Highly Enantioselective Iridium-Catalyzed Hydrogenation of 2-Benzylquinolines and 2-Functionalized and 2,3-Disubstituted Quinolines

Cu(I) Complexes Catalyzed the Dehydrogenation of N-Heterocycles

A copper-catalyzed method for the dehydrogenation of various nitrogen-containing heterocycles to furnish quinolines and indoles has been developed. A range of 1,2,3,4-tetrahydroquinolines underwent dehydrogenation by employing 2 mol % of copper complex Cat 3 as a catalyst and using O2 as an oxidant at 120 °C in 1,2-dichlorobenzene to afford the desired quinolines. The method enables the dehydrogenation of a variety of indolines in the presence of 2 mol % of copper complex Cat 2, using 10 mol % of TEMPO as an additive and O2 as an oxidant under room temperature in tetrahydrofuran to furnish indoles in high yields. Mechanistic studies suggested that the dehydrogenative activity is ascribed to the formation of a copper(II) active species from copper(I) complexes oxidized by O2, which was proved by high-resolution mass spectrometry (HRMS). The copper-catalyzed dehydrogenation reaction proceeds via a superoxide radical anion (·O2-) as proved by electron paramagnetic resonance (EPR) spectrometry. In situ infrared spectroscopy revealed that the dihydroquinoline intermediate was formed in the dehydrogenation of 1,2,3,4-tetrahydroquinolines.

Recent Advances in the Enantioselective Synthesis of Chiral Amines via Transition Metal-Catalyzed Asymmetric Hydrogenation

Chiral amines are key structural motifs present in a wide variety of natural products, drugs, and other biologically active compounds. During the past decade, significant advances have been made with respect to the enantioselective synthesis of chiral amines, many of them based on catalytic asymmetric hydrogenation (AH). The present review covers the use of AH in the synthesis of chiral amines bearing a stereogenic center either in the α, β, or γ position with respect to the nitrogen atom, reported from 2010 to 2020. Therefore, we provide an overview of the recent advances in the AH of imines, enamides, enamines, allyl amines, and N-heteroaromatic compounds.

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).

Enantioselective Synthesis of 2-Functionalized Tetrahydroquinolines through Biomimetic Reduction

Biomimetic asymmetric reduction of 2-functionalized quinolines has been successfully developed with the chiral and regenerable NAD(P)H model CYNAM in the presence of transfer catalyst simple achiral phosphoric acids, providing the chiral 2-functionalized tetrahydroquinolines with up to 99% ee. Using this methodology as a key step, a chiral and potent opioid analgesic containing a 1,2,3,4-tetrahydroquinoline motif was synthesized with high overall yield.

Asymmetric hydrogenation of O-/N-functional group substituted arenes

Asymmetric hydrogenation of aromatic compounds represents one of the most straightforward synthetic methods to construct important chiral cyclic skeletons that are often found in biologically active agents and natural products. So far, the most successful examples in this field are largely limited to aromatics containing alkyl and aryl substituted groups due to the poor functional-group tolerance of hydrogenation. Direct asymmetric hydrogenation of functionalized aromatics provides enormous potential for expanding the structural diversity of reductive products of planar aromatic compounds, which is highly desirable and has not been well studied. This feature article focuses on the progress in catalytic asymmetric hydrogenation and transfer hydrogenation of O/N substituted arenes.

Recent Advances in Homogeneous Catalysts for the Asymmetric Hydrogenation of Heteroarenes

The asymmetric hydrogenation of heteroarenes has recently emerged as an effective strategy for the direct access to enantioenriched, saturated heterocycles. Although several homogeneous catalyst systems have been extensively developed for the hydrogenation of heteroarenes with high levels of chemo- and stereoselectivity, the development of mild conditions that allow for efficient and stereoselective hydrogenation of a broad range of substrates remains a challenge. This Perspective highlights recent advances in homogeneous catalysis of heteroarene hydrogenation as inspiration for the further development of asymmetric hydrogenation catalysts, and addresses underdeveloped areas and limitations of the current technology.

Exploration of chiral diastereomeric spiroketal (SPIROL)-based phosphinite ligands in asymmetric hydrogenation of heterocycles

New and readily available chiral SPIROL-based diphosphinite ligands (SPIRAPO) have been prepared and employed for iridium-catalyzed asymmetric hydrogenations of quinolines, quinoxalines and 2H-1,4-bezoxazin-2-ones. While the structurally similar (R,R,R)-SPIRAPO and (R)-SPINOL-based phosphinites were not the best ligands for these transformations, the (S,R,R)-diastereomer of SPIRAPO was found to be highly effective ligand for the reduction of 20 different heterocyclic systems with loadings as low as S/C = 10 000. This dearomatizative hydrogenation provided direct access to optically active tetrahydroquinolines in high enantioselectivities (up to 94% ee) and excellent yields (up to 99%), and was used to generate 1.75 g of natural alkaloid (-)-(R)-angustureine. This protocol was subsequently extended to achieve asymmetric hydrogenation of quinoxalines and 2H-1,4-benzoxazin-2-ones in good to excellent enantioselectivities.

