Strong Brønsted acid promoted asymmetric hydrogenation of isoquinolines and quinolines catalyzed by a Rh-thiourea chiral phosphine complex via anion binding

Fe Single Atoms and Fe2O3 Clusters Liberated from N-Doped Polyhedral Carbon for Chemoselective Hydrogenation under Mild Conditions

In the area of catalysis, selective reduction of nitro compounds to amino compounds is a colossal challenge due to the existence of competitive reducible functional groups. Herein, an Fe-based catalyst Fe_SAs/Fe₂O_3ACs/N-doped polyhedral carbon (NPC) has been designed and synthesized. As we expected, compared with Fe_SAs and Fe_NPs, Fe_SAs/Fe₂O_3ACs/NPC shows excellent catalytic performance (turnover frequency up to 1923 h^-1, calculated with nitrobenzene), chemoselectivity, and tolerance during the hydrogenation reaction of nitro compounds under room temperature because of the synergistic effects between Fe_SAs and Fe₂O_3ACs. The theoretical calculations show that Fe_SAs prefers to undergo hydrazine decomposition to generate hydrogen and the Fe₂O_3ACs surface is more active toward the nitrobenzene reduction to aniline.

Iridium-Catalyzed Enantioselective and Diastereoselective Hydrogenation of 1,3-Disubstituted Isoquinolines

The development of a general method utilizing a hydroxymethyl directing group for asymmetric hydrogenation of 1,3-disubstituted isoquinolines to provide chiral 1,2,3,4-tetrahydroisoquinolines is reported. The reaction, which utilizes [Ir(cod)Cl]₂ and a commercially available chiral xyliphos ligand, proceeds in good yield with high levels of enantioselectivity and diastereo-selectivity (up to 95% ee and >20:1 dr) on a range of differentially substituted isoquinolines. Directing group studies demonstrate that the hydroxymethyl functional group at the C1-position is more efficient at enabling hydrogenation than other substituents, although high levels of enantioselectivity were conserved across a variety of polar and non-polar functional groups. By utilizing the generated chiral β-amino alcohol as a functional handle, the synthetic utility is further highlighted via the synthesis of 1,2-fused oxazolidine, oxazolidinone, and morpholinone tetrahydroisoquinolines in one step. Additionally, a non-natural analog of the tetrahydroprotoberberine alkaloids was successfully synthesized.

Recent developments in the synthesis and applications of chiral ferrocene ligands and organocatalysts in asymmetric catalysis

This review highlights recent advances in the synthesis and application of ferrocene-containing chiral ligands and organocatalysts in asymmetric catalysis.

A domino reaction for generating β-aryl aldehydes from alkynes by substrate recognition catalysis

The development of universal catalyst systems that enable efficient, selective, and straightforward chemical transformations is of immense scientific importance. Here we develop a domino process comprising three consecutive reaction steps based on the strategy of supramolecular substrate recognition. This approach provides valuable β-aryl aldehydes from readily accessible α-alkynoic acids and arenes under mild reaction conditions, employing a supramolecular Rh catalyst containing an acylguanidine-bearing phosphine ligand. Furthermore, the synthesis of a key intermediate of Avitriptan using this protocol is accomplished. The first step of the reaction sequence is proved to be the regioselective hydroformylation of α-alkynoic acids. Remarkably, molecular recognition of the ligand and the substrate via hydrogen bonding plays a key role in this step. Control experiments indicate that the reaction further proceeds via 1,4-addition of an arene nucleophile to the unsaturated aldehyde intermediate and subsequent decarboxylation.

Highly Enantioselective Catalytic Addition of Grignard Reagents to N-Heterocyclic Acceptors

General methods to prepare chiral N‐heterocyclic molecular scaffolds are greatly sought after because of their significance in medicinal chemistry. Described here is the first general catalytic methodology to access a wide variety of chiral 2‐ and 4‐substituted tetrahydro‐quinolones, dihydro‐4‐pyridones, and piperidones with excellent yields and enantioselectivities, utilizing a single catalyst system.

