Oxidation of alcohols, aldehydes, and carboxylates by the aquachromium(IV) ion

Article abstract of DOI:10.1021/ja00037a025

Four methods have been developed to prepare aquachromium(IV), which we believe to be an oxo ion, CrO²⁺. It readily converts Ph₃P to Ph₃PO (k = 2.1 × 10³ L mol^-1 s^-1) at 25 °C in 85% CH₃CN/H₂O (0.10 M HClO₄). The reactions used to form CrO²⁺ are those between Cr²⁺ and (a) O₂ (b) anaerobic CrO₂²⁺, (c) anaerobic CrOOCr⁴⁺, and (d) anaerobic Tl(III). The CrO²⁺ has a half-life of 30 s in acidic solution at room temperature and will oxidize alcohols, aldehydes, and certain carboxylates as well as diethyl ether. The second-order rate constants (L mol^-1 s^-1) in acidic solution (μ = 1.0 M HClO₄/LiClO₄, 25 °C) are as follows: CH₃OH, 52; CD₃OH, 15; C₂H₅OH, 88; C₂D₅OH, 41; (CH₃)₂CHOH, 12.0; (CD₃)₂CDOH, 4.6; CH₂=CHCH₂OH, 101; CH₃(CH₂)₂CH₂OH, 44; (C₂H₅)(CH₃)CHOH, 41; (CH₃)₃CCH₂OH, 39; C₆H₅CH₂OH, 56; (C₆H₅)(CH₃)CHOH, 30; (C₆H₅)₂CHOH, 10.5;p-CH₃OC₆H₄CH₂OH, 71;p-CH₃C₆H₄CH₂OH, 66; p-CF₃C₆H₄CH₂OH, 60; c-C₄H₇OH, 44; c-C₅H₉OH, 31; HCHO·H₂O, 92; (CH₃)₃CCHO, 37; HCO₂H, 11.6; HCO₂^-, 6.9 × 10³; HC₂O₄^-, 2.2 × 10³; (C₂H₅)₂O, 4.5. Activation parameters were also determined for selected reactions. In all but two of these reactions (cyclobutanol and pivaldehyde), Cr²⁺ is the immediate product as shown by trapping with O₂. On the basis of the kinetic and product data, the mechanism of oxidation by CrO²⁺ is proposed to be hydride transfer. The reactivity order for alcohols (1° > CH₃ > 2°), the small substituent effect for the benzyl alcohols, and the similarity of all the rate constants regardless of the organic substrate are inconsistent with the formation of carbon-centered radicals. The reaction of HCrO₄^- with (CH₃)₂CHOH is also shown to involve CrO²⁺ and Cr²⁺ as intermediates. The latter reacts with HCrO₄^- with a rate constant of 2 × 10⁹ L mol^-1 s^-1 in 2.0 M HClO₄.