Highly Enantioselective Rhodium-Catalyzed Hydrogenation with Monodentate Ligands

P-Stereogenic Phosphorus Ligands in Asymmetric Catalysis

Chiral phosphorus ligands play a crucial role in asymmetric catalysis for the efficient synthesis of useful optically active compounds. They are largely categorized into two classes: backbone chirality ligands and P-stereogenic phosphorus ligands. Most of the reported ligands belong to the former class. Privileged ones such as BINAP and DuPhos are frequently employed in a wide range of catalytic asymmetric transformations. In contrast, the latter class of P-stereogenic phosphorus ligands has remained a small family for many years mainly because of their synthetic difficulty. The late 1990s saw the emergence of novel P-stereogenic phosphorus ligands with their superior enantioinduction ability in Rh-catalyzed asymmetric hydrogenation reactions. Since then, numerous P-stereogenic phosphorus ligands have been synthesized and used in catalytic asymmetric reactions. This Review summarizes P-stereogenic phosphorus ligands reported thus far, including their stereochemical and electronic properties that afford high to excellent enantioselectivities. Examples of reactions that use this class of ligands are described together with their applications in the construction of key intermediates for the synthesis of optically active natural products and therapeutic agents. The literature covered dates back to 1968 up until December 2023, centering on studies published in the late 1990s and later years.

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.

Catalytic asymmetric synthesis of α-stereogenic carboxylic acids: recent advances

Chiral carboxylic acids bearing an α-stereogenic center constitute the backbone of many natural products and therapeutic reagents as well as privileged chiral ligands and catalysts. Hence, it is not surprising that a large number of elegant catalytic asymmetric strategies have been developed toward the efficient synthesis of α-chiral carboxylic acids, such as α-hydroxy acids and α-amino acids. In this review, the recent advances in asymmetric synthesis of α-stereogenic free carboxylic acids via organocatalysis and transition metal catalysis are summarized (mainly from 2010 to 2020). The content is organized by the reaction type of the carboxyl source involved, including asymmetric functionalization of substituted carboxylic acids, cyclic anhydrides, α-keto acids, substituted α,β-unsaturated acids and so on. We hope that this review will motivate further interest in catalytic asymmetric synthesis of chiral α-substituted carboxylic acids.

Concise Six-Step Asymmetric Approach to Ramelteon from an Acetophenone Derivative Using Ir, Rh, Cu, and Ni Catalysis

A concise six-step asymmetric synthesis of nearly enantiomerically pure ramelteon was developed from a monocyclic precursor with a 17% overall yield and a 97% ee in the asymmetric step. The synthetically challenging tricyclic 1,6,7,8-tetrahydro-2H-indeno[5,4-b]furan core of ramelteon was assembled by using Ir-catalyzed O-vinylation and Rh-catalyzed vinyl ether annulation through directed C–H bond activation, while the chirality was introduced with enantioselective reduction of an α,β-unsaturated nitrile moiety under hydrosilylation conditions using a Cu^II/Walphos type catalyst. The presented methodology represents the shortest synthetic approach to ramelteon.

Recent Developments on the Synthesis and Bioactivity of Ilamycins/Rufomycins and Cyclomarins, Marine Cyclopeptides That Demonstrate Anti-Malaria and Anti-Tuberculosis Activity

Ilamycins/rufomycins and cyclomarins are marine cycloheptapeptides containing unusual amino acids. Produced by Streptomyces sp., these compounds show potent activity against a range of mycobacteria, including multidrug-resistant strains of Mycobacterium tuberculosis. The cyclomarins are also very potent inhibitors of Plasmodium falciparum. Biosynthetically the cyclopeptides are obtained via a heptamodular nonribosomal peptide synthetase (NRPS) that directly incorporates some of the nonproteinogenic amino acids. A wide range of derivatives can be obtained by fermentation, while bioengineering also allows the mutasynthesis of derivatives, especially cyclomarins. Other derivatives are accessible by semisynthesis or total syntheses, reported for both natural product classes. The anti-tuberculosis (anti-TB) activity results from the binding of the peptides to the N-terminal domain (NTD) of the bacterial protease-associated unfoldase ClpC1, causing cell death by the uncontrolled proteolytic activity of this enzyme. Diadenosine triphosphate hydrolase (PfAp3Aase) was found to be the active target of the cyclomarins in Plasmodia. SAR studies with natural and synthetic derivatives on ilamycins/rufomycins and cyclomarins indicate which parts of the molecules can be simplified or otherwise modified without losing activity for either target. This review examines all aspects of the research conducted in the syntheses of these interesting cyclopeptides.

