Nickel-Catalyzed Chemoselective Asymmetric Hydrogenation of α,β-Unsaturated Ketoimines: An Efficient Approach to Chiral Allylic Amines

Rhodium-Catalyzed Aromatic C-H Allylation with α,β-Unsaturated Imines

Reaction of 2-arylpyridines with α,β-unsaturated imines in the presence of 2.5 mol % of [Cp*RhCl2]2 and 10 mol % of AgSbF6 in acetone affords allylated 2-arylpyridines with an enamine unit located in the allyl segment. This method features ortho-monoallylation selectivity and Z-selectivity of the C-C double bonds, is applicable to a wide range of substrates, and is compatible with air and functional groups such as halides, CF3, COOMe, OH, MeO, and ketal groups.

Stereoselective Nickel-Catalyzed Iterative 1,2-Reduction of Trisubstituted Enones to Cycloalkanols Bearing Two Contiguous Stereocenters

Herein we first report a nickel-catalyzed asymmetric iterative 1,2-reduction of trisubstituted enones to cycloalkanols with two contiguous stereocenters in high yields with excellent diastereo- and enantioselectivities (36 examples, up to 98.5:1.5 er, >20:1 dr, TON = 500). The combined experimental and computational mechanistic studies suggested energy changes during two consecutive reduction processes and provided a range of unique mechanistic rationales that have not been disclosed in nickel-catalyzed asymmetric hydrogenation-related studies.

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.

Chiral bisphosphine Ph-BPE ligand: a rising star in asymmetric synthesis

Chiral 1,2-bis(2,5-diphenylphospholano)ethane (Ph-BPE) is a class of optimal organic bisphosphine ligands with C2-symmetry. Ph-BPE with its excellent catalytic performance in asymmetric synthesis has attracted much attention of chemists with increasing popularity and is growing into one of the most commonly used organophosphorus ligands, especially in asymmetric catalysis. Over two hundred examples have been reported since 2012. This review presents how Ph-BPE is utilized in asymmetric synthesis and how powerful it is as a chiral ligand or even a catalyst in a wide range of reactions including applications in the total synthesis of bioactive molecules.

Nickel-Catalyzed Chemo- and Enantioselective Hydrogenation and Deuteration of α,β-Unsaturated Ketimines Using Alcohols: Synthesis of Deuterated Chiral Allylic Amines

A nickel-catalyzed chemoselective asymmetric transfer hydrogenation of α,β-unsaturated ketimines using ethanol as a hydrogen donor under mild conditions that avoid using high-pressure hydrogen gas was developed. With this catalyst, C1-selective deuterated chiral allylic amines were efficiently synthesized using only stoichiometric 2-propanol-d8. Mechanism studies demonstrated the formation of a nickel hydride intermediate.

Enantioselective Synthesis of Chiral Cyclic Hydrazines by Ni-Catalyzed Asymmetric Hydrogenation

An efficient Ni-(S,S)-Ph-BPE complex that catalyzed asymmetric hydrogenation of cyclic N-acyl hydrazones has been developed to produce various chiral cyclic hydrazines in high yields with excellent enantioselectivities of up to >99% enantiomeric excess. Moreover, the hydrogenation can not only proceed smoothly on a gram scale under lower catalyst loading (S/C = 3000) without any decrease of enantioselectivity but can also be applied to the asymmetric synthesis of a RIP-1 kinase inhibitor.

BIAN-Aluminium-Catalysed Imine Hydrogenation

Hydrogenations have been dominated by transition metal catalysis, while the use of more abundant and inexpensive main group metal catalysts has remained a great challenge. Here, a bimetallic Li/Al dihydride was successfully applied to catalytic hydrogenations of imines. The catalyst [(DippBIAN)Al(μ-H)2Li(OEt2)2] was easily prepared from the 2e-reduced BIAN derivative and LiAlH4.

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.

C 1-Symmetric diphosphorus ligands in metal-catalyzed asymmetric hydrogenation to prepare chiral compounds

Asymmetric hydrogenation has remained an important and challenging research area in industry as well as academia due to its high atom economy and ability to induce chirality. Among several types of ligands, chiral bidentate phosphine ligands have played a pivotal role in developing asymmetric hydrogenation. Although C₂-symmetric chiral bidentate phosphine ligands have dominated the field, it has been found that several C₁-symmetric ligands are equally effective and, in many cases, have outperformed their C₂-symmetric counterparts. This review evaluates the possibility of the use of C₁-symmetric diphosphorus ligands in asymmetric hydrogenation to produce chiral compounds. The recent strategies and advances in the application of C₁-symmetric diphosphorus ligands in the metal-catalyzed asymmetric hydrogenation of a variety of CC bonds have been summarized. The potential of diphosphorus ligands in asymmetric hydrogenation to produce pharmaceutical intermediates, bioactive molecules, drug molecules, agrochemicals, and fragrances is discussed. Although asymmetric hydrogenation appears to be a problem that has been resolved, a deep dive into the recent literature reveals that there are several challenges that are yet to be addressed. The current asymmetric hydrogenation methods mostly employ precious metals, which are depleting at a fast pace. Therefore, scientific interventions to perform asymmetric hydrogenation using base metals or earth-abundant metals that can compete with established precious metals hold significant potential.

Nickel-Catalyzed Asymmetric Hydrogenation for Kinetic Resolution of [2.2]Paracyclophane-Derived Cyclic N-Sulfonylimines

Nickel-catalyzed asymmetric hydrogenation for kinetic resolution of [2.2]paracyclophane-derived cyclic N-sulfonylimines was successfully developed. High selectivity factors were observed in most cases (s up to 89), providing the recovered materials and hydrogenation products in good yields with high levels of enantiopurity. The recovered materials and hydrogenation products are useful synthetic intermediates for the synthesis of planar chiral [2.2]paracyclophane-based compounds.

Silver Salt-Mediated Allylation Reactions Using Allyl Bromides

A facile, efficient, and chemoselective synthesis of allylic amides has been developed. Allyl bromides were used as the precursors activated by silver triflate. A Ritter-type reaction readily proceeded to give various allyl amides under mild conditions. The reaction protocol was also applicable to different nucleophilic partners to give a wide range of allyl-substituted products in the absence of a base.

Asymmetric hydrogenation catalyzed by first-row transition metal complexes

This review focuses on asymmetric direct and transfer hydrogenation with first-row transition metal complexes. The reaction mechanisms and the models of enantiomeric induction were summarized and emphasized.

Ni-catalyzed asymmetric hydrogenation of N-aryl imino esters for the efficient synthesis of chiral α-aryl glycines

Chiral α-aryl glycines play a key role in the preparation of some bioactive products, however, their catalytic asymmetric synthesis is far from being satisfactory. Herein, we report an efficient nickel-catalyzed asymmetric hydrogenation of N-aryl imino esters, affording chiral α-aryl glycines in high yields and enantioselectivities (up to 98% ee). The hydrogenation can be conducted on a gram scale with a substrate/catalyst ratio of up to 2000. The obtained chiral N-p-methoxyphenyl α-aryl glycine derivatives are not only directly useful chiral secondary amino acid esters but can also be easily deprotected by treatment with cerium ammonium nitrate for further transformations to several widely used molecules including drug intermediates and chiral ligands. Formation of a chiral Ni-H species in hydrogenation is detected by 1H NMR. Computational results indicate that the stereo selection is determined during the approach of the substrate to the catalyst.