Cp* Iridium Precatalysts for Selective C-H Oxidation with Sodium Periodate As the Terminal Oxidant

Sodium periodate (NaIO4) is shown to be a milder and more efficient terminal oxidant for C-H oxidation with Cp*Ir (Cp* = C5Me5) precatalysts than ceric(IV) ammonium nitrate. Synthetically useful yields, regioselectivities, and functional group tolerance were found for methylene oxidation of substrates bearing a phenyl, ketone, ester, or sulfonate group. Oxidation of the natural products (-)-ambroxide and sclareolide proceeded selectively, and retention of configuration was seen in cis-decalin hydroxylation. At 60 degrees C, even primary C-H bonds can be activated: whereas methane was overoxidized to CO2 in 39% yield without giving partially oxidized products, ethane was transformed into acetic acid in 25% yield based on total NaIO4. O-18 labeling was demonstrated in cis-decalin hydroxylation with (OH2)-O-18 and NaIO4. A kinetic isotope effect of 3.0 +/- 0.1 was found in cyclohexane oxidation at 23 degrees C, suggesting C-H bond cleavage as the rate-limiting step. Competition experiments between C-H and water oxidation show that C-H oxidation of sodium 4-ethylbenzene sulfonate is favored by 4 orders of magnitude. In operando time-resolved dynamic light scattering and kinetic analysis exclude the involvement of metal oxide nanoparticles and support our previously suggested homogeneous pathway.