Palladium(II)-catalyzed rearrangement of glycal trichloroacetimidates: application to the stereoselective synthesis of glycosyl ureas

The research on the area of glycosyl urea derivatives, in which the O- and N-glycosidic bonds are replaced with the urea-glycosidic linkages, has recently emerged with applications in the field of aminoglycoside antibiotics. We have developed a novel method for the stereoselective synthesis of alpha- and beta-glycosyl ureas via Pd(II)-catalyzed rearrangement of glycal trichloroacetimidates. In our approach, the alpha- and beta-selectivity at the anomeric carbon of N-glycosyl trichloroacetamides depends on the nature of the palladium-ligand catalyst. While the cationic Pd(II)-L-4 (2-trifluoroacetylphenol) complex promotes alpha-selectivity, the neutral Pd(II)-TTMPP-L-5 (4-chloro-2-trifluoroacetylphenol) complex favors beta-selectivity. The resulting alpha- and beta-N-glycosyl trichloroacetamides were further coupled with a diverse array of primary and hindered secondary nitrogen nucleophiles to provide the corresponding glycosyl ureas in moderate to good yields and with no loss of stereochemical integrity at the anomeric carbon. We have further demonstrated the utility of N-glycosyl trichloroacetamides as robust and versatile intermediates in the synthesis of unsymmetrical urea-linked disaccharides and trisaccharide.     

Investigations of glycosylation reactions with 2-N-acetyl-2N,3O-oxazolidinone-protected glucosamine donors

NIS/AgOTf-promoted glycosylations with ethyl 2,3-N,O-carbonyl-2-deoxy-1-thio-beta-D-glucopyranoside donors can be performed with either alpha- or beta-selectivity by tuning the reaction conditions. Small amounts of AgOTf (0.1 equiv) and short reaction times give beta-selectivity, whereas 0.4 equiv of AgOTf and prolonged reaction times afford alpha-linked products. NMR-monitored glycosylation and anomerization experiments show initial formation of exclusively the beta-linkage, which anomerizes, through an intramolecular mechanism involving an endocyclic C-O bond cleavage, to the alpha-linkage.     

Kinetic studies of complexation reactions of water-soluble hydrazones with nickel(II) and palladium(II) ions

The kinetics of complexation reactions of five water-soluble heterocyclic hydrazones with nickel(II) and palladium(II) ions have been investigated by stopped-flow spectrophotometry. Rates of complexations with nickel(II) and palladium(II) in the absence of chloride ion were found to be proportional to the first order of the ligand and metal ion concentrations and to the inverse first order of the hydrogen ion concentration except for the complexation of alpha-(2-benzimidazolyl)-alpha-(5-nitro-2-pyridyl)hydrazono-3-toluenesulfonic acid with palladium(II). Rates of complexation with palladium(II) in the presence of chloride ion were best described by a two-term expression, both terms being first order in the palladium ion and ligand concentrations and inverse first order in the hydrogen ion concentration. The first term has zero dependence of the chloride ion concentration, whereas the second is first order with respect to the chloride ion concentration. The rate constant for each complexation reaction was determined. The complexation of the hydrazones with nickel(II) was estimated to go according to an Eigen mechanism and that with palladium (II) according to the associative mechanism.     

The 4,6-O-[alpha-(2-(2- iodophenyl)ethylthiocarbonyl)benzylidene] protecting group: stereoselective glycosylation,reductive radical fragmentation,and synthesis of beta-D-rhamnopyranosides and other deoxy sugars

[reaction: see text] In the thioglycoside/BSP/Tf(2)O glycosylation method, the 4,6-O-[alpha-(2-(2-iodophenyl)ethylthiocarbonyl)benzylidene] group enforces beta-selectivity in mannopyranosylations. Following glycosylation, treatment with Bu(3)SnH in toluene at reflux affords regioselective, reductive fragmentation to the 6-deoxy-beta-mannosides (beta-rhamnosides). Applied to glucosides, the radical fragmentation provides 6-deoxyglucosides, whereas 4-deoxygalactosides are the preferred products in the galactose series. The radical fragmentation is fully compatible with the presence of benzyl and p-methoxybenzyl ethers and with acetate esters     

Determination of palladium(II) with alpha-(2-benzimidazolyl)-alpha',alpha'-(N-5-nitro-2-pyridylhydrazone)-toluene by adsorptive cathodic stripping voltammetry

