Stereoselective Synthesis of Ferrocene-BasedC2-Symmetric Diphosphine Ligands: Application to the Highly Enantioselective Hydrogenation of α-Substituted Cinnamic Acids

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.

Ferrocene-based P-chiral amidophosphinate: stereoselective synthesis and X-ray structural study

Racemic and enantiopure ferrocene-based P-chiral amidophosphinates have been simply and stereoselectively synthesized by ortho-lithiation of rac- or (R)-Ugi's amine and further reaction with amidochlorophenylphosphinate Cl-P(O)(Ph)NEt₂. This is the first example of an asymmetric reaction of ortho-lithiated Ugi's amine with tetracoordinated phosphorus(V) chlorides. The structures of rac- and (R)-Ugi's amine ferrocenyl(phenyl)phosphinic acid N,N-diethylamide have been extensively studied experimentally (NMR, X-ray analysis, electrochemistry). The CV first peak refers to the oxidation of the amine fragment, which is clearly seen when (R)-Ugi's amine ferrocenyl(phenyl)phosphinic acid N,N-diethylamide reacts with anhydrous acid. The addition of two equivalents of CF₃COOH leads to the protonation of nitrogen atoms, and a classical reversible wave of oxidation of Fe(II) to Fe(III) is observed.

Development of C2-Symmetric Chiral Diphosphine Ligands for Highly Enantioselective Hydrogenation Assisted by Ion Pairing

A new series of C₂-symmetrical chiral ferrocene-based diphosphino-ethane ligands termed as f-DPE were developed. Assisted by the ion pairing interaction with the ligand, a wide scope of 2-substituted acrylic acids was hydrogenated to obtain chiral propanoic acids with high yields and enantioselectivities. The well-known anti-inflammatory drugs ibuprofen, naproxen, and flurbiprofen could be synthesized efficiently. In addition, the synthetic utilities of the current method were demonstrated by gram-scale experiments.

Enantioselective Synthesis of Chiral Carboxylic Acids from Alkynes and Formic Acid by Nickel-Catalyzed Cascade Reactions: Facile Synthesis of Profens

We report a stereoselective conversion of terminal alkynes to α-chiral carboxylic acids using a nickel-catalyzed domino hydrocarboxylation-transfer hydrogenation reaction. A simple nickel/BenzP* catalyst displayed high activity in both steps of regioselective hydrocarboxylation of alkynes and subsequent asymmetric transfer hydrogenation. The reaction was successfully applied in enantioselective preparation of three nonsteroidal anti-inflammatory profens (>90 % ees) and the chiral fragment of AZD2716.

Facile access to chiral chromanone-2-carboxylic acids enabled by Rhodium-catalyzed chemo- and enantioselective hydrogenation

Rh-catalyzed highly chemo- and enantioselective hydrogenation of chromone-2-carboxylic acids was first successfully established, providing a wide range of enantiopure chromanone-2-carboxylic acids with excellent results (up to 97% yield and 99%...

Iridium-Catalyzed Chemoelective Asymmetric Hydrogenation of Conjugated Enones with Ferrocene-Based Multidentate Phosphine Ligands

A series of novel and readily prepared ferrocene-based multidentate phosphine ligands (f-PNNO) have been developed and successfully used in iridium-catalyzed enantioselective 1,2-reduction of ,-unsaturated ketones, delivering chiral allylic alcohols in...

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.

Synthesis of Planar Chiral 2-Aryl Aroylferrocenes via Palladium-Catalyzed C-C Bond-Cleavage/Ring-Opening Reaction

A palladium-catalyzed ring-opening reaction of optically active ferrocenyl tertiary alcohols for the construction of planar chiral ketones is reported. The stereochemistry of the hydroxyl group in ferrocenyl alcohols markedly affects reaction: ferrocenyl alcohols with a β-hydroxyl group show better reactivity and chemoselectivity than the corresponding α-hydroxyl analogues. The treatment of α-hydroxyl substrates with trifluoroacetic acid successfully realizes the inversion of the orientation of hydroxyl group to the corresponding β-analogues.

