Reaction products of dichlorodiorganostannanes with sodium in liquid ammonia: In-situ investigations with Sn NMR spectroscopy and usage as intermediates for the synthesis of tetraorganostannanes

Dichlorodibutylstannane, dichlorodioctylstannane and dichlorodiphenylstannane were reacted with different amounts of sodium in liquid ammonia. At a molar ratio of R₂SnCl₂/Na of 1:2, polystannanes precipitated, in some cases accompanied by cyclic oligostannanes. The products resulting from mixtures with R₂SnCl₂/Na ratios of 1:3 to 1:10 were soluble and, hence, could be studied in-situ in liquid ammonia with ¹¹⁹Sn NMR spectroscopy. The compounds obtained, tin hydrides of the type R₂SnH⁻ and in certain cases distannides of the composition R₄Sn₂²⁻, formed essentially independent of the R₂SnCl₂/Na ratio; this, in contrast to views expressed in the literature. Our experiments showed that the chemical structure of the in-situ generated species did not permit to draw conclusions about the composition of the reaction products with bromoethane and vice versa - a practice commonly described. Furthermore, we observed migration of the butyl groups both in-situ during the reaction of dichlorodibutylstannane with sodium in liquid ammonia, as well as in the final reaction product. By contrast in the case of the phenyl substituent, migration was detected not during the chemical event in liquid ammonia, but only in the compounds formed. These observations imply a different mechanism for butyl and phenyl group migration.     

C−H Activation and Olefin Insertion in d8 and d0 Complexes: Same Elementary Steps,Different Electronics

Metallocene alkyl complexes with d⁰ electron configuration and d⁸-configured square planar α-diimine late transition metal alkyl complexes show activity in both C−H activation through σ-bond metathesis and olefin insertion. Herein, we show by analysis of their M−CH₃ ¹³C chemical shift tensors that these reactions involve a π(M−C) interaction in the horizontal plane of the complex for both d⁰ and d⁸ systems. While in the case of d⁰ systems the interaction of an empty metal d-orbital and a filled carbon p-orbital causes partial alkylidene character of the M−C bond, the corresponding metal d-orbital is filled in d⁸ systems, thus generating a filled π*(M−C) orbital that increases the anionic character of the methyl group. This entails fundamentally different reaction mechanisms for d⁰ and d⁸ systems, which are reflected in the structures of the transition states: While d⁰ olefin insertion can be viewed as a [2+2] cycloaddition reaction, d⁸ olefin insertion rather resembles methyl group migration onto a positively polarized olefin, thus explaining the observed differences in regioselectivity. These findings are translated to σ-bond metathesis, a reaction which is isolobal to olefin insertion for both early and late transition metals.     

Crystal Structure of<Emphasis Type="SmallCaps"> D</Emphasis>(−)-amino-(4-hydroxyphenyl)acetate,the Zwitter Ionic Form of Biologically Active<Emphasis Type="SmallCaps"> D</Emphasis>(−)-4-hydroxyphenylglycine

Abstract  Crystals of D(−)-amino-(4-hydroxyphenyl)acetate were analyzed by single crystal X-ray diffraction. The compound crystallizes in the chiral space group, P2₁ (No. 4) with one molecule in the asymmetric unit and a = 5.0994(10) ?, b = 9.3169(19) ?, c = 8.6960(17) ?, β = 91.50(3)°. The intermolecular interactions in the crystal have been visualised by plotting the Hirshfeld surface of the molecule. Graphical Abstract  The extensive hydrogen bonding network in D(−)-amino-(4-hydroxyphenyl)acetate has been described and visualised by plotting its Hirshfeld surface.      

