Intramolecular kinetic isotope effects in alkane hydroxylations catalyzed by manganese and iron porphyrin complexes

Article abstract of DOI:10.1021/ja00069a031

Intramolecular kinetic isotope effects (KIEs) in alkane hydroxylations catalyzed by manganese and iron porphyrins have been studied with 1,3-dideuterioadamantane as substrate. This substrate is highly suitable for determination of primary isotope effects in metalloporphyrin-catalyzed hydroxylations: the high chemical reactivity of tertiary C-H (D) bonds avoids side reactions which complicate product analyses, and tertiary C-H and C-D bonds have the same steric and stereochemical environment. Kinetic isotope effects have been determined with adamantane-1,3-d₂ (98 atom % D) and iron and manganese complexes of meso-tetrakis(2,6-dichlorophenyl)porphyrin and mesotetramesitylporphyrin [M^III(TDCPP)Cl and M^III(TMP)Cl,M^III = Fe or Mn] as catalysts. KHSO₅, NaOCl, and PhIO were used as simple oxygen atom donors. All KIE data were calculated taking into account isotope purity corrections and integrated scan calculations. The highest KIE values were obtained with Fe(TMP)Cl/NaOCl and Fe(TMP)-Cl/PhIO: 8.71 ± 0.20 and 7.52 ± 0.21, respectively. With KHSO₅, all KIEs are rather low, between 4.09 ± 0.17 and 4.74 ± 0.17 with Fe(TMP)Cl and Mn(TMP)Cl, respectively (values obtained with benzene as solvent). On one hand, these KIE values obtained with various metalloporphyrin catalysts associated with different oxidants suggest that the nature of the high-valent metal-oxo species is probably a "pure" metal-oxo species when iron porphyrins, especially Fe(TMP)Cl, are activated by NaOCl or PhIO. The leaving group of the oxidant is probably not involved in the rather symmetrical H-transfer transition state in these cases. On the other hand, active metal-oxo species generated by KHSO₅ regardless of the metalloporphyrin or by PhIO or NaOCl activated with manganese porphyrins might qualify as metal-oxo like species, since the leaving group of the oxidant is probably involved in the transition state which is consequently more bent than with catalytic systems generating a pure metal-oxo entity, as is the case for Fe(TMP)Cl associated with PhIO or NaOCl. The temperature dependence on KIE values obtained with different metalloporphyrin catalysts confirms the different transition state geometries between iron and manganese porphyrins and suggests that the high KIEs obtained with Fe(TMP)Cl associated with NaOCl or PhIO might be due to the possible contribution of a tunneling effect.