Aurélien Moncomble

Cobalt-catalyzed vinylation of aromatic halides using β-halostyrene: experimental and DFT studies.

A new protocol for the direct cobalt-catalyzed vinylation of aryl halides using β-halostyrene has been developed in order to form functionalized stilbenes. A variety of aromatic halides featuring different reactive group were employed. This method proceeded smoothly with a total retention of the double bond configuration in the presence of triphenylphosphine as ligand. Preliminary DFT calculations rationalize these results and proposed a reaction pathway in agreement with the experimental conditions. This procedure offers a new route to the stereoselective synthesis of stilbenes.

Enantioselective organocatalytic partial transfer hydrogenation of lactone-fused quinolines.

The first enantioselective synthesis of 4-aza-podophyllotoxin derivatives by partial transfer hydrogenation of lactone-fused quinolines was achieved using a chiral Brønsted acid catalyst. This reaction was extended to a large scope of substrates with good yields and enantioselectivities.

Direct Carboxylation of Aryl Tosylates by CO2 Catalyzed by In situ-Generated Ni(0).

A novel Ni(0) -catalyzed carboxylation of aryl tosylates with carbon dioxide has been achieved under moderate temperatures and atmospheric pressure. In this procedure, the active Ni(0) species is generated in situ by simply mixing the Ni(0) precatalyst [NiBr2 (bipy)] with an excess of manganese metal. This approach requires neither a glove-box nor the tedious preparation of sophisticated intermediate organometallic derivatives. This mild, convenient, and user-friendly process is successfully applied to the valorization of carbon dioxide and the synthesis of versatile reactants with broad tolerance of substituents.

Experimental and computational exploration of the UV-visible properties of hexaniobate and hexatantalate ions

The UV-visible properties of hexaniobate (HxNb6O19x−8) and hexatantalate (HxTa6O19x−8) ions were investigated experimentally and by DFT calculations. Due to the huge discrepancies among the reported values found in prior studies, the extinction coefficients of hexaniobate ions were determined from multiple samples in various media. A simple and low-cost method was then developed for the determination of the niobium content of both synthetic and industrial samples. Taking advantage of the UV spectra of the hexaniobate ions, the first protonation constant of Nb6O198− could be determined experimentally (pKa = 13.4 ± 0.1 at 25 °C, ΔrH = −95 kJ mol−1 in 3 M KOH/KCl) and is in accordance with the reported values previously extrapolated from potentiometric measurements (pKa = 13.3 ± 0.6). UV batch titrations performed at different ionic strengths suggest that the equilibrium between the monoprotonated and the deprotonated forms of the Lindqvist ion is accompanied by an exchange of potassium ions and that the cluster is easier to deprotonate in Na+ media. This study highlights the importance of the ion-pairing on the chemistry of these polyoxometalates. The tremendous difference between the UV spectrum of Nb6O198− and Ta6O198− was investigated by DFT computations. The UV spectra were reproduced and show a good agreement with the experimental data. Moreover this study revealed some insights into the evolution of the spectra together with the pH of the medium by studying the nature of the transitions involved in this spectral domain.

The Complexation of AlIII, PbII, and CuII Metal Ions by Esculetin: A Spectroscopic and Theoretical Approach

UV-visible absorption spectroscopy combined with quantum chemical calculations and, notably, Time-Dependent Density Functional Theory were used to probe the structure of metal complexes with esculetin in dilute aqueous solution, at pH = 5. For the 1:1 complex formation, the studied metal ions can be classified according to their complexing power: aluminum(III) > copper(II) > lead(II). For the three complexes, a chelate is formed with the fully deprotonated catechol moiety and an absorption band is observed at the same wavelength. In all cases, a pronounced ionic character is calculated for metal-ligand bonds. However, the complexes differ in their coordination sphere. Copper and lead are bound to two water molecules leading to a square plane geometry and a hemidirected complex, respectively, whereas aluminum atom has an octahedral environment involving three water molecules and a hydroxide ion. For Al(III) only, a 2:1 complex is observed, and the involvement of an aluminum dimer was evidenced.

Enhancement of the Water Solubility of Flavone Glycosides by Disruption of Molecular Planarity of the Aglycone Moiety

Enhancement of the water solubility by disruption of molecular planarity has recently been reviewed as a feasible approach in small-molecule drug discovery programs. We applied this strategy to some natural flavone glycosides, especially diosmin, a highly insoluble citroflavonoid prescribed as an oral phlebotropic drug. Disruption of planarity at the aglycone moiety by 3-bromination or chlorination afforded 3-bromo- and 3-chlorodiosmin, displaying a dramatic solubility increase compared with the parent compound.

