Acidity of osmium tetroxide (OsO4) towards coordination with pyridine and its derivatives

This article reports on the Lewis acidity of osmium tetroxide (OsO4), the most reliable reagent available for the cis-hydroxylation of alkenes. Three simple and straightforward scales, donor ability of N-donors, the Hammett reaction constant and the Electrostatic-Covalent model (ECW) were used for this purpose. Equilibrium constants (Keq) and enthalpies for the complexation of OsO4 with pyridine and its derivatives have been evaluated by spectrophotometeric measurements. The excellent linear relationships were observed between the equilibrium constants and either the pKa values of the N-donors or the Hammett σ constants of the substituents on the ligands. The slope of the plot of log Keq versus ligand’s pKa and the Hammett ρ value derived suggest the use of these parameters as an index of the Lewis acidity strength for OsO4. The enthalpies of complex formation of N-donors with OsO4 also fit the ECW model to predict the values of EA and CA parameters for OsO4 as 2.01 and 0.25 (kcal/mol)1/2, respectively. The parameters can be used to make an acidity scale for OsO4. These parameters also permit the chemists and biochemists to predict and correlate quantitatively the enthalpies of OsO4 · Lewis base interactions.

Three scales, donor ability of substituted pyridines, the Hammett reaction constant and the ECW model were used to make an acidity scale for OsO4. The results show that the acid strength order towards pyridine is as follow: BMe3 > Zn(TPP) > MTO > I2 > OsO4.Figure optionsDownload as PowerPoint slide