Selective Arene Hydrogenation for Direct Access to Saturated Carbo- and Heterocycles

Arene hydrogenation provides direct access to saturated carbo- and heterocycles and thus its strategic application may be used to shorten synthetic routes. This powerful transformation is widely applied in industry and is expected to facilitate major breakthroughs in the applied sciences. The ability to overcome aromaticity while controlling diastereo-, enantio-, and chemoselectivity is central to the use of hydrogenation in the preparation of complex molecules. In general, the hydrogenation of multisubstituted arenes yields predominantly the cis isomer. Enantiocontrol is imparted by chiral auxiliaries, Brønsted acids, or transition-metal catalysts. Recent studies have demonstrated that highly chemoselective transformations are possible. Such methods and the underlying strategies are reviewed herein, with an emphasis on synthetically useful examples that employ readily available catalysts.

Progress in the Chemistry of Tetrahydroquinolines

Tetrahydroquinoline is one of the most important simple nitrogen heterocycles, being widespread in nature and present in a broad variety of pharmacologically active compounds. This Review summarizes the progress achieved in the chemistry of tetrahydroquinolines, with emphasis on their synthesis, during the period from mid-2010 to early 2018.

Highly efficient Ir-catalyzed asymmetric hydrogenation of benzoxazinones and derivatives with a Brønsted acid cocatalyst

The Ir-catalyzed highly efficient asymmetric hydrogenation of benzoxazinones and derivatives was successfully developed with N-methylated ZhaoPhos L5 as the ligand, affording various chiral dihydrobenzoxazinones and derivatives with excellent results.

The Ir-catalyzed highly efficient asymmetric hydrogenation of benzoxazinones and derivatives was successfully developed with N-methylated ZhaoPhos L5 as the ligand, which may display a new activation mode with a single anion-binding interaction among the substrate, cocatalyst Brønsted acid and ligand. This synthetic approach afforded a series of chiral dihydrobenzoxazinones and derivatives with excellent results (>99% conversion, 88–96% yields, 91–>99% ee, up to 40 500 TON). A key to success is the utilization of a strong Brønsted acid as the cocatalyst, such as hydrochloric acid, to form a possible single anion-binding interaction with the substrate and catalyst, which greatly contributed to the improvement of reactivity and enantioselectivity. Importantly, a creative and efficient synthetic route was developed to construct the important intermediate for the potential IgE/IgG receptor modulator through our catalytic methodology system.

Synthesis of chiral seven-membered β-substituted lactams via Rh-catalyzed asymmetric hydrogenation

Rh/bisphosphine-thiourea ligand (ZhaoPhos)-catalyzed asymmetric hydrogenation of seven-membered β-substituted α,β-unsaturated lactams was successfully developed to prepare various chiral seven-membered β-substituted lactams with good to excellent results (up to >99% conversion, 99% yield, and >99% ee).

Efficient synthesis of chiral 2,3-dihydro-benzo[b]thiophene 1,1-dioxides via Rh-catalyzed hydrogenation

Rh-catalyzed asymmetric hydrogenation of prochiral substituted benzo[b]thiophene 1,1-dioxides was successfully developed, affording various chiral 2,3-dihydrobenzo[b]thiophene 1,1-dioxides with high yields and excellent enantioselectivities (up to 99% yield and >99% ee). In particular, for challenging substrates, such as aryl substituted substrates with sterically hindered groups and alkyl substituted substrates, the reaction proceeded smoothly in our catalytic system with excellent results. The gram-scale asymmetric hydrogenation proceeded well with 99% yield and 99% ee in the presence of 0.02 mol% (S/C = 5000) catalyst loading. The possible hydrogen-bonding interaction between the substrate and the ligand may play an important role in achieving high reactivity and excellent enantioselectivity.

