Control of organoclay structure in hydrocarbon polymers

Two different kinds of organoclays were prepared by mixing a pristine montmorillonite and a double‐chain ammonium salt in many different thermoplastic or elastomeric polymers. Independently of the chemical nature of the considered polymers, the obtained organoclays presented a basal spacing of 4 or 6 nm, when the mixing occurred in the absence or in the presence of a small amount of stearic acid (SA), respectively. X‐ray diffraction and Fourier transform infrared measurements support the hypothesis that these two kinds of organoclays correspond to paraffin‐type tilted and perpendicular bi‐layer intercalates, respectively. The co‐intercalation of SA molecules with the double‐chain amphiphile is suggested, to explain the observed expansion of the clay interlayer distance. The obtained results suggest an easy way to control the organoclay structure in polymer composites. Moreover, the authors on the basis of these results propose a criticism to the extensive literature that systematically explains most d basal spacing increase observed for clays in polymer with the penetration of apolar polymer chains in the clay interlayer space. Copyright © 2010 John Wiley & Sons, Ltd.     

σ‐Alkylpalladium Intermediates in Intramolecular Heck Reactions:Isolation and Catalytic Activity

The isolation of σ‐alkylpalladium Heck intermediates, possible when β‐hydride elimination is inhibited, is a rather rare event. Performing intramolecular Heck reactions on N‐allyl‐2‐halobenzylamines in the presence of [Pd(PPh₃)₄], we isolated and characterized a series of stable bridged palladacycles containing an iodine or bromine atom on the palladium atom. Indolyl substrates were also tested for isolation of the corresponding complexes. X‐ray crystallographic analysis of one of the indolyl derivatives revealed the presence of a five‐membered palladacycle with the metal center bearing a PPh₃ ligand and an iodine atom in a cis position with respect to the nitrogen atom. The stability of the σ‐alkylpalladium complexes is probably a consequence of the strong constraint resulting from the bridged junction that hampers the cisoid conformation essential for β‐hydride elimination. Subsequently, the thus obtained bridged five‐membered palladacycles were proven to be effective precatalysts in Heck reactions as well as in cross‐coupling processes such as Suzuki and Stille reactions.     

A Flexible Pro‐porous Coordination Polymer: Non‐conventional Synthesis and Separation Properties Towards CO2/CH4 Mixtures

The novel coordination polymers [Cu(Hoxonic)(H₂O)]_n ( 1 ) and [Cu(Hoxonic)(bpy)_0.5]_n ? 1.5 n H₂O ( 2?H₂O ) (H₃oxonic: 4,6‐dihydroxy‐1,3,5‐triazine‐2‐carboxylic acid; bpy: 4,4′‐bipyridine) have been isolated and structurally characterised by ab initio X‐ray powder diffraction. The dense phase 1 contains 1D zig‐zag chains in which Hoxonic dianions bridge square‐pyramidal copper(II) ions, apically coordinated by water molecules. On the contrary, 2?H₂O , prepared by solution and solventless methods, is based on 2D layers of octahedral copper(II) ions bridged by Hoxonic ligands, further pillared by bpy spacers. The resulting pro‐porous 3D network possesses small hydrated cavities. The reactivity, thermal, magnetic and adsorptive properties of these materials have been investigated. Notably, the adsorption studies on 2 show that this material possesses unusual adsorption behaviour. Indeed, guest uptake is facilitated by increasing the thermal energy of both the guest and the framework. Thus, neither N₂ at 77 K nor CO₂ at 195 K are incorporated, and CH₄ is only minimally adsorbed at 273 K and high pressures (0.5 mmol g^?1 at 2500 kPa). By contrast, CO₂ is readily incorporated at 273 K (up to 2.5 mmol g^?1 at 2500 kPa). The selectivity of 2 towards CO₂ over CH₄ has been investigated by means of variable‐temperature zero coverage adsorption experiments and measurement of breakthrough curves of CO₂/CH₄ mixtures. The results show the highly selective incorporation of CO₂ in 2 , which can be rationalised on the basis of the framework flexibility and polar nature of its voids.     

Reversible coordinative chain transfer polymerization of styrene by rare earth borohydrides,chlorides/dialkylmagnesium systems

The ability of various rare earth borohydride and chloride complexes/n‐butylethylmagnesium systems to operate styrene chain transfer polymerization in mild conditions has been assessed. Thirteen precatalysts have been considered: the rare earth trisborohydrides Ln(BH₄)₃(THF)_x (x = 3, Ln = Nd (1), La (2), Sm (3), x = 2, Ln = Y (4), Sc (5)), the rare earth chlorides LnCl₃(THF)_x (x = 3, Ln = Nd (6), La (7), Sm (8), Y (9), x = 2, Ln = Sc (10)), the mixed La(BH₄)₂Cl(THF)_2.6 (11) and the half‐lanthanidocenes Cp*Ln(BH₄)₂(THF)₂ (Ln = Nd (12), La (13)). Six systems were found to be active precatalysts for the polymerization of styrene. 1 , 2 , and 11 led to an efficient transmetalation of the growing polystyrene chain with the simultaneous occurrence of βH elimination, whereas 7 , 12 , and 13 led to catalyzed chain growth behavior. It is noteworthy that the catalyzed chain growth obtained with 12 and 13 occurs with significant stereoselectivity. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 802–814, 2010     

Halo-carbonyl complexes of palladium,platinum and gold

In this review we summarize some recent literature data concerning synthetic procedures, properties, structure, reactivity and applications of halo-carbonyl complexes of palladium, platinum and gold, taking into consideration that the organometallic chemistry of these metals, with a particular attention to the halo-carbonyls, has been reviewed 20 years ago [F. Calderazzo, J. Organomet. Chem. 400 (1990) 303]. A brief overview of the early studies is provided.     

