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. One way or the other: Theoretical and experimental studies on enolate transfer between SiMe3 and CuI centers evidenced sensitivity of the thermodynamics of the reaction to the exchanged ligands and of its kinetics to the nature of the enol ether chain and the copper center. Thus, formation of copper enolate is favored when stabilized enolates are used, and replacement of F by Cl reverses the preferred direction of the reaction (see scheme).