Tin-free radical alkoxyamine addition and isomerization reactions by using the persistent radical effect: variation of the alkoxyamine structure

Various C-centered radicals can efficiently be generated through thermal C-O-bond homolysis of alkoxyamines. This method is used to perform environmentally benign radical cyclization and intermolecular addition reactions. These alkoxyamine isomerizations and intermolecular carboaminoxylations are mediated by the persistent radical effect (PRE). In the paper, the effect of the variation of the alkoxyamine structure--in particular steric effects in the nitroxide moiety--on the outcome of the PRE mediated radical reactions will be discussed. Fourteen different nitroxides were used in the studies. It will be shown that reaction times can be shortened about 100 times upon careful tuning of the alkoxyamine structure. Activation energies for the C-O-bond homolysis of the various alkoxyamines are provided. The kinetic data are used to explain the reaction outcome of the PRE-mediated processes.     

High Syn Selectivity of a SE′ Reaction: Acylation of an Optically Active 1,1-Disilyl-2-alkene. Preliminary communication

The disilane/disilylmethane rearrangement of an optically active disilanyl sulfide 9B was used to prepare an optically active disilylalkene 10 whose absolute configuration was established by X-ray analysis of the bromo derivative 13 (P2₁, a = 7.847 (3) Å, b = 9.487 (3) Å, c = 20.010 (8) Å, β = 82.28° (3), Z = 2). Acylation of 10 furnished an optically active ketone 14 , which was degraded to 16 , a compound of known absolute configuration. The enantiomeric excess of 10 was determined by alkylation with an optically active lithium compound and that of 14 by an optically active NMR.-shift reagent. The S_E′ reaction 10 → 14 was thus shown to proceed with 94% (97% syn/3% anti) stereoselectivity.     

Frictional shakedown and ratcheting of an oscillating cylindrical,elastic contact with coulomb friction

We examine frictional shakedown of an elastic contact of a cylinder pressed on a flat substrate. Slight oscillatory rolling of the cylinder varies the pressure distribution and the contact region. Together with the tangential load, this rocking motion causes incremental sliding processes and a macroscopic rigid body motion. In case that the oscillation amplitude is sufficiently small, the slip ceases after the first few periods and a safe shakedown occurs: the residual force in the contact withstands the tangential load. Otherwise ratcheting occurs: one side of the contact alternately sticks, while the other slips. This leads to a continuing rigid body motion. By derivation of the tangential stress distribution and use of the Boussinesq and Cerruti potential functions, we find approximations for the shakedown limits for the tangential load and the oscillation amplitude. This allows the accurate prediction of the displacement and the reduced tangential load capacity in the shakedown state. The results show strong agreement with numerical and experimental data.     

A wear-reduced nanodrive based on oscillating rolling

In this article we introduce a promising new concept for a high precision actuator. It is based on inertia effects and oscillating rolling. A sphere acts as the drive and is pressed on a movable substrate that acts as the runner. A combination of oscillating translation and rotation of the sphere induces motion of the runner. A varying normal force leads to varying indentation depth and contact area. This asymmetry together with the inertia of the runner enables accurate control of its displacement. As slip is completely omitted here, in theory the actuator works principally wearless. We use the method of dimensionality reduction to conduct a quasistatic numerical simulation of the system. In addition we derive analytical expressions for the steady working points of the system that are in perfect agreement with the simulation results.     

A basic course on compiler principles

An attempt to devise a methodology of compiler design is described, and an outline is given for a possible course on this subject. The theoretical basis is formed by the concepts of phrase-structure language, finite-state- and stack-acceptor, and transducer. As their extension capable of processing context-dependent elements of languages, a so-called Table-Transducer is postulated, and it serves as the core-algorithm upon which compilers are based. The developed theory and method of compiler construction is applied to an example of a simple programming language.     

Restricted Liapunov stability and stabilization of ordinary differential equations

Summary For the solutions of a system of ordinary differential equations restricted Liapunov stability and the notion of Liapunov stable sets are discussed, in particular for separable and almost separable Hamiltonian systems. We consider stabilization procedures, i.e. modifications of the original differential equations such that the solutions of the modified are Liapunov stable and thus the new system is better suited for numerical integration. In this paper some theoretical aspects rather than numerical methods are pointed out.

