Stereoselective synthesis of C15C24 and C25C30 fragments of dolabelides

The stereocontrolled synthesis of a C15C24 fragment of dolabelides is reported. The C19 and C21 hydroxyl-bearing stereocenters were installed using ruthenium-mediated asymmetric hydrogenations of cyclic hemiketal 4 and β-keto ester 7. The C25C30 portion of dolabelides was prepared as well by ring opening of chiral epoxy alcohol 12 to set up the C27 stereogenic center.     

Cycloaddition of alkynes RCCR′ with free singlet dimethyl germylene Me2Ge

Alkynes RCCR′ exhibit a high and specific reactivity towards free dimethyl germylene Me₂Ge as a typical singlet heavy carbene analogue. New and novel unsaturated² 4-,5-, and 6-membered germa heterocycles with 1–3 Ge atoms are formed, dependent on R and R′ in the 7 alkynes used. Corresponding reaction mechanisms are proposed.     

Trapping of the coordinatively unsaturated organosilicon anion radical Me3SiCCCCSiMe3− by double ate-complex formation with AIR3

Reaction of 1,4-bis(trimethylsilyl)-1,3-butadiyne with AIR₃ and potassium in THF has led to observation of the first examples of persistent radical di-ate complexes, [(R₃Al)(Me₃Si)CCCC(SiMe₃)(AIR₃)]⁻, which have been characterized by ESR spectroscopy.     

Copper(II)-catalyzed CH oxidation of alkylbenzenes and cyclohexane with hydrogen peroxide

Copper(II) complex 1 efficiently catalyses the oxidation of alkylbenzenes and cyclohexane into the corresponding ketones in moderate to high yields in the presence of 30% H₂O₂. This protocol is simple, clean and generates water as the only by-product.     

Thermodynamics of the URuC system

Thermodynamic measurements by the electromotive force method were made on the binary intermetallic phases URu₃ and U₃Ru₅ and on the ternary carbides URu₃C_0.7 and U₂RuC₂ of the URu and the URuC systems between 950 and 1200 K using galvanic cells with CaF₂ single crystal electrolytes: U, UF₃¦CaF₂¦UF₃, URu₃, Ru; U, UF₃¦CaF₂¦UF₃, U₃Ru₅, URu₃; Ru, URu₃, UF₃¦CaF₂¦UF₃, URu₃C_0.7, Ru, C; U, UF₃¦CaF₂¦UF₃, URu₃C_0.7, U₂RuC₂, C. The Gibbs energies of formation of URu₃, U₃Ru₅, URu₃C_0.7 and U₂RuC₂ were evaluated from the measured electromotive force which give ^fΔG^o〈URu₃〉 = −199 100 + 35.9 T J mol^−1fΔG^o〈U₃Ru₅〉 = −398 600 + 43.6 T J mol^−1fΔG^o〈URu₃C_0.7〉 = −192 600 + 2.5 T J mol^−1fΔG^o〈U₂RuC₂〉 = −380 200 + 52.5 T J mol⁻¹ The implications of these thermodynamic data for the behaviour of the fission product ruthenium in irradiated carbide fuels are discussed.     

C(sp)H?O and C(sp)H?O hydrogen bonds in acyclic- and cyclic-organotellurium carboxylates

The reactions between R₂TeI₂ (R₂=(CH₃)₂, C₄H₈, C₅H₁₀) and AgOCOR′ (R′=C₆H₅, 4-NO₂C₆H₄, CHCHC₆H₅) (molar ratio 1:2) yield diorganotellurium dicarboxylates: (CH₃)₂Te(OCOC₆H₅)₂ (1), C₅H₁₀ Te(OCOC₆H₅)₂ (2), C₄H₈Te(OCO4-NO₂C₆H₄)₂ (3) and C₄H₈Te(OCOCHCHC₆H₅)₂ (4). They are characterized by IR, (¹H, ¹³C, ¹²⁵Te) solution NMR; (¹³C, ¹²⁵Te) solid state NMR spectroscopy. The X-ray structures of 1-4 (the immediate environment about tellurium is that of distorted trigonal bipyramidal geometry with a stereochemically active electron lone pair) are described in the context of their ability to generate intermolecular CH?O hydrogen bonds, which lead to the formation of supramolecular assemblies.     

Allyldialkyl complexes of ruthenium(IV): Preparation and reductive CC bond formation followed by CH bond activation

New η³-allyldimethyl complexes Ru(η⁵-C₅R₅)(η³-C₃H₅)(CH₃)₂, where R = H or CH₃, are prepared from Ru(η⁵-C₅R₅)(η³-C₃H₅)Br₂ by alkylation with trimethyl-aluminium. The Ru^IV dimethyl complex is thermally converted to the Ru^II 1-methylallyl compound, Ru(η⁵-C₅R₅)(η³-CH₂CHCHCH₃)L, where L = CO or t-C₄H₉NC, with evolution of methane. Kinetic and deuteration studies on the reductive process are also discussed.     

