Rhodium(II,II) dimer as an efficient catalyst for aziridination of sulfonamides and amidation of steroids

[reaction: see text] Unsaturated sulfonamides underwent direct intramolecular aziridination catalyzed by Rh(2)(OAc)(4) with PhI(OAc)(2) and Al(2)O(3) to give the corresponding aziridine products in excellent yields (up to 98%) and with good to excellent conversions. High turnovers (up to 1375) were achieved. The intermolecular rhodium-catalyzed amidation of cholesteryl acetate with PhI=NTs or PhI(OAc)(2)/NH(2)R as the nitrogen source exhibited both excellent regio- and alpha-selectivity (alpha/beta ratio up to 9:1).     

Palladium(II) and platinum(II) complexes of Schiff bases derived from 2,6-diacetylpyridine and S-methyldithiocarbazate and the X-ray crystal structure of the [Pd(mdapsme)Cl] complex

Condensation of 2,6-diacetylpyridine (dap) with S-methyldithiocarbazate (smdtc) in a 1:2 molar ratio yields a bicondensed pentadentate Schiff base (H₂dapsme) which reacts with K₂MCl₄ (M = Pd^II, Pt^II) giving stable complexes of empirical formula, [M(dapsme)] · 0.5Me₂CO. These complexes have been characterized by a variety of physico-chemical techniques. Condensation of dap with smdtc in a 1:1 molar ratio also yields the bicondensed Schiff base (H₂dapsme) as the major product, but a mono-condensed one-armed Schiff base (Hmdapsme) is also obtained as a minor product. The latter reacts with K₂PdCl₄ in an EtOH–H₂O mixture yielding a crystalline complex of empirical formula, [Pd(mdapsme)Cl], the crystal structure of which has been determined by X-ray diffraction. The complex has a distorted square-planar structure in which the ligand is coordinated to the palladium(II) ion as a uninegatively charged tridentate chelating agent via the pyridine nitrogen atom, the azomethine nitrogen atom and the thiolate sulfur atom; the oxygen atom of the acetyl group does not participate in coordination.     

Determination of oxygen in alkali halide salts by proton activation and radiochemical separation

A lanthanum fluoride precipitation method for the separation of¹⁸F produced from the¹⁸O(p,n)¹⁸F reaction in alkali halide salts is described. This radiochemical separation method minimizes interferences from other positron emitters produced by proton bombardment and makes the accurate determination of ppm-level¹⁸O in complex alkali halide systems feasible. The interference from the¹⁹F(p,d)¹⁸F reaction is eliminated by keeping the proton energy less than 8.2 MeV. Applications of this technique to studies of dissolved oxide species in molten alkali halide salts are discussed.     

Electron impact mass spectral fragmentation of chiral bisbenzoxazole,bisbenzothiazole and biscarbamide derivatives of 2,2-dimethyl-1,3-dioxolane

The mass spectrometric fragmentation behaviour of five pairs of (R,R)- and (S,S)-4,5-bis(benzoxazol-2-yl)-2,2-dimethyl-1,3-dioxolane derivatives, one pair of (R,R)- and (S,S)-4,5-bis(benzothiazol-2-yl)-2,2-dimethyl-1,3-dioxolanes, and three pairs of (R,R)- and (S,S)-N,N'-bis(2-hydroxyaryl)-2,2-dimethyl-1,3-dioxolane-4,5-dicarbamides, all important compounds for asymmetric catalysis (P. Jiao et al., Tetrahedron Asymmetry 2001; 12: 3081), has been studied with the aid of mass-analyzed ion kinetic energy spectrometry and accurate mass measurements under electron impact ionization conditions. The spectral observations have been rationalized in terms of fragment ion structures and fragmentation mechanisms that will provide an aid to spectral interpretation for new compounds of this type.     

Time-resolved resonance Raman and density functional theory study of the deprotonation reaction of the triplet state of p-hydroxyacetophenone in water solution

[reaction; see text] Picosecond and nanosecond time-resolved resonance Raman (TR(3)) spectroscopy was employed to investigate the deprotonation/ionization reaction of p-hydroxyacetophenone (HA) after ultraviolet photolysis in water solution. The TR(3) spectra in conjunction with density functional theory (DFT) calculations were used to characterize the structure and dynamics of the excited-state HA deprotonation to form HA anions in near neutral water solvent. DFT calculations based on a solute-solvent intermolecular H-bonded complex model containing up to three water molecules were used to evaluate the H-bond interactions and their influence on the deprotonation reaction and the structures of the intermediates. The deprotonation reaction was found to occur on the triplet manifold with a planar H-bonded HA triplet complex as the precursor species. The HA triplet species is generated within several picoseconds and then decays with a approximately 10 ns time constant to produce the HA triplet anion species after 267 nm photolysis of HA in water solution. The triplet anion species was observed to decay with a time constant of about 90 ns into the ground-state anion species that was found to have a lifetime of about 200 ns. The DFT calculations on the H-bonded complexes of the anion triplet and ground-states species suggest that these anion species are H-bonded complexes with planar quinonoidal structures containing two water molecules H-bonded, respectively, with oxygen lone pairs of the carbonyl and deprotonated hydroxyl moieties. A deactivation scheme of the photoexcited HA in regard to the deprotonation reaction in neutral water solutions was proposed. With the above dynamic and structural information available, we briefly discuss the possible implications of the model HA photochemistry in water solutions for the photodeprotection reactions of related p-HP phototrigger compounds in aqueous solutions.     

