Novel and Efficient Chiral Bisphosphorus Ligands for Rhodium-Catalyzed Asymmetric Hydrogenation

Synthesis of 1,2,4-diazaphospholes via base-promoted cyclization reaction of hydrazonoyl chlorides and [Bu4N][P(SiCl3)2]

Aromatic 1,2,4-diazaphospholes featuring distinct hybrid-mode nitrogen atoms (N(λ3σ2), N(λ3σ3)) and low-valent phosphorus atoms (λ3σ2) exhibited the characteristic of serving as unique hybrid ligands. This study presented a one-pot reaction involving the base-promoted stepwise cyclization of hydrazonoyl chlorides and [Bu4N][P(SiCl3)2] to yield 1,2,4-diazaphospholes, providing an effective method for synthesizing such compounds.

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.

Design and Synthesis of Pyrrolidinyl Ferrocene-Containing Ligands and Their Application in Highly Enantioselective Rhodium-Catalyzed Olefin Hydrogenation

Herein, we report the design and synthesis of a series of chiral pyrrolidine-substituted ferrocene-derived ligands. The proficiency of this novel structural motif was demonstrated in the Rh-catalyzed asymmetric hydrogenation of dehydroamino acid esters and α-aryl enamides. The products were obtained with full conversions and excellent levels of enantioselectivities of up to >99.9% ee and 97.7% ee, respectively, using a BINOL-substituted phosphine-phosphoaramidite ligand which possesses planar, central, and axial chirality elements.

The stereoselective conversion of epimerized alkoxyl phosphine-borane to P,C, axial-stereogenic tertiary phosphine via cleavage of P-O bond

The P-O bond of epimerized alkoxyl phosphine-borane was cleaved by naphthalene-lithium, to form two diastereomers of P-anions in a ratio of 86 : 14, which was then converted to secondary phosphine-borane via acidification, and to tertiary phosphines with alkyl halides with enhanced 96 : 4 dr. The isolated tertiary phosphine containing hydroxyl (in >99 : 1 dr) was converted to multi-stereogenic tertiary phosphines via O-alkylation with alkylene dihalides.

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.

Enantioselective hydrogenation of cyclic tetrasubstituted-olefinic dehydroamino acid derivatives

An efficient asymmetric hydrogenation of cyclic tetrasubstituted-olefinic dehydroamino acid derivatives has been achieved with a Rh-ArcPhos catalyst, affording a series of α-acylamino-β-alkyl tetrahydropyranones with two contiguous chiral centers in up to 96% ee and 1000 TON.

Cleavage of Aromatic C-O Bonds via Intramolecular SNAr Reaction and Preparation of P,C,Axial-Stereogenic Menthyl Phosphine Derivatives

Phosphine ligands with up to six chiral sites were prepared, starting from 2-phenylphenol, via O- and P-alkylation, cyclization, and coupling. The chirality was transferred from (L)-menthyl to phosphorus, α-carbon, and axis, to achieve excellent diastereoselectivities. During an intramolecular S_NAr reaction with alkoxyl as the leaving groups, the C-O bond was converted to a C-C bond. Both phosphine boranes and oxides could be used for the conversions, affording a series of cyclic phosphines.

P-Stereogenic N-Phosphine-Phosphite Ligands for the Rh-Catalyzed Hydrogenation of Olefins

We have identified a successful family of simple P-stereogenic N-phosphine-phosphite ligands for the Rh-catalyzed asymmetric hydrogenation of olefins. These catalysts show excellent enantiocontrol for α-dehydroamino acid derivatives and α-enamides (ee's up to >99%) and promising results for the more challenging β-analogues (ee's up to 80%). The usefulness of these catalytic systems was further demonstrated with the synthesis of several valuable precursors for pharmacologically active compounds, with ee's at least as high as the best ones reported previously (up to >99%).

Bis[diphenylphosphino]methane and its bridge-substituted analogues as chemically non-innocent ligands for H2 activation

Interconversion between coordinated diphosphinomethane and diphosphinomethanide moieties can be used as a new tool for metal–ligand cooperation.

