General and Stereoselective Method for the Synthesis of Sterically Congested and Structurally Diverse P-Stereogenic Secondary Phosphine Oxides

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.

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.

Universal and divergent P-stereogenic building with camphor-derived 2,3-diols

The access to P-stereogenic motifs has always been considered a very challenging and high attractive mission in modern organic synthesis. While several chiral auxiliaries employed by the practical Jugé-Stephan-like methodology have been developed, new type of readily accessible bifunctional ligands toward P-stereogenic building still remain much desirable. Herein, we present a powerful chiral template, camphor-derived 2,3-diols named CAMDOL, which were designed and synthesized from the commercially cheap camphorquinone in high yields at 50 grams scale with a column-free purification. Diverse P(III)-chiral compounds and their borane forms including phosphinous acids, phosphinites, and phosphines, as well as the corresponding P(V)-chiral compounds including phosphinates, phosphine oxides, phosphinothioates, phosphine sulfides, and secondary phosphine oxides were afforded in high yields and ee values through the optimal 2,3-diphenyl CAMDOL platform. An unusual C³-OP bond cleavage following the first P-OC² bond breaking was observed during the ring-opening process when quenching by NH₄Cl solution, which generates a unique but valuable camphor-epoxide scaffold as by-product.

Synthesis of Phosphorus(V)-Substituted Six-Membered N-Heterocycles: Recent Progress and Challenges

Heterocycles functionalized with pentavalent phosphorus are of great importance since they include a great variety of biologically active compounds and pharmaceuticals, advanced materials, and valuable reactive intermediates for organic synthesis. Significant progress in synthesis of P(O)R2-substituted six-membered heterocycles has been made in the past decade. This review covers the synthetic strategies towards aromatic monocyclic six-membered N-heterocycles, such as pyridines, pyridazines, pyrimidines, and pyrazines bearing phosphonates and phosphine oxides, which were reported from 2012 to 2022.

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.

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).

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.

Resolution of aryl-H-phosphinates applied in the synthesis of P-stereogenic compounds including a Brønsted acid NMR solvating agent

An enantioseparation method for the preparation of P-stereogenic H-phosphinates was elaborated. In stereoselective reactions, various chiral P-stereogenic compounds were prepared and their applications as chiral NMR solvating agents were assessed.

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.

The Formation of P-C Bonds Utilizing Organozinc Reagents for the Synthesis of Aryl- and Heteroaryl-Dichlorophosphines

Aryl- and heteroaryl-dichlorophosphines are mildly and selectively made in a one-pot synthesis in moderate to good yields starting from the respective aryl bromides or five-membered heterocycles, following lithiation with nBuLi, transmetalation with ZnCl₂, and subsequently the reaction with PCl₃. Selected aryl- and heteroaryl-dichlorophosphines were successfully synthesized using this reaction method and could easily be purified after isolation. The intermediate formation of the organozinc species is essential, as it prevents the formation of multiple substitution products. Important are also the reaction conditions: the usage of the proper solvent for the respective aromatic precursors and removal of the remaining salts by addition of a dioxane/pentane mixture. Depending on the solvent and steric demand of the substituent, mono- and bis-substitution products can be formed but formation also prevented. Hereby, different organozinc species might play an important role.

Enantioseparation of P-Stereogenic Secondary Phosphine Oxides and Their Stereospecific Transformation to Various Tertiary Phosphine Oxides and a Thiophosphinate

Secondary phosphine oxides incorporating various aryl and alkyl groups were synthesized in racemic form, and these products formed the library reported in this study. TADDOL derivatives were used to obtain the optical resolution of these P-stereogenic secondary phosphine oxides. The developed resolution method showed a good scope under the optimized reaction conditions, as 9 out of 14 derivatives could be prepared with an enantiomeric excess (ee) ≥ 79% and 5 of these derivatives were practically enantiopure >P(O)H compounds (ee ≥ 98%). The scalability of this resolution method was also demonstrated. Noncovalent interactions responsible for the formation of diasteromeric complexes were elucidated by single-crystal XRD measurements. (S)-(2-Methylphenyl)phenylphosphine oxide was transformed to a variety of P-stereogenic tertiary phosphine oxides and a thiophosphinate in stereospecific Michaelis–Becker, Hirao, or Pudovik reactions.

