De-racemization of enantiomers versus de-epimerization of diastereomers--classification of dynamic kinetic asymmetric transformations (DYKAT)

Lewis Base-Catalyzed Dynamic Kinetic Asymmetric Transformation of Racemic Chlorosilanes en Route to Si-Stereogenic Silylethers

Enantiopure Si-stereogenic organosilanes are highly valued in the fields of organic synthesis, development of advanced materials, and drug discovery. However, they are not naturally occurring, and their synthesis has been largely confined to resolution of racemic silanes or desymmetrization of symmetric silanes. In contrast, the dynamic kinetic asymmetric transformation (DYKAT) of racemic organosilanes offers a mechanistically distinct approach and would broaden the accessibility of Si-stereogenic silanes in an enantioconvergent manner. In this study, we report a Lewis base-catalyzed DYKAT of racemic chlorosilanes. The chiral isothiourea catalyst, (S)-benzotetramisole, facilitates silyletherification with phenols, yielding (R)-silylethers in good yields with high enantioselectivity (27 examples, up to 86% yield, up to 98:2 er). Kinetic analysis, control experiments, and DFT calculations suggest that a two-catalyst-bound pentacoordinate silicate is responsible for the Si-configurational epimerization of the ion-paired tetracoordinated silicon intermediates.

Enantiodivergent Cyclization of Racemic Cyclohexadienones via Parallel Kinetic Asymmetric Transformation

Strategies that fully convert available racemic substrates into valuable enantioenriched products are urgently needed in organic synthesis. Reported herein is the first parallel kinetic asymmetric transformation of racemic cyclohexadienones. Racemic cyclohexadienones are first diastereoselectively converted into a new pair of racemic transient dienol intermediates, which are then parallel protonated by chiral phosphoric acid to deliver two sets of hydroindole products bearing a quaternary stereocenter with generally excellent enantioselectivity.

Switchable Chemo-, Regio- and Pseudo-Stereodivergence in Palladium-Catalyzed Cycloaddition of Allenes

Here, we report a strategy enabling triple switchable chemo-, regio-, and stereodivergence in newly developed palladium-catalyzed cycloadditions of allenes. An asymmetric pseudo-stereodivergent cycloaddition of allenes bearing a primary leaving group at the α-position, where a dynamic kinetic asymmetric hydroalkoxylation of racemic unactivated allenes was the enantio-determining step, is realized, providing four stereoisomers [(Z,R), (Z,S), (E,S), and (E,R)] containing a di-substituted alkene scaffold and a stereogenic center. By tuning reaction conditions, a mechanistically distinctive cycloaddition is uncovered selectively with the same set of substrates. By switching the position of the leaving group of allenes, a cycloaddition involving an intermolecular O-attack is disclosed. Diverse mechanisms of the cycloaddition reactions of allenes enable rapid access to structurally and stereochemically diverse 3,4-dihydro-2H-1,4-benzoxazines in high efficiency and selectivity.

Stereoretentive enantioconvergent reactions

Enantioconvergent reactions are pre-eminent in contemporary asymmetric synthesis as they convert both enantiomers of a racemic starting material into a single enantioenriched product, thus avoiding the maximum 50% yield associated with resolutions. All currently known enantioconvergent processes necessitate the loss or partial loss of the racemic substrate's stereochemical information, thus limiting the potential substrate scope to molecules that contain labile stereogenic units. Here we present an alternative approach to enantioconvergent reactions that can proceed with full retention of the racemic substrate's configuration. This uniquely stereo-economic approach is possible if the two enantiomers of a racemic starting material are joined together to form one enantiomer of a non-meso product. Experimental validation of this concept is presented using two distinct strategies: (1) a direct asymmetric coupling approach, and (2) a multicomponent approach, which exhibits statistical amplification of enantiopurity. Thus, the established dogma that enantioconvergent reactions require substrates that contain labile stereogenic units is shown to be incorrect.

