Evolution of phosphorus-thioether ligands for asymmetric catalysis

Application of mixed phosphorus/sulfur ligands based on terpenoids in Pd-catalyzed asymmetric allylic substitution and Rh-catalyzed hydrogenation

A small library of easily prepared diamidophosphite-sulfides based on 1,3-thioether alcohols, primarily of terpenoid nature, was developed. Upon complexation with Pd(II) ions, these hemilabile ligands showed the ability to form both P,S-chelates and complexes with two ligands P-monodentately bonded to the metal. The structures of the ligands and their complexes were determined by 2D NMR spectroscopy and X-ray diffraction. The use of these stereoselectors provided up to 95% ee in the classic Pd-catalyzed asymmetric allylic substitution reactions of (E)-1,3-diphenylallyl acetate with C- and N-nucleophiles and up to 80% ee in the Pd-mediated allylic alkylation of cinnamyl acetate with β-ketoesters. In addition, ee values of up to 90% with quantitative conversion were achieved in the Rh-catalyzed asymmetric hydrogenation of methyl esters of unsaturated acids. The effects of the structural parameters, reaction conditions and ligand-to-metal ratio on the catalytic results are discussed.

TADDOL-based P,S-bidentate phosphoramidite ligands in palladium-catalyzed asymmetric allylic substitution

A series of easy-to-prepare and modular chiral P,S-bidentate phosphoramidites were synthesized. With respect to Pd(II), these ligands showed the ability to form stable P,S-chelate allylic complexes. The structures of the ligands and their complexes were confirmed by 2D NMR spectroscopy and single-crystal X-ray diffraction. These chiral inducers provided up to 99% ee in the Pd-catalyzed asymmetric allylic substitution of (E)-1,3-diphenylallyl acetate with C- and N-nucleophiles and up to 94% ee in the Pd-mediated allylic alkylation of cinnamyl esters with β-ketoesters and 2,5-dimethylpyrrole. Furthermore, up to 92% ee with quantitative conversion and chemo- and regioselectivity was achieved in the rare reaction between 2-(diethoxyphosphoryl)-1-phenylallyl acetate and aniline. The effects of the structural parameters, reaction conditions and ligand-to-metal ratio on the catalytic results are discussed. It was shown that the ligands surpass their analogues with different denticity.

Simple Coumarin-Based Fluorescent Probe for Recognition of Pd(II) and Its Live Cell Imaging

A simple coumarin hydrazine Schiff base bearing a thioether recognition fragment (compound CBBS) has been rationally designed and easily prepared. CBBS exhibited an excellent selectivity for Pd(II) and a low detection limit of 65 nM (S/N = 3). The fluorescence emission intensities of CBBS at 495 nm were linear to Pd(II) concentrations in a wide range from 0 to 80 μM. Moreover, CBBS has been well used in fluorescence imaging of Pd(II) in living A549 cells. CBBS as a simple coordination-based fluorescent probe will inspire the researchers to develop a polymer for selective detection and adsorption of Pd(II).

P-Stereogenic Ir-MaxPHOX: A Step toward Privileged Catalysts for Asymmetric Hydrogenation of Nonchelating Olefins

The Ir-MaxPHOX-type catalysts demonstrated high catalytic performance in the hydrogenation of a wide range of nonchelating olefins with different geometries, substitution patterns, and degrees of functionalization. These air-stable and readily available catalysts have been successfully applied in the asymmetric hydrogenation of di-, tri-, and tetrasubstituted olefins (ee's up to 99%). The combination of theoretical calculations and deuterium labeling experiments led to the uncovering of the factors responsible for the enantioselectivity observed in the reaction, allowing the rationalization of the most suitable substrates for these Ir-catalysts.

P,S-Bidentate Phosphoramidites with (Ra)-BINOL Core in Palladium-Catalyzed Asymmetric Allylic Substitution

Abstract

New P,S-bidentate phosphoramidite ligands, including these having a stereogenic phosphorus atom in the 1,3,2-dioxaphosphepine ring, have been synthesized on the basis of (Ra)-BINOL and its adamantanyl derivatives. With their participation in the Pd-catalyzed asymmetric allylic alkylation of (E)-1,3-diphenylallyl acetate with dimethyl malonate and the amination with pyrrolidine involving these ligands have afforded selectivity up to 84 and 75% ee, respectively.