclic α-Aryl Ketones

We report a photoredox-enabled deracemization of cyclic α-aryl ketones that occurs with high stereoselectivity and yield and proceeds by mechanistically distinct proton transfer reactions. This reaction is jointly mediated by a visible-light photocatalyst and a chiral phosphate base cocatalyst under blue light irradiation. Notably, the extent of deracemization for this reaction exhibits an unexpected dependence on the identity of the photocatalyst and the concentration of a chiral base cocatalyst, wherein the extent of deracemization can be increased by employing photocatalysts with more positive ground-state reduction potentials, raising the concentration of the chiral base cocatalyst, or by a combination of these factors. This effect is attributed to two competing processes, back-electron transfer and deprotonation, which consume the same reaction intermediate, and we propose a kinetic model that rationalizes this behavior. We also demonstrate that the redox properties of the photocatalyst impact the stereoselectivity of the product-forming step, which is the dominant stereoselective step in this transformation. Together, these mechanistic insights facilitate a deeper understanding of the complexity of light-driven deracemization reactions involving reversible electron transfer and suggest approaches by which the stereoselectivity of these processes may be increased.

Deracemization of C(sp3)-H Arylated Carbonyl Compounds via Asymmetric Ion-Pairing Photoredox Catalysis

Deracemization of C(sp3)-H arylated carbonyl compounds faces limitations in terms of substrate scope. Through the photoactivation of the aryl group and the stereocontrol of the generated arene radical cation via asymmetric ion-pairing catalysis, we are able to achieve deracemization of carbonyl compounds arylated at both enolizable and unenolizable stereocenters. A diverse range of α-, β-, and γ-aryl ketones and esters, including natural products and medicinal derivatives, can be effectively converted into their enantiomers with high enantioselectivity. Mechanistic investigations through combined experimental and computational studies suggest that the reaction involves single-electron oxidation of electron-rich aryl groups, followed by a kinetic resolution of the resulting radical cation intermediates by the chiral phosphate anion. Deprotonation is identified as the stereodetermining step, while stereoselective back electron transfer and triplet-state quenching of 3 Mes-Acr1+* may also affect the enantioselectivity at the photostationary state.

Catalytic asymmetric photocycloaddition reactions mediated by enantioselective radical approaches

The use of olefins in the construction of cyclic compounds represents a powerful strategy for advancing the pharmaceutical industry. Photocycloaddition has attracted significant interest from chemists due to its ability to exploit simple and readily available olefins along with their reaction patterns under mild conditions. Moreover, the sustainable and versatile pathways for generating highly reactive intermediates can greatly enrich both substrate diversity and reaction patterns. As a result, numerous photocycloaddition reactions have been successfully developed, particularly asymmetric [2+2], [3+2], and [4+2] photocycloadditions mediated by enantioselective radical approaches, achieving remarkable enantioselectivities. This review offers a comprehensive overview of this rapidly evolving field, organizing the discussion into three distinct reaction types that facilitate the construction of enantioenriched derivatives of cyclobutanes, cyclopentanes, and cyclohexanes. Emphasis is placed on analyzing and summarizing established strategies aimed at circumventing the challenges posed by racemic background transformations. Additionally, the exploration of asymmetric [3+2] and [4+2] photocycloaddition reactions will be interwoven with a detailed discussion of the various substrate types involved. This systematic framework seeks to enhance understanding of the strategies employed to manage the high reactivity of radicals while achieving high enantioselectivity. Importantly, it aims to guide readers in identifying uncharted radical-based cycloaddition pathways, which possess significant potential to broaden the diversity of complex cyclic molecules.

Visible Light-Promoted Deracemization of α-Amino Aldehyde by Synergistic Chiral Primary Amine and Hypervalent Iodine Catalysis

α-Amino aldehydes, as versatile chiral synthons, are easily racemized under normal acid or base conditions, seriously limiting their synthetic potentials. We report herein an effective deracemization of α-amino aldehydes by a synergistic chiral primary amine and hypervalent iodine catalysis under visible light. The developed catalytic system allows for the on-demand production of α-Boc- or Cbz-protected amino aldehydes with high enantioselectivity. Mechanistic studies verified a photochemical Z-E isomerization mechanism that drives the deracemization process.

Chiral-at-metal catalysts: history, terminology, design, synthesis, and applications

For decades, advances in chiral transition metal catalysis have been closely tied to the development of customized chiral ligands. Recently, however, an alternative approach to this traditional metal-plus-chiral-ligand method has emerged. In this new strategy, chiral transition metal catalysts are composed entirely of achiral ligands, with the overall chirality originating exclusively from a stereogenic metal center. This "chiral-at-metal" approach offers the benefit of structural simplicity. More importantly, by removing the need for chiral elements within the ligand framework, it opens up new possibilities for designing innovative catalyst architectures with unique properties. As a result, chiral-at-metal catalysis is becoming an increasingly important area of research. This review offers a comprehensive overview and detailed insights into asymmetric chiral-at-metal catalysis, encouraging scientists to explore new avenues in asymmetric transition metal catalysis and driving innovation in both fundamental and applied research.

Photochemical Deracemization of 4,7-Diaza-1-isoindolinones by Unidirectional Hydrogen Atom Shuttling

By coupling a photochemical and a thermal step, a single chiral catalyst can establish a photostationary state in which the enantiopure form of a chiral compound is favored over its racemate. Following this strategy, 3-substituted 4,7-diaza-1-isoindolones were successfully deracemized (74-98% yield, 86-99% ee) employing 2.5 mol % of a photocatalyst. Key to the success of the reaction is the fact that a chiral benzophenone recruits selectively one enantiomer of the substrate for a photoinduced hydrogen atom transfer. A combination of computational and experimental studies suggests that the hydrogen atom is shuttled via the oxygen atom of the catalyst to the 4-nitrogen atom of the substrate.

