Iron(II) Catalysis in Oxidation of Hydrocarbons with Ozone in Acetonitrile

Oxidation of alcohols, ethers, and sulfoxides by ozone in acetonitrile is catalyzed by submillimolar concentrations of Fe(CH3CN)(6) (2+) . The catalyst provides both rate acceleration and greater selectivity toward the less oxidized products. For example, Fe(CH3CN)(6)(2+)-catalyzed oxidation of benzyl alcohol yields benzaldehyde almost exclusively (>95%), whereas the uncatalyzed reaction generates a 1:1 mixture of benzaldehyde and benzoic acid. Similarly, aliphatic alcohols are oxidized to aldehydes/ketones, cyclobutanol to cyclobutanone, and diethyl ether to a 1:1 mixture of ethanol and acetaldehyde. The kinetics of oxidation of alcohols and diethyl ether are first-order in [Fe(CH3CN)(6)(2+)] and [O-3] and independent of [substrate] at concentrations greater than similar to 5 mM. In this regime, the rate constant for all of the alcohols is approximately the same, k(cat) = (8 +/- 1) x 104 M-1 s(-1), and that for (C2H5)(2)O is (5 +/- 0.5) x 104 M-1 s(-1). In the absence of substrate, Fe(CH3CN)(6)(2+) reacts with O-3 with k(Fe) = (9.3 +/- 0.3) x 104 M-1 s(-1). The similarity between the rate constants k(Fe) and k(cat) strongly argues for Fe(CH3CN)(6) (2+)/O-3 reaction as rate-determining in catalytic oxidation. The active oxidant produced in Fe(CH3CN)(6) (2+)/O-3 reaction is suggested to be an Fe(IV) species in analogy with a related intermediate in aqueous solutions. This assignment is supported by the similarity in kinetic isotope effects and relative reactivities of the two species toward substrates.