Selective enhancement effects of silver salts on the transition metal-catalyzed synthesis of gem-difluorinated heterocyclics using 2,2-difluorohomopropargyl amides

The addition of silver salts had an effect on the catalyst activity in the Pd(0)-catalyzed cyclization-coupling tandem reaction, as well as in the Rh(I)-catalyzed Pauson-Khand reaction. The cationic palladium complex generated from Pd(PPh₃)₄ (2.5 mol%) with AgSbF₆ (1.5 equiv.) activates the triple bond of 2,2-difluoropropargylic amides to give the 4,5-disubstituted 3,3-difluoro-γ-lactams, through a sequential 5-endo-dig cyclization and cross-coupling reaction. The γ-lactam was transformed into ring-opened monofluorovinylic compounds after silica-gel chromatography. Pauson-Khand reaction of fluorinated 1,7-enyne amides using catalytic amounts of [Rh(COD)₂]₂ (5 mol%) and AgOTf (20 mol%) gave the corresponding gem-difluorinated bicyclic lactam.     

An efficient Stille cross-coupling reaction catalyzed by Pd(OAc)2/DAB-Cy catalytic system

An efficient palladium-catalyzed Stille cross-coupling reaction has been developed. In the presence of 3 mol% of Pd(dba)₂ and 6 mol% of DAB-Cy (1,4-dicyclohexyl-diazabutadiene), various aryl halides (iodides and bromides) were coupled with organotin compounds to afford the corresponding biaryls and alkyne in good to excellent yields. Furthermore, high TONs [turnover numbers, TONs up to 950,000 for the reaction of 1-iodo-4-nitrobenzene and tributyl(phenyl)stannane] for the Stille cross-coupling reaction were observed.     

Capillary electrophoretic determination of ammonia using headspace single-drop microextraction

A new method involving headspace single-drop microextraction (SDME) and capillary electrophoresis (CE) is developed for the preconcentration and determination of ammonia (as dissolved NH₃ and ammonium ion). An aqueous microdrop (5 μL) containing 1 mmol/L H₃PO₄ and 0.5 mmol/L KH₂PO₄ (as internal standard) was used as the acceptor phase. Common experimental parameters (sample and acceptor phase pH, extraction temperature, extraction time) affecting the extraction efficiency were investigated. Proposed SDME-CE method provided about 14-fold enrichment in about 20 min. The calibration curve was linear for concentrations of NH₄⁺ in the range from 5 to 100 μmol/L (R² = 0.996). The LOD (S / N = 3) was estimated to be 1.5 μmol/L of NH₄⁺. Such detection sensitivity is high enough for ammonia determination in common environmental and biological samples. Finally, headspace SDME was applied to determine ammonia in human blood, seawater and milk samples with spiked recoveries in the range of 96-107%.     

Triaryl phosphine-functionalized N-heterocyclic carbene ligands for Heck reaction

A new type of triaryl phosphine-functionalized imidazolium salts 6 were prepared. Their palladium complexes, generated in situ, were successfully applied in the palladium-catalyzed Heck reaction. Using 1 mol% of Pd(dba)₂ and 1 mol% 6c in the presence of 2 equiv of K₂CO₃ in DMAc has proven to be highly efficient for the coupling of a wide array of aryl bromides and iodides with acrylates in excellent yield. The coupling of 4-bromotoluene with various styrene derivatives catalyzed by Pd/6c complex also gave good results.     

Asymmetric epoxidation of cis-1-propenylphosphonic acid (CPPA) catalyzed by chiral tungsten(VI) and molybdenum(VI) complexes

In the presence of 5.0 mol% chiral tungsten(VI) and molybdenum(VI) complexes, the catalytic asymmetric epoxidation of cis-1-propenylphosphonic acid (CPPA) with 30% aqueous H₂O₂ affording (1R,2S)-(−)-(1, 2)-epoxypropyl phosphonic acid (fosfomycin) was first described. The enantioselectivities of the tungsten and molybdenum catalysts depend strongly on the ligands, reaction temperature and solvent. In CH₂Cl₂ at 0 °C for 72 h, complex MoO₂[(+)-campy]₂ catalyzed the asymmetric epoxidation in a 100% conversion of CPPA with the highest 80% ee. The mechanism of the present epoxidation could be described as direct oxygen transfer occurred on the interface of the biphasic H₂O-nonprotic system.     

