Asymmetric Transfer Hydrogenation Reactions Research Articles

Efficient and Scalable Enantioselective Synthesis of a Key Intermediate for Rimegepant: An Oral CGRP Receptor Antagonist

Rimegepant is a calcitonin gene-related peptide antagonist used for acute treatment and prevention of migraine. We herein attempt to explore an efficient and practiced method for scale-up, regio- and enantioselective synthesis of (R)-9-hydroxy-6,7,8,9-tetrahydro-5H-cyclohepta[b]pyridin-5-one (1), a key intermediate of rimegepant. In this work, a Ru-catalyzed asymmetric transfer hydrogenation (ATH) reaction was a key step. The optimization of the reaction conditions involved exploring the reaction parameters including catalysts, bases, and solvents. The results suggested that the Ru-catalyzed ATH process using formic acid as the hydrogen donor could be operated under mild conditions at a low catalyst loading (0.5 mol%), affording a high yield (92.1% yield with 99.8% purity) and gratifying enantioselectivity (99.9% ee) of the target product (1). This work first reported the Ru-catalyzed ATH process in the synthesis of key intermediates of rimegepant. The optimized ATH process was easy to implement and cost-effective, making it particularly suitable for manufacturing scale production.

Amido-ene(amido) Ni(II)-Catalyzed Highly Enantioselective Transfer Hydrogenations of Ketone: Dual Functions of the Ene(amido) Group

Catalytic asymmetric transfer hydrogenation (ATH) reactions of ketones have become an important tool for producing enantiomerically pure alcohols that are widely used in the syntheses of bioactive compounds. However, the high toxicities and low abundances of traditionally employed precious metals serving as catalyst centers have impeded widespread application of precious metal-based catalysts in asymmetric synthesis. Development of catalysts from abundant and less toxic 3d metals is highly desirable but still challenging. We report herein the development of structurally well-defined (R,R)-amido-ene(amido) nickel catalysts for ATH of ketone substrates, facilitating access to a wide range of secondary (S)-configuration alcohols, many of which are synthetic building blocks of biologically relevant molecules, with up to 95% enantiomeric excess (e.e.) and broad scope of functional group tolerance. Catalytic intermediate isolation and density functional theory calculation ascribed the catalytic activity to stabilization of the electron-rich nickel center with the ene(amido) group via a π-backdonation interaction within the catalytic intermediates, while the stereoinduction mainly arises from a catalyst–substrate π–π interaction in the transition state for enantioselective hydride transfer.

Dosage delivery of chiral ruthenium catalysts using non-ionic surfactants for asymmetric transfer hydrogenation reactions in aqueous media

Dosage delivery of asymmetric ruthenium catalysts for transfer hydrogenation reactions of ketones: nano-reactors in aqueous media.

Coinage Metal-Catalyzed Asymmetric Reactions of ortho-Alkynylaryl and Heteroaryl Aldehydes and Ketones.

Coinage metals have become the metal of choice due to their excellent catalytic activity in organic transformation processes. Combining various chiral ligands and coinage metals became a productive area of research and access to heterocyclic derivatives according to an efficient and sustainable manner. This review was devoted to the various recently developed coinage metal-catalyzed domino processes of ortho-alkynylaryl and heteroaryl aldehydes and ketones leading to functionalized heterocycles. Various gold chiral complexes were presented, and methods of preparations of chromenes along with indoles were covered. Ag-chiral complexes are also prone to interesting activities such as cyclization followed by reduction and functionalization with enolizable ketones or (diazomethyl)phosphonate. Asymmetric Cu-catalyzed domino cyclization and asymmetric transfer hydrogenation reactions efficiently led to functionalized chromenes. Some remarkable examples involving copper associated with ruthenium in the context of a cyclization and asymmetric hydrogenation process were also presented.

Enantiocontrolled Preparation of ϒ-Substituted Cyclohexenones: Synthesis and Kinase Activity Assays of Cyclopropyl-Fused Cyclohexane Nucleosides.

The enantioselective preparation of the two isomers of 4-hydroxy-2-cyclohexanone derivatives 1a,b was achieved, starting from a common cyclohexenone, through asymmetric transfer hydrogenation (ATH) reactions using bifunctional ruthenium catalysts. From these versatile intermediates, a stereoselective route to a cytosine analogue built on a bicyclo [4.1.0]heptane scaffold is described. Nucleoside kinase activity assays with this cyclopropyl-fused cyclohexane nucleoside, together with other related nucleosides (2a–e), were performed, showing that thymine- and guanine- containing compounds have affinity for herpes simplex virus Type 1 (HSV-1) thymidine kinase (TK) but not for human cytosolic TK-1, thus pointing to their selectivity for herpetic TKs but not cellular TKs.