Photochemical Deracemization of Lactams with Deuteration Enabled by Dual Hydrogen Atom Transfer

Photochemical deracemization has emerged as one of the most straightforward approaches to access highly enantioenriched compounds in recent years. While excited-state events such as energy transfer, single electron transfer, and ligand-to-metal charge transfer have been leveraged to promote stereoablation, approaches relying on hydrogen atom transfer, which circumvent the limitations imposed by the triplet energy and redox potential of racemic substrates, remain underexplored. Conceptually, the most attractive method for tertiary stereocenter deracemization might be hydrogen atom abstraction followed by hydrogen atom donation. However, implementing such a strategy poses significant challenges, primarily because the enantioenriched products are also reactive if the chiral catalyst is unable to differentiate between the two enantiomers. Herein we report a distinct dual hydrogen atom transfer strategy for photochemical deracemization of δ- and γ-lactams, achieving high enantioenrichment and deuterium incorporation despite the inherent reactivity of the products. Mechanistic studies reveal that benzophenone enables nonselective hydrogen atom abstraction while a tetrapeptide-derived thiol dictates the enantioselectivity of the hydrogen atom donation step. More importantly, a pyridine-based alcohol was found to play crucial roles in facilitating the hydrogen atom abstraction as well as enhancing the enantioselectivity of the hydrogen atom donation step.

Stereoselective Enesulfinamide-Sulfinylimine Tautomerization of β,β-Disubstituted Enesulfinamides

In the presence of cesium fluoride and organosilicon reagent, β,β-disubstituted NH-enesulfinamides undergo stereoselective enesulfinamide-sulfinylimine tautomerization at room temperature, resulting in the formation of α-branched N-sulfinyl ketimines in good yields with high stereoselectivity. A variety of acyclic ketone surrogates α-substituted with two electronically and sterically similar groups (e.g., methyl and ethyl), which are typically challenging to access through conventional protocols involving stereoselective protonation of enolates and their equivalents, have been effectively synthesized.

Asymmetric Synthesis of Chiral Cyclopropanes from Vinyl Sulfoxonium Ylides Catalyzed by a Chiral-at-Metal Rh(III) Complex

A chiral-at-metal Rh(III) complex-mediated [2+1] cyclization of vinyl sulfoxonium ylides with α,β-unsaturated 2,2-acylimidazoles has been demonstrated for the first time. This work provides a practical approach for assembling 1,2,3-trisubstituted chiral cyclopropane with alkyl structural units, which had advantages such as a wide range of substrates, good functional group tolerance, and mild reaction conditions. In addition, further amplification experiments and transformation of cycloaddition products were carried out to highlight the practicality of the method.

Enantioselective [2 + 2] Photocycloreversion Enables De Novo Deracemization Synthesis of Cyclobutanes

While photochemical deracemization significantly enhances atom economy by eliminating the necessity for additional oxidants or reductants, the laborious presynthesis of substrates from feedstock chemicals is often required, thereby compromising the practicality of this method. In this study, we propose a novel approach known as de novo deracemization synthesis, which involves direct utilization of simple substrates undergoing both photochemical transformation and reversible photochemical transformation. The efficient enantiocontrol of chiral catalysts in the latter process establishes an effective platform for deracemization. This alternative and practical approach to address the challenges of asymmetric photocatalysis has been successfully demonstrated in the photosensitized de novo deracemization synthesis of azaarene-functionalized cyclobutanes featuring three stereocenters, including an all-carbon quaternary center. By exclusively employing a suitable chiral catalyst to enable kinetically controlled [2 + 2] photocycloreversion, we pave a creative path toward achieving more cost-effective photochemical deracemization.

Harnessing the potential of acyl triazoles in bifunctional cobalt-catalyzed radical cross-coupling reactions

Persistent radicals facilitate numerous selective radical coupling reactions. Here, we have identified acyl triazole as a new and versatile moiety for generating persistent radical intermediates through single-electron transfer processes. The efficient generation of these persistent radicals is facilitated by the formation of substrate-coordinated cobalt complexes, which subsequently engage in radical cross-coupling reactions. Remarkably, triazole-coordinated cobalt complexes exhibit metal-hydride hydrogen atom transfer (MHAT) capabilities with alkenes, enabling the efficient synthesis of diverse ketone products without the need for external ligands. By leveraging the persistent radical effect, this catalytic approach also allows for the development of other radical cross-coupling reactions with two representative radical precursors. The discovery of acyl triazoles as effective substrates for generating persistent radicals and as ligands for cobalt catalysis, combined with the bifunctional nature of the cobalt catalytic system, opens up new avenues for the design and development of efficient and sustainable organic transformations.

The sugar cube: Network control and emergence in stereoediting reactions

Stereochemical editing strategies have recently enabled the transformation of readily accessible substrates into rare and valuable products. Typically, site selectivity is achieved by minimizing kinetic complexity by using protecting groups to suppress reactivity at undesired sites (substrate control) or by using catalysts with tailored shapes to drive reactivity at the desired site (catalyst control). We propose "network control," a contrasting paradigm that exploits hidden interactions between rate constants to greatly amplify modest intrinsic biases and enable precise multisite editing. When network control is applied to the photochemical isomerization of hexoses, six of the eight possible diastereomers can be selectively obtained. The amplification effect can be viewed as a mesoscale phenomenon between the limiting regimes of kinetic control in simple chemical systems and metabolic regulation in complex biological systems.

