On the Existence and Hydrolytic Stability of Titanosiloxane Bonds in the System: Glycidoxypropyltrimethoxysilane-Water-Titaniumtetraethoxide

Heterometal materials based on glycidoxypropyltrialkoxysilane and titaniumalkoxide are used for optical applications and require a high homogeneity on the molecular level. The presence of heterometal titanosiloxanes, their distribution and hydrolytic stability should influence the homogeneity of these materials. 29Si and 17O NMR spectroscopy has been used to investigate sols with molar ratios Si : Ti = 1 and H2O : OR (H) = 0.5 – 2.0 and their gels after heat treatment at 130°C. The presence of Si—O—Ti bonds in sols with a low water content (H < 0.2) and in the corresponding gels was identified by the high-field shift of the 29Si NMR signals of T1 and T2 units of up to 2–3 ppm compared to corresponding signals of homo-condensed Si—O—Si bonds. The existence of Si—O—Ti bonds in the sols is supported by 17O NMR spectra which show a characteristic signal around 340 ppm. A cleavage of the Si—O—Ti bonds occurs with increasing water/OR ratio in the sols. The cleavage of the heterometal bonds and the building up of homo-condensed species leads to a separation into areas with predominantly Ti—O—Ti and Si—O—Si bonds resulting in a decreased molecular homogeneity of the materials.     

ESR study of the thermal decomposition of di-tert-butoxy-tert-butyl alumotrioxide formed in the reaction of tri-tert-butoxyaluminum with tert-butyl hydroperoxide

Tri-tert-butoxyaluminum reacts with tert-butyl hydroperoxide to produce di-tert-butoxy-tert-butyl alumotrioxide, which decomposes heterolytically to form singlet dioxygen and homolytically with the O—O bond cleavage. The Bu^tOO^·, (Bu^tO)₂AlOO^·, Bu^tO^·, and (Bu^tO)₂AlO^· radicals were identified by ESR using spin traps. These findings confirm the formation of aluminum-containing trioxide. The above radicals initiate alkylarene oxidation by the tri-tert-butoxyaluminum—tert-butyl hydroperoxide system. The carbon-centered and alkylperoxy radicals originated from the oxidized substrates were identified.     

Reactions of cage metal siloxanes with functionalized organochlorosilanes

The reactions of [(2-acetoxyetoxy)methyl]dimethylchlorosilane and 1-acetoxy-2-(dimethylchlorosilylmethoxy)benzene with the cage phenylcopper and phenylmanganese siloxanes leads to the cleavage of the M—O—Si bond to give metal chlorides and six-unit cyclosiloxanes with an acetoxy group in the organic substituent at the silicon atom. Methanolysis of these acetoxy derivatives does not affect the ring structure and affords the corresponding polyhydric alcohols.     

Reaction of 3,4,6-Triisopropyl-<Emphasis Type="Italic">о</Emphasis>-benzoquinone with 3,4,6-Triisopropylcatechol

Formation of protonated 3,4,6-triisopropylsemiquinone radicals in solution of a mixture of 3,4,6- triisopropyl-о-benzoquinone and 3,4,6-triisopropylcatechol was shown using ESR spectroscopy. Heating the mixture to 110°С leads to the formation of the condensation products, additional amount of 3,4,6-triisopropylcatechol, and evolution of propylene. Mathematical simulation of the kinetics of the reaction was performed.     

EPR 法研究对氯四苯基锰卟啉对异丙苯过氧化氢的催化分解作用

 以 5,5-二甲基-1-吡咯啉-N-氧化物 (DMPO) 为自旋捕捉剂, 采用电子顺磁共振波谱 (EPR) 法研究了对氯四苯基锰卟啉 (T(p-Cl)PPMnⅢCl) 催化分解异丙苯过氧化氢 (CHP) 的反应过程. 结果表明, 在 25 oC 下的初始反应阶段, 在 T(p-Cl)PPMnⅢCl 与 CHP 的反应体系中仅检测到有异丙苯氧自由基的 DMPO 自旋加合物. 随着 CHP 浓度的增大, 还检测到有异丙苯过氧自由基自旋加合物的重排产物信号. 这说明在 T(p-Cl)PPMnⅢCl 的催化作用下, 初始阶段 CHP 是以 O–O 键均裂的方式产生异丙苯氧自由基引发分解反应, 并主要生成 2-苯基-2-丙醇. 在较大的初始 CHP 浓度下, 异丙苯氧自由基进一步与 CHP 反应, 产生异丙苯过氧自由基. 提出了 CHP 分解反应的主要自由基历程.     

