Polycondensation and disproportionation of an oligosiloxanol in the presence of a superbase

Kinetics of reactions of model oligosiloxanols, 1,1,3,3,3-pentamethyldisiloxane-1-ol (MDH) and 1,1,3,3,5,5,5-heptamethyltrisiloxane-1-ol (MD₂H), which occur in the presence of phosphazenium superbase, hexapyrrolidine-diphosphazenium hydroxide, in an acid-base inert solvent, toluene, was studied using sampling and gas chromatographic analysis method. In addition, kinetics of reactions of MDH and MD₂H with trimethylsilanol (MH) was studied. In the MDH and MD₂H systems the oligosiloxanol condensation competes with the oligosiloxanol disproportionation, the latter being the dominating process. The disproportionation products, i.e. MD_n+1H and MD_n−1H, n=1, 2, … undergo analogous consecutive disproportionation and condensation reactions. The kinetic law was derived and rate parameters determined from initial rates and by computer simulation to the best agreement with experimental data. Both competing reactions, the disproportionation and the condensation, conform to the same general kinetic law being first internal order in substrate and first order in catalyst. Activation parameters of these reactions were determined. The results were interpreted in terms of a bimolecular mechanism in which nucleophilic attack of the silanolate anion directed to silicon of the silanol group causes the cleavage of one of its geminal bonds to oxygen, either the one to hydroxyl leading to condensation or the one to siloxane which leads to disproportionation. The latter is faster as the silanolate is a better leaving group compared with OH⁻. Moreover, in the pentacoordinate silicon transition state (or intermediate) the siloxane substituent preferentially enters the apical position, thus driving the OH substituent into the unreactive equatorial position.     

Kinetics and mechanism of oxidation of α,β-unsaturated alcohols by manganese(III) acetate in aqueous sulphuric acid medium

Summary The kinetics of oxidation of ,-unsaturated alcohols (UA's), such as prop-2-ene-1-ol, but-2-ene-1-ol and 3-phenyl-prop-2-ene-1-ol, by manganese(III) acetate in aqueous H₂SO₄ at constant ionic strength and different acidities has been studied. The reaction was found to proceed through an outer sphere mechanism. The reactions were first order with respect to [Mn^III] and fractional order in [UA]. The reaction showed first order dependence in [H⁺], and the rate decreased on addition of [Mn^II]. Added salts, such as Na₂SO₄, had a negligible effect on the rate. The data suggested that disproportionation of the Mn^III-UA complex into free radicals was the rate determining step in the presence of [Mn^II]. A mechanism consistent with the experimental data is proposed. The activation parameters have been evaluated for the temperature range 298–313 K.     

Protic Anions [H(B12X12)]− (X=F,Cl, Br,I) That Act as Brønsted Acids in the Gas Phase

The acidity of protic cations and neutral molecules has been studied extensively in the gas phase, and the gas‐phase acidity has been established previously as a very useful measure of the intrinsic acidity of neutral and cationic compounds. However, no data for any anionic acids were available prior to this study. The protic anions [H(B₁₂X₁₂)]^? (X=F, Cl, Br, I) are expected to be the most acidic anions known to date. Therefore, they were investigated in this study with respect to their ability to protonate neutral molecules in the gas phase by using a combination of mass spectrometry and quantum‐chemical calculations. For the first time it was shown that in the gas phase protic anions are also able to protonate neutral molecules and thus act as Brønsted acids. According to theoretical calculations, [H(B₁₂I₁₂)]^? is the most acidic gas‐phase anion, whereas in actual protonation experiments [H(B₁₂Cl₁₂)]^? is the most potent gas‐phase acidic anion for the protonation of neutral molecules. This discrepancy is explained by ion pairing and kinetic effects.     

Condensation polymerization of oligomeric polydimethylsiloxanols in aqueous emulsion

Polydimethylsiloxane oligomers with silanol ends condense in aqueous dispersion under mild conditions to form high polymers in the presence of sulfonic acid surfactants. The process follows a second-order rate law in silanol if reversibility is taken into account. The second-order rate constant is proportional to the area at the oil–water interface and is a complex function of surfactant concentration. The principal driving force is the heat of condensation of the water produced in the polymerization. A mechanism paralleling surface catalysis is offered in which a termolecular complex that consists of two surfactant molecules and one silanol end group reacts bimolecularly at the oil–water interface.     