Copper-catalyzed asymmetric allylic substitution of racemic/meso substrates

The synthesis of enantiomerically pure compounds is a pivotal subject in the field of chemistry, with enantioselective catalysis currently standing as the primary approach for delivering specific enantiomers. Among these strategies, Cu-catalyzed asymmetric allylic substitution (AAS) is significant and irreplaceable, especially when it comes to the use of non-stabilized nucleophiles (pK a > 25). Although Cu-catalyzed AAS of prochiral substrates has also been widely developed, methodologies involving racemic/meso substrates are highly desirable, as the substrates undergo dynamic processes to give single enantiomer products. Inspired by the pioneering work of the Alexakis, Feringa and Gennari groups, Cu-catalyzed AAS has been continuously employed in deracemization and desymmetrization processes for the synthesis of enantiomerically enriched products. In this review, we mainly focus on the developments of Cu-catalyzed AAS with racemic/meso substrates over the past two decades, providing an explicit outline of the ligands employed, the scope of nucleophiles, the underlying dynamic processes and their practical applications.

Stereodivergent Synthesis of Spiroaminals via Chiral Bifunctional Hydrogen Bonding Organocatalysis

Spiroaminals represent novel structural motifs prevalent in diverse natural products and biologically active molecules. Achieving their enantioselective synthesis is a highly desirable and challenging task in synthetic endeavors due to their intricate molecular frameworks. Herein, we accomplished the first stereodivergent construction of spiroaminals using chiral bifunctional organocatalyzed intramolecular 1,2-addition followed by an oxa-Michael addition cascade in a high atom and step economical pathway. A proper modulation of the cinchona-derived squaramide catalysts efficiently provided access to all the possible stereoisomers with high yield, diastereoselectivity, and excellent enantioselectivity while displaying a broad substrate tolerance. Additionally, we validated the scalability of the reaction and demonstrated the synthesis of variable spiroaminal scaffolds, confirming the viability of our protocol.

Catalytic Synthesis of (S)-CHBE by Directional Coupling and Immobilization of Carbonyl Reductase and Glucose Dehydrogenase

Ethyl (S)-4-chloro-3-hydroxybutyrate ((S)-CHBE) is an important chiral intermediate in the synthesis of the cholesterol-lowering drug atorvastatin. Studying the use of SpyTag/SpyCatcher and SnoopTag/SnoopCatcher systems for the asymmetric reduction reaction and directed coupling coenzyme regeneration is practical for efficiently synthesizing (S)-CHBE. In this study, Spy and Snoop systems were used to construct a double-enzyme directed fixation system of carbonyl reductase (BsCR) and glucose dehydrogenase (BsGDH) for converting 4-chloroacetoacetate (COBE) to (S)-CHBE and achieving coenzyme regeneration. We discussed the enzymatic properties of the immobilized enzyme and the optimal catalytic conditions and reusability of the double-enzyme immobilization system. Compared to the free enzyme, the immobilized enzyme showed an improved optimal pH and temperature, maintaining higher relative activity across a wider range. The double-enzyme immobilization system was applied to catalyze the asymmetric reduction reaction of COBE, and the yield of (S)-CHBE reached 60.1% at 30 °C and pH 8.0. In addition, the double-enzyme immobilization system possessed better operational stability than the free enzyme, and maintained about 50% of the initial yield after six cycles. In summary, we show a simple and effective strategy for self-assembling SpyCatcher/SnoopCatcher and SpyTag/SnoopTag fusion proteins, which inspires building more cascade systems at the interface. It provides a new method for facilitating the rapid construction of in vitro immobilized multi-enzyme complexes from crude cell lysate.

