Ironii 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)62+. The catalyst provides both rate acceleration and greater selectivity toward the less oxidized products. For example, Fe(CH3CN)62+-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)62+] and [O3] and independent of [substrate] at concentrations greater than ~5 mM. In this regime, the rate constant for all of the alcohols is approximately the same, kcat = (8 ± 1) × 104 M-1 s-1, and that for (C2H5)2O is (5 ± 0.5) × 104 M-1 s-1. In the absence of substrate, Fe(CH3CN)62+ reacts with O3 with kFe = (9.3 ± 0.3) × 104 M-1 s-1. The similarity between the rate constants kFe and kcat strongly argues for Fe(CH3CN)62+/O3 reaction as rate-determining in catalytic oxidation. The active oxidant produced in Fe(CH3CN)62+/O3 reaction is suggested to be an Fe(IV) species in analogy with a related intermediate in aqueous solutions. As a result, this assignment is supported by the similarity in kinetic isotope effects and relative reactivities of the two species toward substrates.