A novel and highly efficient synthesis of gem-difluorocyclopropanes

[reaction: see text] A new and highly versatile source of difluorocarbene is reported. Trimethylsilyl fluorosulfonyldifluoroacetate (TFDA) undergoes decomposition in the presence of catalytic fluoride to form difluorocarbene under conditions that allow its addition to relatively electron deficient alkenes in high yield. For example, unprecedented CF2: addition to n-butyl acrylate proceeded in 73% yield.     

Preparation, structure, and anion sensing properties of 1,n-diaza[n]ferrocenophanes

The synthesis of structurally new types of azaferrocenophanes is reported in this note: 4, which comprises a 1,2-fused azaheterocycle to a 1,3-diaza[3]ferrocenophane framework; 1,3-diaza[3]ferrocenophanes 5 and 7, which can be considered as a 1,1'-ferrocenylene N,N'-guanidine or urea, respectively; and 1,3,6,8-tetraza[8]ferrocenophanes 9 and 10, bearing two ureido moieties in the ansa-bridge. These compounds were prepared directly from 1,1'-bis(isocyanato)ferrocene 1 and thoroughly characterized by spectroscopic means. Tetraza[8]ferrocenophanes 9 and 10 show spectral and electrochemical anion-sensing action: they display a selective downfield shift of the urea protons and a cathodic shift of the ferrocene/ferrocinium redox couple with dihydrogenphosphate and fluoride anions. The crystal structure of compound 4 has been determined by single-crystal X-ray methods.     

Multicomponent analysis: Comparison of various graphical and numerical methods

The performance of several graphical (zero-crossing and derivative quotient spectra with standardized divisor) and numerical methods (MULTIC and PLS) for the resolution of binary and ternary mixtures of species is compared. Numerical methods were found to be specially suited to multicomponent analysis, particularly for mixtures containing more than two analytes with highly overlapped spectra. The results obtained by using the compared methods to analyse various synthetic mixtures of acetylsalicylic acid, caffeine and thiamine were quite consistent and errors in the simultaneous quantification of the analytes amounted to less than 5% in all instances.     

Cyclization of interlocked fumaramides into β-lactams: experimental and computational mechanistic assessment of the key intercomponent proton transfer and the stereocontrolling active pocket

A detailed mechanistic study of the diastereoselective CsOH-promoted cyclization of interlocked fumaramides to give β-lactams is described. The mechanistic analysis comprises the experimental evaluation of the structure-reactivity relationship for a wide range of fumaramides [2]rotaxanes (Hammet-plots), KIE studies with deuterium-labelled interlocked fumaramides and computational analysis of two alternative mechanistic pathways for the cyclization process. The obtained results confirm that: (a) the rate-determining step is the deprotonation of the N-benzyl group of the thread by the amidate group of the macrocycle generated by the external base, (b) the polyamide macrocycle plays an important role not only as activating element but also as the stereodifferenciating factor responsible for the observed diastereoselection and (c) the higher flexibility of the adamantyl core speeds up the cyclization process in diadamantyl-derived rotaxanes.

A mechanistic study of the diastereoselective cyclization of interlocked fumaramides to give β-lactams unveils the key factors for successfully taming the process.     

An iminophosphorane-based approach for the synthesis of spiropyrrolidine-imidazole derivatives

A method based on the reaction of an E-phosphazide, an intermediate in the Staüdinger reaction between triphenylphosphine and an azide, with heterocumulenes allows the one-pot, two-component synthesis of a number of pyrrole-imidazole derivatives. The procedure, which involves sequential treatment of the appropriate α-azido ester with triphenylphosphine and isocyanate leads to the hydantoin product after aqueous work-up. The cyclization conditions can also be adapted for the synthesis of thiohydantoins by using isothiocyanates. These hybrids pyrrole-thiohydantoins undergo a novel oxidative spirocyclization by action of DDQ to give a tricyclic derivative (pyrrole-pyrrolidine-imidazole), which displays an interesting cytotoxic activity.     