Nickel-Catalyzed Enantioselective Synthesis of Dienyl Sulfoxide

Sulfoxides are widely used in the pharmaceutical industry and as ligands in asymmetric catalysis. However, the efficient asymmetric synthesis of this structural motif remains limited. In this study, we disclosed a Ni-catalyzed enantioconvergent reaction that utilizes both racemic allenyl carbonates and β-sulfinyl esters. Our method employs cheap and more sustainable Ni(II) as a precatalyst and successfully overcomes the challenging poisoning effect and instability of sulfenate generated in situ. This enables the synthesis of a series of dienyl sulfoxides with enantioselectivity of up to 98 % ee. The product exhibits tremendous potential in various applications, including diastereoselective Diels-Alder reactions, coordination with transition metals, and incorporation into medicinal compounds, among others. Using a combination of experimental and computational methods, we have uncovered an interesting associated outersphere mechanism that contrasts with conventional mechanisms commonly observed in asymmetric transition metal catalysis.

Rhodium-Catalyzed Asymmetric Synthesis of 1,2-Disubstituted Allylic Fluorides

Although there are many methods for the asymmetric synthesis of monosubstituted allylic fluorides, construction of enantioenriched 1,2-disubstituted allylic fluorides has not been reported. To address this gap, we report an enantioselective synthesis of 1,2-disubstituted allylic fluorides using chiral diene-ligated rhodium catalyst, Et3 N ⋅ 3HF as a source of fluoride, and Morita Baylis Hillman (MBH) trichloroacetimidates. Kinetic studies show that one enantiomer of racemic MBH substrate reacts faster than the other. Computational studies reveal that both syn and anti π-allyl complexes are formed upon ionization of allylic substrate, and the syn complexes are slightly energetically favorable. This is in contrast to our previous observation for formation of monosubstituted π-allyl intermediates, in which the syn π-allyl conformation is strongly preferred. In addition, the presence of an electron-withdrawing group at C2 position of racemic MBH substrate renders 1,2-disubstituted π-allyl intermediate formation endergonic and reversible. To compare, formation of monosubstituted π-allyl intermediates was exergonic and irreversible. DFT calculations and kinetic studies support a dynamic kinetic asymmetric transformation process wherein the rate of isomerization of the 1,2-disubstituted π-allylrhodium complexes is faster than that of fluoride addition onto the more reactive intermediate. The 1,2-disubstituted allylic fluorides were obtained in good yields, enantioselectivity, and branched selectivity.

Sugar-Assisted Kinetic Resolutions in Metal/Chiral Amine Co-Catalyzed α-Allylations and [4+2] Cycloadditions: Highly Enantioselective Synthesis of Sugar and Chromane Derivatives

Functionalized triose-, furanose and chromane-derivatives were synthesized by the titled reactions. The sugar-assisted kinetic resolution/C-C bond-forming cascade processes generate a functionalized sugar derivative with a quaternary stereocenter in a highly enantioselective fashion (up to >99 % ee) by using a simple combination of metal and chiral amine co-catalysts. Notably, the interplay between the chiral sugar substrate and the chiral amino acid derivative allowed for the construction of a functionalized sugar product with high enantioselectivity (up to 99 %) also when using a combination of racemic amine catalyst (0 % ee) and metal catalyst.

Organocatalytic (3+3)-cycloaddition of ortho-substituted phenyl nitrones with aryl cyclopropane carbaldehydes: a facile access to enantioenriched 1,2-oxazinanes

The first asymmetric (3+3)-cycloaddition of ortho-substituted phenyl nitrones with aryl cyclopropane carbaldehydes has been demonstrated by secondary amine catalysts. While the other ortho-substituents gave 1,2-oxazinanes, ortho-hydroxy ones provided a novel class of tetrahydrochromeno-1,2-oxazine cores via rare 1,3-aryl migration, followed by cyclization. An unusual type of asymmetric approach was also recognized.