Chiral diene-promoted room temperature conjugate arylation: highly enantioselective synthesis of substituted chiral phenylalanine derivatives and α,α-di(arylmethyl)acetates

A highly enantiocontrolled room temperature rhodium-catalyzed conjugate arylation process was developed. The reaction proceeds through 1,4-addition of α-substituted acrylates followed by enantioselective protonation using a C1-symmetric chiral bicyclo[2,2,2] diene as the ligand and water as the proton source. This exceptionally simple protocol provides a reliable and practical access to structurally important phenylalanine derivatives and α,α-di(arylmethyl)acetates in high yields (up to 99%) with good to excellent ee values (up to 99%).

Nickel-Catalyzed Asymmetric Hydrogenation of Cyclic Alkenyl Sulfones, Benzo[b]thiophene 1,1-Dioxides, with Mechanistic Studies

A highly efficient catalytic system based on the cheap transition metal nickel for the asymmetric hydrogenation of challenging cyclic alkenyl sulfones, 3-substituted benzo[b]thiophene 1,1-dioxides, was first successfully developed. A series of hydrogenation products, chiral 2,3-dihydrobenzo[b]thiophene 1,1-dioxides, were obtained in high yields (95-99%) with excellent enantioselectivities (90-99% ee). According to the results of nonlinear effect studies, deuterium-labeling experiments, and DFT calculation investigations, a reasonable catalytic mechanism for this nickel-catalyzed asymmetric hydrogenation was provided, which displayed that the two added hydrogen atoms of the hydrogenation products could be from H₂ through the insertion of Ni-H and subsequent hydrogenolysis.

Recent developments in asymmetric hydroformylation

This review describes the recent developments in the field of asymmetric hydroformylation. A large variety of ligands is now available, some of which are extremely effective in inducing high enantio- and regioselectivity.

“In situ immobilization” of a multicomponent chiral catalyst (MCC) via non-covalent interactions for heterogeneous asymmetric hydrogenation reactions

Novel hybrid catalysts that resulted from the anchoring of pyrene-tagged Rh(i) complexes onto graphene materials via π–π stacking interactions show excellent catalytic activity towards the hydrogenation of dehydroamino acid.

Asymmetric Synthesis of Chiral Spiroketal Bisphosphine Ligands and Their Application in Enantioselective Olefin Hydrogenation

A series of chiral spiroketal bisphosphine ligands containing 1,1'-spirobi(3 H,3' H)isobenzofuran backbones was accessed through asymmetric synthesis and subsequently tested in enantioselective Rh-catalyzed hydrogenation of α-dehydroamino acid esters. The ligand providing the highest enantioselectivity (up to 99.5%) was obtained in seven steps in an overall 38% yield. The synthesis could be performed on a gram scale, and no kinetic resolution of enantiomers is required. Overall, the developed ligand provides an easily accessible alternative to SDP ligands as well as other chiral bisphosphine ligands.

Direct Asymmetric Reductive Amination for the Synthesis of (S)-Rivastigmine

In this article we demonstrate how asymmetric total synthesis of (S)-rivastigmine has been achieved using direct asymmetric reductive amination as the key transformation in four steps. The route started with readily available and cheap m-hydroxyacetophenone, through esterification, asymmetric reductive amination, N-diphenylmethyl deprotection and reductive amination, to provide the final (S)-rivastigmine in 82% overall yield and 96% enantioselectivity. In the asymmetric reductive amination, catalysed by the iridium–phosphoramidite ligand complex and helped by some additives, the readily prepared 3-acetylphenyl ethyl(methyl)carbamate directly reductively coupled with diphenylmethanamine to yield the chiral amine product in 96% ee and 93% yield.

A comparison of MOP-phosphonite ligands and their applications in Rh(i)- and Pd(ii)-catalysed asymmetric transformations

Six chiral MOP-phosphonites have been synthesised and compared via experimental and computational methods in an effort to quantify their differing structural and electronic profiles. They were found to be electron-poor ligands in comparison to their arylphosphine analogues and have a larger trans influence in square planar Pt(ii) complexes. Four [Rh(L^P)(η²:η²-cod)Cl] complexes were synthesised and characterised by NMR, HRMS and X-ray crystallography. Two [Rh(L^P)₂]BF₄ complexes were prepared where one ligand acts as a chelating P,C-π-donor; detailed NMR studies demonstrated a hemilabile η⁶-coordination mode, which in one case was confirmed by X-ray crystallography. Rh(i) complexes were used as catalysts in asymmetric hydrogenation and hydroformylation reactions and in the addition of phenyl boronic acid to an isatin. Pd(ii) complexes were successfully employed in asymmetric Suzuki-Miyaura cross-coupling reactions yielding binaphthyl products. Two [Pd(L^P)₂Cl₂] complexes were synthesised and characterised by X-ray crystallography, both adopting cis orientations, with one of the complexes crystallising as two pseudo-polymorphs.

