Dual Lewis acid-Lewis base activation in enantioselective cyanation of aldehydes using acetyl cyanide and cyanoformate as cyanide sources

Dual activation by a chiral Lewis acid and an achiral or chiral Lewis base enabled cyanation of both aromatic and aliphatic aldehydes with acetyl cyanide and ethyl cyanoformate to provide direct access to O-acetylated and O-alkoxycarbonylated cyanohydrins, respectively, under mild conditions. With a combination of a Ti-salen catalyst and Et3N, benzaldehyde was converted to the O-acetylated cyanohydrin with 94% ee within 10 h at -40 degrees C in 89% isolated yield.     

Magnetohydrodynamic flow in a rectangular duct with perfectly conducting electrodes

Zusammenfassung Grenzschichtverfahren werden benutzt, um das Problem der magnetohydrodynamischen Strömung einer leitenden Flüssigkeit durch einen unendlichen rechteckigen Kanal bei grosser Hartmannzahl zu lösen für den Fall, dass die Wände, die parallel zum angelegten Magnetfeld sind, perfekt leiten, während die anderen beiden Wände Isolatoren sind. Das Problem ist als Integralgleichung für die Stromdichte an der Elektrode formuliert. Diese Gleichung besitzt als Lösung eine einfache, geschlossene Form.

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Part of this work was done in partial fulfillment of the degree of Doctor of Philosophy at the University of Minnesota.     

Direct Formic Acid Mediated Z‐Selective Reductive Coupling of Dienes and Aldehydes

Methods for the addition of unsaturated nucleophiles to carbonyls to generate Z‐olefin products remain rare and often require either alkyl borane or zinc reductants, limiting their utility. Demonstrated here is that formic acid mediates the Rh‐catalyzed, Z‐selective coupling of dienes and aldehydes. The process is distinguished by broad tolerance towards reducible or electrophilic groups. Kinetic analysis suggests that generation of the catalytically active Rh intermediate by ligand dissociation is the rate‐determining step. The rapid generation and trapping of Rh‐allyl intermediates is key to preventing chain‐walking isomerization events that plague related protocols. Insights gained through this study may have wider implications in selective metal‐catalyzed hydrofunctionalization reactions.     

A characterization of cycle-free unit probe interval graphs

A graph is a probe interval graph (PIG) if its vertices can be partitioned into probes and nonprobes with an interval assigned to each vertex so that vertices are adjacent if and only if their corresponding intervals overlap and at least one of them is a probe. PIGs are a generalization of interval graphs introduced by Zhang for an application concerning the physical mapping of DNA in the human genome project. PIGs have been characterized in the cycle-free case by Sheng, and other miscellaneous results are given by McMorris, Wang, and Zhang. Johnson and Spinrad give a polynomial time recognition algorithm for when the partition of vertices into probes and nonprobes is given. The complexity for the general recognition problem is not known. Here, we restrict attention to the case where all intervals have the same length, that is, we study the unit probe interval graphs and characterize the cycle-free graphs that are unit probe interval graphs via a list of forbidden induced subgraphs.     

Reversible formation of a PdC(x) phase in Pd nanoparticles upon CO and O2 exposure

The structure and chemical composition of Pd nanoparticles exposed to pure CO and mixtures of CO and O(2) at elevated temperatures have been studied in situ by a combination of X-ray Diffraction and X-ray Photoelectron Spectroscopy in pressures ranging from ultra high vacuum to 10 mbar and from room temperature to a few hundred degrees celsius. Our investigation shows that under CO exposure, above a certain temperature, carbon dissolves into the Pd particles forming a carbide phase. Upon exposure to CO and O(2) mixtures, the carbide phase forms and disappears reversibly, switching at the stoichiometric ratio for CO oxidation. This finding opens new scenarios for the understanding of catalytic oxidation of C-based molecules.     

Porous three-dimensional nanorod arrays through selective chemical etching of nanocomposites

Three-dimensional Cu-Si and Cu-SiO(2) nanorod arrays containing ~68 at% Cu have been fabricated by a glancing angle co-deposition technique. By selectively etching Cu in 0.05 M KCN methanol solution, porous nanorods with different shapes form, which are promising for applications in sensors, catalysts, and as medical capsules that are able to be loaded with functional materials.     

Electrochemical trifluoromethylation of aromatic compounds

It was shown that the current yield of the products of anodic trifluoromethylation of arenes correlates with the adsorptivity and the rate of electrooxidation of arenes on the surface of a platinum electrode. Translated fromIzvestiya Akademi Nauk, Seriya Khimicheskaya, No. 6, pp. 1180–1184, June, 1997.     

Acetate formation during the ethanol oxidation on Rh(1 1 1)

The catalytic cracking of ethanol on clean and oxygen precovered Rh(1 1 1) surfaces has been studied by a combination of synchrotron based high-resolution core level spectroscopy and surface infrared spectroscopy. On the clean surface the decomposition of ethanol occurs without any formation of atomic oxygen. The presence of oxygen on the surface changes the reactivity significantly and acetate (CH₃COO) was identified as an important surface intermediate in the ethanol oxidation.