Photoexcited Palladium-Catalyzed Deracemization of Allenes

The different enantiomers of specific chiral molecules frequently exhibit disparate biological, physiological, or pharmacological properties. Therefore, the efficient synthesis of single enantiomers is of particular importance not only to the pharmaceutical sector but also to other industrial sectors, such as agrochemical and fine chemical industries. Deracemization, a process during which a racemic mixture is converted into a nonracemic product with 100% atom economy and theoretical yield, is the most straightforward method to access enantioenriched molecules but a challenging task due to a decrease in entropy and microscopic reversibility. Axially chiral allenes bear a distinctive structure of two orthogonal cumulative π-systems and are acknowledged as synthetically versatile synthons in organic synthesis. The selective creation of axially chiral allenes with high optical purity under mild reaction conditions has always been a very popular and hot topic in organic synthesis but remains challenging. Herein, a photoexcited palladium-catalyzed deracemization of nonprefunctionalized disubstituted allenes is disclosed. This method provides an efficient and economical strategy to accommodate a broad scope of allenes with good enantioselectivities and yields (53 examples, up to 96% yield and 95% ee). The use of a suitable chiral palladium complex with visible light irradiation is an essential factor in achieving this transformation. A metal-to-ligand charge transfer mechanism was proposed based on control experiments and density functional theory calculations. Quantum mechanical studies implicate dual modes of asymmetric induction behind our new protocol: (1) sterically controlled stereoselective binding of one allene enantiomer under the ground-state and (2) facile, noncovalent interaction-driven excited-state isomerization toward the opposite enantiomer. The success of this newly established photochemical deracemization strategy should provide inspiration for expansion to other multisubstituted allenes and will open up a new mode for enantioselective excited-state palladium catalysis.

Desymmetrization-Addition Reaction of Cyclopropenes to Imines via Synergistic Photoredox and Cobalt Catalysis

Herein, we designed a reaction for the desymmetrization-addition of cyclopropenes to imines by leveraging the synergy between photoredox and asymmetric cobalt catalysis. This protocol facilitated the synthesis of a series of chiral functionalized cyclopropanes with high yield, enantioselectivity, and diastereoselectivity (44 examples, up to 93% yield and >99% ee). A possible reaction mechanism involving cyclopropene desymmetrization by Co-H species and imine addition by Co-alkyl species was proposed. This study provides a novel route to important chiral cyclopropanes and extends the frontier of asymmetric metallaphotoredox catalysis.

Organic Synthesis Away from Equilibrium: Contrathermodynamic Transformations Enabled by Excited-State Electron Transfer

ConspectusChemists have long been inspired by biological photosynthesis, wherein a series of excited-state electron transfer (ET) events facilitate the conversion of low energy starting materials such as H2O and CO2 into higher energy products in the form of carbohydrates and O2. While this model for utilizing light-driven charge transfer to drive catalytic reactions thermodynamically "uphill" has been extensively adapted for small molecule activation, molecular machines, photoswitches, and solar fuel chemistry, its application in organic synthesis has been less systematically developed. However, the potential benefits of these approaches are significant, both in enabling transformations that cannot be readily achieved using conventional thermal chemistry and in accessing distinct selectivity regimes that are uniquely enabled by excited-state mechanisms. In this Account, we present work from our group that highlights the ability of visible light photoredox catalysis to drive useful organic transformations away from their equilibrium positions, addressing a number of long-standing synthetic challenges.We first discuss how excited-state ET enabled the first general methods for the catalytic anti-Markovnikov hydroamination of unactivated alkenes with alkyl amines. In these reactions, an excited-state iridium(III) photocatalyst reversibly oxidizes secondary amine substrates to their corresponding aminium radical cations (ARCs). These electrophilic N-centered radicals can then react with olefins to furnish valuable tertiary amine products with complete anti-Markovnikov regioselectivity. Notably, some of these products are less thermodynamically stable than their corresponding amine and alkene starting materials. We next present a strategy for light-driven C-C bond cleavage within various aliphatic alcohols mediated by homolytic activation of alcohol O-H bonds by excited-state proton-coupled electron transfer (PCET). The resulting alkoxy radical intermediates then undergo C-C β-scission to ultimately provide isomeric linear carbonyl products that are often higher in energy than their cyclic alcohol precursors. Applications of this chemistry for the light-driven depolymerization of lignin biomass, commercial phenoxy resin, hydroxylated polyolefin derivatives, and thermoset polymers are presented as well. We then describe a method for the contrathermodynamic positional isomerization of highly substituted olefins by means of cooperative photoredox and chromium(II) catalysis. In this work, generation of an allylchromium(III) species that can undergo highly regioselective in situ protodemetalation enables access to a less substituted and thermodynamically less stable positional isomer. Product selectivity in this reaction is determined by the large differential in oxidation potentials between differently substituted olefin isomers. Lastly, we discuss a light-driven deracemization reaction developed in collaboration with the Miller group, wherein a racemic urea substrate undergoes spontaneous optical enrichment upon visible light irradiation in the presence of an iridium(III) chromophore, a chiral Brønsted base, and a chiral peptide thiol. Excellent levels of enantioselectivity are achieved via sequential and synergistic proton transfer (PT) and H atom transfer (HAT) steps. Taken together, these examples highlight the ability of excited-state ET events to enable access to nonequilibrium product distributions across a wide range of catalytic, redox-neutral transformations in which photons are the only stoichiometric reagents.

