From tropos to atropos: 5,5′-bridged 2,2′-bis(diphenylphosphino)biphenyls as chiral ligands for highly enantioselective palladium-catalyzed hydrogenation of α-phthalimide ketones

Design and Synthesis of Diphosphine Ligands Based on the Chiral Biindolyl Scaffold and Their Application in Transition-Metal Catalysis

An extremely concise, scalable, and stereoselective synthesis of a privileged chiral skeleton based on 2,2'-biindolyl and commercially available chiral building blocks has been developed. This novel skeleton allows for easy access to a range of bisphosphine ligands (decagram scale, up to 58% total yield, only three steps). The synthetic method is characterized by an efficient central-to-axial chirality transfer strategy. In particular, the superior performance of the ligands has been demonstrated in diverse reactions, including several asymmetric hydrogenations, asymmetric conjugate reductions, and cycloisomerization reactions, indicating a great potential for the application of the newly developed chiral backbones in further modifications and exploration of novel chiral ligands and catalysts.

Rh-Catalyzed Enantioselective Desymmetric Hydrogenation of α-Acetamido-1,3-indanediones Using Ether-Bridged Biphenyl Diphosphine Ligands

Novel axially chiral biphenyl diphosphine ligands Enm-BridgePhos, bearing an ether chain bridge at the 5,5'-position of the biphenyl backbone, have been developed and successfully applied in the Rh-catalyzed enantioselective desymmetric hydrogenation of α-acetamido-1,3-indanediones, providing chiral α-acetamido-β-hydroxybenzocyclic pentones in high yields (up to 97%) and with excellent enantioselectivities (up to 99% ee). The reaction could be carried out on a gram scale, and the corresponding products were used as vital intermediates for the synthesis of analogues of chiral spirobenzylisoquinoline alkaloids. Both the crystal structure analysis and the DFT calculations revealed that the large dihedral angle of the Enm-BridgePhos-Rh complexes is highly related to the excellent enantioselectivities.

Rh-Catalyzed Sequential Asymmetric Hydrogenations of 3-Amino-4-Chromones Via an Unusual Dynamic Kinetic Resolution Process

Rh-catalyzed sequential asymmetric hydrogenations of 3-amino-4-chromones have been achieved for the first time via an unprecedented dynamic kinetic resolution under neutral conditions, providing (S,R)-3-amino-4-chromanols in high yields (up to 98%) with excellent enantio- and diastereoselectivities (up to 99.9% ee and 20:1 dr). The mechanistic studies based on control experiments and density functional theory (DFT) calculations suggest that the dynamic kinetic resolution process for the intermediate enantiomers generated in the first hydrogenation step proceeded via a stereomutation (or called chiral assimilation) pathway from an undesired enantiomer to the desired enantiomer rather than via traditional racemization of the undesired enantiomer. The protocol can be performed on a gram scale with a relatively low catalyst loading and offers a practical and convenient pathway for synthesizing a series of bioactive chromanols and their derivatives.

Rhodium-Catalyzed Asymmetric Hydrogenation of 3-Benzoylaminocoumarins for the Synthesis of Chiral 3-Amino Dihydrocoumarins

An asymmetric hydrogenation of 3-benzoylaminocoumarins was achieved for the first time using our BridgePhos-Rh catalytic system, providing chiral 3-amino dihydrocoumarins in high yields (up to 98 %) and with excellent enantioselectivities (up to 99.7 % ee). The relationship between the enantioselectivities of the hydrogenations and the dihedral angles and the resulting π-π stacking effects of the BridgePhos-Rh complexes, which were determined by X-ray diffraction analysis, are discussed. The corresponding hydrogenated products allow for many transformations, providing several chiral skeletons with important physiological and pharmacological activities.

Palladium/Zinc Co-Catalyzed Asymmetric Hydrogenation of γ-Keto Carboxylic Acids

A palladium-catalyzed asymmetric hydrogenation of levulinic acid has been successful developed by using Zn(OTf)₂ as co-catalyst. The present method not only has provided a strategy in the palladium-catalyzed asymmetric hydrogenation of ketone, but also allowed the preparation of a wide range of chiral γ-valerolactones in good yields with excellent enantioselectivities.