A Bulky Three-Hindered Quadrant Bisphosphine Ligand: Synthesis and Application in Rhodium-Catalyzed Asymmetric Hydrogenation of Functionalized Alkenes

A bulky three-hindered quadrant bisphosphine ligand, di-1-adamantylphosphino(tert-butylmethylphosphino)methane, named BulkyP*, has been synthesized via a convergent short pathway with chromatography-free procedures. The ligand is a crystalline solid and can be readily handled in air. Its rhodium(I) complex exhibits very high enantioselectivities and catalytic activities in the asymmetric hydrogenation of functionalized alkenes.

P-Chiral Phosphorus Ligands Based on a 2,3-Dihydrobenzo[ d][1,3]oxaphosphole Motif for Asymmetric Catalysis

Despite the rapid progress in the field of asymmetric catalysis, the search for new, efficient, and practical asymmetric catalytic transformations to facilitate the green synthesis of chiral natural products and drugs will continue to be a major ongoing effort in organic chemistry. Chiral phosphorus ligands have played a significant role in recent advances in transition-metal-catalyzed asymmetric transformations. However, there remain numerous challenging issues of reactivity and selectivity in catalysis. The development of new and efficient chiral phosphorus ligands with new structural motifs remains highly desirable. P-Chiral phosphorus ligands have been overlooked and are underdeveloped, except for the early success of DIPAMP, introduced first by Knowles in the early 1970s. It was not until the late 1990s that the development of P-chiral phosphorus ligands regained attention with the advent of bisP*, TangPhos, etc. Nonetheless, most P-chiral phosphorus ligands were either difficult to prepare or operationally inconvenient. The development of efficient, practical, and operationally convenient P-chiral phosphorus ligands with new structural motifs remains an important subject of research. This Account introduces the design and development of a series of practical and efficient P-chiral bis- and monophosphorus ligands based on a 2,3-dihydrobenzo[ d][1,3]oxaphosphole motif. Their unique structural and physical properties include conformational unambiguousness, high tunability of electronic and steric properties, and operational simplicity as air-stable solids, which make them practical and exceptional ligands for asymmetric catalysis. Chiral bisphosphorus ligands such as MeO-BIBOP (L3), WingPhos (L4), and iPr-BABIBOP (L7) have demonstrated excellent enantioselectivities and unprecedented turnover numbers (TONs) in various asymmetric hydrogenations and other transformations, providing practical and efficient solutions leading to chiral amines, alcohols, carboxylic acids, and α- and β-amino acids. Chiral biaryl monophosphorus ligands, including BI-DIME (L9), AntPhos (L15), iPr-BI-DIME (L11), etc., have proven to be a class of versatile and powerful ligands for a number of catalytic asymmetric transformations, including asymmetric Suzuki-Miyaura coupling, asymmetric palladium-catalyzed dearomative cyclization, asymmetric hydroboration/diboration, asymmetric nickel-catalyzed reductive coupling, asymmetric palladium-catalyzed intramolecular arylation, asymmetric alkene aryloxyarylation, asymmetric α-arylation, asymmetric Heck reaction, and asymmetric nucleophilic addition, providing efficient solutions leading to various synthetically challenging chiral structures such as chiral biaryls, chiral tertiary alcohols, chiral α-amino tertiary boronic esters, and chiral all-carbon quaternary stereocenters. The high enantioselectivities and TONs obtained with these ligands have resulted in the syntheses of several chiral natural products and therapeutic agents in concise and highly efficient manners. While our efforts on the development of P-chiral phosphorus ligands are ongoing, it should be emphasized that the development of ligands and catalysts with new structural motifs should continue in the search for new reactivity and selectivity to tackle current synthetic challenges. Such effort is destined to promote the advances of asymmetric catalysis as well as synthetic organic chemistry.

Synthesis of N-aryl β-amino acid derivatives via Cu(ii)-catalyzed asymmetric 1,4-reduction in air

In the presence of the inexpensive and stable stoichiometric reductant polymethylhydrosiloxane (PMHS) as well as certain amounts of appropriate alcohol and base additives, the non-precious metal copper-catalyzed asymmetric 1,4-hydrosilylation of β-aryl or β-alkyl-substituted N-aryl β-enamino esters was well realized to afford a diverse range of N-aryl β-amino acid esters in high yields and excellent enantioselectivities (26 examples, 90-98% ee). This approach tolerated the handling of both catalyst and reactants in air without special precautions. The chiral products obtained have been successfully converted to the corresponding enantiomerically enriched β-lactam and unprotected β-amino acid ester, which highlighted the synthetic utility of the developed catalytic procedure.

