Palladium Catalyzed Additions to Allylic Hydroxy Phosphonates: Applications in the Enantioselective Synthesis of Enterolactone and Turmerone

Reaction of nonracemic allylic hydroxy phosphonates, prepared by the asymmetric phosphonylation of unsaturated aldehydes, with methyl chloroformate in pyridine yields the corresponding carbonates. The carbonates are excellent substrates for the palladium-catalyzed addition of nucleophiles. Addition of the nucleophile is highly regioselective, resulting in n -substituted vinyl phosphonates. The reaction of the allylic carbonates with aryl stannanes and malonates has been investigated. Progress in the application of these reactions to the synthesis of turmerone and enterolactone is reported.     

Dynamics of interacting edge defects in copolymer lamellae

It is known that terraces at the air-polymer interface of lamella-forming diblock copolymers do not make discontinuous jumps in height. Despite the underlying discretized structure, the height profiles are smoothly varying. The width of a transition region of a terrace edge in isolation is typically several hundreds of nanometres, resulting from a balance between surface tension, chain stretching penalties, and the enthalpy of mixing. What is less well known in these systems is what happens when two transition regions interact with one another. In this study, we investigate the dynamics of the interactions between copolymer lamellar edges. We find that the data can be well described by a model that assumes a repulsion between adjacent edges. While the model is simplistic, and does not include molecular level details, its agreement with the data suggests that some of the the underlying assumptions provide insight into the complex interplay between defects.     

Control of a coupled map lattice model for vortex shedding in the wake of a cylinder

The flow behind a vibrating flexible cable at low Reynolds numbers can exhibit complex wake structures such as lace-like patterns, vortex dislocations and frequency cells. These structures have been observed in experiments and numerical simulations, and are predicted by a previously developed low-order coupled map lattice (CML). The discrete (in time and space) CML models consist of a series of diffusively coupled circle map oscillators along the cable span. Motivated by a desire to modify the complex wake patterns behind flexible vibrating cables we have studied the addition of control terms into the highly efficient CML models and explored the resulting dynamics. Proportional, adaptive proportional and discontinuous non-linear (DNL) control methods were used to derive the control laws. The first method employed occasional proportional feedback. The adaptive method used spatio-temporal feedback control. The DNL method used a discontinuous feedback linearization procedure, and the controller was designed for the resulting linearized system using eigenvalue assignment. These techniques were applied to a modeled vortex dislocation structure in the wake of a vibrating cable in uniform freestream flow. Parallel shedding patterns were achieved for a range of forcing frequency-forcing amplitude combinations studied to validate the control theory. The adaptive proportional and DNL methods were found to be more effective than the proportional control method due to the incorporation of a spatially varying feedback gain across the cylinder span. The DNL method was found to be the most efficient controller of the low-order CML model. The required control level across the cable span was correlated to the 1/1 lock-on behavior of the temporal circle map.     

Unoccupied surface and conduction band states on Ge (111) from energy loss spectroscopy

Unoccupied surface states are observed on Ge (111) by measuring 3d-core level to conduction-band transitions using electron energy loss spectroscopy. These experiments also yield values for conduction band density-of-states features.     

Oxygen equilibration in the electron impact mass spectra of aryl esters and carbamates

The positional integrity of an ¹⁸O label is partially lost in the molecular ion in the electron impact mass spectra of 4-methylphenyl benzoate, 4-methylphenyl 4-nitrobenzoate and 4-methylphenyl N,N-dimethylcarbamate.     

Crosslinking poly[1-(trimethylsilyl)-1-propyne] and its effect on physical stability

Poly[1-(trimethylsilyl)-1-propyne] (PTMSP) has been crosslinked using 3,3′-diazidodiphenylsulfone to improve its solvent resistance and physical stability. This study reports the influence of crosslinking on N₂, O₂ and CH₄ gas permeabilities and fractional free volume (FFV) as a function of time. Crosslinking PTMSP renders it insoluble even in excellent solvents for the uncrosslinked polymer. The gas permeability and FFV of uncrosslinked and crosslinked PTMSP decreased over time, so crosslinking PTMSP does not arrest physical aging. The addition of 10 wt.% polysiloxysilsesquioxanes (POSS) nanoparticles decreased the permeability of PTMSP by 55%, and the permeability and FFV values were stable over time for PTMSP films containing 10 wt.% POSS nanoparticles. The permeability of PTMSP at a given FFV was greater than that of other substituted polyacetylenes, polysulfones or polycarbonates, which is consistent with differences in the arrangement of free volume in these polymers, as probed by positron annihilation lifetime spectroscopy (PALS). Ellipsometry was used to characterize physical aging of thin (400 nm) uncrosslinked and crosslinked PTMSP films supported on silicon wafers. The ellipsometry results showed that crosslinking does not markedly slow physical aging of thin PTMSP films.     

Mechanistic investigation of cyclohexane oxidation by a non-heme iron complex: evidence of product inhibition by UV/vis stopped-flow studies

We report herein studies examining a binuclear non-heme iron model complex that is capable of catalytically oxidizing cyclohexane to cyclohexanol in excess of 200 turnovers, relative to the iron complex, and cyclohexanone (5 turnovers) via heterolytic cleavage of the mechanistic probe peroxide MPPH. Low-temperature stopped-flow electronic spectroscopy was utilized to investigate the mechanism of the reaction of this diiron(II) compound, Fe(2)(H(2)Hbamb)(2)(N-MeIm)(2), (H(2)Hbamb = 2,3-bis(2-hydroxybenzamido)dimethylbutane) (1) with MPPH. In the absence of substrates, the reaction proceeds in three consecutive steps starting with oxygen atom transfer to the diferrous complex to generate a putative [Fe(IV)=O species], thought to be the oxidant in the catalytic cycle. Over time, the rate of catalysis is observed to decrease without consumption of all available peroxide. By utilizing low-temperature stopped-flow UV/vis kinetic studies, the diferrous complex, 1, is shown to undergo product inhibition arising from the interaction of either cyclohexanol or MPP-OL product species to the diiron center, therefore precluding further reaction with MPPH.     

Double Salts Iodide Scintillators: Cesium Barium Iodide,Cesium Calcium Iodide,and Barium Bromine Iodide

In this study we review the state‐of‐the‐art for double salt iodide scintillators, in particular cesium barium iodide (CBI), cesium calcium iodide (CCI) and barium bromine iodide (BBI), as well as report on their scintillation and optical properties. Double salt iodides inherently have high density and atomic number which translates to good stopping power for energetic particles, in particular gamma rays. Light yields of 54,000 ph/MeV for CBI, 51,000 ph/MeV for CCI, and 46,000 ph/MeV for BBI were measured. A FWHM energy resolution for the 662 keV full absorption peak was observed at 5.7% for CBI, 16.3% for CCI and 3.56% for BBI. The principal scintillation decay timing for CBI was 840 ns, 462 ns for BBI, and two distinct time components of 9 ns and 1900 ns were observed for CCI.