Stepwise Oxidation of Anilines by cis-[RuIV(bpy)2(py)(O)]2

The net six-electron oxidation of aniline to nitrobenzene or azoxybenzene by cis-[RuIV(bpy)2(py)(O)]2+ (bpy is 2,2‘-bipyridine; py is pyridine) occurs in a series of discrete stages. In the first, initial two-electron oxidation is followed by competition between oxidative coupling with aniline to gi...

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Published in:Inorganic chemistry 2004-08, Vol.43 (17), p.5205-5215
Main Authors: Seok, Won K, Meyer, Thomas J
Format: Article
Language:English
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Summary:The net six-electron oxidation of aniline to nitrobenzene or azoxybenzene by cis-[RuIV(bpy)2(py)(O)]2+ (bpy is 2,2‘-bipyridine; py is pyridine) occurs in a series of discrete stages. In the first, initial two-electron oxidation is followed by competition between oxidative coupling with aniline to give 1,2-diphenylhydrazine and capture by H2O to give N-phenylhydroxylamine. The kinetics are first order in aniline and first order in Ru(IV) with k(25.1 °C, CH3CN) = (2.05 ± 0.18) × 102 M-1 s-1 (ΔH ⧧ = 5.0 ± 0.7 kcal/mol; ΔS ⧧ = −31 ± 2 eu). On the basis of competition experiments, k H 2 O/k D 2 O kinetic isotope effects, and the results of an 18O labeling study, it is concluded that the initial redox step probably involves proton-coupled two-electron transfer from aniline to cis-[RuIV(bpy)2(py)(O)]2+ (RuIVO2+). The product is an intermediate nitrene (PhN) or a protonated nitrene (PhNH+) which is captured by water to give PhNHOH or aniline to give PhNHNHPh. In the following stages, PhNHOH, once formed, is rapidly oxidized by RuIVO2+ to PhNO and PhNHNHPh to PhNNPh. The rate laws for these reactions are first order in RuIVO2+ and first order in reductant with k(14.4 °C, H2O/(CH3)2CO) = (4.35 ± 0.24) × 106 M-1 s-1 for PhNHOH and k(25.1 °C, CH3CN) = (1.79 ± 0.14) × 104 M-1 s-1 for PhNHNHPh. In the final stages of the six-electron reactions, PhNO is oxidized to PhNO2 and PhNNPh to PhN(O)NPh. The oxidation of PhNO is first order in PhNO and in RuIVO2+ with k(25.1 °C, CH3CN) = 6.32 ± 0.33 M-1 s-1 (ΔH ⧧ = 4.6 ± 0.8 kcal/mol; ΔS ⧧ = −39 ± 3 eu). The reaction occurs by O-atom transfer, as shown by an 18O labeling study and by the appearance of a nitrobenzene-bound intermediate at low temperature.
ISSN:0020-1669
1520-510X
DOI:10.1021/ic0302985