Facile generation of dialkylaminyl radicals from N-hydroxypyridine-2-thione carbamates. Application in kinetic studies of small ring cycloalkylaminyl radical ring openings

The preparation of N-hydroxypyridine-2-thione carbamates and the formation of dialkylaminyl radicals from these species are described and the application of this methodology in kinetic studies of aminyl radical rearrangements is demonstrated.     

Preparation of chlorofluoromethylene olefins via phosphine dechlorination of dichlorofluoromethyltris?dimethylamino?phosphonium chloride

The reaction of dichlorofluoromethyltris?dimethylamino?phosphonium chloride with tertiary phosphines provides a convenient preparation of the chlorofluoromethylenetris?dimethylamino?phosphonium ylide. This ylide provides reasonable yields of chlorofluoroolefins from aldehydes, activated ketones, non-activated ketones, and activated esters. The mechanism of phosphonium salt formation was shown to involve positive chlorine abstraction from CFCl₃ by (Me₂N)₃P followed by recombination of the intermediate ion pair. Dechlorination of the resulting dichlorofluoromethyltris?dimethylamino?phosphonium chloride by triphenylphosphine gave an olefinating solution of reasonable stability. In contrast, the solution obtained by dechlorination of the phosphonium salt by tris?dimethylamino?-phosphine showed no stability.     

Endo-exo and facial stereoselectivity in the Diels-Alder reactions of 3-substituted cyclopropenes with butadiene

A computational examination of the four modes of addition in the Diels-Alder reactions of 3-substituted cyclopropene derivatives (substituents: BH(2), CH(3), SiH(3), NH(2), PH(2), OH, SH, F, and Cl) with butadiene have been carried out at the B3LYP/6-31++G(d)//HF/6-31++G(d) level. The degree of stabilization of these derivatives at the ground state correlates with the electronegativity of the substituent. This attenuation of reactivity and differences in steric interactions are the only factors needed to explain both the high facial selectivity and the differences in the endo-exo selectivity seen in these reactions. Furthermore, evidence is presented that indicates that stabilization by an interaction involving the syn C-3 hydrogen of cyclopropene and butadiene is small or irrelevant in controlling the endo-exo selectivity of the Diels-Alder reaction.     

Molecular dynamics simulations of the oxidation of aluminum nanoparticles

The oxidation of aluminum nanoparticles is studied with classical molecular dynamics and the Streitz-Mintmire (Streitz, F. H.; Mintmire, J. W. Phys. Rev. B 1994, 50, 11996) electrostatic plus (ES+) potential that allows for the variation of electrostatic charge on all atoms in the simulation. The structure and charge distributions of bulk crystalline alpha-Al(2)O(3), a surface slab of alpha-Al(2)O(3) with an exposed (0001) basal plane, and an isolated Al(2)O(3) nanoparticle are studied. Constant NVT simulations of the oxidation of aluminum nanoparticles are also performed with different oxygen exposures. The calculations simulate a thermostated one-time exposure of an aluminum nanoparticle to different numbers of surface oxygen atoms. In the first set of oxidation studies, the overall approximate ratios of Al to O in the nanoparticle are 1:1 and 2:1. The nanoparticles are annealed to 3000 K and are then cooled to 500, 1000, or 1500 K. The atomic kinetic energy is scaled during the simulation to maintain the desired temperature. The structure and charge distributions in the oxidized nanoparticles differ from each other and from those of the bulk Al(2)O(3) phases. In the Al(1)O(1) simulation, an oxide shell forms that stabilizes the shape of the particle, and thus the original structure of the nanoparticle is approximately retained. In the case of Al(1)O(0.5), there is insufficient oxygen to form a complete oxide shell, and the oxidation results in particles of irregular shapes and rough surfaces. The particle surface is rough, and the nanoparticle is deformed.     

Solid colloidal particles inducing coalescence in bitumen-in-water emulsions

Silica particles are dispersed in the continuous phase of bitumen-in-water emulsions. The mixture remains dispersed in quiescent storage conditions. However, rapid destabilization occurs once a shear is applied. Observations under the microscope reveal that the bitumen droplets form a colloidal gel and coalesce upon application of a shear. We follow the kinetic evolution of the emulsions viscosity, eta, at constant shear rate: eta remains initially constant and exhibits a dramatic increase after a finite time, tau. We study the influence of various parameters on the evolution of tau: bitumen droplet size and volume fraction, silica diameter and concentration, shear rate, etc.     

Potent inhibitors of the hepatitis C virus NS3 protease: design and synthesis of macrocyclic substrate-based beta-strand mimics

The virally encoded NS3 protease is essential to the life cycle of the hepatitis C virus (HCV), an important human pathogen causing chronic hepatitis, cirrhosis of the liver, and hepatocellular carcinoma. The design and synthesis of 15-membered ring beta-strand mimics which are capable of inhibiting the interactions between the HCV NS3 protease enzyme and its polyprotein substrate will be described. The binding interactions between a macrocyclic ligand and the enzyme were explored by NMR and molecular dynamics, and a model of the ligand/enzyme complex was developed.