Chiral platinum and palladium complexes containing functionalized C2-symmetric bisaminoaryl ‘Pincer’ ligands

Chiral Tridentate Ligands in Transition Metal-Catalyzed Asymmetric Hydrogenation

Asymmetric hydrogenation (AH) of double bonds has been one of the most effective methods for the preparation of chiral molecules and for the synthesis of important chiral building blocks. In the past 60 years, noble metals with bidentate ligands have shown marvelous reactivity and enantioselectivity in asymmetric hydrogenation of a series of prochiral substrates. In recent years, developing chiral tridentate ligands has played an increasingly important role in AH. With modular frameworks and a variety of functionalities on the side arms, chiral tridentate ligand complexes enable both reactivities and stereoselectivities. Although great achievements have been made for noble metal catalysts with chiral tridentate ligands since the 1990s, the design of chiral tridentate ligands for earth abundant metal catalysts has still been in high demand. This review summarizes the development of chiral tridentate ligands for homogeneous asymmetric hydrogenation. The philosophy of ligand design and the reaction mechanisms are highlighted and discussed as well.

Chiral palladium pincer complexes for asymmetric catalytic reactions

Palladium pincer complexes, containing a monoanionic terdentate ligand composed of an anionic aryl carbon atom and two mutually compatible donor sites, have aroused considerable interest since their first reports in the late 1970s. The high stability of the Pd pincer complexes and particularly their high modularity make these species ideal candidates for catalysis. Furthermore, the nature of the meridional coordination of the pincer ligands, and along with this their ability to enforce a stereo-specific environment around the Pd center, provide a good opportunity for developing chiral Pd pincer catalysts. Thus, a broad variety of chiral Pd pincer complexes have been prepared by the introduction of various stereochemical centers in the pincer skeletons. These chiral Pd pincer complexes have been successfully applied to many asymmetric catalytic reactions such as hydrophosphination reactions, allylation of aldehydes and imines, Michael and aldol reactions, Suzuki-Miyaura reactions as well as reactions of nitrile compounds with imines. This review focuses on the synthetic methods and the applications of chiral Pd pincer complexes in asymmetric catalysis.

1,3-Bis{[(2,6-dimethylphenyl)sulfanyl]methyl}benzene

The structure of the title compound, C24H26S2, an example of a pincer ligand with an SCS-chelation motif, illustrates the steric effects of the methyl groups in the thiophenyl rings at the 2- and 6-positions, forcing a dissimilar spatial orientation of the thiophenyl rings relative to the central aryl group [dihedral angles = 33.58 (7) and 40.49 (7)°]. In the crystal, weak S...S contacts [3.4009 (7) Å] link the molecules into inversion dimers.

New Insights into Aldol Reactions of Methyl Isocyanoacetate Catalyzed by Heterogenized Homogeneous Catalysts

The Hayashi-Ito aldol reaction of methyl isocyanoacetate (MI) and benzaldehydes, a classic homogeneous Au(I)-catalyzed reaction, was studied with heterogenized homogeneous catalysts. Among dendrimer encapsulated nanoparticles (NPs) of Au, Pd, Rh, or Pt loaded in mesoporous supports and the homogeneous analogues, the Au NPs led to the highest yield and highest diastereoselectivity of products in toluene at room temperature. The Au catalyst was stable and was recycled for at least six runs without substantial deactivation. Moreover, larger pore sizes of the support and the use of a hydrophobic solvent led to a high selectivity for the trans diastereomer of the product. The activation energy is sensitive to neither the size of Au NPs nor the support. A linear Hammett plot was obtained with a positive slope, suggesting an increased electron density on the carbonyl carbon atom in the rate-limiting step. IR studies revealed a strong interaction between MI and the gold catalyst, supporting the proposed mechanism, in which rate-limiting step involves an electrophilic attack of the aldehyde on the enolate formed from the deprotonated MI.

Single step, high yield synthesis of para-hydroxy functionalized POCOP ligands and their Ni(ii) pincer derivatives

A facile and efficient method is provided for the production of phosphinito ligands [C₆H₃-5-OH-1,3-(OPR₂)₂] and their pincer complexes [NiCl{C₆H₂-4-OH-2,6-(OPR₂)₂}].

Organometallic benzylidene anilines: donor–acceptor features in NCN-pincer Pt(ii) complexes with a 4-(E)-[(4-R-phenyl)imino]methyl substituent

Inductively the Pt-moiety in these organometallic benzylidene anilines is a very strong electron-withdrawing group, but mesomerically a very strong electron-donating group.