Scope and Mechanistic Probe into Asymmetric Synthesis of α-Trisubstituted-α-Tertiary Amines by Rhodium Catalysis

Asymmetric reactions that convert racemic mixtures into enantioenriched amines are of significant importance due to the prevalence of amines in pharmaceuticals, with about 60% of drug candidates containing tertiary amines. Although transition-metal catalyzed allylic substitution processes have been developed to provide access to enantioenriched α-disubstituted allylic amines, enantioselective synthesis of sterically demanding α-tertiary amines with a tetrasubstituted carbon stereocenter remains a major challenge. Herein, we report a chiral diene-ligated rhodium-catalyzed asymmetric substitution of racemic tertiary allylic trichloroacetimidates with aliphatic secondary amines to afford α-trisubstituted-α-tertiary amines. Mechanistic investigation is conducted using synergistic experimental and computational studies. Density functional theory calculations show that the chiral diene-ligated rhodium promotes the ionization of tertiary allylic substrates to form both anti and syn π-allyl intermediates. The anti π-allyl pathway proceeds through a higher energy than the syn π-allyl pathway. The rate of conversion of the less reactive π-allyl intermediate to the more reactive isomer via π-σ-π interconversion was faster than the rate of nucleophilic attack onto the more reactive intermediate. These data imply that the Curtin-Hammett conditions are met in the amination reaction, leading to dynamic kinetic asymmetric transformation. Computational studies also show that hydrogen bonding interactions between β-oxygen of allylic substrate and amine-NH greatly assist the delivery of amine nucleophile onto more hindered internal carbon of the π-allyl intermediate. The synthetic utility of the current methodology is showcased by efficient preparation of α-trisubstituted-α-tertiary amines featuring pharmaceutically relevant secondary amine cores with good yields and excellent selectivities (branched-linear >99:1, up to 99% enantiomeric excess).

Copper-Phosphido Catalysis: Enantioselective Addition of Phosphines to Cyclopropenes

We describe a copper catalyst that promotes the addition of phosphines to cyclopropenes at ambient temperature. A range of cyclopropylphosphines bearing different steric and electronic properties can now be accessed in high yields and enantioselectivities. Enrichment of phosphorus stereocenters is also demonstrated via a Dynamic Kinetic Asymmetric Transformation (DyKAT) process. A combined experimental and theoretical mechanistic study supports an elementary step featuring insertion of a CuI -phosphido into a carbon-carbon double bond. Density functional theory calculations reveal migratory insertion as the rate- and stereo-determining step, followed by a syn-protodemetalation.

Organocatalytic Activation of Donor-Acceptor Cyclopropanes: A Tandem (3 + 3)-Cycloaddition/Aryl Migration toward the Synthesis of Enantioenriched Tetrahydropyridazines

An organocatalytic enantioselective (3 + 3)-cycloaddition reaction of racemic cyclopropane carbaldehydes and aryl hydrazones has been demonstrated for the first time. A wide range of enantioenriched tetrahydropyridazines with an exocyclic double bond were obtained with moderate to good yields and good to excellent enantiomeric excesses. Mechanistic investigations hinted toward a matched/mismatched kinetic resolution, and control experiments and DFT calculations unveiled that 1,3-aryl migration was concerted and intramolecular and proceeds via a four-membered transition state.

Construction of C-B axial chirality via dynamic kinetic asymmetric cross-coupling mediated by tetracoordinate boron

Catalytic dynamic kinetic asymmetric transformation (DyKAT) provides a powerful tool to access chiral stereoisomers from racemic substrates. Such transformation has been widely employed on the construction of central chirality, however, the application in axial chirality remains underexplored because its equilibrium of substrate enantiomers is limited to five-membered metalacyclic intermediate. Here we report a tetracoordinate boron-directed dynamic kinetic asymmetric cross-coupling of racemic, configurationally stable 3-bromo-2,1-azaborines with boronic acid derivatives. A series of challenging C-B axially chiral compounds were prepared with generally good to excellent enantioselectivities. Moreover, this transformation can also be extended to prepare atropisomers bearing adjacent C-B and C-C diaxes with excellent diastereo- and enantio-control. The key to the success relies on the rational design of a reversible tetracoordinate boron intermediate, which is supported by theoretical calculations that dramatically reduces the rotational barrier of the original C-B axis and achieves the goal of DyKAT.