Enantioselective Syntheses of Tricyclic Benzimidazoles via Intramolecular Allylic Aminations with Chiral-Bridged Biphenyl Phosphoramidite Ligands

The first iridium-catalyzed enantioselective intramolecular allylic aminations of benzimidazole-tethered allylic carbonates were developed, providing three classes of tricyclic benzimidazoles bearing a tertiary carbon stereogenic center in high yields and excellent enantioselectivities (up to 99% yield, 99% ee). Wide substrate scope, excellent catalytic efficiency and mild conditions rendered this protocol particularly superior and practical. Impressively, the chiral bridge with a tunable structure was shown to provide a very good adjustment space for the chiral environment. The excellent catalytic performance of the ligands manifested their advantages over the bisphenol-based and BINOL-derived counterparts in these transformations. It also highlighted the potential application value of the chiral-bridged ligands.

New Developments on the Hirao Reactions, Especially from "Green" Point of View

BACKGROUND

The Hirao reaction discovered ca. 35 years ago is an important P-C coupling protocol between dialkyl phosphites and aryl halides in the presence of Pd(PPh3)4 as the catalyst and a base to provide aryl phosphonates. Then, the reaction was extended to other Preagents, such as secondary phosphine oxides and H-phosphinates and to other aryl and hetaryl derivatives to afford also phosphinic esters and tertiary phosphine oxides. Instead of the Pd(PPh3)4 catalyst, Pd(OAc)2 and Ni-salts were also applied as catalyst precursors together with a number of mono- and bidentate P-ligands.

OBJECTIVE

In our review, we undertook to summarize the target reaction with a special stress on the developments attained in the last 6 years, hence this paper is an update of our earlier reviews in a similar topic.

CONCLUSIONS

"Greener" syntheses aimed at utilizing phase transfer catalytic and microwave-assisted approaches, even under "P-ligand-free. or even solvent-free conditions are the up-to date versions of the classical Hirao reaction. The mechanism of the reaction is also in the focus these days.

Reducing Diastereomorphous Bis(phosphane oxide) Atropisomers to One Atropisomerically Pure Diphosphane: A New Ligand and a Novel Ligand-Preparation Design

1,1'-Biphenyl-2,2'-diphosphanes with an achiral bridge spanning C-5 and C-5' form atropisomers that are enantiomers. Accessing them in an atropisomerically pure form requires resolving a racemic mixture thereof or of a bis(phosphane oxide) precursor. 1,1'-Biphenyl-2,2'-diphosphanes with a homochiral bridge spanning C-5 and C-5' form atropisomers that are diastereomers. We synthesized the first compound of this kind 1) atropselectively and 2) under thermodynamic control-seemingly a first-time exploit in diphosphane synthesis. The selectivity-inducing step was a high-temperature reduction of two non-interconverting bis(phosphane oxide) atropisomers (60:40 mixture). It furnished the desired diphosphane atropisomerically pure (and atropconvergently because the yield was 67 %). This diphosphane proved worthwhile in Tsuji-Trost allylations, the Hayashi addition of phenylboronic acid to cyclohexenone, and the asymmetric hydrogenation of methyl acetoacetate (up to 95 % yield and 95 % ee).

Asymmetric synthesis of 4-aryl-1,2,5-thiadiazolidin-3-one 1,1-dioxides via Pd-catalyzed hydrogenation of cyclic ketimines

An efficient access to optically active sulfahydantoins, 4-aryl-1,2,5-thiadiazolidin-3-one 1,1-dioxides, was developed through palladium-catalyzed asymmetric hydrogenation of the corresponding cyclic N-sulfonylketimines with up to 98% ee.

Efficient access to chiral 1,2-amino alcohols via Ir/f-amphox-catalyzed asymmetric hydrogenation of α-amino ketones

We successfully developed the iridium/f-amphox-catalyzed asymmetric hydrogenation of α-amino ketones to prepare chiral 1,2-amino alcohols with excellent results.

Highly enantioselective asymmetric transfer hydrogenation (ATH) of α-phthalimide ketones

A mild catalyst system for the synthesis of chiral amino alcohols via asymmetric transfer hydrogenation (ATH) of α-phthalimide ketones has been developed by using a chiral Ru-TsDPEN complex as the catalyst in DMF/MeOH at 40 °C. The reaction exhibits high reaction activity and excellent enantioselectivity where up to 96% yield and 99% ee of the product were obtained.