Kinetics and mechanism of rhenium-catalyzed O atom transfer from epoxides

O atom transfer from epoxides cis-stilbene oxide and styrene oxide to triphenylphosphine catalyzed by Tp'ReO(3) (Tp' = hydridotris(3,5-dimethylpyrazolyl)borate) is shown to proceed via an unexpectedly complex combination of mechanisms. Reduction of Tp'ReO(3) with PPh(3) in THF is rapid above room temperature to form a highly reactive species suggested to be Tp'ReO(2). Spectroscopic examination and attempts to isolate this by chromatography lead only to Tp'Re(O)(OH)(2) (1); exposure of the crude reduction mixture to ethanol results in formation of Tp'Re(O)(OEt)(OH) (3). Both 1 and 3 are as efficient catalysts for O atom transfer as the unpurified mixture resulting from reaction of PPh(3) with Tp'ReO(3); all three rhenium reactants give the same turnover frequency to within 10% at identical [Re](total) and [epoxide]. The kinetic behavior of the catalytic system (epoxide:Re = 20) is complex; an initial "burst" of alkene production is seen, which quickly tapers off and falls into a pseudo-zero-order reaction. The majority of rhenium is observed to exist as the syn-Tp'Re(O)(diolate) complex, formed by ring expansion of the epoxide. However, cycloreversion of this diolate is incapable of accounting for the observed catalytic turnover frequency. An additional intermediate, a coordinated epoxide, is proposed to form and partition between ring expansion and direct fragmentation to alkene; eventually a steady-state concentration of diolate forms. Competition between direct atom transfer and ring expansion followed by diolate cycloreversion is demonstrated by reaction of 3 with excess cis-stilbene oxide and styrene oxide in the absence of reductant to give a 4:1 mixture of alkene and syn-diolate from cis-stilbene oxide or a 5.5:1 mixture of alkene and syn-diolate from styrene oxide under conditions where diolate cycloreversion is a negligible contributor.     

Studies in bicyclo[3.1.0]hexane methanolysis. Ring opening of activated cyclopropanes under acidic and basic conditions

A bicyclo[3.1.0]hexane, with one cyclopropane carbon flanked by a ketone and an ester or an aldehyde, undergoes methanolysis with cleavage of one of the two activated cyclopropane bonds, depending on the reaction conditions. Acidic conditions yield primarily or exclusively a 4-methoxycyclohexane, while basic conditions yield a 3-methoxymethylcyclopentanone.     

Mechanistic evaluation of arsenite oxidation in TiO2 assisted photocatalysis

We report herein a detailed assessment of the roles of O2, H2O2, *OH, and O2-* in the TiO2 assisted photocatalytic oxidation (PCO) of arsenite. Although both arsenite, As(III), and arsenate, As(V), adsorb extensively onto the surface of TiO2, past studies relied primarily on the analysis of the arsenic species in solution, neglecting those adsorbed onto the surface of TiO2. We used extraction and analyses of the arsenic species adsorbed onto the surface of the TiO2 to illustrate that the oxidation of As(III) to As(V) occurs in an adsorbed state during TiO2 PCO. The TiO2 photocatalytic oxidation (PCO) of surface adsorbed As(III) in deoxygenated solutions with electron scavengers, Cu2+, and polyoxometalates (POM) yields oxidation rates that are comparable to those observed under oxygen saturation, implying the primary role of oxygen is as a scavenger of the conduction band electron. Pulse radiolysis and competition kinetics were employed to determine a rate constant of 3.6 x 10(6) M(-1) s(-1) for the reaction of As(III) with O2-*. Transient absorption studies of adsorbed hydroxyl radicals, generated by subjecting colloidal TiO2 to radiolytic conditions, provide convincing evidence that the adsorbed hydroxyl radical (TiO2+*OH) plays the central role in the oxidation with As(III) during TiO2 assisted photocatalysis. Our results suggest the reaction of superoxide anion radical does not contribute in the conversion of As(III) when compared to the reaction of As(III) with *OH radical during TiO2 PCO.     

Gemini-induced columnar jointing in vitreous ice. Cryo-HRSEM as a tool for discovering new colloidal morphologies

A gemini surfactant is able to promote columnar jointing in vitreous ice where long pillars, often of hexagonal cross section, are formed. This jointing is visible by cryo-high-resolution scanning electron microscopy (cryo-HRSEM), in which colloidal suspensions in bulk water are cooled rapidly in liquid ethane, thereby avoiding the potential artifacts with other types of EM. The jointing is proposed to arise from a new type of colloidal morphology where the surfactant self-assembles into hexagonal columns. Evidence for this mechanism comes from a cryo-HRSEM photo of an ice-free hexagonal "skeleton" composed of surfactant. Cryo-HRSEM, a method that is just beginning to realize its potential, would seem to have a promising future in the discovery of additional and as yet unimagined colloidal structures.     

