Chloride-Bridged Dinuclear Rhodium(III) Complexes Bearing Chiral Diphosphine Ligands: Catalyst Precursors for Asymmetric Hydrogenation of Simple Olefins

Bisphospholane Josiphos-type Ligands in Rhodium Asymmetric Catalysis

Asymmetric catalysis has become a universal and powerful method for constructing chiral compounds. In rhodium asymmetric catalysis, bisphospholane Josiphos-type ligands and their rhodium complexes are receiving increasing attention. This review provides comprehensive information on the bisphospholane Josiphos-type ligands in rhodium asymmetric catalysis. The scope of the literature covers from 2013 to now. The application of bisphospholane Josiphos-type ligands in rhodium asymmetric catalysis is summarized as follows: (i) asymmetric addition to C(sp2 )-C(sp2 ) bonds, (ii) asymmetric addition to C(sp2 )-C(sp) bonds of allenes, (iii) asymmetric hydrogenation of C(sp2 )-N bonds, C(sp2 )-O bonds and pyridinium salts, and (iv) asymmetric silanization of C-H and O-H bonds.

Two Rhodium(III) Ions Confined in a [18]Porphyrin Frame: 5,10,15,20‐Tetraaryl‐21,23‐Dirhodaporphyrin

Tetraaryl‐21,23‐dirhodaporphyrin and a series of related monorhodaporphyrins have been obtained by tellurium‐to‐rhodium exchange in a reaction of tetraaryl‐21,23‐ditelluraporphyrin with [RhCl(CO)₂]₂. These organometallic metallaporphyrins contain rhodium(III) centers embedded in rhodacyclopentadiene rings, incorporated within the porphyrin frames. The skeletons of 21,23‐dirhodaporphyrin and 21‐rhoda‐23‐telluraporphyrin are strongly deformed in‐plane from the rectangular shape typical for porphyrins, due to rhodium(III) coordination preferences, the large size of the two core atoms, and the porphyrin skeleton constrains. These two metallaporphyrins exhibit fluxional behavior, as studied by ¹H NMR and DFT, involving the in‐plane motion and the switch of the rhodium center(s) between two nitrogen donors. A side product detected in the reaction mixture, 21‐oxa‐23‐rhodaporphyrin, results from tellurium‐to‐oxygen exchange, occurring in parallel to the tellurium‐to‐rhodium exchange. The reaction paths and mechanisms have been analyzed. The title 21,23‐dirhodaporphyrin contains a bridged bimetallic unit, Rh₂Cl₂, in the center of the macrocycle, with two rhodium(III) ions lying approximately in the plane of the porphyrinoid skeleton. The geometry of the implanted Rh₂Cl₂ unit is affected by macrocyclic constrains.

Highly efficient synthesis of 3,4-diarylbutadiene sulfones using Heck–Matsuda reaction

For the first time we describe a general method for the synthesis of previously not synthesized unsymmetrical 3,4-diarylbutadiene sulfones which can be stable convenient precursors for 2,3-diaryl-1,3-butadienes. Our method for arylation of butadiene sulfones via Heck–Matsuda reaction allows to obtain unsymmetrical 3,4-diarylbutadiene sulfones with a variety of alkyl, alkoxy, nitro, ethoxycarbonyl, perfluoroalkyl and halogen substituents (30 examples) in very good yields using readily available reagents and catalysts.

An efficient and facile Pd-catalyzed synthesis of 3,4-diarylbutadiene sulfones with high regioselectivity from aryldiazonium salts and butadiene sulfone was developed.

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-Catalyzed Allyl-Allyl Reductive Coupling of Allylamines or Allylic Alcohols with H2 as Sole Reductant

Catalytic carbon-carbon bond formation building on reductive coupling is a powerful method for the preparation of organic compounds. The identification of environmentally benign reductants is key for establishing an efficient reductive coupling reaction. Herein an efficient strategy enabling H₂ as the sole reductant for the palladium-catalyzed allyl-allyl reductive coupling reaction is described. A wide range of allylamines and allylic alcohols as well as allylic ethers proceed smoothly to deliver the C-C coupling products under 1 atm of H₂. Kinetic studies suggested that the dinuclear palladium species was involved in the catalytic cycle.

