Vanya B. Kurteva

Simple transformation of crystalline chiral natural anions to liquid medium and their use to induce chirality

New chiral ionic liquids can be prepared simply by combining the tetra-n-hexyl-dimethylguanidinium cation with readily available chiral anions and used as an asymmetric inducing agent as demonstrated for catalytic Rh(II) carbenoid C-H insertion and Sharpless dihydroxylation.

Synthesis of Chiral Cyclopentenones

The cyclopentenone unit is a very powerful synthon for the synthesis of a variety of bioactive target molecules. This is due to the broad diversity of chemical modifications available for the enone structural motif. In particular, chiral cyclopentenones are important precursors in the asymmetric synthesis of target chiral molecules. This Review provides an overview of reported methods for enantioselective and asymmetric syntheses of cyclopentenones, including chemical and enzymatic resolution, asymmetric synthesis via Pauson-Khand reaction, Nazarov cyclization and organocatalyzed reactions, asymmetric functionalization of the existing cyclopentenone unit, and functionalization of chiral building blocks.

Exploiting Tautomerism for Switching and Signaling

Herein, we demonstrate a conceptual idea for a tautomeric switch based on implementation of a flexible piperidine unit in 4-(phenyldiazenyl)naphthalen-1-ol. The results show that a directed shift in the position of the tautomeric equilibrium can be achieved through protonation/deprotonation in a number of solvents. The developed molecular switch, in spite of the simple host–guest system, has shown acceptable complexation ability towards small alkaliand alkalineearth-metal ions and can be a promising basis for further development of effective molecular sensors through implementation of azacrown ethers. Organic molecular materials are increasingly recognized as suitable molecular-level elements (such as switching, signaling, and memory elements) for molecular devices, because the wide range of molecular characteristics can be combined with the versatility of synthetic chemistry to alter and optimize molecular structure in the direction of desired properties. Virtually every molecule changes its behavior when acted upon by external fields or other stimuli. True molecular switches undergo reversible structural changes, caused by a number of influences, which give a variety of possibilities for control. Several classes of photoresponsive molecular switches are already known; these operate through processes such as bond formation and bond breaking, cis– trans isomerization, and photoinduced electron transfer upon complexation. A conceptual scheme of a molecular switch based on molecular recognition is shown in Scheme 1. The host–guest system represents, for instance, a crown ether that can bind ions or a cyclodextrin that can bind other small molecules. It is bound to a signal converter. The complexation behavior is monitored by the state of the signal converter, and in turn its optical or electronic properties are determined by the complexation state of the host–guest system. The main requirement in the design of new molecular switches is to provide fast and clean interconversion between structurally different molecular states (on and off). Tautomerism could be a possibility, because change in the tautomeric state can be accomplished by a fast proton transfer reaction between two or more structures, each of them with clear and different molecular properties. Therefore, our aim herein is to show how tautomerism can be exploited for signal conversion. The conceptual idea of such a device is presented in Scheme 1. In this structure, a change in tautomeric state, labeled A and B, is linked to changes in the complexation abilities of the host–guest system by modulating the propensity of the system to hydrogen bond to the antenna. At the same time, engagement of this antenna causes a change in the tautomeric state. The sensitivity of the electronic ground and excited states of the tautomeric forms to environment stimuli (light, pH value, temperature, solvent) and to the presence of a variety of substituents or to hydrogen bonding can be exploited in the design of flexible tools for control. Obviously, such a device should be based on a tautomeric structure with easy proton exchange between the tautomers, which means that they must coexist in solution. At the same time, a main feature of systems of tautomers coexisting in solution is that the overall optical response is a mixture of the optical responses of the individual tautomers. Consequently, in the design of tautomeric switches, conditions for obtaining pure end tautomer in the corresponding off and on states must be provided. Herein we report the properties of two tautomeric switches, namely 3 and 4 (Scheme 2), based on 4-(phenyldiazenyl)phenol (1) and 4-(phenyldiazenyl)naphthalen-1-ol (2). The parent compound 2 is the first dye that was shown to tautomerize by Zincke and Bindewald in 1884. It has been the object of many spectral and theoretical studies because its tautomeric forms coexist in solution and the equilibrium Scheme 1. Molecular switch based on molecular recognition (left) and conceptual idea for a tautomerism-based molecular switch (right).

