Saturated Boronic Acids,Boronates, and Trifluoroborates: An Update on Their Synthetic and Medicinal Chemistry

This review discusses recent advances in the chemistry of saturated boronic acids, boronates, and trifluoroborates. Applications of the title compounds in the design of boron-containing drugs are surveyed, with special emphasis on α-amino boronic derivatives. A general overview of saturated boronic compounds as modern tools to construct C(sp³)−C and C(sp³)-heteroatom bonds is given, including recent developments in the Suzuki-Miyaura and Chan-Lam cross-couplings, single-electron-transfer processes including metallo- and organocatalytic photoredox reactions, and transformations of boron “ate” complexes. Finally, an attempt to summarize the current state of the art in the synthesis of saturated boronic acids, boronates, and trifluoroborates is made, with a brief mention of the “classical” methods (transmetallation of organolithium/magnesium reagents with boron species, anti-Markovnikov hydroboration of alkenes, and the modification of alkenyl boron compounds) and a special focus on recent methodologies (boronation of alkyl (pseudo)halides, derivatives of carboxylic acids, alcohols, and primary amines, boronative C−H activation, novel approaches to alkene hydroboration, and 1,2-metallate-type rearrangements).     

Torsional strain of TaS3 whiskers on the charge-density wave depinning

We find that an electric current I exceeding the threshold value results in torsional strain of o-TaS3 samples with one contact freely suspended. The rotation angle deltavarphi of the free end achieves several degrees and exhibits hysteresis as a function of I. The sign of deltavarphi depends on the I polarity; a polar axis along the conducting chains (the c axis) is pointed out. We associate the effect with surface shear of the charge-density wave (CDW) coupled to the crystal shear. The current-induced torsional strain could be treated in terms of enormous piezoelectric coefficients (>10(-4) cm/V) corresponding to shear. In essence, TaS3 appears to be a ready torsional actuator based on the unique intrinsic property of the CDW.     

Barothermal modification of activated carbon with maleic anhydride

We have used the barothermal treatment method (heat and pressure) to modify the surface of activated charcoal with maleic anhydride. As a result of the modification, about 1 mmol/g of thermally stable carboxyl groups are formed on the surface with a decomposition temperature range of 330–470°C. We show that the proposed procedure makes it possible to obtain materials with a uniform coverage of functional groups.     

Kinetics of the gas-phase chlorination of activated carbon with carbon tetrachloride

The chlorination of activated charcoals KAU and SCN with CCl₄ vapor in the temperature range 300–600 °C and also the thermal stability of the added chlorine has been studied. It is shown that chlorination with CCl₄ vapor led to the addition of up to 22 mass % (6.2 mmol/g) of chlorine to the surface layer of the carbon. The added chlorine is desorbed from the surface with formation of HCl, while at temperatures above 500 °C desorption of chlorine goes practically to completion. The effective rate constants and the energies of activation of the processes of chlorination and desorption have been found.     

Liquid-Vapor Phase Equilibria in Binary and Ternary Subsystems Constituted by Pyrocondensate Components from the Pyrolysis of Straight-Run Naphtha

Liquid-vapor equilibrium in the binary subsystems of the ethylbenzene-o-xylene-α-methylstyrene-p-vinyltoluene-dicyclopentadiene-indene system was predicted using UNIFAC group model and studied experimentally. To assess the reliability of parameters, the liquid-vapor equilibrium was studied experimentally in the ternary systems and compared with the calculated data.__________Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 3, 2005, pp. 403–407.Original Russian Text Copyright © 2005 by Prikhod’ko, Komarova, Gorelova, Gorlova, Bondaletov.     

Four contact electrical measurements on the anisotropic high-Tc superconductors: voltage reversals

We describe three mechanisms for the anomalous voltage reversals which are occasionally observed during four contact current-voltage measurements on the high temperature superconductors. Voltage reversals have been frequently observed for whiskers of the bismuth strontium calcium copper oxide (BSSCO) compounds, and we describe measurements made on high quality whiskers of the 2:2:1:2 and 2:2:2:3 phases. A simple model based on extreme anisotropy plus offset contacts can explain the gross features of four contact measurements in the ohmic flux-flow regime, but not the wealth of reversal phenomena found for the IVVI configuration in the lower temperature irreversibility region.     

Peculiarities of carrier scattering at the boundary of a normal metal and a quasi-one-dimensional conductor with a charge-density wave

Nonlinear effects at the interface between a normal metal and the quasi-one-dimensional conductor with a charge-density wave K_0.3MoO₃ are investigated experimentally. It is found that the scattering of the normal carriers with energy E less than the Peierls gap Δ_p gives rise to an excess resistance of the N-CDW boundary. The character of the scattering is similar to Andreev reflection. In contrast to superconductors, however, the sign of the charge of the reflected particle does not change. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 4, 259–264 (25 August 1996)     

Chemical Modification of Activated Carbons

Mesoporous activated carbons and silica gel (Kieselgel 100, Merck) were modified as follows: (i) treatment with vinyltrimethoxysilane; or (ii)chlorination with CCl₄ followed by the reaction with a Grignard reagent. Modification of silica gel was proved by methods FTIR, NMR-¹³C and element analysis on carbon. The chemical modification of coals by alkans and olefins was supported by comparison of results of thermogravimetry for modified coals and modified silica gels. Polymerization of vinyl groups on carbon surface is shown by methods thermogravimetric analysis and differential scanning calorimetry. This revised version was published online in August 2006 with corrections to the Cover Date.     

Local spectroscopy of carrier reflection at the interface between a normal metal and the Peierls conductor K0.3MoO3

The nature of carrier reflection from a normal metal-Peierls conductor interface is clarified by studying the characteristics of point contacts formed with an intermediate metallic layer sputtered onto the (010) face of K_0.3MoO₃ crystals (injection along the chains) or the face (injection perpendicular to the chains). In the Peierls state, for bias voltages eV smaller than the Peierls gap Δ_P, an excess differential resistance is observed with a local minimum at V=0. The magnitude of the excess resistance is proportional to a ²/d ², where a is the contact diameter and d is the thickness of the metal film. The excess resistance is much higher for injection along the chains than for injection perpendicular to them. A comparative analysis of the data for different injection directions indicates that the dominant contribution to the excess resistance for injection along the chains is from normal reflection of carriers without changes in the sign of their charge and with a momentum transfer 2p _F to the condensate of electron-hole pairs carried away from the interface. Zh. éksp. Teor. Fiz. 113, 1830–1842 (May 1998)