Structure and photochemistry of 18-diazo-1,4,7,10,13,16-hexaoxacyclononadeca-17,19-dione and its sodium and potassium complexes. Control of the ground-state conformation of 2-diazo-1,3-dicarbonyl fragment via host-guest complexation

[reaction: see text] The macrocyclic 18-diazo-1,4,7,10,13,16-hexaoxacyclononadeca-17,19-dione (3-diazo-2,4-dioxo-19-crown-6, 1) readily forms complexes with potassium (2, stability constant in methanol is K(K+) = 229 +/- 25 M(-1)) and sodium ions (3, K(Na+) = 84.2 +/- 7.9 M(-1) in methanol). According to B3LYP/6-31G+(d,p) calculations and temperature-dependent 1H NMR spectroscopy, the predominant conformation of 1 has a Z,Z arrangement of the diazo and carbonyl groups. The X-ray crystal structure analysis showed that the potassium complex (2) has the same Z,Z arrangement, while the sodium analogue (2) exists in conformation with Z,E geometry of the diazodicarbonyl moiety. Direct 254 nm photolysis of diazo compounds 1-3 in methanol results in the formation of 3-methoxy-2,4-dioxo-19-crown-6 (5), the product of the insertion of corresponding alpha,alpha'-dicarbonylcarbene into the O-H bond of the solvent. The triplet-sensitized photolysis of diazomalonates 1-3 produces 2,4-dioxo-19-crown-6 (6), which is apparently formed via the triplet state of the intervening carbene.     

Synthesis of gold nanorod/single-wall carbon nanotube heterojunctions directly on surfaces

This contribution describes the synthesis of gold nanorod (Au NR)/single-wall carbon nanotube (SWCNT) heterojunctions assembled directly on Si/SiOx substrates. SWCNTs are attached to amine-functionalized Si/SiOx substrates, and Au monolayer-protected clusters (MPCs) are adsorbed to the surface of SWCNTs through hydrophobic interactions. Seed-mediated reduction of HAuCl4 with ascorbic acid in the presence of cetyltrimethylammonium bromide (CTAB) onto the Au MPCs leads to the growth of larger Au nanostructures directly on the SWCNTs. Au NRs account for 19% of the nanostructures, some of which are attached directly to the sidewall and some at the ends of the SWCNTs. Raman spectroscopic measurements of SWCNTs before and after growth of the Au nanostructures reveal that the presence of Au leads to an approximately 50-fold enhancement of the Raman scattering signal. Combining 1D nanostructures of different materials (Au and carbon in this example) is of fundamental interest and may find use in nanoelectronics, chemical sensing, electrochemical, and spectroscopy applications.     

Strategy for the synthesis of polymeric supports with hydrazone linkers for solid-phase alkylation of ketones and aldehydes

A new approach to polymeric supports useful for the immobilization of aldehydes and ketones via hydrazone linkers is reported. The new strategy gives supports with better properties and is effective for the synthesis of all supports previously used for the alkylation of ketones anchored as hydrazones. In contrast to other approaches, the new strategy also provided a polymer with an economical C2 spacer linker. The supports were used for immobilization of ketones 3-pentanone, acetone, N-benzylpiperidone, and aldehydes hexanal and 3-phenylpropanal in the form of their hydrazones. The polymer-supported hydrazones were subjected to alpha-alkylation (LDA/RX) followed by acidic, reductive, or oxidative cleavage/workup procedures to provide alpha-alkylated aldehydes or ketones as well as corresponding primary amines, alcohols, nitriles or acids.     

Optically active imidazole N-oxides derived from l-prolinamine1

Starting with (S)-1-benzylprolinamine and α-hydroxyimino ketones, enantiomerically pure bisheterocyclic imidazole N-oxides bearing the (S)-configured N-benzyl(pyrrolidin-2-yl)methyl residue were prepared. These N-oxides reacted with 2,2,4,4-tetramethylcyclobutane-1,3-dithione to give the corresponding optically active imidazole-2-thione derivatives via a sulfur transfer reaction. Reduction of the N-oxides with Raney-nickel led to deoxygenation, whereas catalytic hydrogenation (Pd/C) in ethanol occurred with simultaneous deoxygenation and debenzylation, leading to optically active 1-(pyrrolidin-2-yl)methyl-1H-imidazoles. Alkylation of the prepared imidazole N-oxides and their respective imidazoles with butyl and hexyl bromide and subsequent anion exchange gave optically active N-alkoxy- and N-alkylimidazolium tetrafluoroborates, respectively, with the properties of ‘room temperature ionic liquids’.     

Impact of the ΔPhe configuration on the Boc-Gly-ΔPhe-NHMe conformation: experiment and theory

Conformational propensities of N-t-butoxycarbonyl-glycine-(E/Z)-dehydrophenylalanine N′-methylamides (Boc-Gly-(E/Z)-ΔPhe-NHMe) in chloroform were investigated by NMR and IR techniques. The low-temperature crystal structure of the E isomer was determined by single crystal X-ray diffraction and the experimental data were elaborated by theoretical calculations using DFT (B3LYP, M06-2X) and MP2 approaches. The β-turn tendencies for both isomers were determined in the gas phase and in the presence of solvent. The obtained results reveal that the configuration of ΔPhe residue significantly affects the conformations of the studied dehydropeptides. The tendency to adopt β-turn conformations is significantly lower for the E isomer (Boc-Gly-(E)-ΔPhe-NHMe), both in gas phase and in chloroform solution.

     

Equilibrium and stability of relativistic stars in extended theories of gravity

We study static, spherically symmetric equilibrium configurations in extended theories of gravity (ETG) following the notation introduced by Capozziello et al. We calculate the differential equations for the stellar structure in such theories in a very generic form i.e., the Tolman–Oppenheimer–Volkoff generalization for any ETG is introduced. Stability analysis is also investigated with special focus on the particular example of scalar–tensor gravity.     

Optimal design of bars under nonuniform tension with respect to mixed creep rupture time

The first attempt of finding of optimal shape for bars in presence of body forces with respect to mixed creep rupture is made. For given volume of the bar, distribution of initial cross-section, ensuring the longest life-time to mixed rupture is sought. The finite strain theory and physical law in form of Norton's law generalized for true stresses and logarithmic strains are applied. Using the method of parametric optimization, the best of linear and quadratic functions describing the initial shape of the bar are found. The shape of initial strength is corrected in a way leading to longer life-time. Results of both approaches are compared.     

Carbon nanotube-ionic liquid nanocomposite modified carbon-ceramic electrode for determination of dopamine in real samples

Room temperature 1-butyl-3-methylimidazolium tetraflouroborate ([BMIM][BF₄]) ionic liquid was employed for dispersion of multi walled carbon nanotubes (MWCNTs) and the formation of nanocomposite on the surface of a carbon-ceramic electrode. The surface of the modified electrode was characterized using scanning electron microscopy and electrochemical impedance spectroscopy. The modified electrode exhibited excellent electrochemical activity to oxidation of dopamine (DA); whereas electro oxidation of ascorbic acid (AA) was not seen and electro oxidation of uric acid (UA) appeared at a more positive potential than DA. The multi walled carbon nanotube-ionic liquid nanocomposite modified carbon-ceramic electrode was used for the selective determination of DA in the presence of high levels of AA and UA using differential pulse voltammetry. The calibration curve for DA was linear in the range of 3.00 to 130 µM with the detection limit (S/N=3) of 0.87 µM. The present electrode was successfully applied to the determination of DA in some commercial pharmaceutical samples and human blood serum.