Peroxycarbenium-mediated C-C bond formation: applications to the synthesis of hydroperoxides and peroxides

The Lewis acid-mediated reaction of alkene nucleophiles with peroxyacetals provides an effective route for the synthesis of homologated peroxides and hydroperoxides. In the presence of Lewis acids such as TiCl(4), SnCl(4), and trimethylsilyl triflate, peroxyacetals and peroxyketals undergo reaction with allyltrimethylsilane, silyl enol ethers, and silyl ketene acetals to afford homoallyl peroxides, 3-peroxyketones, and 3-peroxyalkanoates, respectively. Reactions of peroxyacetals are Lewis acid dependent; TiCl(4) promotes formation of ethers while SnCl(4) and trimethylsilyl triflate promote formation of peroxides. Lewis acid-promoted reactions of silylated hydroperoxyacetals furnish silylated hydroperoxides, which can be deprotected to homologated hydroperoxides. Hydroperoxyketals undergo Lewis acid-mediated allylation to furnish 1,2-dioxolanes via attack of hydroperoxide on the intermediate carbocation. Lewis acid-mediated cyclization of unsaturated peroxyacetals furnishes 1,2-dioxanes, 1,2-dioxepanes, and 1,2-dioxacanes through 6-endo/exo, 7-endo/endo, and 8-endo/endo pathways. The corresponding reactions involving 6-endo/endo and 5-endo/exo pathways were unsuccessful.     

Hydrogen Peroxide and Di(hydroperoxy)propane Adducts of Phosphine Oxides as Stoichiometric and Soluble Oxidizing Agents

Aqueous hydrogen peroxide is widely used as an oxidizing agent in industry and academia. Herein, the hydrogen peroxide adducts of phosphine oxides, [tBu₃PO ? H₂O₂]₂ and [Ph₃PO ? H₂O₂]₂ ? H₂O₂, are described. Additionally, the corresponding di(hydroperoxy)propane adducts R₃PO ? (HOO)₂CMe₂ (R=Cy, Ph) were synthesized and characterized. All adducts could be obtained as large single crystals suitable for structural characterization by X‐ray crystallography and solid‐state NMR spectroscopy. The di(hydroperoxy)propane adducts are soluble in organic solvents which enables oxidation reactions in one phase. As the adducts are solid and molecular, they can easily be applied stoichiometrically. No loss of oxidizing power occurs upon long‐term storage of the single crystals at room temperature or the powders at ?20 °C.     

Adsorption of Metallocenes on Silica

The adsorption of the metallocenes ferrocene, doubly deuterated ferrocene, cymantrene and nickelocene, as well as molybdenum hexacarbonyl, proceeds in the absence of a solvent. Large single pieces of silica gel were placed in contact with the solid metallocenes and the adsorption process was visualized on a macroscopic scale and the maximal loadings were determined. ¹H, ²H, and ¹³C solid‐state NMR studies confirmed fast isotropic reorientation of the surface‐adsorbed metallocene molecules within the pores of the silica. All prevalent anisotropic solid‐state interactions were averaged out. The solid diamagnetic and paramagnetic materials were amenable to measurements with a standard solution NMR instrument. All metallocenes adsorbed in a monolayer. In the case of ferrocene and cymantrene, different ¹³C MAS signals were obtained for the cyclopentadienyl ring carbon nuclei and assigned to one ring interacting with the surface and one ring pointing away from it. The relative adsorption strengths of ferrocene on different silica supports, nanotubes, and activated carbon were determined by a novel straightforward method recording the desorption temperature. The reversibility of adsorption has been demonstrated by competition experiments using ferrocene, doubly deuterated ferrocene, and cymantrene. Adsorbed nickelocene could be reduced to small Ni⁰ aggregates on the surface and the catalytic activity of the resulting material for the cyclotrimerization of phenylacetylene was proven.     

Neural correlates of semantic and syntactic processes in the comprehension of case marked pronouns: Evidence from German and Dutch

Background  

It is well known that both semantic and syntactic information play a role in pronoun resolution in sentences. However, it is unclear what the relative contribution of these sources of information is for the establishment of a coreferential relationship between the pronoun and the antecedent in combination with a local structural case constraint on the pronoun (i.e. case assignment of a pronoun under preposition governing). In a prepositional phrase in German and Dutch, it is the preposition that assigns case to the pronoun. Furthermore, in these languages different overtly case-marked pronouns are used to refer to male and female persons. Thus, one can manipulate biological/syntactic gender features separately from case marking features.     

Microwave heating of ceramic composites

The microwave heating of a ceramic composite is modelled andanalysed. The composite consists of many small ceramic particlesembedded in a ceramic cement. The composite is assumed to bewell insulated, and each particle is assumed to be in imperfectthermal contact with the surrounding cement. Based on thesetwo assumptions an asymptotic theory exploiting the small Biotnumber and small non-dimensional contact conductance is developed.Our asymptotic theory yields a set of nonlinear partial differentialequations which govern the temperature in the composite. Theseare reduced to a set of coupled nonlinear ordinary differentialequations in which the surface area of each particle entersas a parameter. Recent experiments with such composites haveshown that the steady-state temperature of the composite isstrongly dependent upon the radii of the embedded particles.Our model captures this effect. In fact, our analysis showsthat the assumption of imperfect thermal contact between theparticles and the ceramic cement is essential for this trendto be established.