The determination of palladium(II) complexed with alpha-(2-benzimidazolyl)-alpha',alpha'-(N-5-nitro-2-pyridylhydrazone)-Toluene (BINPHT) was investigated by adsorptive cathodic stripping voltammetry using hanging mercury drop electrode. Palladium(II) in the sample solution can be determined in BINPHT and ethylenediaminetetraacetic acid (EDTA). Accumulation is achieved by adsorption of Pd(II)-BINPHT complex on a hanging mercury drop electrode. Optimal conditions were found to be: supporting electrolyte; 0.01 M sodium acetate buffer at pH 5.0, accumulation potential; -590 mV versus Ag/AgCl, accumulation time; 180 s, scan rate; 50 mV s(-1), concentration of BINPHT; 2x10(-5) M. The linear range of Pd(II) was observed over the concentration range 20-100 ng ml(-1) The detection limit (S/N=3) is 2 ng ml(-1). A good reproductivity shows RSD of 2.0% (n=7). This procedure offers high selectivity with the presence of EDTA masking some metallic ions. River water sample spiking with palladium was determined.     

Catalytic Oxidation of Iron(II) Aqua Complex by Oxygen in the Presence of Palladium(II) Tetraaqua Complex

The catalytic oxidation of iron(II) with oxygen occurs along with an autocatalytic reaction between palladium(II) tetraaqua complex and iron(II) aqua complex in an oxygen atmosphere. The reaction is catalyzed by a compound of palladium in an intermediate oxidation state, presumably by a small palladium cluster formed in the course of the reduction of palladium(II) tetraaqua complex with iron(II) aqua complex.     

Precipitation of bis(1,2-cyclohexanedionedioximato)palladium(II) from homogeneous solution

The reaction between hydroxylamine and cyclohexanedione in the presence of palladium ions has been made the basis of the precipitation of bis(1,2-cyclohexanedionedioximato)palladium(II) from homogeneous solution. The procedure provides a means of separating palladium from Cu(II), Co(II), Ni(II) and Pt(IV), and is a simple, rapid and accurate method for determining palladium.     

Extraction of palladium(II) from hydrochloric acid solutions with triacylated ethyleneamines

Extraction of palladium(II) from hydrochloric acid solutions with novel efficient extractants, triacylated ethyleneamines, was studied. The most effective extraction of palladium(II) was observed from 0.5–1 M HCl solutions. Extraction of palladium(II) from 1 M HCl solutions was found to occur through mixed (coordination and anion-exchange) mechanism. In the field of dominance of the anion-exchange mechanism of the extraction of palladium(II) with triacylated pentaethylenehexamine the concentration constant of palladium(II) extraction was calculated, and thermodynamic parameters of extraction were evaluated.     

Studies on Pd(II)-catalyzed coupling-cyclization of alpha- or beta-amino allenes with allylic halides

The palladium-catalyzed coupling-cyclization of alpha- or beta-amino allenes with allylic halides leading to 3-allylic 2,5-dihydropyrroles and 1,2,3,6-tetrahydropyridines, respectively, was studied. The starting materials are easily available. The skeletons of both two classes of products were established by the X-ray diffraction studies of 7i and 9b. Through the study of the reaction of 2b with 3-chloro-1-butene, 1-chloro-2-butene, and pi-allyl palladium species and the stereochemical outcome of the coupling cyclization of (S)-2m and (R)-2n, it is believed that the current transformation most likely proceeded via a Pd(II)-catalyzed pathway, although a Pd(0) pathway cannot be completely excluded.     

Synthesis and evaluation of isourea-type glycomimetics related to the indolizidine and trehazolin glycosidase inhibitor families