From poly(dialkylstannane)s to poly(diarylstannane)s: comparison of synthesis methods and resulting polymers

The efficiency and applicability of three different methods to synthesize polystannanes with different side chains are described. By means of dehydrogenative coupling utilizing the transition metal catalyst RhCl(PPh₃)₃ (Wilkinson's catalyst), n‐Bu₂SnH₂ reached the highest molar masses. Dehydrogenetive coupling in the presence of tetramethylethylenediamine could be best employed for (4‐n‐BuPh)₂SnH₂. Wurtz coupling using sodium in liquid ammonia was best suited for Ph₂SnCl₂. Next to the above‐mentioned educts, n‐Bu(Ph)SnX₂ (X = H or Cl (as appropriate for the particular route) was used for polymerization resulting in one of so far rare example of asymmetric polystannanes with high molecular masses. Copyright © 2011 John Wiley & Sons, Ltd.     

Statistical copolymers of N-vinylpyrrolidone and benzyl methacrylate via RAFT: Monomer reactivity ratios,thermal properties and kinetics of thermal decomposition

Statistical copolymers of N-vinylpyrrolidone (NVP) with benzyl methacrylate (BzMA) were prepared by Reversible Addition-Fragmentation chain Transfer Polymerization (RAFT), employing three different xanthates as RAFT agents: [(O-ethylxanthyl)methyl]benzene, [1-(O-ethylxanthyl)ethyl]benzene and O-ethyl S-(phthalimidylmethyl) xanthate. The reactivity ratios were estimated using the Fineman-Ross, inverted Fineman-Ross, Kelen-Tüdos and extended Kelen-Tüdos linear graphical methods, as well as the computer program COPOINT. Structural parameters of the copolymers were obtained by calculating the dyad sequence fractions and the mean sequence length. All the methods are in agreement that the probability for the incorporation of BzMA units is much greater than for that of NVP units. The glass-transition temperature (Tg) values of the copolymers were measured by Differential Scanning Calorimetry and examined by means of several theoretical equations, allowing for the prediction of these Tg values. Subsequently, a systematic and detailed investigation of the thermal degradation of these copolymers compared with that of the respective homopolymers, was carried out by Thermogravimetric Analysis, within the framework of the Ozawa-Flynn-Wall and Kissinger methodologies. As expected, the thermal stability of the copolymers is influenced by the structure of the substituents of the thiocarbonylthio end groups due to the RAFT agents.     

Polystannanes—Synthesis and Properties

Abstract

Polystannanes were prepared from H₂SnR₂ with the catalyst [RhCl(PPh₃)₃], or by reaction of Cl₂SnR₂ with Na in liquid ammonia. The polymers showed a characteristic peak in ¹¹⁹Sn NMR and UV-vis spectra, and a yellow color. Using common techniques, the polymers could be processed into films and oriented, the latter leading to a dichroism. The polystannanes are sensitive to light, more pronounced for R = alkyl than for R = aryl.

GRAPHICAL ABSTRACT     

From Si nanowire to SiC nanotube

Si nanowires (NWs), with diameters of about 800 nm and lengths of about 10 ??m, previously synthesized by the VLS method with gold catalyst, were carburized at 1,100 °C under methane for conversion into SiC nanostructures. These experiments have shown that Si NWs have been transformed into SiC nanotubes (NTs) with approximately the same sizes. Nanotubes?? sidewall thickness varies from 20 to 150 nm depending on the NTs?? height. These SiC nanotubes are hexagonal in shape and polycrystalline. A model of growth based on the out-diffusion of Si through the SiC layer was proposed to explain the transformation from Si nanowires to SiC nanotubes. This model was completed with thermodynamic calculations on the Si?CH₂?CCH₄?CO₂ system and with results from complementary experiment using propane precursor. Routes for obtaining crystalline SiC NTs using this reaction are proposed.     

1H and 13C NMR investigation of 20‐hydroxyecdysone dioxolane derivatives,a novel group of MDR modulator agents

The synthesis, structure elucidation and the complete ¹H and ¹³C signal assignment of a series of dioxolane derivatives of 20‐hydroxyecdysone, synthesized as novel modulators of multidrug resistance, are presented. The structures and NMR signal assignment were established by comprehensive one‐dimensional and two‐dimensional NMR spectroscopy supported by mass spectrometry. Copyright © 2013 John Wiley & Sons, Ltd.