Thermal analytical and infrared spectroscopic investigations on polymorphic organic compounds-II

At room temperature, three of the six polymorphic compounds described are not present in their highest melting-point form, owing to enantiotropy. These three are triphenyltin chloride (3 modifications), 1-(4,4-dimethyl-2-oxo-3-tetrahydrofuryl)thiourea (4 modifications), and 2-aminoethyl diphenylborinate (3 modifications). In the case of 4-amino-1-(2-phenylethyl)-1,2,4-triazoline-5-thione, in addition to the absolutely stable modificationI, a further six unstable crystal forms have been observed. The three modifications of,-dimethyl-[1,1-biphenyl]-4-methanol are also monotropic in character. 2,9-Dimethyl-1,10-phenanthroline, the three polymorphic forms of which are unstable, is a special case, since it absorbs water from the air and changes into a semi-hydrate.     

Principle of a new immunoassay based on electrophoretic mobility of poly(styrene/alpha-tert-butoxy-omega-vinylbenzyl-polyglycidol) microspheres: application for the determination of helicobacter pylori IgG in blood serum

The principle of a novel latex diagnostic test for the determination of antibodies against Helicobacter pylori in blood sera is described. The test is based on the measurement of the electrophoretic mobility of the microspheres with immobilized H. pylori antigens. The electrophoretic mobility of these microspheres depends on the concentration of the antibodies against H. pylori in suspending medium. Particles with hydrophilic polyglycidol in the surface layer were used for the test. The microspheres were obtained by an emulsifier-free emulsion copolymerization of styrene and alpha-tert-butoxy-omega-vinylbenzyl-polyglycidol macromonomer (D(n) = 220 nm, diameter polydispersity factor D(w)//D(n) = 1.02). Activation of polyglycidol hydroxyl groups with cyanuric chloride allowed for covalent immobilization of H. pylori antigens. The fraction of H. pylori not specifically adsorbed onto the microspheres was negligible. Changes of the electrophoretic mobility of the microspheres with the surface concentration of the covalently immobilized H. pylori antigens Gamma = (1.6 +/- 0.3) . 10(-3) g m(-2) were suitable for the detection of the antibodies in the sera of patients with titer in the range (determined by the indirect ELISA test) from 1:500 to 1:32 000.     

Zinc-catalyzed Williamson ether synthesis in the absence of base

Zinc powder was found to be a highly efficient catalyst for the synthesis of aromatic ethers using microwave heating in the presence of N,N-dimethylformamide as well as under stirring in an oil-bath using tetrahydrofuran as solvent without any inorganic base. This method can be used for selective mono-, di- or tri-O-alkylations.     

1,3-dialkyl- and 1,3-diaryl-3,4,5,6-tetrahydropyrimidin-2-ylidene rhodium(i) and palladium(II) complexes: synthesis, structure, and reactivity

