Stereoselective intramolecular nicholas reaction using epoxides as nucleophiles

[reaction: see text] The intramolecular nucleophilic attack of the epoxides on the exo-Co(2)(CO)(6)-propargylic cations provided cyclic ethers in good yields. The use of substrates with stereochemically defined oxiranes provided polysubstituted tetrahydropyrans and oxepanes with a high degree of stereocontrol. The cyclization is sensitive to the nature of the protecting group used at the primary alcohol, the use of tert-butyl carbonates being highly effective in terms of regioselectivity and yields.     

1‐(2‐Aminophenyl)‐3‐(4‐pyridyl)prop‐2‐en‐1‐one: a three‐dimensional framework structure built from N—H...N,C—H...O and C—H...π(arene) hydrogen bonds

The intramolecular dimensions of the title compound, C₁₄H₁₂N₂O, provide evidence for a polarized electronic structure. The molecule, which is almost completely planar, contains an intramolecular N—H...O hydrogen bond, and the molecules are linked by a combination of N—H...N, C—H...O and C—H...π(arene) hydrogen bonds to form a three‐dimensional framework structure.     

Mechanism of Ni2+ and NiOH+ interaction with hydroxamic acids in SDS: evaluation of the contributions to the equilibrium and rate parameters in the aqueous and micellar phase

The equilibria and kinetics (stopped-flow) of the binding of Ni(II) to salicylhydroxamic acid (SHA) and phenylbenzohydroxamic acid (PBHA) have been investigated in aqueous solutions containing SDS micelles. The two ligands are fairly distributed between the two pseudophases present, so the binding reaction occurs in both phases. The contributions to the total reaction from each phase has been evaluated, following a procedure where use is made of the experimentally determined partition coefficients of the reactants involved. The mechanism of the reaction occurring on the micelle surface has been derived and comparison with the mechanism in water shows that the step Ni(2+) + HL ? NiHL(2+) is operative in both pseudophases, whereas the step Ni(2+) + L(-)? NiL(+), which is operative in water, is replaced in SDS by the step NiOH(+) + HL ? NiL(+). The analysis of the equilibrium and of the kinetic data enabled the evaluation of the equilibrium and the rate constants of the individual steps taking part in the binding process over the micelle surface. Interestingly, the first hydrolysis constant of the Ni(H(2)O)(6)(2+) ion in SDS is more than two orders of magnitude higher than in water. The agreement between the equilibrium constants derived from kinetics and those obtained by static measurements confirms the validity of the proposed mechanism.     

Metal-free highly luminescent silica nanoparticles

Stable, cost-effective, brightly luminescent, and metal-free organosilica nanoparticles (NPs) were prepared using the St?ber method without any thermal treatment above 318 K. The white-light photoluminescence results from a convolution of the emission originated in the NH(2) groups of the organosilane and oxygen defects in the silica network. The time-resolved emission spectra are red-shifted, relative to those acquired in the steady-state regime, pointing out that the NPs emission is governed by donor-acceptor (D-A) recombination mechanisms. Moreover, the increase of the corresponding lifetime values with the monitored wavelength further supports that the emission is governed by a recombination mechanism typical of a D-A pair attributed to an exceptionally broad inhomogeneous distribution of the emitting centers peculiar to silica-based NPs. These NPs exhibit the highest emission quantum yield value (0.15 ± 0.02) reported so far for organosilica biolabels without activator metals. Moreover, the emission spectra and the quantum yield values are quite stable over time showing no significant aging effects after exposure to the ambient environment for more than 1 year, stressing the potential of these NPs as metal-free biolabels.     

Preferred Conformations of Stabilized Phosphorus Ylides

The stabilized phosphorus ylides, Ph₃P=C(CO.R′)CO.OR; 1, R=Et, R′=CH₂P⁺Ph₃; 2, R=R′=Me; 3, R=Et, R′=Me; 4, R=Prⁱ; R′=Me; 5, R=Bu^t; R′=Me, adopt a near planar conformation in the crystal which allows extensive electronic delocalization. The keto and alkoxylic oxygens are oriented and align favorably with the cationoid phosphorus. These conformations bring methyl hydrogens in the ester residue into proximity with the face of a phenyl group and lead to π-shielding and upfield shifts of the ¹HNMR signals of 3 over a wide temperature range (-50–95°C) in (CD₃)₂CO, CDCl₃ and DMSO_d-6. Geometries of 2 and 3, optimized by using the HF 3-21 (G^*) or 6-31 (G^*) basis sets, are very similar to those in the crystal, but semiempirical treatments generate structures in which either the ester or keto moiety is twisted out of plane.