Response of a benzoxainone derivative linked to monoaza-15-crown-5 with divalent heavy metals

The response of a monoaza-15-crown-5 with an optically active aminobenzoxazinone moiety to divalent cations was investigated. The crown ether was found to undergo a strong emission shift to the blue when complexed with specific divalent metals that have ionic diameters between 1.9-2.4 A. Consequently the photoactive macrocycle is responsive to Mg(2+), Ca(2+), Ba(2+), Sr(2+), Cd(2+), and particularly responsive to Hg(2+)and Pb(2+). Macrocycle emission spectra are shown to be a function of cation concentration. Alkaline metal cations and smaller transition metals ions such as Ni(2+), Co(2+)and Zn(2+)do not cause significant changes in the macrocycle emission spectra. Emission, absorption, and complex stability constants are determined. Mechanisms of cation selectivity and spectral emission shifts are discussed. Challenges involving immobilization of the macrocycle while preserving its spectral response to cations are explored.     

Evaluation of dithiocarbamates and beta-diketones as chelating agents in supercritical fluid extraction of Cd, Pb, and Hg from solid samples

The use of four dithiocarbamates and three fluorinated β-diketones as potential chelating agents for three transition metal ions (Cd²⁺, Pb²⁺, and Hg²⁺) extracted from spiked sand and filter paper samples by supercritical fluid extraction (SFE) was investigated. The extractions were performed at 45°C and 250 atm for spiked sand samples and at 60°C and 200 atm for filter paper samples using supercritical carbon dioxide modified with 5% methanol. At 250 atm and using carbon dioxide modified with 5% methanol, the recoveries of Cd²⁺, Pb²⁺, and Hg²⁺ ions from spiked sand samples were 95% with lithium bis(trifluoroethyl)dithiocarbamate (LiFDDC) as the chelating agent; they ranged from 83–97% with diethylammonium diethyldithiocarbamate and from 87–97% with sodium di-ethyldithiocarbamate as chelating agents, and from 68–96% with trifluoracetylacetone, hexafluoroacetylacetone, and thenoylfluoroacetone as chelating agents. Ammonium pyrrolidinedithiocarbamate was not effective in the chelation SFE of Cd²⁺, Pb²⁺, and Hg²⁺ ions from either spiked sand or spiked filter paper samples under the extraction conditions used. Supercritical carbon dioxide alone gave consistently lower analyte recoveries than supercritical carbon dioxide modified with 5% methanol. The results suggest that the solubility of the metal chelate in the supercritical fluid plays a more important role than the solubility of the chelating agent in the supercritical fluid, as long as sufficient chelating agent is present in the fluid phase. Fluorination of the chelating agent, as in the case of LiFDDC, increases the solubility of the metal chelate, and subsequently enhances the extraction efficiency for the metal ions.     

Ln(Ln=Y/Gd)VO4:Er3+/Nd3+的制备及发光性能

采用高温固相法制备了Ln（Ln=Y／Gd）VO4掺Er^3＋或Nd^3＋的近红外发光材料。通过X射线粉末衍射（XRD）和光致发光（PL）对样品进行了表征。结果表明：所得产品结晶良好，属于四方晶系，锆石结构。研究了Er^3＋，Nd^3＋的含量、煅烧时间、煅烧温度等对材料近红外发光性质的影响。在Ln（Ln=Y／Gd）VO4：Er^3＋／Nd^3＋中，存在明显的从VO4^3-向Er^3＋／Nd^3＋的能量传递。两种不同的LnVO4（Ln=Y／Gd）基质对发光性质也有一定的影响。小浓度Bi^3＋的掺人可以明显提高YVO4：Er^3＋／Nd^3＋的近红外发光强度。     

Preconcentration of lanthanides from natural waters with a lipophilic crown ether carboxylic acid

Lipophilic crown ether carboxylic acids such as 2-(sym-dibenzo-16-crown-5-oxy)stearic acid with a cavity size comparable to the ionic radius of rare-earth elements are selective chelation agents for preconcentration and separation of lanthanides from natural waters for NAA. Interfering matrix elements such as sodium and bromine can be simultaneously eliminated during the extraction. The lanthanides can be back-extracted into a dilute acid solution for NAA, thus providing a large preconcentration factor. This two-step extraction method appears suitable for the determination of lanthanides in natural waters and in biological samples.     

Competition between pi and non-pi cation-binding sites in aromatic amino acids: a theoretical study of alkali metal cation (Li+, Na+, K+)-phenylalanine complexes

To understand the cation-pi interaction in aromatic amino acids and peptides, the binding of M(+) (where M(+) = Li(+), Na(+), and K(+)) to phenylalanine (Phe) is studied at the best level of density functional theory reported so far. The different modes of M(+) binding show the same order of binding affinity (Li(+)>Na(+)>K(+)), in the approximate ratio of 2.2:1.5:1.0. The most stable binding mode is one in which the M(+) is stabilized by a tridentate interaction between the cation and the carbonyl oxygen (O[double bond]C), amino nitrogen (--NH(2)), and aromatic pi ring; the absolute Li(+), Na(+), and K(+) affinities are estimated theoretically to be 275, 201, and 141 kJ mol(-1), respectively. Factors affecting the relative stabilities of various M(+)-Phe binding modes and conformers have been identified, with ion-dipole interaction playing an important role. We found that the trend of pi and non-pi cation bonding distances (Na(+)-pi>Na(+)-N>Na(+)-O and K(+)-pi>K(+)-N>K(+)-O) in our theoretical Na(+)/K(+)-Phe structures are in agreement with the reported X-ray crystal structures of model synthetic receptors (sodium and potassium bound lariat ether complexes), even though the average alkali metal cation-pi distance found in the crystal structures is longer. This difference between the solid and the gas-phase structures can be reconciled by taking the higher coordination number of the cations in the lariat ether complexes into account.