Publications by authors named "Tsuneo Imamoto"

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.

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A new series of -symmetric P-chirogenic bisphosphine ligands of the type ()-5,8--Quinox-Bu (Silyl = SiMe, SiEt, SiMePh) 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--silyl-boryl-cyclopropanes than the other possible isomers, -cyclopropane and allylboronate (up to 97% yield; 98% ee; / = >99:1; cyclopropane/allylboronate = >99:1).

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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.

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Article Synopsis
  • - A new ligand called (,)-5,8-TMS-QuinoxP* was created, featuring silyl groups and showing improved reactivity and enantioselectivity compared to its predecessor, achieving 95% enantiomeric excess (ee) in borylation reactions.
  • - The ligand was effectively used in the borylative kinetic resolution of linear allyl electrophiles, reaching up to 90% ee, indicating strong performance in asymmetric synthesis.
  • - X-ray crystallography and computational studies illustrated how the silyl groups enhance the ligand structure and affect reactivity, with detailed calculations showing that the ligand promotes enantioselective recognition of double bonds, largely independent of the chiral position
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A bulky three-hindered quadrant bisphosphine ligand, di-1-adamantylphosphino(-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.

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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.

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Article Synopsis
  • Researchers have synthesized quasi-stable trimer models of the Aβ40 protein with potentially toxic conformations using a specific linker at various positions.
  • Out of the synthesized trimers, only the one linked at position 38 showed weak neurotoxicity against SH-SY5Y cells.
  • This finding indicates that the trimer model may be less likely to form toxic oligomers compared to the corresponding dimer model.
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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%).

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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.

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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.

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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 C 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.

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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.

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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.

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A new catalytic system has been developed for the asymmetric hydrogenation of β-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.

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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.

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A pincer-Pd complex was utilized in the chemoselective transfer hydrogenation of α,β-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.

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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.

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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.

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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.

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Article Synopsis
  • The study focuses on creating new chiral bicyclic imidazoles and converting them into imidazolium salts.
  • These imidazolium salts serve as precursors for chiral N-heterocyclic carbenes.
  • The researchers demonstrated the application of these carbenes by synthesizing a C-N-C pincer Ni-complex, which was characterized using single-crystal X-ray analysis.
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This article describes recent developments in the understanding of the mechanism of enantioselection in one of the most efficient artificial catalytic reactions--Rh-catalyzed asymmetric hydrogenation. Numerous experimental and computational data are consistent with the idea that the enantioselection takes place through the reversible coordination of the double bond in octahedral dihydride complexes, which can happen only if the resulting chelate cycle is formed in the less hindered quadrant. In its main features, this mechanism of generating chirality resembles the recently uncovered three-stage recognition process occurring in enzymatic reactions.

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Article Synopsis
  • A new screening technique for asymmetric catalysts utilizes a product from one reaction as a catalyst for another, enhancing efficiency.
  • The method involves metal-containing, optically-active products that can act as chiral catalysts.
  • This approach significantly speeds up the preparation and testing of catalysts, reducing the overall time needed for the screening process.
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Optically active dinuclear palladium complexes containing a Pd-Pd bond were prepared by using (R,R)-bis(tert-butylmethylphosphino)methane ((R,R)-t-Bu-MiniPHOS). The dinuclear palladium complexes coupled with silver triflate exhibited good to excellent enantioselectivities up to 99% in palladium-catalyzed alkylative ring-opening reactions of azabenzonorbornadienes.

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New synthetic routes to substituted 3-hydroxypyridines 6 are presented. Ring-closing olefin metathesis (RCM)/elimination and RCM/oxidation/deprotection of nitrogen-containing dienes 4 are the key processes of the routes. An application of RCM/oxidation/deprotection to the synthesis of 3-aminopyridine 13f is also described.

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