The first catalytic asymmetric addition of diethylzinc to aldehyde promoted by chiral thiourea

A series of C₂-symmetric and asymmetric chiral thiourea derivatives were synthesized from commercial L-phenylalanine.All of the new compounds have been fully characterized by IR,¹H NMR,¹³C NMR,MS spectra and elemental analyses.The chiral thioureas were used as chiral ligands in the catalytic enantioselective ethylation of aldehydes with diethylzinc,the corresponding sec-alcohols were gained with excellent enantioselectivities（up to 87.1%ee） and high yields（up to 76.7%） after the conditions were optimized.     

Pd(II)-Catalyzed Tandem Enantioselective Methylene C(sp3)−H Alkenylation–Aza-Wacker Cyclization to Access β-Stereogenic γ-Lactams

Herein, we describe an unprecedented cascade reaction to β-stereogenic γ-lactams involving Pd(II)-catalyzed enantioselective aliphatic methylene C(sp³)−H alkenylation–aza-Wacker cyclization through syn-aminopalladation. Readily available 3,3′-substituted BINOLs are used as chiral ligands, providing the corresponding γ-lactams with broad scope and high enantioselectivities (up to 98 % ee).     

Copper(II) complexes derived from substituted 2,2′-bis-(4-isopropyl-4-methyl-4,5-dihydro-1H-imidazol-5-one) ligands: Synthesis,structure and catalytic activity

New chiral N,N-bidentate 2,2′-bis-(4-isopropyl-4-methyl-4,5-dihydro-1H-imidazol-5-one) ligands have been prepared and characterised by their ¹H and ¹³C NMR spectra and/or optical rotation. The ligands prepared were then tested for their ability to form complexes with copper(II) salts. It was found that the most stable complex is formed from the 2,2′-bis-(4-isopropyl-1,4-dimethyl-4,5-dihydro-1H-imidazol-5-one) ligand and copper(II) chloride. The structure of this complex was determined by means of quantum-chemical computations at the B3LYP or UB3LYP/6-31G(d,p) level. According to the computations, the geometry of the copper atom most resembles a tetrahedral arrangement, which was also confirmed by means of X-ray structural analysis. It was found that the structure of this copper(II) complex does not allow the copper atom to coordinate to additional ligands; therefore, it is catalytically inactive in the asymmetric Henry reaction.     

Modular bipyridine ligands coupled with Cinchona alkaloids for Cu(II)-catalyzed asymmetric Henry reactions

A new series of bipyridine ligands coupled with Cinchona alkaloids were prepared in relatively high yields in only one step starting from commercially or readily available materials, and have been developed as chiral ligands for copper(II)-catalyzed asymmetric Henry reactions. The catalytic system works well with a wide range of aromatic, aliphatic, and heteroaromatic aldehydes to afford the corresponding nitroalkanols with high enantioselectivity (up to 92%) in good yields (up to 89%). A transition state model was proposed based on ¹H NMR spectroscopy, X-ray structure, and computational calculations.     

Pd(II)‐Catalyzed Tandem Enantioselective Methylene C(sp3)−H Alkenylation–Aza‐Wacker Cyclization to Access β‐Stereogenic γ‐Lactams

Herein, we describe an unprecedented cascade reaction to β‐stereogenic γ‐lactams involving Pd(II)‐catalyzed enantioselective aliphatic methylene C(sp³)?H alkenylation–aza‐Wacker cyclization through syn‐aminopalladation. Readily available 3,3′‐substituted BINOLs are used as chiral ligands, providing the corresponding γ‐lactams with broad scope and high enantioselectivities (up to 98 % ee).     

