Chemical and Electrochemical Asymmetric Dihydroxylation of Olefins in I2−K2CO3−K2OsO2(OH)4 and I2−K3PO4/K2HPO4−K2OsO2(OH)4 Systems with Sharpless' Ligand

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Abstract
Iodine-assisted chemical and electrochemical asymmetric dihydroxylation of various olefins in I2−K2CO3−K2OsO2(OH)4 and I2−K3PO4/K2HPO4−K2OsO2(OH)4 systems with Sharpless' ligand provided the optically active glycols in excellent isolated yields and high enantiomeric excesses. Iodine (I2) was used stoichiometrically for the chemical dihydroxylation, and good results were obtained with nonconjugated olefins in contrast to the case of potassium ferricyanide as a co-oxidant. The potentiality of I2 as a co-oxidant under stoichiometric conditions has been proven to be effective as an oxidizing mediator in electrolysis systems. Iodine-assisted asymmetric electro-dihydroxylation of olefins in either a t-BuOH/H2O(1/1)−K2CO3/(DHQD)2PHAL−(Pt) or t-BuOH/H2O(1/1)−K3PO4/K2HPO4/(DHQD)2PHAL−(Pt) system in the presence of potassium osmate in an undivided cell was investigated in detail. Irrespective of the substitution pattern, all the olefins afforded the diols in high yields and excellent enantiomeric excesses. A plausible mechanism is discussed on the basis of cyclic voltammograms as well as experimental observations.