Mechanochemical Asymmetric Transfer Hydrogenation of Diketones to Access Chiral 1,3-Diols under Solvent-Free Conditions

AbstractA mechanochemical asymmetric transfer hydrogenation (ATH) of diketones in the presence of a ruthenium complex under solvent-free conditions was developed to provide chiral 1,3-diol derivatives. This protocol benefits from rapid reaction kinetics, no use of solvents, and excellent enantioselectivity. In addition, the mechanochemical ATH reaction can easily be performed on a gram scale.

Review Study of Chiral N-Heterocyclic Carbene (NHC) Ligands in Stereoselective Metal-Catalyzed Reduction Reactions

Stereoselective metal-catalyzed reactions using N-heterocyclic carbene (NHC) ligands have shown significant recent advances, due to the ability of NHC ligands as strong σ-donor species to coordinate with a wide variety of transition metals. Therefore, the design of new ligands and the subsequent strategies for their synthesis enables new applications of their metal complexes in catalysis to be investigated. This study focuses on the applications of different classes of Ir-, Pd-, Au- and Rh-NHC ligand complexes as promising catalysts in the asymmetric hydrogenation, hydrosilylation and transfer hydrogenation reactions.

Iridium-Catalyzed Asymmetric Transfer Hydrogenation of Quinolines in Biphasic Systems or Water.

An asymmetric transfer hydrogenation (ATH) of quinolines in water or biphasic systems was developed. This ATH reaction proceeds smoothly without the need for inert atmosphere protection in the presence of a water-soluble iridium catalyst, which bears an easily available aminobenzimidazole ligand. This ATH system can work at a catalyst loading of 0.001 mol % (S/C = 100 000, turnover number (TON) of up to 33 000) under mild reaction conditions. The turnover frequency (TOF) value can reach as high as 90 000 h-1. A variety of quinoline and N-heteroaryl compounds are transformed into the desired products in high yield and up to 99% enantiomeric excess (ee).

Chemoenzymatic Stereoselective Synthesis of trans-Flavan-4-ols via Lipase-Catalyzed Kinetic Resolutions

Flavan-4-ols are a subclass of flavonoids that are present in complex molecules with application in the industrial sector as pigments, antioxidants, or antimitotics, among many others. The most traditional way to achieve their synthesis is from naturally abundant flavanones, asymmetric transfer hydrogenation reactions or bioreduction being well known strategies, while their preparation from racemic flavan-4-ols has been less explored. In this article, we have focused on the synthesis of a series of trans-flavan-4-ols bearing different substitution patterns in the aromatic ring to explore later the potential of lipases as biocatalysts for stereoselective acylation reactions. Therefore, a series of flavanones have been chemically prepared, starting from the corresponding benzaldehydes by aldol condensation with 2′-hydroxyacetophenone in a strongly basic medium, and later transformed into the corresponding racemic trans-flavan-4-ols following a carbonyl reduction, Mitsunobu reaction, and ester deprotection sequence. A screening of lipases and optimization of the reaction conditions for the stereoselective acylation of racemic 2-phenylchroman-4-ol were performed before expanding the best reaction conditions to the kinetic resolution of other 2-arylchroman-4-ols. Interestingly, the combination of AK lipase from Pseudomonas fluorescens as enzyme and vinyl acetate as both acyl donor and solvent allowed the performance of highly asymmetric transformations (E > 200, 50–99% eeS and >99% eeP) under mild reaction conditions (30 °C and 250 rpm).

Electrochemically Promoted Asymmetric Transfer Hydrogenation of 2,2,2-Trifluoroacetophenone.

The study reported an electrochemically promoted asymmetric hydrogen transfer reaction of 2,2,2-trifluoroacetophenone with a chiral Ru complex. (R)-α-(Trifluoromethyl) benzyl alcohol with a 96% yield and 94% ee could be obtained with only a 0.5 F mol-1 charge amount at room temperature and normal pressure.

Ru-TsDPEN catalysts and derivatives in asymmetric transfer hydrogenation reactions.

This review summarizes current developments, novel synthetic routes for Ruthenium tethered chiral catalyst, and its derivatives along with its application in asymmetric synthesis. The review also covers derivatization in tethering unit, modification in N-monofunctionalized ligand as well as ligation of other ligand with Ru metal in chiral catalyst. Apparently, the effect of a modified tethered catalyst in the enantioselective synthesis of chiral products as well as in synthetic chemistry is also discussed in detail.