Comparing extraction, synergism and separation of lanthanoids using acidic and neutral compounds in chloroform and one ionic liquid: is the latter always “better”?

The complexation properties of a pyrazolone derivative, 3-methyl-1-phenyl-4-(4-trifluoromethylbenzoyl)-pyrazol-5-one (HL), in chloroform were examined and it was found that it possesses improved complexation ability in comparison with 4-benzoyl, 4-(4-methylbenzoyl) and 4-(4-fluorobenzoyl) derivatives. Mixed ligand chelate extractions of trivalent lanthanoids (La, Nd, Eu, Ho and Lu) with HL and 5,11,17,23-tetra-tert-butyl-25,26,27-tris(dimethylphosphinoylpropoxy)-28-hydroxy-calix[4]arene (S1) or 5,11,17,23-tetra-tert-butyl-25,27-bis(dimethylphosphinoylpropoxy)-26,28-dihydroxy-calix[4]arene (S2) as synergistic agents were also carried out in chloride medium at constant ionic strength μ = 0.1 and CHCl3 as organic phase. The interactions between the extractants in deuterochloroform were studied by 1H, 13C, and 31P NMR. The composition of the extracted species was established as LnL3·HL (L− represents HL anion) with HL as a single extractant, and as LnL3·S in the presence of the phosphorus-containing calix[4]arene. On the basis of the experimental data, the values of the equilibrium constants were calculated. The influence of the number of PO groups of tert-butyl-calix[4]arene on the extraction process was discussed. The synergistic enhancement and separation factors between metals were evaluated. Finally, the extraction of La(III) and Eu(III) ions was performed by using an ionic liquid, 1-butyl-3-methylimidazolium-bis(trifluoromethanesulfonyl)imide, as a solvent and varying the type of ligands. By comparison to CHCl3, the ionic liquid offers increased distribution ratios, at the expense of a levelling off of the separation factors. Reasons for such a phenomenon are discussed.

Synergism as a phenomenon in solvent extraction of 4f-elements with calixarenes

The main objective of the present review is to provide an overview on the synergistic solvent extraction of lanthanoids, in particularly on the use of β-diketone/calixarene i.e. acidic/neutral mixture, as well as the analysis of data hitherto reported in this scientific topic. Based on the previously presented results, the extraction efficiency has been careful examined and discussed mainly in terms of the corresponding equilibrium constants among other outlined extraction parameters. Furthermore, the evaluation of the chelating ligand structures and pKa values are emphasized. Major conclusions on the role of calixarenes ligating groups towards the mechanism, the improved extraction enhancement and selectivity are additionally provided. Consequently, it is very important that the obtained remarkable architecture of the formed complexes with very high coordination numbers (10/12) including a few cyclic molecules is in accordance with the well-established principles of coordination 4f-ion chemistry. Finally, a critical analysis of the lanthanoid intra-group selectivity (light/middle, heavy/middle and light/heavy 4f-ion pairs) versus the extraction performance of the acidic/neutral combinations and, on the other hand, the nature of the water-immiscible molecular or ionic diluent used have also been carried out and are here presented and critically addressed.

Microwave accelerated facile synthesis of fused polynuclear hydrocarbons in dry media by intramolecular Friedel–Crafts alkylation

Fused polynuclear tetrahydro arenes are synthesised in a fast, simple, high yielding and regiospecific procedure by an intramolecular Friedel-Crafts alkylation in dry media under microwave irradiation of the corresponding 1-bromo-4-arylbutanes immobilised on silica. The observed reactivity is rationalised by molecular modelling studies which suggest the occurrence of a concerted mechanism.

