Of the ortho effect in palladium/norbornene-catalyzed reactions: a theoretical investigation

Mechanistic questions concerning palladium and norbornene catalyzed aryl-aryl coupling reactions are treated in this paper: how aryl halides react with the intermediate palladacycles, formed by interaction of the two catalysts with an aryl halide, and what is the rational explanation of the "ortho effect" (caused by an ortho substituent in the starting aryl halide), which leads to aryl-aryl coupling with a second molecule of aryl halide rather than to aryl-norbornyl coupling. Two possible pathways have been proposed, one involving aryl halide oxidative addition to the palladacycle, the other passing through a palladium(II) transmetalation, also involving the palladacycle, as previously proposed by Cardenas and Echavarren. Our DFT calculations using M06 show that, in palladium-catalyzed reaction of aryl halides, not containing ortho substituents, and norbornene, the intermediate palladacycle formed has a good probability to undergo transmetalation, energetically favored over the oxidative addition leading to Pd(IV). The unselective sp(2)-sp(2) and sp(2)-sp(3) coupling, experimentally observed in this case, can be explained in the framework of the transmetalation pathway since the energetic difference between aryl attack onto the aryl or norbornyl carbon of the palladacycle intermediate is quite small. On the other hand, according to the experimentally observed "ortho effect", selective aryl-aryl coupling only occurs in the reactions of ortho-substituted metallacycles. The present work offers the first possible rationalization of this finding. These in situ formed palladacycles containing an ortho substituent could more easily undergo oxidative addition of an aryl halide rather than reductive elimination from the transmetalation intermediate as a result of a steric clash in the transition state of the latter. The now energetically accessible Pd(IV) intermediate, featuring a Y-distorted trigonal bipyramidal structure, can account for the reported selective aryl-aryl coupling through a reductive elimination which is easier than aryl-norbornyl coupling. Thus, the steric effect represents the main factor that dictates the energetic convenience of the system to follow the Pd(IV) or the transmetalation pathway. Ortho substituents cause a higher energy transition state for reductive elimination from the transmetalation intermediate than for oxidative addition to the metallacycle palladium(II) and the pathway based on the latter predominates.     

A comparison of lower bounds for the symmetric circulant traveling salesman problem

When the matrix of distances between cities is symmetric and circulant, the traveling salesman problem (TSP) reduces to the so-called symmetric circulant traveling salesman problem (SCTSP), that has applications in the design of reconfigurable networks, and in minimizing wallpaper waste. The complexity of the SCTSP is open, but conjectured to be NP-hard, and we compare different lower bounds on the optimal value that may be computed in polynomial time. We derive a new linear programming (LP) relaxation of the SCTSP from the semidefinite programming (SDP) relaxation in [E. de Klerk, D.V. Pasechnik, R. Sotirov, On semidefinite programming relaxation of the traveling salesman problem, SIAM Journal of Optimization 19 (4) (2008) 1559-1573]. Further, we discuss theoretical and empirical comparisons between this new bound and three well-known bounds from the literature, namely the Held-Karp bound [M. Held, R.M. Karp, The traveling salesman problem and minimum spanning trees, Operations Research 18 (1970) 1138-1162], the 1-tree bound, and the closed-form bound for SCTSP proposed in [J.A.A. van der Veen, Solvable cases of TSP with various objective functions, Ph.D. Thesis, Groningen University, The Netherlands, 1992].     

Reversible gelatin-based hydrogels: Finetuning of material properties

The present work reports on the synthesis and evaluation of a crosslinkable thiolated gelatin derivative. The effect of varying two parameters including the pH of the reaction buffer and the thiolating agent applied (i.e. N-acetylhomocysteine thiolactone versus Traut’s reagent) on the obtained modification degree was studied in a first part. The gelatin derivatives synthesized starting from N-acetylhomocysteine thiolactone and Traut’s reagent were characterized in depth using size exclusion chromatography and UV–VIS spectrophotometry. In a subsequent part of the present work, hydrogel films were prepared starting from the thiolated gelatin derivative developed using N-acetylhomocysteine thiolactone. The contributions of both the chemical and the physical crosslinking of the hydrogels developed were studied in depth using rheology, swelling experiments and texturometry. The results indicate that the physical structuring, inherent to gelatin, contributes to a large extent to the mechanical properties. However, the chemical crosslinking mostly determines the final hydrogel properties and can be controlled to a large extent. The gelatin-based gels are flexible, strong and transparent. A major advantage of disulfide-crosslinked hydrogels is the fact that the crosslinking is reversible. The latter could be interesting in view of future applications as cell carriers for tissue engineering.     

