RESUMEN
The configurational instability of allenyl copper reagents is unveiled. An experimental study highlights the crucial role of Li+ and of the reaction temperature in the control of the configurational stability of allenyl copper reagents. A judicious choice of the reaction conditions allows efficient dynamic kinetic resolution processes and gives a one-pot access to homopropargylic alcohols or amines bearing up to four contiguous defined stereogenic centers.
RESUMEN
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.
RESUMEN
We study the ionization energy, electron affinity, and the π â π(∗) ((1)La) excitation energy of the anthracene molecule, by means of variational quantum Monte Carlo (QMC) methods based on a Jastrow correlated antisymmetrized geminal power (JAGP) wave function, developed on molecular orbitals (MOs). The MO-based JAGP ansatz allows one to rigorously treat electron transitions, such as the HOMO â LUMO one, which underlies the (1)La excited state. We present a QMC optimization scheme able to preserve the rank of the antisymmetrized geminal power matrix, thanks to a constrained minimization with projectors built upon symmetry selected MOs. We show that this approach leads to stable energy minimization and geometry relaxation of both ground and excited states, performed consistently within the correlated QMC framework. Geometry optimization of excited states is needed to make a reliable and direct comparison with experimental adiabatic excitation energies. This is particularly important in π-conjugated and polycyclic aromatic hydrocarbons, where there is a strong interplay between low-lying energy excitations and structural modifications, playing a functional role in many photochemical processes. Anthracene is an ideal benchmark to test these effects. Its geometry relaxation energies upon electron excitation are of up to 0.3 eV in the neutral (1)La excited state, while they are of the order of 0.1 eV in electron addition and removal processes. Significant modifications of the ground state bond length alternation are revealed in the QMC excited state geometry optimizations. Our QMC study yields benchmark results for both geometries and energies, with values below chemical accuracy if compared to experiments, once zero point energy effects are taken into account.