Coordination behavior of ligand based on NNS and NNO donors with ruthenium(III) complexes and their catalytic and DNA interaction studies

Reactions of 2-acetylpyridine-thiosemicarbazone HL(1), 2-acetylpyridine-4-methyl-thiosemicarbazone HL(2), 2-acetylpyridine-4-phenyl-thiosemicarbazone HL(3) and 2-acetylpyridine-semicarbazone HL(4) with ruthenium(III) precursor complexes were studied and the products were characterized by analytical and spectral (FT-IR, electronic, EPR and EI-MS) methods. The ligands coordinated with the ruthenium(III) ion via pyridine nitrogen, azomethine nitrogen and thiolate sulfur/enolate oxygen. An octahedral geometry has been proposed for all the complexes based on the studies. All the complexes are redox active and display an irreversible and quasireversible metal centered redox processes. Further, the catalytic activity of the new complexes has been investigated for the transfer hydrogenation of ketones in the presence of isopropanol/KOH and the Kumada-Corriu coupling of aryl halides with aryl Grignard reagents. The DNA cleavage efficiency of new complexes has also been tested.

Ruthenium(II) hydrazone Schiff base complexes: synthesis, spectral study and catalytic applications

Ruthenium(II) hydrazone Schiff base complexes of the type [RuCl(CO)(B)(L)] (were B=PPh(3), AsPh(3) or Py; L=hydrazone Schiff base ligands) were synthesized from the reactions of hydrazone Schiff base ligand (obtained from isonicotinoylhydrazide and different hydroxy aldehydes) with [RuHCl(CO)(EPh(3))(2)(B)] (where E=P or As; B=PPh(3), AsPh(3) or Py) in 1:1 molar ratio. All the new complexes have been characterized by analytical and spectral (FT-IR, electronic, (1)H, (13)C and (31)P NMR) data. They have been tentatively assigned an octahedral structure. The synthesized complexes have exhibited catalytic activity for oxidation of benzyl alcohol to benzaldehyde and cyclohexanol to cyclohexanone in the presence of N-methyl morpholine N-oxide (NMO) as co-oxidant. They were also found to catalyze the transfer hydrogenation of aliphatic and aromatic ketones to alcohols in KOH/Isopropanol.

Pincer oxazolines: emerging tools in coordination chemistry and catalysis--where to next?

A brief overview of the coordination chemistry aspects of pincer ligands and their complexes containing at least one oxazoline (i.e., 4,5-dihydro-2-oxazole) unit is presented. This historical perspective is placed into a context of the possible future direction(s) in this particular arena of pincer chemistry. These ideas are compared and contrasted to the overall direction of pincer chemistry since the first such complexes were reported in the 1970s.

Mononuclear ruthenium(III) complexes containing chelating thiosemicarbazones: synthesis, characterization and catalytic property

Mononuclear ruthenium(III) complexes of the type [RuX(EPh(3))(2)(L)] (E=P or As; X=Cl or Br; L=dibasic terdentate dehydroacetic acid thiosemicarbazones) have been synthesized from the reaction of thiosemicarbazone ligands with ruthenium(III) precursors, [RuX(3)(EPh(3))(3)] (where E=P, X=Cl; E=As, X=Cl or Br) and [RuBr(3)(PPh(3))(2)(CH(3)OH)] in benzene. The compositions of the complexes have been established by elemental analysis, magnetic susceptibility measurement, FT-IR, UV-vis and EPR spectral data. These complexes are paramagnetic and show intense d-d and charge transfer transitions in dichloromethane. The complexes show rhombic EPR spectra at LNT which are typical of low-spin distorted octahedral ruthenium(III) species. All the complexes are redox active and display an irreversible metal centered redox processes. Complex [RuCl(PPh(3))(2)(DHA-PTSC)] (5) was used as catalyst for transfer hydrogenation of ketones in the presence of isopropanol/KOH and was found to be the active species.

Ruthenium(II) chalconate complexes: Synthesis, characterization, catalytic, and biological studies

A series of new hexa-coordinated ruthenium(II) carbonyl complexes of the type [RuCl(CO)(EPh(3))(B)(L)] (E = P or As; B = PPh(3), AsPh(3) or Py; L = 2'-hydroxychalcones) have been prepared by reacting [RuHCl(CO)(EPh(3))(2)(B)] (E = P or As; B = PPh(3), AsPh(3) or Py) with 2'-hydroxychalcones in benzene under reflux. The new complexes have been characterized by analytical and spectral (IR, electronic, (1)H, (31)P and (13)C NMR) data. Based on the above data, an octahedral structure has been assigned for all the complexes. The new complexes exhibit catalytic activity for the oxidation of primary and secondary alcohols into their corresponding aldehydes and ketones in the presence of N-methylmorpholine-N-oxide (NMO) as co-oxidant and also found efficient catalyst in the transfer hydrogenation of ketones. The antifungal properties of the complexes have also been examined and compared with standard Bavistin.