Application of pyranoside phosphite-pyridine ligands to enantioselective metal-catalyzed allylic substitutions and conjugate 1,4-additions

A series of glucopyranoside phosphite–pyridine ligands have been applied in the metal-catalyzed allylic substitution and conjugate 1,4-addition reactions of several substrate types. We have been able to identify ligands that provided promising enantioselectivities in the Pd-catalyzed intermolecular allylic substitution of cyclic substrates (ee’s up to 86%) and desymmetrization (ee’s up to 94%) and in the Cu-catalyzed conjugate addition of challenging aliphatic enones (ee’s up to 90%).     

Dioxygen Activation and Methane Hydroxylation by Soluble Methane Monooxygenase: A Tale of Two Irons and Three Proteins A list of abbreviations can be found in Section 7

Methanotrophic bacteria are capable of using methane as their sole source of carbon and energy. The first step in methane metabolism, the oxidation of methane to methanol, is catalyzed by a fascinating enzyme system called methane monooxygenase (MMO). The selective oxidation of the very stable C-H bond in methane under ambient conditions is a remarkable feat that has not yet been repeated by synthetic catalysts and has attracted considerable scientific and commercial interest. The best studied MMO is a complex enzyme system that consists of three soluble protein components, all of which are required for efficient catalysis. Dioxygen activation and subsequent methane hydroxylation are catalyzed by a hydroxylase enzyme that contains a non-heme diiron site. A reductase protein accepts electrons from NADH and transfers them to the hydroxylase where they are used for the reductive activation of O(2). The third protein component couples electron and dioxygen consumption with methane oxidation. In this review we examine different aspects of catalysis by the MMO proteins, including the mechanisms of dioxygen activation at the diiron site and substrate hydroxylation by the activated oxygen species. We also discuss the role of complex formation between the different protein components in regulating various aspects of catalysis.     

GC-FID/MS method for the impurity profiling of synthetic d-allethrin

A GC/FID/MS method was developed for the identification and quantification of d-allethrin (DA) and its major impurities in commercial samples. Optimisation of the experimental conditions was carried out considering such important requirements as resolution, reproducibility, detection limits of 0.1% (m/m) for the impurities, and short analysis time. Under the optimised final conditions the method was validated for specificity, precision (CV% = 0.133 at 2.10 mg/mL and CV% = 0.035 at 3.00 mg/mL), linearity (0-3.00 microg injected), limits of detection (0.09 ng injected) and quantitation (0.28 ng injected), and robustness. The DA related impurities were identified by using a GC/MS method with ion trap mass detection and also by comparison with synthesised standards. The most abundant impurities were crysolactone, allethrolone, chrysanthemic acid, and chloro-derivatives of DA.     

Calorimetric investigation of the adsorption of nitrogen bases and nucleosides on a hydrophobic interaction sorbent

Heats of adsorption for nitrogen bases and nucleosides on Sepharose CL-6B, a hydrophobic interaction adsorbent, were collected through flow microcalorimetry in order to ascertain the thermodynamic driving force for adsorption in each case. It was determined that enthalpy changes associated with base stacking self-interactions can contribute significantly to the observed heats of adsorption. Accordingly, the observed heats were the net effect of the adsorbate/adsorbent interactions and the adsorbate stacking self-interactions. Since base stacking proceeds beyond the dimer stage, multi-layer adsorption of these compounds is possible, even at low solution concentrations.     

Azidotetrakis(diarylformamidinate)diruthenium(II,III) Compounds: Synthesis,Molecular Structures and Voltammetric Properties; Linear Free Energy Relationships in Dinuclear Compounds VII*<Superscript>,</Superscript>†

A series of Ru₂(DArF)₄(N₃) compounds have been prepared from the metathesis reaction between Ru₂(DArF)₄Cl and NaN₃, where DArF is diarylformamidinate, and aryl is X–Ph with X as 4-MeO (1), 4-Me (2), H (3), 3-MeO (4), 4-Cl (5), 3-Cl (6), 3-CF₃ (7), 4-CF₃ (8), 3,4-Cl₂ (9) and 3,5-Cl₂ (10). Single crystal X-ray diffraction studies of 3 and 10 revealed the expected paddlewheel motif for the first coordination sphere of Ru₂ and axial ligation of azide. Voltammetric studies revealed a (quasi)reversible oxidation (A) and a irreversible reduction (B) for all compounds, and a second (quasi)reduction (C) for compounds 6–10. The half-wave potentials of couple A span a remarkable range of 640 mV, and they correlate linearly with the Hammett constant (σ) of the aryl substituent.¹Dedicated to Professor F. Al Cotton, mentor, friend, and pioneer, on the occasion of his 75th birthday. ²For part VI, see C. Lin, T. Ren, E.J. Valente, and J.D. Zubkowski (1999). J. Organomet. Chem. 579, 114.     

Rapid palladium-catalyzed cross-coupling in the synthesis of aryl thioethers under microwave conditions

A Pd-catalyzed coupling of aromatic iodides or bromides and tin-thiolates under microwave conditions was developed to synthesize aromatic thioethers without concomitant formation of the reduced products. Ligand screening revealed DiPPF and BINAP-Tol as the most generally useful ligands for this transformation. A variety of iodides or bromides were coupled to give the thioethers rapidly (10 min) in 60-95% isolated yield.     

Spatially-directed oxidation of gold nanoparticles by Au(III)-CTAB complexes

Gold nanoparticles are readily oxidized by Au(III) in the presence of cetyl-trimethylammonium bromide (CTAB). Oxidation occurs preferentially at surface sites with higher curvature. Conversely, oxidation with cyanide ions in the absence of CTAB leads to uniform oxidation over the whole surface. Examples of the spatially directed oxidation are provided using large, irregular spheres, nanocubes, and nanorods. We conclude that the mechanism of oxidation depends on whether the oxidant is attached to CTAB micelles. It is postulated that the CTAB micelles approach the nanoparticles preferentially at the tips, leading to spatially directed oxidation.