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.

Asymmetric Synthesis of P-Stereogenic Secondary Phosphine ­Oxides (SPOs)

P-Stereogenic secondary phosphine oxides [SPOs, RR′P(O)H], valuable ligands for metal complexes in asymmetric catalysis, are also building blocks for other chiral phosphorus derivatives. This short review summarizes methods used for asymmetric synthesis of P-stereogenic SPOs.

1 Introduction

2 Configurational Stability of P-Stereogenic SPOs

3 Classical Resolution, HPLC Separation, and Dynamic Resolution

4 Synthesis via Chiral Auxiliaries

5 Kinetic Resolution

6 Conclusions and Outlook

A facile and practical preparation of P-chiral phosphine oxides

A practical and cost-effective synthetic method of P-chiral diarylalkyl, aryldialkyl, and triaryl phosphine oxides by using readily available chiral diphenyl-2-pyrrolidinemethanol as the auxiliary is developed. The long-standing racemization issue during solvolysis has been addressed and well controlled by employing a suitable solvent, a low reaction temperature, and an appropriate reaction time.

The conversion of ether bonds to hydroxyl via a base-promoted rearrangement of cyclic phosphine oxides

Biphenyl derived secondary phosphine oxides having 2′-hydroxyl were prepared.

Diastereoselective Synthesis of P-Stereogenic Secondary Phosphine Oxides (SPOs) Bearing a Chiral Substituent by Ring Opening of (+)-Limonene Oxide with Primary Phosphido Nucleophiles

Kinetic separation of the commercially available cis/trans-(+)-limonene oxide mixture by ring opening with primary phosphido nucleophiles LiPHR (R = ferrocenyl, Ph, Cy, t-Bu, Mes* (Mes* = 2,4,6-(t-Bu)₃C₆H₂)), followed by treatment with aqueous NH₄Cl and H₂O₂, gave unreacted cis-(+)-limonene oxide and diastereoenriched mixtures of the secondary phosphine oxides (SPOs) PHR(trans-(+)-Lim-OH)(O), which could be separated by chromatography and/or recrystallization. This one-pot synthesis uses a cheap chiral material and commercially available primary phosphines to control the configuration of the new P-stereogenic SPOs, which are potentially useful as ligands for metal complexes in asymmetric catalysis.

Focusing on the Catal. of the Pd- and Ni-Catalyzed Hirao Reactions

The Hirao reaction involving the phosphinoylation or phosphonation of aryl halides by >P(O)H reagents is a P–C bond forming transformation belonging to the recently very hot topic of cross-couplings. The Pd- or Ni-catalyzed variations take place via the usual cycle including oxidative addition, ligand exchange, and reductive elimination. However, according to the literature, the nature of the transition metal catalysts is not unambiguous. In this feature article, the catalysts described for the Pd(OAc)₂-promoted cases are summarized, and it is concluded that the “(HOY₂P)₂Pd(0)” species (Y = aryl, alkoxy) is the real catalyst. In our model, the excess of the >P(O)H reagent served as the P-ligand. During the less studied Ni(II)-catalyzed instances the “(HOY₂P)(−OY₂P)Ni(II)Cl⁻” form was found to enter the catalytic cycle. The newest conclusions involving the exact structure of the catalysts, and the mechanism for their formation explored by us were supported by our earlier experimental data and theoretical calculations.

Palladium-catalyzed asymmetric hydrophosphorylation of alkynes: facile access to P-stereogenic phosphinates

Despite the importance of P-chiral organophosphorus compounds in asymmetric catalysis, transition metal-catalyzed methods for accessing P-chiral phosphine derivatives are still limited. Herein, a catalytic enantioselective method for the synthesis of P-stereogenic alkenylphosphinates is developed through asymmetric hydrophosphorylation of alkynes. This process is demonstrated for a wide range of racemic phosphinates and leads to diverse P-stereogenic alkenylphosphinates directly.