Wadsworth Emmons cyclopropanation mediated diastereoselective Syntheses of α-epoxy- trans-disubstituted cyclopropane and α-hydroxy-trans-disubstituted cyclopropane

The present work describes the diastereoselective Syntheses of α -epoxy- trans -disubstituted cyclopropane and α -hydroxy- trans -disubstitutedcyclopropane. The genesis of chirality in the targeted products is based on the exploitation of catalytic asymmetric transfer hydrogenation (CATHy) reaction. Asymmetric Cyclopropanation was carried out using Wadsworth–Emmons protocol. Among the surveyed conditions, employing TEPA, epoxide, NaH in toluene under reflux furnished the desired cyclopropane with 85% yield.

Chiral salen - Ni (II) based spherical porous silica as platform for asymmetric transfer hydrogenation reaction and synthesis of potent drug intermediate montekulast

Heterogeneous catalyst has an edge over homogeneous systems in terms of recyclability, activity, stability and recovery. Silica has evolved as a good support material in heterogeneous systems due to its stability and ability to get modified as per the end application. Herein, we report a novel chiral Ni-Schiff base derived catalyst and its immobilization into mesoporous silica which was synthesized by post-grafting process. The chiral catalyst demonstrated remarkably high catalytic activity, enantioselectivity (up to 99 % enantiomers excess) for heterogeneous asymmetric transfer hydrogenation of various ketones. The developed catalyst was characterized by Ultraviolet-visible spectroscopy (UV–vis), Fourier-Transform Infrared spectroscopy (FT-IR), X-ray Powder Diffraction (XRD), Brunauer-Emmett-Teller (BET isotherm), Scanning Electron Microscopy – Energy Dispersive X-ray Spectroscopy (SEM-EDX), High Resolution – Transmission Electron Microscopy (HR-TEM), Vibrating Sample Magnetometer (VSM), X-ray Photoelectron Spectroscopy (XPS) and elemental analysis. The catalyst could be recovered and reused for multiple consecutive runs without losing the enantioselectivity. The chiral catalyst was used in asymmetric transfer hydrogenation reaction for synthesizing enantiomerically pure drug intermediate Montekulast.

Chiral cyclometalated iridium complexes for asymmetric reduction reactions.

A series of chiral cyclometalated iridium complexes have been synthesised by cyclometalating chiral 2-aryl-oxazoline and imidazoline ligands with [Cp*IrCl2]2. These iridacycles were studied for asymmetric transfer hydrogenation reactions with formic acid as the hydrogen source and were found to display various activities and enantioselectivities, with the most effective ones affording up to 63% ee in the asymmetric reductive amination of ketones and 77% ee in the reduction of pyridinium ions.

Asymmetric transfer hydrogenation of unsaturated ketones; factors influencing 1,4- vs 1,2- regio- and enantioselectivity, and alkene vs alkyne directing effects

A detailed study has been completed on the asymmetric transfer hydrogenation (ATH) of a series of enones using Ru(II) catalysts. Electron-rich rings adjacent to the CO group reduce the level of CO reduction compared to CC. The ATH reaction can readily discriminate between double and triple bonds adjacent to ketones, reducing the double bond but leaving a triple bond intact in the major product.

Neutrophil-Membrane-Directed Bioorthogonal Synthesis of Inflammation-Targeting Chiral Drugs

Summary Targeted bioorthogonal catalysis holds great potential for localized prodrug activation. Although enantiomerically pure drugs are of vital importance in the clinic, chiral drug synthesis by bioorthogonal reactions in living systems is scarcely reported. Toward this goal, we constructed chirally modified Pd catalysts for the asymmetric transfer hydrogenation (ATH) reaction by using sodium formate as the biocompatible reductant. By combining the ATH reaction and chemotaxis of neutrophil membrane, we achieved inflammation site-selective chiral drug synthesis in living cells. As a proof of concept, we synthesized the chiral model drug ibuprofen in situ by a targeted ATH reaction to alleviate inflammation in the mouse paw as an in vivo model. Compared with controls, the neutrophil-membrane-coated chiral Pd catalysts exhibited inflammation-targeted capability and enantioselectivity simultaneously in the anti-inflammatory action. This study could offer a novel perspective for bioorthogonal catalysis in targeted prodrug activation.