Molecular Ratchets and Kinetic Asymmetry: Giving Chemistry Direction

Over the last two decades ratchet mechanisms have transformed the understanding and design of stochastic molecular systems-biological, chemical and physical-in a move away from the mechanical macroscopic analogies that dominated thinking regarding molecular dynamics in the 1990s and early 2000s (e.g. pistons, springs, etc), to the more scale-relevant concepts that underpin out-of-equilibrium research in the molecular sciences today. Ratcheting has established molecular nanotechnology as a research frontier for energy transduction and metabolism, and has enabled the reverse engineering of biomolecular machinery, delivering insights into how molecules 'walk' and track-based synthesisers operate, how the acceleration of chemical reactions enables energy to be transduced by catalysts (both motor proteins and synthetic catalysts), and how dynamic systems can be driven away from equilibrium through catalysis. The recognition of molecular ratchet mechanisms in biology, and their invention in synthetic systems, is proving significant in areas as diverse as supramolecular chemistry, systems chemistry, dynamic covalent chemistry, DNA nanotechnology, polymer and materials science, molecular biology, heterogeneous catalysis, endergonic synthesis, the origin of life, and many other branches of chemical science. Put simply, ratchet mechanisms give chemistry direction. Kinetic asymmetry, the key feature of ratcheting, is the dynamic counterpart of structural asymmetry (i.e. chirality). Given the ubiquity of ratchet mechanisms in endergonic chemical processes in biology, and their significance for behaviour and function from systems to synthesis, it is surely just as fundamentally important. This Review charts the recognition, invention and development of molecular ratchets, focussing particularly on the role for which they were originally envisaged in chemistry, as design elements for molecular machinery. Different kinetically asymmetric systems are compared, and the consequences of their dynamic behaviour discussed. These archetypal examples demonstrate how chemical systems can be driven inexorably away from equilibrium, rather than relax towards it.

Deracemization of Atropisomeric Biaryls Enabled by Copper Catalysis

Atropisomeric biaryls have found crucial applications in versatile chiral catalysts as well as in ligands for transition metals. Herein, we have developed an efficient crystallization-induced deracemization (CID) method to access chiral biaryls from their racemates with a chiral ammonium salt under copper catalysis including BINOL, NOBIN, and BINAM derivatives. After being significantly accelerated by its bidentate diamine ligand, the copper catalyst exhibits high efficiency and selectivity in racemizing biaryl skeletons, and the cocrystal complex would be enantioselectively formed together with chiral ammonium salt, which on acid-quenching would directly deliver chiral biaryl without further chromatographic purification. This CID process is easily scalable, and the chiral ammonium salt was nicely recoverable. Ligand effect studies showed that bulky alkyl substitution was an indispensable element to ensure efficient racemization, which probably proceeds via a radical-cation intermediate and further allows axial rotation by forming a delocalized radical.

Ratcheting synthesis

Synthetic chemistry has traditionally relied on reactions between reactants of high chemical potential and transformations that proceed energetically downhill to either a global or local minimum (thermodynamic or kinetic control). Catalysts can be used to manipulate kinetic control, lowering activation energies to influence reaction outcomes. However, such chemistry is still constrained by the shape of one-dimensional reaction coordinates. Coupling synthesis to an orthogonal energy input can allow ratcheting of chemical reaction outcomes, reminiscent of the ways that molecular machines ratchet random thermal motion to bias conformational dynamics. This fundamentally distinct approach to synthesis allows multi-dimensional potential energy surfaces to be navigated, enabling reaction outcomes that cannot be achieved under conventional kinetic or thermodynamic control. In this Review, we discuss how ratcheted synthesis is ubiquitous throughout biology and consider how chemists might harness ratchet mechanisms to accelerate catalysis, drive chemical reactions uphill and programme complex reaction sequences.

Asymmetric Synthesis of Tertiary α-Hydroxylation-Cyclopentanones via Synergetic Catalysis of Chiral-at-Metal Rhodium(III) Complexes/Pyrrolidine

Catalytic and asymmetric domino Michael/aldol reaction of 1,2-dicarbonyl compounds with α,β-unsaturated ketones under the synergetic catalysis of chiral-at-metal rhodium complexes and pyrrolidine to deliver tertiary α-hydroxylation-cyclopentanones (45-89% yields with 81-99% ee and up to >20:1 dr) bearing three contiguous stereogenic centers had been established. Moreover, the scalability and practical utility of this protocol were well demonstrated by employing a gram-scale reaction and some representative transformations.

Catalytic asymmetric conjugate addition of coumarins to unsaturated ketones catalyzed by a chiral-at-metal Rh(III) complex

The first catalytic asymmetric vinylogous Michael addition of coumarins to unsaturated ketones catalyzed by chiral rhodium catalysts has been established. This strategy allowed the synthesis of a variety of highly enantioenriched compounds containing coumarin skeletons in 41-99% yields and 84-99% ee. The developed reaction enriches the chemistry of catalytic asymmetric vinylogous Michael additions of 3-cyano-4-methylcoumarins. Furthermore, the protocol showed obvious advantages in reaction enantioselectivity. When the chiral rhodium catalyst was reduced to 0.06 mol%, a Gram-level reaction was still achieved to provide the desired products with 99% ee.