Concise total syntheses of (-)-jorunnamycin A and (-)-jorumycin enabled by asymmetric catalysis

The bis-tetrahydroisoquinoline (bis-THIQ) natural products have been studied intensively over the past four decades for their exceptionally potent anticancer activity, in addition to strong Gram-positive and Gram-negative antibiotic character. Synthetic strategies toward these complex polycyclic compounds have relied heavily on electrophilic aromatic chemistry, such as the Pictet-Spengler reaction, that mimics their biosynthetic pathways. Herein, we report an approach to two bis-THIQ natural products, jorunnamycin A and jorumycin, that instead harnesses the power of modern transition-metal catalysis for the three major bond-forming events and proceeds with high efficiency (15 and 16 steps, respectively). By breaking from biomimicry, this strategy allows for the preparation of a more diverse set of nonnatural analogs.

Versatile Synthesis of Functionalized Tetrahydroisoquinolines by Ring Transformation of 2H-Pyran-2-ones

Functionalized tetrahydroisoquinolines are convenient precursors for the construction of numerous heterocyclic compounds of therapeutic importance. In this paper we have illustrated an efficient synthesis of highly substituted tetrahydroisoquinolines from 2H-pyran-2-ones via nucleophile-mediated ring transformation with tert-butyl-4-oxopiperidine-1-carboxylate followed by acid-mediated cleavage of the tert-butyloxycarbonyl group. The products were achieved smoothly in high yields with flexibility of various substituents.

Asymmetric hydrogenation of α,β-unsaturated sulfones by a rhodium/thiourea–bisphosphine complex

We herein report a method to synthesize chiral sulfone compounds.

Pd nanoparticles on green support as dip-catalyst: a facile transfer hydrogenation of olefins and N-heteroarenes in water

Chemo- and regioselective hydrogenation methods using highly green sources, particularly from metal nanoparticles on plant stem as support and water, is an intensive research area, which is highly relevant to the development of green chemistry and technology in the 21^st century. Here, the synthesis and activity of a heterogeneous catalytic system (called “dip-catalyst”) for the transfer hydrogenation of a series of styrenyl, unfunctionalized olefins, quinoline and other N-heteroarenes, are presented. It consists of Pd nanoparticles (15–20 nm) anchored on bio-processed jute plant (Corchorus genus) stem as the support. Pd nanoparticles were decorated on the green support (GS) jute stem by the in situ reduction of K₂PdCl₄ in aqueous medium at 70 °C, using formic acid as the reductant. The Pd@GS was characterized using SEM, EDS, XRD, FTIR, XPS and TEM. Elemental mapping revealed uniform distribution of Pd on the cellulose matrix of the jute stem. The catalyst was successfully applied to the chemoselective transfer hydrogenation of numerous styrenyl, unfunctionalized olefins. Its high functional group tolerance was investigated during the olefins hydrogenation in water. Furthermore, Pd@GS was capable of quantitative hydrogenation to selectively produce 1,2,3,4-tetrahydroquinoline (THQ) in water using stoichiometric amounts of tetrahydroxydiboron (THDB) at 60 °C with turn over frequency (TOF) 4938 h⁻¹. This system is stable in water and displays excellent recyclability; it could be used for 32 consecutive cycles, without losing its original crystallinity or requiring replenishment.

A dip catalyst consisting of Pd nanoparticles immobilized on the surface of thin slices of jute-stem was fabricated. Its versatility as a hydrogen transfer agent for styrenyl, cyclic and unfunctionalized olefins and N-heterocycles was investigated.

Ruthenium(II)-Catalyzed Regioselective C-8 Hydroxylation of 1,2,3,4-Tetrahydroquinolines

Ru(II)-catalyzed chelation-assisted highly regioselective C8-hydroxylation of 1,2,3,4-tretrahydroquinolines has been developed. Various 1,2,3,4-tetrahydroquinolines underwent smooth C8-H hydroxylation with cheap and safe K₂S₂O₈ as the oxidant and oxygen source to furnish the corresponding products in good to excellent yields with high tolerance of the functional groups. The choice of a readily installable and removable N-pyrimidyl directing group is the key to catalysis. Mechanistic studies suggest the involvement of a six-membered ruthenacycle intermediate in the catalytic cycle. The method can also be extended to the direct hydroxylation of other (hetero)arene C-H bonds.