Construction of three contiguous stereocenters through amine-catalyzed asymmetric aldol reactions

Three contiguous stereocenters were constructed by an amino acid-catalyzed asymmetric aldol reaction of α-siloxyketones with racemizable α-haloaldehydes via dynamic kinetic resolution. One-pot catalytic asymmetric synthesis of the highly functionalized products could also be accomplished by the α-bromination of simple aldehydes and the subsequent asymmetric aldol reaction.

Role of Noncovalent Interactions in Inducing High Enantioselectivity in an Alcohol Reductive Deoxygenation Reaction Involving a Planar Carbocationic Intermediate

The involvement of planar carbocation intermediates is generally considered undesirable in asymmetric catalysis due to the difficulty in gaining facial control and their intrinsic stability issues. Recently, suitably designed chiral catalyst(s) have enabled a guided approach of nucleophiles to one of the prochiral faces of carbocations affording high enantiocontrol. Herein, we present the vital mechanistic insights from our comprehensive density functional theory (B3LYP-D3) study on a chiral Ir-phosphoramidite-catalyzed asymmetric reductive deoxygenation of racemic tertiary α-substituted allenylic alcohols. The catalytic transformation relies on the synergistic action of a phosphoramidite-modified Ir catalyst and Bi(OTf)₃, first leading to the formation of an Ir-π-allenyl carbocation intermediate through a turn-over-determining S_N1 ionization, followed by a face-selective hydride transfer from a Hantzsch ester analogue to yield an enantioenriched product. Bi(OTf)₃ was found to promote a significant number of ionic interactions as well as noncovalent interactions (NCIs) with the catalyst and the substrates (allenylic alcohol and Hantzsch ester), thus providing access to a lower energy route as compared to the pathways devoid of Bi(OTf)₃. In the nucleophilic addition, the chiral induction was found to depend on the number and efficacy of such key NCIs. The curious case of reversal of enantioselectivity, when the α-substituent of the allenyl alcohol is changed from methyl to cyclopropyl, was identified to originate from a change in mechanism from an enantioconvergent pathway (α-methyl) to a dynamic kinetic asymmetric transformation (α-cyclopropyl). These molecular insights could lead to newer strategies to tame tertiary carbocations in enantioselective reactions using suitable combinations of catalysts and additives.