Novel benzoferrocenyl chiral ligands: Synthesis and evaluation of their suitability for asymmetric catalysis

Four benzoferrocenyl phosphorus chiral ligands were conveniently prepared in good overall yields. These ligands were found to be stable in solid form and in solution. Two of the four ligands were resolved by chiral HPLC. Unlike a reported bis(phosphino-η⁵-indenyl)iron(II) complex, in which the indenyl ligands undergo ring flipping through an η¹-intermediate, these two ligands were found to be configurationally stable in solution and in solid state. The suitability of these ligands for enantioselective catalysis was assessed in studies on allylic alkylation reactions. When the two less sterically hindered ligands were used, excellent chemical yields were obtained, but the other two more sterically hindered ones gave lower yields. When the two enantiopure ligands were used, enantioselectivity of up to 51% ee was observed. These findings suggest that benzoferrocene derivatives may be used as chiral ligands for asymmetric catalysis.     

Enantioselective Diels–Alder reactions with left-handed G-quadruplex DNA-based catalysts

Since the discovery of left-handed G-quadruplex (L-G4) structure formed by natural DNA, there has been a growing interest in its potential functions. This study utilised it to catalyse enantioselective Diels-Alder reactions, considering its different optical rotation compared to an ordinary G4. It was determined that when L-G4 was used with a combination of copper(II) ions, there was a good enantioselectivity (?52% ee) without further addition of ligands. When further consideration was given by adding G4 ligands, G4 was further stabilised, even obtaining a better enantioselectivity (up to ?80% ee). Moreover, when using ligands that have regulatory effects on G4, the ee value can be adjusted. In this work, a minimal left-handed G4 was reported. A follow-up study was also conducted, which recovers that the minimal left-handed G4 remains its catalytic effect and enantioselectivity, but is not so effective as the former case. This indicates that a complete G4 structure is relatively conducive to chiral catalysis.     

Binuclear schiff-base complexes of copper(II) linked by phthaloyl as a bridging group

Binuclear Schiff-base complexes were prepared by bridging an unsymmetrical tetradentate Schiff-base complex of copper(II) with m- or p-phthaloyl. The complexes were characterized by means of elemental analyses, molecular weights, UV, IR and ¹H NMR (for metal-free ligands) spectra. ESR and magnetic susceptibility measurements show that the copper(II)copper(II) interaction is negligibly small.     

Stereoselective aldol reactions of dihydroxyacetone derivatives catalyzed by chiral Zn2+ complexes

The direct aldol reaction between a protected dihydroxyacetone derivative and 4-nitrobenzaldehyde catalyzed by chiral Zn²⁺ complexes of 1-(n-carboxylalkyl)-7-aminoacyl-1,4,7,10-tetraazacyclododecane is reported. New Zn²⁺ complexes containing l-histidine and carboxylalkyl chains that mimic a class II aldolase, carboxypeptidase A and a serine protease were designed and synthesized. Syn-aldol products were mainly formed by an aldol reaction of acetonide-protected dihydroxyacetone with benzaldehydes and other benzaldehydes in N-methylpyrrolidone (NMP)/alcohol (MeOH, EtOH or 2-PrOH) in good yields with a high degree of diastereo- and enantioselectivity (56%～quant., 57～>99% ee). Mechanistic aspect based on ESI-HRMS, elemental analysis and pH titrations of model ligands is also discussed.     

Novel chiral C2-symmetric multidentate aminophosphine ligands for use in catalytic asymmetric reduction of ketones

A series of novel chiral C₂-symmetric multidentate aminophosphine ligands have been successfully synthesized by Schiff-base condensation of bis（o-formylphenyl）phenylphosphane and easily available monoprotected（1R,2R）-diaminocyclohexane.The catalytic properties of these ligands were investigated in Ir-catalyzed asymmetric transfer hydrogenation of various aromatic ketones,giving the corresponding optical active alcohols with up to 98%conversion and good ee under mild reaction conditions.     

