Thermodynamic and kinetic control in selective ligand transfer in conjugate addition of mixed organocuprate Me(X)CuLi.
ABSTRACT Since the proposal of the dummy ligand concept by Corey, it has been widely accepted that the ligand transfer selectivity of a mixed organocuprate (Me(X)CuLi) depends on the Cu-X bond strength. The present B3LYP density functional studies on the Me(2)(X)Cu(III).OMe(2), pi-allyl Cu(III), and Me(X)Cu(III)LiCl.LiCl reacting with acrolein showed that the ligand transfer selectivity of the conjugate addition depends on two factors, thermodynamic stability (X = tert-butyl, ethynyl, cyano, and thiomethyl groups) and kinetic reactivity ((trimethylsilyl)methyl and vinyl groups) of the Cu(III) intermediate formed by complexation of the cuprate and the alpha,beta-unsaturated carbonyl compound. For the typical dummy ligands (X = alkynyl, cyano, and heteroatom ligands), the trans effect and the strong Li-X affinity are the reasons why these ligands stay on the copper atom. In contrast, for the (trimethylsilyl)methyl and vinyl groups, the selectivity depends on the kinetics of reductive elimination of the Cu(III) intermediate. The (trimethylsilyl)methyl transfer is retarded by repulsive four-electron interaction between the lone pair Cu 3d(xy)() orbital and the C-Si sigma-orbital.