The synthesis of novel 1,3-diaryl- and 1,3-dialkylpyrimidin-2-ylidene-based N-heterocyclic carbenes (NHCs) and their rhodium(i) and palladium(II) complexes is described. The rhodium compounds bromo(cod)[1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene]rhodium (7), bromo(cod)(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)rhodium (8) (cod=eta(4)-1,5-cyclooctadiene, mesityl=2,4,6-trimethylphenyl), chloro(cod)(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)rhodium (9), and chloro(cod)[1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene]rhodium (10) were prepared by reaction of [[Rh(cod)Cl](2)] with lithium tert-butoxide followed by addition of 1,3-dimesityl-3,4,5,6-tetrahydropyrimidinium bromide (3), 1,3-dimesityl-3,4,5,6-tetrahydropyrimidinium tetrafluoroborate (4), 1,3-di-2-propyl-3,4,5,6-tetrahydropyrimidinium bromide (6), and 1,3-di-2-propyl-3,4,5,6-tetrahydropyrimidinium tetrafluoroborate, respectively. Complex 7 crystallizes in the monoclinic space group P2(1)/n, and 8 in the monoclinic space group P2(1). Complexes 9 and 10 were used for the synthesis of the corresponding dicarbonyl complexes dicarbonylchloro(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)rhodium (11), and dicarbonylchloro[1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene]rhodium (12). The wavenumbers nu(CO I)/nu(CO II) for 11 and 12 were used as a quantitative measure for the basicity of the NHC ligand. The values of 2062/1976 and 2063/1982 cm(-1), respectively, indicate that the new NHCs are among the most basic cyclic ligands reported so far. Compounds 3 and 6 were additionally converted to the corresponding cationic silver(i) bis-NHC complexes [Ag(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(2)]AgBr(2) (13) and [Ag[1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene](2)]AgBr(2) (14), which were subsequently used in transmetalation reactions for the synthesis of the corresponding palladium(II) complexes Pd(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(2) (2+)(Ag(2)Br(2)Cl(4) (4-))(1/2) (15) and Pd[1,3-bis(2-propyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene)(2)]Cl(2) (16). Complex 15 crystallizes in the monoclinic space group P2(1)/c, and 16 in the monoclinic space group C(2)/c. The catalytic activity of 15 and 16 in Heck-type reactions was studied in detail. Both compounds are highly active in the coupling of aliphatic and aromatic vinyl compounds with aryl bromides and chlorides with turnover numbers (TONs) up to 2000000. Stabilities of 15 and 16 under Heck-couplings conditions were correlated with their molecular structure. Finally, selected kinetic data for these couplings are presented.     

Synthesis of TiO2 nanoparticles on the Au(111) surface

The growth of titanium oxide nanoparticles on reconstructed Au(111) was investigated by scanning tunneling microscopy and x-ray photoelectron spectroscopy. Ti was deposited by physical-vapor deposition at 300 K. Regular arrays of titanium nanoparticles form by preferential nucleation of Ti at the elbow sites of the herringbone reconstruction. The titanium oxide nanoclusters were synthesized by subsequent exposure to O(2) at 300 K. Two-and three-dimensional titanium oxide nanocrystallites form during annealing in the temperature range from 600 to 900 K. At the same time, the Au(111) surface assumes a serrated 110-oriented step-edge morphology suggesting step-edge pinning by titanium oxide nanoparticles. The oxidation state of the titanium oxide nanoparticles varies with annealing temperature. Specifically, annealing to 900 K results in the formation of stoichiometric TiO(2) nanocrystals as judged by the Ti(2p) binding energies measured in the x-ray photoelectron data. The nanodispersed TiO(2) on Au(111) is an ideal system to test the various models proposed for the enhanced catalytic reactivity of supported Au nanoparticles.     

Use of FTIR spectroscopy in the investigation of catalytic reactions in zeolites—Possibilities and limitations

FTIR-spectroscopic investigations of catalytic reactions yield detailed information about the interaction of adsorbed molecules with the catalyst and kinetic data of the surface reaction, if the participating molecules show vibrations whose position and intensity in the IR-spectrum depend on the sorption state and the quantity adsorbed. In this paper, the possibilities and limitations of the method are represented by two examples.     

Tale of the Breslow intermediate,a central player in N-heterocyclic carbene organocatalysis: then and now

N-Heterocyclic carbenes (NHCs) belong to the popular family of organocatalysts used in a wide range of reactions, including that for the synthesis of complex natural products and biologically active compounds. In their organocatalytic manifestation, NHCs are known to impart umpolung reactivity to aldehydes and ketones, which are then exploited in the generation of homoenolate, acyl anion, and enolate equivalents suitable for a plethora of reactions such as annulation, benzoin, Stetter, Claisen rearrangement, cycloaddition, and C–C and C–H bond functionalization reactions and so on. A common thread that runs through these NHC catalyzed reactions is the proposed involvement of an enaminol, also known as the Breslow intermediate, formed by the nucleophilic addition of an NHC to a carbonyl group of a suitable electrophile. In the emerging years of NHC catalysis, enaminol remained elusive and was largely considered a putative intermediate owing to the difficulties encountered in its isolation and characterization. However, in the last decade, synergistic efforts utilizing an array of computational and experimental techniques have helped in gaining important insights into the formation and characterization of Breslow intermediates. Computational studies have suggested that a direct 1,2-proton transfer within the initial zwitterionic intermediate, generated by the action of an NHC on the carbonyl carbon, is energetically prohibitive and hence the participation of other species capable of promoting an assisted proton transfer is more likely. The proton transfer assisted by additives (such as acids, bases, other species, or even a solvent) was found to ease the kinetics of formation of Breslow intermediates. These important details on the formation, in situ detection, isolation, and characterization of the Breslow intermediate are scattered over a series of reports spanning well over a decade, and we intend to consolidate them in this review and provide a critical assessment of these developments. Given the central role of the Breslow intermediate in organocatalytic reactions, this treatise is expected to serve as a valuable source of knowledge on the same.

