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 H-1-NMR study

Alcohols and 1,2-diols oxidation by o-iodoxybenzoic acid (IBX) has been examined by H-1-NMR spectroscopy. Reversible formation of reactive intermediates, iodic esters 5, has been observed, and their structures in DMSO-d(6) 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 preequilibrium is largely dependent on initial water concentration as a consequence of a particularly high k(2) value. The influence of the alcohol structure on measured equilibrium (K-eq) and rate constants (k(2)) and the effect of water on the overall reaction rate are discussed.