Pentamethylcyclopentadienyl Iridium Complexes in Selective Carbon-Hydrogen Oxidation [electronic resource].

9781267853080

1 online resource (139 p.)

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While C--H oxidation by ruthenium oxo compounds has been broadly applied in organic synthesis, examples of C--H oxidation by metal oxo complexes from the rest of the platinum group are still rare. Nevertheless, a series of Cp* iridium (Cp*Ir, Cp* = C5Me5 or pentamethylcyclopentadienyl) complexes can catalyze C--H oxidation, with ceric ammonium nitrate (CAN) as the terminal oxidant and H2O as the source of oxygen. Remarkably the hydroxylation of cis-decalin and cis-1,4-dimethylcyclohexane proceeds with retention of stereochemistry. cis-Decalin oxidation gave 18O incorporation into the product cis-decalol with heavy oxygen (18O) H2O. The reaction is also effective for the selective oxidation of cyclooctane and heterocycles. Surprisingly, high selectivity for mono-oxidation of cyclooctane to cyclooctanone was observed. Kinetic isotope effect (KIE) experiments in the C--H oxidation of ethylbenezene to acetophenone yield kH/kD = 15.4 +/- 0.8 at 23 °C and 17.8 +/- 1.2 at 0 °C, which are consistent with C--H oxidation involving a significant tunneling contribution. The nature of the active species was investigated by Transmission Electron Microscopy (TEM), UV-vis spectroscopy, microfiltration and its reactivity in cis-decalin hydroxylation. Density Functional Theory (DFT) calculations showed that the C--H oxidation of cis-decalin by an Ir(V) oxo complex, [Cp*Ir(phpy)(O)]+ (phpy = o-phenylpyridine), follows a direct oxygen insertion mechanism on the singlet potential energy surface, rather than the radical rebound route that would be seen for the triplet, in good agreement with the retention of stereochemistry observed in this reaction. The reaction was improved with sodium periodate (NaIO4), a milder and more efficient terminal oxidant for C--H oxidation with Cp*Ir catalysts than CAN. 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 proceed selectively, and retention of configuration was again seen in cis-decalin hydroxylation. At 60 °C, even primary C--H bonds can be activated: whereas methane was over-oxidized to CO 2 in 39% yield, ethane in excess amount was selectively transformed into acetic acid in 25% yield based on NaIO4. 18O labeling was again demonstrated in cis-decalin hydroxylation with heavy oxygen (18O) H2O and NaIO4. A KIE of 3.0 +/- 0.1 was found in cyclohexane oxidation at 23 °C, suggesting C--H bond cleavage as rate-limiting step. Competition experiments between C--H and water oxidation show that C--H oxidation of sodium 4-ethylbenzene sulfonate is favored by four orders of magnitude. Time-resolved dynamic light scattering (DLS) and kinetic analysis exclude the involvement of metal oxide nanoparticles, and support our previously suggested mono-nuclear pathway.

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English

July 24, 2014

Thesis (Ph.D.)--Yale University, 2012.

Yale University. Chemistry.