Weak carbon-hydrogen-nitrogen interactions affect the heterocyclic ligand bonding modes in barium complexes containing eta2-tetrazolato and eta2-pentazolato ligands

Treatment of Ba[N(SiMe3)2]2(THF)2 with 2 equiv of dimethylaminotetrazole or diisopropylaminotetrazole and 1 equiv of 18-crown-6 afforded Ba[CN4(NMe2)]2(18-crown-6) (87%) and Ba[CN4(NiPr2)]2(18-crown-6) (79%) as colorless crystalline solids. Ba[CN4(NMe2)]2(18-crown-6) contains two 1,2-eta2-tetrazolato ligands and one eta6-18-crown-6 ligand. The molecular structure of Ba[CN4(NiPr2)]2(18-crown-6) is similar to that of Ba[CN4(NMe2)]2(18-crown-6), except that the tetrazolato ligands exhibit the isomeric 2,3-eta2-coordination mode and the tetrazolato ligand CN4 cores are bent significantly toward the 18-crown-6 ligands. Molecular orbital calculations were carried out on the model complexes Ba(azolate)2(18-crown-6) (azolate = 1,2-eta2-CHN4, 2,3-eta2-CHN4, and eta2-N5) and demonstrate that the ligand coordination modes are influenced by intramolecular interactions between filled nitrogen orbitals on the azolato ligands and empty C-H sigma* orbitals on the 18-crown-6 ligands.     

Chemistry of 1,4-dioxene II : A new method for the two-carbon homologation of aldehydes and ketones to α-hydroxymethyl ketones

1,4-Dioxen-2-yl lithium $\text{2}$ reacts with ketones or aldehydes to give alcohols $\text{3}$ which lead after an allylic rearrangement under mild conditions, followed by reduction and hydrolysis to α-hydroxymethyl ketones in fair yields.     

Formal synthesis of (+/-)-guanacastepene A: a tandem ring-closing metathesis approach

A concise route to a key intermediate in the total synthesis of guanacastepene A is described. The main features include the simultaneous construction of the seven- and six-membered rings, using a tandem ring-closing metathesis and a stereoselective introduction of the oxygenated function at the C5 position. [reaction: see text]     

Physicochemical Principles and Applications of Supercritical Fluid Chromatography (SFC). New analytical methods (19)

In supercritical fluid chromatography (SFC) compressed gases in the region of their critical temperature are used as mobile phases. SFC has important advantages over gas chromatography (GC) for the separation of low-volatile or thermally unstable substances. Like high pressure liquid chromatography (HPLC) and gel chromatography, it is used for various special applications and preparative separations, e.g. in the petroleum industry and in the separation of oligomers. SFC is of great interest in fundamental research on fluid extraction and for the determination of the physicochemical properties of fluid systems. In this contribution the most important physicochemical, methodological, and instrumental principles of SFC are summarized; characteristic physicochemical applications are the determination of capacity ratios, partition coefficients, partial molar volumes, interaction second virial coefficients, and difusion coefficients.     

Rate acceleration of anionic oxy-cope rearrangements induced by an additional unsaturation

[reaction: see text] In sharp contrast to their isopropyl counterparts, a variety of (Z)-isopropenyl tertiary bicyclo[2.2.2]octenols undergo facile anionic oxy-Cope rearrangements allowing the stereoselective incorporation of an isopropenyl group into polycyclic skeletons such as the tricyclic system of vinigrol, bicyclo[5.3.1]undecane, and cis-decalin frameworks. This rate acceleration is probably due to the stabilization of the transition state by the additional unsaturation on the terminal position.     

Calix[4]azacrowns: self-assembly and effect of chain length and O-alkylation on their metal ion-binding properties

Both the consequences of 1,3-cyclization of calix[4]arenes by diamide formation, and the effect of 2- and 2,4-alkoxycarbonylmethylation of two 1,3-bridged calix[4]azacrowns on their metal ion-binding capacities have been studied by synthesis of the derivatives in their cone conformation and measurement of the stability constants for their complexation in a range of alkaline earth and transition metal cations. In these two cases, the conformation of the ligand in the solid state has been defined in detail by a crystal structure determination. In the first case, a tubular architecture performing methanol channels is displayed, however, in the second case a zigzag architecture generated by a chloroform network connecting calixarene molecules is present. Speciation in solution appears to be limited to the formation of 1:2 and/or 1:1 and/or 2:1 (M-L) complexes, depending on the system, with values of the stability constants determined by absorption spectroscopy in acetonitrile, lying in the ranges log β₁₂ ca. 10, log β₁₁ ca. 2-5, and log β₂₁ ca. 7-9. There is evidence for selectivity toward Cu(II) and for some unexpected anion effects. None of these ligands appear to be an efficient extractant for the metals as their picrate.     

Uniform in bandwidth consistency of the kernel-type estimator of the Shannon's entropy

We establish uniform-in-bandwidth consistency for kernel-type estimators of the differential entropy. Our proofs rely on the methods of Einmahl and Mason (2005) [10].     

A simple model for a two-stage chemical reaction with diffusion

We analyse the evolution of a two-stage chemical reaction betweentwo neighbouring plumes of reactants. Under the assumption thatthe plumes are approximately Gaussian we derive a system ofordinary differential equations for the total amount, the centroidand the variance of each reactant. We compare the solution ofthese equations with full numerical simulation of the reaction.Excellent agreement is obtained, with solution of the near-Gaussianmodel requiring considerably less computational effort thanthe full simulations. Of key importance is the yield of thereaction, and we discuss this feature in particular.     

Dichroisme circulaire d'olefines VI alkylidene-17 androstanes

The Cotton effect of 17-alkylidene (5α) androstanes is discussed. The large variations observed in the C.D. values indicated that the role of olefin torsion in the ground state is not significant. A semi quantitative approach based on A.B.P. model is suggested.