Pd-Ga intermetallic compounds as highly selective semihydrogenation catalysts

The intermetallic compounds Pd(3)Ga(7), PdGa, and Pd(2)Ga are found to be highly selective semihydrogenation catalysts for acetylene outperforming established systems. The stability of the crystal and electronic structure under reaction conditions allows the direct relation of structural and catalytic properties and a knowledge-based development of new intermetallic catalyst systems. In the crystal structure of PdGa palladium is exclusively surrounded by gallium atoms. The alteration of the Pd coordination in PdGa leads to a strong modification of the electronic structure around the Fermi level in comparison to elemental Pd. Electronic modification and isolation of active sites causes the excellent catalytic semihydrogenation properties.     

A Gram‐Scale Batch and Flow Total Synthesis of Perhydrohistrionicotoxin

The total synthesis of the spiropiperidine alkaloid (?)‐perhydrohistrionicotoxin (perhydro‐HTX) 2 has been accomplished on a gram scale by employing both conventional batch chemistry as well as microreactor techniques. (S)‐(?)‐6‐Pentyltetrahydro‐pyran‐2‐one 8 underwent nucleophilic ring opening to afford the alcohol 10 , which was elaborated to the nitrone 13 . Protection of the nitrone as the 1,3‐adduct of styrene and side‐chain extension to the unsaturated nitrile afforded a precursor 17 , which underwent dipolar cycloreversion and 1,3‐dipolar cycloaddition to give the core spirocyclic precursor 18 that was converted into perhydro‐HTX 2 . The principal steps to the spirocycle 18 have successfully been transferred into flow mode by using different types of microreactors and in a telescoped fashion, allowing for a more rapid access to the histrionicotoxins and their analogues by continuous processing.     

Photoprogrammable Organic Light‐Emitting Diodes

Enlightening the memory : The integration of a crosslinkable photochromic dithienylperfluorocyclopentene (DTE) into organic light‐emitting diodes (OLED) allows for the individualization of the emissive area of the OLED device, for example, for signage applications. The operation principle is based on switching the injection barrier for holes (positive charge carriers). Very large ON/OFF ratios of up to 3000 for current as well as electroluminescence have been achieved.

     

Sterically crowded azulene-based dication salts as novel guests: synthesis and complexation studies with crown ethers and calixarenes in solution and in the gas phase

The 1,4-bis(3-guaiazulenylmethylium)benzene and 1,4-bis[1-(4,6,8-trimethylazulenylmethylium)]benzene dication salts were synthesized via an acid-catalyzed condensation/dehydration protocol with guaiazulene-terephthalaldehyde (2 : 1 ratio), and 4,6,8-trimethylazulene-terephthalaldehyde (2 : 1 ratio) respectively in one-pot processes. A similar condensation reaction with the parent azulene led to an insoluble oligomer that was shown by MALDI-TOF-MS to contain 1,4-bis[(diazulenyl)methylium]benzene as a repeating unit. Dication salts and were fully characterized by 2D NMR and NOE techniques and by electrospray-MS (ES-MS) and MALDI-TOF-MS. NMR studies confirm that the dications are best represented as bis-tropylium species. A delicate balance of electronic (inductive stabilization) and steric influence of the alkyl groups on the seven-membered ring seems to influence the chemo-/regioselectivity of the co-condensation process. NMR titration and T(1) measurements established that, despite its highly crowded structure, dication forms host-guest HG complexes with dibenzo-30-crown-10 (DB30C10) and dibenzo-24-crown-8 (DB24C8) in solution, but fails to complex with the smaller dibenzo-18-crown-6 (DB18C6). The corresponding HG cation-molecule cluster ions were also detected in the gas phase by ES-MS, showing the formation of both dication-crown 1 : 1 and 1 : 2 complexes. Similar complexation of dication salt with DB30C10 was observed via NMR titration and T(1) measurements in solution and by ES-MS in the gas phase. Although solution complexation studies (NMR titration) did not indicate stable complex formation between and p-tert-butyl-methoxycalix[8]arene, their [HG](2+) and [H(2)G](2+) clusters were detectable by ES-MS. Solution decomplexation experiments (HG(2+) --> H + G(2+)) were performed on -crown complex by addition of DMSO, acetone, silver tosylate, and tropylium cation salt. Complexation of with DB30C10 was also studied by microcalorimetric titration.     

Sequencing mixed-model assembly lines: Survey,classification and model critique

Manufacturers in a wide range of industries nowadays face the challenge of providing a rich product variety at a very low cost. This typically requires the implementation of cost efficient, flexible production systems. Often, so called mixed-model assembly lines are employed, where setup operations are reduced to such an extent that various models of a common base product can be manufactured in intermixed sequences. However, the observed diversity of mixed-model lines makes a thorough sequence planning essential for exploiting the benefits of assembly line production. This paper reviews and discusses the three major planning approaches presented in the literature, mixed-model sequencing, car sequencing and level scheduling, and provides a hierarchical classification scheme to systematically record the academic efforts in each field and to deduce future research issues.     