Catalytic Photochemical Deracemization via Short-Lived Intermediates

Upon irradiation in the presence of a suitable chiral catalyst, racemic compound mixtures can be converted into enantiomerically pure compounds with the same constitution. The process is called photochemical deracemization and involves the formation of short-lived intermediates. By opening different reaction channels for the forward reaction to the intermediate and for the re-constitution of the chiral molecule, the entropically disfavored process becomes feasible. Since the discovery of the first photochemical deracemization in 2018, the field has been growing rapidly. This review comprehensively covers the research performed in the area and discusses current developments. It is subdivided according to the mode of action and the respective substrate classes. The focus of this review is on the scope of the individual reactions and on a discussion of the mechanistic details underlying the presented reaction.

Creating a Defined Chirality in Amino Acids and Cyclic Dipeptides by Photochemical Deracemization

2,5-Diketopiperazines are cyclic dipeptides displaying a wide range of applications. Their enantioselective preparation has now been found possible from the respective racemates by a photochemical deracemization (53 examples, 74 % to quantitative yield, 71-99 % ee). A chiral benzophenone catalyst in concert with irradiation at λ=366 nm enables to establish the configuration at the stereogenic carbon atom C6 at will. If other stereogenic centers are present in the diketopiperazines they remain unaffected and a stereochemical editing is possible at a single position. Consecutive reactions, including the conversion into N-aryl or N-alkyl amino acids or the reduction to piperazines, occur without compromising the newly created stereogenic center. Transient absorption spectroscopy revealed that the benzophenone catalyst processes one enantiomer of the 2,5-diketopiperazines preferentially and enables a reversible hydrogen atom transfer that is responsible for the deracemization process. The remarkably long lifetime of the protonated ketyl radical implies a yet unprecedented mode of action.

Photoisomerization of "Partially Embedded Dihydropyridazine" with a Helical Structure

Herein, we report the synthesis of two "partially embedded fused-dihydropyridazine N-aryl aza[5]helicene derivatives" (PDHs) and the demonstration of their intrinsic photo-triggered multi-functional properties based on a Kekulé biradical structure. Introducing bulky electron-withdrawing trifluoromethyl or pentafluoroethyl groups into the aza[5]helicene framework (PDH-CF3 and -C2 F5 ) gives PDH axial chirality based on the helicity of the P and M forms, even at room temperature. Upon photo-irradiation of PDH-CF3 in a frozen solution, an ESR signal from the triplet biradical with zero-field splitting values, generated by N-N bond dissociation, was observed. However, when the irradiation was turned off, the ESR signal became silent, thus indicating the existence of two equilibria: between the biradical and quinoidal forms based on the Kekulé structure, and between N-N bond cleavage and recombination. The observed photo- and thermally induced behaviors indicate that T-type photochromic molecules are involved in the photoisomerization mechanism involving the two equilibria. Inspired by the photoisomerization, chirality control of PDH by photoracemization was achieved. Multiple functionalities, such as T-type photochromism, photo-excitation-mediated triplet biradical formation, and photoracemization, which are attributed to the "partially embedded dihydropyridazine" structure, are demonstrated.

Iridium-Catalyzed Enantioselective Alkene Hydroalkylation via a Heteroaryl-Directed Enolization-Decarboxylation Sequence

Upon exposure to a cationic Ir(I)-complex modified with the chiral diphosphine DuanPhos, hydroalkylations of styrenes and α-olefins with diverse heteroaryl tert-butyl acetates occur with complete branched selectivity and very high enantioselectivity. The initial adducts undergo acid promoted decarboxylation in situ to provide alkylated heteroarenes possessing defined β-stereocenters. The processes are postulated to proceed via a stereodefined chiral Ir-enolate, which arises upon heteroarene directed enolization of the heteroaryl acetate precursor. The method can be classified as an enantioselective decarboxylative C(sp3)-C(sp3) cross-coupling.

Multiplicative enhancement of stereoenrichment by a single catalyst for deracemization of alcohols

Stereochemical enrichment of a racemic mixture by deracemization must overcome unfavorable entropic effects as well as the principle of microscopic reversibility; recently, photochemical reaction pathways unveiled by the energetic input of light have led to innovations toward this end, most often by ablation of a stereogenic C(sp3)-H bond. We report a photochemically driven deracemization protocol in which a single chiral catalyst effects two mechanistically different steps, C-C bond cleavage and C-C bond formation, to achieve multiplicative enhancement of stereoinduction, which leads to high levels of stereoselectivity. Ligand-to-metal charge transfer excitation of a titanium catalyst coordinated by a chiral phosphoric acid or bisoxazoline efficiently enriches racemic alcohols that feature adjacent and fully substituted stereogenic centers to enantiomeric ratios up to 99:1. Mechanistic investigations support a pathway of sequential radical-mediated bond scission and bond formation through a common prochiral intermediate and reveal that, although the overall stereoenrichment is high, the selectivity in each individual step is moderate.

Recent Advances in Photochemical Asymmetric Three-Component Reactions

Over the past decades, asymmetric photochemical synthesis has garnered significant attention for its sustainability and unique ability to generate enantio-enriched molecules through distinct reaction pathways. Photochemical asymmetric three-component reactions have demonstrated significant potential for the rapid construction of chiral compounds with molecular diversity and complexity. However, noteworthy challenges persist, including the participation of high-energy intermediates such as radical species, difficulties in precise control of stereoselectivity, and the presence of competing background and side reactions. Recent breakthroughs have led to the development of sophisticated strategies in this field. This review explores the intricate mechanisms, synthetic applications, and limitations of these methods. We anticipate that it will contribute towards advancing asymmetric catalysis, photochemical synthesis, and green chemistry.

