Reactivity of 13,13-dibromo-2,4,9,11-tetraoxadispiro[5.0.5.1]tridecane toward organolithiums: remarkable resistance to the DMS rearrangement

Reactions of dibromocyclopropane 2a, containing two spiro-fused 1,3-dioxane rings, with MeLi gave only the methylation products 8 and 9 even at elevated temperatures. In contrast, the cyclohexane analogue 2b treated with MeLi underwent a smooth rearrangement to bicyclo[1.1.0]butane 11b at -78, -10, or +35 degrees C. Treatment of 2a with PhLi gave the alpha-Ph anion 13 as the only product, which underwent smooth methylation with MeI to give 14. Under the same conditions, 2b with PhLi gave bicyclo[1.1.0]butane 11b accompanied by bromophenyl derivative 8b. Treatment of either dibromide with t-BuLi gave a mixture of products including debrominated cyclopropanes 12. Experimental results were augmented with DFT calculations for salts 23 and MP2//DFT-level calculations for carbenes 22. They demonstrated a higher stability of the dioxane alpha-bromo anion with respect to alpha-elimination by 4.8 kcal/mol and also a lower tendency of the carbene 22a to undergo rearrangement by 4.0 kcal/mol than the cyclohexane analogues. These differences have been attributed to the inductive effect of the four oxygen atoms, which results in lower LUMO energy, the higher positive charge at the carbenic center, and the overall more electrophilic character of carbene 22a as compared to the cyclohexane derivative 22b. The rearrangement of carbenes 22 to the corresponding allenes 1, the thermodynamic products, requires a higher activation energy DeltaG(double dagger)(298) by 4.2 kcal/mol for dioxane and 6.4 kcal/mol for cyclohexane derivatives than for the formation of the bicyclo[1.1.0]butanes 11. The DeltaG(double dagger)(298) for intramolecular insertions to the CH bond is low and calculated as 6.0 kcal/mol for dioxane 22a and 2.0 kcal/mol for the formation of cyclohexane 22b.     

Numerical investigations of the porosity effect on the shock focusing process

The effect of cylindrical obstacles and the porosity in between them along the path of a converging cylindrical shock is studied through numerical simulations. An initially cylindrical converging shock wave is perturbed by cylindrical obstacles placed radially in its path. High pressures and temperatures are achieved as the shock wave is focused. Results show that the shape of the shock wave close to the point of convergence as well as the porosity and type of shock wave reflection the converging shock undergoes influence the peak values. Various configurations of the obstacle size and number are considered. The Guderley constant for each case is compared with previous reported experimental values.     

A tentative study of urban and suburban fine particles (PM2.5) collected in Ouagadougou,Burkina Faso

The aim of the study was to determine the mass, black carbon (BC), and elemental concentrations in fine particles (PM_2.5) and their variations at two sites in Ouagadougou, the capital city of Burkina Faso. The weather situation in Ouagadougou during the field campaign was dominated by high pressure with variable cloudiness and no precipitation. Diurnal temperatures varied between 19 and 38 °C and relative humidity between 10 and 60%. Winds in Ouagadougou were generally coming in from the north, showing a diurnal pattern with gusts of up to 6 m/s during daytime, while evenings and nights were calmer with very stable atmospheric conditions. However, during part of this field campaign, a period of nighttime moderately stable atmospheric conditions occurred with increased wind speed and more easterly winds. Cyclones were used for the PM_2.5 particle collection at both sites. The elemental analysis was done using energy dispersive x‐ray fluorescence (EDXRF) spectroscopy. Cl, K, Ca, Ti, Mn, Fe, Cu, Zn, Br, Rb, Sr, and Pb were identified and quantified in most of the samples. The particle mass concentration was 27–164 µg/m³ while BC varied between 1.3 and 8.2 µg/m³. No influence of leaded gasoline was found. Soil dust was identified as a major component of the particles, which was confirmed by comparing with the elements in a soil sample. A significant difference in elemental, BC, and mass concentrations was seen between periods with very stable and moderately stable atmospheric conditions. Copyright © 2009 John Wiley & Sons, Ltd.     