A physico-chemical investigation of poly(ethylene oxide)-block-poly(L-lysine) copolymer adsorption onto silica nanoparticles

The adsorption behavior of poly(ethylene oxide)-b-poly(L-lysine) (PEO(113)-b-PLL(10)) copolymer onto silica nanoparticles was investigated in phosphate buffer at pH 7.4 by means of dynamic light scattering, zeta potential, adsorption isotherms and microcalorimetry measurements. Both blocks have an affinity for the silica surface through hydrogen bonding (PEO and PLL) or electrostatic interactions (PLL). Competitive adsorption experiments from a mixture of PEO and PLL homopolymers evidenced greater interactions of PLL with silica while displacement experiments even revealed that free PLL chains could desorb PEO chains from the particle surface. This allowed us to better understand the adsorption mechanism of PEO-b-PLL copolymer at the silica surface. At low surface coverage, both blocks adsorbed in flat conformation leading to the flocculation of the particles as neither steric nor electrostatic forces could take place at the silica surface. The addition of a large excess of copolymer favoured the dispersion of flocs according to a presumed mechanism where PLL blocks of incoming copolymer chains preferentially adsorbed to the surface by displacing already adsorbed PEO blocks. The gradual addition of silica particles to an excess of PEO-b-PLL copolymer solution was the preferred method for particle coating as it favoured equilibrium conditions where the copolymer formed an anchor-buoy (PLL-PEO) structure with stabilizing properties at the silica-water interface.     

Zone–edge optical transitions in GaAs/AlAs lateral superlattices grown on vicinal surfaces by molecular beam epitaxy

We report on new features in the photoluminescence excitation (PLE) spectra and PLE linear polarization spectra of GaAs/AlAs lateral superlattices grown by molecular beam epitaxy (MBE). These lines appear systematically as the tilt angle of the lateral superlattice is varied. They are identified as zone–edge excitonic transitions by comparison between experimental data and detailed numerical calculations of optical transitions including valence-band mixing and tilt effects.     

Théorème de De Rham pour les variétés stratifées

Sans résumé     

Stoichiometry effects on surface properties of GaAs{100} grown in situ by MBE

In this paper, we report results dealing with the effects of stoichiometry on surface properties of GaAs(001) layers grown by MBE. Three aspects of surface properties were investigated: crystallography, electronic properties and chemical reactivity. Surface crystallography was studied mainly by LEED. The reconstruction of the surface was found to be drastically dependent on the composition of the uppermost atomic layer, i.e. the surface stoichiometry. According to the arsenic surface coverage, many structures from the c(8 × 2) Ga rich to the (1 × 1) arsenic saturated surface have been observed. The influence of stoichiometry on surface electronic properties has been studied by electron loss spectroscopy (ELS) and contact potential difference (CPD) measurements. In the electron loss spectra, two peaks, at about 10.3 and 20.2 eV are very sensitive to the surface composition: they gradually disappear when the arsenic coverage increases, and consequently are associated with surface states on gallium atoms. On the other hand, the CPD measurements have shown that the variation of the work function with the arsenic surface coverage is not monotonic: in particular, an abrupt change of work function of about 300 meV occurs between the (1 × 6) and c(2 × 8) structures which are very similar as far as the arsenic surface coverage (about 0,5 and 0,6 respectively) is concerned. Therefore, it seems that the work function is strongly dependent on the atomic reconstruction occurring at the surface, and not only on its stoichiometry. The connection between stoichiometry and chemical reactivity of the surface is illustrated by the study of H₂S adsorption: a large difference (factor of 10³) in sticking coefficient has been found between surfaces with different arsenic coverages.     

Evaluation of the activation energy for the oligomerization of ethylene using a microreactor in a DTA

The theoretical considerations and the experimental data to calculate the activation energy of the ethylene oligomerization using a microreactor in a differential thermal analyzer are presented.