A versatile catalyst system for enantioselective synthesis of 2-substituted 1,4-benzodioxanes

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.

Azole-Directed Cobalt-Catalyzed Asymmetric Hydrogenation of Alkenes

The azole-directed cobalt-catalyzed asymmetric hydrogenation of alkenes has been developed with high efficiency. With this approach, chiral pyrazole compounds were obtained in quantitative yields and excellent enantioselectivities (up to 99 % ee) under mild conditions, and the hydrogenation was conducted on a gram scale with up to 2000 TON. Several useful applications were demonstrated including the convenient introduction of β-chirality to a drug intermediate containing an azole ring.

Asymmetric inverse-electron-demand 1,3-dipolar cycloadditions of cyclopentadienones and thiophene-1,1-dioxide with C,N-cyclic azomethine imines

The normal 1,3-dipolar cycloaddition between the carbonates of 4-hydroxy-2-cyclopentenones and C,N-cyclic azomethine imines can be switched to an inverse-electron-demand version under Pd(0) catalysis, by in situ generation of HOMO-raised η²-Pd(0)-cyclopentadienone complexes. An array of fused heterocyclic architectures are constructed with high levels of diastereo and enantioselectivity, and diastereodivergent synthesis is well realised by tuning the bifunctional phosphine ligands. In addition, similar reaction with in situ formed thiophene-1,1-dioxide is compatible by using a chiral bisphosphine ligand, and the fused cyclic sulfone frameworks are afforded with high stereoselectivity.

Chiral Indolizidine Synthesis through the Ir-Catalyzed Asymmetric Hydrogenation of Cyclic Pyridinium Salts

The Ir-catalyzed asymmetric hydrogenation of cyclic pyridinium salts is presented as a new strategy for the convenient and efficient synthesis of chiral indolizidines. The asymmetric hydrogenation of cyclic pyridinium salts derived from 2-(2-acylphenyl)pyridines proceeded smoothly in the presence of [Ir(cod)Cl]₂ and (R)-DM-SegPhos to provide the desired chiral 7,8-benzoindolizidines 6 in high to excellent yields with moderate enantioselectivity (up to 86:14 er) and excellent diastereoselectivity (>20:1 dr). The enantiomeric purity of 6j was increased to 92:8 through recrystallization.

Nitro and Other Electron Withdrawing Group Activated Ruthenium Catalysts for Olefin Metathesis Reactions

Advanced applications of the Nobel Prize winning olefin metathesis reaction require user-friendly and highly universal catalysts. From many successful metathesis catalysts, which belong to the two distinct classes of Schrock and Grubbs-type catalysts, the subclass of chelating-benzylidene ruthenium complexes (so-called Hoveyda-Grubbs catalysts) additionally activated by electron-withdrawing groups (EWGs) provides a highly tunable platform. In the Review, the origin of the EWG-activation concept and selected applications of the resulting catalysts in target-oriented synthesis, medicinal chemistry, as well as in the preparation of fine-chemicals and in materials chemistry is discussed. Based on the examples, some suggestions for end-users regarding minimization of catalyst loading, selectivity control, and general optimization of the olefin metathesis reaction are provided.

Copper-Catalyzed Asymmetric Hydroamination of Styrenes with piv ZPhos as Ligand

A copper-catalyzed hydroamination of styrenes using piv ZPhos as ligand is reported. Enantioselectivities up to 94% are achieved under optimized conditions with aryl and heteroaryl styrenes. A variety of electrophilic O-benzoylhydroxylamines are well tolerated.

Exploration of chiral diastereomeric spiroketal (SPIROL)-based phosphinite ligands in asymmetric hydrogenation of heterocycles

New and readily available chiral SPIROL-based diphosphinite ligands (SPIRAPO) have been prepared and employed for iridium-catalyzed asymmetric hydrogenations of quinolines, quinoxalines and 2H-1,4-bezoxazin-2-ones. While the structurally similar (R,R,R)-SPIRAPO and (R)-SPINOL-based phosphinites were not the best ligands for these transformations, the (S,R,R)-diastereomer of SPIRAPO was found to be highly effective ligand for the reduction of 20 different heterocyclic systems with loadings as low as S/C = 10 000. This dearomatizative hydrogenation provided direct access to optically active tetrahydroquinolines in high enantioselectivities (up to 94% ee) and excellent yields (up to 99%), and was used to generate 1.75 g of natural alkaloid (-)-(R)-angustureine. This protocol was subsequently extended to achieve asymmetric hydrogenation of quinoxalines and 2H-1,4-benzoxazin-2-ones in good to excellent enantioselectivities.

Cu-Catalyzed Asymmetric Aminoboration of E-Vinylarenes with pivZPhos as the Ligand

A Cu-catalyzed enantioselective aminoboration of E-vinylarenes with ^pivZPhos as a ligand is reported. Enantioenriched aminoborates are prepared with excellent regio- and enantioselectivities up to >99:1 er under the optimized conditions. The utility of the current method was further established by rapid conversion of an adduct to a chiral benzo[f][1,4]oxazepine. A model for the stereochemistry of the asymmetric aminoboration process, which agrees with the experimental outcomes, was generated by computational analysis of the systems.