Asymmetric Hydrogenation of β-Aryl Alkylidene Malonate Esters: Installing an Ester Group Significantly Increases the Efficiency

Herein, we report a practical method for efficient asymmetric hydrogenation of β-aryl alkylidene malonates. With a site-specifically tailored chiral spiro iridium catalyst, a series of β-aryl alkylidene malonate esters were hydrogenated to afford chiral malonate esters with good to excellent enantioselectivities (up to 99% ee) and high turnover numbers (up to 19000). The results showed that installing an ester group in α,β-unsaturated carboxylic esters significantly increased the efficiency of their asymmetric hydrogenation reactions.

The core of the matter - arene substitution determines the coordination and catalytic behaviour of tris(1-phosphanyl-1'-ferrocenylene)arene gold(I) complexes

Changing the aromatic core of C3-symmetric tris(ferrocenyl)arene-based tris-phosphanes has profound effects on their coordination behaviour towards gold(i). Depending on the arene (s-triazine, benzene, or trifluorobenzene), four different coordination modes can be distinguished and their preference has been rationalised using computational methods. The corresponding 1 : 1 ligand-to-metal complexes, studied by variable-temperature NMR spectroscopy, revealed fluctional behaviour in solution. Given the presence of up to three or six ferrocenylene spacers per complex, their electrochemistry was investigated. The redox-responsive nature of the complexes can be advantageously exploited in the catalytic ring-closing isomerisation of N-(2-propyn-1-yl)benzamide, where the benzene-based 2 : 3 ligand-to-metal complex has been shown to display multiple activity states depending on the degree of (reversible) oxidation in a preliminary trial.

Recent Developments in Enantioselective Transition Metal Catalysis Featuring Attractive Noncovalent Interactions between Ligand and Substrate

Enantioselective transition metal catalysis is an area very much at the forefront of contemporary synthetic research. The development of processes that enable the efficient synthesis of enantiopure compounds is of unquestionable importance to chemists working within the many diverse fields of the central science. Traditional approaches to solving this challenge have typically relied on leveraging repulsive steric interactions between chiral ligands and substrates in order to raise the energy of one of the diastereomeric transition states over the other. By contrast, this Review examines an alternative tactic in which a set of attractive noncovalent interactions operating between transition metal ligands and substrates are used to control enantioselectivity. Examples where this creative approach has been successfully applied to render fundamental synthetic processes enantioselective are presented and discussed. In many of the cases examined, the ligand scaffold has been carefully designed to accommodate these attractive interactions, while in others, the importance of the critical interactions was only elucidated in subsequent computational and mechanistic studies. Through an exploration and discussion of recent reports encompassing a wide range of reaction classes, we hope to inspire synthetic chemists to continue to develop asymmetric transformations based on this powerful concept.

Noncovalent Interaction-Assisted Ferrocenyl Phosphine Ligands in Asymmetric Catalysis

Noncovalent interactions are ubiquitous in nature and are responsible for the precision control in enzyme catalysis via the cooperation of multiple active sites. Inspired by this principle, noncovalent interaction-assisted transition metal catalysis has emerged recently as a powerful tool and has attracted intense interest. However, it is still highly desirable to develop efficient and operationally convenient ligands along this line with new structural motifs. Based on the specific nature of hydrogen bonding and ion pairing interactions, we developed a series of noncovalent interaction-assisted chiral ferrocenyl phosphine ligands, including Zhaophos, Wudaphos, and miscellaneous SPO-Wudaphos. Due to the assistance of noncovalent interactions, this catalytic mode is capable of achieving transition metal catalyzed asymmetric hydrogenation and other transformations with remarkable improvement of reactivity and selectivity. In some specific challenging cases, this probably represents one of the most productive methods. Moreover, these ligands are easily prepared, air stable, and highly tunable, meeting the requirements of industrial application.In this Account, we give a concise review of recent advances in asymmetric catalysis. By means of hydrogen bonding interactions, Rh- and Ir-Zhaophos complexes exhibited excellent activities and enantioselectivities in asymmetric hydrogenation of a wide range of substrates: C═C bonds of substituted conjugate alkenes with neutral hydrogen bond acceptors, including nitro groups, carbonyl groups (ketones, esters, amides, maleinimides, and anhydrides), ethers, and sulfones; C═N bonds of substituted iminium salts with chloride as an anionic hydrogen bond acceptor, including N-H imines and cyclic imines; N-heteroaromatic compounds with HCl as an additive, including unprotected quinolines, isoquinolines, and indoles; carbocation of substituted oxocarbenium ions. By means of ion pairing interactions, Rh-Wudaphos complexes enabled the catalytic asymmetric hydrogenation of α-substituted unsaturated carboxylic acids, carboxy-directed α,α-disubstituted terminal olefins, and sodium α-arylethenylsulfonates. Rh-SPO-Wudaphos utilized both hydrogen bonding and ion pairing interactions in asymmetric hydrogenation of α-substituted unsaturated carboxylic acids and phosphonic acids. In addition, Zhaophos has achieved highly selective intramolecular reductive amination and inter- and intramolecular asymmetric decarboxylative allylation. Investigations into mechanism implied that noncovalent interactions were involved in the catalytic cycle and played a critical role for both high reactivity and selectivity. Notably, a rare ionic hydrogenation pathway has been proposed in some cases. Furthermore, these catalytic systems have been used in the gram-scale synthesis of natural products and pharmaceuticals.