H NMR and IR study of temperature-induced phase transition of negatively charged poly(N-isopropylmethacrylamide-co-sodium methacrylate) copolymers in aqueous solutions

¹H NMR and IR spectroscopies were used to investigate the temperature-induced phase transition behaviour of poly(N-isopropylmethacrylamide-co-sodium methacrylate) [P(IPMAAm/MNa)] copolymers, containing in aqueous solutions negatively charged MNa units (i = 1-10 mol%), and the obtained results were compared with those obtained for poly(N-isopropylmethacrylamide) (PIPMAAm) homopolymer. For PIPMAAm/H₂O solution, IR spectra indicate that the transition temperatures for the hydrophilic CO groups are slightly higher (by ∼ 2 K) in comparison with hydrophobic CH₃ groups. The decreasing values of phase-separated fraction p_max and the decrescent hysteresis during gradual heating and cooling, both with increasing content of MNa units i in the copolymer, show that for copolymers with i ? 5 mol% the globular-like structures formed at temperatures above the respective LCST are rather porous and disordered with relatively low degree of polymer-polymer hydrogen bonding. While for P(IPMAAm/MNa) copolymers with i ? 5 mol% most water molecules are expelled from globular structures, for i < 5 mol% a certain portion of water (HDO) molecules is rather tightly bound in globular structures; at the same time no releasing process was detected for the bound water even for 90 h.     

Efficient Suzuki coupling of aryl chlorides catalyzed by tricyclohexylphosphine adducts of cyclopalladated ferrocenylimines

The air and moisture stable tricyclohexylphosphine (PCy₃) adducts of dimeric cyclopalladated ferrocenylimines 5 and 6 have been easily synthesized and successfully used in palladium-catalyzed Suzuki cross-coupling of aryl chlorides. Using 0.1 mol% of 6 in the presence of 2 equivalent of Cs₂CO₃ as base in dioxane at 100 °C provided coupled products in excellent yields in the reaction of non-activated and deactivated aryl chlorides with phenylboronic acid. For activated chlorides such as 4-chloronitrobenzene and 4-chloroacetophenone, the catalyst loadings could be lowered to 0.01 mol% without loss of activity.     

Microstructure and electrical-optical properties of cesium tungsten oxides synthesized by solvothermal reaction followed by ammonia annealing

Cesium tungsten oxides (Cs_xWO₃) were synthesized by solvothermal reactions using ethanol and 57.1 vol% ethanol aqueous solution at 200 °C for 12 h, and the effects of post annealing in ammonia atmosphere on the microstructure and electrical-optical properties were investigated. Agglomerated particles consisting of disk-like nanoparticles and nanorods of Cs_xWO₃ were formed in the pure ethanol and ethanol aqueous solutions, respectively. The samples retained the original morphology and crystallinity after annealing in ammonia atmosphere up to 500 °C, while a small amount of nitrogen ion were incorporated in the lattice. The as-prepared Cs_xWO₃ sample showed excellent near infrared (NIR) light shielding ability as well as high transparency in the visible light region. The electrical resistivity of the pressed pellets of the powders prepared in pure ethanol and 57.1 vol% ethanol aqueous solution greatly decreased after ammonia annealing at 500 °C, i.e., from 734 to 31.5 and 231 to 3.58 Ω cm, respectively.     

