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.

Conformationally Fixed Chiral Bisphosphine Ligands by Steric Modulators on the Ligand Backbone: Selective Synthesis of Strained 1,2-Disubstituted Chiral cis-Cyclopropanes.

A new series of C1-symmetric P-chirogenic bisphosphine ligands of the type (R)-5,8-Si-Quinox-tBu3 (Silyl = SiMe3, SiEt3, SiMe2Ph) have been developed. The bulky silyl modulators attached to the ligand backbone fix the phosphine substituents to form rigid chiral environments that can be used for substrate recognition. The ligand showed high performances for a copper(I)-catalyzed asymmetric borylative cyclopropanation of bulky silyl-substituted allylic electrophiles to afford higher disfavored 1,2-cis-silyl-boryl-cyclopropanes than the other possible isomers, trans-cyclopropane and allylboronate (up to 97% yield; 98% ee; cis/trans = >99:1; cyclopropane/allylboronate = >99:1). Detailed computational studies suggested that the highly rigid phosphine conformation, which is virtually undisturbed by the steric interactions with the bulky silyl-substituted allyl electrophiles, is key to the high stereo- and product-selectivities. Furthermore, the detailed computational analysis provided insight into the mechanism of the stereoretention or -inversion of the chiral alkylcopper(I) intermediate in the intramolecular cyclization.

Synthesis and applications of high-performance P-chiral phosphine ligands.

Metal-catalyzed asymmetric synthesis is one of the most important methods for the economical and environmentally benign production of useful optically active compounds. The success of the asymmetric transformations is significantly dependent on the structure and electronic properties of the chiral ligands coordinating to the center metals, and hence the development of highly efficient ligands, especially chiral phosphine ligands, has long been an important research subject in this field. This review article describes the synthesis and applications of P-chiral phosphine ligands possessing chiral centers at the phosphorus atoms. Rationally designed P-chiral phosphine ligands are synthesized by the use of phosphine-boranes as the intermediates. Conformationally rigid and electron-rich P-chiral phosphine ligands exhibit excellent enantioselectivity and high catalytic activity in various transition-metal-catalyzed asymmetric reactions. Recent mechanistic studies of rhodium-catalyzed asymmetric hydrogenation are also described.

Backbone-Modified C2-Symmetrical Chiral Bisphosphine TMS-QuinoxP*: Asymmetric Borylation of Racemic Allyl Electrophiles.

A new C2-symmetrical P-chirogenic bisphosphine ligand with silyl substituents on the ligand backbone, (R,R)-5,8-TMS-QuinoxP*, has been developed. This ligand showed higher reactivity and enantioselectivity for the direct enantioconvergent borylation of cyclic allyl electrophiles than its parent ligand, (R,R)-QuinoxP* (e.g., for a piperidine-type substrate: 95% ee vs 76% ee). The borylative kinetic resolution of linear allyl electrophiles was also achieved using (R,R)-5,8-TMS-QuinoxP* (up to 90% ee, s = 46.4). An investigation into the role of the silyl groups on the ligand backbone using X-ray crystallography and computational studies displayed interlocking structures between the phosphine and silyl moieties of (R,R)-5,8-TMS-QuinoxP*. The results of DFT calculations revealed that the entropy effect thermodynamically destabilizes the dormant dimer species in the catalytic cycle to improve the reactivity. Furthermore, in the direct enantioconvergent case, detailed calculations indicated a pronounced enantioselective recognition of carbon-carbon double bonds, which is virtually unaffected by the chirality at the allylic position, as a key for the borylation from both enantiomers of racemic allyl electrophiles.

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.

Copper(I)-Catalyzed Enantioconvergent Borylation of Racemic Benzyl Chlorides Enabled by Quadrant-by-Quadrant Structure Modification of Chiral Bisphosphine Ligands.

The first copper(I)-catalyzed enantioselective borylation of racemic benzyl chlorides has been realized by a quadrant-by-quadrant structure modulation of QuinoxP*-type bisphosphine ligands. This reaction converts racemic mixtures of secondary benzyl chlorides into the corresponding chiral benzylboronates with high enantioselectivity (up to 92 % ee). The results of mechanistic studies suggest the formation of a benzylic radical intermediate. The results of DFT calculations indicate that the optimal bisphosphine-copper(I) catalyst engages in noncovalent interactions that efficiently recognize the radical intermediate, and leads to high levels of enantioselectivity.

Synthesis and biochemical characterization of quasi-stable trimer models of full-length amyloid ß40 with a toxic conformation.

