Synthesis of novel planar chiral Ag and Rh N-heterocyclic carbene complexes derived from [2.2]paracyclophane and their application in ultrasound assisted asymmetric addition reactions of organoboronic acids to aldehydes

Three novel planar chiral N-heterocyclic carbene silver and rhodium complexes based on [2.2]paracyclophane have been prepared. These could be used as catalysts/precatalysts for the asymmetric 1,2-addition of organoboronic acids to aldehydes. We optimized the reaction conditions and have applied ultrasonic irradiation in the asymmetric arylation for the first time. Under ultrasound irradiation, the combination of planar chiral NHC–Ag complex 5 and RhCl₃ can achieve higher catalytic activities in the asymmetric addition of organoboronic acids to aldehydes.     

Enantioselective reduction of aliphatic ketones using NaBH4 and TarB-NO2, a chiral boronic ester

High enantioselectivities are obtained using a tartaric acid-derived boronic ester (TarB-NO₂) in combination with NaBH₄ for the asymmetric reduction of aliphatic ketones. The resulting alcohols are obtained in enantiomeric excesses ranging from 56% to 94%.     

Asymmetric intramolecular C–H insertion of sulfonyldiazoacetates catalyzed by Rh(II)

The asymmetric C–H insertion of alkyldiazosulfones has been studied. High selectivity was achieved using a combination of a chiral catalyst and a chiral auxiliary (Rh₂(S-pttl)₄ and menthyl ester).     

Diastereo- and enantioselective synthesis of densely functionalized cyclohexanones via double Michael addition of curcumins with nitroalkenes

The asymmetric double Michael additions of curcumins to nitroalkenes to afford highly functionalized cyclohexanones have been carried out for the first time. A combination of a dihydrocinchonine-thiourea organocatalyst and K₂CO₃ was found to be the most effective in obtaining the desired cyclohexanones in good yield, diastereoselectivity and enantioselectivity.     

Studies on Cu-catalyzed asymmetric alkynylation of tetrahydroisoquinoline derivatives

Enantioselective C–C bond formations between the sp³ C–H bond of prochiral CH₂ and terminal alkynes via the cross-dehydrogenative coupling (CDC) reaction were studied. Efficient asymmetric syntheses of alkynyl tetrahydroisoquinoline derivatives were achieved by using a catalytic amount of CuOTf together with PyBox chiral ligand. When dihydroisoquinolinium salts were used as electrophiles, the combination of CuBr/QUINAP provided the best results for asymmetric syntheses of alkynyl tetrahydroisoquinoline derivatives. The factors influencing the enantioselectivity were studied.     

Catalytic Enantioselective Amination of Alcohols by the Use of Borrowing Hydrogen Methodology: Cooperative Catalysis by Iridium and a Chiral Phosphoric Acid

The catalytic asymmetric reduction of ketimines has been explored extensively for the synthesis of chiral amines, with reductants ranging from Hantzsch esters, silanes, and formic acid to H₂ gas. Alternatively, the amination of alcohols by the use of borrowing hydrogen methodology has proven a highly atom economical and green method for the production of amines without an external reductant, as the alcohol substrate serves as the H₂ donor. A catalytic enantioselective variant of this process for the synthesis of chiral amines, however, was not known. We have examined various transition‐metal complexes supported by chiral ligands known for asymmetric hydrogenation reactions, in combination with chiral Brønsted acids, which proved essential for the formation of the imine intermediate and the transfer‐hydrogenation step. Our studies led to an asymmetric amination of alcohols to provide access to a wide range of chiral amines with good to excellent enantioselectivity.     

Highly stable asymmetric supercapacitors based on Ni(OH)<Subscript>2</Subscript> deposited on electro-etched carbon paper

Herein, we introduce the application of nickel hydroxide nanosheets on the electro-etched carbon fiber (ECF) formed via a direct electrodeposition, for fabrication of asymmetric supercapacitor. To confirm the practical applicability of prepared Ni(OH)₂–ECF, an asymmetric device was assembled using Ni(OH)₂–ECF in combination with an activated carbon (AC) electrode. Our results showed a substantial cycling stability (96% capacitance retention after 10000 cycles) and considerable rate capability at large discharge currents (60% capacitance retention at 8 A g^??1) for this asymmetric supercapacitor that may have originated from the good contact between Ni(OH)₂ and ECF. A maximum specific capacitance of 88.1 F g^??1 was achieved for Ni(OH)₂–ECF//AC/CF device and showed considerable rate capability at large discharge currents (60% capacitance retention at 8 A g^??1). The results of this study suggest the Ni(OH)₂–ECF electrode is an excellent material for fabrication of supercapacitor electrodes.     