Molecular insights on the formation, detection, and even isolation of the Breslow intermediate, which is the most important species in N-heterocyclic carbene (NHC) catalysis, as obtained from experimental and computational studies, are presented.     

Rhodium complexes with dioximes as catalysts of hydroformylation and hydrogenation of 1-hexene

Rhodium(II) complexes with dioximes [Rh(Hdmg)₂(PPh₃)]₂ [I] (Hdmg=monoanion of dimethylglyoxime) and [Rh(Hdmg)(ClZndmg)(PPh₃)]₂ [II] catalyse hydroformylation and hydrogenation reactions of 1-hexene at 1 MPa CO/H₂ and 0.5 MPa H₂ at 353 K, respectively. Hydroformylation with complex [I] produces 94% of aldehydes (n/iso=2.2) and 6% 2-hexene whereas the second catalyst [II] gives ca. 40% of aldehydes (n/iso=2.1) and 60% of 2-hexene. Corresponding Rh(III) complexes are inactive in hydroformylation except of RhH(Hdmg)₂(PPh₃) [III], which shows activity similar to [I]. Complexes [Rh(Hdmg)₂(PPh₃)]₂ [I], [Rh(Hdmg)(ClZndmg)(PPh₃)]₂ [II], RhH(Hdmg)₂(PPh₃) [III] and [Rh(Hdmg)₂(PPh₃)₂]ClO₄ [V] catalyse 1-hexene hydrogenation with an average TON ca. 18 cycles/mol [Rh]×min. Complex [II] has also been found to catalyse hydrogenation of cyclohexene, 1,3-cyclohexadiene and styrene.     

Tamarindus indica mucilage and its acrylamide-grafted copolymer as flocculants for removal of dyes

The application of Tamarindus indica seed mucilage (Tam), a food grade polysaccharide, and its acrylamide grafted copolymer as flocculants was assessed for the first time for removal of various types of dyes from model textile wastewater containing azo, basic, and reactive dyes. Acrylamide grafted T. indica mucilage (Tam-g-PAM) was obtained by ceric ion initiated polymerization technique. A series of contact time experiments were conducted to assess the system variables such as concentrations of mucilage and dyes and pH. These flocculants reduce the dye concentration by flocculation and settling. The grafted copolymer, Tam-g-PAM showed better results for dye removal. The optimal flocculant concentration, which was required to effect flocculation, was independent of dye concentration within the range examined. Both the flocculants performed better for removal of azo dyes than for reactive and basic dyes.     

Bioaccumulation study of acrylates in algae (<Emphasis Type="Italic">Chlorella kessleri</Emphasis>) by Py-GC and Py-GC/MS

Acrylate monomers methylmethacrylate (MMA) and cyclohexylmethacrylate (CHMA) bioaccumulation has been determined in aquatic organism, algae (Chlorella kessleri). Algae were collected in amount of 0.4 mg and directly injected to the pyrolytical cell. In algae bodies accumulated monomers were analysed by pyrolysis gas chromatography (Py-GC) and pyrolysis gas chromatography coupled with mass spectrometry (Py-GC/MS). Traces of the accumulated monomers in algae body can be determined after 1-, 2-, 3-weeks of incubation. Maximum content of MMA was determined after 3-week of experiment, contrariwise in the case of CHMA after 2-week exposition. Relationship with pyrolysis temperature has also been studied.