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

Stereocontrolled Hydrogenation of Conjugated Enones to Alcohols via Dual Iridium-Catalysis

The concept of dual catalysis is an emerging area holding high potential in terms of preparative efficiency, yet faces severe challenges in compatibility of reaction conditions and interference of catalysts. The transition-metal catalyzed stereoselective hydrogenation of olefins and ketones typically proceeds under different reaction conditions and/or uses a different reductant. As a result, these two types of hydrogenations can normally not be performed in the same pot. Herein, the stereocontrolled hydrogenation of enones to saturated alcohols is described, enabled by orthogonal dual iridium catalysis, using molecular hydrogen for both reductions. In this one-pot procedure, N,P-iridium catalysts (hydrogenation active towards olefins) and NHC,P-iridium catalysts (hydrogenation active towards ketones) operated independently of one another allowing the construction of two contiguous stereogenic centers up to 99 % ee, 99/1 d.r. Ultimately, by simple selection of the chirality of either ligands, the enone could be efficiently reduced to all four stereoisomers of the saturated alcohol in equally high stereopurity. This degree of stereocontrol for the synthesis of different stereoisomers by dual transition-metal catalyzed hydrogenation was previously not attained. The generality in substituted enones (alkyl, aryl, heteroaryl) demonstrate the wide applicability of this concept.

Dynamic Kinetic Resolution of β-Cyano α-Ketoesters via Asymmetric Transfer Hydrogenation

An efficient rhodium-catalyzed asymmetric transfer hydrogenation of β-cyano α-ketoesters via dynamic kinetic resolution has been developed. Despite the challenge posed by multiple functional groups, the reaction proceeded smoothly under mild conditions, generating versatile synthons with two adjacent stereocenters in high yields with excellent enantio- and diastereoselectivities. Furthermore, the power of this strategy is highlighted by the scale-up reaction and the follow-up synthesis of cytoxazone and paclitaxel intermediates.

Improving reproducibility through condition-based sensitivity assessments: application, advancement and prospect

The fluctuating reproducibility of scientific reports presents a well-recognised issue, frequently stemming from insufficient standardisation, transparency and a lack of information in scientific publications. Consequently, the incorporation of newly developed synthetic methods into practical applications often occurs at a slow rate. In recent years, various efforts have been made to analyse the sensitivity of chemical methodologies and the variation in quantitative outcome observed across different laboratory environments. For today's chemists, determining the key factors that really matter for a reaction's outcome from all the different aspects of chemical methodology can be a challenging task. In response, we provide a detailed examination and customised recommendations surrounding the sensitivity screen, offering a comprehensive assessment of various strategies and exploring their diverse applications by research groups to improve the practicality of their methodologies.

An Aza-Enolate Strategy Enables Iridium-Catalyzed Enantioselective Hydroalkenylations of Minimally Polarized Alkenes en Route to Complex N-Aryl β2-Amino Acids

Cationic Ir(I)-complexes modified with homochiral diphosphines promote the hydroalkenylative cross-coupling of β-(arylamino)acrylates with monosubstituted styrenes and α-olefins. The processes are dependent on the presence of an NH unit, and it is postulated that metalation of this generates an iridium aza-enolate that engages the alkene during the C-C bond forming event. The method offers high branched selectivity and enantioselectivity and occurs with complete atom economy. Diastereocontrolled reduction of the products provides β2-amino acids that possess contiguous stereocenters.

RuPHOX-Ru Catalyzed Asymmetric Cascade Hydrogenation of 3-Substituted Chromones for the Synthesis of Corresponding Chiral Chromanols

An efficient RuPHOX-Ru catalyzed asymmetric cascade hydrogenation of 3-substituted chromones has been achieved under mild reaction conditions, affording the corresponding chiral 3-substituted chromanols in high yields with excellent enantio- and diastereoselectivities (up to 99 % yield, >99 % ee and >20 : 1 dr). Control reactions and deuterium labelling experiments revealed that a dynamic kinetic resolution process occurs during the subsequent hydrogenation of the C=O double bond, which is responsible for the high performance of the asymmetric cascade hydrogenation. The resulting products allow for several transformations and it was shown that the protocol provides a practical and alternative strategy for the synthesis of chiral 3-substituted chromanols and their derivatives.

Synthesis of Chiral 1,2-Amino Alcohol-Containing Compounds Utilizing Ruthenium-Catalyzed Asymmetric Transfer Hydrogenation of Unprotected α-Ketoamines

Herein, we disclose a facile synthetic strategy to access an important class of drug molecules that contain chiral 1,2-amino alcohol functionality utilizing highly effective ruthenium-catalyzed asymmetric transfer hydrogenation of unprotected α-ketoamines. Recently, the COVID-19 pandemic has caused a crisis of shortage of many important drugs, especially norepinephrine and epinephrine, for the treatment of anaphylaxis and hypotension because of the increased demand. Unfortunately, the existing technologies are not fulfilling the worldwide requirement due to the existing lengthy synthetic protocols that require additional protection and deprotection steps. We identified a facile synthetic protocol via a highly enantioselective one-step process for epinephrine and a two-step process for norepinephrine starting from unprotected α-ketoamines 1b and 1a, respectively. This newly developed enantioselective ruthenium-catalyzed asymmetric transfer hydrogenation was extended to the synthesis of many 1,2-amino alcohol-containing drug molecules such as phenylephrine, denopamine, norbudrine, and levisoprenaline, with enantioselectivities of >99% ee and high isolated yields.