Featuring Xantphos

This review highlights the use of the bisphosphine ligand group in homogeneous catalysis.

Ligand libraries for high throughput screening of homogeneous catalysts

This review describes different approaches to construct ligand libraries towards high throughput screening of homogeneous metal catalysts.

In Situ-Generated Glycinyl Chloroaminals for a One-Pot Synthesis of Non-proteinogenic α-Amino Esters

An acetyl chloride-mediated cascade transformation involving a primary carbamate, ethyl glyoxylate, and various types of nucleophiles is reported for the synthesis of orthogonally protected α-amino esters. These reactions proceeded rapidly to afford the pivotal α-chloroglycine intermediate in excellent yields, which can be directly functionalized in situ with various types of nucleophiles. A mild and unique AcOH(cat.)/AcCl system was found to promote an autocatalytic-like condensation and facilitate the multicomponent assembly of non-proteinogenic α-amino esters. To better understand this one-pot transformation and the orchestration of the components' condensations, the investigation of a broader scope of nucleophiles and some kinetic studies are presented. Our findings suggest that the halogenation step toward the formation of α-chloroglycine is the rate-determining step likely proceeding through the formation of N-carbamoyl iminium. Also, the initial kinetic profiling for the nucleophilic substitution supports an S_N1-like (S_N2C+) mechanism in which nucleophiles add to the iminium-chloride tight ionic pair. These results lead ultimately to the design of a new protocol in which an achiral hydrogen bond donor thiourea catalyst was utilized to enhance the reaction scope and enable silylated nucleophiles to be efficiently exploited to synthesize novel non-proteinogenic α-amino esters.

Direct Catalytic Asymmetric Reductive Amination of Aliphatic Ketones Utilizing Diphenylmethanamine as Coupling Partner

The highly efficient direct catalytic reductive amination of ketones with diphenylmethanamine catalyzed by iridium-phosphoramidite complexes is described. As an effective coupling partner, diphenylmethanamine is suitable for a wide range of ketones to provide chiral amines in high yields and enantioselectivity. The chiral monodentate phosphoramidite ligands are tunable and competent to accommodate substrates with different structures.

Iridium-catalyzed direct asymmetric reductive amination of aromatic ketones

Diphenylmethanamine offers excellent stereocontrol and easy deprotection to provide primary amines in the studied asymmetric reductive amination.

By-design enantioselective self-amplification based on non-covalent product-catalyst interactions

The synthesis of enantiomerically pure compounds is of great importance in pharmaceuticals, fragrances and biological applications, and functions as a key to many processes in nature. Asymmetric catalysis using enantiomerically pure catalysts represents an efficient synthetic method to achieve this goal. The enantiomeric excess of the reaction product correlates with the enantiomeric purity of the catalysts, except for nonlinear behaviour, therefore the use of stereochemically flexible catalysts seems to complicate the control of stereoselectivity. Self-amplifying catalytic reactions are attractive, but a general rational design is highly challenging. Here we show that product interaction with chiral recognition sites attached to structurally flexible phoshoramidite-type catalysts can sense the chirality and induce enantioselectivity in the catalyst. Structural flexibility along with sensing of the chirality of the product molecules results in a rapid increase of enantioselectivity of the dynamic catalysts (Δe.e. of up to 76%) and a shift out of equilibrium. In contrast to stereodynamic catalysts controlled with cleavable chiral auxiliaries, the enantioselectivity does not decrease.

Total synthesis of cyclomarins A, C and D, marine cyclic peptides with interesting anti-tuberculosis and anti-malaria activities

Cyclomarins are cyclic heptapeptides containing four unusual amino acids.

A one-step, modular route to optically-active diphos ligands

A chlorosilane elimination reaction has been developed that allows the efficient synthesis of optically pure C1-symmetric, C1-backboned diphosphines with a wide variety of stereoelectronic characteristics.