Cobalt-catalyzed highly enantioselective hydrogenation of α,β-unsaturated carboxylic acids

Asymmetric hydrogenation of α,β-unsaturated acids catalyzed by noble metals has been well established, whereas, the asymmetric hydrogenation with earth-abundant-metal was rarely reported. Here, we describe a cobalt-catalyzed asymmetric hydrogenation of α,β-unsaturated carboxylic acids. By using chiral cobalt catalyst bearing electron-donating diphosphine ligand, high activity (up to 1860 TON) and excellent enantioselectivity (up to >99% ee) are observed. Furthermore, the cobalt-catalyzed asymmetric hydrogenation is successfully applied to a broad spectrum of α,β-unsaturated carboxylic acids, such as various α-aryl and α-alkyl cinnamic acid derivatives, α-oxy-functionalized α,β-unsaturated acids, α-substituted acrylic acids and heterocyclic α,β-unsaturated acids (30 examples). The synthetic utility of the protocol is highlighted by the synthesis of key intermediates for chiral drugs (6 cases). Preliminary mechanistic studies reveal that the carboxy group may be involved in the control of the reactivity and enantioselectivity through an interaction with the metal centre.

A large number of marketed drugs contains a chiral carboxylic acid scaffold. Here, the authors report the asymmetric hydrogenation of α,β-unsaturated carboxylic acids to α-chiral carboxylic acids using a cobalt catalyst bearing an electron-donating chiral diphosphine ligand.

Highly Chemo- and Enantioselective Hydrogenation of 2-Substituted-4-oxo-2-alkenoic Acids

The highly chemo- and enantioselective hydrogenation of (E)-2-substituted-4-oxo-2-alkenoic acids was established for the first time using the Rh/JosiPhos complex, affording a series of chiral α-substituted-γ-keto acids with excellent results (up to 99% yield and >99% ee) and high efficiency (up to 3000 TON). In addition, the importance of this methodology was further demonstrated by a concise and gram-scale synthesis of the anti-inflammatory drug (R)-flobufen.

Unusual Racemization of Tertiary P-Chiral Ferrocenyl Phosphines

Tertiary phosphines are generally known to withstand inversion under moderate conditions. In this work, a remarkable racemization process of three P-chiral ferrocenyl phosphines is reported. Subjected to conventional column chromatography as highly enantioenriched compounds, they greatly experienced racemization when collected at the column outlet within minutes. Initially, attention was drawn to this unusual inversion behavior after observing that the superb enantiomeric excess of these ligands (>95 % ee in all cases) was almost lost in their corresponding ruthenium(II) complexes. Successively excluding possible racemization causes, these P-chiral ferrocenyl phosphines were found to undergo a significant, acid-catalyzed racemization process at room temperature within a few minutes. This process is mainly observed during standard column chromatography by using conventional silica or alumina, but can also be triggered deliberately by addition of certain acids. Therefore, the stereochemical preservation of P-chiral phosphines during their purification may per se not always be guaranteed, since column chromatography is the most frequently used technique for purifying such types of compounds.

A dienamine-mediated deconjugative addition/cyclization cascade of γ,γ-disubstituted enals with carboxylic acid-activated enones: a rapid access to highly functionalized γ-lactones

An aminocatalytic deconjugative addition/cyclization cascade of γ,γ-disubstituted enals with carboxylic acid-activated enones was realized, giving rise to highly functionalized γ-lactones with excellent enantioselectivities.