Uniform Ln (Eu, Tb) doped NaLa(WO4)2 nanocrystals: Synthesis, characterization, and optical properties

Uniform shuttle-like Ln³⁺ (Eu³⁺, Tb³⁺) doped NaLa(WO₄)₂ nanocrystals have been solvothermally synthesized, and the size of the nanocrystals could be easily controlled by adjusting the volume ratio of ethylene glycol (EG) to water. Doped with 5 mol% Eu³⁺ and Tb³⁺ ions, the NaLa(WO₄)₂ nanocrystals showed strong red and green emissions with lifetimes of 0.8 and 1.40 ms, respectively. A high quenching concentration of 15 mol% was observed in Eu³⁺-doped NaLa(WO₄)₂ nanocrystals and 35 mol% in Tb³⁺-doped NaLa(WO₄)₂ nanocrystals. The emission intensity measurements of Eu³⁺-doped NaLa(WO₄)₂ with different sizes indicated that the emission intensity of shuttles with length of 300 nm in average was stronger than that of shuttles with length of 900 nm in average, but was weaker than that of needles with length of 4 and 9 μm in average.     

A highly active catalyst for the reductive cyclization of ortho-nitrostyrenes under mild conditions

A mild and efficient method for the palladium-catalyzed reductive cyclization of ortho-nitrostyrenes to afford indoles is reported. Treatment of ortho-nitrostyrenes with 0.1 mol% palladium (II) trifluoroacetate [Pd(TFA)₂] and 0.7 mol% 3,4,7,8-tetramethyl-1,10-phenanthroline (tm-phen) in DMF at 15 psig CO and 80 °C afforded indoles in good to excellent yields. When the reaction was conducted in toluene, the corresponding N-hydroxyindole was isolated. A mechanism that accounts for the formation of N-hydroxyindole is proposed.     

The substituent effects of the leaving groups on the aminolysis of phenyl acetates: DFT studies

The density functional theory B3LYP/6-31G(d, p) method is employed to study the mechanism of aminolysis reaction of p-substituted phenyl acetates (CH₃C(O)OC₆H₄X, X = H, NH₂, and NO₂) with ammonia in the gas phase. Two reaction pathways are considered: the concerted process and the stepwise pathway through neutral intermediates. The substituent effects of the leaving groups on the reactivity of phenyl acetates are discussed. The solvent effect of acetonitrile on the title reaction is also assessed by the polarizable continuum model (CPCM model) at B3LYP/6-31++G(d, p) level of theory. The calculated results show that the activation barriers of the concerted pathways are lower than those of the rate-controlling steps of the stepwise processes for all the three aminolysis reactions. This aminolysis of phenyl acetates is more favorable for X = NO₂ than for X = H and NH₂ in the gas phase and in acetonitrile.     

Triethylborane as an efficient promoter for palladium-catalyzed allylation of active methylene compounds with allyl alcohols

Without prior activation of allyl alcohols, allylation of a variety of active methylene compounds with allyl alcohols proceeds smoothly at rt-50°C in the presence of catalytic amounts of Pd(OAc)₂ (1-10 mol%), Et₃B (30-240 mol%), a phosphine ligand (1-20 mol%), and a base (0 to 50-60 mol%).     

K2Li(NH2)3 and K2Na(NH2)3—synthesis and crystal structure of two crystal-chemically isotypic mixed-cationic amides

K₂Li(NH₂)₃ (1) was the only crystalline product obtained from the reaction of potassium with dilithium decahydro-closo-decaborate Li₂B₁₀H₁₀ in liquid ammonia at −38 °C. The compound crystallizes in the space group P4₂/m with Z=4, a=6.8720(5) Å, c=11.706(1) Å and V=552.81(7) Å³. The investigated crystal-chemically isotypic sodium compound K₂Na(NH₂)₃ (2) was merohedrally twinned and crystallized from a reaction mixture containing potassium and disodium decahydro-closo-decaborate Na₂B₁₀H₁₀ in liquid ammonia with a=7.0044(5) Å, c=12.362(1) Å and V=606.48(9) Å³. The compounds contain pairs of edge sharing tetraamidolithium or tetraamidosodium tetrahedra which are interconnected by potassium ions forming three-dimensional infinite networks.     

