Hélène Gérard

Sulfonyl vs. carbonyl group: which is the more electron-withdrawing?

Unexpectedly high reactivity of nitrogenated aromatics protected as amides or carbamates, when compared to sulfonamides, can be explained by a decrease of the aromaticity due to a greater ability of the carbon-centered groups to achieve delocalisation of the nitrogen lone pair, resulting in stronger global withdrawing effects.

A computational study of the effects of ancillary ligands on copper(I)–ethylene interaction

A set of [Cu(I)Ln(C2H4)]q (q = −1, 0, or 1) complex modelling systems of experimental interest were studied by DFT calculations to analyze the Cu(I)–ethylene bonding using NBO and CDA analyses. All complexes are better viewed as donor–acceptor complexes between a d10 Cu(I) center and ethylene. Back-donation depends significantly on the nature and number of ancillary ligands, and hence on the coordination sphere at copper. Back-donation is shown to vary more with the nature of the ligands than donation and to increase significantly with the number of ancillary ligands. However, even with strongly donating ligands such as alkyl (modelled by CH3), there is no tendency of forming a metallacyclopropane. This can lead to revisiting the mechanisms of alkylation of olefin catalyzed by copper complexes.

Asymmetric Sequential Cu-Catalyzed 1,6/1,4-Conjugate Additions of Hard Nucleophiles to Cyclic Dienones: Determination of Absolute Configurations and Origins of Enantioselectivity

The first stereocontrolled Cu-catalyzed sequential 1,6/1,4-asymmetric conjugate addition (ACA) of C-metalated hard nucleophiles to cyclic dienones is reported. The use of DiPPAM (diphenylphosphinoazomethinylate) followed by a phosphoramidite as the stereoinducing ligands facilitated both high ee values for the 1,6-ACA and high de values for the 1,4-ACA reaction components, which thus gave enantioenriched 1,3-dialkylated moieties. The absolute configurations were determined by using vibrational circular dichroism (VCD) and optical rotatory dispersion (ORD) spectroscopy, in combination with DFT calculations and X-ray analysis. Interestingly, DFT calculations for the mechanism of enantioselective 1,6-addition by using an unprecedented Cu-Zn bimetallic catalytic system confirmed this attribution. Lastly, exploring intramolecular cyclization avenues for enantioenriched 1,3-dialkylated products provided access to the challenging drimane skeleton.

Interactions of copper(II) and zinc(II) with chlorophyll: insights from density functional theory studies

The reaction of chlorophyll (a) with Cu2+ or Zn2+ in the presence of acetonitrile has been studied on a slightly simplified model using DFT computations of three potential processes: the substitution of the central Mg2+ cation, the chelation of the peripheral keto-ester, associated or not with deprotonation. Mg-substitution is found to be highly favored in the case of copper, whereas chelation without deprotonation is proposed to occur for zinc. It is proposed that chelation associated with deprotonation could become competitive if water is present in the reaction medium. The resulting compounds are shown to have interesting electronic properties as metal → ligand spin transfer is observed in the case of Cu2+.

Mechanism of Enolate Transfer between Si and Cu

Exchange of X (F, Cl, OMe) and a substituted enolate chain between SiMe3 and various CuI complexes was examined. Reaction mechanisms pass through a cyclic transition state in which the reaction coordinate is associated with rotation of the SiMe3 moiety. The dependence of the thermodynamic and kinetic features on the nature of the active and ancillary ligands was examined. Formation of copper enolate is shown to be favored when stabilized enolates are used. Replacement of F by Cl reverses the preference of the reaction. This is associated with the small difference between the Cu-Cl and Si-Cl bond energies, in contrast to other Si-X bonds, which are systematically stronger than their Cu-X analogues.

DFT and TD-DFT Studies of Mg-Substitution in Chlorophyll by Cr(II), Fe(II) and Ni(II)

Electronic contributions in electronic transitions and energetic data associated to the Mg-substitution in chlorophyll by three transition metals: chrom (Cr2+), iron (Fe2+) and nickel (Ni2+) have been studied theoretically using density functional theory and time dependent density functional theory (TD-DFT) methods. The binding energies are stronger than for Mg2+ in the case of all three cations especially in the case of Ni2+. The Mg-substitution process is found to be exergonic for all title elements in gas phase and in acetonitrile using both implicit and explicit models of solvation. The natural population analysis results, which estimated by natural bond orbital analysis, showed significant charge transfer from pheophytin ligand to the central cation. The UV–visible proprieties of the different substitution compounds have been studied using the TD-DFT method evidencing that substitution of Mg by Cr, Fe or Ni is associated to a blue shift of the Q-band for the three cations.