Enantioconvergent 6π Electrocyclization Enabled by Photoredox Racemization

This study presents a novel photoredox-enabled enantioconvergent catalytic strategy used to construct chiral 2H-1,3-benzoxazines via an unprecedented oxa-6π electrocyclization utilizing racemic α-substituted glycinates as substrates. The approach leverages a cobalt-based chiral Lewis acid catalyst, which promotes the transformation under thermal or photoredox conditions. While the thermal reaction selectively converts only the (S)-configured glycinates into enantioenriched 2H-1,3-benzoxazines (up to 96:4 e.r.), the addition of 0.5 mol % of a commercially available iridium photocatalyst under visible light irradiation transforms the reaction into an enantioconvergent process. Detailed mechanistic and time course studies of optically pure α-deuterated substrates revealed the presence of an enantiospecific kinetic isotope effect, which helped to clarify the role of both the photo- and chiral Lewis acid catalyst in the reaction sequence. In this dual catalytic system, the photocatalyst promotes a dynamic interconversion between the substrate enantiomers─a process not accessible via ground-state chemistry─while the chiral Lewis acid selectively transforms only the (S)-configured substrates. Further mechanistic evidence for the proposed mechanism is provided by linear free energy relationship analysis, which suggests that the stereodetermining step involves a 6π electrocyclization under both thermal and photoredox conditions.

Light-enabled deracemization of cyclopropanes by Al-salen photocatalysis

Privileged chiral catalysts-those that share common structural features and are enantioselective across a range of reactions-continue to transform the chemical-research landscape1. In recent years, new reactivity modes have been achieved through excited-state catalysis, processes activated by light, but it is unclear if the selectivity of ground-state privileged catalysts can be matched. Although the interception of photogenerated intermediates by ground-state cycles has partially addressed this challenge2, single, chiral photocatalysts that simultaneously regulate reactivity and selectivity are conspicuously scarce3. So far, precision donor-acceptor recognition motifs remain crucial in enantioselective photocatalyst design4. Here we show that chiral Al-salen complexes, which have well-defined photophysical properties, can be used for the efficient photochemical deracemization5 of cyclopropyl ketones (up to 98:2 enantiomeric ratio (e.r.)). Irradiation at λ = 400 nm (violet light) augments the reactivity of the commercial catalyst to enable reactivity and enantioselectivity to be regulated simultaneously. This circumvents the need for tailored catalyst-substrate recognition motifs. It is predicted that this study will stimulate a re-evaluation of many venerable (ground-state) chiral catalysts in excited-state processes, ultimately leading to the identification of candidates that may be considered 'privileged' in both reactivity models.

Asymmetric Olefin Isomerization via Photoredox Catalytic Hydrogen Atom Transfer and Enantioselective Protonation

Asymmetric olefin isomerization can be appreciated as an ideal synthetic approach to access valuable enantioenriched C═C-containing molecules due to the excellent atom economy. Nonetheless, its occurrence usually requires a thermodynamic advantage, namely, a higher stability of the product to the substrate. It has thus led to rather limited examples of success. Herein, we report a photoredox catalytic hydrogen atom transfer (HAT) and enantioselective protonation strategy for the challenging asymmetric olefin isomerization. As a paradigm, by establishing a dual catalyst system involving a visible light photosensitizer DPZ and a chiral phosphoric acid, with the assistance of N-hydroxyimide to perform HAT, a wide array of allylic azaarene derivatives, featuring α-tertiary carbon stereocenters and β-C═C bonds, was synthesized with high yields, ees, and E/Z ratios starting from the conjugated α-substituted alkenylazaarene E/Z-mixtures. The good compatibility of assembling deuterium on stereocenters by using inexpensive D2O as a deuterium source further underscores the broad applicability and promising utility of this strategy. Moreover, mechanistic studies have provided clear insights into its challenges in terms of reactivity and enantioselectivity. The exploration will robustly inspire the development of thermodynamically unfavorable asymmetric olefin isomerizations.

Catalytic Deracemization Reactions

Deracemization, which converts a racemate into its single enantiomer without separation of the intermediate, has gained renewed interest in asymmetric synthesis with its inherent atomic economy and high efficiency. However, this ideal process requires selective energy input and delicate reaction design to surmount the thermodynamical and kinetical constraints. With the rapid development of asymmetric catalysis, many catalytic strategies in concert with exogenous energy input have been exploited to facilitate this nonspontaneous enantioenrichment. In this perspective, we will discuss the basic ideas to accomplish catalytic deracemization, categorized by the three major exogenous energy sources including chemical (redox)-, photo- and mechanical energy from attrition. Emphasis will be given to the catalytic features and the underlying deracemization mechanism together with perspectives on future development.

Conversion of Racemic Alkyl Aryl Sulfoxides into Pure Enantiomers Using a Recycle Photoreactor: Tandem Use of Chromatography on Chiral Support and Photoracemization on Solid Support

Chiral sulfoxides are valuable in the fields of medicinal chemistry and organic synthesis. A recycle photoreactor utilizing the concept of deracemization, where a racemate is converted into a pure enantiomer, is developed and successfully applied in the syntheses of chiral alkyl aryl sulfoxides. The recycling system consists of rapid photoracemization using an immobilized photosensitizer and separation of the enantiomers via chiral high-performance liquid chromatography, and the desired pure chiral sulfoxides are obtained after 4-6 cycles. The key to the success of the system is the photoreactor site, wherein the photosensitizer 2,4,6-triphenylpyrylium is immobilized on the resin and irradiated (405 nm) to enable the rapid photoracemizations of the sulfoxides. As the green recycle photoreactor requires no chiral components, it should be a useful alternative system for application in producing chiral compounds.