Here, we report the first synthesis of quasi-stable trimer models of full-length Aß40 with a toxic conformation using a 1,3,5-phenyltris-l-alanyl linker at position 34, 36, or 38. The only trimer to exhibit weak neurotoxicity against SH-SY5Y cells was the one which was linked at position 38. This suggests that such a propeller-type trimer model is not prone to forming oligomers with potent neurotoxicity, which is in contrast with its corresponding dimer model.

Pd(OAc)2-catalyzed asymmetric hydrogenation of sterically hindered N-tosylimines.

Asymmetric hydrogenation of sterically hindered substrates still constitutes a long-standing challenge in the area of asymmetric catalysis. Herein, an efficient palladium acetate (an inexpensive Pd salt with low toxicity) catalyzed asymmetric hydrogenation of sterically hindered N-tosylimines is realized with high catalytic activities (S/C up to 5000) and excellent enantioselectivities (ee up to 99.9%). Quantum chemical calculations suggest that uniformly high enantioselectivities are observed due to the structurally different S- and R-reaction pathways.

Computational design of high-performance ligand for enantioselective Markovnikov hydroboration of aliphatic terminal alkenes.

Finding optimal chiral ligands for transition-metal-catalyzed asymmetric reactions using trial-and-error methods is often time-consuming and costly, even if the details of the reaction mechanism are already known. Although modern computational analyses allow the prediction of the stereoselectivity, there are only very few examples for the attempted design of chiral ligands using a computational approach for the improvement of the stereoselectivity. Herein, we report a systematic method for the design of chiral ligands for the enantioselective Markovnikov hydroboration of aliphatic terminal alkenes based on a computational and experimental evaluation sequence. We developed a three-hindered-quadrant P-chirogenic bisphosphine ligand that was designed in accordance with the design guidelines derived from this method, which allowed the Markovnikov hydroboration to proceed with high enantioselectivity (up to 99% ee).

1,3-Dithianes as Acyl Anion Equivalents in Pd-Catalyzed Asymmetric Allylic Substitution.

A Pd-catalyzed asymmetric allylic substitution with 1,3-dithianes as acyl anion equivalents has been developed in high yields and excellent enantioselectivities. The reaction was performed on a gram scale, and the corresponding alkylated products were conveniently converted into several biologically active products. This work provides an alternative strategy utilizing electrophilic carbonyl compounds as nucleophilic species in a Pd-catalyzed allylic substitution.

Searching for Practically Useful P-Chirogenic Phosphine Ligands.

In this account, the design, synthesis, and application of P-chirogenic phosphine ligands that have been mainly carried out in our laboratory over the last three decades are described. Various enantiopure P-chirogenic phosphine ligands have been efficiently prepared by using phosphine boranes as intermediates. Conformationally rigid and electron-rich P-chirogenic phosphine ligands possessing C2 symmetry as well as a bulky alkyl group and a small group at the phosphorus atoms exhibit excellent enantioselectivities and catalytic efficiency in a variety of transition-metal-catalyzed asymmetric reactions. Enantiopure 2,3-bis(tert-butylmethylphosphino)quinoxaline (QuinoxP*) is an air-stable crystalline solid that shows superior enantioinduction ability in catalytic asymmetric syntheses. Mechanistic studies of Rh-catalyzed asymmetric hydrogenation using structurally simple P-chirogenic phosphine ligands, such as tBu-BisP*, are briefly described.

Catalyst-Free and Regioconvergent Substitution Reactions of Bromothiophenes with a BH3-Substituted Phosphide Anion.

A range of bromothiophenes reacted with lithium boranato(tert-butyl)methylphosphide in the absence of transition-metal catalysts under mild conditions to provide the same 2,5-disubstituted and 2-monosubstituted products regardless of the substitution patterns of the starting bromothiophenes.

ZnCl2 -promoted asymmetric hydrogenation of ß-secondary-amino ketones catalyzed by a P-chiral Rh-bisphosphine complex.

A new catalytic system has been developed for the asymmetric hydrogenation of ß-secondary-amino ketones using a highly efficient P-chiral bisphosphine-rhodium complex in combination with ZnCl2 as the activator of the catalyst. The chiral γ-secondary-amino alcohols were obtained in 90-94 % yields, 90-99 % enantioselectivities, and with high turnover numbers (up to 2000 S/C; S/C=substrate/catalyst ratio). A mechanism for the promoting effect of ZnCl2 on the catalytic system has been proposed on the basis of NMR spectroscopy and HRMS studies. This method was successfully applied to the asymmetric syntheses of three important drugs, (S)-duloxetine, (R)-fluoxetine, and (R)-atomoxetine, in high yields and with excellent enantioselectivities.