Asymmetric Intramolecular Hydroalkoxylation of Unactivated Alkenes Catalyzed by Chiral N‐Triflyl Phosphoramide and TiCl4†

By using a combination of a chiral N‐triflyl phosphoramide and TiCl₄ as the catalyst, a new process for asymmetric intramolecular hydroalkoxylation of unactivated alkenes was developed, producing various chiral tetrahydrofuran derivatives in 51%—99% yields with 30%—71% ee's.     

Desilylation‐Activated Propargylic Transformation: Enantioselective Copper‐Catalyzed [3+2] Cycloaddition of Propargylic Esters with β‐Naphthol or Phenol Derivatives

A copper‐catalyzed asymmetric [3+2] cycloaddition of 3‐trimethylsilylpropargylic esters with either β‐naphthols or electron‐rich phenols has been realized and proceeds by a desilylation‐activated process. Under the catalysis of Cu(OAc)₂?H₂O in combination with a structurally optimized ketimine P,N,N‐ligand, a wide range of optically active 1,2‐dihydronaphtho[2,1‐b]furans or 2,3‐dihydrobenzofurans were obtained in good yields and with high enantioselectivities (up to 96 % ee). This represents the first desilylation‐activated catalytic asymmetric propargylic transformation.     

Ir(III) complexes of diamine ligands for asymmetric ketone hydrogenation

The use of a combination of IrCl₃ with a series of ligands derived from the C2-symmetric diamine diphenylethanediamine (DPEN) forms a catalyst capable of the asymmetric hydrogenation of ketones in up to 85% ee.     

Synthesis of chiral mesoporous silica and its potential application to asymmetric separation

Chiral mesoporous silica (CMS) has been successfully synthesized in the presence of basic amino acids; the use of basic amino acids in combination with the chiral anionic surfactant is advantageous for the formation of CMS in terms of uniformity in the twisted morphology. We first demonstrate that thus obtained chiral mesoporous silicas can be used for the enantioselective separation of racemic compounds; the helical rod-shaped CMS is found to be capable of asymmetric separation of racemic N-trifluoroacetylalanine ethyl ester (CF₃CO-Ala-OEt). The left handedness-rich CMS shows asymmetric preferential adsorption of the L isomer and vice versa.     

Asymmetric Desymmetrization via Metal‐Free C−F Bond Activation: Synthesis of 3,5‐Diaryl‐5‐fluoromethyloxazolidin‐2‐ones with Quaternary Carbon Centers

We disclose the first asymmetric activation of a non‐activated aliphatic C?F bond in which a conceptually new desymmetrization of 1,3‐difluorides by silicon‐induced selective C?F bond scission is a key step. The combination of a cinchona alkaloid based chiral ammonium bifluoride catalyst and N,O‐bis(trimethylsilyl)acetoamide (BSA) as the silicon reagent enabled the efficient catalytic cycle of asymmetric C_sp3?F bond cleavage under mild conditions with high enantioselectivities. The ortho effect of the aryl group at the prostereogenic center is remarkable. This concept was applied for the asymmetric synthesis of promising agrochemical compounds, 3,5‐diaryl‐5‐fluoromethyloxazolidin‐2‐ones bearing a quaternary carbon center.     

Asymmetric cyclizations via a sequential Michael addition/Conia-ene reaction by combining multifunctional quaternary phosphonium salt and silver catalysis

A one-pot asymmetric Michael addition/Conia-ene reaction sequence, catalyzed by combination of a dipeptide-derived multifunctional quaternary phosphonium salt and Ag₂CO₃ has been developed, which provides a series of synthetically important chiral methylenecyclopentane derivatives in moderate to excellent yields (up to 97%) and enantioselectivities (up to 93%).     

Enantioselective Zinc‐Catalyzed Hydrosilylation of Ketones using Pybox or Pybim Ligands

The combination of ZnEt₂ and chiral pyridinebisoxazoline (pybox) or pyridinebisimidazoline (pybim) ligands catalyzed the asymmetric hydrosilylation of aryl, alkyl, cyclic, heterocyclic, and aliphatic ketones. Under mild conditions, high yields and good enantioselectivities were achieved. ESI measurements allowed for the characterization of the active catalyst.     

