C-H BOND ACTIVATION BY METAL OXO SPECIES - CHROMYL CHLORIDE OXIDATIONS OF CYCLOOCTANE, ISOBUTANE, AND TOLUENE

Chromyl chloride, CrO2Cl2, oxidizes cyclooctane, isobutane, and toluene under mild conditions (25-60 degrees C). The reactions give chlorinated products (chlorocyclooctane, tert-butyl chloride, and benzyl chloride) and a dark chromium-containing precipitate. Hydrolysis of the precipitate yields oxygenated products, such as ketones, aldehydes, chloro ketones, epoxides, and alcohols. Kinetic data show that all of the reactions are first order in CrO2Cl2 and first order in substrate, with no sign of an induction period. Primary isotope effects have been observed for t-d(1)-isobutane and d(8)-toluene. The kinetic and mechanistic data indicate that the reactions proceed by initial hydrogen atom transfer from the substrate to CrO2Cl2. The rates of hydrogen atom abstraction by CrO2Cl2 vary in the order cyclohexane < cyclooctane congruent to isobutane < toluene and are directly related to the strength of the C-H bond being cleaved. A correlation is observed between Delta H double dagger and Delta S double dagger, indicating a common mechanism for the four substrates. The pathways leading from the initially formed alkyl radicals to the observed products are described. The ability of CrO2Cl2 to abstract a hydrogen atom from alkanes is remarkable, as it is a closed-shell, diamagnetic species, not a radical. It is proposed that the hydrogen atom abstracting ability derives from the strong O-H bond formed on hydrogen atom transfer, in [Cl-2(O)Cr(OH)]. The rates of the CrO2Cl2 reactions correlate with rates of hydrogen atom abstraction-by oxygen radicals, assuming a CrO-H bond strength of 83 kcal/mol (similar to that in HMnO4-). The implications of this perspective for transition metal mediated hydrogen atom transfer reactions are discussed.