Synthesis and reaction of Schiff base 4-(pyridin-3-ylmethylimino)-pent-2-en-2-ol with FeCl3

A potentially tridentate Schiff base 4-(pyridin-3-ylmethylimino)-pent-2-en-2-ol was synthesized in the condensation reaction of 3-picolylamine and acetylacetone. The compound was characterized by ¹H,¹³C-NMR and IR spectra. The reaction of Schiff base with Fe(III) in the acetone solution and the presence of pyridine led to its hydrolysis and the formation of the octahedral complex [Fe(acac)Cl₂(py)₂]. The structure of the complex was determined by the single crystal X-ray diffraction analysis. The magnetic properties and the molar conductivity of the complex compound were also derived.     

Gas phase amination of octan-1-ol over a cu-cr catalyst

Summary The reductive amination of octan-1-ol by ammonia in the gas phase has been studied. From the shape of the conversion curve it was shown that in a certain conversion interval the amount of secondary and tertiary amines is higher than the equilibrium composition of disproportionation reactions.     

Understanding the Synthesis of Supported Vanadium Oxide Catalysts Using Chemical Grafting

The complexity of variables during incipient wetness impregnation synthesis of supported metal oxides precludes an in-depth understanding of the chemical reactions governing the formation of the dispersed oxide sites. This contribution describes the use of vapor phase deposition chemistry (also known as grafting) as a tool to systematically investigate the influence of isopropanol solvent on VO(OⁱPr)₃ anchoring during synthesis of vanadium oxide on silica. The availability of anchoring sites on silica was found to depend not only on the pretreatment of the silica but also on the solvent present. H-bond donors can reduce the reactivity of isolated silanols whereas disruption of silanol nests by H-bond acceptors can turn unreactive H-bonded silanols into reactive anchoring sites. The model suggested here can inform improved syntheses with increased dispersion of metal oxides on silica.     

ChemInform Abstract: Protic Anions [H(B12X12)]‐ (X: F,Cl, Br,I) that Act as Broensted Acids in the Gas Phase.

The protic anions [H(B₁₂X₁₂)]^‐ (X: F, Cl, Br, I) are investigated with respect to their ability to protonate neutral molecules in the gas phase by using a combination of tandem mass spectrometry and quantum‐chemical calculations.     

2,3-Epoxyperfluoro-2-methylpentane in reactions with urea and thiourea

2,3-Epoxyperfluoro-2-methylpentane reacts with thiourea in protic (methanol, 2-propanol) and aprotic (dioxane) solvents, and also with urea in acetonitrile affording unexpected products: 1-(2,2,3,3,3-pentafluoropropionyl)-2-(2,2,2-trifluoro-1-trifluoro-methylethyl)isothiourea, and 1-(2,2,3,3,3-pentafluoropropionyl)-3-(2,2,2-trifluoro-1-trifluoro-methylethyl)urea respectively that result from the rearrangement of the intermediately formed ketone in the process of the intramolecular “haloform” cleavage. At the same time in dioxane the 2,3-epoxyperfluoro-2-methylpentane reacts with urea with the formation of a heterocyclic compound, 2-amino-4-pentafluoroethyl-5,5-bis(trifluoromethyl)-4,5-dihydrooxazol-4-ol. From 1-(2,2,3,3,3-pentafluoropropionyl)-2-(2,2,2-trifluoro-1-trifluoromethylethyl)isothiourea and Cu(OAc)₂ a stable fluorine-containing chelate complex was obtained.     

Complex Formation of Crown Ethers and Cations in Water–Organic Solvent Mixtures: Part XI. Effects of the Preferential Solvation of Benzo-15-Crown-5 and Acid-Base Properties of the Mixtures on the Thermodynamic Functions for Complex Formation of Benzo-15-Crown-5 with Na+ in Propan-1-ol–Water Mixtures at 298.15 K

The thermodynamic functions of complex formation of benzo-15-crown-5 ether (B15C5) and sodium cation (Na⁺) in the mixtures of propan-1-ol (PrOH) with water at 298.15 K have been calculated from experimental measurements. The equilibrium constants of B15C5/Na⁺ complex formation have been determined by conductivity measurements. The enthalpic effect of complex formation has been measured by a calorimetric method. The complexes are enthalpy stabilized but entropy destabilized in the PrOH–H₂O mixtures. The effects of preferential solvation of B15C5 by molecules of the organic solvent, solvation of the sodium cation, as well as the acid-base properties of propan-1-ol–water mixtures on the complex formation processes are discussed.     

Kinetics and Mechanism of the Oxidation of Propan-1-ol and Propan-2-ol by Permanganate in the Absence and Presence of Tween-20 in Perchloric Acid Medium

The kinetics of the oxidation of propan-1-ol and propan-2-ol by KMnO₄ in HClO₄ medium has been studied in absence and presence of Tween-20. In the absence of Tween-20 the reaction is of first order with respect to each of permanganate and H⁺, but of complex order with respect to substrate. The active oxidant species HMnO₄ reacts with the alkanol molecule to form an intermediate complex, which decomposes in the rate-determining step to form the respective product and Mn^v. In the presence of Tween-20, both the oxidant and the substrate are distributed between the aqueous phase and the micellar pseudo-phase and then react. Different kinetic and thermodynamic parameters have been evaluated. Compensation between water structure destruction and substrate–micelle interaction plays an important role in the presence of the surfactant.     