Gold(I)‐Catalyzed Asymmetric Desymmetrization of meso‐Alkynyl Diols and Kinetic Resolution of the Corresponding dl‐Diols: Effects of Celite Filtration and Silver Salts

The asymmetric desymmetrization of meso‐2‐alkynylbenzenediols through the use of a combination of axially chiral diphosphine(AuCl)₂ precatalysts and silver salt co‐catalysts gave optically active isochromene compounds with high enantioselectivities in good yields. The corresponding dl ‐diol isomers underwent efficient kinetic resolution to give the cyclized isochromenes and recovered diols with high enantioselectivities under similar conditions. The high reactivity and selectivity in the desymmetrization of the meso‐diols is independent of the combination of axially chiral diphosphine(AuCl)₂ precatalyst and silver salt co‐catalyst, whereas the corresponding tricarbonylchromium complexes of alkynylbenzenediols were affected by the combination of the diphosphine(AuCl)₂ and silver salt. The reactivity was largely dependent on the nature of the gold(I) species.     

Enantioselective cyanosilylation of aldehydes catalyzed by Mn(salen) complex/triphenyl phosphine oxide

The activation of chiral Mn(salen) complexes with Ph₃PO has been found to provide a good strategy for the asymmetric cyanosilylation of aldehydes. Aromatic aldehydes have been converted into the corresponding cyanohydrin trimethylsilylether in yields up to 95% and ee up to 67% using 0.25 mol% chiral Mn(salen) complex in combination with 10 mol% of achiral Ph₃PO as additive. Copyright © 2007 John Wiley & Sons, Ltd.     

ZnCl2‐Promoted Asymmetric Hydrogenation of β‐Secondary‐Amino Ketones Catalyzed by a P‐Chiral Rh–Bisphosphine Complex

A new catalytic system has been developed for the asymmetric hydrogenation of β‐secondary‐amino ketones using a highly efficient P‐chiral bisphosphine–rhodium complex in combination with ZnCl₂ as the activator of the catalyst. The chiral γ‐secondary‐amino alcohols were obtained in 90–94 % yields, 90–99 % enantioselectivities, and with high turnover numbers (up to 2000 S/C; S/C=substrate/catalyst ratio). A mechanism for the promoting effect of ZnCl₂ on the catalytic system has been proposed on the basis of NMR spectroscopy and HRMS studies. This method was successfully applied to the asymmetric syntheses of three important drugs, (S)‐duloxetine, (R)‐fluoxetine, and (R)‐atomoxetine, in high yields and with excellent enantioselectivities.     

Asymmetric hydrosilylation,transfer hydrogenation and hydrogenation of ketones catalyzed by iridium complexes

Iridium-based asymmetric reduction of ketones to chiral enantiomerically enriched alcohols has recently attracted attention by a number of research groups and interest in this area is growing. This review presents the different catalytic systems based on iridium complexes that have been used in asymmetric hydrosilylation, in asymmetric transfer hydrogenation (ATH) with alcohols or formic acid derivatives as reducing agents, and in asymmetric hydrogenation (H₂ as reducing agent). A large variety of chiral ligands of various denticities and bearing various combination of coordinating atoms (N, P, S, O, C, …) have been used and will be presented. The last part critically reviews the mechanistic understanding of all the above transformations with specific reference to iridium catalysts.     

Ruthenium‐Catalyzed Tandem‐Isomerization/Asymmetric Transfer Hydrogenation of Allylic Alcohols

A one‐pot procedure for the direct conversion of racemic allylic alcohols to enantiomerically enriched saturated alcohols is presented. The tandem‐isomerization/asymmetric transfer hydrogenation process is efficiently catalyzed by [{Ru(p‐cymene)Cl₂}₂] in combination with the α‐amino acid hydroxyamide ligand 1 , and performed under mild conditions in a mixture of ethanol and THF. The saturated alcohol products are isolated in good to excellent chemical yields and in enantiomeric excess up to 93 %.     

Naphthalene‐1,7‐diol

The asymmetric unit of the title compound, C₁₀H₈O₂, contains two planar symmetry‐independent mol­ecules linked by an O—H⋯O hydrogen bond. In the crystal structure, mol­ecules are linked into infinite chains of rings, formed by a combination of O—H⋯O and C—H⋯O hydrogen bonds, and additionally reinforced by π–π stacking inter­actions. Adjacent chains are connected by weak C—H⋯π inter­actions.