Copper(II) complexes containing chiral substituted 2-(4-isopropyl-4-methyl-4,5-dihydro-1H-imidazol-5-one-2-yl)pyridine ligands: Synthesis, X-ray structural studies and asymmetric catalysis

New chiral N,N-bidentate ligands derived from substituted 2-(4-isopropyl-4-methyl-4,5-dihydro-1H-imidazol-5-one-2-yl)pyridines have been prepared and characterised by means of ¹H, ¹³C NMR spectroscopy and optical rotation. Their Cu(II) complexes were characterized by means of elemental analysis, ¹H NMR spectroscopy and MS. By means of X-ray diffraction, molecular geometry of the complex of 2-(1-methyl-4-isopropyl-4-methyl-4,5-dihydro-1H-imidazol-5-one-2-yl)pyridine with copper(II) chloride was determined. The complex exhibits heterochiral dimeric arrangement of two square pyramids with one terminal and one bridge-forming chlorine atoms and two nitrogen atoms in the bases of the pyramids. The tops of these pyramids are formed by the remaining chlorine atoms. The complexes prepared catalyse the Henry reaction with the overall yields of 41-97% and with the maximum enantioselective excess of 19%.     

Novel nitrogen ligands based on imidazole derivatives and their application in asymmetric catalysis

Recently prepared chiral amines have been used in the preparation of novel tridentate ligands based on an imidazole ring with an additional (hetero)ring. The synthesis was carried out by the reaction of chiral amines with suitable aldehydes (2-phenylimidazole-4-carbaldehyde, 2-hydroxybenzaldehyde or pyridine-2-carbaldehyde) under reductive conditions (H₂/Pd or NaBH₄). All ligands prepared showed strong hydrogen bonds in d₆-DMSO solution, which resulted in hindered imidazole tautomerism. The observed hindered tautomerism was studied by ¹H NMR spectroscopy. The structures of the prepared ligands were also confirmed by APCI mass spectroscopy. Both chiral amines and tridentate compounds have been applied as ligands in copper (II)-catalyzed nitroaldol reactions (Henry reaction). Various reaction conditions for the Henry reaction have been studied (influence of temperature, molar ratio, solvent or copper (II) precursors). The compounds prepared with the two imidazole rings showed fast reaction times and a reversal in enantioselectivity compared to other chiral amines.     

A new type of L-Tertiary leucine-derived ligand: Synthesis and application in Cu(II)-catalyzed asymmetric Henry reactions

A new series of Schiff bases derived from amino acids were developed as chiral ligands for Cu(II)-catalyzed asymmetric Henry reactions. The optimum ligand 7d exhibited outstanding catalytic efficiency in the Cu(II)-catalyzed asymmetric Henry additions of four nitroalkanes to different kinds of aldehydes to produce 76 desired adducts in high yields (up to 96%) with excellent enantioselectivities, up to 99% enantiomeric excess (ee).     

Asymmetric synthesis of α-amino acids using polymer-supported Cinchona alkaloid-derived ammonium salts as chiral phase-transfer catalysts

Cinchonidine and cinchonine have been N-alkylated with Merrifield resin and employed as chiral phase-transfer catalysts for the enantioselective alkylation of enolates from N-(diphenylmethylene)glycine esters with activated electrophiles (up to 90% ee). The use of the polymer-supported cinchonidine ammonium salt afforded the corresponding (S)-isomers, whereas the (R)-isomers were obtained using related cinchonine-supported polymers.     

New phosphoramidite and phosphito-N chiral ligands based on 8-substituted quinolines and (S)-binaphthol; applications in the Cu-catalyzed enantioselective conjugate addition of diethylzinc to 2-cyclohexen-1-one

The copper(II)-catalyzed enantioselective conjugate addition of diethylzinc to 2-cyclohexen-1-one, in the presence of phosphoramidite and of phosphito-N chiral ligands, derived from 8-chloroquinoline or 8-hydroxyquinoline and (S)-4-chloro-3,5-dioxa-4-phosphacyclohepta[2,1-a;3,4-a′]binaphthalene, resulted in ee's of 70 and 51%, respectively.     