Reduction stereospecifique de complexes aziridinyl-cetones-bromure de zinc

Specific reduction of ketoaziridine is obtained by NaBH₄ in protic solvent. The ketoaziridine must be previously complexed by ZnBr₂. Crystal structure determination shows that the complex is formed by two molecules of ketoaziridine and that no complexation between the nitrogen and the ketogroup is possible.     

Unsymmetrical Pincer‐Type Ruthenium Complex Containing β‐Protic Pyrazole and N‐Heterocyclic Carbene Arms: Comparison of Brønsted Acidity of NH Groups in Second Coordination Sphere

A reaction of a 2‐(imidazol‐1‐yl)methyl‐6‐(pyrazol‐3‐yl)pyridine with [RuCl₂(PPh₃)₃] resulted in tautomerization of the imidazole unit to afford the unsymmetrical pincer‐type ruthenium complex 2 containing a protic pyrazole and N‐heterocyclic carbene (NHC) arms. Deprotonation of 2 with one equivalent of a base led to the formation of the NHC–pyrazolato complex 3 , indicating that the protic NHC arm is less acidic. When 2 was treated with two equivalents of a base under H₂ or in 2‐propanol, the hydrido complex 4 containing protic NHC and pyrazolato groups was obtained through metal–ligand cooperation.     

Catalytic Hydrodehydration of Cyclohexanone,Hydrogenation of 2-Cyclohexen-1-one,and Dehydrogenation of Cyclohexene over a Mo Chloride Cluster with an Octahedral Metal Framework

A silica gel-supported molybdenum halide cluster, (H₃O)₂[(Mo₆Cl₈)Cl₆]·6H₂O (1), developed selective catalytic activity for the condensation of cyclohexanone to cyclohexylbenzene and cyclohex-1-enylbenzene, when it was allowed to react in a stream of helium at 300°C. Halide clusters of Nb, Ta, and W of the same metal framework supported on SiO₂ also catalyzed the condensation at 400°C. However, at 400°C, 1 catalyzed disproportionation, and selectivity increased with increasing temperature, yielding cyclohexene and its dehydrogenation products, 1,3-cyclohexadiene and benzene, and 2-cyclohexen-1-one and its dehydrogenation product, phenol. When the same reaction was performed in a stream of hydrogen above 400°C, hydrodehydration proceeded almost exclusively, producing cyclohexene and its dehydrogenation products. 2-Cyclohexen-1-one was hydrogenated to cyclohexanone under the same reaction conditions utilizing hydrogen molecules, whereas cyclohexene was dehydrogenated via 1,3-cyclohexadiene to benzene. The active site developed on 1 was assumed to be a molybdenum atom, which becomes isoelectronic with the platinum metals by accepting two or more electrons from the ligands.     

Preparation of high temperature stable glass capillary columns coated with PS-090 (20% diphenyl substituted CH3O-terminated polysiloxane) a selective stationary phase for the direct analysis of metal-porphyrin complexes

In this report we describe the coating of delonized glass surfaces with commercially available CH₃O-terminated diphenyl-dimethylpolysiloxane (PS-090). This new type of reactive, high temperature stable stationary phases withstands temperatures up to 430°C. As already reported for OH-terminated polymers, the underlying stabilization process is a condensation of methoxy groups of the phase with surface silanol groups arising from high temperature silylation. The good selectivity of this medium polar coating for substrates bearing π-electron systems is demonstrated by the separation of various metalloporphyrins.     

Thermogravimetric Investigations During the Synthesis of Silica-based MCM-41

MCM-41 material was synthesized starting from hydrogel containing colloidal fumed silica, sodium silicate, cetyltetramethylammonium bromide(CTMABr) as surfactant, and distilled water as solvent. These reactants were mixed to obtain a gel with the following composition: 4SiO₂:1Na₂O:1CTMABr:200H₂O. The hydrogel with pH=14 was hydrothermally treated at100°C, for 4 days. Each day, the pH was measured, and then adjusted to 9.5–10 by using 30%acetic acid solution. Thermogravimetry was the main technique, which was used to monitor the participation of the surfactant on the MCM-41 nanophase, being possible to determine the temperature ranges relative to water desorption as well as the surfactant decomposition and silanol condensation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal analysis of boron complexes containing ligands of the type R-C(CH2OH)3

The thermal behaviours of certain salts of the complex “ethriol-” and “methriolboric” acids have been investigated by means of Derivatograph and IR spectra. It is shown that compounds of this type decompose in three main stages on heating: 1) the crystallization water is lost, 2) the condensation water is lost and dimers are formed containing bridge bonds B₄-O-B₄; 3) up to 600–700? all the organic matter is lost; the residue is the corresponding borate (1 ∶ 1). The possibility of performing a fast full analysis by means of the TG and DTG curves has been demonstrated.     

