Development of Ruthenium Catalysts for the Enantioselective Synthesis of P-Stereogenic Phosphines via Nucleophilic Phosphido Intermediates

Enantiopure P-Chiral Secondary Phosphines (P*HRR') from the Catalytic Asymmetric Hydrogenation of P═C Bonds

We report the first bottleable enantiopure P-chiral secondary phosphines from the rhodium-catalyzed asymmetric hydrogenation of phosphaalkenes. Catalytic asymmetric hydrogenation, a reaction of broad academic and industrial importance for C═C, N═C, and O═C bonds, has not previously been reported for the P═C bond. The hydrogenation of ArP═CR2 (Ar = Mes, m-Xyl and TMOP; R = Ph, 4-C6H4F) affords four unprecedented P-stereogenic secondary phosphines in 76%-90% isolated yields with 91%-97% enantiomeric excess (ee). These isolable P-chiral secondary phosphines are configurationally stable indefinitely in the solid state and show only modest loss in ee when kept in solution for over a month at room temperature.

N-Heterocyclic Carbene Enabled Copper Catalyzed Asymmetric Synthesis of Pyrimidinyl Phosphine with both Axial and P-Stereogenicity

P-stereogenic phosphines, renowned for their utility as ligands and catalysts, have been instrumental in the field of asymmetric catalysis. However, the catalytic asymmetric synthesis of chiral ligands possessing both axial and phosphine chirality remains a significant challenge. Here, we present the successful demonstration of a Cu-catalyzed asymmetric C-P construction using in situ generated secondary phosphine and heteroaryl chloride. By introducing a chiral NHC ligand and an achiral diphosphine auxiliary ligand, we effectively alleviated the poisoning effect caused by phosphine(III) compounds and suppressed the nonenantioselective background reaction. The reaction exhibited excellent enantioselectivity, with up to 96 % ee, and good diastereoselectivity, with up to 14 : 1 dr, when employing less sterically hindered secondary phosphines. This particular substrate poses a significant challenge due to its strong poisoning effect in copper catalysis.

Palladium-catalyzed regio- and stereo-selective phosphination of cyclic biarylsulfonium salts to access atropoisomeric phosphines

A palladium-catalyzed regio- and stereo-selective phosphination of cyclic biarylsulfonium salts (racemic) with HPAr3Ar4 for straightforward synthesis of atropoisomeric phosphines (P,S-ligands) bearing a stereogenic axis or both a stereogenic axis and a P-stereogenic center is reported. The high reactivity and regio- and stereo-selectivity originate from the torsional strain release and palladium catalysis, and the construction of a P-stereogenic center is enabled by an efficient dynamic kinetic resolution. The high performance of the nascent P,S-ligands has been demonstrated in palladium-catalyzed asymmetric allylic substitutions, indicating the great potential of the present methodology.

Direct Synthesis of Tertiary Phosphines via Alkoxide-Mediated Deborylative Phosphination of Organoboronates

The direct transformation of alkylboron has emerged as a versatile and powerful methodology for creating carbon-carbon and carbon-heteroatom bonds. However, its potential application in the formation of carbon and phosphorus remains unexplored. In this study, we present an alkoxide base-promoted reaction system that enables deborylative phosphination of benzylic organoboronates and geminal bis(boronates) via selective C-B bond cleavage. This approach allows for the synthesis of valuable tertiary phosphines in good yields under mild conditions. The practicality and industrial potential of this approach are underscored by the operational simplicity, broad substrate scope, and easy scalability.

Pd(II)-Catalyzed enantioselective C-H olefination toward the synthesis of P-stereogenic phosphinamides

P-Stereogenic phosphorus compounds are important structural elements in chiral ligands or organocatalysts. Herein, we report a Pd(II)-catalyzed enantioselective C-H olefination toward the synthesis of P-stereogenic phosphinamides using cheap commercially available L-pGlu-OH as a chiral ligand. A broad range of P-stereogenic phosphinamides were gained in good yields with high enantioselectivities (33 examples, up to 77% yield, 99% ee) via desymmetrization and kinetic resolution.