A practical strategy to access chiral α-aryloxy carboxylic acids through ion-pairing directed asymmetric hydrogenation

A series of optically active α-aryloxy functionalized carboxylic acids were obtained via non-covalent interaction assisted highly efficient asymmetric hydrogenation.

Highly Enantioselective Asymmetric Hydrogenation of Carboxy-Directed α,α-Disubstituted Terminal Olefins via the Ion Pair Noncovalent Interaction

The t-Bu-Wudaphos was successfully applied into Rh-catalyzed asymmetric hydrogenation of α,α-disubstituted terminal olefins bearing a carboxy-directed group with excellent reactivities and enantioselectivities via the ion pair noncovalent interaction (up to >99% conversion, 98% yield, 98% ee) under mild reaction conditions without base. In addition, control experiments were conducted, and the results demonstrated that the ion pair noncovalent interaction between ligand and substrate played an important role in achieving an outstanding performance in this asymmetric hydrogenation.

Rh/SPO-WudaPhos-Catalyzed Asymmetric Hydrogenation of α-Substituted Ethenylphosphonic Acids via Noncovalent Ion-Pair Interaction

Asymmetric hydrogenation of α-substituted ethenylphosphonic acids has been successfully achieved by Rh/ferrocenyl chiral bisphosphorus ligand (SPO-Wudaphos) through noncovalent ion-pair interaction between the substrate and catalyst under mild reaction conditions without base. A series of chiral phosphonic acids were obtained with excellent results (up to 98% ee, >99% conversion, 2000 TON). Moreover, the control experiments showed that the noncovalent ion-pair interaction was critical in this asymmetric hydrogenation.

Iridium-Catalyzed Asymmetric Hydrogenation of Unsaturated Carboxylic Acids

Chiral carboxylic acid moieties are widely found in pharmaceuticals, agrochemicals, flavors, fragrances, and health supplements. Although they can be synthesized straightforwardly by transition-metal-catalyzed enantioselective hydrogenation of unsaturated carboxylic acids, because the existing chiral catalysts have various disadvantages, the development of new chiral catalysts with high activity and enantioselectivity is an important, long-standing challenge. Ruthenium complexes with chiral diphosphine ligands and rhodium complexes with chiral monodentate or bidentate phosphorus ligands have been the predominant catalysts for asymmetric hydrogenation of unsaturated acids. However, the efficiency of these catalysts is highly substrate-dependent, and most of the reported catalysts require a high loading, high hydrogen pressure, or long reaction time for satisfactory results. Our recent studies have revealed that chiral iridium complexes with chiral spiro-phosphine-oxazoline ligands and chiral spiro-phosphine-benzylamine ligands exhibit excellent activity and enantioselectivity in the hydrogenation of α,β-unsaturated carboxylic acids, including α,β-disubstituted acrylic acids, trisubstituted acrylic acids, α-substituted acrylic acids, and heterocyclic α,β-unsaturated acids. On the basis of an understanding of the role of the carboxy group in iridium-catalyzed asymmetric hydrogenation reactions, we developed a carboxy-group-directed strategy for asymmetric hydrogenation of olefins. Using this strategy, we hydrogenated several challenging olefin substrates, such as β,γ-unsaturated carboxylic acids, 1,1-diarylethenes, 1,1-dialkylethenes, and 1-alkyl styrenes in high yield and with excellent enantioselectivity. All these iridium-catalyzed asymmetric hydrogenation reactions feature high turnover numbers (up to 10000) and turnover frequencies (up to 6000 h^-1), excellent enantioselectivities (greater than 95% ee with few exceptions), low hydrogen pressure (<12 atm), and operational simplicity. These features make chiral iridium catalysts superior or comparable to well-established chiral ruthenium and rhodium catalysts for asymmetric hydrogenation of unsaturated carboxylic acids. A number of chiral natural products and pharmaceuticals have been prepared by concise routes involving an iridium-catalyzed asymmetric hydrogenation of an unsaturated carboxylic acid as a key step. As part of a mechanistic study of iridium-catalyzed asymmetric hydrogenation of unsaturated acids, we isolated, for the first time, the migratory insertion intermediate in the iridium-catalyzed asymmetric hydrogenation of olefins, and this result strongly supports the involvement of an Ir(III)/Ir(V) catalytic cycle. The rigid, bulky scaffold of the chiral spiro-P,N-ligands of the catalysts not only prevents them from undergoing deactivating aggregation under the hydrogenation conditions but also is responsible for the efficient chiral induction. The carboxy group of the substrate acts as an anchor to ensure coordination of the substrate to the iridium center of the catalyst during the reaction and makes the hydrogenation proceed smoothly.