Solving the car sequencing problem via Branch & Bound

In a mixed-model assembly line, varying models of the same basic product are to be produced in a facultative sequence. This results to a short-term planning problem where a sequence of models is sought which minimizes station overloads. In practice – e.g. the final assembly of cars – special sequencing rules are enforced which restrict the number of models possessing a certain optional feature k to r_k within a subsequence of s_k successive models. This problem is known as car sequencing. So far, employed solution techniques stem mainly from the field of Logic and Constraint Logic Programming. In this work, a special Branch & Bound algorithm is developed, which exploits the problem structure in order to reduce combinatorial complexity.     

1,4‐Di(isopropyl)‐1,4‐diazabutadien als Abfangreagenz für monomere Bruchstücke des Tetragalliumclusters Ga4[C(SiMe3)3]4 – Bildung eines ungesättigten GaN2C2‐Heterocyclus und eines Oxidationsprodukts mit Ga‐O‐O‐Ga‐Gruppe

1,4‐Di(isopropyl)‐1,4‐diazabutadiene as a Reagent for the Trapping of Monomeric Fragments of the Tetragalliumcluster Ga₄[C(SiMe₃)₃]₄ – Formation of an Unsaturated GaN₂C₂ Heterocycle and an Oxidation Product Containing a Ga‐O‐O‐Ga Group The tetrahedral tetragallium cluster Ga₄[C(SiMe₃)₃]₄ ( 1 ) dissociates upon dissolution to yield the monomeric fragments Ga‐R [R = C(SiMe₃)₃]. These monomers could be trapped now by the treatment of their solutions with 1,4‐di(isopropyl)‐1,4‐diazabutadiene. The product of the cycloaddition reaction ( 2 ) possesses a five‐membered GaN₂C₂ heterocycle with a coordinatively unsaturated gallium atom and an endocyclic C=C double bond. 2 is rather sensitive towards oxidation by traces of air. The contact with oxygen yielded a digallium peroxide [(C₂N₂iPr₂)RGa‐O‐O‐GaR(C₂N₂iPr₂)] ( 3 ) which was isolated in a very low yield only and which has a gallium atom attached to each oxygen atom of the inner peroxo group. Both chelating ligands of 3 possess an unpaired electron.     

Synthesis of [Ru(phen)(2)dppz](2+)-tethered oligo-DNA and studies on the metallointercalation mode into the DNA duplex

To explore the binding properties of [Ru(phen)(2)dppz](2+) complex (phen = 1,10-phenanthroline, dppz = dipyrido[3,2-a:2',3'-c]phenazine) in a sequence-specific manner in DNA duplex, it was tethered through the dppz ligand to a central position as well as both at the 3'- and 5'-ends of oligodeoxyribonucleotide (ODN). The middle [Ru(phen)(2)dppz](2+)-ODN tethered was resolved and isolated as four pure diastereomers, while the 3'- or 5'-[Ru(phen)(2)dppz](2+)-ODNs were inseparable on RP-HPLC. Thermal stability of the (Ru(2+)-ODN).DNA duplexes is found to increase considerably (DeltaT(m) = 12.8-23.4 degrees C), depending upon the site of the covalent attachment of the tethered [Ru(phen)(2)dppz](2+) complex, or the chirality of the [Ru(phen)(2)dppz](2+)-linker tethered at the middle of the ODN, compared to the unlabeled counterpart. Gross differences in CD between the [Ru(phen)(2)dppz](2+)-tethered and the native DNA duplexes showed that the global duplex conformation of the former has considerably altered from the B-type, but is still recognized by DNase I. The thermal melting studies, CD measurements, as well as DNase I digestion data, are interpreted as a result of intercalation of the dppz moiety, which is realized by threading of the Ru(phen)(2) complex part through the DNA duplex core. DNase I footprinting with four diastereomerically pure middle ([Ru(phen)(2)dppz](2+)-ODN).DNA duplexes furthermore showed that the tethered [Ru(phen)(2)dppz](2+)-linker chirality dictates the stereochemical accessibility of various phosphodiester moieties (around the intercalation site) toward the cleavage reaction by the enzyme. The diastereomerically pure ruthenium-modified duplexes, with the well-defined pi-stack, will be useful to explore stereochemistry-dependent energy- and electron-transfer chemistry to understand oxidative damage to the DNA double helix as well as the long-range energy- and electron-transfer processes with DNA as a reactant.     

Selective propargylic C(sp3)–H activation of methyl-substituted alkynes versus [2 + 2] cycloaddition at a titanium imido template

The reaction of the titanium imido complex 1b with 2-butyne leads to the formation of the titanium azadiene complex 2a at ambient temperature instead of yielding the archetypical [2 + 2] cycloaddition product (titanaazacyclobutene) which is usually obtained by combining titanium imido complexes and internal alkynes. The formation of 2a is presumably caused by an initial propargylic C(sp³)–H activation step and quantum chemical calculations suggest that the outcome of this unexpected reactivity is thermodynamically favored. The previously reported titanaazacyclobutene I (which is obtained by reacting 1b with 1-phenyl-1-propyne) undergoes a rearrangement reaction at elevated temperature to give the corresponding five-membered titanium azadiene complex 2b.

An unexpected reactivity between a titanium imido complex and internal alkynes was unveiled yielding titanaazacyclobutenes instead of the expected [2 + 2] cycloaddition products.