Metal-free dihydroxylation of alkenes using cyclobutane malonoyl peroxide

Cyclobutane malonoyl peroxide (7), prepared in a single step from the commercially available diacid 6, is an effective reagent for the dihydroxylation of alkenes. Reaction of a chloroform solution of 7 with an alkene in the presence of 1 equiv of water at 40 °C followed by alkaline hydrolysis leads to the corresponding diol (30-84%). With 1,2-disubstituted alkenes, the reaction proceeds with syn-selectivity (3:1 → 50:1). A mechanism consistent with experimental findings is proposed, which is supported by deuterium and oxygen labeling studies and explains the stereoselectivity observed. Alternative reaction pathways that are dependent on the structure of the starting alkene are also described leading to the synthesis of allylic alcohols and γ-lactones.     

Xanthates as synthetic equivalents of oxyacyl radicals: access to lactones under tin-free conditions

In addition to their utility in Barton-McCombie deoxygenations, xanthates can engage in 5-exo-trig radical cyclizations to afford lactones after oxidative workup. In this paper, we describe a tin-free protocol that provides direct access to lactones via hydrolysis of labile thioketal intermediates. Analysis of several systems of varying complexity reveals that the reaction is most applicable for constrained systems in which the reacting center is prepositioned near the radical-accepting alkene.     

C2 hydroxyl group governs the difference in hydrolysis rates of methyl-α-D-glycero-D-guloseptanoside and methyl-β-D-glycero-D-guloseptanoside

A computational investigation into the hydrolysis of two methyl septanosides, methyl-α-D-glycero-D-guloseptanoside and methyl-β-D-glycero-D-guloseptanoside was undertaken. These septanosides were chosen as model compounds for comparison to methyl pyranosides and allowed direct comparison of α versus β hydrolysis rates for a specific septanoside isomer. Results suggest that hydrolysis takes place without proceeding through a transition state, an observation that was suggested in previous computational studies on exocyclic bond cleavage of carbohydrates. A conformational analysis of α- and β-anomers 1 and 2 and their corresponding oxocarbenium 3, coupled with relaxed potential energy surface (PES) scans (M06-2X/6-311+G**, implicit methanol), indicated that hydrolysis of the α-anomer is favored by 1-2 kcal/mol over the β-anomer, consistent with experiment. Model systems revealed that the lowest energy conformations of the septanoside ring system destabilize the β-anomer by 2-3 kcal/mol relative to the α-anomer, and the addition of a single hydroxyl group at the C2-position on a minimal oxepane acetal can reproduce the PES for the septanoside 1. These results suggest that the C2 hydroxyl plays a unique role in the hydrolysis mechanism, destabilizing the septanoside via its proximity to the anomeric carbon and also through its interaction with the departing methanol from the α-anomer via hydrogen-bonding interactions.     

Stereoselection in the diels–alderase ribozyme: A molecular dynamics study

The Diels‐Alderase ribozyme is an in vitro‐evolved ribonucleic acid enzyme that catalyzes a [4 + 2] cycloaddition reaction between an anthracene diene and a maleimide dienophile. The ribozyme can in principle be used to selectively synthesize only one product enantiomer, depending on which of the two entrances to the catalytic pocket, “front” or “back”, the substrate is permitted to use. Here, we investigate stereoselection and substrate recognition in the ribozyme by means of multiple molecular dynamics simulations, performed on each of the two substrates individually in the pocket, on the reactant state, and on the product state. The results are consistent with a binding mechanism in which the maleimide likely binds first followed by the anthracene, which enters preferentially through the front door. The free energy profiles for anthracene binding indicate that the pre‐(R,R)‐enantiomer conformation is slightly preferred, in agreement with the experimentally observed small enantiomeric excess of the (R,R)‐enantiomer of the product. The reactant state is stabilized by the simultaneous presence of both substrates bound to their binding sites in the hydrophobic pocket as well as by stacking interactions between them. © 2012 Wiley Periodicals, Inc.     

High-density amine-terminated monolayers formed on fluorinated CVD-grown graphene

There has been considerable interest in chemically functionalizing graphene films to control their electronic properties, to enhance their binding to other molecules for sensing, and to strengthen their interfaces with matrices in a composite material. Most reports to date have largely focused on noncovalent methods or the use of graphene oxide. Here, we present a method to activate CVD-grown graphene sheets using fluorination followed by reaction with ethylenediamine (EDA) to form covalent bonds. Reacted graphene was characterized via X-ray photoelectron spectroscopy (XPS), attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and Raman spectroscopy as well as measurements of electrical properties. The functionalization results in stable, densely packed layers, and the unbound amine of EDA was shown to be active toward subsequent chemical reactions.     

