OXIDATION OF ALCOHOLS, ALDEHYDES, AND CARBOXYLATES BY THE AQUACHROMIUM(IV) ION

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 x 10(3) L mol-1 s-1) at 25-degrees-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 (mu = 1.0 M HClO4/LiClO4, 25-degrees-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 x 10(3); HC2O4-, 2.2 x 10(3); (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-degrees > CH3 > 2-degrees), the small substituent effect for the benzyl alcohols, and the similarity of all the rate constants regardless of the organic substrate are inconsistent with thc 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 x 10(9) L mol-1 s-1 in 2.0 M HClO4.