Effect of Titanium on Fluxional Behavior of Unbridged Metallocene Catalysts

Summary: Thanks to the use of a ligand consisting of a saturated ring fused to the cyclopentadienyl moiety, which sensibly increases the stability of titanocenes, it was possible to investigate the oscillating behavior of titanium catalysts in propylene polymerization for the first time. The titanium‐based catalysts yield poly(propylene)s with new and interesting microstructures. Indeed, in spite of the significantly lower isotactic pentad content [mmmm], the average lengths of the isotactic blocks (N_iso) are similar or even higher than that obtained with the zirconium‐based homologues. Accordingly, differential scanning calorimetry (DSC) analysis shows melting peaks which are more evident than those obtained with the zirconium homologues. The study of the microstructure of the poly(propylene)s obtained with these catalysts allowed us to participate in the still open discussion on the oscillating mechanism of unbridged metallocenes.

The titanium‐based catalysts employed in this study.     

Preparation of novel analgesics via diastereoselective nucleophilic addition to 1-dimethylamino-2-methylpentan-3-one

The diastereoselective synthesis of naphthyl amino alcohols via nucleophilic addition to racemic 1-dimethylamino-2-methylpentan-3-one was studied. The use of the appropriate experimental conditions allowed the synthesis of both diastereoisomers. The relative configurations were established via NOESY experiments.     

Chirality in the Absence of Rigid Stereogenic Elements: The Absolute Configuration of Residual Enantiomers of C3‐Symmetric Propellers

Two new tris(aryl)phosphane oxides existing as configurationally stable residual enantiomers have been synthesised and their racemates resolved by semipreparative HPLC on a chiral stationary phase (CSP HPLC). One of them, recognised as a conglomerate, could be resolved by fractional crystallisation at a preparative scale level. In this case, the absolute configuration of the propeller‐shaped molecule was determined by anomalous X‐ray scattering. The problem of the correlative assignment of the absolute configuration to all known C₃‐symmetric three‐bladed propeller‐shaped molecules existing as stable residual enantiomers is discussed. The configurational stability of the new chiral phosphane oxides and of the corresponding phosphanes was evaluated by CD signal decay kinetics and dynamic ¹H NMR spectroscopy. The racemisation barriers in phosphanes were found about 10 kcal mol^?1 lower than those found for the corresponding oxides, though geometry and inter‐ring gearing would be very similar in the two series. Configurational stability of residual tris(aryl)phosphanes was found to be influenced by the electronic availability of the phosphorus centre, as evaluated by electrochemical CV experiments.     

Carbon and diamond paste microelectrodes based on Mn(III) porphyrins for the determination of dopamine

Three Mn(III) porphyrins were used for the design of carbon paste and diamond paste based microelectrodes, which were employed for the determination of dopamine in pharmaceutical and biological samples using differential pulse voltammetry (DPV). The limits of detection lie between 1.6 × 10⁻¹³ and 2.0 × 10⁻⁶ mol L⁻¹ while the sensitivities were between 230 pA μmol L⁻¹ and 3.24 μA mol L⁻¹. Dopamine was recovered reliable from pharmaceutical and biological samples in percentages higher than 91.00% and 92.00%, respectively. The surface of the microelectrodes can easily be renewed by simple polishing, obtaining a fresh surface ready for use in a new assay.     

Development and validation of an HPLC-MS/MS method to quantify clopidogrel acyl glucuronide, clopidogrel acid metabolite, and clopidogrel in plasma samples avoiding analyte back-conversion

A new sensitive and fast quantitative analytical method for the simultaneous determination of clopidogrel, its main metabolite clopidogrel carboxylic acid, and the newly described acyl glucuronide metabolite, in human plasma samples, is presented. The analytical procedures (plasma storage, handling, and extract storage in the autosampler) were optimized in order to avoid back-conversion; a known drawback in measurements of clopidogrel. Clopidogrel acyl glucuronide was confirmed as a major source of back-conversion to the parent drug in the presence of methanol, and thorough stability experiments were carried out to find the most appropriate conditions for an accurate analysis of clopidogrel and the two metabolites. The method was validated by assessing selectivity, sensitivity, linearity, accuracy, and precision for all three analytes, in accordance to Food and Drug Administration guidelines. Spiked quality controls in plasma as well as incurred samples were used to verify back-conversion in the selected conditions, with results meeting European Medicines Agency acceptance criteria (concentrations within 80–120% of the first reading). The method was then applied to a pharmacokinetic study, and for the first time, a pharmacokinetic curve of clopidogrel acyl glucuronide in human plasma is presented. The concentrations ranged up to 1,048.684 ng/mL, with a mean of 470.268 ng/mL, while clopidogrel had a mean C _max of 1.348 ng/mL; these orders of magnitude show how much the back-conversion of this metabolite may influence clopidogrel quantification if it is not properly controlled.