Design and anti-tumor evaluation of new platinum(II) and copper(II) complexes of nitrogen compounds containing selenium moieties

Three ligands containing selenium were synthesized by refluxing 3-acetylcoumarin (AC), 3-acetylbenzocoumarin (ABC) and acetobenzylsulfonamide (ABS) with selenosemicarbazide. The synthesized ligands were reacted with two metal salts namely; copper(II) nitrate and potassium teterachloroplatinate(II). The obtained copper and platinium complexes were characterized by different spectroscopic techniques including, ¹H NMR, ¹³C NMR, IR, UV–visible, ESR and MS. The biological activity of newly synthesized compounds were evaluated using different testes like in-vitro antimicrobial screening, anticancer, glutathione-S-transferase and Catalase activities. The in-vitro cytotoxicity against human breast cancer cell line (MCF-7), human liver cancer cell line (HepG-2) and human colon cancer cell line (HCT-116) and Human normal melanocytes (HFB 4) was investigated, where some of the tested compounds were equipotent, while the others were more potent compared with 5-flurouracil and cis-platin as reference drugs. The obtanined results showed that the best results were for copper(II) complexes and especially for benzocoumarin ligand.     

Diastereoselective cyclocarbonylation of isopulegol by palladium(II) complexes containing no chiral ligands

The cyclocarbonylation of isopulegol catalyzed by palladium(II) complexes containing no chiral ligands produces the two compounds (1R,5R or 5S,6S,9R)-5,9-dimethyl-2-oxabicyclo[4.4.0]decan-3-one with a diastereoisomeric excess up to 60%. X-Ray diffraction of the 5S stereoisomer, ¹H and ¹³C NMR, and NOE measurements have allowed complete characterization of the two diastereoisomers. These results gave more details about the mechanism of cyclocarbonylation, and particularly the role of the OH group in the substrate.     

Studies on asymmetric synthesis of bicyclomycin precursors. A chemoenzymatic route to chiral 2,5-diketopiperazines and 2-oxa-bicyclo[4.2.2]decane-8,10-diones

A novel asymmetric route to bicyclomycin analogues, 2-oxa-bicyclo[4.2.2]decane-8,10-diones, is described. The key chiral synthons 3-(ω-hydroxyalkyl)-2,5-diketopiperazines 3a-c were obtained via enzymatic kinetic resolution of their respective acetates 2a-c using hydrolases (up to >98% ee, E > 200). The chiral 2,5-diketopiperazines were then transformed into their bicyclic derivatives in a stereospecific manner. Circular dichroism and NMR studies were performed to determine the absolute and relative configuration of the obtained products. The biocatalytic approach gave high stereoselectivities in comparison to the chiral pool synthesis from glutamic acid (58% ee) and thus demonstrated the ability of hydrolases to discriminate a remote stereocenter.     

Iridium and rhodium complexes with the planar chiral thioether ligands in asymmetric hydrogenation of ketones and imines

Rhodium complexes with the planar chiral phosphinoferrocenyl thioether ligands [Rh(P,SR)(diene)X] (R = Me, Bu^t, Ph, Bn, diene is cyclooctadiene (COD) or norbornadiene (NBD), X = Cl, BF₄) catalyze hydrogenation of ketones, imines, and heteroaromatic compounds; in the case of acetophenone, the enantioselectivity reached 60% ee. Similar iridium complexes demonstrate a good activity in the hydrogenation of imines, the maximal enantioselectivity in the case of N-phenyl-N-(1-phenylethylidene)amine was about 40% ee.     

Application of chiral N,N′-dialkyl-1,2-cyclohexanediamine derivatives in asymmetric copper(II)-catalyzed Henry reactions

A series of chiral N,N′-dialkyl-1,2-cyclohexanediamine derivatives were designed, synthesized, and applied as ligands in asymmetric copper(II)-catalyzed Henry reactions. The catalysts based on such ligands and copper(II) acetate were found to promote asymmetric Henry reactions between aromatic/aliphatic aldehydes and nitromethane efficiently, and could provide the corresponding β-nitroalcohols in very good yields and with enantioselectivities of up to 93.6%.