Catalytic Asymmetric Heterogeneous Aziridination of Styrene Using CuHY/bis(oxazoline): Comments on the Factors Controlling Enantioselectivity

The copper-catalyzed aziridination of styrene is described using both heterogeneous, copper-exchanged zeolite HY, and homogeneous, copper (II) triflate catalysts using both [N-(p-tolylsulfonyl)imino]phenyliodinane (PhI=NTs) and [N-(p-nosylsulfonyl)imino]phenyliodinane (PhI=NNs) as nitrene donors. The key differences observed for the two catalysts when modified by chiral bis(oxazoline) ligands are discussed in detail. In particular, the heterogeneously catalyzed asymmetric reaction can give much higher enantioselection than the comparable homogeneously catalyzed reaction. The structure of the bis(oxazoline) ligand is the critical factor, and bis(oxazoline) ligands that are ineffective with the homogeneous catalysts are highly effective for the Cu²⁺ cation constrained within the zeolite micropores. The consequences of this observation for the design of chiral ligands for asymmetric heterogeneous catalysis are discussed. The effect of the degree of styrene conversion on the enantioselectivity is described in detail using PhI=NNs as a nitrene donor. The reaction shows a significant enhancement in ee with conversion at 25°C, and the possible origin of this effect is discussed.     

Heterogeneous Catalytic Sulfimidation using Immobilized Cu(acac)2

The heterogeneous sulfimidation of various sulfides by microencapsulated copper(II) acetylacetonate, [MC‐Cu(acac)₂], and Cu(acac)₂ immobilized in ionic liquids using [N‐(p‐tolylsulfonyl)imino]phenyliodinane (PhINTs) as nitrene donor has been developed to afford the corresponding sulfimides in good to excellent yields. In the presence of a chiral bis(oxazoline) as ligand, asymmetric induction occurs to afford the chiral sulfimides (up to 50% ee). The ionic liquid containing the immobilized bis(oxazoline)‐copper catalyst can be reused for several cycles with consistent activity and enantioselectivity.     

Pyrrole Functionalization by Copper-Catalyzed Nitrene Transfer Reactions

The catalytic functionalization of pyrroles by incorporation of a nitrene group is reported. The Cα-H bond of 1H-pyrrole is amidated upon the formal insertion of the NTs (Ts=p-toluenesulfonyl) group catalyzed by Tp^Br3Cu(NCMe) (Tp^Br3=hydrotris(3,4,5-tribromo-pyrazolyl)borate). N-substituted pyrroles also verify the same transformation. The mechanism proposal is similar to that previously described for benzene amidation with the same catalyst and PhI=NTs, which takes place through aziridine formation, ring opening and 1,2-hydrogen shift. A cascade reaction involving the coupling of 2,5-dimethylfuran, 1,2,3-trimethyl-pyrrole and a nitrene NTs group is also described, leading to a 1,2-dihydropyridine-imine compound.     

Heterogeneous Enantioselective Synthesis of a Dihydropyran Using Cu-Exchanged Microporous and Mesoporous Materials Modified by Bis(oxazoline)

The formation of dihydropyran from the Diels–Alder reaction between E-ethyl-2-oxo-3-pentenonate and vinyl ethyl ether is investigated using copper (II) bis(oxazoline) as catalyst. The homogeneously and heterogeneously catalyzed reactions are contrasted. Immobilization using mesoporous materials (Cu-MCM-41, Cu-AlSBA-15, Cu-MSU-2) and zeolite Y is found to produce an effective heterogeneous catalyst. Although the level of enantioselection is not high in this initial study, the CuH-zeolite Y/bis(oxazoline) catalyst gives the highest ee (41% ee), which is significantly higher than that observed for the Cu(OTf)₂ homogeneous catalyst (20% ee) under comparable conditions. In addition, with the heterogeneously catalyzed reaction, the enantioselection changes from the initial 2R,4S product to the 2S,4R diastereoisomer. This behavior is not observed with the homogeneously catalyzed reaction, which always yields the 2R,4S product. These results are discussed in terms of the confinement of the catalyst complex within the pores of the heterogeneous catalyst.     

Ionic Liquid Effect on the Reversal of Configuration for the Magnesium(II) and Copper(II) Bis(oxazoline)‐Catalysed Enantioselective Diels–Alder Reaction

Ionic liquids have been used to support a range of magnesium‐ and copper‐based bis(oxazoline) complexes for the enantioselective Diels–Alder reaction between N‐acryloyloxazolidinone and cyclopentadiene. Compared with reaction performed in dichloromethane or diethyl ether, an enhancement in ee is observed with a large increase in reaction rate. In addition, for non‐sterically hindered bis(oxazoline) ligands, that is, phenyl functionalised ligands, a reversal in configuration is found in the ionic liquid, 1‐ethyl‐3‐methylimidazolium bis[(trifluoromethanesulfonyl)imide], compared with molecular solvents. Supported ionic liquid phase catalysts have also been developed using surface‐modified silica which show good reactivity and enantioselectivity for the case of the magnesium‐based bis(oxazoline) complexes. Poor ees and conversion were observed for the analogous copper‐based systems. Some drop in ee was found on supporting the catalyst due a drop in the rate of reaction and, therefore, an increase in the contribution from the uncatalysed achiral reaction.     