Biocatalytic Enantioselective Synthesis of Atropisomers

Atropisomeric compounds are found extensively as natural products, as ligands for asymmetric transition-metal catalysis, and increasingly as bioactive and pharmaceutically relevant targets. Their enantioselective synthesis is therefore an important ongoing research target. While a vast majority of known atropisomeric structures are (hetero)biaryls, which display hindered rotation around a C-C single bond, our group's long-standing interest in the control of molecular conformation has led to the identification and stereoselective preparation of a variety of other classes of "nonbiaryl" atropisomeric compounds displaying restricted rotation around C-C, C-N, C-O, and C-S single bonds.Biocatalytic transformations are finding increasing application in both academic and industrial contexts as a result of a significant broadening of the range of biocatalytic reactions and sources of enzymes available to the synthetic chemist. In this Account, we summarize the main biocatalytic strategies currently available for the asymmetric synthesis of biaryl, heterobiaryl, and nonbiaryl atropisomers. As is the case with more traditional synthetic approaches to these compounds, most biocatalytic methodologies for the construction of enantioenriched atropisomers follow one of two distinct strategies. The first of these is the direct asymmetric construction of atropisomeric bonds. Synthetically applicable biocatalytic methodologies for this type of transformation are limited, despite the extensive research into the biosynthesis of (hetero)biaryls by oxidative homocoupling or cross-coupling of electron-rich arenes. The second of these is the asymmetric transformation of a molecule in which the bond that will form the axis already exists, and this approach represents the majority of biocatalytic strategies available to the synthetic organic chemist. This strategy encompasses a variety of stereoselective techniques including kinetic resolution (KR), desymmetrization, dynamic kinetic resolution (DKR), and dynamic kinetic asymmetric transformation (DYKAT).Nondynamic kinetic resolution (KR) of conformationally stable biaryl derivatives has provided the earliest and most numerous examples of synthetically useful methodologies for the enantioselective preparation of atropisomeric compounds. Lipases (i.e., enzymes that mediate the formation or hydrolysis of esters) are particularly effective and have attracted broad attention. This success has led researchers to broaden the scope of lipase-mediated transformations to desymmetrization reactions, in addition to a limited number of DKR and DYKAT examples. By contrast, our group has used redox enzymes, including an engineered galactose oxidase (GOase) and commercially available ketoreductases (KREDs), to desymmetrize prochiral atropisomeric diaryl ether and biaryl derivatives. Building on this experience and our long-standing interest in dynamic conformational processes, we later harnessed intramolecular noncovalent interactions to facilitate bond rotation at ambient temperatures, which allowed the development of the efficient DKR of heterobiaryl aldehydes using KREDs. With this Account we provide an overview of the current and prospective biocatalytic strategies available to the synthetic organic chemist for the enantioselective preparation of atropisomeric molecules.

Alcohol Dehydrogenases with anti-Prelog Stereopreference in Synthesis of Enantiopure Alcohols

Biocatalytic production of both enantiomers of optically active alcohols with high enantiopurities is of great interest in industry. Alcohol dehydrogenases (ADHs) represent an important class of enzymes that could be used as catalysts to produce optically active alcohols from their corresponding prochiral ketones. This review covers examples of the synthesis of optically active alcohols using ADHs that exhibit anti-Prelog stereopreference. Both wild-type and engineered ADHs that exhibit anti-Prelog stereopreference are highlighted.

Iridium-Catalyzed Enantioselective and Chemodivergent Allenylic Alkylation of Vinyl Azides for the Synthesis of α-Allenylic Amides and Ketones

The first enantioselective synthesis of α-allenylic amides and ketones through allenylic alkylation of vinyl azides is reported. In these chemodivergent reactions, cooperatively catalyzed by a Ir^I /(phosphoramidite,olefin) complex and Sc(OTf)₃ , vinyl azides act as the surrogate for both amide enolates and ketone enolates. The desiccant (molecular sieves) plays a crucial role in controlling the chemodivergency of this enantioconvergent and regioselective reaction: Under otherwise identical reaction conditions, the presence of the desiccant led to α-allenylic amides, while its absence resulted in α-allenylic ketones. Utilizing racemic allenylic alcohols as the alkylating agent, the overall process represents a dynamic kinetic asymmetric transformation (DyKAT), where both the products are formed with the same absolute configuration. To the best of our knowledge, this is the first example of the use of vinyl azide as the ketone enolate surrogate in an enantioselective transformation.

Heavy-Atom Kinetic Isotope Effects: Primary Interest or Zero Point?

Chemists have many options for elucidating reaction mechanisms. Global kinetic analysis and classic transition-state probes (e.g., LFERs, Eyring) inevitably form the cornerstone of any strategy, yet their application to increasingly sophisticated synthetic methodologies often leads to a wide range of indistinguishable mechanistic proposals. Computational chemistry provides powerful tools for narrowing the field in such cases, yet wholly simulated mechanisms must be interpreted with great caution. Heavy-atom kinetic isotope effects (KIEs) offer an exquisite but underutilized method for reconciling the two approaches, anchoring the theoretician in the world of calculable observables and providing the experimentalist with atomistic insights. This Perspective provides a personal outlook on this synergy. It surveys the computation of heavy-atom KIEs and their measurement by NMR spectroscopy, discusses recent case studies, highlights the intellectual reward that lies in alignment of experiment and theory, and reflects on the changes required in chemical education in the area.

