Combined Theoretical and Experimental Studies Unravel Multiple Pathways to Convergent Asymmetric Hydrogenation of Enamides

Asymmetric Synthesis of N-Hydroxyethyl Amino Indane Derivatives Catalyzed by an Engineered Imine Reductase

Asymmetric synthesis of chiral N-hydroxyethyl amino indane derivatives remains challenging. In this study, an imine reductase mutant (IR262-F185E/F229L) was identified with high enantioselectivity toward various N-hydroxyethyl imino derivatives. Furthermore, a continuous fed-batch strategy was designed to avoid the hydrolysis of imines, and up to 200 mM 1-((2-hydroxyethyl)imino)-2,3-dihydro-1H-indene-4-carbonitrile could be completely converted into (S)-1-((2-hydroxyethyl)amino)-2,3-dihydro-1H-indene-4-carbonitrile in 70% isolated yield and >99% ee, demonstrating a great potential for the synthesis of the ozanimod intermediate in industrial applications.

Enantioselective Synthesis of Chiral β2-Amino Phosphorus Derivatives via Nickel-Catalyzed Asymmetric Hydrogenation

Compared with chiral β3-amino phosphorus compounds, which can be easily derived from natural optically pure α-amino acids, obtaining chiral β2-amino phosphorus derivatives remains a challenge. These derivatives, which cannot be derived from chiral natural amino acids, possess unique biological activities or potential catalytic activities. Herein, highly enantioselective hydrogenation for the preparation of chiral β2-amino phosphorus derivatives from E-β-enamido phosphorus compounds is reported by using a green and low-cost earth-abundant metal nickel catalyst (13 examples of 99% ee). In particular, this catalytic system provides the same enantiomer product from the E- and Z-alkene substrates, and the E/Z-substrate mixtures provide good results (up to 96% ee). The products can be diversely derivatized, and the derivatives exhibit good catalytic activities as novel chiral β2-aminophosphine ligands. Density functional theory calculations reveal that the weak attractive interactions between the nickel catalyst and the substrate are crucial for achieving perfect enantioselectivities. In addition, the different coordination modes between the E- or Z-substrates and the catalyst may result in the formation of the same enantiomer product.

Asymmetric and Chemoselective Iridium Catalyzed Hydrogenation of Conjugated Unsaturated Oxime Ethers

Research on the chemoselective metal-catalyzed hydrogenation of conjugated π-systems has mostly been focussed on enones. Herein, we communicate the understudied asymmetric hydrogenation of enimines catalyzed by N,P-iridium complexes and chemoselective toward the alkene. A number of enoxime ethers underwent hydrogenation smoothly to yield the desired products in high yield and stereopurity (up to 99 % yield, up to 99 % ee). No hydrogenation of the C=N π-bond was observed under the applied reaction conditions (20 bar H2, rt, DCM). It was demonstrated that the chiral oxime ether could be hydrolyzed into the ketone with complete preservation of the installed stereogenity at the α-carbon. At last, a binding mode of the substrate to the active iridium catalyst and the consequence for the stereoselective outcome was proposed.

The Effect of Conformational Freedom vs Restriction on the Rate in Asymmetric Hydrogenation: Iridium-Catalyzed Regio- and Enantioselective Monohydrogenation of Dienones

Transition metal-catalyzed asymmetric hydrogenation constitutes an efficient strategy for the preparation of chiral molecules. When dienes are subjected to hydrogenation, control over regioselectivity still presents a large challenge and the fully saturated alkane is often yielded. A few successful monohydrogenations of dienes have been reported, but hitherto these are only efficient for dienes comprised of two distinctly different olefins. Herein, the reactivity of a conjugated carbonyl compound as a function of their conformational freedom is studied, based on a combined experimental and theoretical approach. It was found that alkenes in the (s)-cis conformation experience a large rate acceleration while (s)-trans restrained alkenes undergo hydrogenation slowly. Ultimately, this reactivity aspect was exploited in a novel method for the monohydrogenation of dienes based on conformational restriction ((s)-cis vs (s)-trans). This mode of discrimination conceptually differs from existing monohydrogenations and dienones constructed of two olefins similar in nature could efficiently be hydrogenated to the chiral alkene (up to 99 % ee). The extent of regioselection is even powerful enough to overcome the conventional reactivity order of substituted olefins (di>tri>tetra). This high yielding and atom-economical protocol provides an interesting opportunity to instal a stereogenic center on a carbocycle, while leaving a synthetically useful alkene untouched.

Axis-Unfixed Biphenylphosphine-Oxazoline Ligands: Design and Applications in Asymmetric Catalytic Reactions

The design and synthesis of chiral ligands plays an important role in asymmetric catalytic reactions. Over the past decades, various types of chiral phosphine-oxazolines (PHOX ligands) have been developed and have greatly advanced the field of asymmetric catalysis. Novel chiral PHOX ligand with an axis-unfixed biphenyl backbone, developed by our group, have shown interesting coordination behavior and excellent chiral inducing ability in various transition-metal-catalyzed asymmetric reactions. This personal account focuses on our developed axis-unfixed biphenylphosphine-oxazoline ligand (BiphPHOX), including an overview of its design and applications, which will provide inspiration for the exploration of novel ligands and related reactions.