Reaction of phenol‐ and thiophenol‐terminated compounds with bisoxazolines

Bulk reactions of phenolic compounds (bisphenol‐A and α,ω‐diphenol oligosulfone) or thiols (thiophenol and bis(4‐mercaptophenyl)sulfide) with bisoxazoline coupling agents, namely 2,2'‐(1,3‐phenylene)bis(2‐oxazoline) ( mbox ), 2,2'‐(1,4‐phenylene)bis(2‐oxazoline) ( pbox ), and 2,2'‐(2,6‐pyridylene)bis(2‐oxazoline) ( pybox ), were carried out in the bulk at 140–240°C. The reactions were followed by viscosimetry, size exclusion chromatography, and ¹H‐ and ¹³C‐NMR spectroscopy. The phenol/bisoxazoline bulk reactions at 240°C required the presence of sodium methoxide catalyst. Bisoxazoline pybox gave the best results in this case. Thiol and dithiol/bisoxazoline reactions were faster and did not require any catalyst. High‐molar‐mass polymers were obtained within 5 min at 200°C while using bis(4‐mercaptophenyl)sulfide (BMPS) and any of the bisoxazolines. The NMR spectra of model compounds and polymers were fully assigned, showing that the oxazoline/phenol and oxazoline/thiophenol (tph) polyaddition reactions proceed in the expected way, without any noticeable side reaction. All polymers were amorphous and displayed good thermal stability. Bisoxazolines were also used as coupling agents for the preparation of copolymers of BMPS and α,ω‐dicarboxy polyamide‐12 and for the preparation of polysulfone‐polyamide‐12 block copolymers. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Surface functionalization of unsized carbon fiber using nitrenes derived from organic azides

The surface of both oxidized and unoxidized unsized carbon fiber was functionalized using an aziridine linking group derived from reactive nitrenes. The aziridine functionality arose from the cyclization of a reactive nitrene species onto the highly electron rich graphitic surface of the carbon fibers; the nitrene species evolved from thermal N₂ elimination from the corresponding (room temperature stable) azide. Surface functionalization using the nitrene approach was supported by X-ray Photoelectron Spectroscopy, in both oxidized and unoxidized carbon fiber. Attempts were also made to functionalize using amide chemistry, the two-step acid chloride coupling being successful for oxidized fibers by utilizing the carboxylic acid rich defect sites on the carbon fiber. None of the chemical treatment pathways had a significant impact on the tensile strength of the individual fibers, and atomic force microscopy revealed that fibers undergoing these treatment methodologies remained intact, without creating additional surface defects.     

Increasing of Photocatalytic Performance of TiO2 Nanotubes by Doping AgS and CdS

Highly ordered titanium nanotubes (TiO₂ NTs) photocatalyst was prepared by the anodic oxidation method, and AgS, CdS, and AgS/CdS nanoparticles were doped on the surface of TiO₂ NTs by the successive ion adsorption and reaction (SILAR) method. The photocatalysts were characterized by SEM, EDS, XRD, and potentiostat system. The SEM and EDS analyses respectively show that the average outer diameter of prepared photocatalysts is in the range of 50–120?nm, and the presence of Ti, O, Ag, and Cd is successfully proved. The photocatalytic properties of TiO₂ NTs and doped TiO₂ NTs were studied by measuring the degradation of Methylene Blue (MB) solution. The experimental results show that AgS/CdS/TiO₂ photocatalyst exhibited most efficient photocatalytic activity with 340?µA/cm² photocurrent value. AgS/CdS/TiO₂ NTs photocatalyst shows up to 22.20% higher than TiO₂ NTs, 16.42% higher than CdS/TiO₂ NTs, and 4.3% higher than AgS/TiO₂ NTs.     