The combination of asymmetric hydrogenation of olefins and direct reductive amination

Asymmetric hydrogenation (AH) and direct reductive amination (DRA) are both efficient transformations frequently utilized in industry. Here we combine the asymmetric hydrogenation of prochiral olefins and direct reductive amination of aldehydes in one step using hydrogen gas as the common reductant and a rhodium-Segphos complex as the catalyst. With this strategy, the efficiency for the synthesis of the corresponding chiral amino compounds is significantly improved. The practical application of this synthetic approach is demonstrated by the facile synthesis of chiral 3-phenyltetrahydroquinoline and 3-benzylindoline compounds.

Highly Diastereo- and Enantioselective Ir-Catalyzed Hydrogenation of 2,3-Disubstituted Quinolines with Structurally Fine-Tuned Phosphine-Phosphoramidite Ligands

A highly diastereo- and enantioselective Ir-catalyzed hydrogenation of unfunctionalized 2,3-disubstituted quinolines, especially 3-alkyl-2-arylquinolines, has been realized. The success of this hydrogenation is ascribed to the use of a structurally fine-tuned chiral phosphine-phosphoramidite ligand with a (S_a)-3,3'-dimethyl H₈-naphthyl moiety and (R_c)-1-phenylethylamine backbone. The hydrogenation displayed broad functional group tolerance, thus furnishing a wide range of optically active 2,3-disubstituted tetrahydroquinolines in up to 96% ee and with perfect cis-diastereoselectivity.

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.

Deconstructive di-functionalization of unstrained, benzo cyclic amines by C–N bond cleavage using a recyclable tungsten catalyst

With H₂WO₄ as the catalyst and H₂O₂ as the oxidant, we herein report a deconstructive difunctionalization of the C–N bond in unstrained, benzo cyclic amines to generate an ester group and nitro group simultaneously.

Divergent Coupling of Anilines and Enones by Integration of C-H Activation and Transfer Hydrogenation

Cp*Rh^III /Ir^III complexes are known to play important roles in both C-H activation and transfer hydrogenation (TH). However, these two areas evolved separately. They have been integrated in redox- and chemodivergent coupling reactions of N-pyridylanilines with enones. The iridium-catalyzed coupling with enones leads to the efficient synthesis of tetrahydroquinolines through TH from ⁱ PrOH. Counterintuitively, ⁱ PrOH does not serve as the sole hydride source, and the major reaction pathway involves disproportionation of a dihydroquinoline intermediate, followed by the convergent and iterative reduction of quinolinium species.

Dual-Excitation Polyoxometalate-Based Frameworks for One-Pot Light-Driven Hydrogen Evolution and Oxidative Dehydrogenation

Dehydrogenation of the tetrahydroisoquinoline derivatives coupled with hydrogen production is important for hydrogen storage applications. Herein, we formulated a new system that embedded Dawson-type polyoxometalates as efficient photosensitizers into the pores of redox-active coordination polymers for the light-driven photocatalytic oxidative Mannich reaction and hydrogen evolution. In the designed Co-POM polymer, UV light excitation gives the excited state of the Dawson-type polyoxometalate first to oxidize electron donors or substrates; the reduced form (i.e., heteropolyblue) adsorbs visible light to achieve a new excited state, which reduced the cobalt redox sites and facilitates hydrogen evolution reaction. The photosensitizer recovered to the ground state, completing the catalytic cycle. Under the optimized conditions, Co-POM enabled the hydrogen evolution and dehydrogenation of tetrahydroisoquinoline without the presence of any other additives. The high catalytic efficiency and robustness indicated the advantages of the combining functional polyoxometalate-based catalysts and porous characters of the coordination polymers for the development of highly active heterogeneous catalysts.

Enantioselective Synthesis of α-(Hetero)aryl Piperidines through Asymmetric Hydrogenation of Pyridinium Salts and Its Mechanistic Insights

Enantioselective synthesis of α-aryl and α-heteroaryl piperidines is reported. The key step is an iridium-catalyzed asymmetric hydrogenation of substituted N-benzylpyridinium salts. High levels of enantioselectivity up to 99.3:0.7 er were obtained for a range of α-heteroaryl piperidines. DFT calculations support an outersphere dissociative mechanism for the pyridinium reduction. Notably, initial protonation of the final enamine intermediate determines the stereochemical outcome of the transformation rather than hydride reduction of the resultant iminium intermediate.

Highly efficient asymmetric construction of novel indolines and tetrahydroquinoline derivatives via aza-Barbier/C–N coupling reaction

An efficient approach towards stereoselective synthesis of chiral indolines and tetrahydroquinolines via Zn-mediated allylation of chiral-N-tert-butanesulfinyl imines and intramolecular C–N cross-coupling is described.

Kinetic resolution of racemic 2-substituted 1,2-dihydroquinolines via asymmetric Cu-catalyzed borylation

A highly efficient kinetic resolution of racemic 2-substituted 1,2-dihydroquinolines via asymmetric Cu-catalyzed borylation has been realized for the first time. Under mild conditions, a variety of chiral 3-boryl-1,2,3,4-tetrahydroquinolines containing two vicinal stereogenic centers as well as the recovered 2-substituted 1,2-dihydroquinolines were afforded after 30 minutes in high yields with up to 99% ee (dr > 99 : 1) and over 98% ee values, respectively, corresponding to kinetic selectivity factors of up to 569. Moreover, this protocol was successfully applied to the asymmetric synthesis of a selective estrogen receptor modulator.