Rethinking Borole Cycloaddition Reactivity

Boroles are attracting broad interest for their myriad and diverse applications, including in synthesis, small molecule activation and functional materials. Their properties and reactivity are closely linked to the cyclic conjugated diene system, which has been shown to participate in cycloaddition reactions, such as the Diels-Alder reaction with alkynes. The reaction steps leading to boranorbornadienes, borepins and tricyclic boracyclohexenes from the thermal reaction of boroles with alkynes are seemingly well understood as judged from the literature. Herein, we question the long-established mechanistic picture of pericyclic rearrangements by demonstrating that seven-membered borepins (i. e., heptaphenylborepin and two derivatives substituted with a thienyl and chloride substituent on boron) exist in a dynamic equilibrium with the corresponding bicyclic boranorbornadienes, the direct Diels-Alder products, but are not isolable products from the reactions. Heating gradually converts the isomeric mixtures into fluorescent tricyclic boracyclohexenes, the most stable isomers in the series. Results from mechanistic DFT calculations reveal that the tricyclic compounds derive from the boranorbornadienes and not the borepins, which were previously believed to be intermediates in purely pericyclic processes.     

Analysis of Non-Aromatic Organic Acids in Beer by CE and Direct Detection Mode with Diode Array Detection

A method for the analysis of the main non-aromatic organic acids in beer using capillary electrophoresis is presented. In this work, malic, citric, succinic, pyruvic, acetic and lactic acids are separated using a sodium hydrogen phosphate background electrolyte with direct detection mode with a diode array detector. The separation exhibits lower sensitivity than equivalent methods with indirect detection mode, however, the risk of co-migration with unknown compounds in beer matrixes is significantly reduced. This is due to (i) a higher efficiency (250,000–400,000 theoretical plates), (ii) a higher selectivity than any equivalent method using an indirect detection mode, and (iii) the possibility to monitor other wavelengths in parallel (260 nm for example) to check for possible co-migration with phenolic or benzoic acids. This was critical when working with beer samples as an unknown compound absorbing at 200 and 260 nm was detected in the neighbourhood of malic, citric and succinic acids. Such co-migration will not have been detected using single wavelength detection below 200 nm or indirect detection mode.     

Dibarium zirconium tetraoxalate trihydrate

A new mixed barium zirconium oxalate, tri­aqua­tetra‐μ‐oxalato‐dibarium(II)­zirconium(IV), Ba₂Zr(C₂O₄)₄·3H₂O or [Ba₂Zr(C₂O₄)₄(H₂O)₃]_n, has been synthesized. The complex is built from eightfold‐coordinated Zr atoms and eleven‐ and sixfold‐coordinated Ba atoms, linked by oxalate groups. The Zr atom, the two Ba atoms and one water O atom lie on crystallographic twofold axes, so that each coordination polyhedron has imposed C2 symmetry. Packing in the crystal is also assumed through hydrogen bonds.     

A fair sampling test for EPR-Bell experiments

The low efficiency of detectors in all EPR experiments with optical photons makes the use of the fair sampling assumption unavoidable. This assumption is reputed to be both reasonable and impossible to test experimentally. We argue that there is, in fact, little evidence supporting the fair sampling assumption, and we propose an experiment capable of putting this crucial hypothesis to a test.     

Pseudo-domino palladium-catalyzed allylic alkylation/Mizoroki-Heck coupling reaction: a key sequence toward (±)-podophyllotoxin

A formal synthesis of podophyllotoxin was carried out in nine steps. The key pseudo-domino step was accomplished through the succession of an intermolecular palladium-catalyzed allylic alkylation and an intramolecular Mizoroki-Heck coupling reaction.     

Electrochemical "switching" of Si(100) modular assemblies

We report on a modular approach for producing well-defined and electrochemically switchable surfaces on Si(100). The switching of these surfaces is shown to change a Si(100) surface from resistant to cell adsorption to promoting cell adhesion. The electrochemical conversion of the modified electrode surface is demonstrated by X-ray photoelectron spectroscopy, X-ray reflectometry, contact angle and cell adhesion studies.     

Oxidative ipso-Rearrangement Performed by a Hypervalent Iodine Reagent and Its Application

An oxidative ipso-rearrangement mediated by a hypervalent iodine reagent that enables rapid generation of a functionalized dienone system containing a quaternary carbon center connected to several sp(2) centers has been developed. The process occurs through transfer of an aryl group from a silyl segment present on the lateral chain. As an illustration of the potential of this transformation, a total synthesis of sceletenone, a small alkaloid, is described.     

Thioketone‐Mediated Polymerization with Dithiobenzoates: Proof for the Existence of Stable Radical Intermediates in RAFT Polymerization

A novel dithioester control agent [dimethyltetrathioterephtalate (DMTTT)] is presented for the thioketone‐mediated radical polymerization (TKMP) of n‐butyl acrylate. The rate of polymerization is significantly decreased in the presence of DMTTT indicating formation of dormant radical species. During polymerization, molar masses increase linearly with monomer conversion with reasonably narrow initial molar mass distributions (PDI between 1.3 and 1.8), whereas the dispersity increases during the course of the polymerization due to irreversible termination of both propagating and dormant radicals. The present results thus highlight the possibility of a mixed mechanism operating in RAFT polymerization, which combines slow fragmentation (long‐lived intermediates) and intermediate radical termination.     

Entanglement Thresholds for Random Induced States

For a random quantum state on obtained by partial tracing a random pure state on , we consider the question whether it is typically separable or typically entangled. For this problem, we show the existence of a sharp threshold of order roughly . More precisely, for any and for d large enough, such a random state is entangled with very large probability when , and separable with very large probability when . One consequence of this result is as follows: for a system of N identical particles in a random pure state, there is a threshold such that two subsystems of k particles each typically share entanglement if k > k₀, and typically do not share entanglement if k < k₀. Our methods also work for multipartite systems and for “unbalanced” systems such as , . The arguments rely on random matrices, classical convexity, high‐dimensional probability, and geometry of Banach spaces; some of the auxiliary results may be of reference value. © 2013 Wiley Periodicals, Inc.     

Darboux theory of integrability in the sparse case

Darboux's theorem and Jouanolou's theorem deal with the existence of first integrals and rational first integrals of a polynomial vector field. These results are given in terms of the degree of the polynomial vector field. Here we show that we can get the same kind of results if we consider the size of a Newton polytope associated to the vector field. Furthermore, we show that in this context the bound is optimal.