A Mechanistically Deceiving Formation of Aryl(1-indanyl)ketones via Acid-Catalyzed Cyclization of ortho-Alkynylarylmethanols

The generation of reactive carbocation intermediates from ortho-alkynylarylmethanol substrates was utilized as a means for the synthesis of aryl(1-indanyl)ketones . Substrates with a tertiary carbon at the β-position to the arene generated a carbocation intermediate via dehydration/protonation, followed by cyclization and hydration to give indanylketone products. For substrates with a quaternary carbon at that position, a carbocation intermediate was generated by protonation/elimination of water, followed by a 1,2-shift and a subsequent cyclization/hydration to give highly substituted indanylketones.

Stereochemical Editing

Stereochemical editing has recently risen to prominence, allowing the direct editing of organic molecules with stereocenter(s) to adjust their relative stereochemistry at a late-stage. Several seminal light-driven stereochemical editing reactions such as deracemization and epimerization have been successively developed. Recently, Wendlandt and co-workers reported a versatile photochemical epimerization of unactivated tertiary stereogenic centers to rapidly prepare the stereoisomers that were previously challenging to access.

Stereospecific Acylative Suzuki–Miyaura Cross-Coupling: General Access to Optically Active α-Aryl Carbonyl Compounds

A novel strategy for the stereospecific Pd-catalyzed acylative cross-coupling of enantiomerically enriched alkylboron compounds has been developed. The protocol features an extremely high level of enantiospecificity to allow facile access to synthetically challenging and valuable chiral ketones and carboxylic acid derivatives. The use of a sterically encumbered and electron-rich phosphine ligand proved to be crucial for the success of the reaction. Furthermore, on the basis of experimental and computational studies, a unique mechanism for the transmetalation, assisted by the noncovalent interactions of the C(sp³)-based organoboron reagent, has been identified.

Light-driven redox deracemization of indolines and tetrahydroquinolines using a photocatalyst coupled with chiral phosphoric acid

The integration of oxidation and enantioselective reduction enables a redox deracemization to directly access enantioenriched products from their corresponding racemates. However, the solution of the kinetically microscopic reversibility of substrates used in this oxidation/reduction unidirectional event is a great challenge. To address this issue, we have developed a light-driven strategy to enable an efficient redox deracemization of cyclamines. The method combines a photocatalyst and a chiral phosphoric acid in a toluene/aqueous cyclodextrin emulsion biphasic co-solvent system to drive the cascade out-of-equilibrium. Systemic optimizations achieve a feasible oxidation/reduction cascade sequence, and mechanistic investigations demonstrate a unidirectional process. This single-operation cascade route, which involves initial photocatalyzed oxidation of achiral cyclamines to cyclimines and subsequent chiral phosphoric acid-catalyzed enantioselective reduction of cyclimines to chiral cyclamines, is suitable for constructing optically pure indolines and tetrahydroquinolines.

Multifaceted View on the Mechanism of a Photochemical Deracemization Reaction

Upon irradiation in the presence of a chiral benzophenone catalyst (5 mol %), a racemic mixture of a given chiral imidazolidine-2,4-dione (hydantoin) can be converted almost quantitatively into the same compound with high enantiomeric excess (80-99% ee). The mechanism of this photochemical deracemization reaction was elucidated by a suite of mechanistic experiments. It was corroborated by nuclear magnetic resonance titration that the catalyst binds the two enantiomers by two-point hydrogen bonding. In one of the diastereomeric complexes, the hydrogen atom at the stereogenic carbon atom is ideally positioned for hydrogen atom transfer (HAT) to the photoexcited benzophenone. Detection of the protonated ketyl radical by transient absorption revealed hydrogen abstraction to occur from only one but not from the other hydantoin enantiomer. Quantum chemical calculations allowed us to visualize the HAT within this complex and, more importantly, showed that the back HAT does not occur to the carbon atom of the hydantoin radical but to its oxygen atom. The achiral enol formed in this process could be directly monitored by its characteristic transient absorption signal at λ ≅ 330 nm. Subsequent tautomerization leads to both hydantoin enantiomers, but only one of them returns to the catalytic cycle, thus leading to an enrichment of the other enantiomer. The data are fully consistent with deuterium labeling experiments and deliver a detailed picture of a synthetically useful photochemical deracemization reaction.

Enantioselective Radical Addition to Ketones through Lewis Acid-Enabled Photoredox Catalysis

Photocatalysis opens up a new window for carbonyl chemistry. Despite a multitude of photochemical reactions of carbonyl compounds, visible light-induced catalytic asymmetric transformations remain elusive and pose a formidable challenge. Accordingly, the development of simple, efficient, and economic catalytic systems is the ideal pursuit for chemists. Herein, we report an enantioselective radical photoaddition to ketones through a Lewis acid-enabled photoredox catalysis wherein the in situ formed chiral N,N'-dioxide/Sc(III)-ketone complex serves as a temporary photocatalyst to trigger single-electron transfer oxidation of silanes for the generation of nucleophilic radical species, including primary, secondary, and tertiary alkyl radicals, giving various enantioenriched aza-heterocycle-based tertiary alcohols in good to excellent yields and enantioselectivities. The results of electron paramagnetic resonance (EPR) and high-resolution mass spectrum (HRMS) measurements provided favorable evidence for the stereocontrolled radical addition process involved in this reaction.

