Heats of solution and neutralization ofLewis acids and bases in acetic anhydride

Heats solution of someLewis acids and bases in acetic anhydride have been determined and the following order of their relative strengths is proposed: SbCl₅>SO₃>SnCl₄>TiCl₄>AsCl₃ and piperidine> n-butylamine>potassium acetate>sodium acetate -picoline>quinoline. Heats neutralization of theseLewis acids and bases in acetic anhydride suggest that the major enthalpy change in these neutralization reactions is due to the combination of a proton and the (CH₂COOCOCH₃)-ion, resulting in the formation of acetic anhydride.

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Lösungs- und Neutralisationswärmen von Lewis-Säuren und-Basen in Essigsäureanhydrid

Zusammenfassung Es wurden die Lösungswärmen einigerLewis-Säuren in Essigsäureanhydrid bestimmt und folgende Reihung nach ihrer relativen Stärke vorgeschlagen: SbCl₅>SO₃>SnCl₄>TiCl₄>AsCl₃ und Piperidin> n-Butylamin>KAc>NaAc-Picolin>Chinolin. Die Neutralisationswärmen dieserLewis-Säuren und-Basen legen nahe, daß der Hauptanteil daran auf die Reaktion eines Protons mit (CH₂–COOCOCH₃)-zurückzuführen ist.

     

Stabilization of platinum(II) hydrides by tri-t-butylarsine; facile syntheses of stable trans-PtH2[As(t-Bu)3]2 and trans-PtHX[As(t-Bu)3]2 complexes

trans-PtH₂[As(t-Bu)₃]₂ was prepared in very good yield by afacile reaction of K₂PtC1₄ with As(t-Bu)₃ in alkaline ethanol. Treatment of trans-PtH₂[As(t-Bu)₃]₂ with CF₃CO₂H or HCI afforded trans-PtH(O₂CCF₃)[As(t-Bu)₃]₂ or trans-PtHCl[As(t-Bu)₃]₂. respectively, in almost quantitative yield.     

Synthesis of polyhydroquinoline derivatives through unsymmetric Hantzsch reaction using organocatalysts

A novel and green approach for efficient and rapid synthesis of polyhydroquinoline derivatives via unsymmetric Hantzsch reaction using organocatalysts at room temperature was reported. The process is a simple, environmentally friendly, rapid, and high yielding reaction for the synthesis of polyhydroquinoline derivatives. The catalytic efficiency of various small organocatalysts such as l-proline, trans-4-hydroxy-l-proline, l-thiaproline, dl-phenylglycine, and (−)-cinchonidine was studied under aqueous, organic, and solvent free conditions.     

Organic phase conversion of bulk (wurtzite) ZnO to nanophase (wurtzite and zinc blende) ZnO

We describe the all-organic phase conversion of bulk commercial ZnO in the wurtzite modification to sub-30 nm ZnO that we find to be partially in the zinc blende [, a=4.568(3) Å] modification. The conversion involves refluxing ZnO in 2,4-pentanedione (acetylacetone) at 413 K to form the zinc 2,4-pentanedionate, which is decomposed by heating at 573 K in an appropriate high-temperature solvent such as dibenzylether to form nanophase ZnO. This nanophase, partially zinc blende ZnO can also be obtained in a single step by heating commercial zinc 2,4-pentanedionate in refluxing dibenzylether. Thermodiffractometry suggests that the conversion of zinc blende ZnO to wurtzite ZnO commences near 650 K.     

Some compounds of iron(III) with bidentate bases

Summary The iron(III) compound, FeCl(OH)(MeCO₂) · MeCO₂H · 0.5 H₂O, (1), interacts with the sample bidentate ligand, 2,2-bipyridyl (bipy), to form an aduct, Fe₂Cl₂(OH)₂(MeCO₂)₂ · (bipy). By the loss of protons, anthranilic (anthH) and salicylic (salH₂) acids behave as uninegative charged ligands to give FeCl(anth)₂ · H₂O and FcCl(salH)₂ · H₂O, respectively. The former decomposes on heatingin vacuo to form Fe₂O(anth)₄ while the latter yields Fe₂O(sal)₂. Acetylacetone (acacH) and 8-hydroxyquinoline (quinH) give FeCl(acac)₂ and FeCl(quin)₂ · (quinH). The latter desolvates at 140°/10^–4 torr to form FeCl(quin)₂. l.r. spectra, thermal decomposition, molar conductance and magnetic susceptibility studies at room temperature have been used to characterize these compounds.     

Heterogeneous colorimetric sensor for mercuric salts

A novel chemical sensor for the colorimetric detection of mercuric salts is described. The sensor is based on a mesoporous nanocrystalline TiO2 film sensitised with a ruthenium dye; immersion of this film in an aqueous solution of Hg2+ results in a rapid colorimetric response, with both a high selectivity and a sub-micromolar sensitivity.     

