Stereoselective Lewis acid-catalyzed alpha-acylvinyl additions

Silyloxyallenes derived from alpha-hydroxypropargylsilanes undergo efficient addition to aldehydes with catalytic amounts of Lewis acids. The allenes are accessed from the corresponding propargylsilanes in a base-catalyzed 1,2-Brook rearrangement/SE2' process. Enantioenriched propargylsilanes are synthesized by a new zinc-promoted addition of alkynes to acylsilanes in up to 74% ee. This work demonstrates that conversion to the silyloxyallenes occurs with minimal erosion in optical activity. The use of a chiral chromium(III) Lewis acid effects the catalytic asymmetric addition of racemic silyloxyallenes to aromatic aldehydes in up to 92% ee. The overall reaction is broad in scope and accommodates a wide variety of aromatic and aliphatic substituents on both the propargylsilane and aldehyde.     

On the discrepancy between two Zagreb indices

We examine the quantity

$S\left(G\right)=\sum _{uv\in E\left(G\right)}min(degu,degv)$

over sets of graphs with a fixed number of edges. The main result shows the maximum possible value of $S\left(G\right)$ is achieved by three different classes of constructions, depending on the distance between the number of edges and the nearest triangular number. Furthermore we determine the maximum possible value when the set of graphs is restricted to be bipartite, a forest, or to be planar given sufficiently many edges. The quantity $S\left(G\right)$ corresponds to the difference between two well studied indices, the irregularity of a graph and the sum of the squares of the degrees in a graph. These are known as the first and third Zagreb indices in the area of mathematical chemistry.     

Managing new and differentiated remanufactured products

We study a firm that makes new products in the first period and uses returned cores to make remanufactured products (along with new products) in future periods. The remanufactured product is differentiated from the new product, so the firm needs to choose differentiated prices. We analyze the monopoly environment in two-period, multi-period (three, four and five) and infinite planning horizons, and characterize the optimal remanufacturing and pricing strategy for the firm. In the process, we identify remanufacturing savings thresholds that determine the production and pricing strategy for the firm. Among other results, we find—counter to intuition—that in a finite-horizon, multi-period setting, the optimal policy is not necessarily monotone in remanufacturing savings.     

NMR Imaging Using Second Order Quadrupole Broadened Resonances

Theoretical and experimental results for NMR imaging measurements of powdered materials using the + to − transition of -integerspin nuclei in the presence of a very large second-order electric quadrupolar broadening are presented. An “effective spin-” formalism is developed to account for additional effects due to the presence of quadrupolar interactions comparable in size to the Zeeman interaction. A large (7.9 mT/cm-A, with a maximum current of ≈20 A), rapid (≈30 μs) pulsed linear gradient field is used with echoes and phase encoding techniques to obtain images in the limit γH₁is much narrower than the NMR linewidth. A one-dimensional projection of the second-order quadrupolar perturbed, 4-MHz-wide, + ↔ − transition for⁶³Cu in Cu₂O powder is presented as an example. An experimental one-dimensional projection of a sample containing Cu₂O and YBa₂Cu₃O_6.7is also presented.     

A transannular reaction in an anionic oxy-cope system9

The carbinol $\text{8}$ is found to rearrange to the bicyclo (2,2,2) octane derivative $\text{10}$ in the presence of base; the rearrangement involves a transannular reaction in a ten membered ring resulting from an Oxy-Cope system.     

Quantitative separation of carrier-free cerium from fission products

The extraction of Ce (IV) by di-(2-ethylhexyl)-phosphoric acid (HDEHP) has been studied as a function of nitric acid concentration. Using the distribution coefficient data, the optimum conditions for recovery of Ce (IV) from nitric acid medium were arrived at. Under the conditions employed for Ce(IV), a small percentage of Ru was also found to be extracted. Cerium could be selectively stripped from the organic phase with 8M HNO₃/30% H₂O₂ solution. This procedure led to the recovery of¹⁴⁴Ce free of¹⁰⁶Ru. Based on the solvent extraction data, an extraction-chromatographic procedure employing HDEHP (40% w/w) loaded on Amberlite XAD-7 as the stationary phase was developed for the isolation of pure, carrier-free¹⁴⁴Ce from the spent fuel solution.     

A new synthesis of angulary substituted octalins via an anionic oxy-cope rearrangement

The divinyl carbinols $\text{3a}$ and $\text{3b}$ furnished octalinols $\text{4a}$ and $\text{4b}$ respectively on treatment with base.     

Roles of surface Te, Nb, and Sb oxides in propane oxidation to acrylic acid over bulk orthorhombic Mo-V-O phase

The outermost surfaces and subsurface layers of the orthorhombic (M1) Mo-V-O catalysts promoted with Te, Nb, and Sb oxide species at submonolayer surface coverage were examined by low-energy ion scattering (LEIS). This study indicated that the Nb oxide species was preferentially located at the topmost surface, while the subsurface Te and Sb concentrations declined gradually into the bulk. Although the original Mo-V-O catalyst was essentially unselective in propane oxidation to acrylic acid, significant improvement in the selectivity to acrylic acid was observed when Te, Nb, and Sb oxides were present as the surface species at submonolayer coverage. These findings further suggested that the formation of the surface V-O-M bonds (M = Nb, Te, or Sb) was highly beneficial for both the activity and selectivity of the orthorhombic Mo-V-O catalysts in propane oxidation to acrylic acid. The highest selectivity was observed when both Nb and Te (or Sb) oxide species were present at the surface. The selectivity trends established for the surface-promoted Mo-V-O catalyst parallel those found previously for the corresponding bulk Mo-V-M-O catalysts. These results further indicated that the introduction of surface metal oxide species is a highly promising method to prepare well-defined model catalysts for studies of the structure-activity/selectivity relationships as well as optimize the catalytic performance of the bulk mixed Mo-V-M-O catalysts for selective (amm)oxidation of propane.     

Use of N-trifluoroacetyl-protected amino acid chlorides in peptide coupling reactions with virtually complete preservation of stereochemistry

The use of protected amino acid chlorides for peptide coupling reactions has long been avoided due to the extensive racemization that commonly occurs during either the acid chloride formation or the coupling reaction itself. Conditions are described which allow N-trifluoroacetyl-protected amino acid chlorides to be generated in high purity and with high retention of stereochemical integrity. Control of temperature is the predominant factor in controlling racemization, and rapid formation of acid chlorides under low temperature can be conveniently achieved using Vilsmeier reagent. Stereochemical integrity is further maintained when coupling of N-trifluoroacetyl acid chlorides is carried out with amino acid esters under Schotten-Baumann conditions using specific controls on pH, temperature, and agitation. Second order rate constants for coupling and the azlactone formation associated with racemization were measured to be 4260 and 3.6 L/mol s, respectively. This high rate differential allows for the reaction to be run with a minimum excess of amine ester, and makes it suitable for continuous processing. The applicability of the preferred coupling conditions to a range of amino acid couplings is described.