Molecular structure,conformational mobility,vibrational spectra,and thermochemistry of peroxyacetic acid: an ab initio and density functional study

The structure of the peroxyacetic acid (PAA) molecule and its conformational mobility under rotation about the peroxide bond was studied by ab initio and density functional methods. The free rotation is hindered by the trans-barrier of height 22.3 kJ mol^–1. The equilibrium molecular structure of AcOOH (C _s symmetry) is a result of intramolecular hydrogen bond. The high energy of hydrogen bonding (46 kJ mol^–1 according to natural bonding orbital analysis) hampers formation of intermolecular associates of AcOOH in the gas and liquid phases. The standard enthalpies of formation for AcOOH (–353.2 kJ mol^–1) and products of radical decomposition of the peroxide — AcO^· (–190.2 kJ mol^–1) and AcOO^· (–153.4 kJ mol^–1) — were determined by the G2 and G2(MP2) composite methods. The O—H and O—O bonds in the PAA molecule (bond energies are 417.8 and 202.3 kJ mol^–1, respectively) are much stronger than in alkyl hydroperoxide molecules. This provides an explanation for substantial contribution of non-radical channels of the decomposition of peroxyacetic acid. The electron density distribution and gas-phase acidity of PAA were determined. The transition states of the ethylene and cyclohexene epoxidation reactions were located (E _a = 71.7 and 50.9 kJ mol^–1 respectively).     

Photooxidation of Ethylbenzene with TiO<Subscript>2</Subscript> and Metal Coated TiO<Subscript>2</Subscript> and its Kinetics

Summary. Photooxidation of ethylbenzene with oxygen to give ethylbenzene hydroperoxide has been achieved in a stirred photochemical reactor that was cooled by a water system by irradiation with a 400W high-pressure mercury lamp and using TiO₂ powder and metal coated TiO₂. The effects of the amount of copper or silver coated on TiO₂ and of the temperature on the rate of oxidation have been investigated. It is suggested that thermal cleavage of the O–O bond and photochemically generated singlet oxygen should be considered as the initiating step in a radical chain mechanism. An optimum loading of 6% Ag or 4–5% Cu was observed for photooxidation of ethylbenzene.     

Photooxidation of Ethylbenzene with TiO 2 and Metal Coated TiO 2 and its Kinetics

Photooxidation of ethylbenzene with oxygen to give ethylbenzene hydroperoxide has been achieved in a stirred photochemical reactor that was cooled by a water system by irradiation with a 400W high-pressure mercury lamp and using TiO₂ powder and metal coated TiO₂. The effects of the amount of copper or silver coated on TiO₂ and of the temperature on the rate of oxidation have been investigated. It is suggested that thermal cleavage of the O–O bond and photochemically generated singlet oxygen should be considered as the initiating step in a radical chain mechanism. An optimum loading of 6% Ag or 4–5% Cu was observed for photooxidation of ethylbenzene.     

Semiempirical Analysis of the Homolytic Decomposition of Peresters with the Concerted Cleavage of Two Bonds

The parabolic model of a bimolecular reaction is modified to study the monomolecular decomposition of molecules into radicals by the cleavage of several bonds. Together with the oscillation model of molecule decomposition with the concerted cleavage of several bonds, this model is used to analyze the kinetic data on the decomposition of 16 peresters with the simultaneous cleavage of C–C and O–O bonds. Parameters characterizing this decomposition are obtained and multiple variants in representing such decomposition in terms of the parabolic model are discussed.     

Estimation of Enthalpies of Alkoxy Radical Formation and Bond Strengths in Alcohols and Ether

Kinetic data on the thermal decomposition of peroxides were analyzed, and energies of the O–O bond dissociation were calculated. Enthalpies of formation of various alkoxy radicals and peroxides were determined. The dissociation energies for the O–H bonds in alcohols and C–O bonds in ethers were estimated. Comparative analysis of literature and obtained data was performed.     

Acyl Iodides in Organic Synthesis: VI. Reactions with Vinyl Ethers

Reactions of acetyl iodide with butyl vinyl ether, 1,2-divinyloxyethane, phenyl vinyl ether, 1,4-di-vinyloxybenzene, and divinyl ether were studied. Vinyl ethers derived from aliphatic alcohols (butyl vinyl ether and 1,2-divinyloxyethane) react with acetyl iodide in a way similar to ethyl vinyl ether, i.e., with cleavage of both O–Csp2 and Alk–O ether bonds. From butyl vinyl ether, a mixture of vinyl iodide, butyl acetate, vinyl acetate, and butyl iodide is formed, while 1,2-divinyloxyethane gives rise to vinyl iodide, vinyl acetate, and 2-iodoethyl acetate. The reaction of acetyl iodide with divinyl ether involves cleavage of only one O–Csp2 bond, yielding vinyl acetate and vinyl iodide. In the reactions of acetyl iodide with phenyl vinyl ether and 1,4-divinyloxybenzene, only the O–CVin bond is cleaved, whereas the O–CAr bond remains intact.     