Reversible colorimetric probes for mercury sensing

The selectivity and sensitivity of two colorimetric sensors based on the ruthenium complexes N719 [bis(2,2'-bipyridyl-4,4'-dicarboxylate)ruthenium(II) bis(tetrabutylammonium) bis(thiocyanate)] and N749 [(2,2':6',2' '-terpyridine-4,4',4' '-tricarboxylate)ruthenium(II) tris(tetrabutylammonium) tris(isothiocyanate)] are described. It was found that mercury ions coordinate reversibly to the sulfur atom of the dyes' NCS groups. This interaction induces a color change in the dyes at submicromolar concentrations of mercury. Furthermore, the color change of these dyes is selective for mercury(II) when compared with other ions such as lead(II), cadmium(II), zinc(II), or iron(II). The detection limit for mercury(II) ions--using UV-vis spectroscopy--in homogeneous aqueous solutions is estimated to be approximately 20 ppb for N719 and approximately 150 ppb for N749. Moreover, the sensor molecules can be adsorbed onto high-surface-area mesoporous metal oxide films, allowing reversible heterogeneous sensing of mercury ions in aqueous solution. The results shown herein have important implications in the development of new reversible colorimetric sensors for the fast, easy, and selective detection and monitoring of mercuric ions in aqueous solutions.     

Totally selective ring-opening of amino epoxides with ketones: a general entry to enantiopure (2R,3S)- and (2S,3S)-3-aminoalkano-1,2-diols

[Reaction: see text] Transformation of enantiopure diastereoisomers (2R,1'S)- and (2S,1'S)-2-(1-aminoalkyl)epoxides into the corresponding 4-(1-aminoalkyl)-1,3-dioxolanes is achieved by reaction with different ketones in the presence of BF3.Et2O. The conversion takes place in very high yields, total selectivity, and without epimerization. A mechanism to explain this transformation is proposed. The obtained 1,3-dioxolanes can be deprotected, and (2R,3S)- and (2S,3S)-3-aminoalkano-1,2-diols were isolated.     

Synthesis, structure and conformation of partially-modified retro- and retro-inverso psi[NHCH(CF3)]Gly peptides

Partially modified retro- (PMR) and retro-inverso (PMRI) psi[NHCH(CF(3))]Gly peptides, a conceptually new class of peptidomimetics, have been synthesized in wide structural diversity and variable length by aza-Michael reaction of enantiomerically pure alpha-amino esters and peptides with enantiomerically and geometrically pure N-4,4,4-trifluorocrotonoyl-oxazolidin-2-ones. The factors underlying the observed moderate to good diastereocontrol have been investigated. The conformations of model PMR-psi[NHCH(CF(3))]Gly tripeptides have been studied in solution by (1)H NMR spectroscopy supported by MD calculations, as well as in the solid-state by X-ray diffraction. Remarkable stability of turn-like conformations, comparable to that of parent malonyl-based retropeptides, was evidenced, as a likely consequence of two main factors: 1) severe torsional restrictions about sp(3) bonds in the [CO-CH(2)-CH(CF(3))-NH-CH(R)-CO] module, which is biased by the stereoelectronically demanding CF(3) group and the R side chain; 2) formation of nine-membered intramolecularly hydrogen-bonded rings, which have been clearly detected both in CHCl(3) solution and in some crystal structures. The former factor seems to be more important, as turn-like conformations were found in the solid-state even in the absence of intramolecular hydrogen bonding. The relative configuration of the -C*H(CF(3))NHC*H(R)- stereogenic centers has a major effect on the stability of the turn-like conformation, which seems to require a syn stereochemistry. X-ray diffraction and ab initio computational studies showed that the [-CH(CF(3))NH-] group can be seen as a sort of hybrid between a peptide bond mimic and a proteolytic transition state analogue, as it combines some of the properties of a peptidyl -CONH- group (low NH basicity, CH(CF(3))-NH-CH backbone angle close to 120 degrees, C-CF(3) bond substantially isopolar with the C=O) with some others of the tetrahedral intermediate [-C(OX)(O(-))NH-] involved in the protease-mediated hydrolysis reaction of a peptide bond (high electron density on the CF(3) group, tetrahedral backbone carbon).     

Studies on electroosmotic and electrophoretic behavior of cholesterol in aqueous solutions

Electrical properties of cholesterol interfaces have been investigated. For this purpose electroosmotic and hydrodynamic permeation of water, sodium chloride, barium chloride, aluminum chloride, and urea solutions across a cholesterol plug have been investigated. Dependence of electroosmotic permeability on concentration of electrolytes has also been studied. Electrophoretic mobility of cholesterol particles of known size distribution and dispersed in solutions of varying concentrations of electrolytes and urea has also been studied. The data have been used to estimate ζ potentials in order to have a plausible picture of the electrical double layer at the cholesterol/ solution interfaces.