Hydride Abstraction from 1,2-Diols by the Pentaaqua(oxo)chromium(IV) Ion

Abstract

Kinetic studies of the oxidation of 1,2-diols by the pentaaqua(oxo)chromium(IV) ion, (H{sub 2}O){sub 5}Cr{sup IV}=O{sup 2+}, were carried out in dilute aqueous solutions of perchloric acid. These reactions yield the superoxochromium(III) ion CrOO{sup 2+}, providing evidence that the two-electron reduction of Cr{sup IV}O{sup 2+} to Cr{sup 2+} occurred in a single step. The rate constants for the primary and secondary diols are comparable to each other and approximately 10 times larger than that for pinacol (k = 3.7 L mol{sup {minus}1}s{sup {minus}1}). The kinetic isotope effect of the O-H hydrogens is negligible, k{sub H}/k{sub D} = 0.95-1.3, whereas that for the C-H hydrogens is substantial, k{sub H}/k{sub D} = 3.7-4.8. A 2-equiv mechanism is proposed, in which CrO{sup 2+} abstracts a hydride ion from an {alpha}-position in the primary and secondary diols and from a {beta}-position in pinacol.