Directing Group Repositioning Strategy Enabled Site- and Enantioselective Addition of Heteroaromatic C-H Bonds to Acyclic Internal Alkenes

Despite the notable advances achieved in the Murai-type hydroarylations, highly enantioselective catalytic addition of native (hetero)arenes to internal alkenes remains a prominent challenge. Herein, we report a directing group repositioning strategy, which enables the iridium-catalyzed enantioselective addition of heteroarenes including furan, benzofuran, and thiophene to internal enamides. The C-H bond at the C2 position of the heteroarene is site-selectively cleaved and added regioselectively to the β-position of an enamide, affording a valuable β-heteroaryl amide with high enantioselectivity. Mechanistic studies indicate that the rate and the enantioselectivity are determined by separate elementary steps.

Enantiodivergent Hydrogenation of Exocyclic α,β-Unsaturated Lactams Enabled by Switching the N-Chirality of Iridium Catalyst

Central chirality is an important chiral element used in the design of chiral ligands and catalysts. Mostly, the attention of organic chemists is focused on developing of chiral ligands with stable stereogenic centers. However, the N-chirality in chiral ligand design has been rarely explored due to its flexibility. Here we demonstrate the design, synthesis, and application of a class of simple P,N-ligands with flexible N-chirality and their derived iridium complexes with fixed N-chiral stereocenters. Both fixed configurations of the N-stereocenter of the iridium complexes could be selectively formed from the same chiral ligand. This pair of diastereoisomeric iridium complexes showed good performance in the enantiodivergent asymmetric hydrogenation of exocyclic α,β-unsaturated lactams. The N-H group plays an impressive role in catalytic activity. Computational studies emphasized the importance of N-chirality and N-H group.

Usefulness of the Global E Factor as a Tool to Compare Different Catalytic Strategies: Four Case Studies

The global E factor (EG factor) has recently been introduced, in the context of asymmetric organocatalysis, as a new green chemistry metric to take into consideration the synthesis of the catalyst in the overall economy of the synthetic process of a given chiral molecule in optically pure form. Herein, its further usefulness in comparing diverse catalytic systems, even different from organocatalysts, is shown by the analysis of four case studies.

Enantioselective Hydrosilylation of β,β-Disubstituted Enamides to Construct α-Aminosilanes with Vicinal Stereocenters

Despite the advances in the area of catalytic alkene hydrosilylation, the enantioselective hydrosilylation of alkenes bearing a heteroatom substituent is scarce. Here we report a rhodium-catalyzed hydrosilylation of β,β-disubstituted enamides to directly afford valuable α-aminosilanes in a highly regio-, diastereo-, and enantioselective manner. Stereodivergent synthesis could be achieved by regulating substrate geometry and ligand configuration to generate all the possible stereoisomers in high enantio-purity.

Rh-Catalyzed Sequential Asymmetric Hydrogenations of 3-Amino-4-Chromones Via an Unusual Dynamic Kinetic Resolution Process

Rh-catalyzed sequential asymmetric hydrogenations of 3-amino-4-chromones have been achieved for the first time via an unprecedented dynamic kinetic resolution under neutral conditions, providing (S,R)-3-amino-4-chromanols in high yields (up to 98%) with excellent enantio- and diastereoselectivities (up to 99.9% ee and 20:1 dr). The mechanistic studies based on control experiments and density functional theory (DFT) calculations suggest that the dynamic kinetic resolution process for the intermediate enantiomers generated in the first hydrogenation step proceeded via a stereomutation (or called chiral assimilation) pathway from an undesired enantiomer to the desired enantiomer rather than via traditional racemization of the undesired enantiomer. The protocol can be performed on a gram scale with a relatively low catalyst loading and offers a practical and convenient pathway for synthesizing a series of bioactive chromanols and their derivatives.

The Implications of the Brønsted Acidic Properties of Crabtree-Type Catalysts in the Asymmetric Hydrogenation of Olefins

Chiral iridium complexes derived from Crabtree's catalyst are highly useful in modern hydrogenations of olefins attributed to high reactivity, stereoselectivity, and stability. Despite that these precatalysts are pH neutral, the reaction mixtures turn acidic under hydrogenation conditions. This Perspective is devoted to the implications of the intrinsic Brønsted acidity of catalytic intermediates in asymmetric hydrogenation of olefins. Despite that the acidity has often been used only as a rationale for side-product formation, more recent methodologies have started to use this property advantageously. We hope that this Perspective serves as a stimulant for the development of such compelling and new asymmetric hydrogenations. The inherent scientific opportunities in utilizing or annihilating the generated Brønsted acid are enormous, and potential new innovations are outlined toward the end.

Catalytic enantioselective synthesis of fluoromethylated stereocenters by asymmetric hydrogenation

Fluoromethyl groups possess specific steric and electronic properties and serve as a bioisostere of alcohol, thiol, nitro, and other functional groups, which are important in an assortment of molecular recognition processes. Herein we report a catalytic method for the asymmetric synthesis of a variety of enantioenriched products bearing fluoromethylated stereocenters with excellent yields and enantioselectivities. Various N,P-ligands were designed and applied in the hydrogenation of fluoromethylated olefins and vinyl fluorides.