ABSTRACT
Ligand coupling takes place between an equatorial and an axial ligand. S. Oae have found many examples of ligand coupling reactions within σ-sulfurane intermediates which are formed by reacting both heteroaryl and aryl sulfoxides with Grignard reagents. Hypervalent sulfur atoms, sulfuranes, have expanded valence shells such that the molecule as a whole is usually unstable. Even the ligand coupling of 2-pyridyl with sp2-centered ligands has been shown to proceed in a concerted manner without isomerization of the coupling ligands. Slight changes in the electronic environment of the S-substituted ligand appear to change the mode of coupling, primarily due to changes in the ease of pseudorotation in the σ-sulfurane intermediate. Ligand coupling is the last, and the least well-known reaction of hypervalent species. In hypervalent species, the axial p orbitals are occupied by electronegative ligands, while the equatorial sp2 hybridized orbitals are taken up by π-ligands or electron-donating ligands.