Chiral metal-organic frameworks with tunable catalytic selectivity in asymmetric transfer hydrogenation reactions

Metal-organic frameworks (MOFs) have achieved great success in the field of heterogeneous catalysis, however, it’s still challenging to design MOF catalysts with enhanced selectivity. Here, we demonstrated a combination strategy of metal design and ligand design on the enantioselectivity—that is the enantioselectivities of chiral MOF (CMOF) catalysts could be significantly enhanced by the rational choice of metal ions with higher electronegativities and introducing sterically demanding groups into the ligands. Four isostructural Ca-, Sr- and Zn-based CMOFs were prepared from enantiopure phosphono-carboxylate ligands of 1,1′-biphenol that are functionalized with 2,4,6-trimethyl- and 2,4,6-trifluoro-phenyl groups at the 3,3’-position. The uniformly distributed metal phosphonates along the channels could act as Lewis acids and catalyze the asymmetric transfer hydrogenation of heteroaromatic imines (benzoxazines and quinolines). Particularly, the Ca-based MOF 1 with 2,4,6-trimethyl groups at the substituents exhibited enhanced catalytic performance, affording the highest enantioselectivity (up to 97%). It is also the first report of the heterogeneous catalyst with chiral non-noble metal phosphonate active sites for asymmetric transfer hydrogenation reactions with Hantzsch ester as the hydrogen source. The catalyst design strategy demonstrated here is expected to develop new types of chiral materials for asymmetric catalysis and other chiral applications.

Asymmetric one-pot transformation of isoflavones to pterocarpans and its application in phytoalexin synthesis

Phytoalexins have attracted much attention due to their health-promoting effects and their vital role in plant health during the last years. Especially the 6a-hydroxypterocarpans glyceollin I and glyceollin II, which may be isolated from stressed soy plants, possess a broad spectrum of bioactivities such as anticancer activity and beneficial contributions against western diseases by anti-oxidative and anti-cholesterolemic effects. Aiming for a catalytic asymmetric access to these natural products, we establish the asymmetric syntheses of the natural isoflavonoids (−)-variabilin, (−)-homopterocarpin, (−)-medicarpin, (−)-3,9-dihydroxypterocarpan, and (−)-vestitol by means of an asymmetric transfer hydrogenation (ATH) reaction. We successfully adapt this pathway to the first catalytic asymmetric total synthesis of (−)-glyceollin I and (−)-glyceollin II. This eight-step synthesis features an efficient one-pot transformation of a 2′-hydroxyl-substituted isoflavone to a virtually enantiopure pterocarpan by means of an ATH and a regioselective benzylic oxidation under aerobic conditions to afford the susceptible 6a-hydroxypterocarpan skeleton.

Synthetic and computational assessment of a chiral metal-organic framework catalyst for predictive asymmetric transformation.

Understanding and controlling molecular recognition mechanisms at a chiral solid interface is a continuously addressed challenge in heterogeneous catalysis. Here, the molecular recognition of a chiral peptide-functionalized metal–organic framework (MOF) catalyst towards a pro-chiral substrate is evaluated experimentally and in silico. The MIL-101 metal–organic framework is used as a macroligand for hosting a Noyori-type chiral ruthenium molecular catalyst, namely (benzene)Ru@MIL-101-NH-Gly-Pro. Its catalytic perfomance toward the asymmetric transfer hydrogenation (ATH) of acetophenone into R- and S-phenylethanol are assessed. The excellent match between the experimentally obtained enantiomeric excesses and the computational outcomes provides a robust atomic-level rationale for the observed product selectivities. The unprecedented role of the MOF in confining the molecular Ru-catalyst and in determining the access of the prochiral substrate to the active site is revealed in terms of highly face-specific host–guest interactions. The predicted surface-specific face differentiation of the prochiral substrate is experimentally corroborated since a three-fold increase in enantiomeric excess is obtained with the heterogeneous MOF-based catalyst when compared to its homogeneous molecular counterpart.

Room-Temperature Asymmetric Transfer Hydrogenation of Biomass-Derived Levulinic Acid to Optically Pure γ-Valerolactone Using a Ruthenium Catalyst.

This study presents a first report on ruthenium-catalyzed asymmetric transfer hydrogenation (ATH) of levulinic acid (LA) to chiral γ-valerolactone (GVL). ATH of LA has been explored with Noyori’s chiral catalyst (Ru–TsDPEN) in methanol solvent. Efficacy of ATH reaction of LA was investigated under different reactions conditions such as temperature, catalyst, and hydrogen donor concentration. The effect of various organic tertiary bases along with formic acid (FA) as a hydrogen donor was studied, and N-methylpiperidine with FA (1:1 molar ratio) was revealed as an efficient hydrogen donor for ATH of LA to GVL furnishing chiral GVL with complete conversion and 93% enantiomeric excess (ee). This operationally simple and mild ATH protocol was tested for practical applicability of ATH of LA obtained from biomass waste (rice husk and wheat straw) and furnished chiral GVL with 82% ee.