A practical synthesis of reducing isourea-derived azasugar glycomimetics related to the indolizidine and trehazolin glycosidase inhibitor families with different pK(a) values is disclosed. The polyhydroxylated bicyclic system was built from readily accessible hexofuranose derivatives through a synthetic scheme that involves the preparation of a 5-deoxy-5-carbodiimido adduct by triphenylphosphine-mediated tandem Staudinger--aza-Wittig-type coupling of azide and isothiocyanate precursors, intramolecular cyclization of a transient vic-hydroxycarbodiimide derivative, and nucleophilic addition of the endocyclic nitrogen atom of the generated 2-amino-2-oxazoline intermediate, with a pseudo-C-nucleoside structure, to the masked aldehyde group of the monosaccharide. The last step is pH-dependent so that the final compounds can pivot between the furanose and the 2-oxaindolizidine forms. Nevertheless, the indolizidine tautomer having the R configuration at the aminoacetalic center, fitting the anomeric effect, was the only species detected in solution at neutral or slightly acidic pH when starting from solutions at basic pH. Glycosidase inhibition tests (K(i) values down to 1.9 microM) showed a marked dependence of the selectivity and potency toward alpha- and beta-glucosidases upon the nature of the substituent at the exocyclic isourea nitrogen, shifting from alpha- to beta-selectivity when going from hydrophilic to hydrophobic substituents. Enzyme inhibition is also pH dependent, supporting a dominant role for the uncharged form of the polyhydroxyiminoindolizidine system in the inhibition of beta-glucosidases.     

Co-precipitation of metal dimethylglyoximates generated from biacetyl monoxime or biacetyl and hydroxylamine

A study has been made of some aspects of the co-precipitation of gold and platinum(II) and (IV) with palladium(II) precipitated from homogeneous solution initially containing biacetyl and hydroxylamine in 0.3M hydrochloric acid. The co-precipitation of nickel(II) with palladium(II) and vice versa from near neutral solutions initially containing biacetyl and hydroxylamine was also explored; the solid is enriched in the minor constituent in both cases. As with biacetyl monoxime-hydroxylamine systems, both nickel(II) and palladium(II) promote formation of dimethylglyoxime from such solutions between pH 7 and 8. Factors leading to this unusual behaviour in co-precipitation are discussed and a qualitative explanation tentatively advanced. The co-precipitation of nickel and palladium(II) is different when biacetyl monoxime solutions are used; the former comes out of solution faster than the latter irrespective of their concentration ratio. This disparity in the reaction rates can be used to separate nickel directly from palladium(II) by precipitation or extraction of the dimethylglyoximate into chloroform.     

Reduction of palladium(II) monoglycinate complexes at rotating disc palladium electrode in acid media

Reduction of palladium(II) glycinate complexes in strongly acid 0.5 M NaClO₄ solutions (pH 0.6 and 1.0) with variable palladium(II) complex and free glycine concentration was studied by the taking of cyclic voltammograms at palladium rotating disc electrode. It is shown that it was a chelate monoglycinate palladium(II) complex that was present in all studied solutions and underwent the reduction. The diffusion coefficient of the chelate monoglycinate palladium(II) complex D = (6.5 ± 0.5) × 10⁻⁶ cm²/s was determined from the limiting diffusion current of the complex reduction. The monoglycinate palladium(II) complex reduction occurred in the double-layer segment of the palladium charging curve; it was not complicated by hydrogen adsorption at electrodes. The palladium(II) complex reduction half-wave potential was determined (E _1/2 = ∼0.300 to 0.330 V (SCE)). It is shown that the decreasing of the number of ligands coordinated by palladium via nitrogen atom facilitates the complex reduction process. In particular, the reduction potentials of palladium(II) complexes with different ligand number at palladium electrode shifted markedly toward negative potentials in the series: Pdgly⁺ < Pd(gly)2 < Pd(gly)₄²⁻.     

Insertion of molecular oxygen into a palladium(II) hydride bond

Insertion of molecular oxygen into a palladium(II) hydride bond to form an (eta1-hydroperoxo)palladium(II) complex is reported. The hydroperoxo palladium(II) product has been crystallographically characterized. A second-order rate law (first-order in palladium and first-order in oxygen) is observed for the reaction and a large kinetic isotope effect implicates Pd-H bond cleavage in the rate-determining step. The results of studies with radical inhibitors and light suggest that the reaction does not proceed by a radical chain mechanism.     

Polystyrene impregnated with beta-diphenylglyoxime, a selective reagent for palladium

alpha- and beta-diphenylglyoximes impregnated on macroreticular polystyrene (XAD-2) were tested as adsorptive reagents and one of them (the beta-isomer) was found to adsorb palladium in preference to platinum (selectivity factor 20) and nickel (factor > 100).     

Interaction of Dinitrogentrioxide with Platinum(0) and Palladium(0) Complexes

Dinitrogentrioxide reacts with tetrakis(triphenylphosphine) platinum and palladium under nitrogen to give dinitrobis(triphenylphosphine) platinum(II) and palladium(II) complexes, respectively. In the presence of oxygen these reactions afford the formation of nitro-nitrato complexes of platinum(II) and palladium(II). The products are characterized by the elemental analyses, i.r. spectra, conductivity and magnetic measurements.     