Design and Synthesis of Diphosphine Ligands Based on the Chiral Biindolyl Scaffold and Their Application in Transition-Metal Catalysis

An extremely concise, scalable, and stereoselective synthesis of a privileged chiral skeleton based on 2,2'-biindolyl and commercially available chiral building blocks has been developed. This novel skeleton allows for easy access to a range of bisphosphine ligands (decagram scale, up to 58% total yield, only three steps). The synthetic method is characterized by an efficient central-to-axial chirality transfer strategy. In particular, the superior performance of the ligands has been demonstrated in diverse reactions, including several asymmetric hydrogenations, asymmetric conjugate reductions, and cycloisomerization reactions, indicating a great potential for the application of the newly developed chiral backbones in further modifications and exploration of novel chiral ligands and catalysts.

Asymmetric Hydrogenation of Racemic 2-Substituted Indoles via Dynamic Kinetic Resolution: An Easy Access to Chiral Indolines Bearing Vicinal Stereogenic Centers

The asymmetric hydrogenation (AH) of N-unprotected indoles is a straightforward, yet challenging method to access biologically interesting NH chiral indolines. This method has for years been limited to 2/3-monosubstituted or 2,3-disubstituted indoles, which produce chiral indolines bearing endocyclic chiral centers. Herein, we have reported an innovative Pd-catalyzed AH of racemic α-alkyl or aryl-substituted indole-2-acetates using an acid-assisted dynamic kinetic resolution (DKR) process, affording a range of structurally fascinating chiral indolines that contain exocyclic stereocenters with excellent yields, diastereoselectivities, and enantioselectivities. Mechanistic studies support that the DKR process relies on a rapid interconversion of each enantiomer of racemic substrates, leveraged by an acid-promoted isomerization between the aromatic indole and nonaromatic exocyclic enamine intermediate. The reaction can be performed on a gram scale, and the products can be derivatized into non-natural β-amino acids via facile debenzylation and amino alcohol upon reduction.

Nickel-Catalyzed Enantioselective Decarboxylative Acylation: Rapid, Modular Access to α-Amino Ketones

A new approach to the enantiocontrolled synthesis of α-amino ketone derivatives is disclosed by employing a decarboxylative acylation strategy. Thus, when an acyl chloride and an α-amido-containing redox-active ester are exposed to a nickel catalyst, chiral ligand, and metal reductant, α-amido ketones are produced in good yield and high ee. The reaction exhibits broad substrate scope, can be easily scaled up, and is applied to dramatically simplify the synthesis of several known structures.

Iridium Catalysts with f-Amphbinol Ligands: Highly Stereoselective Hydrogenation of a Variety of Ketones

A series of novel and modular ferrorence-based amino-phosphine-binol (f-amphbinol) ligands have been successfully synthesized. The f-amphbinol ligands exhibited extremely high air stability and catalytic efficiency in the Ir-catalyzed stereoselective hydrogenation of various ketones to afford corresponding stereodefined alcohols with excellent results (full conversions, cis/trans >99:1, and 83% → 99% ee, TON up to 500 000). Control experiments have shown that -OH and -NH groups played a key role in this stereoselective hydrogenation.

Iridium-Catalyzed Remote Site-Switchable Hydroarylation of Alkenes Controlled by Ligands

An iridium-catalyzed remote site-switchable hydroarylation of alkenes was reported, delivering the products functionalized at the subterminal methylene and terminal methyl positions on an alkyl chain controlled by two different ligands, respectively, in good yields and with good to excellent site-selectivities. The catalytic system showed good functional group tolerance and a broad substrate scope, including unactivated and activated alkenes. More importantly, the regioconvergent transformations of mixtures of isomeric alkenes were also successfully realized. The results of the mechanistic studies demonstrate that the reaction undergoes a chain-walking process to give an [Ar-Ir-H] complex of terminal alkene. The subsequent processes proceed through the modified Chalk-Harrod-type mechanism via the migratory insertion of terminal alkene into the Ir-C bond followed by C-H reductive elimination to afford the hydrofunctionalization products site-selectively.

Rh-Catalyzed Enantioselective Desymmetric Hydrogenation of α-Acetamido-1,3-indanediones Using Ether-Bridged Biphenyl Diphosphine Ligands

Novel axially chiral biphenyl diphosphine ligands Enm-BridgePhos, bearing an ether chain bridge at the 5,5'-position of the biphenyl backbone, have been developed and successfully applied in the Rh-catalyzed enantioselective desymmetric hydrogenation of α-acetamido-1,3-indanediones, providing chiral α-acetamido-β-hydroxybenzocyclic pentones in high yields (up to 97%) and with excellent enantioselectivities (up to 99% ee). The reaction could be carried out on a gram scale, and the corresponding products were used as vital intermediates for the synthesis of analogues of chiral spirobenzylisoquinoline alkaloids. Both the crystal structure analysis and the DFT calculations revealed that the large dihedral angle of the Enm-BridgePhos-Rh complexes is highly related to the excellent enantioselectivities.