Rh-Catalyzed Asymmetric Hydrogenation of α,β- and β,β-Disubstituted Unsaturated Boronate Esters

A highly enantioselective hydrogenation of α,β-unsaturated boronate esters catalyzed by Rh-(S)-DTBM-Segphos complex has been developed. Both (Z)-α,β- and β,β-disubstituted substrates can be successfully hydrogenated to afford chiral boronates with excellent enantioselectivities, up to 98 % ee. Furthermore, the obtained chiral boronate esters, as important versatile synthetic intermediates are successfully transformed to the corresponding chiral alcohols, amines and other important derivatives with maintained enantioselectivities.

Asymmetric Hydrogenation of Aryl Perfluoroalkyl Ketones Catalyzed by Rhodium(III) Monohydride Complexes Bearing Josiphos Ligands

The asymmetric hydrogenation of 2,2,2-trifluoroacetophenones and aryl perfluoroalkyl ketones was developed using a unique, well-defined chloride-bridged dinuclear rhodium(III) complex bearing Josiphos-type diphosphine ligands. These complexes were prepared from [RhCl(cod)]₂ , Josiphos ligands, and hydrochloric acid. As catalyst precursors, they allow for the efficient and enantioselective synthesis (up to 99 % ee) of chiral secondary alcohols with perfluoroalkyl groups. This system does not require an activating base for the hydrogenation of 2,2,2-trifluoroacetophenones. Additionally, the enantioselective C=O hydrogenations of 2-phenyl-3-(haloacetyl)-indoles, a class of privileged structures in medicinal chemistry, is reported for the first time.

Efficient synthesis of 3-sulfolenes from allylic alcohols and 1,3-dienes enabled by sodium metabisulfite as a sulfur dioxide equivalent

We present herein an efficient and practical method for a gram scale synthesis of 3-sulfolenes using sodium metabisulfite as a safe, inexpensive, and easy to handle sulfur dioxide equivalent. Diversely-substituted 3-sulfolenes can be prepared by reacting a variety of 1,3-dienes or allylic alcohols with sodium metabisulfite in aqueous hexafluoroisopropanol (HFIP) or in aqueous methanol in the presence of potassium hydrogen sulfate. Advantageously, the method enables conversion of allylic alcohols directly to 3-sulfolenes, bypassing intermediate 1,3-dienes.

Ir/BiphPHOX-catalyzed asymmetric hydrogenation of 3-substituted 2,5-dihydropyrroles and 2,5-dihydrothiophene 1,1-dioxides

An efficient asymmetric hydrogenation of 3-substituted 2,5-dihydropyrroles and 2,5-dihydrothiophene 1,1-dioxides was developed using an Ir catalyst with an axially flexible chiral phosphine-oxazoline ligand.

Triply Halide-Bridged Dinuclear Iridium(III) Complexes with Chiral Diphosphine Ligands as New Easy-to-Handle Iridium Catalysts for Asymmetric Hydrogenation of Imines and N-Heteroaromatics

Iridium(III) complexes bearing chiral ligands have proved to be active species in asymmetric hydrogenation of C=N bonds, though there are only a few iridium(III) precursors. We prepared triply halide-bridged dinuclear iridium complexes bearing chiral diphosphine ligands by simple treatment of the iridium(I) precursor, chiral diphosphine, and aqueous hydrogen halide. The strong advantage of these dinuclear iridium complexes is that they are air and moisture stable, leading to easy handling in asymmetric synthesis. The dinuclear iridium complexes exhibited high catalytic activity toward asymmetric hydrogenation of imines and N-heteroaromatics. Moreover, we demonstrated the application of triply halide-bridged dinuclear ruthenium(II) and rhodium(III) catalyst precursors for the asymmetric hydrogenation of ketonic substrates and simple olefins, respectively.