Pressure‐Induced Enhancement of Magnetic‐Ordering Temperature in an Organic Radical to 70 K: A Magnetostructural Correlation

The structure of the canted antiferromagnet β‐p‐NCC₆F₄CNSSN ( 1 ) was determined from synchrotron powder‐diffraction studies in the pressure range 0–21.6 kbar. Radical 1 crystallizes in the orthorhombic space group Fdd2, but undergoes an asymmetric contraction of the unit‐cell size with increasing pressure. At the molecular level, this contraction of the unit cell is simultaneously accommodated by: 1) an increase in twist angle between aryl and heterocyclic rings; and 2) a shortening of the intermolecular S ??? N contacts, which propagate the magnetic‐exchange pathway. DFT calculations based on the structures in this pressure range revealed an increase in the magnetic‐exchange interaction (J) with increasing pressure, and an excellent correlation was observed between J and the magnetic‐ordering temperature, which increased from 36 K at ambient pressure up to 70 K at 16 kbar.     

Ferromagnetic Interactions in a 1D Alternating Linear Chain of π‐Stacked 1,3‐Diphenyl‐7‐(thien‐2‐yl)‐1,4‐dihydro‐1,2,4‐benzotriazin‐4‐yl Radicals

X‐ray studies show that 1,3‐diphenyl‐7‐(thien‐2‐yl)‐1,4‐dihydro‐1,2,4‐benzotriazin‐4‐yl ( 6 ) adopts a distorted, slipped π‐stacked structure of centrosymmetric dimers with alternate short and long interplanar distances (3.48 and 3.52 Å). Cyclic voltammograms of 7‐(thien‐2‐yl)benzotriazin‐4‐yl 6 show two fully reversible waves that correspond to the ?1/0 and 0/+1 processes. EPR and DFT studies on radical 6 indicate that the spin density is mainly delocalized over the triazinyl fragment. Magnetic susceptibility measurements show that radical 6 obeys Curie–Weiss behavior in the 5–300 K region with C=0.378 emu K mol^?1 and θ=+4.72 K, which is consistent with ferromagnetic interactions between S=1/2 radicals. Fitting the magnetic susceptibility revealed the behavior is consistent with an alternating ferromagnetic chain (g=2.0071, J₁=+7.12 cm^?1, J₂=+1.28 cm^?1).     

Building a metal binding domain,one half at a time

The recently determined structure of a zinc binding peptide reveals that a particular sequence can adopt one stable fold as an isolated peptide but adopt an alternative structure as part of a larger protein domain.     

Measurement and energy correction due to charge carrier lifetimes in large HPGe spectrometers

This work addresses the energy spectrum correction due to increased charge carrier collection times in larger HPGe spectrometers. The energy of the radiation interaction is expected to be proportional to the total collected charge. This is increasingly not true with larger HPGe spectrometers. Some charge is lost as the total charge travels from the interaction location to the collection electrode. This path dependent loss of charge results in decreased energy resolution. In HPGe spectrometers, this process is characterized by the charge carrier lifetime constant and is given as an exponential function of the charge carrier collection time divided by this constant. Thus large detectors can experience exponential decrease in energy resolution as charge carrier collection time increases. We studied the effect of charge carrier lifetime on energy resolution for a p-type point contact HPGe spectrometers using pulse shape analysis. We present a method using the rise time to correct for the charge carrier lifetime on a pulse by pulse basis for a given HPGe spectrometer.     

Experimental measurements of short-lived fission products from uranium,neptunium, plutonium and americium

Fission yields are especially well characterized for long-lived fission products. Modeling techniques incorporate numerous assumptions and can be used to deduce information about the distribution of short-lived fission products. This work is an attempt to gather experimental (model-independent) data on short-lived fission products. Fissile isotopes of uranium, neptunium, plutonium and americium were irradiated under pulse conditions at the Washington State University 1 MW TRIGA reactor to achieve ~10⁸ fissions. The samples were placed on an HPGe (high purity germanium) detector to initiate counting in less than 3 min post irradiation. The data was analyzed to determine which radionuclides could be quantified and compared to the published fission yield data.