Rearrangements and stereomutations of metallacycles derived from allenes and imidozirconium complexes

The mechanisms of the rearrangements and stereoinversion of azametallacyclobutenes generated via [2+2] cycloaddition of allenes and imidozirconium complexes have been studied. Metallacycles derived from allenes bearing beta-hydrogen atoms racemize at room temperature by reversible beta-hydride elimination, a process which is also responsible for their eventual conversion to monoazadiene complexes. Metallacycles derived from diarylallenes racemize by reversible thermal bond homolysis at 95 degrees C; racemization of these metallacycles is also catalyzed by mild oxidants.     

Palladium assisted synthesis of 1-azaspirocycles

A facile, general route to 1-azaspirocycles utilizing a π-allyl palladium complex in the key cyclization step has been developed.     

Land cover change using an energy transition paradigm in a statistical mechanics approach

This paper explores a statistical mechanics approach as a means to better understand specific land cover changes on a continental scale. Integrated assessment models are used to calculate the impact of anthropogenic emissions via the coupling of technoeconomic and earth/atmospheric system models and they have often overlooked or oversimplified the evolution of land cover change. Different time scales and the uncertainties inherent in long term projections of land cover make their coupling to integrated assessment models difficult. The mainstream approach to land cover modelling is rule-based methodology and this necessarily implies that decision mechanisms are often removed from the physical geospatial realities, therefore a number of questions remain: How much of the predictive power of land cover change can be linked to the physical situation as opposed to social and policy realities? Can land cover change be understood using a statistical approach that includes only economic drivers and the availability of resources? In this paper, we use an energy transition paradigm as a means to predict this change. A cost function is applied to developed land covers for urban and agricultural areas. The counting of area is addressed using specific examples of a Pólya process involving Maxwell–Boltzmann and Bose–Einstein statistics. We apply an iterative counting method and compare the simulated statistics with fractional land cover data with a multi-national database. An energy level paradigm is used as a basis in a flow model for land cover change. The model is compared with tabulated land cover change in Europe for the period 1990–2000. The model post-predicts changes for each nation. When strong extraneous factors are absent, the model shows promise in reproducing data and can provide a means to test hypothesis for the standard rules-based algorithms.     

4-Chloro-2,6-bis(hydroxymethyl)pyridinium Chloride and 4-Dimethylamino-2,6-bis(hydroxymethyl)pyridinium Chloride Hemihydrate

Abstract  Two substituted pyridinium chloride salts, namely 4-chloro-2,6-bis(hydroxymethyl)pyridinium chloride (triclinic, P-1, a = 6.0651(8) ?, b = 8.4393(8) ?, c = 8.6554(9) ?, α = 78.845(1)°, β = 83.156(1)°, γ = 89.047(1)°; V = 431.55(8) ?³; Z = 2) and 4-dimethylamino-2,6-bis(hydroxymethyl)pyridinium chloride hemihydrate (orthorhombic, Aba2, a = 17.2179(14) ?, b = 17.6332(15) ?, c = 7.2068(6) ?; V = 2188.0(3) ?³; Z = 4), have been structurally characterized. Hydrogen bonding and van der Waals contacts are evident in both structures. Furthermore, the 4-dimethylamino derivative features a trigonal planar dimethylamino group with some π interaction between the nitrogen atom and the pyridine ring. Graphical Abstract  Two compounds, namely 4-chloro-2,6-bis(hydroxymethyl)pyridinium chloride and 4-dimethylamino-2,6-bis(hydroxymethyl)pyridinium chloride hemihydrate, have been spectroscopically and structurally characterized.      

Spin-dependent electronic states of the ferromagnetic semimetal EuB6

The spin polarization of EuB6 has been measured by using Andreev reflection spectroscopy. Analyses of the conductance spectra of the EuB6/Pb junctions yield a spin polarization of about 56%. The results demonstrate that the ferromagnetic EuB6 is not half-metallic. Combined with the Hall effect and magnetoresistivity data, the results indicate a semimetallic band structure with a fully spin-polarized hole band and an unpolarized electron band. The values and the spread of the measured spin polarization are quantitatively consistent with the experimentally determined Fermi surface and carrier densities.     

Zero-shift values of automatic and inexpensive strain-gage instrumentation systems

Computer-controlled data-acquisition systems are being used extensively for gathering strain-gage data. This paper explores the relative merits of using modern solidstate digital multi-meters (DDM) to measure the strain-gage resistance directly rather than using a conventional Wheatstone bridge. Both a direct-resistance measurement scheme and a reversed current scheme are compared over long measurements terms of 6 and 12 days using a 6 1/2-digit multimeter. The results show that the reversed current method is superior in maintaining the zero-gage resistance reading at the cost of using several meters. Possibly the direct-resistance method can be improved so that the operation and equipment can be simplified.Paper was presented at the 1990 SEM Spring Conference on Experimental Mechanics held in Albuquerque, NM on June 4–6.     

Ruthenium-Catalyzed C−C Coupling of Terminal Alkynes with Primary Alcohols or Aldehydes: α,β-Acetylenic Ketones (Ynones) via Oxidative Alkynylation

The first metal-catalyzed oxidative alkynylations of primary alcohols or aldehydes to form α,β-acetylenic ketones (ynones) are described. Deuterium labelling studies corroborate a novel reaction mechanism in which alkyne hydroruthenation forms a transient vinylruthenium complex that deprotonates the terminal alkyne to form the active alkynylruthenium nucleophile.     

Exploiting an inherent neighboring group effect of alpha-amino acids to synthesize extremely hindered dipeptides

The creation of highly hindered peptides that contain combinations of non-natural N-alkyl amino acids and N-alkyl-alpha,alpha-disubstituted amino acids presents a formidable challenge. Hindered, non-natural amino acids are of interest because they import resistance to proteolysis and unusual conformational properties to peptides that contain them. Toward a solution to this problem, we describe a new approach to creating extremely hindered dipeptides that is operationally simple and uses mild conditions and commercially available amino acids. The approach reduces the need for protecting groups and yields urethane-protected dipeptide acids that can be used as building blocks in the synthesis of larger peptides. We propose that the reaction proceeds through a previously unexploited intramolecular O,N-acyl transfer pathway.