Iridium-Catalyzed Asymmetric Hydrogenation of 4,6-Disubstituted 2-Hydroxypyrimidines

An efficient iridium-catalyzed hydrogenation of 4,6-disubstituted 2-hydroxypyrimidines has been achieved, giving chiral cyclic ureas with excellent diastereoselectivities and up to 96% ee of enantioselectivities. In the presence of the in situ generated hydrogen halide, the equilibrium of the lactame-lactime tautomerism of 2-hydroxypyrimidine is more toward the oxo form with lower aromaticity, which effectively improves the reactivity to facilitate hydrogenation. Moreover, the cyclic ureas could be readily converted into chiral 1,3-diamine derivatives without loss of optical purity.

Enantioselective Access to Chiral 2-Substituted 2,3-Dihydrobenzo[1,4]dioxane Derivatives through Rh-Catalyzed Asymmetric Hydrogenation

Rh-catalyzed asymmetric hydrogenation of various benzo[ b][1,4]dioxine derivatives was successfully developed to prepare chiral 2-substituted 2,3-dihydrobenzo[1,4]dioxane derivatives using ZhaoPhos and N-methylation of ZhaoPhos ligands with high yields and excellent enantioselectivities (up to 99% yield, >99% enantiomeric excess (ee), turnover number (TON) = 24 000). Moreover, this asymmetric hydrogenation methodology, as the key step with up to 10 000 TON, was successfully applied to develop highly efficient synthetic routes for the construction of some important biologically active molecules, such as MKC-242, WB4101, BSF-190555, and ( R)-doxazosin·HCl.

Enantioselective synthesis of 1,2,3,4-tetrahydroquinoline-4-ols and 2,3-dihydroquinolin-4(1H)-ones via a sequential asymmetric hydroxylation/diastereoselective oxidation process using Rhodococcus equi ZMU-LK19

A cascade biocatalysis system involving asymmetric hydroxylation and diastereoselective oxidation was developed using Rhodococcus equi ZMU-LK19, which gave chiral 2-substituted-1,2,3,4-tetrahydroquinoline-4-ols (2) (up to 57% isolated yield, 99 : 1 dr, and >99% ee) and chiral 2-substituted-2,3-dihydroquinolin-4(1H)-ones (3) (up to 25% isolated yield, and >99% ee) from (±)-2-substituted-tetrahydroquinolines (1). In addition, a possible mechanism for this cascade biocatalysis was tentatively proposed.

Ruthenium-Catalyzed Chemo- and Enantioselective Hydrogenation of Isoquinoline Carbocycles

A chemoselective hydrogenation of isoquinoline carbocycles was achieved by using the catalyst prepared from Ru(methallyl)₂(cod) and trans-chelate chiral ligand PhTRAP. The unique chemoselectivity achieved in this hydrogenation could be ascribed to the trans-chelation of the chiral ligand. The procedure for preparing the catalyst strongly affects the reproducibility of the carbocycle hydrogenation. Various 5-, 6-, 7-, and 8-substituted isoquinolines were selectively hydrogenated at their carbocycles to afford 5,6,7,8-tetrahydroisoquinolines as major products in high yields with moderate or good enantioselectivities. Some mechanistic studies suggested that the stereogenic center was created during the initial addition of H₂ to the aromatic ring in the hydrogenation of 5-substituted isoquinolines. In other words, the stereochemical control was accompanied by the dearomatization.

Rhodium-catalyzed asymmetric hydrogenation of β-cyanocinnamic esters with the assistance of a single hydrogen bond in a precise position

The first asymmetric hydrogenation of β-cyanoacrylate esters has been developed to furnish chiral β-cyano esters with excellent yields and excellent enantioselectivities. Notably, the catalyst with a single H-bond donor in a precise position performed better than that with double H-bond donors.