The excess enthalpies of liquid GaGeTe and GaSnTe alloys

The excess enthalpies of the liquid alloys GaGeTe and GaSnTe were measured in a heat-flow calorimeter at 1203 K. The enthalpy surface in the ternary space in both systems is characterized by a valley stretching from the exothermic minimum in the GaTe system to the minima of the GeTe and SnTe systems. The minima in the ternary systems were found in this valley, i.e. on the sections Ga₂Te₃-GeTe and Ga₂Te₃-SnTe. A comparison of the experimental data with those calculated from the excess enthalpies of the constituent binaries with the aid of the Bonnier model, reveals only small deviations. A preliminary investigation was made into the ternary phase diagram of GaSnTe. This system contains the two quasibinary sections Ga₂Te₃-SnTe, GaTe-SnTe and the ternary compound Ga₆SnTe₁₀. The previously reported compounds Ga₂SnTe₃ and GaSnTe₂ do not exist.     

High-Tc superconductivity in the ErBaCuO and related systems

ErBa₂Cu₃O₇ and Er_0.5Y_0.5Ba₂Cu₃O₇ are both high-T_c superconductors attaining zero resistance above 80 K. Preliminary studies indicate that Yb_1−xY_xBa₂Cu₃O₇ also exhibits zero resistance above 77 K.     

Stepwise TiCl,TiCH3, and TiC6H5 bond dissociation enthalpies in bis(pentamethylcyclopentadienyl)titanium complexes

Reaction-solution calorimetric studies involving the complexes Ti[η⁵-C₅(CH₃)₅]₂-(CH₃)₂, Ti[η⁵-C₅(CH₃)₅]₂(CH₃), Ti[η⁵-C₅(CH₃)₅]₂(C₆H₅), Ti[η⁵-C₅(CH₃)₅]₂Cl₂, and Ti[η⁵-C₅(CH₃)₅]₂Cl, have enabled derivation of titaniumcarbon and titaniumchlorine stepwise bond dissociation enthalpies in these species.     

Integral intensities and polarity of vinylic CH bonds of 3,3-disubstituted cyclopropenes

The influence of substituents on the polarity of vinylic CH bonds of 3,3-disubstituted cyclopropenes was studied by analysis of the integral absorption intensities of infrared bands. The influence of intermolecular interactions in solutions of cyclopropenes in non-polar solvents on integral intensities has been determined. The dipole moments of the vinylic bonds were determined starting from infrared band intensities. The experimental data were compared with CNDO/S3 calculations. High sensitivity of intensities and frequencies of the absorption bands to the changes in electron acceptor character of substituent has been found. Correlations were established between dipole moment values of vinylic bonds, determined from experiment and on the basis of CNDO/S3 calculations, as well as between stretching vibrational frequencies on the one hand and values of substituent constants on the other.     

First principles search of hard materials within the SiCN ternary system

Starting from C₃N₄ and Si₃N₄ stoichiometries and from the pseudocubic model structure of the former, intermediate phases SiC₂N₄ and Si₂CN₄ are proposed and geometry optimised within density functional built pseudopotential method using both local density (LDA) and generalised gradient approximations (GGA). The ternary compounds are found to be less stable than the two binary systems but the trends in the calculated magnitudes of the bulk moduli B₀ from the fit of the E(V) curves with Birch equation of state: B₀ (SiC₂N₄)=334.5 GPa and B₀ (Si₂CN₄)=270.3 GPa can be interpolated from those of the two extreme compounds: B₀ (C₃N₄)=424.1 GPa and B₀ (Si₃N₄)=219.8 GPa. This translates the chemical role of the substituting element on one hand and allows validating Cohen's semiempirical law relating B₀ to the inverse powers of the average interatomic distances on the other hand. From a mismatch of the chemical bonding in Si(C)NC(Si) chain observed by the electron localisation function (ELF) plot we propose an interpretation for the instability of the intermediate ternary phases. The electronic structure (density of states and band structures) obtained from augmented spherical wave (ASW) calculations of the relaxed structures point to semiconducting behaviour with smaller band gaps for the intermediate phases (∼2 eV, compared with the ∼4 eV gap of binaries).     