Synthesis,structure and antitumor activity of the binuclear tetranitrosyl iron complex with 2-mercaptobenzthiazole – the nitric oxide donor (NO)

Single-crystals of binuclear neutral sulfur-nitrosyl iron complex with 2-mercaptobenzthiazole ligand [Fe₂(С₇н₄NS₂)₂(NO)₄] (BTz) have been obtained. From X-ray data, BTz has a centrosymmetric dimeric structure, with the iron atoms being linked through the µ-N-C-S bridge. As follows from mass-spectra of the BTz gas phase, the complex is stable in the crystalline state under normal conditions (ambient temperature and standard humidity) and releases nitric oxide upon heating. Using amperometry, kinetics of NO release by BTz in 1% dimethyl sulfoxide aqueous solution has been studied: release of NO was shown to occur immediately upon dissolution in protic media. BTz inhibited growth of K562 leukemia and SKOV₃ ovarian carcinoma cells in vitro and induced apoptosis via activation caspases 3 and 7.     

Temperature- and Pressure-induced Structural Transition of Binary Clathrate Hydrates

We discover new structure II (sII) hydrate forming agents of two C₄H₈O molecules (2-methyl-2-propen-1-ol and 2-butanone) and report the abnormal structural transition of binary C₄H₈O+CH₄ hydrates between structure I (sI) and sII with varying temperature and pressure conditions. In both (2-methyl-2-propen-1-ol+CH₄) and (2-butanone+CH₄) systems, the phase boundary of the two different hydrate phases (sI and sII) exists at the slope change of the phase-equilibrium curve in the semi-logarithmic plots. We confirm the crystal structures of two hydrates synthesized at low (278 K and 6 MPa) and high (286 K and 15 MPa) temperature and pressure conditions by using high-resolution powder diffraction and Raman spectroscopy. 2-Methyl-2-propen-1-ol and 2-butanone can occupy the large cages of sII hydrate at low temperature and pressure conditions; however, they are excluded from the hydrate phase at high temperature and pressure conditions, resulting in the formation of pure sI CH₄ hydrate.     

Oxetane from A-nor-steroides: 2-alpha, 5-oxido-A-nor-5-alpha-cholestane, 2-beta, 5-oxido-A-nor-5-beta-cholestane and 2-alpha-ethyl-2-beta,2'-oxido-A-nor-5-alpha-cholestane

Treatment of 2β-tosyloxy-A-nor-5α-cholestane-5-ol ( 2 ) with t-butoxide in t-butanol gave 2α, 5-epoxy-A-nor-5α-cholestane ( 3 ) in quantitative yield. When A-nor-5β-cholestane-2α, 5-diol ( 4 ) was treated with tosyl chloride in pyridine 2β-chloro-A-nor-5β-cholestane-5-ol ( 7 ) and 2α-tosyloxy-A-nor-5β-cholestane-5-ol ( 8 ) were obtained. Whereas the chloride 7 was resistant to t-butoxide the tosylate 8 was transformed into an 1 : 1 mixture of 2α, 5-epoxy-5β-cholestane ( 10 ) and 2ξ-t-butoxy-A-nor-5β-cholestane-5-ol ( 11 ). In 2α-tosyloxy-A-nor-5α-cholestane-5-ol ( 12 ) substitution occurred as the only reaction. Both oxetanes 3 and 10 isomerize after heating above 50° and in polar or protic solvents to form A-nor-Δ³⁽⁵⁾-cholestene-2α-ol ( 6 ) and -2β-ol ( 14 ) respectively. Also, 2, 5-diols are encountered. 2α-Ethyl-2β, 2′-epoxy-A-nor-5α-cholestane ( 23 ) was synthesized starting from A-nor-5α-cholestane-2-one ( 17 ). The intermediates were the ester 16 , the diol 18 , the hydroxy-tosylate 19 and the chlorhydrin 20 . The spirocyclic oxetane 23 was reduced by LiAlH₄ in dioxane (not in ether). By chromatography on silica gel 23 was isomerized to the homoallylic alcohol 21 and transformed into 2-methylene-A-nor-5α-cholestane ( 24 ) by fragmentation. The IR. and NMR. spectra of the new oxetanes were compared with those of a series of known oxetanes.