With the assistance of hydrogen bonds, the first asymmetric hydrogenation of β-cyanocinnamic esters is developed, affording chiral β-cyano esters with excellent enantioselectivities (up to 99% ee). This novel methodology provides an efficient and concise synthetic route to chiral GABA-derivatives such as (S)-Pregabalin, (R)-Phenibut, (R)-Baclofen. Interestingly, in this system, the catalyst with a single H-bond donor performs better than that with double H-bond donors, which is a novel discovery in the metalorganocatalysis area.

Asymmetric hydrogenation of maleic anhydrides catalyzed by Rh/bisphosphine-thiourea: efficient construction of chiral succinic anhydrides

Asymmetric hydrogenation of various 3-substituted maleic anhydrides catalyzed by Rh/bisphosphine-thiourea (ZhaoPhos) under mild conditions was successfully developed. A wide range of 3-alkyl and 3-aryl maleic anhydrides were hydrogenated well to provide the desired products 3-substituted succinic anhydrides in one hour with excellent results (full conversions, up to 99% yield, 99% ee, 3000 TON). Importantly, we developed a creative and efficient synthetic route to construct the key intermediate of the hypoglycemic drug mitiglinide through our catalytic system.

Synthesis and kinetic resolution of substituted tetrahydroquinolines by lithiation then electrophilic quench

Treatment of N-Boc-2-aryl-1,2,3,4-tetrahydroquinolines with n-butyllithium in THF at -78 °C resulted in efficient lithiation at the 2-position and the organolithiums were trapped with a variety of electrophiles to give substituted products. Variable temperature NMR spectroscopy gave kinetic data that showed that the rate of tert-butoxycarbonyl (Boc) rotation was fast (ΔG‡ ≈ 45 kJ mol-1 at -78 °C) and in situ ReactIR spectroscopy showed fast lithiation at -78 °C. By carrying out the lithiation in the presence of the chiral ligand sparteine, kinetic resolutions with very high levels of enantioselectivity were achieved. The resulting enantioenriched N-Boc-2-aryltetrahydroquinolines were converted to 2,2-disubstituted products without significant loss in enantiopurity. Most electrophiles add at the 2-position and the chemistry provides a way to access tetrahydroquinolines that are fully substituted alpha to the nitrogen atom. Notably, either enantiomer of the 2,2-disubstituted tetrahydroquinolines can be obtained with high selectivity from the same enantiomer of the chiral ligand. Unusually, when methyl cyanoformate was used as the electrophile, substitution occurred in the ortho position of the aryl ring attached at C-2. This change in regioselectivity on changing the electrophile was probed by deuterium isotope studies and by DFT calculations which suggested that the binding of the cyanoformate altered the structure of the intermediate organolithium. Secondary amine products can be prepared by removing the Boc group with acid or by inducing the Boc group to rearrange to the 2-position in the presence of triethylborane and this carbonyl N-to-C rearrangement occurs with retention of configuration from the intermediate enantiomerically enriched organolithium species.

A one-pot process for the enantioselective synthesis of tetrahydroquinolines and tetrahydroisoquinolines via asymmetric reductive amination (ARA)

Asymmetric reductive amination for the synthesis of both chiral tetrahydroquinolines and tetrahydroisoquinolines has been realized with an Ir/ZhaoPhos catalytic system via a one-pot N-Boc deprotection/intramolecular asymmetric reductive amination (ARA) sequence.

Facile hydrogenation of N-heteroarenes by magnetic nanoparticle-supported sub-nanometric Rh catalysts in aqueous medium

This work describes the preparation and systematic characterization of a reusable magnetic heterogeneous nanocatalyst (Rh@Fe₃O₄) for the hydrogenation of N-heterocycles and simple aromatics.

Reductive Etherification via Anion-Binding Catalysis

Reductive condensations of alcohols with aldehydes/ketones to generate ethers are catalyzed by a readily accessible thiourea organocatalyst that operates in combination with HCl. 1,1,3,3-tetramethyldisiloxane serves as a convenient reducing reagent. This strategy is applicable to challenging substrate combinations and exhibits functional group tolerance. Competing reductive homocoupling of the carbonyl component is suppressed.