Palladium-(II) and -(IV) complexes as intermediates in catalytic CC bond-forming reactions

Palladium-(II) and -(IV) species active as catalysts in CC bond-forming reactions have been stabilized by adding phenanthroline as a ligand. The complexes formed have been characterized by chemical and spectroscopic methods and their formation and subsequent fate have been monitored by ¹H NMR spectroscopy.     

Divergent Coupling of Benzocyclobutenones with Indoles via C−H and C−C Activations

Highly selective divergent coupling reactions of benzocyclobutenones and indoles, in which the chemoselectivity is controlled by catalysts, are reported herein. The substrates undergo C2(indole)–C8(benzocyclobutenone) coupling to produce benzylated indoles and benzo[b]carbazoles in the Ni- and Ru-catalyzed reactions. A completely different selectivity pattern C2(indole)–C2(benzocyclobutenone) coupling to form arylated indoles is observed in the Rh-catalyzed reaction. Preliminary mechanistic studies suggest C−H and C−C activations in the reaction pathway. Synthetic utility of this protocol is demonstrated by the selective synthesis of three different types of carbazoles from the representative products.     

Oxidation resistance of LnSiON powders (Ln=Y,Gd and La)

For application of LnSiON (Ln=Y, Gd and La) oxynitride materials, e.g. as host-lattices for lamp phosphors, oxidation resistance up to about 600 °C in air is a prerequisite. In this study we prepared LnSiON (Ln=Y, Gd and La) powders by solid state reaction and observed via TGA/DTA-experiments that most compounds are oxidation resistant up to 600 °C in air. The stability in air at high temperatures increases going from Ln₅(SiO₄)₃N, Ln₄Si₂O₇N₂, LnSiO₂N, Ln₂Si₃O₃N₄ to Ln₃Si₈O₄N₁₁. This is explained by an increasing cross-linking between the siliconoxygennitrogen tetrahedra in this sequence. For the lattices with less cross-linking between the siliconoxygennitrogen tetrahedra we observed that the oxidation resistance decreases slightly going from Y and Gd to La. For these lattices, also, an additional weight gain was observed during the oxidation reaction, which was higher than expected for complete oxidation. The additional weight gain was ascribed to an intermediate phase in which nitrogen retention takes place.     

Torsional potentials and conformational structures in the halogen substituted molecules HOCCH2X,HOCCHX2, XOCCH2X,XOCCHX2, XH2CCOCH3, X2HCCOCH3, (XH2C)2CO, (X2HC)2CO,and (X3C)2CO (X = Cl,Br) as determined by molecular-mechanics calculations

Parameter values of the non-blonded atom⋯atom interaction Br⋯OC have been established from gas-phase data. Conformational energies, structures, torsional barrier heights and torsional force constants of the title compounds have been estimated. For BrOCCH₂Br, (Cl₃C)₂CO and (ClH₂C)₂CO the calculated results are compared with gas-phase observations. It is suggested that hexachloroacetone ought to be reinvestigated by electron diffraction.     

Gas phase substitution reactions by radical cations Part 5: Reaction of the CC ring-opened oxirane radical cation with 1,2-, 1,3- and 1,4-dichlorobenzene

Under conditions of chemical ionization in the high pressure source of a mass spectrometer, the α-distonic CC ring-opened oxirane radical cation transfers a methylene group to 1,2-, 1,3- and 1,4-dichlorobenzene. The structures of the M + 14]^·+ product ions have been established by collisionally induced dissociation of these ions compared with reference ions and application of principal component analysis. 1,2-Dichlorobenzene yields 80% 2-chlorobenzyl chloride, 5% 2,3-dichlorotoluene and 15% 3,4-dichlorotoluene. The [M + 14]^·+ ions from 1,3-dichlorobenzene are 64–67% 3-chlorobenzyl chloride, 27–28% 2,6-dichlorotoluene and 7% 2,4- or 3,5-dichlorotoluene. From 1,4-dichlorobenzene mainly 4-chlorobenzyl chloride is formed, together with some 2,5-dichlorotoluene. In this case there is also an unidentified contribution, probably by 1,4-dichlorocycloheptatriene ions. Possible formation of distonic product ions does not occur in the cases of 1,2- and 1,3-dichlorobenzene, and from 1,4-dichlorobenzene it is considered to be unlikely.     

Template-directed CH activation: development and application to the total synthesis of 7-episordidin

The development of a template-directed CH activation strategy and its application to the diastereoselective synthesis of (±)-7-episordidin, an aggregation pheromone from the male banana weevil, Cosmopolites sordidus Germar, is reported. The key step of the synthetic route is a regioselective rhodium(II)-catalyzed diazocarbonyl CH activation reaction that simultaneously generates three of the four stereocenters present in the natural product.