Nickel-Catalyzed Asymmetric Synthesis of P-Stereogenic Phosphanyl Hydrazine Building Blocks

Catalytic asymmetric methods for the synthesis of synthetically versatile P-stereogenic building blocks offer an efficient and practical approach for the diversity-oriented preparation of P-chiral phosphorus compounds. Herein, we report the first nickel-catalyzed synthesis of P-stereogenic secondary aminophosphine-boranes by the asymmetric addition of primary phosphines to azo compounds. We further demonstrate that the P-H and P-N bonds on these phosphanyl hydrazine building blocks can be reacted sequentially and stereospecifically to access various P-stereogenic compounds with structural diversity.

Copper-Phosphido Catalysis: Enantioselective Addition of Phosphines to Cyclopropenes

We describe a copper catalyst that promotes the addition of phosphines to cyclopropenes at ambient temperature. A range of cyclopropylphosphines bearing different steric and electronic properties can now be accessed in high yields and enantioselectivities. Enrichment of phosphorus stereocenters is also demonstrated via a Dynamic Kinetic Asymmetric Transformation (DyKAT) process. A combined experimental and theoretical mechanistic study supports an elementary step featuring insertion of a CuI -phosphido into a carbon-carbon double bond. Density functional theory calculations reveal migratory insertion as the rate- and stereo-determining step, followed by a syn-protodemetalation.

Synthesis of the Brivaracetam Employing Asymmetric Photocatalysis and Continuous Flow Conditions

An original total synthesis of the antiepileptic drug brivaracetam (BRV) is reported. The key step in the synthesis consists of an enantioselective photochemical Giese addition, promoted by visible-light and the chiral bifunctional photocatalyst Δ-RhS. Continuous flow conditions were employed to improve the efficiency and allow an easy scale-up of the enantioselective photochemical reaction step. The intermediate obtained from the photochemical step was converted into BRV by two different pathways, followed by one alkylation and amidation, thus giving the desired active pharmaceutical ingredients (API) in 44% overall yield, 9:1 diastereoisomeric ratio (dr) and >99:1 enantiomeric ratio (er).

Copper-Catalyzed Dynamic Kinetic Resolution of Secondary Phosphine Oxides

Copper-catalyzed dynamic kinetic resolution of secondary phosphine oxides has been successfully developed, providing a general method for the gram-scale enantioselective synthesis of P-stereogenic cyclic phosphine oxides with high yields and high enantioselectivities. The products could be easily reduced to the corresponding useful P(III)-stereogenic cyclic phosphines. A mechanism of the dynamic kinetic resolution involving the unusual rapid racemization of SPOs has been proposed.

A Ni-catalyzed asymmetric C(sp)-P cross-coupling reaction for the synthesis of P-stereogenic alkynylphosphines

Due to the high reactivity of the triple bond, P-stereogenic alkynylphosphines could be easily derivatized, serving as universal building blocks for structurally diverse phosphine compounds. However, the synthesis of alkynylphosphines via direct P-C bond formation was unprecedented. Here, we report an efficient method for the synthesis of P-stereogenic alkynylphosphines with high enantioselectivity via a Ni-catalyzed asymmetric cross-coupling reaction. The reaction could tolerate a variety of functional groups, affording products that can be converted into useful phosphine derivatives.

Access to Enantiomerically Pure P-Stereogenic Primary Aminophosphine Sulfides under Reductive Conditions

Stereochemically pure phosphines with phosphorus-heteroatom bonds and P-centered chirality are a promising class of functional building blocks for the design of chiral ligands and organocatalysts. A route to enantiomerically pure primary aminophosphine sulfides was opened through stereospecific reductive C-N bond cleavage of phosphorus(V) precursors by lithium in liquid ammonia. The chemoselectivity of the reaction as a function of reaction time, substrate pattern, and chiral auxiliary was investigated. In the presence of exclusively aliphatic groups bound to the phosphorus atom, all competing reductive side reactions are totally prevented. The absolute configurations of all P-stereogenic compounds were determined by single-crystal X-ray diffraction analysis. Their use as synthetic building blocks was demonstrated. The lithium salt of (R)-BINOL-dithiophosphoric acid proved to be a useful stereochemical probe to determine the enantiomeric purity. Insights into the coordination mode of the lithium-based chiral complex formed in solution was provided by NMR spectroscopy and DFT calculations.