Salvinicins A and B, new neoclerodane diterpenes from Salvia divinorum

[reaction: see text] Two new neoclerodane diterpenes, salvinicins A (4) and B (5), were isolated from the dried leaves of Salvia divinorum. The structures of these compounds were elucidated by spectroscopic techniques, including (1)H and (13)C NMR, NOESY, HMQC, and HMBC. The absolute stereochemistry of these compounds was assigned on the basis of single-crystal X-ray crystallographic analysis of salvinicin A (4) and a 3,4-dichlorobenzoate derivative of salvinorin B.     

Functionalized organosilica microspheres via a novel emulsion-based route

Thiol-functionalized organosilica microspheres were synthesized via a two-step process: (1) acid-catalyzed hydrolysis and condensation of 3-mercaptopropyltrimethoxysilane (MPTMS), followed by (2) base-catalyzed condensation, which led to the rapid formation of emulsion droplets with a narrow size distribution. These droplets continued to condense to form solid microspheres. Solution (29)Si NMR and optical microscopy were applied to study the mechanism of this novel synthetic route. Solid-state (29)Si NMR, SEM, zeta potential titration, and Coulter counter measurements were used to study the bulk and surface properties and to determine the particle size distributions of the final microspheres. Compared to conventional St?ber silica particles, these microspheres were shown to have a lower degree of cross-linking (average degree of condensation, r = 1.25), a larger average size (up to 6 microm), and a higher isoelectric point (pH = 4.4). Confocal microscopy of dye-labeled microspheres showed an even distribution of dye molecules throughout the interior, characteristic of a readily accessible and permeable organosilica network. These findings have implications for the production of functionalized solid supports for use in catalysis and biological applications, such as optically encoded carriers for combinatorial synthesis.     

Optimization of matrix-assisted laser desorption/ionization time-of-flight collision-induced dissociation using poly(ethylene glycol)

Details of the optimization of the collision-induced dissociation (CID) process, using a collision cell on a matrix-assisted laser desorption/ionization (MALDI) time-of-flight (TOF) mass spectrometer, are described using poly(ethylene glycol) 1000 (PEG 1000) as a model analyte. The effects of collision gas identity (helium, air, and argon), as well as collision gas pressure, on the resulting MS/MS data were investigated. With PEG 1000, helium was found to give the best results with respect to signal-to-noise (S/N) ratio. The optimum pressure for each gas was found to be in the range where the precursor ion signal was attenuated to approximately 30-50% for helium and 40-60% for argon. The effect of cation choice (Li, Na, and K) on the CID of PEG was also studied. CID spectra were produced for each, but PEG cationized with lithium was found to produce the spectra with the highest S/N ratio. The MALDI-TOF CID spectra that were generated for PEG were compared with the high-energy and low-energy MS/MS spectra obtained from a sector mass spectrometer and from a triple quadrupole mass spectrometer, respectively. The results observed for PEG confirm that CID on a MALDI-TOF mass spectrometer is a high-energy MS/MS technique.     

Sorption of triethyleneglycol and tetraethyleneglycol by silicalite at fixed water activities

The sorption of tetraethyleneglycol by silicalite-1 and triethyleneglycol by silicalite-1 and a silicalite-1/silicalite-2 intergrowth has been studied at 25°C and at fixed water activities in the range 0.111<a _w<0.902 by an isopiestic method. All of the glycol uptakes are between 0.95 and 1.2 molecules per unit cell and in all cases they are essentially independent of water activity. The water content of the silicalite-1 was more sensitive to water activity and glycol content than was that of the silicalite-1/2 intergrowth.     

The first intermolecular Friedel-Crafts acylation with beta-lactams

The first intermolecular Friedel-Crafts acylation of a variety of aromatic substrates with azetidinones is described. The Friedel-Crafts acylations are performed under very mild conditions, using trifluoromethanesulfonic acid to produce beta-amino aromatic ketones in excellent yields.     

The thiolate anion as a nucleophile Part XII. Reactions of Lead(II) Benzenethiolate

The reactions of various fluoroaromatics, C₆F_6?xH_x, and C₆F₅X (X = C₆F₆, Cl, Me, NO₂, CF₃, COCl, CH₂Br, OMe, and NH₂) with lead(II) benzenethiolate in DMF have been examined. Lead thiolate acted as an excellent source of benzenethiolate anions and displacement of fluorine, chlorine or the nitro group was observed. The new products have been characterized by elemental analysis, and NMR (H?1 and F?19), infrared and mass spectroscopy.