TfOH-Catalyzed Phosphinylation of 2,3-Allenols into γ-Ketophosphine Oxides

The first facile and efficient acid-catalyzed direct coupling of a wide range of unprotected 2,3-allenols with arylphosphine oxides was developed, offering a general, one-step approach for the synthesis of structurally diverse γ-ketophosphine oxides accompanied by concurrent C-P/C═O bond formation with remarkable functional group tolerance and complete atom-economy under metal- and additive-free conditions. Mechanistic studies showed that this transformation involved a rearrangement and a phospha-Michael reaction.

Preparation of Enantiomerically Enriched P-Stereogenic Dialkyl-Arylphosphine Oxides via Coordination Mediated Optical Resolution

Optical resolution of several dialkyl-arylphosphine oxides was elaborated using the Ca2+ salt of (−)-O,O’-dibenzoyl-(2R,3R)-tartaric acid as the resolving agent. The conditions of crystallization and purification of the enantiomerically enriched phosphine oxides were optimized. Ethyl-phenyl-propylphosphine oxide and butyl-methyl-phenylphosphine oxide were prepared with an enantiomeric excess higher than 93%, whereas, three other dialkyl-arylphosphine oxides were obtained with an enantiomeric excess of 37–85%. It was also found that the sterically demanding alkyl chains hinder the formation of stable diastereomeric complexes, which consequently led to less efficient resolution procedures.

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).

The construction of three C–P bonds of P-stereogenic tertiary phosphines containing (l)-menthyl

A wide range of R_P or S_P-(l)-menthyl-containing SPOs were prepared via the reaction of 1,1′-bi(2-naphthol) derived phosphonite with a metallic reagent with excellent yield and dr.

Experimental study on the microreactor-assisted synthesis of phosphinic chlorides with varying steric hindrance

An efficient and catalyst-free method for the preparation of optically active and racemic mono-substituted t-butyl phosphinic chlorides ^tBuRP(O)Cl under flow conditions was reported.

P-Chiral, N-phosphoryl sulfonamide Brønsted acids with an intramolecular hydrogen bond interaction that modulates organocatalysis

Brønsted acids exemplified by OttoPhosa I (5c) were designed and evaluated in the asymmetric transfer hydrogenation of quinolines. Their catalytic properties are modulated by an intramolecular hydrogen bond that rigidifies their catalytic cavity, accelerates the reaction rate and improves enantioselectivity.

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.

Coordination chemistry and catalysis with secondary phosphine oxides

Review on synthesis, coordination chemistry and catalysis with secondary phosphine oxides.

New Developments on the Hirao Reactions, Especially from "Green" Point of View

BACKGROUND

The Hirao reaction discovered ca. 35 years ago is an important P-C coupling protocol between dialkyl phosphites and aryl halides in the presence of Pd(PPh3)4 as the catalyst and a base to provide aryl phosphonates. Then, the reaction was extended to other Preagents, such as secondary phosphine oxides and H-phosphinates and to other aryl and hetaryl derivatives to afford also phosphinic esters and tertiary phosphine oxides. Instead of the Pd(PPh3)4 catalyst, Pd(OAc)2 and Ni-salts were also applied as catalyst precursors together with a number of mono- and bidentate P-ligands.

OBJECTIVE

In our review, we undertook to summarize the target reaction with a special stress on the developments attained in the last 6 years, hence this paper is an update of our earlier reviews in a similar topic.

CONCLUSIONS

"Greener" syntheses aimed at utilizing phase transfer catalytic and microwave-assisted approaches, even under "P-ligand-free. or even solvent-free conditions are the up-to date versions of the classical Hirao reaction. The mechanism of the reaction is also in the focus these days.