Secondary Phosphine Sulfide-Enabled Iridium-Catalyzed Asymmetric Allylic Substitution

An iridium-catalyzed asymmetric synthesis of branched allylic phosphine compounds under mild conditions is reported. Products bearing various functional groups can be synthesized with excellent stereoselectivity (up to 99.9 % ee) and regioselectivity. The employment of phosphine sulfides with relatively low deactivation capacity against metal catalysts is crucial for the success of this reaction.

Direct visible-light-induced synthesis of P-stereogenic phosphine oxides under air conditions

Over the past two decades, visible-light-induced transformations have been regarded as being among the most environmentally benign and powerful strategies for constructing complex molecules and diverse synthetic building blocks in organic synthesis. However, the development of efficient photochemical processes for assembling enantiomerically pure molecules remains a significant challenge. Herein, we describe a simple and efficient visible-light-induced C-P bond forming reaction for the synthesis of P-chiral heteroaryl phosphine oxides in moderate to high yields with excellent ee values (97-99% ee). Even in the absence of transition metal or photoredox catalysts, a variety of P-chiral heteroaryl phosphine oxides, including chiral diphosphine oxide 41, have been directly obtained under air conditions. Density functional theory (DFT) calculations have shown that the reaction involves intersystem crossing and single electron transfer to give a diradical intermediate under visible light irradiation.

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.

Ni-Catalyzed Enantioselective Benzylation of Secondary Phosphine Oxide

A nickel-catalyzed benzylic substitution of secondary phosphine oxide was described, affording the dialkylated P-stereogenic tertiary phosphine oxides with high to excellent enantioselectivities. The reaction was performed under mild conditions with commercially available benzyl chlorides and bench stable secondary phosphine oxides, exhibiting broad functional group tolerance. It represented a practical example for the preparation of P-stereogenic phosphine compounds.

Cobalt-Catalysed Asymmetric Addition and Alkylation of Secondary Phosphine Oxides for the Synthesis of P-Stereogenic Compounds

The catalytic asymmetric synthesis of P-chiral phosphorus compounds is an important way to construct P-chiral ligands. Herein, we report a new strategy that adopts the pyridinyl moiety as the coordinating group in the cobalt-catalysed asymmetric nucleophilic addition/alkylation of secondary phosphine oxides. A series of tertiary phosphine oxides were generated with up to 99 % yield and 99.5 % ee, and with broad functional-group tolerance. Mechanistic studies reveal that (R)-secondary phosphine oxides preferentially interact with the cobalt catalysts to produce P-stereogenic compounds.

Nickel-Catalyzed Asymmetric Synthesis of P-Stereogenic Vinyl Phosphines

Addition reaction to alkynes is an efficient strategy for constructing valuable alkenyl compounds. However, the elusive regioselectivity has been a persistent challenge. In the context of hydrophosphination reaction which could afford valuable P-stereogenic phosphines, the control of enantioselectivity as well as regioselectivity were especially tricky. Here, we highlighted our recent work on the nickel-catalyzed regio- and enantioselective hydrophosphination of unactivated alkynes with in situ generated secondary phosphines.

1 Introduction

2 Hydrophosphination of Alkynes

3 Derivatization Reactions

4 Mechanism Research

5 Summary and Outlook

Racemization mechanism of lithium tert-butylphenylphosphido-borane: A kinetic insight

The racemization mechanism of tert-butylphenylphosphido-borane is investigated experimentally and theoretically. Based on this converging approach, it is shown, first, that several phosphido-borane molecular species coexist at the time of the reaction and, second, that one particular of both initially assumed reactive routes most significantly contribute to the overall racemization process. From our converging modeling and experimental measurement, it comes out that the most probable species to be here encountered is a phosphido-borane-Li (THF)₂ neutral solvate, whose P-stereogenic center monomolecular inversion through a Y-shaped transition structure (Δ_r G°^≠ : 81 kJ mol^-1 ) brings the largest contribution to the racemization process.

Palladium/Xiao-Phos-Catalyzed Kinetic Resolution of sec-Phosphine Oxides by P-Benzylation

P-stereogenic tert- and sec-phosphines have wide applications in asymmetric catalysis, materials, and pharmaceutical chemistry, however, their practical synthesis still constitutes a significant challenge. Herein, a successful kinetic resolution of rac-secondary phosphine oxides via the enantioselective P-benzylation process catalyzed by the palladium/Xiao-Phos was designed. Both tert- and sec-phosphine oxides were delivered in good yield and excellent enantiopurity (selectivity factor up to 226.1). The appealing synthetic utilities are further demonstrated by the facile preparation of several valuable P-chiral compounds, precursors of bidentate ligands, as well as transition metal complexes.

