Homogeneous pyrolysis kinetics of ethyl 3-hydroxy-3-methylbutanoate in the gas phase

The pyrolysis kinetics of ethyl 3-hydroxy-3-methylbutanoate have been examined over the temperature range of 286–330°C and pressure range of 29–108 Torr. In a seasoned vessel and in the presence of the free radical inhibitor cyclohexene or toluene the reaction is homogeneous, unimolecular and obeys a first-order rate law. The elimination products are mainly acetone and ethyl acetate, and very small amounts of ethyl 3-butenoate, acetic acid, ethylene and H₂O. The rate coefficient is expressed by the following equation: log k₁(s^–1)=(12.39±0.46)–(174.5±5.2) kJ mol^–1 (2.303RT)^–1. The mechanism appears to proceed via a six-membered cyclic transition state, where polarization of the (CH₃)C(OH)^+...-CH₂COOCH₂CH₃ bond is rate determining.     

Heteronuclear Nickel‐Iron Complexes and the Crystal Structure of [Fe2(CO)6(μ3‐S)2{Ni(dppe)}]

A series of heteronuclear nickel‐iron complexes [Fe₂(CO)₆(μ‐SH)(μ₃‐S){NiCl(PPh₃)₂}] ( 1 ), [Fe₂(CO)₆(μ‐SH)(μ₃‐S){NiCl(dppe)}] ( 2 ), [Fe₂(CO)₆(μ₃‐S)₂{Ni(PPh₃)₂}] ( 3 ), [Fe₂(CO)₆(μ₃‐S)₂{Ni(dppe)}] ( 4 ) and [Fe₂(CO)₆(μ‐SPh)(μ₃‐S){NiCl(dppe)}] ( 5 ) have been prepared. The structure of 4 has been determined by X‐ray crystallography. The central metal‐sulfur core of 4 has a trigonal bipyramidal shape with a NiFe₂ base plane with two axial sulfur atoms. Each iron atom is 5‐coordinate forming a distorted square pyramid; the nickel is square planar coordinated by two sulfur atoms and two phosphorus atoms.     

Réactions de germylènes avec divers oxirannes et le thiiranne. Mise en évidence de germanones et germathiones intermédiaires. Communication préliminaire

Complexed dialkylgermylenes R₂GeNR₃ or R₂GePy react with oxiranes or thiirane and lead to dialkylgermanone and germathione via germaoxetanes or germathiacyclobutane. The formation of germadioxolanes or germadithiolane finally observed arises from condensation of dialkylgermanone (or germathione) on oxiranes (or thiirane).     

An unprecedented highly efficient solvent-free oxidation of alkynes to α,β-acetylenic ketones with tert-butyl hydroperoxide catalyzed by water-soluble copper complex

The catalytic system composed of CuCl₂ and 2,2′-biquinoline-4,4′-dicarboxylic acid dipotassium salt (BQC) was found to be highly efficient for the selective α-oxidation of internal alkynes to the corresponding α,β-acetylenic ketones, with aqueous tert-butyl hydroperoxide under mild conditions. For the first time, full conversions of alkynes were reached with excellent selectivities, and propargylic tert-butylperoxy ethers were observed and suggested as the reaction intermediates. In the case of terminal alkynes, the oxidations are sluggish and low yields ranging from 32% to 40% were obtained.     

Tailoring the cavities of hydrogen-bonded amphidynamic crystals using weak contacts: towards faster molecular machines

This work describes the use of C–H⋯F–C contacts in the solid-state from the stator towards the rotator to fine-tune their internal motion, by constructing a set of interactions that generate close-fitting cavities in three supramolecular rotors 1–3I. The crystal structures of these rotors, determined by synchrotron radiation experiments at different temperatures, show the presence of such C–H⋯F–C contacts between extended carbazole stators featuring fluorinated phenyl rings and the 1,4-diazabicyclo[2.2.2]octane (DABCO) rotator. According to the ²H NMR results, using deuterated samples, and periodic density functional theory computations, the rotators experience fast angular displacements (preferentially 120° jumps) due to their low rotational activation energies (E_a = 0.8–2.0 kcal mol⁻¹). The higher rotational barrier for 1 (2.0 kcal mol⁻¹) is associated with a larger number of weak C–H⋯F–C contacts generated by the stators. This strategy offers the possibility to explore the correlation among weak intermolecular forces, cavity shape, and internal dynamics, which has strong implications in the design of future fine-tuned amphidynamic crystals.

This work describes the use of C–H⋯F–C contacts in the solid-state from the stator towards the rotator to fine-tune their internal motion, by constructing a set of interactions that generate close-fitting cavities in three supramolecular rotors 1–3I.     

Short synthesis of the CDEF ring system of lactonamycin

[reaction: see text]. A synthesis of the CDEF fragment of lactonamycin is achieved in eight steps (six pots) from the known and readily available anhydride 4 via a Diels-Alder reaction between tricycle 13 and 2,3-dimethylbenzoquinone.     