Are fancy acidic or neutral ligands really needed for synergism in ionic liquids? A comparative study of lanthanoid extraction in CHCl3 and an ionic liquid

The study of trivalent lanthanoid (La, Nd, Eu, Ho and Lu) extraction by two individual ligands, one acidic such as a pyrazolone derivative, 3-methyl-1-phenyl-4-(4-phenylbenzoyl)-pyrazol-5-one (HL), and one neutral such as 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis-(dimethylphosphinoylmethoxy)calix[4]arene (SIV), was performed in CHCl3 and in one ionic liquid, IL 1-methyl-1,3-butylimidazolium-bis(trifluoromethanesulfonyl)imide ([C1C4im][Tf2N]), in order to establish and examine the impact of the diluent on individual and synergistic extraction processes. Chloride aqueous medium and constant ionic strength μ = 0.1 M were set in both cases. The parameters of the extraction processes were determined. Comparison between the two diluents was made on the basis of (i) composition of the extracted species: LnL3·HL in CHCl3vs. LnL3 in IL with HL as a single extractant (L− represents the HL anion), and LnL3·SIV in the presence of the phosphorus-containing calix[4]arene in both diluents; and (ii) values of the equilibrium constants and the synergistic enhancements; the separation factors between metals were evaluated as well. The IL medium appears far superior to the molecular solvent in terms of individual and overall synergistic extraction efficiencies. However, the huge differences in extraction efficiencies observed in CHCl3 for chemically different ligands are erased in the IL medium, thus modifying the lever arms on which chemists should rely in order to obtain even better extracting systems in ionic liquid media.

Solvent-free synthesis of melamines under microwave irradiation

A fast, highly efficient and environmentally friendly solvent-free procedure under microwave irradiation, using silica gel supported reagents, for the synthesis of melamines, including ones with a broad range of biological activities, is developed. The main advantages of the synthetic approach presented here are the cheap and easily available starting material, cyanuric chloride, and the considerable rate enhancement in comparison with a thermal reaction. The scope of the reaction, towards the amine used, is also studied and it is shown that the method is valid as a highly effective one when moderately bulk amines are used. The by-product of the reaction, hydrogen chloride, is quenched as ammonium salts, preventing its release into the environment.

The influence of nanoparticle architecture on latex film formation and healing properties

We present a study of chain interdiffusion in films formed by specially architectured PBMA nanoparticles by Förster Resonance Energy Transfer -FRET. Polymer nanoparticles contained linear chains with narrower molecular weight distributions than other previous reports, allowing a more detailed study. Apparent fractions of mixing and diffusion coefficients, determined from the quantum efficiency of energy transfer, were used to characterize the interdiffusion mechanism in the different films. The resistance of the films to dissolution by a good solvent was finally correlated with the interdiffusion results, in order to get information about film healing. We concluded that whenever interdiffusion occurs between nanoparticles containing linear chains and fully cross-linked nanoparticles, healing becomes more effective in spite of showing slower interdiffusion. We also observed that particles with longer chains are more effective for film healing. Finally, we concluded that interdiffusion occurs both ways across interfaces in blends formed by particles swollen with linear chains of different molecular weights.

Gas-phase tautomerism in hydroxy azo dyes – from 4-phenylazo-1-phenol to 4-phenylazo-anthracen-1-ol†

The tautomeric constants of a series of azo dyes were estimated in the gas phase by using electron ionization mass spectrometry. It was shown that the relative amount of the keto tautomer increases from 4-phenylazo-1-phenol to 4-phenylazo-anthracen-1-ol, thus confirming the quantum-chemical predictions. The existence of the enol tautomer of 4-phenylazo-anthracen-1-ol is shown for the first time by mass spectrometry in the gas phase. This finding is supported by flash photolysis measurements in solution.