755 research outputs found
Magnesium Diimidosulfinates – Conformational Studies in the Solid State and in Solution
No full textThe magnesium diimidosulfinates [(thf)(2)MgX{(NR)(2)SR'}] (X = Cl, Br; R = tBu, SiMe(3); R' = Ph, Bz, Me, nBu; 1-5) have been synthesised by treating sulfur diimides with different Grignard reagents and characterized by X-ray crystallography and NMR spectroscopy. The structural analyses of compounds 1-5 show that they contain either monomeric or dimeric structural motifs, depending on the steric requirements of the sulfur-bound organic substituent. NMR experiments suggest a temperature-dependent dynamic equilibrium between different conformers for all compounds
Access to new Janus head ligands: linking sulfur diimides and phosphanes for hemilabile tripodal scorpionates
No full textReactions of lithium dialkyl/phenyl phosphanylmethylides, RR'PCH(X)Li (R, R' = Me, Et, Ph and R = Me, R; = Ph; X = H or Me), with sulfur diimides S(NR '')(2) (R '' = (t)Bu or SiMe(3)) in an equimolar ratio yielded Janus head complexes with the structural motif [Li{RR'PCH(X)S(NR '')(2)}](2) (R '' = (t)Bu, SiMe(3)). The basic core of these dimeric complexes is composed of a (LiN)(2) four-membered ring containing two four-coordinated lithium atoms. A lithium complex of the new Janus head ligand with another structural motif [TMEDA center dot Li{Ph(2)PCH(2)S(NSiMe(3))(2)}] (6) could be isolated from the reaction of [Ph(2)PCH(2)Li center dot TMEDA] with S(NSiMe(3))(2). Two monomeric complexes [Mg{Me(2)PCH(2)S(NR '')(2)}(2)] (7, 8) were synthesised by a straightforward reaction of [Li{Me(2)PCH(2)S(NR '')(2)}(2)] with MgCl(2) in pentane. The magnesium atom is chelated by one phosphorus atom and two nitrogen atoms of each unit of the hemilabile ligand in a tripodal manner, leading to octahedral geometry around the magnesium cation. A complete analysis of [Ph(2)PCH(2)(SNSiMe(3))(HNSiMe(3))] (9) is also described in which one nitrogen atom of the imido moiety is protonated.DFG [1178]; Center of Materials Crystallography; INS
‐Bis(1,3‐disila‐2‐lithiumamido)‐ethylensilylaminen – Synthesen, Kristallstrukturen, Reaktionen
No full textEthylenediamine reacts with the chloro-functional tris(silyl)amines ClSiMe2N(SiMe3)SiMe2R in a molar ratio of 1:2 to give the N,N'-bis(1,1,3-trisila-2-aza)-ethylenediamines, [CH2-NH-SiMe2-N(SiMe3)SiMe2R](2), R = Me (1) and CMe3 (2). A structural isomer of 2 (CH2-NSiMe2CMe3-SiMe2-NHSiMe3 = 3) is formed thermally, via 1,3-silyl group migration from the ME3CSiMe2-N-SiMe3 group, at 200 degrees C. 1 reacts with butyllithium at 0 degrees C in a molar ratio of 1:2, forming the diamide 4, which was characterized as the monomer [CH2-NiSMe3-SiMe2-N(SiMe3)Li](2). Again two silyl groups migrate from the N(SiMe3)(2) groups to the ethyleneamine nitrogen atoms. The lithium ions are coordinated by both amido groups forming a planar four-membered ring. With THF as a solvent, each lithium ion of 4 coordinates one THF molecule. [CH2-NSiMe3-SiMe2-N-(SiMe3)Li-THF](2) (5) is obtained. With TMEDA as the donor base the cage structures of 4 and 5 are broken up to give an open-chain dilithium salt [CH2-NiSMe3-SiMe2-N(SiMe3)Li-TMEDA](2) (6) with each lithium ion coordinated by one TMEDA molecule. Halosilanes and haloboranes react with 4, 5, and 6 to give an N,N'-bis(1,1,3-trisila-2-aza)-ethylenesilylamine [CH2-NiSMe3-SiMe2-N(SiMe3)R](2), R= SiF2CMe3 (7), and N,N'-bis(1-bora-1,3-disila-2-aza)-ethylene-silylamines, R = B(F)N(SiMe3)(2) (8). B(F)[N(CMe2CH2)(2)CH2] (9). Thermally, 9 forms (FSiMe2-NSiMe3-CH2)(2) and via transient iminoborene. Me3SiN=BR, the four-membered ring (Me3SiN-BR)(2), (10). R = [N(CMe2CH2)(2)CH2]. Ring closure occurs and cyclononanes are obtained in the reaction of 4 with BF3 (11) and SiF4 (12). The crystal structures 4-10 are discussed