One out of Four: Kinetic Resolution of Stereoisomeric Mixtures of Secondary Alcohols with a Quaternary Carbon Atom in the β-Position by Cu-H-Catalyzed Enantioselective Silylation

An enantio- and diastereoselective Cu-H-catalyzed silylation of acyclic secondary alcohols with a vicinal quaternary stereocenter is reported. The reaction kinetically selects one out of four stereoisomers, affording the fastest-reacting stereoisomer as the silyl ether in enantio- and diastereomerically enriched form. The obtained motif with a quaternary carbon atom in the β-position of the hydroxy group is otherwise not easy to access.

Visible-Light-Driven Catalytic Deracemization of Secondary Alcohols

Deracemization of racemic chiral compounds is an attractive approach in asymmetric synthesis, but its development has been hindered by energetic and kinetic challenges. Here we describe a catalytic deracemization method for secondary benzylic alcohols which are important synthetic intermediates and end products for many industries. Driven by visible light only, this method is based on sequential photochemical dehydrogenation followed by enantioselective thermal hydrogenation. The combination of a heterogeneous dehydrogenation photocatalyst and a chiral molecular hydrogenation catalyst is essential to ensure two distinct pathways for the forward and reverse reactions. These reactions convert a large number of racemic aryl alkyl alcohols into their enantiomerically enriched forms in good yields and enantioselectivities.

Dynamic Kinetic Asymmetric Transformation of Racemic Diastereomers: Diastereo- and Enantioconvergent Michael-Henry Reactions to Afford Spirooxindoles Bearing Furan-Fused Rings

Dynamic kinetic asymmetric transformation (DYKAT) reactions of racemic diastereomer mixtures that afford the products as essentially single diastereomers with high enantioselectivities are described. We demonstrated the DYKAT in the diastereo- and enantioselective synthesis of spirooxindoles bearing furan-fused rings. The starting materials of the DYKAT, dihydrobenzofuranone derivatives, were synthesized in racemic diastereomer mixtures, and these were transformed to the spirooxindole derivatives in high yields with high diastereo- and enantioselectivities through Michael-Henry cascade reactions with nitrostyrenes under organocatalytic conditions. In the reactions, regardless the stereochemistry of the starting materials, all the four isomers were transformed to single diastereomers with high enantioselectivities, and four new chiral centers were created.

Additions to Racemates: A Strategy for Developing Asymmetric Cross-Coupling Reactions

In this Account, the authors describe their progress in developing catalytic asymmetric C(sp3)–C(sp3) and C(sp3)–C(sp2) cross-coupling reactions. Whereas most catalytic enantioselective transformations rely on prochiral or meso starting materials, strategies that use racemic starting materials are rare. Key features of these reactions are efficient mechanisms for deracemization. Here, the authors present copper-catalyzed alkylation and rhodium-catalyzed Suzuki–Miyaura-type arylation reactions, their underlying mechanisms, and their applications in complex-molecule syntheses.

Catalytic asymmetric and stereodivergent oligonucleotide synthesis

We report the catalytic stereocontrolled synthesis of dinucleotides. We have demonstrated, for the first time to our knowledge, that chiral phosphoric acid (CPA) catalysts control the formation of stereogenic phosphorous centers during phosphoramidite transfer. Unprecedented levels of diastereodivergence have also been demonstrated, enabling access to either phosphite diastereomer. Two different CPA scaffolds have proven to be essential for achieving stereodivergence: peptide-embedded phosphothreonine-derived CPAs, which reinforce and amplify the inherent substrate preference, and C2-symmetric BINOL-derived CPAs, which completely overturn this stereochemical preference. The presently reported catalytic method does not require stoichiometric activators or chiral auxiliaries and enables asymmetric catalysis with readily available phosphoramidites. The method was applied to the stereocontrolled synthesis of diastereomeric dinucleotides as well as cyclic dinucleotides, which are of broad interest in immuno-oncology as agonists of the stimulator of interferon genes (STING) pathway.

