Photophysics of monodisperse platinum-acetylide oligomers: delocalization in the singlet and triplet excited states

A series of monodisperse Pt-acetylide polymers that contain the [-CC-(p-C6H4)-CC-(t-Pt(PBu3)2)-]n repeat unit has been prepared for n = 1, 2, 3, 4, 5, and 7. The photophysical properties of the series provide information concerning the relationship between the oligomer length and delocalization in the singlet and triplet excited states of the pi-conjugated electron system. The results imply that the singlet excited state is delocalized over approximately 6 repeat units; however, the triplet state is considerably more localized. The triplet energy is almost invariant with oligomer length, but the phosphorescence spectra and triplet nonradiative decay rates indicate that the electron-vibrational coupling in the triplet state decreases with increasing oligomer length.     

Highly efficient antibody-catalyzed deuteration of carbonyl compounds

Antibody 38C2 efficiently catalyzes deuterium-exchange reactions at the alpha position of a variety of ketones and aldehydes, including substrates that have a variety of sensitive functional groups. In addition to the regio- and chemoselectivity of these reactions, the catalytic rates (kcat) and rate-enhancement values (kcat/kun) are among the highest values ever observed with catalytic antibodies. Comparison of the substrate range of the catalytic antibody with highly evolved aldolase enzymes, such as rabbit-muscle aldolase, highlights the much broader practical scope of the antibody, which accepts a wide range of substrates. The hydrogen-exchange reaction was used for calibration and mapping of the antibody active site. Isotope-exchange experiments with cycloheptanone reveal that the formation of the Schiff base species (as concluded from the 16O/18O exchange rate at the carbonyl oxygen) is much faster than the formation of the enamine intermediate (as concluded from the H/D exchange rate), and both steps are faster than the antibody-catalyzed aldol addition reaction.     

Electron transfer initiated heterogenerative cascade cyclizations: polyether synthesis under nonacidic conditions

[reaction: see text] Single-electron oxidation has been employed to initiate heterogenerative cascade cyclization reactions that form polyether compounds under essentially neutral conditions. The reactions proceed through mesolytic benzylic carbon-carbon bond cleavages of homobenzylic ether-derived radical cations followed by intramolecular epoxonium ion formation, leading to further cyclizations. Both oligotetrahydrofuran and tetrahydropyran structures can be prepared by altering substrate topography.     

Supramolecular polymer networks based on calix[5]arene chained poly(p-phenyleneethynylene) and C60 fulleropyrrolidine

Abstract

A poly(p-phenyleneethynylene) polymer (PCF[5]), bearing two π-rich cone-like calix[5]arene cavities (assembling cores) attached to a rigid p-phenyleneethynylene spacer, was synthesised by a Pd-catalysed cross-coupling reaction. UV–vis absorption and fluorescence spectroscopies combined with dynamic light scattering measurements provide evidence for the self-assembly of PCF[5] (homopolytopic host molecule) with a complementary C₆₀ fulleropyrrolidine (C₆₀-Pyr) guest in solution, in the construction of a supramolecular polymer network. Atomic force microscopy analysis of PCF[5]/C₆₀-Pyr highlights the formation of a bicontinuous network consisting of a uniform distribution of prominent structures, within a polymeric background forming a biphasic structure.     

A simplified [11C]phosgene synthesis

A new flow-through system for the production of [¹¹C]phosgene, a versatile labelling agent in radiochemistry for PET, is described. Cyclotron-produced [¹¹C]CH₄ is mixed with Cl₂ and converted into [¹¹C]CCl₄ by passing the mixture through an empty quartz tube at 510 °C. The outflow is directed through a Sb-filled guard that takes out Cl₂ and then, without intentional O₂ addition, through a second empty quartz tube at 750 °C, giving rise to [¹¹C]phosgene in 30–35% radiochemical yield.     

Intramolecular Diels-Alder and tandem intramolecular Diels-Alder/1,3-dipolar cycloaddition reactions of 1,3,4-oxadiazoles

The scope of intramolecular Diels-Alder and a novel tandem Diels-Alder/1,3-dipolar cycloaddition cascade of 1,3,4-oxadiazoles is disclosed. In the cases examined, the tandem cycloadditions construct three new rings with formation of four new C-C bonds and set all six stereocenters about a central six-membered ring in a single step including three contiguous and four total quaternary centers without a trace of a second diastereomer.     