P-stereogenic PNP pincer-Pd catalyzed intramolecular hydroamination of amino-1,3-dienes.

A new P-stereogenic PNP pincer-Pd complex was readily prepared from optically pure 2,6-bis[(boranato(tert-butyl)methylphosphino)methyl]pyridine. It was used in the asymmetric intramolecular hydroamination of amino-1,3-dienes, with the desired products being obtained in good yields and with excellent regioselectivities and up to moderate enantioselectivities. The absolute configuration of one of the hydroamination products was determined by X-ray crystallography studies. This simple and efficient procedure can be used for the synthesis of allyl-type chiral pyrrolidine derivatives.

P-Stereogenic PCP pincer-Pd complexes: synthesis and application in asymmetric addition of diarylphosphines to nitroalkenes.

Novel P-stereogenic PCP pincer-Pd complexes were designed and prepared in short steps from optically pure tert-butylmethylphosphine-borane. These optically active Pd-complexes were successfully applied in asymmetric addition of diarylphosphines to nitroalkenes with high yields and good enantioselectivity.

Chemoselective transfer hydrogenation of α,ß-unsaturated ketones catalyzed by pincer-Pd complexes using alcohol as a hydrogen source.

A pincer-Pd complex was utilized in the chemoselective transfer hydrogenation of α,ß-unsaturated ketones using n-BuOH as a hydrogen source and solvent. Good to excellent yields were obtained for various substrates even with reducible groups. Based on deuterium-labeling experiments, the reaction mechanism is proposed to occur via a pincer-Pd-hydride intermediate.

Three-hindered quadrant phosphine ligands with an aromatic ring backbone for the rhodium-catalyzed asymmetric hydrogenation of functionalized alkenes.

The three-hindered quadrant phosphine ligands (R)-1-tert-butylmethylphosphino-2-(di-tert-butylphosphino)benzene ((R)-3H-BenzP*) and (R)-2-tert-butylmethylphosphino-3-(di-tert-butylphosphino)quinoxaline ((R)-3H-QuinoxP*) exhibited good to excellent enantioselectivities in the rhodium-catalyzed asymmetric hydrogenation of selected dehydroamino acid derivatives, enamides, and ethenephosphonates.

Rigid P-chiral phosphine ligands with tert-butylmethylphosphino groups for rhodium-catalyzed asymmetric hydrogenation of functionalized alkenes.

Both enantiomers of 2,3-bis(tert-butylmethylphosphino)quinoxaline (QuinoxP*), 1,2-bis(tert-butylmethylphosphino)benzene (BenzP*), and 1,2-bis(tert-butylmethylphosphino)-4,5-(methylenedioxy)benzene (DioxyBenzP*) were prepared in short steps from enantiopure (S)- and (R)-tert-butylmethylphosphine-boranes as the key intermediates. All of these ligands were crystalline solids and were not readily oxidized on exposure to air. Their rhodium complexes exhibited excellent enantioselectivities and high catalytic activities in the asymmetric hydrogenation of functionalized alkenes, such as dehydroamino acid derivatives and enamides. The practical utility of these catalysts was demonstrated by the efficient preparation of several chiral pharmaceutical ingredients having an amino acid or a secondary amine component. A rhodium complex of the structurally simple ligand BenzP* was used for the mechanistic study of asymmetric hydrogenation. Low-temperature NMR studies together with DFT calculations using methyl α-acetamidocinnamate as the standard model substrate revealed new aspects of the reaction pathways and the enantioselection mechanism.

Enantiopure 1,2-bis(tert-butylmethylphosphino)benzene as a highly efficient ligand in rhodium-catalyzed asymmetric hydrogenation.

An electron-rich P-stereogenic bisphosphine ligand named "BenzP*" was conveniently prepared from o-dibromobenzene and enantiopure tert-butylmethylphosphine-borane. Its rhodium complex exhibited excellent enantioselectivities of up to 99.9% and high catalytic activity of up to 10,000 h(-1) TOF in asymmetric hydrogenations of various functionalized alkenes.

Bicyclic imidazoles for modular synthesis of chiral imidazolium salts.

The preparation of new chiral bicyclic imidazoles 5 and their transformation into imidazolium salts 6 and 7 are reported. The utility of the salts as precursors for chiral N-heterocyclic carbenes was demonstrated by the synthesis of C-N-C pincer Ni-complex 13h, the structure of which was characterized by single-crystal X-ray analysis.