Mixed conductivity, structural and microstructural characterization of titania-doped yttria tetragonal zirconia polycrystalline/titania-doped yttria stabilized zirconia composite anode matrices

Taking advantage of the fact that TiO₂ additions to 8YSZ cause not only the formation of a titania-doped YSZ solid solution but also a titania-doped YTZP solid solution, composite materials based on both solutions were prepared by solid state reaction. In particular, additions of 15 mol% of TiO₂ give rise to composite materials constituted by 0.51 mol fraction titania-doped yttria tetragonal zirconia polycrystalline and 0.49 mol fraction titania-doped yttria stabilized zirconia (0.51TiYTZP/0.49TiYSZ). Furthermore, Y₂(Ti_1−yZr_y)₂O₇ pyrochlore is present as an impurity phase with y close to 1, according to FT-Raman results. Lower and higher additions of titania than that of 15 mol%, i.e., x=0, 5, 10, 20, 25 and 30 mol% were considered to study the evolution of 8YSZ phase as a function of the TiO₂ content. Furthermore, zirconium titanate phase (ZrTiO₄) is detected when the titania content is equal or higher than 20 mol% and this phase admits Y₂O₃ in solid solution according to FE-SEM-EDX.The 0.51TiYTZP/0.49TiYSZ duplex material was selected in this study to establish the mechanism of its electronic conduction under low oxygen partial pressures. In the pO₂ range from 0.21 to 10^−7.5 atm. the conductivity is predominantly ionic and constant over the range and its value is 0.01 S/cm. The ionic plus electronic conductivity is 0.02 S/cm at 1000 °C and 10^−12.3 atm. Furthermore, the onset of electronic conductivity under reducing conditions exhibits a −1/4 pO₂ dependence. Therefore, it is concluded that the n-type electronic conduction in the duplex material can be due to a small polaron-hopping between Ti³⁺ and Ti⁴⁺.     

Humidity effects in Fe16N2 fine powder preparation by low-temperature nitridation

Low-temperature nitridation has been reported to produce ferromagnetic α″-Fe₁₆N₂ by ammonia nitridation of α-Fe fine powder, which was obtained from the reduction of vapor-grown γ-Fe₂O₃. The effects of humidity during this preparation were investigated in the present study. α″-Fe₁₆N₂ was inconsistently obtained, and at low yield, from Fe₃O₄ fine powder (MT-40) prepared from aqueous solution. Reducing the adsorbed water content in the iron oxide starting powder resulted in improved reproducibility of the α″-Fe₁₆N₂ yield of the nitridation. The use of a smaller-diameter reaction tube, less than 25 mm in diameter, enabled more reproducible preparation from vapor-grown γ-Fe₂O₃ powder (CI-30). The reaction yield was further improved by using high-quality ammonia with a water content of ≤0.05 ppm. Minimizing the humidity made it possible to obtain a fine powder with a high α″-Fe₁₆N₂ content. Enhancement of the magnetization to 210 emu g⁻¹ at room temperature was observed from a nitrided mixture of α″-Fe₁₆N₂ with residual α-Fe, compared to 199 emu g⁻¹ for an α-Fe fine powder reduced from γ-Fe₂O₃. However, excess nitrogen and residual oxygen in the nitrided products reduced the magnetization below the value of α-Fe powder after nitridation. The magnetization was enhanced in nitrided products with a nitrogen content slightly below the stoichiometric amount for α″-Fe₁₆N₂.     

