Advances in Transition Metal-Catalyzed Asymmetric Hydrogenation of Heteroaromatic Compounds

Enantioselective synthesis of 2-oxazolidinones by ruthenium(ii)-NHC-catalysed asymmetric hydrogenation of 2-oxazolones

A highly enantioselective synthesis of optically active 4-substituted 2-oxazolidinones was described via ruthenium(ii)–NHC-catalysed asymmetric hydrogenation of 2-oxazolones.

An efficient synthesis of optically active 4-substituted 2-oxazolidinones by the ruthenium(ii)–NHC-catalysed asymmetric hydrogenation of 2-oxazolones was developed. Excellent enantioselectivities (up to 96% ee) and yields (up to 99%) were obtained for a variety of substrates, bearing a range of functional groups and useful motifs. The hydrogenation reaction was successfully scaled up to gram scale using low catalyst loading. Moreover, the utility of this methodology was demonstrated by the transformation of the enantioenriched product into the corresponding chiral β-amino alcohol, a bisoxazoline ligand, and the formal synthesis of (–)-aurantioclavine.

Recent advances in rhodium-catalyzed asymmetric synthesis of heterocycles

Heterocycles are crucial structural motifs that are ubiquitously present in biologically active natural products and pharmaceutically important compounds. Over the past few decades, great attention has been paid to develop efficient methodologies for the construction of diverse enantioenriched heterocyclic frameworks. This review focuses on the recent impressive progress and advances in the asymmetric synthesis of heterocycles under rhodium catalysis.

Enantioselective Hydrogenation of Imidazo[1,2-a]pyridines

The enantioselective synthesis of tetrahydroimidazo[1,2-a]pyridines by direct hydrogenation was achieved using a ruthenium/N-heterocyclic carbene (NHC) catalyst. The reaction forgoes the need for protecting or activating groups, proceeds with complete regioselectivity, good to excellent yields, enantiomeric ratios of up to 98:2, and tolerates a broad range of functional groups. 5,6,7,8-Tetrahydroimidazo[1,2-a]pyridines, which are found in numerous bioactive molecules, were directly obtained by this method, and its applicability was demonstrated by the (formal) synthesis of several functional molecules.

Facile synthesis of chiral indolines through asymmetric hydrogenation of in situ generated indoles

A concise synthesis of chiral indolines has been developed through intramolecular condensation, deprotection and palladium-catalyzed asymmetric hydrogenation in a one-pot process with up to 96% ee. A strong Brønsted acid played an important role in both the formation of indoles and asymmetric hydrogenation process.

Iridium-catalyzed asymmetric hydrogenation of cyclic iminium salts

An iridium-catalyzed asymmetric hydrogenation of cyclic iminium salts has been developed, affording products with up to 96% ee.

Ru-Catalyzed asymmetric transfer hydrogenation of substituted dibenzo[b,f][1,4]oxazepines in water

A phosphine free, additive free catalytic ATH protocol of dibenzo[b,f][1,4]oxazepine compounds using an (R,R)-Ru-Ts-DPEN complex was developed for the first time using HCOOH–HCOONa as a green hydrogen source.

B2(OH)4-mediated one-pot synthesis of tetrahydroquinoxalines from 2-amino(nitro)anilines and 1,2-dicarbonyl compounds in water

A practical one-pot synthesis of tetrahydroquinoxalines from readily available 2-amino(nitro)anilines and 1,2-dicarbonyl compounds mediated by diboronic acid with water as both a solvent and a hydrogen donor under metal-free conditions has been discovered.

Regio- and Enantioselective Cobalt-Catalyzed Sequential Hydrosilylation/Hydrogenation of Terminal Alkynes

A highly regio- and enantioselective cobalt-catalyzed sequential hydrosilylation/hydrogenation of alkynes was developed to afford chiral silanes. This one-pot method is operationally simple and atom economic. It makes use of relatively simple and readily available starting materials, namely alkynes, silanes, and hydrogen gas, to construct more valuable chiral silanes. Primary mechanistic studies demonstrated that highly regioselective hydrosilylation of alkynes with silanes occurred as a first step, and the subsequent cobalt-catalyzed asymmetric hydrogenation of the resulting vinylsilanes showed good enantioselectivity.

Highly Enantioselective Synthesis of Indolines: Asymmetric Hydrogenation at Ambient Temperature and Pressure with Cationic Ruthenium Diamine Catalysts

A highly enantioselective synthesis of indolines by asymmetric hydrogenation of 1H-indoles and 3H-indoles at ambient temperature and pressure, catalyzed by chiral phosphine-free cationic ruthenium complexes, has been developed. Excellent enantio- and diastereoselectivities (up to >99 % ee, >20:1 d.r.) were obtained for a wide range of indole derivatives, including unprotected 2-substituted and 2,3-disubstituted 1H-indoles, as well as 2-alkyl- and 2-aryl-substituted 3H-indoles.

Synthesis of Chiral Piperazines via Hydrogenation of Pyrazines Activated by Alkyl Halides

A facile method has been developed for the synthesis of chiral piperazines through Ir-catalyzed hydrogenation of pyrazines activated by alkyl halides, giving a wide range of chiral piperazines including 3-substituted as well as 2,3- and 3,5-disubstituted ones with up to 96% ee. The high enantioselectivity, easy scalability, and concise drug synthesis demonstrate the practical utility.

Iridium-Catalyzed Intramolecular Asymmetric Allylic Dearomatization Reaction of Pyridines, Pyrazines, Quinolines, and Isoquinolines

The first Ir-catalyzed intramolecular asymmetric allylic dearomatization reaction of pyridines, pyrazines, quinolines, and isoquinolines has been developed. Enabled by in situ formed chiral Ir-catalyst, the dearomatized products were isolated in high levels of yield (up to 99% yield) and enantioselectivity (up to 99% ee). It is worth noting that the Me-THQphos ligand is much more efficient than other tested ligands for the dearomatization of pyrazines and certain quinolines. Mechanistic studies of the dearomatization reaction were carried out, and the results suggest the feasibility of an alternative process which features the formation of a quinolinium as the key intermediate. The mechanistic findings render this reaction a yet unknown type in the chemistry of Reissert-type reactions. In addition, the utility of this method was showcased by a large-scale reaction and formal synthesis of (+)-gephyrotoxin.

Pd-catalyzed asymmetric hydrogenation of fluorinated aromatic pyrazol-5-ols via capture of active tautomers

An efficient palladium-catalyzed asymmetric hydrogenation of fluorinated aromatic pyrazol-5-ols has been developed via capture of the active tautomers. A wide variety of 2,5-disubstituted and 2,4,5-trisubstituted pyrazolidinones have been synthesized with up to 96% and 95% ee, respectively. The hydrogenation pathway includes Brønsted acid promoted tautomerization of pyrazol-5-ols and Pd-catalyzed asymmetric hydrogenation of the active tautomer.

Reactive toughening of polyvinyl alcohol hydrogel and its wastewater treatment performance by immobilization of microorganisms

Toughened PVA hydrogel beads were prepared by co-crosslinking with glycerol, resulting in the formation of a uniform and dense network gel structure, and the tensile property and hydraulic impact resistance were efficiently improved.

Asymmetric hydrogenation of 3-substituted 2H-1,4-benzoxazines with chiral cationic Ru-MsDPEN catalysts: a remarkable counteranion effect

The achiral counteranions of chiral cationic Ru-diamine complexes regulated both the enantioselectivity and regioselectivity in the asymmetric hydrogenation of 3-substituted-2H-1,4-benzoxazines.