Asymmetric transfer hydrogenation using amino acid derivatives; further studies and a mechanistic proposal

Design of Iridium N-Heterocyclic Carbene Amino Acid Catalysts for Asymmetric Transfer Hydrogenation of Aryl Ketones

A series of chiral complexes of the form Ir(NHC)2(aa)(H)(X) (NHC = N-heterocyclic carbene, aa = chelated amino acid, X = halide) was synthesized by oxidative addition of α-amino acids to iridium(I) bis-NHC compounds and screened for asymmetric transfer hydrogenation of ketones. Following optimization of the reaction conditions, NHC, and amino acid ligands, high enantioselectivity was achieved when employing the Ir(IMe)2(l-Pro)(H)(I) catalyst (IMe = 1,3-dimethylimidazol-2-ylidene), which asymmetrically reduces a range of acetophenone derivatives in up to 95% enantiomeric excess.

Catalytic Nanoassemblies Formed by Short Peptides Promote Highly Enantioselective Transfer Hydrogenation

Self-assembly enables formation of incredibly diverse supramolecular structures with practically important functions from simple and inexpensive building blocks. Here, we show how a semirational, bottom-up approach to create emerging properties can be extended to a design of highly enantioselective catalytic nanoassemblies. The designed peptides comprising as few as two amino acid residues spontaneously self-assemble in the presence of metal ions to form supramolecular, vesicle-like nanoassemblies that promote transfer hydrogenation of ketones in an aqueous phase with excellent conversion rates and enantioselectivities (>90% ee).

Ruthenium amino carboxylate complexes as asymmetric hydrogen transfer catalysts

The synthesis and characterization of optically active amino carboxylate complexes of formula [(η(6)-arene)Ru(Aa)Cl] (arene = C(6)H(6), C(6)Me(6), Aa = amino carboxylate) as well as those of the related trimers [{(η(6)-arene)Ru(Aa)}(3)][BF(4)](3) are reported. Trimerization takes place with chiral self-recognition: only diastereomers equally configured at the metal, R(Ru)R(Ru)R(Ru) or S(Ru)S(Ru)S(Ru), are detected. The crystal structures of the complexes [(η(6)-C(6)H(6))Ru(Pip)Cl] and [{(η(6)-C(6)Me(6))Ru(Pro)}(3)][BF(4)](3) have been determined by X-ray diffraction methods. Both types of complexes catalyse the hydrogen transfer reaction from 2-propanol to ketones with moderate enantioselectivity (up to 68% ee). The enantiodifferentiation achieved can be accounted for by assuming that Noyori's bifunctional mechanism is operating.

Novel improved ruthenium catalysts for the generation of hydrogen from alcohols

The dehydrogenation reaction of alcohols to generate hydrogen at ambient conditions has been studied, it is shown for the first time that ruthenium in the presence of easily available amine ligands constitutes an active catalysts for this reaction: high turnover frequencies (TOF) up to 519 h(-1) (after 2 h) and catalyst stability (>250 h) were achieved for the dehydrogenation of isopropanol with a combination of [RuCl2(p-cymene)]2/TMEDA.

An Environmentally Benign Process for the Hydrogenation of Ketones with Homogeneous Iron Catalysts

Iron complexes generated in situ catalyze homogeneously the transfer hydrogenation of aliphatic and aromatic ketones by utilizing 2-propanol as a hydrogen donor in the presence of base. The influence of different reaction parameters on the catalytic activity is investigated in detail by applying a three-component catalyst system composed of an iron salt, 2,2':6',2''-terpyridine, and PPh3. The scope and limitations of the described catalyst is shown in the reduction of 11 different ketones. In most cases, high conversion and excellent chemoselectivity are obtained. Mechanistic studies indicate a monohydride reaction pathway for the homogeneous iron catalyst.

The “Reverse-Tethered” Ruthenium (II) Catalyst for Asymmetric Transfer Hydrogenation: Further Applications

The attachment of a tethering group from the basic nitrogen atom to the arene ligand of a ruthenium(II) catalyst greatly improves its ability to catalyze asymmetric transfer hydrogenation (ATH) reactions. In this paper, we describe further applications of this versatile system to an extended substrate range.