Hydrogen-bonded networks from η-semiquinone complexes of manganese tricarbonyl

The cationic manganese tricarbonyl complexes containing η⁶-2-methylhydroquinone (2a), η⁶-2,3-dimethylhydroquinone (3a), η⁶-2-t-butylhydroquinone (4a), η⁶-tetramethylhydroquinone (5a) and η⁶-4,4′-biphenol (6a) are readily deprotonated to the corresponding neutral (η⁵-semiquinone)Mn(CO)₃ (2b-6b) and anionic (η⁴-quinone)Mn(CO)₃⁻ (2c-5c) complexes. The X-ray structures of 2b-6b feature strong intermolecular hydrogen bonding interactions that result in the formation of supramolecular organometallic networks. Significantly, the substitution pattern at the semiquinone ring affects the stereochemistry of the hydrogen bonding interactions. NMR spectra of 2b, 3b and 5b reveal dynamic hydrogen bonding in solution.     

Theoretical insights into the mechanism of acetylcholinesterase-catalyzed acylation of acetylcholine

Acylation of acetylcholine (ACh) catalyzed by acetylcholinesterase (AChE) has been studied using high-level theoretical calculations on a model system that mimics the reaction center of the enzyme, and compared with uncatalyzed acylation reaction. The geometries of all the intermediates and transition states, activation energies, and solvent effects have been calculated. The calculations predict simultaneous formation of two short-strong hydrogen bonds (SSHB) in the rate-determining transition state structures [the first SSHB involves the hydrogen atom of Ser-200 (H(s)) and another involves the hydrogen atom of His-440 (H(h))]. In the intermediate states, the H-bond corresponding to H(h) involves SSHB, whereas the one corresponding to H(s) does not.     

Preparation of water‐soluble,syndiotacticity‐rich,low molecular weight poly(vinyl alcohol) microfibrils in high yields with the low‐temperature polymerization of vinyl pivalate in tetrahydrofuran and saponification

To prepare water‐soluble, syndiotacticity‐rich poly(vinyl alcohol) (PVA) microfibrils for various industrial applications, we synthesized syndiotacticity‐rich, low molecular weight PVA by the solution polymerization of vinyl pivalate (VPi) in tetrahydrofuran (THF) at low temperatures with 2,2′‐azobis(2,4‐dimethylvaleronitrile) (ADMVN) as an initiator and successive saponification of poly(vinyl pivalate) (PVPi). Effects of the initiator and monomer concentrations and the polymerization temperature were investigated in terms of the polymerization behaviors and molecular structures of PVPi and the corresponding syndiotacticity‐rich PVA. The polymerization rate of VPi in THF was proportional to the 0.91 power of the ADMVN concentration, indicating the heterogeneous nature of THF polymerization. The low‐temperature solution polymerization of VPi in THF with ADMVN proved to be successful in obtaining water‐soluble PVA with a number‐average degree of polymerization (P_n) of 300–900, a syndiotactic dyad content of 60–63%, and an ultimate conversion of VPi into PVPi of over 75%. Despite the low molecular weight of PVA with P_n = 800, water‐soluble PVA microfibrillar fibers were prepared because of the high level of syndiotacticity. In contrast, for PVA with P_n = 330, shapeless and globular morphologies were observed, indicating that molecular weight has an important role in the in situ fibrillation of syndiotacticity‐rich PVA. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1103–1111, 2002     

Stereoselective One-Pot Synthesis of Vinyl Sulfones from Jcthyl Phenyl Sulfone

Vinyl sulfones are stereoselectively prepared in high yields in a one-pot by the Wittig-Horner reaction via phosphorylation of methyl phenyl sulfone at low temperature.     

Tetrachlorosilane Catalyzed Dithioacetalization

Tetrachlorosilane was found to be a mild as well as selective catalyst for dithioacetalization of the carbonyl compounds.     

Effect of Dissolved Oxygen Tension and Agitation Rates on Sulfur-Utilizing Autotrophic Denitrification: Batch Tests

The effect of dissolved oxygen (DO) and agitation rate in open and closed reactors was examined for sulfur-utilizing autotrophic denitrification. The reaction rate constants were determined based on a half-order kinetic model. Declining denitrification rate constants obtained for open reactors those of 8.46, 8.03, and 2.18 for 50 mg NO₃ ^?-N/L, while 11.12, 9.14, and 0.12 mg^1/2/L^1/2?h were for 100 mg NO₃ ^?-N/L at agitation speeds of 0, 100, and 200 rpm. In closed reactors, the ever-increasing denitrification rates were 10.13, 22.56, and 37.03, whereas for the same nitrate concentrations and speeds the rates were 13.17, 15.63, and 26.67 mg^1/2/L^1/2?h. The rate constants correlated well (r ² ?=?0.89–0.99) with a half-order kinetic model. In open reactors, high SO₄ ^2?/N ratios (8.02–75.10) while in closed reactors comparatively low SO₄ ^2?/N ratios (6.10–13.39) were obtained. Sulfur oxidation occurred continuously in the presence of DO, resulting in mixed cultures acclimated to sulfur and nitrate. SO₄ ^2? was produced as an end product, which reduced alkalinity and lowered pH over time. Furthermore, DO inhibited sulfur denitrification in open reactors, while agitation in closed reactors increased the rate of denitrification.     

Quantitative determination of duloxetine and its metabolite in rat plasma by HPLC‐MS/MS

The major metabolite of duloxetine is a glucuronide conjugate of 4‐hydroxy duloxetine (4‐HD). However, interestingly, there have been no reports determining concentrations of 4‐HD and no fully validated method has been established for measuring duloxetine and 4‐HD in rat plasma. We developed a method for the simultaneous quantification of duloxetine and its metabolite in rat plasma using high‐performance liquid chromatography tandem mass spectrometry. Duloxetine and 4‐HD were analyzed on a reverse‐phase C₁₈ analytical column after protein precipitation of the plasma sample with methanol, using carbamazepine as an internal standard. The isocratic mobile phase of 5 mm ammonium acetate–methanol (4:6, v/v) was eluted at 0.4 mL/min. Quantification was performed on a triple‐quadrupole mass spectrometer using electrospray ionization, and the ion transition monitored in selective reaction monitoring mode. The coefficient of variation for assay precision was <18.0%, and the accuracy was 84.0–118.0%. This method was successfully used to measure the concentrations of duloxetine and its metabolite in plasma following the oral administration of a single 40 mg/kg dose in rats. Copyright © 2013 John Wiley & Sons, Ltd.     

Preparation of Water-Soluble Polymers via the Mannich Reaction

The Mannich reaction, defined as the condensation of ammonia or a primary or secondary amine with formaldehyde and a compound containing at least one hydrogen atom of pronounced activity, was first shown to yield polymers from suitably selected reactants as early as 1946 [l]. Surprisingly, very few further attempts to utilize this interesting reaction in polymer formation have been reported in the literature.