Extraction of Co-Products from Biomass: Example of Thermal Degradation of Silymarin Compounds in Subcritical Water

In an effort to increase revenues from a given feedstock, valuable co-products could be extracted prior to biochemical or thermochemical conversion with subcritical water. Although subcritical water shows significant promise in replacing organic solvents as an extraction solvent, compound degradation has been observed at elevated extraction temperatures. First order thermal degradation kinetics from a model system, silymarin extracted from Silybum marianum, in water at pH 5.1 and 100, 120, 140, and 160 °C were investigated. Water pressure was maintained slightly above its vapor pressure. Silymarin is a mixture of taxifolin, silichristin, silidianin, silibinin, and isosilibinin. The degradation rate constants ranged from 0.0104 min⁻¹ at 100 °C for silichristin to a maximum of 0.0840 min⁻¹ at 160 °C for silybin B. Half-lives, calculated from the rate constants, ranged from a low of 6.2 min at 160 °C to a high of 58.3 min at 100 °C, both for silichristin. The respective activation energies for the compounds ranged from 37.2 kJ/gmole for silidianin to 45.2 kJ/gmole for silichristin. In extracting the silymarin with pure ethanol at 140 °C, no degradation was observed. However, when extracting with ethanol/water mixtures at and 140 °C, degradation increased exponentially as the concentration of water increased. An erratum to this article can be found at     

Ethynyl and Propynylpyrene Inhibitors of Cytochrome P450

Abstract  

The single-crystal X-ray structures and in vivo activities of three aryl acetylenic inhibitors of cytochromes P450 1A1, 1A2, 2A6, and 2B1 have been determined and are reported herein. These are 1-ethynylpyrene, 1-propynylpyrene, and 4-propynylpyrene. To investigate electronic influences on the mechanism of enzyme inhibition, the experimental electron density distribution of 1-ethynylpyrene has been determined using low-temperature X-ray diffraction measurements, and the resulting net atomic charges compared with various theoretical calculations. A total of 82,390 reflections were measured with Mo Kα radiation to a (sinθ/λ)_max = 0.985 Å⁻¹. Averaging symmetry equivalent reflections yielded 8,889 unique reflections. A least squares refinement procedure was used in which multipole parameters were added to describe the distortions of the atomic electron distributions from spherical symmetry. A map of the model electron density distribution of 1-ethynylpyrene was obtained. Net atomic charges calculated from refined monopole population parameters yielded charges that showed that the terminal acetylenic carbon atom (C18) is more negative than the internal carbon (C17). Net atomic charges calculated by ab initio, density functional theory, and semi-empirical methods are consistent with this trend suggesting that the terminal acetylenic carbon atom is more likely to be the site of oxidation. This is consistent with the inhibition mechanism pathway that results in the formation of a reactive ketene intermediate. This is also consistent with assay results that determined that 1-ethynylpyrene acts as a mechanism-based inhibitor of P450s 1A1 and 1A2 and as a reversible inhibitor of P450 2B1. Crystallographic data: 1-ethynylpyrene, C₁₈H₁₀, P2₁/c, a = 14.571(2) Å, b = 3.9094(5) Å, c = 20.242(3) Å, β = 105.042(2)°, V = 1,113.5(2) Å³; 1-propynylpyrene, C₁₉H₁₂, P2₁/n, a = 8.970(2) Å, b = 10.136(1) Å, c = 14.080(3) Å, β = 99.77(2)°, V = 1,261.5(4) Å³; 4-propynylpyrene, C₁₉H₁₂, Pbca, a = 9.904(1) Å, b = 13.174(2) Å, c = 19.401(1) Å, V = 2,531.4(5) Å³.     

Reactivite d'organometalliques vis a vis des enynes conjugues α, α'-bifonctionnels: III. Enynes a groupement alcool en α de la triple liaison: Synthese d'allenes et de dienes mono- ou bifonctionnels

Organolithium, organomagnesium and organozinc compounds react with α,α'-difunctional enynes: HOCH₂CCCHCHCH₂Y (Y  OH, OCH₃, N(CH₃)₂). This reaction produces one or two derivatives, according to the nature of the metallic group: mono- or di-functional allenes, mono- or di-functional dienes.