Neutral iridium catalysts with chiral phosphine-carboxy ligands for asymmetric hydrogenation of unsaturated carboxylic acids

We developed neutral iridium catalysts with chiral spiro phosphine-carboxy ligands (SpiroCAP) for asymmetric hydrogenation of unsaturated carboxylic acids. Different from the cationic Crabtree-type catalysts, the iridium catalysts with chiral spiro phosphine-carboxy ligands are neutral and do not require the use of a tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BAr_F^-) counterion, which is necessary for stabilizing cationic Crabtree-type catalysts. Another advantage of the neutral iridium catalysts is that they have high stability and have a long lifetime in air. The new iridium catalysts with chiral spiro phosphine-carboxy ligands exhibit unprecedented high enantioselectivity (up to 99.4% ee) in the asymmetric hydrogenations of various unsaturated carboxylic acids, particularly for 3-alkyl-3-methylenepropionic acids, which are challenging substrates for other chiral catalysts.

A new ferrocenyl bisphosphorus ligand for the asymmetric hydrogenation of α-methylene-γ-keto-carboxylic acids

A new chiral ferrocenyl bisphosphorus ligand t-Bu-Wudaphos exhibited excellent performance in the asymmetric hydrogenation of α-methylene-γ-keto-carboxylic acids (up to >99% conversion, >99% ee).

Iridium catalysts with modular axial-unfixed biphenyl phosphine–oxazoline ligands: asymmetric hydrogenation of α,β-unsaturated carboxylic acids

Axial-unfixed biphenyl phosphine–oxazoline ligands were successfully applied to Ir-catalyzed asymmetric hydrogenation of α,β-unsaturated carboxylic acids.

Highly stereoselective synthesis and application of P-chiral ferrocenyl bisphosphorus ligands for asymmetric hydrogenation

New, efficient and highly stereoselective methods were developed for the synthesis/systematic modification of Wudaphos type ligands.

Ferrocenyl chiral bisphosphorus ligands for highly enantioselective asymmetric hydrogenation via noncovalent ion pair interaction

A new class of ferrocenyl chiral bisphosphorus ligand, Wudaphos, was developed, and exhibits excellent ee and activity (ee up to 99%, TON up to 20 000) for the asymmetric hydrogenation of both 2-aryl and 2-alkyl acrylic acids through ion pair noncovalent interaction under base free and mild reaction conditions. Well-known anti-inflammatory drugs such as naproxen and ibuprofen together with the intermediate for the preparation of Roche ester and some bioactive compounds were also efficiently obtained with excellent ee. Control experiments were conducted and revealed that the ion pair noncovalent interaction and chain length played important roles.

Asymmetric Hydrogenation of β-Aryloxy/Alkoxy Cinnamic Nitriles and Esters

A highly efficient and enantioselective hydrogenation of β-aryloxy/alkoxy cinnamic nitriles and esters under mild conditions has been realized by using a rhodium catalyst with a chiral f-spiroPhos ligand. The method provides efficient access to the asymmetric synthesis of a variety of chiral β-oxy-functionalized nitriles and esters with excellent enantioselectivities (up to 99.9% ee) and high turnover numbers (TON of up to 50000). This methodology has also been successfully applied to the concise and practical synthesis of the chiral pharmaceutical nisoxetine.

Highly enantioselective hydrogenation of α-oxy functionalized α,β-unsaturated acids catalyzed by a ChenPhos–Rh complex in CF3CH2OH

A Chenphos–Rh complex is demonstrated to be a highly efficient catalyst for asymmetric hydrogenation of α-oxy functionalized α,β-unsaturated carboxylic acidsviaan ionic interaction between the ligand and the substrate.

Nickel-catalyzed asymmetric transfer hydrogenation of conjugated olefins

A nickel catalyst is used for asymmetric hydrogenation of electron-deficient olefins using formic acid as hydrogen source.