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Zusammenfassung Wir diskutieren die eingeschränkte Liapunov-Stabilität von Lösungen eines Systems von gewöhnlichen Differentialgleichungen und den Begriff der Liapunov-stabilen Menge, insbesondere für separable und fastseparable Hamilton'sche Systeme. Dann betrachten wir Stabilisierungs-methoden, d.h. Abänderungen der Differentialgleichungen, die bewirken, dass die Lösungen des modifizierten Systems Liapunov-stabil sind und die numerische Integration dadurch verbessert wird. Das Gewicht liegt auf der Klärung von einigen theoretischen Zusammenhängen und nicht auf der Erläuterung von numerischen Methoden.

     

Microwave-assisted free radical chemistry using the persistent radical effect

Environmentally benign radical carboaminoxylations of various nonactivated olefins and difficult radical cyclization reactions are performed in good to excellent yields and with short reaction times under microwave irradiation.     

Alterations in enamel remineralization in vitro induced by blue light

Blue light, especially from LED devices, is a very frequently used tool in dental procedures. However, the investigations of its effects on dental enamel are focused primarily on enamel demineralization and fluoride retention. Despite the fact that this spectral region can inhibit enamel demineralization, the effects of the irradiation on demineralized enamel are not known. For this reason, we evaluated the effects of blue LED on remineralization of dental enamel. Artificial lesions were formed in bovine dental enamel blocks by immersing the samples in undersaturated acetate buffer. The lesions were irradiated with blue LED (455 nm, 1.38 W/cm², 13.75 J/cm², and 10 s) and remineralization was induced by pH-cycling process. Cross-sectional hardness was used to asses mineral changes after remineralization. Non-irradiated enamel lesions presented higher mineral content than irradiated ones. Furthermore, the mineral content of irradiated group was not significantly different from the lesion samples that were not submitted to the remineralization process. Results obtained in the present study show that the blue light is not innocuous for the dental enamel and inhibition of its remineralization can occur.     

Compact,diode-side-pumped and Q-switched Nd:YLiF<Subscript>4</Subscript> laser cavity operating at 1053 nm with diffraction limited beam quality

This work reports a diode-side-pumped and passively Q-switched Nd:YLiF₄ (YLF) laser operating at 1053 nm using a new laser resonator concept. Very stable pulses of 1 mJ energy with less than 10 ns pulse duration are obtained at 1 kHz repetition rate in a very simple, compact, and robust cavity that uses a double bounce configuration to achieve TEM₀₀ operation.     

Steric and electronic effects in cyclic alkoxyamines--synthesis and applications as regulators for controlled/living radical polymerization

The synthesis of new six- and seven-membered cyclic alkoxyamines bearing ethyl groups at the alpha-N position of the alkoxyamines is described. The key step in the synthesis of the sterically hindered six-membered cyclic alkoxyamines is a Wadsworth-Horner-Emmons olefination with bisphosphonate 1. The seven-membered cyclic alkoxyamines were prepared from the corresponding six-membered keto alkoxyamines by ring-enlargement with trimethylsilyl(TMS)-diazomethane. The use of the new alkoxyamines as regulators/initiators for radical polymerization is discussed. Efficient controlled and living polymerization of styrene and n-butyl acrylate was obtained with the six-membered tetraethyl alkoxyamine 13. Controlled polymerizations can be conducted even at 90 degrees C. In addition, alkoxyamine 13 can be used for the preparation of AB diblock and ABA triblock copolymers with narrow polydispersities. The influence of the replacement of methyl groups in the alpha-position of the N atom in cyclic alkoxyamines by larger ethyl groups on the styrene polymerization (reaction time, PDI, kinetics of the C-O bond homolysis) is discussed. In addition, thermal decomposition of the new alkoxyamines was studied. Furthermore, the synthesis of N,N-bissilylated alkoxyamines is described. The silylated alkoxyamines are not suitable as regulators/initiators for the controlled/living radical polymerization. The C-O bonds in silylated alkoxyamines are stronger than the C-O bonds in analogous N,N-dialkylated alkoxyamines. The experimental results are verified by calculations with Gaussian 98 (A. 9).