Light-empowered contra-thermodynamic stereochemical editing

Creating, conserving and modifying the stereochemistry of organic compounds has been the subject of significant research efforts in synthetic chemistry. Most synthetic routes are designed according to the stereoselectivity-determining step. Stereochemical editing is an alternative strategy, wherein the chiral-defining or geometry-defining steps are independent of the construction of the major scaffold or complexity. It enables late-stage alterations of stereochemistry and can generate isomers from a single compound. However, in many instances, stereochemical editing processes are contra-thermodynamic, meaning the transformation is unfavourable. To overcome this barrier, photocatalysis uses photogenerated radical species and introduces thermochemical biases. A range of synthetically valuable contra-thermodynamic stereochemical editing processes have been invented, including deracemization of chiral molecules, positional alkene isomerization and dynamic epimerization of sugars and diols. In this Review, we highlight the fundamental mechanisms of visible-light photocatalysis and the general reactivity modes of the photogenerated radical intermediates towards contra-thermodynamic stereochemical editing processes.

Photocatalytic deracemisation of cobalt(III) complexes with fourfold stereogenicity

The deracemisation of fourfold stereogenic cobalt(III) diketonates with a chiral photocatalyst is described. With only 0.5 mol% menthyl Ru(bpy)32+ photocatalyst, an enantiomeric enrichment of up to 88 : 12 e.r. was obtained for the major meridional diastereomers. Moreover, a distribution of configurationally stable diastereomers distinct from the thermodynamic ratio was observed upon reaching the photostationary state.

Stereochemical editing logic powered by the epimerization of unactivated tertiary stereocenters

The stereoselective synthesis of complex targets requires the precise orchestration of chemical transformations that simultaneously establish the connectivity and spatial orientation of desired bonds. In this work, we describe a complementary paradigm for the synthesis of chiral molecules and their isomers, which tunes the three-dimensional structure of a molecule at a late stage. Key to the success of this strategy is the development of a mild and highly general photocatalytic method composed of decatungstate polyanion and disulfide cocatalysts, which enable the interconversion of unactivated tertiary stereogenic centers that were previously configurationally fixed. We showcase the versatility of this method-and the implementation of stereoediting logic-by the rapid construction of chiral scaffolds that would be challenging to access using existing tools and by the late-stage stereoediting of complex targets.

Energy- and atom-efficient chemical synthesis with endergonic photocatalysis

Endergonic photocatalysis is the use of light to perform catalytic reactions that are thermodynamically unfavourable. While photocatalysis has become a powerful tool in facilitating chemical transformations, the light-energy efficiency of these processes has not gathered much attention. Exergonic photocatalysis does not take full advantage of the light energy input, producing low-energy products and heat, whereas endergonic photocatalysis incorporates a portion of the photon energy into the reaction, yielding products that are higher in free energy than the reactants. Such processes can enable catalytic, atom-economic syntheses of reactive compounds from bench-stable materials. With respect to environmental friendliness and carbon neutrality, endergonic photocatalysis is also of interest to large-scale industrial manufacturing, where better energy efficiency, less waste and value addition are highly sought. We therefore assess here the thermochemistry of several classes of reported photocatalytic transformations to showcase current advances in endergonic photocatalysis and point to their industrial potential.

Asymmetric Synthesis of Chiral Cyclopropanes from Sulfoxonium Ylides Catalyzed by a Chiral-at-Metal Rh(III) Complex

An enantioselective cyclopropanation reaction of sulfoxonium ylides with β,γ-unsaturated ketoesters catalyzed by a chiral rhodium catalyst has been realized. A variety of optically pure 1,2,3-trisubstituted cyclopropanes was synthesized in 48-89% yields, with up to 99% ee, and with dr >20:1. Furthermore, research shows that a weak coordination between the chiral rhodium catalyst and β,γ-unsaturated ketoesters was responsible for the high diastereoselectivity and enantioselectivity of the corresponding products.

Photochemical Deracemization of Chiral Alkenes via Triplet Energy Transfer

A visible-light-mediated, enantioselective approach to axially chiral alkenes is described. Starting from a racemic mixture, a major alkene enantiomer is formed due to selective triplet energy transfer from a catalytically active chiral sensitizer. A catalyst loading of 2 mol % was sufficient to guarantee consistently high enantioselectivities and yields (16 examples, 51%-quant., 81-96% ee). NMR studies and DFT computations revealed that triplet energy transfer is more rapid within the substrate-catalyst complex of the minor alkene enantiomer. Since this enantiomer is continuously racemized, the major enantiomer is enriched in the photostationary state.

Enantioselective [2 + 2] photocycloaddition of quinolone using a C1-symmetric chiral phosphoric acid as a visible-light photocatalyst

The enantioselective intra- and intermolecular [2 + 2] photocycloaddition of quinolone using a C₁-symmetric chiral phosphoric acid as a visible-light photocatalyst is developed. The thioxanthone chromophore on phosphoric acid plays an important role in both phototransformation and enantioselectivity.

Synthesis of 7-Arylthiomethyl Dibenzo[b,d]azepines through Imidoylative Heck Cyclization and CPA-Catalyzed Thio-Michael Addition/Enantioselective Protonation

A chiral phosphoric acid-catalyzed thio-Michael addition/enantioselective protonation has been developed for the first time. The reaction applies 7-methylene-6-aryl-7H-dibenzo[b,d]azepines, products of Pd-catalyzed imidoylative Heck cyclization, as Michael acceptors in reactions with a wide range of aryl thiols. Diversified 7-[(arylthio)methyl]-7H-dibenzo[b,d]azepines bearing a benzylic stereocenter and a thermodynamically regulated biaryl axis were produced with good to excellent enantioselectivity and 14-25:1 diastereoisomeric ratios.