Dynamic Kinetic Resolution of α-Trifluoromethyl Hemiaminals without α-Hydrogen via NHC-Catalyzed O-Acylation

Following the well-recognized dynamic kinetic resolution (DKR) of hemiaminals with α-hydrogen under lipase and chiral DMAP catalysis, the unprecedented DKR of hemiaminals without α-hydrogen was developed via N-heterocyclic carbene catalyzed O-acylation of 3-hydroxy-3-trifluoromethylbenzosultams. The racemic hemiaminals without α-hydrogen were effectively racemized and differentiated by chiral NHCs under basic conditions. The resulting esters were obtained in high yields with good to high enantioselectivities.

Kinetic Resolution of Aziridines Enabled by N-Heterocyclic Carbene/Copper Cooperative Catalysis: Carbene Dose-Controlled Chemo-Switchability

Catalytic kinetic resolution (KR) and dynamic kinetic asymmetric transformation (DyKAT) are alternative and complementary avenues to access chiral stereoisomers of both starting materials and reaction products. The development of highly efficient chiral catalytic systems for kinetically controlled processes has therefore been one of the linchpins in asymmetric synthesis. N-heterocyclic carbene (NHC)/copper cooperative catalysis has enabled highly efficient KR and DyKAT of racemic N-tosylaziridines by [3+3] annulation with isatin-derived enals, leading to highly enantioenriched N-tosylaziridine derivatives (up to >99 % ee) and a large library of spirooxindole derivatives with high structural diversity and stereoselectivity (up to >95:5 d.r., >99 % ee). Mechanistic studies suggest that the NHC can bind reversibly to the copper catalyst without compromising its catalytic activity and regulate the catalytic activity of the copper complex to switch the chemoselection between KR and DyKAT.

Enantioconvergent Cu-Catalyzed Intramolecular C-C Coupling at Boron-Bound C(sp3) Atoms of α-Aminoalkylboronates Using a C1-Symmetrical 2,2'-Bipyridyl Ligand Attached to a Helically Chiral Macromolecular Scaffold

Enantioconvergent intramolecular coupling of α-(2-bromobenzoylamino)benzylboronic esters was achieved using a copper catalyst having helically chiral macromolecular bipyridyl ligand, PQXbpy. Racemic α-(2-bromobenzoylamino)benzylboronic esters were converted into (R)-configured 3-arylisoindolinones with high enantiopurity using right-handed helical PQXbpy as a chiral ligand in a toluene/CHCl₃ mixed solvent. When enantiopure (R)- and (S)-configured boronates were separately reacted under the same reaction conditions, both afforded (R)-configured products through formal stereoinvertive and stereoretentive processes, respectively. From these results, a mechanism involving deracemization of organocopper intermediates in the presence of PQXbpy is assumed. PQXbpy switched its helical sense to left-handed when a toluene/1,1,2-trichloroethane mixed solvent was used, resulting in the formation of the corresponding (S)-products from the racemic starting material.

Asymmetric Dearomatization of Indole by Palladium/PC-Phos-Catalyzed Dynamic Kinetic Transformation

A palladium-catalyzed intermolecular dynamic kinetic asymmetric dearomatization of 3-arylindoles with internal alkynes was developed with the use of achiral Xantphos and chiral sulfinamide phosphine ligand (PC-Phos) as the co-ligands. This method could deliver various spiro[indene-1,3'-indole] compounds in good yields (up to 95 % yield) with up to 98 % ee. The salient features of the transformation include the use of readily available substrates, ease of scale-up and the versatile functionalization of the products. The mechanistic experiments gave some insights on active intermediates.