Understanding the Nonlinear Hammett Relationship in Osmylation of Olefins with OsO-Amine Ligands: Importance of Singlet-Diradical Character.

Although the concerted [3 + 2] mechanism of osmium-catalyzed asymmetric dihydroxylation has been generally accepted, the unusual nonlinear Hammett relationship induced by amine-type ligands remains unexplained. To understand this, we carried out a density functional theory (DFT) study for the osmylation of substituted styrenes by the following: OsO, OsO-pyridine, OsO-4-cyanopyridine, OsO-4-pyrrolidinopyridine, and OsO-quinuclidine. Calculations using the M06 functional successfully reproduce the experimentally observed nonlinear relationships. The transition states exhibit considerable singlet-diradical character, which causes the nonlinear Hammett relationship. Regardless of the presence or absence of an amine-type ligand, an electron donation from styrene to OsO is observed, indicating no mechanistic change. Calculations indicate that the electronic interaction between the amine-type ligand and styrene also influences the reaction rate.