A Novel Chemoenzymatic Synthesis of Sulfated Type 2 Tumor‐Associated Carbohydrate Antigens by Transglycosylation of Sulfated Lewis X Oxazoline Catalyzed by Keratanase II

Sulfated type 2 carbohydrate chains are known tumor‐associated carbohydrate antigens (TACAs). Many reports on cancer vaccines employing TACAs as specific antigens have been published, but structurally specified sulfated TACAs have not been used because of the low natural abundance and difficulties in chemical synthesis. We demonstrate for the first time the synthesis of the sulfated type 2 TACAs with an l ‐fucose branch by keratanase‐II‐catalyzed transglycosylation of the sulfated Lewis X (Galβ(1→4)[Fucα(1→3)]GlcNAc(6‐OSO₃^?); su‐Le^x) oxazoline derivative. Two keratanase IIs (from Bacillus sp. Ks36 and Bacillus circulans KsT202) efficiently catalyzed the transglycosylation reaction of the su‐Le^x oxazoline derivative, thereby giving the su‐Le^x dimer as the main product in good yields. Structural analysis of the oligomers confirmed exclusive formation of the β(1→3) glycosidic bond.     

Polymer‐Bound Pyridine‐Bis(oxazoline). Preparation through Click Chemistry and Evaluation in Asymmetric Catalysis

A pyridine‐bis(oxazoline) ligand was efficiently immobilized by copper(I)‐catalyzed azide‐alkyne cycloaddition onto a polystyrene resin. The so obtained click‐pybox resin 1a was associated with various metal salts (YbCl₃, LuCl₃, CuOTf) and the resulting resin‐bound catalysts were explored in ring‐opening of cyclohexene oxide, silylcyanation of benzaldehyde and alkynylation of imines. These new polymer‐supported catalysts exhibit good to excellent performances in terms of catalytic activity, enantioselectivity and recyclability.     

Nanoflower-like CdS and SnS2 loaded TiO2 nanotube arrays for photocatalytic wastewater treatment and hydrogen production

In this work, nanoflower-like CdS/SnS₂/TiO₂ NTs ternary heterojunction photocatalysts were synthesized by a hydrothermal method, the relationship between the morphology, microscopic morphology, crystallinity, elemental presence state and hydrogen production performance of the ternary photocatalysts were investigated by SEM, TEM, XRD and XPS, respectively. The photocatalytic performance, electrochemical property and hydrogen production capacity of CdS/SnS₂/TiO₂ NTs were compared with pure TiO₂ NTs, CdS/TiO₂ NTs and SnS₂/TiO₂ NTs. After 2 h of photocatalytic reaction, the removal efficiency of MB wastewater reached 100%, and the photocatalytic efficiencies toward RhB and Cr(VI) removal reached 86.08% and 80.93% after 3 h, respectively. The electron spin resonance (ESR) technique certified the active radical groups that played a role in the catalytic process and further investigated the possible photocatalytic mechanism. Hydrogen production per unit time achieved 97.14 μmol h^?1 cm^?2, this work provides the new technique to achieve solar energy conversion for hydrogen generation.     

Non-covalent immobilization of chiral copper complexes on Al-MCM41: Effect of the nature of the ligand

Cu-Al-MCM41, prepared by deposition of copper(II) triflate with incipient wetness impregnation and thermal treatment under air flow, can be modified with different chiral ligands of the bis(oxazoline) family. The efficiency of the supported chiral catalysts in the enantioselective cyclopropanation of styrenes with ethyl diazoacetate depends on the nature of the ligand. The azabis(oxazolines) perform much better than the bis(oxazolines) and give stable catalysts that can be used for at least five consecutive runs, with productivities that can reach values close to 1000 molecules of cyclopropane per copper site. The best enantioselectivities obtained with these catalysts are in the range of 60–70% ee in the reaction at 90 °C.     

Highly Efficient Catalytic Dimerization of Styrenes via Cationic Palladium(II) Complexes

A highly efficient head‐to‐tail dimerization of a styrene was developed using a cationic palladium(II)‐catalyzed selective C C bond forming reaction. The complex [AllylPd(PPh₃)]⁺OTf⁻, which is believed to generate ‘palladium hydride’ (Pd H), catalyzed the dimerization of various styrenes in excellent yields as single isomers. This Pd(II)‐catalyzed reaction provides a new economical C C bond forming method.

     

Remarkable Efficiency Improvement in the Preparation of Insoluble Polymer‐Bound (IPB) Enantioselective Catalytic Systems by the Use of Silicone Chemistry

The use of platinum‐catalyzed hydrosilylation chemistry of silicones greatly simplifies the preparation of bis‐oxazoline (box) ligands covalently bound to an insoluble polymeric support. The use of such immobilized chiral ligands in different copper‐catalyzed asymmetric transformations (carbonyl‐ene, Mukaiyama aldol and olefin cyclopropanation reactions) allows the attainment of high levels of enantioselectivity (91–99 % ee).     

Imidazolium ion tethered TsDPENs as efficient water-soluble ligands for rhodium catalyzed asymmetric transfer hydrogenation of aromatic ketones

An imidazolium ion tethered TsDPEN has been synthesized readily and used as a water-soluble ligand for [Cp*RhCl₂]₂ catalyzed asymmetric transfer hydrogenation (ATH) of aromatic ketones in water. This process provided the secondary alcohols in moderate to excellent conversions (up to 100%) with high enantioselectivities (up to 98% ee) under mild reaction conditions without adding any surfactants. The catalytic system is highly effective with the substrate to catalyst (S/C) ratio of 500 and low hydride donor loading of 1.5 equiv. of HCO₂Na. The procedure presented is simple and makes this method suitable for practical use.     

