C-H bond activation by metal oxo species: Oxidation of cyclohexane by chromyl chloride

Article abstract of DOI:10.1021/ja00084a029

Chromyl chloride (CrO₂Cl₂) reacts with cyclohexane solvent at 75 °C to give a dark precipitate along with chlorocyclohexane and a small amount of cyclohexene (in 10.0 and ca. 0.3% yields based on chromium). Hydrolysis of the precipitate, or treatment with a coordinating organic solvent such as acetonitrile, yields cyclohexanone (8.0%) and chlorocyclohexanone (2.5%). Spectroscopic studies of the precipitate indicate that the ketone products are present intact as σ-only ligands. Iodometric titrations of the complex show the average chromium oxidation state to be 3.41. The observed organic products account for only 26% of the chromium oxidizing equivalents used in the reaction; the remainder are most likely consumed in the formation of ring-opened products such as adipic acid. The three major organic products grow in concurrently during the course of the reaction and are not substantially oxidized further under the reaction conditions. Cyclohexene, however, is readily oxidized by CrO₂Cl₂ to give mostly ring-opened products with some 2-chlorocyclohexanone and cyclohexanone. The rate of reaction of CrO₂Cl₂ with cyclohexane, monitored via the optical absorbance of the CrO₂Cl₂ vapor above the solution, is first order in chromium; assuming first-order behavior in cyclohexane as well gives k = 1.07 × 10^-5 M^-1 s^-1 at 75 °C, ΔH* = 26.6 (8) kcal/mol, and ΔS* = -5 (2) eu. Rates were reproducible with no sign of an induction period. Reaction in the presence of the radical trap CBrCl₃ gave some bromocyclohexane. The data indicate that the reaction proceeds by initial hydrogen atom transfer from cyclohexane to CrO₂Cl₂. The cyclohexyl radical is rapidly trapped by oxidizing chromium species via one of three pathways: (i) chlorine atom abstraction, (ii) formation of a C-O bond, and (iii) transfer of a second hydrogen atom. The mass balance of the reaction and results from reactions of cyclohexane-d₁₂ are consistent with this mechanism. The ability of CrO₂Cl₂ to abstract a hydrogen atom from cyclohexane 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 ability of CrO₂Cl₂ to make a strong O-H bond by accepting a hydrogen atom. The strength of the analogous O-H bond made by permanganate can be calculated from a thermodynamic cycle to be 80 kcal/mol. Using this value for CrO₂Cl₂ provides both a qualitative and a quantitative understanding of the hydrogen atom transfer step. The implications of this perspective for transition metal mediated hydrogen atom transfer reactions are discussed.