Radical-Pair Formation in Hydrocarbon (Aut)Oxidation

The reaction profiles for the uni- and bimolecular decomposition of benzyl hydroperoxide have been studied in the context of initiation reactions for the (aut)oxidation of hydrocarbons. The unimolecular dissociation of benzyl hydroperoxide was found to proceed through the formation of a hydrogen-bonded radical-pair minimum located +181 kJ mol⁻¹ above the hydroperoxide substrate and around 15 kJ mol⁻¹ below the separated radical products. The reaction of toluene with benzyl hydroperoxide proceeds such that O−O bond homolysis is coupled with a C−H bond abstraction event in a single kinetic step. The enthalpic barrier of this molecule-induced radical formation (MIRF) process is significantly lower than that of the unimolecular O−O bond cleavage. The same type of reaction is also possible in the self-reaction between two benzyl hydroperoxide molecules forming benzyloxyl and hydroxyl radical pairs along with benzaldehyde and water as co-products. In the product complexes formed in these MIRF reactions, both radicals connect to a centrally placed water molecule through hydrogen-bonding interactions.     

Correlations between bond lengths and stretching frequencies in the O—D...O hydrogen bridge and their consequences

Based on modern neutron diffraction data and the known empirical correlations between the geometric and spectroscopic parameters of hydrogen bonds, the analytical expression describing the relation between the O—D covalent and D...O hydrogen bond lengths in the O—D...O hydrogen bridge was obtained. The distribution functions of the interatomic and nearest intermolecular distances in heavy water were calculated from the Raman band shapes in the 10 to 90 °C temperature interval in the framework of the fluctuation theory of hydrogen bonding.     

Sulfate Ions in Titania Polymorphs

Sol-gel titania was sulfated by using sulfuric acid as hydrolysis catalyst, or by impregnating with ammonium sulfate fresh samples prepared with nitric acid or ammonium hydroxide as hydrolysis catalyst. Samples were characterized with X-ray powder diffraction, infrared spectroscopy, thermogravimetry and atomic absorption spectroscopy. Sulfate ions were found anchored to brookite and anatase phases, because they have short O—O atomic bond lengths slightly larger than the largest O—O bond length of sulfate ion. Since the shortest O—O atomic bond in anatase is smaller than the one in brookite, the sulfate ions are then less deformed when they are anchored to anatase, and consequently more stable. Therefore when the sample temperature is raised, the brookite with sulfate ions was transformed mainly to anatase and not into rutile, which is the most probably transformation when these ions are not involved. Sulfate ions also hindered anatase and brookite crystallite growing and stabilized the crystalline structure of anatase. When the sulfate ions are lost the crystalline anatase phase is transformed into rutile, leaving a large number of vacancies that favored atom diffusion and consequently the growing of rutile crystallites. The crystalline evolution of the samples as a function of the annealing temperature is almost independent of the sulfating method.     

The nature of the O—O bond in hydroperoxides

Quantum-chemical calculations of the H₂O₂ and F₂ molecules using different computational schemes, basis sets, and procedures for the inclusion of electron correlation were performed. High-resolution X-ray diffraction study of the electron density distribution in the crystals of 2,5-dimethyl-2,5-dihydroperoxyhexane and 2,5-dimethyl-2,5-dihydroperoxyhex-3-yne was carried out. Joint analysis of the results obtained showed that the formally covalent O—O and F—F bonds correspond to a specific type of interatomic interaction. This type is intermediate between the shared and closed-shell interactions (the latter are typical of the ionic systems and van der Waals molecules).     

Design of Homogeneous Hybrid Materials through a Careful Control of the Synthetic Procedure

Sols for the synthesis of hybrid organic-inorganic materials have been prepared by mixing zirconium n-propoxide and methacryloxypropyltrimethoxysilane (MPS). The synthesis was done in two steps: a 15 minute hydrolysis of a MPS : H₂O : EtOH 1 : 1 : 2 mixture and then addition of 0.5 molar equivalent of zirconium alkoxide. All the experimental parameters—hydrolysis ratio, pH, dilution, pre-hydrolysis time—have been optimized through a detailed ²⁹ Si and ¹⁷O NMR analysis. Immediately after the addition, 94% of the initial water was consumed for the formation of Si–O–Zr bridges. Cleavage of these bonds, associated with formation of Si–O–Si and Zr–O–Zr bridges are then observed during the aging time.     