Spectrophotometric determination of palladium(II) based on its catalytic effect on the reduction of azure I by sodium hypophosphite

A catalytic-spectrophotometric method for the determination of traces of palladium(II) is proposed. The reaction is based on the catalytic action of palladium(II) on the reduction of azure I (λ_max = 647 nm) by sodium hypophosphite. The various variables affecting the sensitivity were studied, and a study of interfering ions was also carried out. The reaction gave a detection limit of 4.3 ng/mL palladium(II) and good reproducibility with a relative standard deviation of 1.53–1.98% in the palladium(II) concentration range 40–200 ng/mL. The method yielded another linear range (5–40 ng/mL) when using slightly different conditions. In this case, the detection limit was 0.78 ng/mL palladium(II), and the relative standard deviation for ten replicate analyses of 20 ng/mL palladium(II) was 2.05%. The method was applied to the determination of palladium in a sample of activated charcoal. The text was submitted by the authors in English.     

Flow-Injection Study and Analytical Applications of the Reactions of Palladium(II) and Platinum(IV) with Tin(II) Chloride in Hydrochloric Acid Solutions

The interaction of palladium(II) and platinum(II) with tin(II) chloride in hydrochloric acid solutions was studied by flow-injection (FI) spectrophotometry. It was found using kinetic measurements in the stopped flow mode that the composition of detected products and the rate of their formation depend on the concentrations of tin(II) and chloride ions in the reaction zone and on the acidity of the solution. Optimal FI conditions were found, and the selectivity of interaction of palladium(II) with tin(II) chloride was estimated for the detection of the signal at 407 nm (yellow form) and 646 nm (green form). It was demonstrated that the reaction of the formation of yellow platinum(IV) complexes is slower than that for palladium(II), especially at rather low concentrations of hydrochloric acid in the reaction flow. Based on the detection of green complexes of palladium(II) with tin(II) chloride, a flow injection method was proposed for the selective spectrophotometric determination of palladium(II) in the presence of other platinum-group metals. The height of the recorded peak is directly proportional to the concentration of palladium(II) in the injected solution in the range of 0.028–0.300 mM. The method was used for the analysis of pharmaceuticals and industrial catalysts.     

Electrochemical behaviour ofcis-dichloro(arylisocyanide)(tertiaryphosphine) platinum (II) and palladium(II) complexes in an aprotic solvent

Summary The electrochemical behaviour of neutral platinum(II) and palladium(II) isocyanide complexes has been investigated in an aprotic medium at platinum and mercury electrodes. Platinum(II) derivatives are reduced to platinum(0) species, Palladium(II) compounds give rise to palladium(0) species at room temperature, while at 0° it is possible to obtain palladium(I) compounds.     

Oscillopolarography of Palladium(II)–Tropeolin 0 Complexes

The electrochemical behavior of palladium(II) complexes of Tropeolin 0 (Tr0) was studied by linear-potential-sweep voltammetry in acetate–ammonia buffer solutions. The optimum conditions for determining palladium(II) with Tr0 were found. The composition of the complex was found to be Pd : Tr0 = 1 : 2. A procedure was proposed for determining palladium(II) with a detection limit of 2.54 × 10^–7M. The procedure was used for determining palladium(II) in capacitors.     

Palladium(II) Extraction by Pyridinecarboxylic Acid Esters

The commercial extractant Acorga CLX-50 and model individual di-2-ethylhexyl pyridine-3,5-dicarboxylate and 2-ethylhexyl pyridine-3-carboxylate in toluene were used for palladium(II) extraction from aqueous HCl solutions. The studies of extraction rate and equilibrium were carried out in systems containing palladium(II) ions in 3.0, 0.1, and 0.1M HCl in the presence of 0.5M sodium chloride and in 0.1M HCl in the presence of 0.1–6.0M lithium chloride and in 0.1M HCl in the presence 0.1–3.5M sodium nitrate. The examined extractants can efficiently extract palladium(II) from aqueous hydrochloric acid and nitrate solutions. The extraction is slow and equilibrium is obtained after 2 hours. The best extraction of palladium(II) is observed from 0.1M HCl solution in the presence of 3.5M sodium nitrate. A spontaneous transfer of palladium(II) to the toluene phase without any phase mixing is also observed.