Chromatography-Free Multicomponent Synthesis of Thioureas Enabled by Aqueous Solution of Elemental Sulfur

The development of a new three-component chromatography-free reaction of isocyanides, amines and elemental sulfur allowed us the straightforward synthesis of thioureas in water. Considering a large pool of organic and inorganic bases, we first optimized the preparation of aqueous polysulfide solution from elemental sulfur. Using polysulfide solution, we were able to omit the otherwise mandatory chromatography, and to isolate the crystalline products directly from the reaction mixture by a simple filtration, retaining the sulfur in the solution phase. A wide range of thioureas synthesized in this way confirmed the reasonable substrate and functional group tolerance of our protocol.     

Studies aimed at the synthesis of morphine V an economic approach to (±)-reticuline from 3,4-dihydropapaverine

Regioselective demethylation of 3,4-dihydropapaveraldine ($\text{2}$a) at 7 and 3' positions affords a properly substituted diphenolic key intermidiate ($\text{2}$d) for the synthesis of reticuline and N-norreticuline.     

Non-Trivial t-Intersection in the Function Lattice

The function lattice, or generalized Boolean algebra, is the set of ℓ-tuples with the ith coordinate an integer between 0 and a bound n_i. Two ℓ-tuples t-intersect if they have at least t common nonzero coordinates. We prove a Hilton–Milner type theorem for systems of t-intersecting ℓ-tuples.Received September 29, 2004     

Improved customer choice predictions using ensemble methods

In this paper various ensemble learning methods from machine learning and statistics are considered and applied to the customer choice modeling problem. The application of ensemble learning usually improves the prediction quality of flexible models like decision trees and thus leads to improved predictions. We give experimental results for two real-life marketing datasets using decision trees, ensemble versions of decision trees and the logistic regression model, which is a standard approach for this problem. The ensemble models are found to improve upon individual decision trees and outperform logistic regression.     

The Neumann problem,cylindrical outlets and Sobolev spaces

The Neumann problem is crucial in mathematical physics. Nevertheless, as far as cylindrical domains are concerned there is still the open question how to construct solutions to data in usual Sobolev spaces since the standard Kondratiev theory does not apply. In this paper that unsatisfactory gap is filled and moreover, data with polynomial asymptotic behaviour are considered. As interesting special case we find solutions with bounded Dirichlet integral. Copyright © 2002 John Wiley & Sons, Ltd.     

Microkinetic modelling of the formation of C1 and C2 products in the Fischer-Tropsch synthesis over cobalt catalysts

The heats of adsorption of different C₁ and C₂ molecules assumed to be present during the initial steps of the Fischer-Tropsch synthesis and activation energies for elementary steps envisioned to occur in the synthesis are calculated for Co by using the unity bond index-quadratic exponential potential (UBI-QEP) method. The preexponential factors for the elementary steps are calculated from transition-state theory, and the rate constants are calculated according to the Arrhenius equation. The activation barrier for hydrogenation of CO is found to be lower compared to hydrogen assisted dissociation of CO, which has a smaller activation barrier than direct dissociation of CO. The reaction steps with high activation barriers are eliminated. Based on this elimination two sets of elementary steps for formation of C₁ and C₂ alkenes and alkanes in the Fischer-Tropsch synthesis are established: one based on hydrogen assisted CO dissociation (carbide mechanism) and one based on CO hydrogenation (CO insertion mechanism). In addition, one mechanism of producing CO₂ from the water-gas shift reaction is proposed. The resulting mechanisms are combined and used in the microkinetic model, which are fitted to experimental results at methanation conditions (T = 483 K or 493 K, p = 1.85 bar and H₂/CO = 10) over a Co/Al₂O₃ Fischer-Tropsch catalyst. A good tuning is obtained by adjusting the C-Co and H-Co binding strengths. The microkinetic modelling based on these assumptions indicates that CO is mainly converted through hydrogenation of CO and that C₂ compounds are mainly produced by insertion of CO into a metal-methyl bond. Thus, from the surface coverages and reaction rates predicted by the microkinetic modelling the mechanism can be further reduced to only include the CO insertion mechanism. Hydrogenation of CHO to CH₂O is found to be the rate determining initiation step, and insertion of CO into a metal-methyl bond is found to be the rate determining step for chain growth. By using the UBI-QEP method for calculation of activation energies, the activation barriers for dissociation of CO and hydrogenation of surface carbon are found to be too large for the carbide mechanisms to occur. However, experimental data or another theoretical method is necessary in order to support or disprove the calculated activation energies in this work.