Computationally Assisted Mechanistic Investigation and Development of Pd-Catalyzed Asymmetric Suzuki-Miyaura and Negishi Cross-Coupling Reactions for Tetra-ortho-Substituted Biaryl Synthesis

Metal-catalyzed cross-coupling reactions are extensively employed in both academia and industry for the synthesis of biaryl derivatives for applications to both medicine and material science. Application of these methods to prepare tetra-ortho-substituted biaryls leads to chiral atropisomeric products that introduces the opportunity to use catalyst-control to develop asymmetric cross-coupling procedures to access these important compounds. Asymmetric Pd-catalyzed Suzuki-Miyaura and Negishi cross-coupling reactions to form tetra-ortho-substituted biaryls were studied employing a collection of P-chiral dihydrobenzooxaphosphole (BOP) and dihydrobenzoazaphosphole (BAP) ligands. Enantioselectivities of up to 95:5 and 85:15 er were identified for the Suzuki-Miyaura and Negishi cross-coupling reactions, respectively. Unique ligands for the Suzuki-Miyaura reaction vs the Negishi reaction were identified. A computational study on these Suzuki-Miyaura and Negishi cross-coupling reactions enabled an understanding in the differences between the enantiodiscriminating events between these two cross-coupling reactions. These results support that enantioselectivity in the Negishi reaction results from the reductive elimination step, whereas all steps in the Suzuki-Miyaura catalytic cycle contribute to the overall enantioselection with transmetalation and reductive elimination providing the most contribution to the observed selectivities.

Catalytic applications of small bite-angle diphosphorus ligands with single-atom linkers

Diphosphorus ligands connected by a single atom (R₂PEPR₂; E = CR₂, CCR₂and NR) give chelating ligands with very small bite-angles as well as enable access to other properties such as bridging modes and hemilability. ThisPerspectivereviews the properties of diphosphorus ligands featuring a single-atom linker and their applications in catalysis, including transformations of alkenes and transfer hydrogenation and hydrogen-borrowing reactions.

Sequential C-H Arylation and Enantioselective Hydrogenation Enables Ideal Asymmetric Entry to the Indenopiperidine Core of an 11β-HSD-1 Inhibitor

A concise asymmetric synthesis of an 11β-HSD-1 inhibitor has been achieved using inexpensive starting materials with excellent step-economy at low catalyst loadings. The catalytic enantioselective total synthesis of 1 was accomplished in 7 steps and 38% overall yield aided by the development of an innovative, sequential strategy involving Pd-catalyzed pyridinium C-H arylation and Ir-catalyzed asymmetric hydrogenation of the resulting fused tricyclic indenopyridinium salt highlighted by the use of a unique P,N-ligand (MeO-BoQPhos) with 1000 ppm of [Ir(COD)Cl]₂.

Applications and stereoselective syntheses of P-chirogenic phosphorus compounds

This review reports the best stereoselective or asymmetric syntheses, the most efficient P*-building blocks and functionalisation of P-chirogenic compounds, in the light of chiral phosphorus compound applications.

Resolution of P-stereogenic P-heterocycles via the formation of diastereomeric molecular and coordination complexes (a review)

TADDOL derivatives and the Ca²⁺-salts of tartaric acid derivatives were found to be versatile and generally applicable resolving agents for the preparation of the enantiomers of P-stereogenic heterocyclic phosphine oxides and phosphinates.

Rh-catalysed asymmetric conjugate addition of boronic acids to nitroalkenes employing a P-chiral P,π-hybrid ligand

A P-chiral P,π-hybrid ligand was applied to the asymmetric conjugate addition of aryl boronic acids to β-substituted nitroalkenes.

Borane as an efficient directing group. Stereoselective 1,2-addition of organometallic reagents to borane P-stereogenic N-phosphanylimines

The 1,2-addition of organolithium reagents to borane P-stereogenic N-phosphanylimines was found to be a highly selective solvent-dependent stereodivergent process.

Preparation of highly substituted (β-acylamino)acrylates via iron-catalyzed alkoxycarbonylation of N-vinylacetamides with carbazates

A simple and efficient iron-catalyst system was applied for the synthesis of various (β-acylamino)acrylate derivatives under mild reaction conditions.