MP2 study of substituent effects of 2-substituted alkyl ethyl methylcarbamates in homogeneous, unimolecular gas phase elimination reaction

Møller-Plesset MP2/6-31G method was used to examine the gas-phase elimination of 2-substituted alkyl ethyl N,N-dimethylcarbamates. The results of these calculations support a concerted non-synchronous six-membered cyclic transition state mechanism for carbamates containing a C_β–H bond at the alkyl side of the ester. These substrates produce the N,N-dimethylcarbamic acid and the corresponding olefin. The unstable intermediate, N,N-dimethylcarbamic acid, rapidly decomposes through a four-membered cyclic transition state to dimethylamine and CO₂ gas. Correlation of the logarithm of theoretical rate coefficients against original Taft's σ* values gave an approximate straight line (ρ*=−1.39, r=0.9558 at 360 °C). In addition to this fact, when log k_rel is plotted against the theoretical log k_rel for 2-substituted ethyl N,N-dimethylcarbamates a reasonable straight line (r=0.9919 at 360 °C) is obtained, suggesting similar mechanism.     

Detection of genetically modified corn (Bt176) in spiked cow blood samples by polymerase chain reaction and immunoassay methods

The fate of DNA and protein transgenic sequences in products derived from animals fed transgenic crops has recently raised public interest. Sensitive molecular tests targeting the Bt176 genetic construct and the transgenic Cry1Ab protein were developed to determine whether plant sequences, especially transgenic sequences, are present in animal products. A protocol for total DNA extraction and purification from cow whole blood samples was first drawn up and assessed by spiking with known amounts of DNA from Bt176 maize. The limit of detection for transgenic sequences (35S promoter and Bt176-specific junction sequence) was determined by both the polymerase chain reaction-enzyme-linked immunosorbent assay (PCR-ELISA) and the 5'-nuclease PCR assay. Four additional PCR systems were built to substantiate the results. The first detects a mono-copy maize-specific sequence (ADH promoter). Two others target multi-copy sequences from plant nucleus (26S rRNA gene) and chloroplast (psaB gene). The last one, used as a positive control, targets a mono-copy animal sequence (alpha(s1)-casein gene). Both methods detected a minimum spiking at 25 copies of Bt176 maize/mL in 10 mL whole blood samples. The sandwich ELISA kit used detected down to 1 ng transgenic Cry1Ab protein/mL spiked whole blood.     

Bis(2,4,6-trifluorophenyl) derivatives of palladium(II)

Summary The organopalladium(II) complexes: Pd(2,4,6-C₆F₃H₂)₂L₂ [L=triphenylphosphine(PPh₃), methyldiphenylphosphine(PPh₂Me), dimethylphenylphosphine-(PPhMe₂) or pyridine(py); L₂=1,2-bis(diphenylphosphino)ethane(dpe), 2,2-bipyridine(bipy), 1, 10-phenanthroline(phen) or ethylenediamine(en)] have been prepared by addition of the appropriate compound to the THF-dioxane solution resulting from the arylation of potassium tetrachloropalladate(II) with (2,4,6-C₆F₃H₂)MgBr. The i.r. data suggest that the py and PPhMe₂ compounds are thecis-isomers, whereas the PPh₃ and PPh₂Me compounds have thetrans configuration.¹H- and¹⁹F-n.m.r. data for the compounds are reported.     

An improved method for the determination of the input applied to a linear system by deconvolution of the output. Application in thermokinetics

A simplified deconvolution procedure which can be used to remove the distortion, due to thermal lags, from calorimetric data is described and tested. The procedure is based on the state function theory. Noise in the deconvoluted data can be abated by smoothing the experimental data and by a careful choice of the data-sampling frequency. The method is adaptable to on-line processing by means of a microprocessor.

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Zusammenfassung Ein vereinfachtes Dekonvolutionsverfahren, das sich zur Behebung von infolge thermischer Lag-Erscheinungen auftretenden Zerrungen kalorimetrischer Daten eignet, wird beschrieben und getestet. Das Verfahren beruht auf der Theorie der Zustandsfunktion. Geräusche der Dekonvolutionsdaten können durch Ausgleichen der Versuchsdaten und durch sorgfältige Wahl der Datennahme-Frequenz gedämpft werden. Die Methode kann mittels eines Mikro-Verarbeiters zur on-line Verarbeitung eingesetzt werden.

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Résumé Une procédure simplifiée de déconvolution qui peut être utilisée pour éliminer des courbes calorimétriques, la distorsion causée par l'inertie thermique du calorimètre, est décrite et testée. La procédure est basée sur la théorie des variables d'état. Le bruit de fond des courbes corrigées peut être diminué par le lissage des données expérimentales et par le choix d'une fréquence d'échantillonnage appropriée. La méthode peut être adaptée au traitement en ligne des données au moyen d'un microprocesseur.

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The authors gratefully acknowledge the assistance of Dr. J. Van Bokhoven, Chemisch Laboratorium TNO, Rijswyk, the Netherlands, during the experimental part of this work.