Ni-Catalyzed Asymmetric Hydrophosphination of Unactivated Alkynes

The practical synthesis of P-stereogenic tertiary phosphines, which have wide applications in asymmetric catalysis, materials, and pharmaceutical chemistry, represents a significant challenge. A regio- and enantioselective hydrophosphination using cheap and ubiquitous alkynes catalyzed by a nickel complex was designed, in which the toxic and air-sensitive secondary phosphines were prepared in situ from bench-stable secondary phosphine oxides. This methodology has been demonstrated with unprecedented substrate scope and functional group compatibility to afford electronically and structurally diversified P(III) compounds. The products could be easily converted into various precursors of bidentate ligands and organocatalysts, as well as a variety of transition-metal complexes containing both P- and metal-stereogenic centers.

Copper(I)-Catalyzed Asymmetric Alkylation of Unsymmetrical Secondary Phosphines

A copper(I)-catalyzed asymmetric alkylation of HPAr¹Ar² with alkyl halides is uncovered, which provides an array of P-stereogenic phosphines in generally high yield and enantioselectivity. The electrophilic alkyl halides enjoy a broad substrate scope, including allyl bromides, propargyl bromide, benzyl bromides, and alkyl iodides. Moreover, 11 unsymmetrical diarylphosphines (HPAr¹Ar²) serve as competent pronucleophiles. The present methodology is also successfully applied to catalytic asymmetric double and triple alkylation, and the corresponding products were obtained in moderate diastereo- and excellent enantioselectivities. Some ³¹P NMR experiments indicate that bulky HPPhMes exhibits weak competitively coordinating ability to the Cu(I)-bisphosphine complex, and thus the presence of stoichiometric HPAr¹Ar² does not affect the enantioselectivity significantly. Therefore, the high enantioselectivity in this reaction is attributed to the high performance of the unique Cu(I)-(R,R_P)-TANIAPHOS complex in asymmetric induction. Finally, one monophosphine and two bisphosphines prepared by the present reaction are employed as efficient chiral ligands to afford three structurally diversified Cu(I) complexes, which demonstrates the synthetic utility of the present methodology.

Asymmetric Synthesis of P-Stereogenic Secondary Phosphine-Boranes by an Unsymmetric Bisphosphine Pincer-Nickel Complex

The first highly enantioselective catalytic synthesis of P-stereogenic secondary phosphine-boranes was realized by the asymmetric addition of primary phosphine to electron-deficient alkenes with a newly developed unsymmetric bisphosphine (PCP') pincer-nickel complex. Various P-stereogenic secondary phosphine-boranes were obtained in 57-92% yields with up to 99% ee and >20:1 dr. The follow-up alkylation upon P-C bond formation with alkyl halides provided a practical way to access P-chiral compounds with diverse functional groups.

Phosphorus Ligands in Hydroformylation and Hydrogenation: A Personal Account

Metal-catalyzed hydroformylation and hydrogenation heavily rely on ligands, among which phosphorous ligands play a pivotal role. This personal account presents a selection of three distinct classes of phosphorous ligands, namely, monodentate meta-substituted phosphinites, bis-phosphites, and P-chiral supramolecular phosphines, developed in our group. The synthesis of these ligands, isolation, characterization, and their performance in transition metal-catalyzed hydroformylation, isomerizing hydroformylation, and asymmetric hydrogenation of olefins is summarized. The state of the art development in iron-catalyzed hydroformylation of alkenes and our contributions to the field is discussed. Use of phosphines enabled iron-catalyzed hydroformylation of alkenes under mild conditions. Thus, this account demonstrates the central role of phosphorus ligands in industrially relevant transformations such as hydrogenation and hydroformylation. The seemingly matured field of ligand discovery still holds significant potential and will steer the field of homogeneous catalysis.