Angular momentum transport produced by shear flow driven drift waves in a collisional magnetoplasma

A shear flow instability in a nonuniform partially ionized magnetoplasma has been investigated and the angular momentum transport of charged particles caused by drift waves has been estimated. The angular momentum transport of the electrons and ions is not equal because the system is not conservative. The present investigation should help to understand the origin of fluctuations and associated parallel momentum transport of charged particles in nonuniform laboratory and astrophysical plasmas with sheared ion flow.     

Hexachloroacetone as a precursor for a tetrachloro-substituted oxyallyl intermediate: [4+3] cycloaddition to cyclic 1,3-dienes

The enol phosphate 2,2-dichloro-1-(trichloromethyl)ethenyl diethyl phosphate (1), easily available by a Perkow reaction between hexachloroacetone and triethyl phosphite, reacts with sodium trifluoroethoxide/trifluoroethanol in the presence of cyclic 5-membered 1,3-dienes to furnish alpha,alpha,alpha',alpha'-tetrachloro-substituted[3.2.1]bicyclic ketones 2. A [4+3] cycloaddition of a tetrachloro-oxyallyl intermediate is postulated.     

Tropyliumionen und tropilidene—V: Synthese und ambidente reaktivität von 7-chlor- und 7-brombenzocycloheptenyliumkationen

7H-Benzocyclohepten-7-one (1) reacts with oxalyl bromide to yield 7-bromobenzocycloheptenylium bromide (4a), a stable carbenium salt. In contrast, the reaction of oxalyl chloride or phosgene with 1 produces a covalent compound, 7,7-dichloro-7H-benzocycloheptent (3b), which ionizes in liquid sulfur dioxide to 7-chloro-benzocycloheptenylium chloride (4b). Nucleophiles are preferably added to C-5 ( C-9) of cation 4a; the resultant products are 7-bromo-5H-benzocycloheptenes (7b-h). The HMO model shows that the positive charge density in 4 is highest at C-5 ( C-9) thus favoring formation of addition products at this position under conditions of kinetic control. In those cases favorable to reversible addition, the competitive nucleophilic substitution at C-7 is the main reaction. The latter reaction occurs between 4a and ?-toluidine or N-methylaniline which produces 7-arylamino-benzocycloheptenylium bromides 13a,b. A further example is the hydrolysis of 4a and 3b to form 1.     

The influence of different parameters on the mercerisation of cellulose for viscose production

A quantitative analysis of degree of transformation from a softwood sulphite dissolving pulp to alkalised material and the yield of this transformation as a function of the simultaneous variation of the NaOH concentration, denoted [NaOH], reaction time and temperature was performed. Samples were analysed with Raman spectroscopy in combination with multivariate data analysis and these results were confirmed by X-ray diffraction. Gravimetry was used to measure the yield. The resulting data were related to the processing conditions in a Partial Least Square regression model, which made it possible to explore the relevance of the three studied variables on the responses. The detailed predictions for the interactive effects of the measured parameters made it possible to determine optimal conditions for both yield and degree of transformation in viscose manufacturing. The yield was positively correlated to the temperature from room temperature up to 45 °C, after which the relation was negative. Temperature was found to be important for the degree of transformation and yield. The time to reach a certain degree of transformation (i.e. mercerisation) depended on both temperature and [NaOH]. At low temperatures and high [NaOH], mercerisation was instantaneous. It was concluded that the size of fibre particles (mesh range 0.25–1 mm) had no influence on degree of transformation in viscose processing conditions, apparently due to the quick reaction with the excess of NaOH.     

Infinite Töplitz–Lipschitz matrices and operators

We introduce a class of infinite matrices , which are asymptotically (as |s| + |s′| → ∞) close to Hankel–T?plitz matrices. We prove that this class forms an algebra, and that flow-maps of nonautonomous linear equations with coefficients from the class also belong to it.