HYDRIDE ABSTRACTION FROM 1,2-DIOLS BY THE PENTAAQUA(OXO)CHROMIUM(IV) ION

Kinetic studies of the oxidation of 1,2-diols by the pentaaqua(oxo)chromium(IV) ion, (H2O)5Cr(IV)=O2+, were carried out in dilute aqueous solutions of perchloric acid. These reactions yield the superoxochromium(III) ion CrOO2+, providing evidence that the two-electron reduction of Cr(IV)O2+ to Cr2+ 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-1 s-1). The kinetic isotope effect of the O-H hydrogens is negligible, k(H)/k(D) = 0.95-1.3, whereas that for the C-H hydrogens is substantial, k(H)/k(D) = 3.7-4.8. A 2-equiv mechanism is proposed, in which CrO2+ abstracts a hydride ion from an a-position in the primary and secondary diols and from alpha,beta-position in pinacol.