Access to P-chiral sec- and tert-phosphine oxides enabled by Le-Phos-catalyzed asymmetric kinetic resolution

The synthesis of P-stereogenic building blocks is extremely difficult. Herein we report an efficient kinetic resolution of secondary phosphine oxides via a Le-Phos-catalyzed asymmetric allylation reaction with Morita-Baylis-Hillman carbonates. This method provides facile access to enantioenriched secondary and tertiary P-chiral phosphine oxides with broad substrate scope, both of which could serve as P-stereogenic synthons, and can be rapidly incorporated into a given scaffold bearing a P-stereocenter. The highly desirable late stage modifications demonstrate the practicability of our method and can be a critical contribution to obtaining optimal P-chiral catalysts and ligands.

Stereoselective preparation of P,axial-stereogenic allenyl bisphosphine oxides via chirality-transfer

P,C-Stereogenic propargyl alcohols RC-3/SC-3' were prepared by the addition of (L)-menthyl-derived SPOs to propynals, which were converted to P,axial-stereogenic allenyl bisphosphine oxides. The chirality transfer was controlled by α-carbon via syn [2,3]-sigmatropic rearrangement. For SC-3' linking weak WDG on the alkynyl moiety, the chirality on the axis depended on stereogenic phosphorus.

Enantioselective synthesis of P-chiral tertiary phosphine oxides with an ethynyl group via Cu(i)-catalyzed azide-alkyne cycloaddition

We report the highly enantioselective synthesis of P-chiral tertiary phosphine oxides featuring an ethynyl group via Cu(i)-catalyzed azide-alkyne cycloaddition. Newly developed chiral pyridinebisoxazolines (PYBOX) bearing a bulky C4 shielding group play an important role in achieving excellent enantioselectivity while suppressing side bis-triazoles formation in desymmetrizing prochiral diethynylphosphine oxides. Notably, by tuning the size of the C4 shielding group, it is possible to achieve excellent remote enantiofacial control in desymmetrizing phosphole oxide-diynes with the prochiral P-center farther from the ethynyl group by four covalent bonds. Time-dependent enantioselectivity is observed for these desymmetric CuAAC reactions, suggesting a synergic combination of a desymmetrization and a kinetic resolution, and our ligands prove to be better than unmodified PYBOX in both steps. This finding contributes to a highly enantioselective kinetic resolution of racemic ethynylphosphine oxides. The resulting chiral ethynylphosphine oxides are versatile P-chiral synthons, which can undergo a number of diversifying reactions to enrich structural diversity.

Asymmetric Electrophilic Reactions in Phosphorus Chemistry

This review is devoted to the theoretic and synthetic aspects of asymmetric electrophilic substitution reactions at the stereogenic phosphorus center. The stereochemistry and mechanisms of electrophilic reactions are discussed—the substitution, addition and addition-elimination of many important reactions. The reactions of bimolecular electrophilic substitution SE2(P) proceed stereospecifically with the retention of absolute configuration at the phosphorus center, in contrast to the reactions of bimolecular nucleophilic substitution SN2(P), proceeding with inversion of absolute configuration. This conclusion was made based on stereochemical analysis of a wide range of trivalent phosphorus reactions with typical electrophiles and investigation of examples of a sizeable number of diverse compounds. The combination of stereospecific electrophilic reactions and stereoselective nucleophilic reactions is useful and promising for the further development of organophosphorus chemistry. The study of phosphoryl group transfer reactions is important for biological and molecular chemistry, as well as in studying mechanisms of chemical processes involving organophosphorus compounds. New versions of asymmetric electrophilic reactions applicable for the synthesis of enantiopure P-chiral secondary and tertiary phosphines are discussed.

Synthesis of P-chiral phosphine compounds by palladium-catalyzed C–P coupling reactions

An efficient C–P coupling reactions of enantiopure tert-butylmethylphosphine-boranes with aryl and heteroaryl halides is developed by using Pd(OAc)₂/dppf as a catalyst, affording a series of P-chiral phosphines (up to 99% ee).

Functional Phosphine Derivatives Having Stationary and Flexible Chiralities: Their Preparation and Chirality Controlling

Various functional secondary and tertiary phosphines, or their derivatives, containing stationary chiral phosphorus and flexible chiral axis were prepared, which could be further modified to afford diversely chelating ligands. The flexible axial chirality was fixed by stereogenic phosphorus via a cyclic linkage of chemical bonds or coordination with a metallic ion.