[(thf)Li-21{H2CS((NBu)-Bu-t)(2)}](2): Synthesis, polymorphism, and experimental charge density to elucidate the bonding properties of a lithium sulfur ylide
No full textSulfur ylides (R2S+--CR2) are widely used in organic synthesis for stereoselective epoxidations, cyclopropane formations, and ring expansion reactions. Nevertheless, their electronic properties are still under debate, because their ylenic textbook formulation (R2S=CR2) contradicts the reactivity. In order to elucidate the electronic situation in a sulfur ylide, we present an experimental charge density study via multipole refinement and subsequent topological analysis based on high-resolution X-ray data of [(thf)Li-2{H2CS((NBu)-Bu-t)(2)}](2) (1). The title compound is of special interest, since additionally the formal hypervalency can be investigated along with the controversial interaction between a carbanion with a Li-3 triangle. As a prerequisite for these studies, the polymorphism and synthesis of I were investigated. The findings clearly support the ylidic, non-hypervalent description of the molecule. The ylidic carbanion was determined to be 6-fold coordinated with three single bonds to the sulfur atom and both hydrogen atoms and three closed-shell interactions to the lithium atoms of the Li-3 triangle
Competing reactions of hypercoordinate silicon dichelates
No full textNeutral hexacoordinate silicon complexes derived from hydrazide chelating ligands with imino-donor groups, and their pentacoordinate ionic dissociation products, undergo facile intramolecular aldol-type condensation catalyzed by their chloride counterion leading to formation of a third chelate ring. In analogous silacyclobutane dichelates, in the absence of halide counterion, a similar uncatalyzed rearrangement takes place, accompanied by opening of the four-membered ring. In the absence of et-protons necessary for the condensation, the four-membered ring residue adds directly to one of the imino-carbon atoms forming a new C-C bond and closing a different chelate ring. This latter addition to the imino carbon is the preferred reaction pathway, even in the presence of 12 alpha-protons, when cyanide ion replaces the chloride counterion and acts as nucleophile. The cyanide reactivity is rationalized in terms of the HSAB concept. An unusual intramolecular rearrangement involving the migration of a t-butyl group from silicon to carbon, while enabling the unprecedented attachment of a third hydrazide chelating agent, leading to a hexacoordinate trichelate complex, is presented. Copyright (C) 2008 John Wiley & Sons, Ltd.Israel Science Foundation [ISF-139/05]; INTAS [03-51-4164
1-Phenyl-1,2-cyclohexadiene: Generation, Interception by Activated Olefins, Dimerisation and Trimerisation
No full textFour possible precursors of 1-phenyl-1,2-cyclohexadiene (2) were examined, namely, 6,6-dibromo-1-phenylbicyclo[3.1.0]hexane, (1 alpha,5 alpha,6 alpha)-6-bromo-6-fluoro-1-phenylbicyclo-[3.1.0]hexane, 1-bromo-2-phenylcyclohexene and 1-bromo-6-phenylcyclohexene. All four compounds could be converted into 2, as demonstrated by the products of the interception of 2 with activated olefins. Styrene, 1,1-diphenyl-ethene, indene, furan and 2,5-dimethylfuran were employed as such. Whereas the first three gave [2+2] cycloadducts of 2, the last two provided one [4+2] cycloadduct each. To create the [2+2] cycloadducts, the it bond of 2 that is more remote from the phenyl group reacted, whereas the pi bond of 2 conjugated with the phenyl group exclusively produced the [4+2] cycloadducts. The generation of 2 in the absence of a trapping reagent brought about relatively good yields of a dimer or a trimer of 2 depending on the mode of the liberation of 2. Being derivatives of triphenylene, the dimer as well as the trimer have unusual structures, thereby indicating that a phenyl group is participating in the formation of these compounds. The most surprising structure of the trimer was elucidated by X-ray crystal diffraction. As to the mechanisms, diradical intermediates are proposed both for the cycloadditions and for the dimerisation. The initial steps of the latter seem to proceed also in the trimerisation.Deutsche Forschungsgemeinschaft; Fonds der Chemischen Industri