Elucidation of complexation multi-equilibrium with MgII and a multisite ligand. A combined electronic spectroscopies and DFT investigation

The study of the formation of complexes between quercetin, a multisite ligand, and MgII cation in solution is reported in this article. A methodology combining spectroscopic measurements and TD-DFT computations was used to study the associated monosite flavonoids and showed that the hydroxy-keto chelating power towards MgII is higher than the catechol one. In each case, the complexation occurs with one hydroxyl deprotonation. Then, by combination of absorption and emission spectra measurements, chemometric treatments and quantum chemistry calculations, it was shown that MgII cation is involved in an equilibrium between the two hydroxy-keto sites of quercetin. The coexistence in solution of two complexes C3 and C5 has been highlighted. Very different Stokes shifts were observed for these two species reflecting the structural rearrangements that occur upon complexation and excitation.

A quantum chemistry evaluation of the stereochemical activity of the lone pair in PbII complexes with sequestering ligands

The stereochemical activity of the lone pair on PbII complexes is assessed using several theoretical methods, including structural analyses, computations of Fukui functions, natural bond orbitals, electron localization function, investigation of the electron density and of its laplacian. The attention is focused on four octadentate N-carbamoylmethyl-substituted tetraazamacrocycles of various ring sizes ranging from 8 to 14 atoms associated with the PbII cation. The theoretical study illustrates the geometrical constraints imposed by the ring structure which limits the spatial development of the lone pair but without fully preventing it. For a given coordination number, the lone pair activity is strongly correlated to the geometry of the ligand and in particular to the size of the cage that the ligand forms around the PbII cation. Some limitations of the theoretical tools used are also evidenced, among them the necessity to sample around a critical point instead of just analyzing its nature. In the case of the laplacian of the electron density, a visualization method is introduced to moderate the results based only on the nature of a critical point. These limitations should also be related to the difficulty to extend the lone pair concept for the heaviest atoms of the classification.

Conformational and structural studies of N-methylacetohydroxamic acid and of its mono- and bis-chelated uranium(VI) complexes.

The thermodynamics and kinetics of the cis/trans isomerism of N-methylacetohydroxamic acid (NMAH) and its conjugated base (NMA(-)) have been reinvestigated in aqueous media by (1)H NMR spectroscopy. Hindered rotation around the central C-N bond due to electronic delocalization becomes slow enough on the NMR time scale to observe both rotamers in equilibrium in D2O at room temperature. By properly assigning the methyl group resonances, evidence for the prevalence of the E over the Z form is unambiguously provided [K300=[E]/[Z]=2.86(2) and 9.63(5) for NMAH and NMA(-), respectively], closing thereby a long-lasting dispute about the most stable conformer. To that end, calculations of the chemical shifts by density functional theory (DFT), which accurately reproduced the experimental data, turned out to be a much more reliable method than the direct computation of the relative energy for each conformer. The Z ⇌ E interconversion dynamics was probed at 300 K in D2O by 2D exchange-correlated spectroscopy (EXSY), affording the associated rate constants [kZE=9.0(2) s(-1) and kEZ=3.14(5) s(-1) for NMAH, kZE=0.96(3) s(-1) and kEZ=0.10(2) s(-1) for NMA(-)] and activation barriers at 300 K [ΔG(≠)ZE=68.0 kJ mol(-1) and ΔG(≠)EZ=70.6 kJ mol(-1) for NMAH, ΔG(≠)ZE=73.6 kJ mol(-1) and ΔG(≠)EZ=79.2 kJ mol(-1) for NMA(-)]. For the first time, mono- and bis-chelated uranium(VI) complexes of NMA(-) have been isolated. Crystals of [UO2(NMA)(NO3)(H2O)2] and [UO2(NMA)2(H2O)] have been characterized by X-ray diffractometry, infrared and Raman spectroscopies.

In-Silico Calculations as a Helpful Tool for Designing New Extractants in Liquid-Liquid Extraction

Due to new challenges, new extraction solvents based on innovative extractants are needed in hydrometallurgy for specific tasks. Thus, the aim of the present article is to discuss the potential and limits of Quantitative Structure–Properties Relationship (QSPR) and molecular modeling for identifying new extractants. QSPR methods may have useful applications in such a complex problem as the design of ligands for metal separation. Nevertheless, the degree of reliability of the predictions is still limited and, in the present state of the art, these techniques are likely more useful for optimization within a given family of extractants than to build in-silico new reagents. The molecular modeling techniques provide binding energies between target metals and given ligands, as well as optimized chemical structures of the formed complexes. Thus, in principle, the information, which can be deduced from the molecular modeling computations are richer than that provided by QSPR methods. Nevertheless, an effort should be made to establish more tangible links between the calculated binding energies and the physical parameters used by the hydrometallurgists, such as the complexation constants in aqueous phase (βMAn) or better the extraction constants (Kex).