C2对称双噁唑啉的新合成方法

在脱水剂PCl3的存在下,噻吩-2,5-二甲酸与乙醇胺反应直接生成噻吩-2,5-双噁唑啉及未环化产物1和2。副产物1和2碱性关环生成噻吩-2,5-双噁唑啉,总收率达63%。此方法的优点是合成路线短,操作简单。     

Stabilization of native penicillin G acylase by ionic liquids

Five different ionic liquids, based on dialkylimidazolium cations associated with perfluorinated and bis{(trifluoromethyl)sulfonyl}imide anions, were used to investigate the scope and limitations of these new solvents as media for penicillin G acylase‐catalyzed reactions. Deactivation of the native enzyme in ionic liquids (ILs) and in organic solvents (toluene, dichloromethane and 2‐propanol) at low water content and 40 °C was investigated using the hydrolysis of penicillin G as activity test. Native penicillin G acylase shows greater stability in IL media than in organic solvents. For example, a half‐life time of 23 h was obtained in 1‐ethyl‐3‐methylimidazolium bis{(trifluoromethyl)sulfonyl}imide, [emim⁺][Tf₂N^?], which was about 2000‐fold higher than that in 2‐propanol. An enhancement of the PGA stability was observed by the presence of substrate in ionic liquids based on tetrafluoroborate and hexafluorophosphate anions, achieving the highest increase of the half‐life time in 1‐butyl‐3‐methylimidazolium hexafluorophosphate ([bmim⁺][PF₆^?]), which was about 9‐fold higher than the half‐life time in the absence of substrate. Copyright © 2007 Society of Chemical Industry     

Synthesis of vinyl substitute poly(silphenylene‐siloxane) via silyl hydride‐dialkoxysilane process

The synthesis of vinyl‐substituted silphenylene‐siloxane polymer through B(C₆F₅)₃ catalyzed polycondensation of 1,4‐bis(dimethylsilyl)benzene and vinylmethyldimethoxysilane is described. ¹H‐NMR, ²⁹Si‐NMR, and UV spectroscopy indicate that the vinyl groups remain undamaged during the polycondensation reaction. No hydrosilylation side reaction is observed under the reaction conditions. The microstructure of the polymer is not perfectly alternating with a randomization of 20%. The temperature for 5% mass loss is 430°C in inert atmosphere and 417°C in oxidative atmosphere with a residue of 56% at 700°C. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Alumina supported copper nanoparticles for aziridination and cyclopropanation reactions

Alumina supported copper (Cu–Al₂O₃) nanoparticles were prepared and applied for the aziridination of olefins employing [N-(p-tolylsulfonyl)imino]phenyliodinane (PhI = NTs) as nitrene source to afford the corresponding 3-membered cyclic compounds in moderate to excellent yields. Cyclopropanation of several olefins was also carried out by using Cu–Al₂O₃ nanoparticles and ethyl diazoacetate as a carbene source. The catalyst was recovered by simple centrifugation and reused for four cycles.     

Air‐Stable and Highly Active Dendritic Phosphine Oxide‐ Stabilized Palladium Nanoparticles: Preparation,Characterization and Applications in the Carbon‐Carbon Bond Formation and Hydrogenation Reactions

Dendrimer‐stabilized palladium nanoparticles were formed in the reduction of palldium bis(acetylacetonate) [Pd(acac)₂] in the presence of phosphine dendrimer ligands using hydrogen in tetrahydrofuran. The resulting Pd nanoparticles were characterized by TEM, ³¹P NMR and ³¹P MAS NMR. The results indicated that the dendritic phosphine ligands were oxidized to phosphine oxides. These dendrimer‐stabilized Pd nanoparticles were demonstrated to be efficient catalysts for Suzuki and Stille coupling reactions and hydrogenations. The dendritic wedges served as a stabilizer for keeping the nanoparticles from aggregating, and as a vehicle for facilitating the separation and/or the recycling of the Pd catalyst. In the case of the Suzuki coupling reaction, these Pd nanoparticles exhibited high catalytic efficiency (TON up to 65,000) and air stability as compared with the commonly used homogeneous catalyst tetrakis(triphenylphosphine)palladium [Pd(PPh₃)₄]. In addition, the results obtained from the bulky dendritic substrate suggest that the Pd nanoparticles might act as reservoir of catalytically active species, and that the reaction is actually catalyzed by the soluble Pd(0) and/or Pd(II) species leached from the nanoparticle surface.