Biogeochemical redox proxies in sediments from Dotternhausen during the Toarcian (Early Jurassic)
No full textAuthor contributions:
The lead author is Angela L. Coe. Measurements were performed by Stephan M. Harding, with supervision of Angela L. Coe and Anthony S. Cohen. Measurements were gathered, processed and analysed by Itzel Ruvalcaba Baroni
Biogeochemical redox proxies in sediments from Yorkshire during the Toarcian (Early Jurassic)
No full textAuthor contributions:
The lead author is Angela L. Coe. Measurements were performed by Stephan M. Harding, with supervision of Angela L. Coe and Anthony S. Cohen. Measurements were gathered, processed and analysed by Itzel Ruvalcaba Baroni
Competitive molecular rearrangements in hexacoordinate cyano-silicon dichelates
No full textPentacoordinate siliconium chloride or neutral hexacoordinate silicon complexes with imino-nitrogen donor groups react with cyanotrimethylsilane in two competing reactions, leading either to addition of the cyano group to the imino carbon or to hexacoordinate cyano-silicon complexes. The latter may further transform to a rearranged tricyclic pentacoordinate complex. The common driving force for these reactions seems to be the conversion of one of the two initial N -> Si dative bonds, present in the starting complexes, to a shorter formal covalent bond
‐Naphthalene (2,3‐Didehydro‐1,2‐dihydronaphthalene)
No full textThe bromofluorocarbene adduct rac-1 of indene, possessing the fluorine atom at the endo position, is a useful substrate for the generation of the isonaphthalene 4 by the Doering-Moore-Skattebol reaction. By resolution of rac-1, an enantiomerically pure precursor of a six-membered cyclic allene was obtained for the first time. The treatment of (+)- or (-)-1, dissolved in 2,5-dimethyl-, 2-tert-butyl-5-methyl-, or 2,5-bis(tert-butyl)furan, with methyllithium gave rise to the [4+2] cycloadducts 6 of 4 to the furans. It was shown by means of HPLC on Chiralcel OD that the formation of 6 proceeded with about 40 % ee, and that this value was independent of the type and concentration of the furan, and the reaction temperature. The absolute configurations of the enantiomers 1, aswell as those of the enantiomers 6, were determined by simulation of the CD spectra by quantum chemical methods and by the comparison of them with the experimental spectra. In the case of (+)-1, the reliability of this procedure was checked by X-ray crystal structure analysis. On the basis of these results, a model is proposed for the steric course of the reaction sequence, leading from a pure enantiomer 1 to the 70:30 mixtures of the product enantiomers 6. The use of indene as trapping reagent for 4 furnished the [2+2] cycload-duct 15. For the preparation of rac-15, the particularly simple one-pot procedure was employed, in which indene, tetrabromomethane, and methyllithium were combined and in which the dibromocarbene adduct 2 of indene serves as precursor of 4. Compound 2 could not be isolated, but was characterised by low-temperature H-1 NMR spectra. The conversion of (+)- or (-)-1 into 4 in the presence of indene afforded 15 only with a very low enantioselectivity. The constitutions and the relative configurations of 15 as well as the compounds 8 and 16, which resulted from thermolysis of the cycloadducts 6b and 15, respectively, were elucidated by X-ray crystal diffraction. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)
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