Hypervalent Iodine Oxidants:  Structure and Kinetics of the Reactive Intermediates in the Oxidation of Alcohols and 1,2-Diols by o-Iodoxybenzoic Acid (IBX) and Dess−Martin Periodinane. A Comparative ¹H-NMR Study

Alcohols and 1,2-diols oxidation by o-iodoxybenzoic acid (IBX) has been examined by ¹H-NMR spectroscopy. Reversible formation of reactive intermediates, iodic esters 5, has been observed, and their structures in DMSO-d₆ solution have been defined as 10-I-4 axial alkoxyiodinane oxides by comparison of the chemical shift difference data with those obtained for Dess−Martin periodinane (DMP)−alcoholate and −diolate adducts. The dichotomous behavior exhibited by IBX and DMP with 1,2-diols can be explained in terms of the different architecture of the reactive intermediates involved in the oxidation. With aliphatic alcohols, kinetic evidences support a two-step reaction mechanism involving a fast pre-equilibrium step leading to 5, followed by a rate-determining disproportionation step. With electronically activated benzyl alcohol, the attainment of pre-equilibrium is largely dependent on initial water concentration as a consequence of a particularly high k₂ value. The influence of the alcohol structure on measured equilibrium (K_eq) and rate constants (k₂) and the effect of water on the overall reaction rate are discussed.