     

Biomimetic synthesis of diversifolin

The conversion of a germacranolide structure (tagitinin C) into a furanoheliangolide one (diversifolin) was achieved by hydride conjugate addition using Stryker’s reagent.     

Dissociative double photoionization of benzene molecules in the 26-33 eV energy range

A time-of-flight mass spectrometer with a position sensitive ion detector was used to study the dissociative double ionization of benzene by UV synchrotron radiation. The threshold energy for the main dissociative processes, leading to CH(3)(+) + C(5)H(3)(+), C(2)H(3)(+) + C(4)H(3)(+) and C(2)H(2)(+) + C(4)H(4)(+) ion pairs were characterized by exploiting a photoelectron-photoion-photoion-coincidence technique, giving 27.8 ± 0.1, 29.5 ± 0.1, and 30.2 ± 0.1 eV, respectively. The first reaction also proceeds via the formation of a metastable C(6)H(6)(2+) dication. The translational kinetic energy of the ionic products was evaluated by measuring the position of ions arriving to the detector. Theoretical calculations of the energy and structure of dissociation product ions were performed to provide further information on the dynamics of the charge separation reactions following the photoionization event.     

Phase-dependent photochromism of a lactone-stabilized chromene from a flavylium reaction network

New trans‐2‐hydroxychalcones bearing a carboxylate group at position 2′ ( Ct ^?) were synthesized (compounds 2 and 3 ). These compounds lead to a network of chemical reactions depending on pH value, light, and solvent. In water, when the pH value is lowered, the ionized trans‐chalcone is protonated and the flavylium cation A H⁺ is formed at very acidic pH values through hemiketal B and cis‐chalcone Cc , with global acidity constants of pK′_a ≤?1 and ≈0.1, respectively, for 2 and 3 . The electron‐acceptor character of the carboxylic substituent not only increases the observed acidity of the flavylium cation, but also decreases the rate of the ring‐opening/‐closing from a subsecond timescale to hours relative to model compound 1 (without carboxylate). The photochemistry of the network was studied in detail by means of continuous irradiation, monitored by UV/Vis absorption and ¹H and ¹³C NMR spectroscopic analysis. Although compound 3 is only slightly photoactive, compound 2 ( Ct^? ) reacts in aqueous solutions (λ_irr=313 nm) to form B^? and Cc^? , with a global quantum yield of 0.15, and fully reverts back to Ct^? with a rate constant of k=6.7×10^?5 s^?1. The flavylium cation is no longer formed in methanol, and irradiation of Ct^? leads to the formation of B ^? and the new lactone‐trapped chromene species La . The formation of La takes place through a sequence of three photochemical steps: photoisomerization of Ct ^?, photo‐ring‐closing reaction of Cc ^?, and photolactonization of B ^?. Only the cis/trans isomerization and ring‐closing reactions are thermally reversible on a timescale of seconds and hours, respectively. A photochromic system was achieved in rigid matrices of methanol (at 77 K) and 1‐dodecanol (5 °C) by irradiating lactone La to give a red ortho‐quinone allide through a photo‐ring‐opening reaction; the color disappears with a rate constant of k=1.25×10^?2 s^?1 in 1‐dodecanol at 5 °C.     

Coaxial metal and magnetic alloy nanotubes in polycarbonate templates by electroless deposition

We present a novel technique for the preparation of coaxial metal and magnetic alloy nanotubes, which is demonstrated for the coaxial nanotubes of Ni/Co and Ni/CoNiFe alloys deposited in activated polycarbonate templates using electroless plating. For each metal or alloy the tube wall thickness was controlled to be less than 100 nm. The process involved two consecutive deposition steps from hypophosphite and/or borane reducing agent based electroless plating solutions. We further characterise the magnetic properties of the ternary magnetic alloy films and coaxial nanotubes. The coaxial tubes show homogenous wall thickness and composition, which is delineated from the magnetic measurements.     

Theoretical study of [Hg3(o-C6F4)3]n · {benzene} (n = 1, 2) complexes

The electronic structure and spectroscopic properties of [Hg₃(o-C₆F₄)₃]_n · {benzene} (n = 1, 2) were studied at the HF, MP2 and PBE levels. The interaction between [Hg₃(o-C₆F₄)₃] and benzene at the HF and MP2 levels was analyzed. Secondary π-interactions (Hg–benzene) were found to be the main contribution short-range stability in the [Hg₃(o-C₆F₄)₃] · {benzene} complex. At the MP2 and PBE levels equilibrium Hg–C distances of 338.4 and 361.4 pm; and interaction energies of 46.6 and 29.2 kJ/mol were found, respectively. The absorption spectra of these complexes were calculated by the single excitation time-dependent method at PBE level.