Algebraic study of chiral anomalies?

The algebraic structure of chiral anomalies ismade globally valid on non-trivial bundles by the introduction of a fixed background connection. Some of the techniques used in the study of the anomaly are improved or generalized, including a systematic way of generating towers of ??descent equations??.     

Specific features of electrical properties of porous biocarbons prepared from beech wood and wood artificial fiberboards

This paper reports on comparative investigations of the structural and electrical properties of biomorphic carbons prepared from natural beech wood, as well as medium-density and high-density fiberboards, by means of carbonization at different temperatures T _carb in the range 650–1000°C. It has been demonstrated using X-ray diffraction analysis that biocarbons prepared from medium-density and high-density fiberboards at all temperatures T _carb contain a nanocrystalline graphite component, namely, three-dimensional crystallites 11–14 Å in size. An increase in the carbonization temperature T _carb to 1000°C leads to the appearance of a noticeable fraction of two-dimensional graphene particles with the same sizes. The temperature dependences of the electrical resistivity ρ of the biomorphic carbons have been measured and analyzed in the temperature range 1.8–300 K. For all types of carbons under investigation, an increase in the carbonization temperature T _carb from 600 to 900°C leads to a change in the electrical resistivity at T = 300 K by five or six orders of magnitude. The dependences ρ(T) for these materials are adequately described by the Mott law for the variable-range hopping conduction. It has been revealed that the temperature dependence of the electrical resistivity exhibits a hysteresis, which has been attributed to thermomechanical stresses in an inhomogeneous structure of the biocarbon prepared at a low carbonization temperature T _carb. The crossover to the conductivity characteristic of disordered metal systems is observed at T _carb ? 1000°C.     

Sensitivity of ultrasonic hydrophone probes below 1 MHz

Frequency responses of different PVDF polymer hydrophone probes, including membrane and needle designs, were measured and are presented in terms of end-of-cable voltage sensitivity versus frequency over a wide, 4.5 octave bandwidth ranging from 0.25 to 2.5 MHz. The probes are seldom, if at all, characterized in this frequency range due to the difficulties associated with a lack of adequate and readily implementable calibration techniques. To this end, a technique, which uses a combination of swept frequency chirp and reciprocity, so that both the relative and absolute plots of sensitivity versus frequency can be obtained, was developed and tested. Salient features of the technique including the design of a 6 octave auxiliary acoustic source are described. The experimental data indicate that a majority of the PVDF membrane hydrophones exhibit a relatively uniform (to within +/- 2 dB) response. While, in general, this is not the case for commercially available needle hydrophone probes, it is evidenced that a careful attention to the PVDF probe design results in frequency characteristics fairly close to those achievable with a membrane design. The overall uncertainty of the calibration technique was estimated to be better than +/- 1.5 dB in the considered frequency range. The results of this work are important to implement procedures for adequate determination of the Mechanical Index (MI) of ultrasound imaging devices. MI is widely accepted as a predictor of potential bioeffects associated with cavitation phenomena. Current efforts are focused on extending the applicability of the technique to frequencies below 100 kHz.     

L-extensions and L-boundary of conformal spacetimes

The notion of L-boundary, a new causal boundary proposed by R. Low based on constructing a ‘sky at infinity’ for any light ray, is discussed in detail. The analysis of the notion of L-boundary will be done in the 3-dimensional situation for the ease of presentation. The proposed notion of causal boundary is intrinsically conformal and, as it will be proved in the paper, under natural conditions provides a natural extension \({\overline{M}}\) of the given spacetime M with smooth boundary \(\partial M = {\overline{M}} {\backslash } M\). The extensions \({\overline{M}}\) of any conformal manifold M constructed in this way are characterised exclusively in terms of local properties at the boundary points. Such extensions are called L-extensions and it is proved that, if they exist, they are essentially unique. Finally it is shown that in the 3-dimensional case, any L-extension is equivalent to the canonical extension obtained by using the L-boundary of the manifold.