DFT study of the mechanism of alkane hydrogenolysis by transition metal hydrides. 1. Interaction of silica-supported zirconium hydrides with methane

Model reactions of silica-supported zirconium hydrides (Si—O—)₃ZrH and (Si—O—)₂ZrH₂ with methane, resulting in cleavage of a C—H bond in the methane molecule and the formation of (Si—O—)₃ZrCH₃ and (Si—O—)₂Zr(H)CH₃ as products were studied using the DFT approach with the PBE density functional. The processes proceed as bimolecular reactions without preliminary formation of agostic complexes. According to calculations, zirconium dihydrides (Si—O—)₂ZrH₂ are more reactive toward the methane C—H bonds than zirconium monohydrides (Si—O—)₃ZrH. The calculated activation energies of the reactions with participation of zirconium dihydrides (Si—O—)₂ZrH₂ are in better agreement with the known experimental data for the Yermakov—Basset catalytic system.     

Synthesis of trichloro-1,4-benzoquinonyl-substituted 2,2′-bithiazoles

2,2-Bi[5-(2,5-dihydroxy-3,4,6-trichlorophenyl)thiazole] and 6-(2,5-dihydroxy-3,4,6-trichlorophenyl)-2-imino-3,4-dihydro-4H-1,4-thiazin-3-thione are synthesized from 2,5-dihydroxy-3,4,6,7-tetrachloro-2,3-dihydrobenzo[b]furan and dithiooxalyldiamide. The products are oxidized by FeCl₃ to the corresponding trichloro-1,4-benzoquinonyl-substituted sulfur-containing heterocycles. N-Methylation of 2,2-bithiazole by dimethyl sulfate gives 2,2-bi[5-(2,5-dihydroxy-3,4,6-trichlorophenyl)-3-methylthiazolium] bismonomethyl sulfate. The last compound is readily transformed to 2,2-bi[5-(2-hydroxy-5-oxido-3,4,6-trichlorophenyl)-3-methylthiazolium] bisbetaine.Riga Technical University, Riga LV-1048, Latvia, Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 988–992, July, 1999.     

Synthesis of 1-functionalized-6-hydroxy-4-methyl and 6,11-dihydroxy-4-methylnaphtho[2,3-g]isoquinoline-5,12-quinones

Using 2-methoxy- and 2,5-dimethoxyacetophenones 8a and 8b as starting materials, 1-chloro-4-methylisoquinoline-5,8-quinone ( 6 ) and its 6-bromo derivative 7 were obtained via multistep sequences. Whereas Diels-Alder condensation of the former compound with homophthalic anhydride ( 22 ) led to a mixture of the two possible isomers: 1-chloro-11-hydroxy-4-methylnaphtho[2,3-g]isoquinoline-5,12-quinone ( 23 ) and 1-chloro-6-hydroxy-4-methylnaphtho[2,3-g]isoquinoline-5,12-quinone ( 24 ), this last tetracyclic chloroquinone was specifically obtained from 6-bromo-1-chloro-4-methylisoquinoline-5,8-quinone ( 7 ) and homophthalic anhydride. The 6,11-dihydroxy derivative was then prepared by ammonium nitrate oxidation or photochemically by cycloaddition of benzocyclobutenedione ( 28 ) and 1-chloro-4-methylisoquinoline-5,8-quinone ( 6 ). Chloro compounds were easily substituted by diamines to provide corresponding 1-amino substituted hydroxy tetracyclic quinones.     

Oxidation of triphenylantimony with 4-hydroperoxy-2-hydroxy-3,4,6-triisopropylcyclohexa-2,5-dienone or 3,4,6-triisopropyl-1,2-benzoquinone

According to the NMR spectroscopic data, the oxidation of triphenylantimony with 4-hydroperoxy-2-hydroxy-3,4,6-triisopropylcyclohexa-2,5-dienone involves three steps. The first step affords the 2,4,10,12-tetraoxa-3,11-distibatricyclo[11.3.1.1^5,9]octadecatetraene derivative. The latter is rearranged into benzodioxastibolone derivatives followed by the rearrangement into 4,6,7-triisopropyl-2,2,2-triphenyl-1,3,2-benzodioxastibol-5-ol. The transformation of the latter depends on the presence of oxygen and the mode of its dosing. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1750–1757, September, 2007.