Titanium-catalyzed Reformatsky-type reaction

A catalytic titanium mediated Reformatsky reaction is presented. The technique employs cyclopentadienyltitaniumdichloride(IV) (10 mol%) in conjunction with Mn (2 equiv.), as the stoichiometric reductant, and (CF₃CO)₂O (1.5 equiv.), as the scavenger, and it is based on the formation of titanocene(III) chloride as a mild and homogeneous single-electron reductant. The reaction gives moderate yields of the desired Reformatsky adduct, and it works well with aromatic and aliphatic aldehydes.     

Visible and infrared luminescence properties of Er-doped Y2Ti2O7 nanocrystals

Er³⁺-doped Y₂Ti₂O₇ nanocrystals were fabricated by the sol-gel method. While the annealing temperature exceeds 757 °C, amorphous pyrochlore phase Er_xY_2−xTi₂O₇ transfers to well-crystallized nanocrystals, and the average crystal size increases from ∼70 to ∼180 nm under 800-1000 °C/1 h annealing. Er_xY_2−xTi₂O₇ nanocrystals absorbing 980 nm photons can produce the upconversion (526, 547, and 660 nm; ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_15/2, respectively) and Stokes (1528 nm; ⁴I_13/2→⁴I_15/2) photoluminescence (PL). The infrared PL decay curve is single-exponential for Er³⁺ (5 mol%)-doped Y₂Ti₂O₇ nanocrystals but slightly nonexponential for Er³⁺ (10 mol%)-doped Y₂Ti₂O₇ nanocrystals. For 5 and 10 mol% doping concentrations, the mechanism of up-converted green light is the two-photon excited-state absorption. Much stronger intensity of red light relative to green light was observed for the sample with 10 mol% dopant. This phenomenon can be attributed to the reduced distance between Er³⁺-Er³⁺ ions, resulting in the enhancement of the energy-transfer upconversion and cross-relaxation mechanisms.     

Preparation and characterization of mesoporous TiO2-CeO2 nanopowders respond to visible wavelength

Mesoporous TiO₂-CeO₂ nanopowders responding to visible wavelength were synthesized by using a surfactant assisted sol-gel technique. They were obtained using metal alkoxide precursors modified with acetylacetone (ACA) and laurylamine hydrochloride (LAHC) as surfactant. The samples were characterized by XRD, nitrogen adsorption isotherm, SEM, TEM, and selected area electron diffraction (SAED), respectively. The 95 mol% TiO₂-5 mol% CeO₂ system yielded single anatase phase, however, further addition of the CeO₂ formed cubic CeO₂ structure while anatase TiO₂ decreased. Additions of 5 and 10 mol% CeO₂ increased the surface area, but those of 25, 50, and 75 mol% CeO₂ did not affect it very much. By using this mixed metal oxides system, TiO₂ can be modified to respond to the visible wavelength. The mixed metal oxides had catalytic activity (evaluating the formation rate of I₃⁻) about 2-3 times higher than pure CeO₂, while nanosize anatase type TiO₂ materials had no catalytic activity under visible light. The catalytic activity was almost proportional to the specific surface area. The formation rate of I₃⁻ was much improved by changing the calcination temperature and calcination period. Highest catalytic activity in this study was obtained for the 50 mol% TiO₂-50 mol% CeO₂ nanopowders calcined at 250 °C for 24 h.     

The first example of atropisomeric amide-derived P,O-ligands used for an asymmetric Heck reaction

Atropisomeric N,N-diisopropyl 2-diphenylphosphino- and 2-di(tert-butyl)phosphino-1-naphthamides were used, for the first time, as bidentate P,O-ligands for intermolecular asymmetric Heck reactions of 2,3-dihydrofuran with aryl triflates. The reactions were carried out in the presence of 4 mol% Pd(OAc)₂, 8 mol% of the axially chiral ligand, and 3 equiv. of (i-Pr)₂NEt in THF at 60 °C for 3 days. Optically active 2-aryl-2,5-dihydrofurans were obtained as the major products along with the rearranged 2-aryl-2,3-dihydrofurans. Enantioselectivity up to 55.2% ee was obtained for the major product.